Questions and Answers about Combat Robotics
from Team Run Amok

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Aaron's Wisdom  I've said this often but builders don't want to believe me:

The weapon may be the least important system on a combat robot.
If you're not winning matches it isn't because you have a poor weapon.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: So, I've got a question...

I'm working on the designs for an antweight (US) lifter, and was running the calculations of using a Silverspark motor to power the weapon. However, the math seems to be a bit... off. Here's what I have:

Lifter Arm Length: .33 feet

Maximum Lifting Weight: 2 lbs

Maximum Torque at Gearbox (ft-lb): (.33 feet x 2 lbs) = .66 lb-ft

Max Load At Gearbox (in-oz): (.66 lb-ft x 192) = 126.72 oz-in

Motor Stall Torque: 0.64 oz-in (Fingertech Silverspark @ 6v)

Torque Overage Factor: 2

Gear Ratio Required: ((126.72 / 166) x 2) = ~1.5:1

Is that really it in terms of a gear ratio? All I need is about a 1.5:1 reduction to lift another ant with a Silverspark motor at 6v? I feel like I've done something wrong.

Also, I plan on running the motors at 12v - 14v rather than the recommended 6v. How does this affect the stall torque, or does it remain constant regardless of the amount of voltage applied to the motors? [Newton, Illinois]

A: [Mark J.] You were doing fine right up to the end. That last line should be:

Gear Ratio Required: ((126.72 / 0.64) x 2) = 396:1
It seems that you inadvertently inserted the 166 oz-in stall torque of the Banebots RS-775 18v @ 18v that you used in your earlier calculation for a featherweight lifter instead of the 0.64 oz-in for the Silver Spark @ 6v.

Stall torque, stall current, and no-load RPM of a brushed PMDC motor increase proportionally with voltage. Increasing voltage from 6 volts to 12 volts will double both the speed and the torque of the motor, so your new calculation will be:

Gear Ratio Required: ((126.72 / 1.28) x 2) = 198:1
At 12 volts, the 200:1 Silver Spark gearmotor should do.

Q: Hey there, Mark! This might be a dumb question, but I'll ask it anyways. Over on Facebook people mentioned the Fingertech vertical spinner which is apparently [expletive deleted]. I noticed, however, that somebody mentioned if it was crafted from AR400 instead of Titanium, it would hit harder. Why? Is it because of the mass increase? Is that really enough to make this spinning weapon more effective? [Kankakee, Illinois]

A: [Mark J.] Changing the material of the FingerTech vertical spinner blade from Grade 5 Titanium to AR400 Steel would result in a heavier, stiffer weapon that would in fact 'hit harder' -- but the improvement would be too small to help this specific weapon.

  • The FingerTech titanium vertical blade is a VERY small weapon blade. With a 1" spin radius and a weight of 10 grams, I calculate that it stores about 1.5 joules of energy at 11,000 RPM. A blade the same size and shape made from steel would weigh about 17 grams and store about 2.5 joules of energy. A typical antweight spinner weapon will store more than ten times that much energy.
  • Titanium is only about 60% as stiff as steel. That means that a titanium weapon of a given design will flex more on impact and will be less effective at transmitting the impact energy to your opponent -- like a fist in a boxing glove versus a bare fist. A steel version of the weapon blade would 'hit harder' than the titanium version, but there is so little energy involved here that impact flexing would be negligable for either material.
Read thru the Ask Aaron Spinner Weapon FAQ for more info on effective spinner weapon design.
Q: Hey Mark! This is probably a "cheerleader" question, but I wanted to at least try and give it a chance.

My team and I want to build a heavyweight. We know we want a strong chassis, now it's just time to figure out the weapon! We want to try a front-hinged flipper, but we aren't quite sure of where to start. Is there a "good" number to shoot for in terms of joules per pound, like spinners? We aren't necessarily trying to get the same power as bronco, as it's going more forward versus upward; we just want enough to get them over the wall.

Sorry if the hamburger is bad, we're mostly theorizing at this stage. [Akron, Ohio]

A: [Mark J.] Yes, it's a question. First I'll explain why your team shouldn't build a heavyweight flipper, then I'll point you to answers to your 'starting point' question in case you choose to make the mistake of going ahead and building it.

Why not? If you decided to take up mountain climbing, it would be a poor idea to catch a plane to Nepal and start walking up the southwest face of Mt. Everest; you should start with a smaller mountain. A heavyweight flipper is Mt. Everest -- expensive, dangerous, and not for the inexperienced. The questions you're asking make it clear that you have no experience with pneumatics and no experience with big combat robots. It's failrly certain that the selection committee for the only heavyweight robot tournament currently operating in North America will not look kindly upon your application to compete. Build some smaller combat robots, work out the finer points of pneumatic flippers on a featherweight, and put together a combat résumé that will get you to your goal.

Ignore my advice? A pneumatic flipper is not a 'one number' system (and neither are spinners). You need a system with enough theoretical force to accelerate your opponent into an arc that will achieve your purpose, but you also need to flow gas from the storage tank into your actuator as quickly as possible to get the speed 'pop' that separates a flipper from a lifter. That last part isn't easy. I have a couple of tutorials and a spreadsheet tool that you should find useful.

Q: I seemed to have forgotten one REALLY IMPORTANT word when writing my flipper question: E L E C T R I C. My team has built several robots in the past (a 4 bar lifter and some spinners) with decent success in smaller weight classes. It's true it'll be our first heavyweight, which is why we don't want to build something we aren't familiar with, as you said.

A: Heavyweight... ELECTRIC... Flipper. Woof. The cheerleader had one look at this and took the day off, so I guess it's up to me.

There are a few insect-class flippers using exotic electric servos, but the square-cube law pretty much rules out heavier flippers directly driven from an electric motor. Sub-lightweight electric-kinetic flippers store up energy in a flywheel to get enough 'pop' to launch opponents. The only remotely successful heavy electric flipper has been Team Whyachi's 'Warrior SKF' (aka 'Warrior Clan' and 'Warrior Dragon'), although many of their victories have been via direct attacks with their flywheel/spinning disk rather than attacks from their anemic kinetic energy flipper. Here are the primary problems:

  1. A flywheel flipper system requires a custom machined 'dog clutch' as explained by Dale Hetherington at his Flip-o-Matic page. Dale developed the Flip-o-Matic for use in hobby and featherweight 'tabletop' competitions that don't allow exposed high-speed spinner weapons. Dale has had good success under those conditions, but in open heavyweight competition you're going to be badly outclassed by more powerful weaponry.
  2. All electric/kinetic flippers share an unusual attribute: they have very short actuation ranges. For example, the flipper on 'Warrior SKF' moves only a few inches. This allows the flipper to expend all of the limited stored energy an a short but explosive burst. Extending this stored energy over a longer 'throw' -- like the large angular throw of a front hinged flipper -- means less energy per unit of distance travel and would require a HUGE flywheel to store enough energy to be effective. A pneumatic system can simply tap into more pressurized gas to continue the motion, but the kinetic energy stored in a flywheel depletes quickly and there won't be any more 'til it spins up again. It really isn't the right choice to power a long-throw, front-hinge flipper.
Build a nice featherweight to test your exotic design. The cost to build and perfect a feather will be less than the cost of a single redesign on a heavy. Besides, BattleBots reboot season 4 isn't guaranteed...

Q: My 1lb antweight is exhibiting some odd behavior. The weapon blade is the fingertech titanium blade. I have a V-spec 2205 motor, a 450mah 4s 70c battery, a 20a brushless ESC and two fingertech ESCs with accompanying silversparks. When I spin my weapon up to max or near max throttle one or more of these behaviors *sometimes* exhibit themselves: The power LED starts flickering, the robot starts doing donuts, power to the weapon motor gets cut and it spins down. As soon as I turn the throttle down it fixes itself and its free to spin back up again. Is this a problem with the weapon ESC not being able to handle the weapon? [St. Louis, Missouri]

A: [Mark J.] Your primary problem is not your weapon ESC; the symptom of an overstressed ESC is a puff of smoke and unrecoverable loss of motor power. You didn't mention your weapon layout, but assuming that you're running the weapon direct-drive your problem is the weapon motor.

The FingerTech weapon blade as used as an add-on weapon for the Viper robot kit is designed to spin at 8000 to 12000 RPM under the power of an 1100 Kv direct drive motor. You're attempting to spin that blade at about 34,000 RPM with a 2350 Kv motor that has a lower torque constant than the design motor. The motor simply isn't capable of spinning the weapon to that speed given the maximum current output (about 30 amps) of the battery pack you've chosen. The motor 'bogs', you get serious 'voltage sag' from the battery, and your electronics 'brown out' -- which gives you the symptoms you're reporting.

Your battery won't put up with this level of abuse for long, and may have been damaged already. If the plastic coating is at all 'puffy' and no longer tight, the battery shoud be safely disposed of immediately -- under no circumstances either charged or used.

A larger battery pack capable of supplying the 60 or 80 amps the motor might require operating bogged down to perhaps 15,000 RPM in this application would simply transfer the failure point to the weapon ESC or the the motor itself, which is rated for only around 25 amps continuous current.

Scrap your weapon motor and pick a lower Kv unit that will be able to deliver enough torque to spin the blade at a more reasonable terminal speed. Don't over-do it. You'll likely want a battery with a bit more peak current output and - depending on the selected motor - a larger capacity ESC as well.

Q: The battery seems fine and that sounds like a big overhaul that I don't really have time for, as a short term "hot fix" could I simply off-set the throttle in my transmitter settings to prevent the motor from revving up to full speed?

A: The problem is that your weapon motor can't rev up to full speed because of the aerodynamic drag of the weapon at such unrealistic speeds. Applying less throttle will reduce the current to the motor, but it also proportionally cuts torque. You might well need to cut throttle to around half to keep all the components in their safe zones. Spin-up times will greatly suffer. Try a new throttle setting in a practice session and check temperatures carefully. I'm also concerned that your battery may not have the capacity needed for a full match with a bogged weapon motor sucking down big amps. Best luck.

Q: How come I never had this problem with a 3s 65c battery and my emax 2204 motor? Similar KV to current motor.

A: Increasing voltage has rather large consequences.

Wattage increases by the square of voltage; the increased voltage also increases the current. Bumping up from 3 cells to 4 cells (a 33.3% increase) results in 1.333 * 1.333 = 1.78 a 78% increase in wattage, motor output power, and peak battery drain.

A similar effect is seen on weapon aero drag. Increasing the weapon speed by 33% means that it has to move 33% more air out of the way per unit time and it has to move each unit of air out of the way 33% quicker. That's the same formula highlighted above. The motor now has to work 78% harder to maintain the faster weapon speed -- if it has enough torque to maintan that speed at all.

Q: Do you think just stepping back down to the old 3s lipo could solve this?

A: Quite likely, yes.

Q: Just a quick update for anyone in the future reading who has a similar issue, I tried the 3s and it didn't fix the problem, it just made my robot weaker. I'm going to try offsetting the throttle so its max signal is like 75% throttle. I can't see any reason this wouldn't work, as the robot works fine if I manually keep the throttle under 80% or so, so if I do not give any future updates assume that this fix worked... or alternatively that the blade flew off and lodged itself in my trachea.

A: The failure of the three-cell battery to solve the problem casts suspicion on a new component in addition to the high current draw issue. You haven't mentioned the specifics of your weapon ESC except to say that it has a 20 amp rating, but it's possible that it's faulty or has been damaged by the current load.

  • If your weapon ESC supports calibration to your radio signal range, run that calibration sequence and try the 3s battery again.
  • If calibration doesn't help, consider replacing the weapon ESC.
From 1,734 miles away that's about all I can suggest for a true 'fix'.

Q: This is the ESC: DYS XSD 20A Brushless Speed Controller. Are you suggesting that upgrading the current rating to say 30a could help or just that it may be faulty and in need of replacement.

A: The conversation has wandered around a bit -- lets recap:

  • I'm confident that your 'brown out' problem stems from attempting to spin a 6-inch weapon bar to 34,000 RPM with a low-torque/high-RPM V-Spec 2205 motor powered by a 450mah 4s 70c battery via a 20a brushless ESC. This set-up is pulling more current than any of these components can handle.
  • Based on your prior 'bot that ran a similar set-up on a 3s pack, replacing the 4s pack with a 3s pack should eliminate the problem -- but it does not. This leads me to believe that the weapon ESC may be faulty or damaged.
  • Running the weapon motor on the 4s pack with a reduced max-throttle weapon setting on your transmitter won't not work, but it doesn't address the actual fault.
  • I'm suggesting that you replace the weapon ESC and try it again on a 3s pack largely because I don't like leaving a mystery unsolved, but also because a faulty/damaged ESC may completely fail in combat without additional warning. If it passes this test you can go back to running it on 4s at reduced throttle with increased confidence.
  • If you want to run a 4s pack with full weapon throttle, upgrading just the ESC won't get you there. Neither your motor or battery pack are likely to survive on a 4s pack with your weapon load. If you plan on upgrading the motor and battery at some point you can certainly move to a higher capacity ESC now while running at reduced throttle -- it won't hurt.

Q: It's also been suggested to me that the BEC on fingertech ESC's isn't the best and this could also be part of the problem.

A: That was my first thought when I read your 'flickering power light' symptom, but overstressed BECs on tinyESCs behave like this. BEC output and receiver current demands don't increase with raising weapon motor speed, so why would the problem only manifest above a certain throttle level? Try an alternate source of receiver power if you like, but I'm not optimistic.

Q: THE STANDALONE BEC FIXED IT! I CAN GO UP TO 100% LUDICROUS SPEED WITH NO POWER ISSUES!!!!!!!!!!!!! (at least on the 3s battery, I'll update with the 4s later hopefully I don't run into an Icarus situation)

90 minutes later...

On further consideration I've decided to leave well enough alone and run on 3s, the 4s battery is heavier and it does more to improve top speed than spin up times. Top speed is something that I don't need (It's already stupid fast at 3s, 26000 RPM if the calculations hold true) and would decrease bite, spin up is good enough already and way better than last years set up. I'll use the extra weight for wheel guards. I do feel bad for having spent the money on the bigger batteries but I think I may just need to accept that this hobby is a money sinkhole by nature.

A: Super awesome! I think that's a solid decision -- but there is a troubling question remaining: what's drawing so much 5 volt power?

Q: Hey, I'm not to sure if this has been asked, but how I should go about heat treating my s7 weopaon? [McAllen, Texas]

A: [Mark J.] It's not practical to heat treat S7 tool steel in your home workshop. Heat Treating S7 is a multi-step process requiring precise temperature control up to 1750° fahrenheit with carefully monitored heating and cooling rates. You need a specialized, fully instrumented furnace plus experience in the art. Backyard 'hillbilly' metalurgy will simply turn your weapon into a warped, brittle, useless piece of scrap.

Find a commercial heat treating company nearby (like Texas Heat Treating in Round Rock) and explain what you're doing. They just might be BattleBots fans and be willing to slip your little piece in with a larger commercial order.

Q: I know Hellachopper isnt the best bot to be asking questions about, but I was reading about two of its apparent abilities on their Facebook Page and they have me thinking more than I should:

EXTENDABLE FLAILS - They apparently have the ability to extend and retract their flails at will. First off, what would happen if they extended or retracted at full speed? How would that effect the bot, forces applied to the robot, spin speed, etc? Second, what would happen if they simply let go all of a sudden, letting the flails pop out quickly and violently?

SELF-BALANCING ARMS - How does that work? How can the arms simply balance themselves with a hammer missing? [Naperville, Illinois]

A: [Mark J.] I generally won't discuss combat robots that have never even fought a match. 'Hellachopper' is best known for catching fire during its 2016 BattleBots safety check - twice. BattleBots leads builders to try some crazy things.

The physics involved are a little beyond the usual short answers I give here. I've spent quite some time writing and re-writing the answers to keep them as brief as possible...

Extendable Flails - Extending or retracting the flails by itself will not change the total rotational energy of the weapon -- but since the Moment of Inertia of the weapon changes as the flails move in or out, the weapon speed will change.

  • Spinning at constant speed with the flails 'in': extending the flails will slow the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a larger circle. If the weapon motor has additional torque available at the lower weapon speed, it can apply that torque to spin the weapon to a higher speed. Calculations for 'Hellachopper' presented on the Reddit forum claim that aerodynamic drag with the flails extended would prevent the weapon from attaining full target speed in that configuration.
  • Spinning at best speed with the flails 'out': retracting the flails (not easy because there's a lot of cetrifugal force acting on them) would increase the weapon RPM. Tip speed remains nearly constant, but the the impactors now travel in a smaller circle. If the weapon motor cannot maintain the weapon at that increased speed, the speed will fall to the level the motor can sustain.
Watch this video on Conservation of Angular Momentum to see this effect in action. The rate at which the flails extend will have no impact on the above changes -- unless the flail cables snap from the sudden load.

Self-Balancing Arms - Self balancing systems are fairly common. Some washing machines have a 'donut' shaped tube partially filled with liquid wrapped around the tub that will compensate for an imbalanced load of clothes during the spin cycle. Somewhat counter-intuitively, the liquid will shift position in the tube to position itself at the location of lowest energy, which will be opposite the 'heavy' side of the washer load as it spins.

Several combat robots with hollow drum weapons have used a product made for commercial truck tires that works the same way. Tiny ceramic beads act like the fluid in the washing machine system to redistribute mass to maintain balance, even if the weapon is damaged and balance changes.

Hellachopper's balance system is based on this same principle, but it relies on flail anchor points in sliding grooves moving to change the distribution of mass. Clever, but I'm not entirely convinced that it will work; the balancing 'fluid' needs to be free to move independent of the rotation of the weapon, and the flail anchors are not entirely 'fluid'.

Q: Hi Mark,
This is more of a theoretical question, and I've searched the archives for an answer, I think I've got it partially resolved but I wanted to ask you directly (please don't make this a Bad Hamburger):

I'm imagining two scenarios where two identical brushless motor and ESC combinations are spinning up geometrically identical rotary weapons (be it a beater, drum, bar, disk, etc.) but one is made out of hardened S7 tool steel and one is made of 6160 aluminum. If my math and intuition are correct, the two weapons should spin at the same angular speed after a sufficient amount of time has passed; the only difference is that the aluminum weapon will have a lower spin-up time (neglecting contact friction forces from bearings or anything dependent on weight).

I say this because the motors will supply the same torque, which is equal to the product of angular acceleration and moment of inertia (MOI). Of course the steel bar is going to have a higher MOI so it will have a lower acceleration, longer spin-up time. However the final speed of each weapon will occur when the torque produced by aerodynamic drag is equal to that which the motor can output. If the shapes are identical, shouldn't they have the same drag force acting on them, and then they'd have the same rotational speed?

As a side note, I'm currently reading through the RioBotz tutorial and will likely return with more questions after I finish that.

Thanks, Jack from Lake Charles, Louisiana.

A: [Mark J.] You're correct, Jack -- but I'm not sure where you're going with this.

  • The aluminum and steel bars will have equal aerodynamic drag and will have the same terminal speed;
  • The aluminum bar will spin up to that speed faster than the steel bar because of its lower MOI;
  • But when spinning at equal speeds the weapon with greater MOI will store more kinetic energy and will hit harder.
You can verify this with one of the Run Amok Spinner Weapon Calculators. Example:
For a spinning bar weapon 450mm by 75mm by 24mm in the usual spinner orientation:
  • Made of steel, the bar weighs 6.32 kilos with an MOI of 0.11 kg-m2. It stores 6200 joules of energy at 3200 RPM.
  • Made of aluminum, the bar weighs 2.24 kilos with an MOI of 0.04 kg-m2. It stores 2200 joules of energy at 3200 RPM.

Q: Why would anyone build a symmetric bar spinner? Don't asymmetric bars deliver more force at lower RPMs and save weight? [The Aether]

A: [Mark J.] You've made an incorrect inference about asymmetric weapons -- they don't deliver more force at lower RPM, they have better 'bite' at high RPM.

An asymmetric spinner bar reduces the number of impact teeth from 2 to 1 in order to improve weapon 'bite' (see: RioBotz Combat Tutorial section 6.3). Too little 'bite' results in a spinner weapon 'grinding' along your opponent's armor without delivering a solid impact. Improved 'bite' allows you to design a lighter bar and spin it at higher speed to attain the same energy storage as a heavier bar without sacrificing the ability to deliver a solid 'hit' on your opponent. If your design features a long bar that needs to be thick and heavy for structural strength and it spins slowly enough for good 'byte' (think 'Tombstone') there is little advantage to going asymmetric.

Q: Probably just an insane idea but in theory if you stacked two horizontal spinning blades on top of each other and spun them in opposite directions? What, if anything, could one expect to happen when they hit? [Pittsburgh, Pennsylvania]

A: [Mark J.] It is fairly insane, Pittsburg -- but that hasn't prevented people from building such weapons. Their combat records are not great. We've discussed both horizontal and vertical counter-rotating spinners in this archive -- search here for hozizontal spinner 'Double Dutch' and vertical spinners 'Counter Revolution' and 'CounterStryker' for full discussions. Here's a sample:

An alternative twin-spinner system design is employed by 2018 BattleBots competitor 'Double Dutch'. Their design features counter-rotating bar spinners placed above and below the robot body. Although both weapon bars can attack an opponent at the same time, it's likely that one bar with strike before the other and throw the opponent clear before the second bar can impact. Almost certainly the two potential weapon impacts will strike different locations on the opponent, which will also limit their effectiveness.

Q: on the excellent spinner spreadsheet the formula for getting a rough calculation of the stall torque is given as ((1352/Kv)*(Volts/(mOhms/1000)))/141.69

i think i must be mixed up, because it seems like this formula is producing some crazy numbers - so maybe i'm doing something wrong and you can help me out. for example, if you look at the spec of a brushless motor like the Scorpion HKII-7050-330KV and plug in the numbers:

Volts = 50
internal resistance in mOhms = 5
Kv = 330

if you use the spreadsheet formula, you get a stall torque of an incredibly powerful 289 N-m!

but if you calculate torque using some other formulas that i've learned about, like Kv/Kt where Kt is 9.549/Max Amps or using the formula PeakPower/w, where w is given by (2*pi*Max RPM)/60 then you find that both of those formulas get you a more reasonable number of 8 N-m of torque, which is way different than 289 N-m.

so am i misunderstanding something fundamental here about brushless motors and torque? did i mess up the numbers or formulas somehow? am i being dumb? i think i need some help.

-josh in salt lake city, utah

A: [Mark J.] Let's start by applying the formula from the spreadsheet to a motor with known specs. The brushed AmpFlow A28-400 will do nicely:

  • 24 volts
  • 44 mΩ
  • 205 Kv
The spreadsheet formula gives a stall torque of: (1352/205)*(24/0.044)/141.69 = 25.16 N-m. That's really close to the published 25.13 N-m stall torque for the motor -- the formula is correct.

The Scorpion HKII-7050-330KV can deliver 15,000 watts of output power while sucking down 300 amps of current. If you pay $999.99 for a brushless motor you're entitled to 'incredibly powerful' results, but does it actually deliver the 289 N-m of stall torque the spreadsheet formula says it will? Absolutely not, because brushless motors are different than brushed motors in how they operate at low speeds. The concept of 'stall torque' really doesn't even apply to hobby brushless motors. See this archived post about brushless hammer drive for greater detail on low-speed brushless torque.

So, why do the other formulas you cite give a much different torque number?:

  • Torque = Power / Angular Speed -- calculates torque at a specific speed where true power output is known. You're assuming 'Peak Power' and 'Max RPM', which never occur together in an electric motor. Stall torque occurs at zero speed, where output power is also zero, giving a 'division by zero' error; the formula cannot directly calculate stall torque.
  • Torque = Kt / Kv -- it's 2 AM and my brain has gone to mush, but this method of calculating stall torque isn't making any sense at all to me. This webpage may offer some additional help on the relationship between Kt and Kv: Learning RC - Brushless Motor Kv Constant Explained.

I don't think that you're using the most recent version of the Run Amok Excel Spinner Spreadsheet. The current version (19-D) is optimized for brushless motors and has a more sophisticated method for approximating true low-speed torque that takes motor controller software into account. Download it from the Run Amok tools page.

Q: thanks for the explanation and the pointer to the new spinner spreadsheet. great stuff (!!) but i think the new spreadsheet seems to still have a major limitation related to brushless motors, and i think it might be because there is no ability to plug in the maximum amperage (maybe?) and so instead it goes to some unrealistic theoretical maximum which is so unrealistic as to not be useful to model - or maybe i'm screwing something up.

for example, if you take the stats from that fancy scorpion motor:

50 volts
330 Kv
5 mΩ
10 magnet poles

and say we use a gear reduction of 5 to 1 and then let's say our horizontal spinner is made of steel, 0.5 meters long, 0.125 meters wide, 25mm thick, which is about 12kg (about 27 lbs).

so the results from the calculator are that this blade achieves 3,000 rpm in less than 0.3 seconds (my gut tells me that this is probably off by a factor of 10 or more) and that it will peak at 8,500 amps, which at 50 volts is almost half a megawatt of power. i doing something wrong, or is this spreadsheet unable to model a big brushless motors like this very well?

thanks again - josh in salt lake city

A: You're planning on buying a $1000 brushless weapon motor capable of 20+ horsepower output and then strangling it with an inadequate power supply? Well, to each their own...

The spreadsheet assumes that the weapon motor will be provided with all the current the brushless controller chooses to pass to it, without voltage sag. There is no adjustment for a fixed 'current maximum' because brushless controllers generally do not have true current limiting, but the spreadsheet is capable of making specific adjustments for changes in the controller firmware that limit control the motor speed at which the 'soft start' feature cuts out and the maximum power pulse percentage during that soft start period.

In SimonK firmware these features are known as 'Commutation Max' (commutation time in u-sec below which 'soft start' restriction is active) and 'Power Max' (maximum power pulse length during 'soft start') but other firmware will typically have similar features. You can 'play' with these settings in the spreadsheet to see their effect on current consumption and performance, then transfer the settings to the controller firmware to implement those power restrictions.

Date marker: June 2019
Q: Hi Mark, I'm trying to spin a bar with impactors with a moi of 0.055 kg*m2 with a Scorpion HKIV-4025-1100KV outrunner. I believe it will spec out to be 13,200 rpm with 13.9 Nm of torque at 12V. I was going to gear it down 3:1 so the weapon would spin at about 4,000 rpm maxed out. My question is about the amp peak when using the spinner tool. I read in the forums about your explanation of ohms law saying this was only theoretical but it still concerns me.

Is my design flawed and is that causing the high peak? Should I be using a brushed motor instead? What ESC should I be using? Would the current setup do - instantly fry, twitch back and forth then fry? I hate electronics so your help is greatly appreciated. This is an overhead spinner for the 12lb class. [Rock Hill, South Carolina]

A: [Mark J.] Sounds like you're in way over your head here. Take a deep breath, relax, and try to forget all about those disturbing numbers.

The Run Amok Excel Spinner Spreadsheet is a complex tool that requires the addition of a healthy scoop of 'real world' to temper its results.

The most effective use of the spinner spreadsheet is to model an existing spinner weapon system that is known to function well, then model a new design to see how the outputs differ. You can then make a reasonable judgement about the viability of the new design.
That 1400 amp current spike you're looking at is a theoretical value which assumes that the weapon motor is the only resistance in the circuit. Your ESC will add in at least as much resistance as that particular motor, and the internal resistance of your battery is somewhere in the same ballpark. Put that all together and your real world peak current tops out at around 300 amps and falls off quickly with increasing RPM. Feeling better now?

Part two: since amps equal torque and that high current spike is theoretical, that 'wishful thinking' 13.9 Nm of stall torque goes away as well. Because of the black magic any specific brushless ESC goes thru to get its associated motor to spin up from a stand-still you'd never see that amount of torque even in theory. The concept of 'stall torque' really doesn't even apply for hobby brushless motors; spin-up time for a brushless spinner will be longer than the theoretical output of the spreadsheet.

Lets take a look at some of the specifics of your design:

  • You didn't share actual dimensions, but my best guess says a bar with a Moment of Inertia around 0.055 Kg*M2 will weigh about seven pounds. That's way too much for a 12-pound robot. Spinning that heavy bar at 4000 RPM will store about 5000 joules of energy. That's healthy for a featherweight -- overkill for a hobbyweight. Excess ≠ Success
  • You've picked a $200 helicopter outrunner motor for your weapon. Nice motor, but it's designed to run at twice the voltage you intend to supply. That's like buying a Ferrari and sticking a block of wood under the gas pedal to limit output to 25% power*. If you're gonna do that you should just buy a Hyundai to start with.
* Electric motor power increases with the square of voltage: double the voltage equals four times the power.
Standard Advice If you don't know what you're doing, copy a successful design. Look around at hobbyweight spinners and find motor/ESC combinations actually in use that are powering effective bar spinners. Size your bar reasonably and make improvements over time as you find weaknesses.

Q: Hi Mark, I had a follow up question about the spinner spreadsheet. Your explanation was very helpful and brought much comfort. I know the 7lb bar is overkill but I wanted to try something like that at least once just to see what would happen in the area upon contact. Overkill is underrated. Maybe...

You said ballpark, the resistance would be about 3x the amount listed. This changes the calculation some on the battery requirements, right? Couldn't I use a much higher voltage battery with a much lower mAh? I ask because as always weight, but particularly space, is an issue. Many thanks!

A: My resistance explanation above was intended to explain why the big start-up current surge the spreadsheet shows isn't really that big in 'real world' terms. As the weapon speed starts to rise the electrical characteristics of the motor start to change. Here's a more complete explanation.

At theoretical 'stall' the combined resistance of the this particular motor and a typical controller and battery is about three times the resistance of the motor alone -- but it doesn't stay that way. The internal resistance of your battery pack and controller do not vary with weapon speed, but the effective resistance of your motor increases linearly with speed due to the rise of back EMF generated by the motor. With the weapon spinning at full speed the effective resistance of the Scorpion motor will have risen from 0.006 ohm to nearly 2 ohms and the added 0.012 ohm resistance from the battery and controller will have long been rendered insignificant.

For purposes of the Spinner Spreadsheet you should still enter the resistance value for the motor alone because the spreadsheet takes into account the rise of back EMF with increasing RPM. The additional resistance at start-up lengthens spin-up just a bit and the huge theoretical current spike at start-up is diminished, but the total draw on the battery to spin the weapon bar up to 5000 joules is essentially the same. Just remember that even though the big current spike at start-up isn't really as big as the spreadsheet shows the total current consumption is close to correct. There's no such thing as a free lunch; you still need a battery with the same watt-hours of capacity.

Q: Btw, what ever happened to the Texas bar spinner?

A: The conversation I had with Tex broke off abruptly and I have no more recent news. Here's where the 'Texas Spinner' page leaves off:

It's been about a week since I last heard from Tex. I have, however, heard from a number of applicants for the second season of BattleBots on ABC about their rejection calls. One note was from a BattleBots veteran who's design was rejected for not having enough 'bling':

ABC was picking robots based on appearance. We got thrown into the bar spinner category, and Trey [Roski] said they had a ton of entries that looked exactly like Tombstone...

Given that Tex was pitching a Tombstone clone I suspect that his 'hook' wasn't tempting enough to get a positive response from BattleBots. I hope he builds a 'bot anyhow.

Q: Hi Mark,
Probably a dumb question but I figured I'd have to ask. So you know how a 540 size motor attaches to a p60 gearbox to drive a bot... why isn't a similar set up used for powering the weapon? Attach a motor to a gearbox and use that to drive the weapon. It would certainly be easier to mount and you'd have a keyed shaft instead of a 'D' profile. It seems like a no brainer in the hobbyweight since you usually take that size or similar motor and gear it down 3/4/5 or so. Then you could do a 1:1 v belt or flat belt from keyed shaft to weapon to allow for slippage. Does the gearbox get too hot for this? Couldn't you put a fan on it if it does? I can't imagine I'm that smart and would like to know where my thinking is wrong. Many Thanks! [Raleigh, North Carolina]

A: [Mark J.] I've seen this done, Raleigh. It's not a horrible design, but you lose more than you gain:

  • Simplicity Your design still needs a belt to save the gearmotor from destructive shock loads on weapon impact. If you do the reduction with the belt drive you leave out complex parts that could become failure points.
  • Efficiency Gearbox heat comes from internal frictional losses. Leave out the gearbox and that lost energy can be spinning your weapon quicker and faster.
  • Weight A 4:1 P60 gearbox weighs in at 6.5 ounces -- plus grease. Scrap the gearbox and you can put that weight directly into the weapon where it will do some good.
  • Cost A 4:1 P60 gearbox runs about $50 -- delivered. A larger driven pulley to get that reduction costs much less.
Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

Q: I am running a Dr. MadThrust 1700 kv motor for my weapon on my 15 lbs robot. I need to find a battery pack or make my own that will be able to run that motor and I cannot use Li-Poly batteries. I know that I need a pack around 3000 mAh but I don't know about the discharge rate. Do you have any advice? [Dublin, Ohio]

A: [Mark J.] The standard solution for robot events that do not allow Li-Poly batteries is a switch to safer and universally allowed lithium iron phosphate packs -- also known as 'LiFe' as a contraction of their LiFePO4 cathode material.

LiFe packs have lower discharge rates than Li-Polys, so you'll want to go with larger capacity packs to obtain higher amp peak drain. A pair of 3S 30C 4200mAh packs in series will get you in the ballpark. LiFe cells have different charge requirements than Li-Polys, so make certain that your charger has a specific LiFe setting.

The Dr. MadThrust 1700 kv motor is rated 105 amps of current, and the 30C 4200mAh packs are rated for 126 amps continuous current output with a 168 amp peak. The peak current draw of your weapon motor under load depends as much on weapon size/weight/gearing as it does on the motor but you gave no weapon info. I think you'll be fine -- LiFe packs are more forgiving than Li-Poly packs, and a spinner weapon will pull high current only briefly while spinning up before settling at a much lower level. Best luck.

Q: I'm looking through your info on spring flipper mechanisms and it's very well done & informative. My question is, how would I adjust a "choo choo' mechanism to make it shoot down, like for an axe? [Woburn, Massachusetts]

A: [Mark J.] Thanks. I'm happy to hear that you like our guide to spring flipper designs, but I must admit that I'm surprised that you looked at the four mechanisms on that page and decided that the 'choo-choo' would be a good choice for an overhead axe weapon.

The demands of flippers and overhead axes are quite a bit different. An axe can build energy gradually over a long arc before striking its target, while a flipper needs a big burst of energy released instantly. A modified flipper mechanism is not your best choice to swing an axe, which is why you haven't seen them used for that purpose.

  • The reaction a flipper 'bot experiences from the quick upward burst of energy just presses it down onto the arena surface where it is well supported. Everything remains stable.
  • That same quick burst of energy applied to a downward striking axe weapon will produce a large reaction that lifts the front of your 'bot and tries to flip it over backwards before the axe even strikes your opponent.
You've got plenty of 'swing' to build axe speed with a simple gearmotor powering the axe directly -- don't add the complexity of a spring load plus a mechanical linkage to wind and release it.That said, it is certainly possible to orient a 'choo-choo' to pull back and release a spring loaded hammer: see sketch at left for a simple solution. It will work, it just isn't a good idea.

Q: This isn't a techy question and it unapologeticaldly fits in the cheerleading section but I just had to ask it anyways. I've been debating asking this question on this site for weeks now and I just had to ask. If it's a garbage question through and through just toss it lol!

Okay, so I've read this website for a while now and I am aware of the cheerleading section and this belongs there but if you have the time I would love your opinion. Ima cut right to the chase and say this: Weaponized Turbojet Engine Afterburner.

You see all of these crazy jet engines on youtube that spit white hot flames and enough noise to wake the dead and we know from bots like ICEwave that crowds love loud bots. Now there are MANY design challenges flaws for using a jet engine in combat robotics. From the reliability of a 100,000 RPM engine when hit by a massive spinner to the weight of the jet not to mention the heat problem... In my mind the weapon would either be a automotive turbo outfitted with a flame tube or a custom made turbojet (which would package a lot better). Maybe even an EDF throwing air through to the afterburner instead of the heavy jet engine (although it would sound like a vacuum). It would be very hard to get working right, as I'm sure most non electric powered weapons are, and the robot wouldn't ever be competitive but what are your thoughts on a weapon like this?

Have a great one guys! [London, Ontario]

A: [Mark J.] The next time you're on YouTube looking for dangerous machines you might pop 'Suvival Research Laboratories' into the search box. SRL puts on destructive machine performance art shows featuring devices that include a big pulse jet very much like you suggest here. Their outdoor shows are the right place for these creations; a robot combat arena is not suited to such displays.

  1. In the very few events that allow robots to carry flame weapons there are strict fuel volume limits. BattleBots allows a 0.5 litre tank of propane or butane, which I suspect would would power a worthy turbojet for perhaps 10 seconds.
  2. A heavyweight-class combat arena is a very expensive item made largely from thick sheets of very meltable plastic. Imagine what happens to the remains of your white-hot jet engine when a spinner weapon hits it with enough energy to rip a water heater in half. No event organizer is going to allow your turbojet inside their precious arena.
Book a flight to the next SRL performance and watch their pulsejet melt a Volkswagen. That oughta get this out of your system. If not, here's the button:
Q: Hey there Mark, I was hoping to ask you about two seperate but semi-related questions concerning some bots I'm building (beetle and feather), both using HDPE.

1) All of my bots have been made of HDPE from 5 to 20mm thick, cut with a mixture of a handsaw, a jigsaw, a chop saw, and a circular/skil saw. I notice however with making curved cuts, the resulting cut tends not to be right-angled, getting more and more drastic the further you go up in thickness. The jigsaw gives cleaner finishes but bends more often, and the hacksaw gives squarer finishes but tends not to curve as well and gives a rougher finish. Do you have any tips for getting cleaner curved cuts with plastics like HDPE/UMHW that don't involve machining?

A: [Mark J.] Given your selection of tools, I think the jigsaw is your best option. Several tips:

  • Buy a brand new blade for your jigsaw and use it only for soft plastic.
  • Several manufacturers offer special blades for soft plastics that reduce heating and make for smoother cuts -- example.
  • Blade tooth count: about 10 TPI works well for HDPE.
  • For tight curves: rough cut about a centimeter from your final cut line, then come back with a second cut to trim away the thin edge.
  • A little wax on the blade helps reduce friction and heating.
  • Try lower blade speeds, and keep the material moving.
  • Square up edges with a sanding block.
2) My second question concerns a snail cam design for a spring flipper. I'm aware that you've been asked questions concerning this topic multiple times and have read through all relevant posts, but for my design I've attempted to adjust the parabolic spiral to allow for a continuous length for the final 1/4 of the rotation to allow for leeway when winding and to prevent misfiring. The motor still seems to struggle to wind spring loads it should be perfectly capable of, and while there may be other issues in the design that could cause that, I believe the cam contributes to the problem.

I've included a render of the cam in question, winding down springs 15mm from their original position. The tolerances in the drawing (eg. the cam not making contact with the spring when the flipper is fired) is to prevent shock damage to key components when the weapon is fired. The cam was also drawn by hand essentially, with me trying to compact the parabolic spiral over 3/4's of the circle. Would you know of an equation that would allow me to draw a more accurate parabolic spiral to fit these tolerances, and would a cam with it's last 1/8th being even put less stress on the motor than the current design?

A: I've been trying to avoid banging out equations for parabolic snail cams, but I suppose I've put it off long enough. Let me find a pad of scratch paper and have a go at it...

One hour passes

OK, I've got it. The general equation for a parabolic sprial cam is... wait... wait... %#!@*&%$!!!

Another hour passes

Right! It's important to get the shape of the parabolic spiral correct to even out the torque load on the motor. The spring becomes harder to compress as it compresses farther, so the cam has to provide a lot of compression at the start of rotation (half the total is in the first 25% of rotation) and taper to less compression as the load increases (the last 12.5% takes the final 25% of rotation).

The formula for the cam in polar coordinates is: Radius = k · θ 0.5 + minimum cam radius where 'k' is a growth variable that defines the lift of the cam; a 'k' of 0.0527 will give one unit of lift over 360 degrees of rotation. Here's an example for a small snail cam with a 5mm minimum radius and 15mm total lift:

Radius = (15 * 0.0527) * θ 0.5 + 5

0 degrees rotation
(15 * 0.0527) * 0 0.5 + 5 = (0.79 * 0.0) + 5 = 5.0mm

90 degrees rotation
(15 * 0.0527) * 90 0.5 + 5 = (0.79 * 9.5) + 5 = 12.5mm

180 degrees rotation
(15 * 0.0527) * 180 0.5 + 5 = (0.79 * 13.4) + 5 = 15.6mm

270 degrees rotation
(15 * 0.0527) * 270 0.5 + 5 = (0.79 * 16.4) + 5 = 18.0mm

360 degrees rotation
(15 * 0.0527) * 360 0.5 + 5 = (0.79 * 18.9) + 5 = 20.0mm

Note The mathematically correct spiral (below left) has far too steep a drop and rise at the start of its lift to allow a cam follower to drop in and correctly follow the profile. The exact needs of that 'landing area' will depend on the details of your lifter mechanism, but it will require some modification (below right) to be mechanically functional.

Mark gets himself a celebration beer

So far, so good. Now, the torque calculations I've given in previous snail cam posts assumes that spring compression occurs evenly throughout the full 360 degree rotation of the cam. You've modified that in two ways:

  1. Compression reaches maximum at 270 degrees of rotation, and
  2. There is no compression during the first ~60 degrees of rotation due to the cam follower being held away from the cam.
The combination requires the motor to fully compress the spring in about 210 degrees of rotation, which requires ( 360 / 210 ) - 1 = 71% more torque from the motor. No wonder the poor thing is struggling. Yes, reducing the no-lift zones on the cam will reduce the motor torque requirement and result in smoother operation.

How to get a proper cam profile that includes a no-lift 'flat sector' at the end of rotation? Increase the growth variable until you get the full lift you need at the degrees of rotation you want your flat sector to start and freeze the radius at that point. The smaller the flat sector, the less torque your motor will need to put out to wind the spring.

3) Finally, for a question that ties everything together, what would be the best way of cutting a cam like this out of 10mm HDPE be? I believe my first attempt was too rough, as it seems the cam gets caught on the smallest of differences during the winding. I'd love to try to make a good cam myself without having to resort to machining.

Thanks for reading. [Galway, Ireland]

A: I have concerns about 10mm HDPE being too thin for the purpose, even for a beetle. A wider cam will spread the loading over a larger area and prevent the cam follower from deforming the surface and increasing turning resistance -- that may be one of the reasons your motor is struggling. Take a look at the width of the snail cam on 450-gram antweight 'Jännä' (video) for comparison. You might also consider a larger 'flat' cam follower to provide greater surface area that would be less affected by surface irregularities.

I think you can make an entirely satisfactory cam by following the jigsaw tips I gave above. A little work with a sanding block can remove any troublesome irregularities, and a flat cam follower will be less sensitive to catching. Best luck!

Comment: Hey again Mark. Cheers for the equations, that's a great deal of useful information! I've attempted a new updated design for the cam based on the recommendations made, with a better gradual increase, less of a loss of torque, and a thicker cam with a recessed hub to compensate for the size increase (space is at a premium within the mechanism). Comparing the old cam design to the new one, I noticed that where the difference between the two was at its greatest was where the gears in my motor sheared, so your theories on load seem bang on! The fact that it's not a direct pull down is still an issue, but that would require more adjustments to equations, and the old mechanism was able to wind that part fine with a similar profile, so I should be alright there. I'll also keep your tips for HDPE in mind when working on the new cam.

Cheers again, if I'm ever in the area I reckon I'd owe you another celebratory beer!

Update In response to a bushel of off-line questions on this topic I've put together a new page devoted to all things snail cam: The Snail Cam Files. In addition to sections on cam profiles, drive motor torque, spring rates, and control circuits -- the page links to a Google Files spreadsheet I authored that will assist in designing your mechanism.

Q: I'm designing a front-hinged pneumatic flipper and I can't figure out how to calculate the initial force at the flipper (see sketch). What is the formula to calculate the red force vector? [Facebook]

A: [Mark J.] The force will change as the flipper extends and the angle of intersection changes, but the initial angle that your pneumatic cylinder intersects the flipper is 180 - 145 = 35 degrees which makes the formula for the initial force at the flipper: sin(35) * 300 = 0.5736 * 300 = 172 pounds

Q: So, the remaining force (300 - 172 = 128 pounds) goes into compressive load in the direction of the flipper hinge?

A: No. The compressive load on the flipper hinge is: cos(35) * 300 = 0.8192 * 300 = 246 pounds

Q: Wait, 172 pounds plus 246 pounds equals 418 pounds of force. How do you get 418 pounds of force from a pneumatic ram with only 300 pounds of output force?

A: We're summing 'force vectors' that have both magnitudes and directions, and you can't sum just the magnitudes. Your ram is producing 300 pounds of force in one direction and 300 pounds of force in the opposite direction as well. In a closed system like this lifter, the full set of force vectors will sum to zero, which isn't of much use in answering your question. Fortunately, trig functions are based on right triangles, and Pythagoras figured out that in a right triangle the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse. We can use that as a shortcut to verify that we are calculating the specific set of forces we're interested in correctly:

172.12 + 245.72 = 29,610 + 60,390 = 90,000

90,000 = 300 pounds at the actuator

Q: Hey guys! Its a little specific and I'm really sorry if its not the kind of question you want to receive :/

I run a 3lbs Beetleweight hammer bot and for some reason my brushless drive weapon system always has like a quarter to a half second of delay on it! For the first version of the hammer I used basically the same weapon setup as an earlier hammer bot "Dain" did. I ran a Fingertech 2838 22 max amp, 300W 22 max amp, 300W 2970kv motor hooked up to the next generation R/C car Trackstar ESC that Dain had. I powered it all by a 850mah 3S battery, a Fingertech receiver and 20AWG wires (whoops). The entire time the hammer had this bad input lag. Despite being hooked up to the same drive receiver as a lag-less drive system

Since the first iteration I've swapped to a bigger motor (650W, 50 something-ish max amps) gotten and programmed a bigger 60A Trackstar ESC, have bought a higher 80C rating battery and I have even changed the radio and receiver and upped the wires to 16AWG and yet I still swing with 0.25 to 0.5 seconds of lag when compared to Dain throughout the entire match! It's maddening!

I've never been great with electronics and I'm honestly out of ideas as to what causes this. It was the exact same problem with the 30A ESC as the 60A, the same with 2 entirely different radio and the same with a bigger C rating and lower gauge wiring!! I really feel like I've cycled out all of the electrical components and still have the same issue. I would love your opinion on this.

Again genuinely sorry if this isn't the kinda question you're looking for on this forum. I've tried to include as much info to avoid the bad hamburger. Have a good one! [Ontario, Canada]

A: [Mark J.] Your question is exactly the type of question we do like to see, Ontario. You've got a problem that has you stumped and it's a problem that other builders might run into as well.

The bad news is that you've put a lot of time and effort trying to fix the electronics when the electronics aren't causing the delayed response. The good news is that there's a little secret about brushless motors that I'm willing to tip. Follow along...

Brushed and brushless motors are very different:

  • Brushed motors are simple. Hook a brushed motor directly to a battery and it spins right up, producing huge chunks of start-up torque.
  • Brushless motors are not simple. Hook one directly to a battery and it will twitch and quickly melt; they require an intelligent external motor controller to supply each of their three input leads with power of the correct polarity at just the right instant and then change everything an instant later as the motor rotates.
Getting a 'sensorless' style brushless motor (like yours) to start rotating is a tricky problem. The correct current polarity to apply to each of the leads to spin the motor in the right direction depends on the relative positions of the magnets and coils in the motor, but with the motor at rest the sensorless controller does not know what that position is. The controller has to 'guess' and send a small current pulse thru one pair of the motor leads, then it waits to detect and analyse the electrical feedback from the motor when the motor starts to rotate in response to that current pulse.

This start sequence works well if the motor has a light load like an airplane propeller, but if the motor has a heavy load (like an overhead hammer) the small current pulse may not cause the motor to move enough to provide feedback to the controller. The controller will wait a few milliseconds and try again... and again... and again until the motor eventually twitches enough to allow the controller to figure out the correct polarity sequence and provide more current to properly spin the motor.

If there is enough free play ('slop') in the gearbox and/or belt drive between the motor and the hammer so that the motor can spin freely for about one full revolution before it takes up the load of lifting the hammer the whole start-up process takes only an instant. However, if the drive train beween the motor and load is very 'tight' and there is not enough free play to allow the motor to start its spin-up without load you're going to get the type of delay you describe.

You've given me a complete description of your hammer electronics but you have provided no details about the mechanics of the hammer drive train. Twisted Sick Robotics' beetleweight 'Dain' has the weapon motor mated to a 14:1 gearbox that then goes to a 3:1 belt drive for a combined 42:1 reduction. It's a good bet that the combined play in the two-stage gearbox along with a fairly loose timing belt allow the weapon motor enough no-load slop to get thru the startup sequence muy pronto. Given your description of the problem it's also a good bet that your unspecified weapon drive train does not have enough slop. Add some.

Reply: Hi Mark! I appreciate the in-depth tips. I run the exact same gearbox and additional reduction as 'Dain' and - credit where credit is due - WOW it works well! The only difference is that I run the additional reduction using specially cut gears, not belts. I'll be looking into a sensored weapon motor setup or swapping to a belt drive system now. Thanks for all your help! Cheers!

A: Very pretty, Ontario! It would be a pity to remove those bespoke gears; a sensored motor and controller will cure your response delay. May your opponents all have fragile top armor.

Q: I'm designing a single-tooth drum but I'm having some trouble. I started with an Archimedean spiral drum cross-section for correct bite clearance, then I shifted the rotation axis to the CAD-calculated center of gravity to balance it. What I got was a drum with a big lump sticking out opposite the tooth that will interfere with the tooth 'bite'. Is there a simple way to reduce the size of that lump and keep the drum in balance like the RioBotz 'snail drum'? [On-line Forum]

A: [Mark J.] You started out well -- the ideal profile will fit snugly inside an Archimedean spiral with nothing sticking out to interfere with maximum tooth 'bite'. Unfortunately, simply starting with a spiral and shifting the spin axis to the CG leaves that lump you described outside the ideal spiral (red spiral line in the diagrams below). You need to modify the mass distribution to pull the profile back inside the spiral.

The RioBotz 'snail drum' is optimized for that purpose, but it is highly challenging to reproduce. Fortunately, there is another Brazilian solution that's a little easier to design and make. Uai!rrior's 'Federal MT' and 'General' have interesting single-tooth disc profiles that could be used to make an effective drum with properties comparable to the RioBotz 'snail drum'.

Start your design with a basic Archimedean spiral cross section, then take slices off the bottom until the CAD center of sall mass stops getting closer to the spiral axis. A few additional design tweaks will bring the CG right onto the spiral axis.

Q: I just got my first disk weapon back from the metal shop and I think it has some problems. Builders are telling me that the sharp internal angles on the spokes are places that will start fractures. Worse, the water jet cutter overshot and left some little notches in the corners. I think I'm in trouble.

Is there anything I can do about this or do I have to start over? [Walsall, England]

A: [Mark J.] Those sharp corners concentrate and focus stress on a small area, and if that area is already weakened by a flaw caused by poor cutting technique you're begging for a failure. Yes, you're in trouble -- but it's not hopeless.

You need to smooth out those corners. The preferred way to avoid a 'stress riser' sharp angle is to carve a smooth, rounded 'fillet' curve when designing the disk -- but that ship has sailed. You can actually strengthen the problem region by removing a little material to create a smooth radius curve. Don't go crazy -- even a small radius relief cut will improve survivability.

Q: Hey Mark,

I'm putting together an antweight with a somewhat experimental weapon. The weapon would be a slightly different take on the overhead saw. Instead of a single abrasive disk trying to cut its way inside the opponent, it would be 4 or 5 abrasive disks put together to create a single thick disk. The idea would be that the thick abrasive disk wouldn't necessarily cut through, but rather take large scoops out of the opponents top armor. Admittedly, it wouldn't be the most efficient way to get to the insides, but in theory it would cause quite a bit of visible damage on the opponents' topside and hopefully create a terrific spark show...if done right.

So let's jump in on some questions about how to "do it right". For this scenario we'll assume the weapon consists of four combined abrasive cutoff wheels with 3" diameter and the motor driving the weapon is a V-spec 2205 2350kv motor.

Abrasive cutoff wheels are pretty light. I do not know the exact weight, but I'd wager that a 2205 brushless motor would have no problem spinning them up on 1:1 gearing. I'd imagine that most overhead saw bots try to find a sweet spot with their gearing where the weapon has good speed, but at the same time, enough torque to keep spinning while making contact with the opponent.

With 4 abrasive disks, that presents 4x the width of the cutting edge, and presumably, 4x the amount of friction that will attempt to slow the weapon down as it makes contact. Would I be correct in assuming that this would also require 4x the amount of torque (4:1 gearing) to pull off than a saw bot only running a single disk on 1:1 with the same specs?

Now, as I mentioned before, many saw bots try to find the happy medium of sufficient torque and good speed. With the inevitable gearing down that would have to happen, the speed would take a dive, but the torque would remain sufficient. At this rate, we could probably see a steady decline in effectiveness because we have to compensate for the extra friction that multiple abrasive disks create. Of course, we're not even getting into the amount of pressure that the overhead arm would be trying to put on the opponent as it comes down! Let's just assume that in this case it is simply "a decent amount".

Do you think the thick abrasive cutting disk would still be effective with such a sharp decline in speed, or would I be better off approaching this by using motors that tend to be a step up in terms of power than what we normally see in antweights?

Thanks, David R. [Livermore, CA]

A: [Mark J.] You've spent some time thinking about this, David. Let me see if I can redirect your thinking just a bit...

  • Consider the relationship between tires and the arena surface. Doubling the width of the tire does not appreciably increase the traction (friction) between the tire and the surface, but doubling the downward force (weight) on the tire will double the traction.
  • A similar relationship exists between a grinding wheel and the top armor of your opponent: increasing the width of the grinding wheel will not increase the friction attempting to slow the wheel, but increasing the downward force applied will proportionally increase that friction.
  • The rate of material removal is a function of the friction (applied downward force) and the rotational speed. For a given downward force and speed, a change in abrasion wheel width will give the same material removal over a larger area -- and the same number of sparks.
I think judges would be more impressed by you cutting a narrow slit thru the armor than grinding a wide but shallow divot across it.

What's the 'chicken-and-a-half' reference?

Q: When designing a heavyweight [electric] hammer (potentially applies to other weight classes) I've seen builders use a slip clutch at both the motor output or at end of the reduction the hammer shaft/sprocket connection. Is this just preference or is there a practical advantage to one vs the other? [Seattle, Washington]

A: [Mark J.] The design choice usually comes down to the availability of suitable torque limiters. If you're planning an off-the-shelf solution you'll find that affordable sprocket slip clutches are most commonly available is sizes better suited to the hammer end of the system.

There is a small mechanical advantage to placing the clutch on the motor end: it allows the inertia of the full chain and sprocket to contribute to the hammer impact rather than dissipating that energy into the slip clutch. That's typically not a lot of energy, but if you're after every last available erg of impact you might justify a custom motor-side clutch. I wouldn't bother.

Q: So I'm about to take the leap into the 30 lb sportsman's class. I've had a few designs bouncing around in my head and came up with on that I've (prematurely) gotten excited about. I thought of it last night so I don't have a lot of details but I'll pitch the basic concept.

It's a pneumatically driven rack and pinion hammer bot. As I'm sure you can imagine, the piston drives the rack, which rotates the pinion gear, which is attached to a devastating hammer that will smash the opponent into tiny little pieces (as is the case with all hammer bots).

I have a full machine shop to work with, CAD and combat experience, and a decent budget. Is this a viable design that I should tackle, or should I hit the cheerleader button? [Albany, Oregon]

A: [Mark J.] Viable? The most feared and successful combat robots to ever swing a hammer/axe have been pneumatic rack and pinion designs.

  • Pioneering hammerbot 'The Judge' is a member of the Combat Robot Hall of Fame with a record of 24 wins / 12 losses. The 'rack and pinion' assembly in 'The Judge' was actually created from chain belts and sprockets, but was functionally identical to a conventional rack/pinion. When the giant overhead hammer came down, the whole building shook and the crowd shouted "GUILTY!"
  • Overhead axebot TerrorHurtz used a true pneumatic rack and pinion to power its way to multiple UK tournament championships. Yes, they are also in the Hall of Fame.
Start with this archived post on pneumatic rack/pinion design, and follow the links therein to more info. Electric hammers are well and good, but if you want to rattle the box you need to go pneumatic!
Q: Would a fingertech chipper blade work on a direct drive overhead 1lb spinner, or would a blade that has a "bow tie" design (think bow ties made of AR400) similar to Cobalt's large impactor, except there's one on each side work? I think it will work better due to the outward weight distribution.

The blade diameter of the theoretical blade is similar to the aforementioned chipper. If its important, here is the motor: Fingertech D2822/17 1100kV Outrunner. [Charleston, West Virginia]

A: [Mark J.] You're correct in thinking that more mass out toward the outer edge is better for energy storage. A simple bar is poor, a 'bow tie' bar with flared ends would be better, and a disk would be better still. However, there are other considerations: strength, ease of construction, and 'bite' for a few.

Consider also the benefit to a 'single toothed' weapon. A weapon with a single impactor can spin twice as fast while retaining the same 'bite'. Since kinetic energy increases with the square of speed, doubling the speed increases energy storage by a factor of four! Alternately, you might spin the single-toothed weapon at the same speed and benefit from twice the bite.

The illustration shows a relatively simple modification to the FingerTech chipper blade: trimming off one impactor and drilling a balance hole near the other end, turning it into a single-toothed weapon. I think that might make for a simple and very effective overhead spinner. I should warn you that a long direct-drive spinner design is very hard on the motor. An unmodified outrunner really isn't designed to absorb the large side-loading it will take from weapon impacts. Pack a few spare weapon motors if you go that route.

Q: Planning for a almost featherweight(12KG). I got these gearmotors 555 sized from a Chinese supplier for 10$ each. They do about 800 RPM in 12V, but I plan to go with slight over-volting, 14.4v. It will be a 4 wheeled robot, powered by 4 of these motors.

Question is, Drum. I want to make a drum spinner, and taking the suggestion from riobotz tutorial book,it should weigh 3KG, just the drum alone. And drum will be powered by a 775 with a high current relay, will convert to brushless later. Does this plan have a big flaws.??

Thank you [Dhaka, Bangladesh]

A: [Mark J.] Yes, the weapon has a very large flaw -- it's a dangerous spinner weapon that will fight in an arena like the one pictured below. I answered some questions about a wedge/lifter 'bot from Bangladesh a couple months ago, but I won't offer any help with spinner weapons that will fight in open arenas. The risk of serious injury is far too great for 'Ask Aaron' to support.

Q: Hi! I was wondering if there were any advantages of 'Kraken' having a Pneumatic Crusher over a hydraulic or battery powered one. Ty and Robots ftw! [Providence, Rhode Island]

A: [Mark J.] 'Kraken' has a non-conventional weapon system that uses a heavy lift airbag instead of the usual pneumatic cylinder to power the jaw. The large area of the lifting bag can produce a great deal of force from relatively low air pressure, but requires a lot of air to pass thru a small port to inflate. That small port is a serious problem that will slow down weapon actuation. It also requires a pair of stiff plates to sandwich the bag and some custom links carry the force to the weapon jaw.

Pneumatics are comparatively simple and robust compared to other options:

  • Their solution is simpler and more reliable than a hydraulic system that requires a motor, motor controller, fluid reservoir, high-pressure pump, 5-port valve, and a bunch of pressure hoses.
  • An electric crusher requires a very heavy and expensive gearbox to survive the massive torque, and electric motors don't survive long when bogged down near stall to provide their maximum torque.
Based on performance I don't think I can recommend the airbag approach. The team claimed 10,000 pounds of force out at the piercing end of the jaws, but I don't think the weapon performed up to expectations.
Q: Hello! I just had a quick question about lifting mechanisms. I've noticed that some lifting mechanisms, like the ones on the featherweights Banana Bender (AUS) and Mad Rush (UK), as well as a number of lifting mechanisms on UK ants that use an angled piece that connects to the lifting arm rather than having the motor connected to the weapon itself.

Other than protecting the motor or servo, are there any other advantages to using this over a conventional lifting mechanism of the same size? What are the drawbacks? How does one calculate what kind of gearing ratio is needed in order to lift opponents?

((I know the last question sounds silly, but I didn't know if calculating the ratio needed to lift an opponent with a particular motor is different with these designs than with normal lifting arms)) [Jacksonville, Illinois]

A: [Mark J.] What you've got there is a '4-bar Mechanism'. The design is widely used in combat flippers and axes: 'BioHazard', 'Ziggy'. 'Pad Thai Doodle Ninja', 'Shunt' and a great many more. A search of this archive yields more than 100 hits for '4-bar'.

The calculations required to optimize a 4-bar mechanism and to determine the motor torque requirements are quite complex: take a look. There is a 4-Bar Simulator written by Adam Wrigley of T.i. Combat Robotics that will give the motor torque requirement for the common layouts of lifters using a 4-bar mechanism, but I will point out that the design used by 'Banana Bender' is a non-standard use of the 4-bar and would not be covered by the T.i. simulator.

Take a look thru the archives for discussions of the advantages of the standard 4-bar design. Briefly: the 4-bar allows greater choice in the 'arc' of the lifter, may reduce the peak torque requirements of the gearmotor, and can increase lifter speed by spreading the power input more evenly over the full sweep of the lift. A custom 4-bar design like 'Banana Bender' is something that third-year mechanical engineering students might undertake just to show off.

Q: Hello, Mark! I was thinking about the question I had a while back about how people turned mixing bowls into full-body spinners, and remembered that both Ziggo and Blendo were made from wok pots (IIRC). As with that question, I am simply wondering: how on earth do you turn a wok pot into a spinning shell of amassed destruction? I'd assume their cast-iron structures would gave them more problems than it'd be worth. [Jacksonville, Illinois]

A: [Mark J.] Not cast iron! Cast iron shatters. Iron woks exist but traditional woks are made of a sheet of carbon steel, hand hammered into a deep bowl shape. 'Ziggo', 'Blendo' and other 'wok-based' FBS use the modern equivalent of that style, with woks made of one or multiple layers of thin steel stamped to shape.

The design is the same as for the insect-class spinners using small steel mixing bowls you asked about in your earlier question; the addition of a thick bottom ring and reinforcing plates under any blades/impactors on the wok itself spread impact load over a large area of the thin material. 'Ziggo' adds a square steel cross-tube that sticks out either side of the spinning wok.

Q: Hello, I was curious as to how friction and RPM relate to each other when talking about a spinning weapon? If I increase my RPM to increase energy storage will I also loose more energy to friction in the process? Also, is this increase linear or exponential? [Grove City, Pennsylvania]

A: [Mark J.] It takes more power to spin a given weapon up to a higher speed, and it takes more power to maintain the weapon spin at a higher speed. This places a greater load on the electric drive motor: that load will reduce motor speed and increase current draw. So, doubling the calculated weapon speed by doubling voltage or halving the belt drive reduction won't really get you to twice the actual speed because the motor will spin a bit slower than expected to generate enough torque to handle the increased load.

Back to your question -- you have several sources of energy loss in a spinning weapon system, and they don't all behave in the same fashion. Without getting to deep into engineering exceptions:

  • Bearings Frictional losses from bearings are essentially linear when operating inside their design parameters.
  • Belt Drive If used, a properly selected and set-up belt drive is quite efficient -- upwards of 98%. Losses are fairly linear. See this PDF on belt efficiency if you want to nitpick.
  • Aerodynamic Drag Unlike bearing and belt drag, aerodynamic losses are exponential. When moving thru the air at twice the speed your weapon must move twice the volume of air out of the way and it must move each unit of air twice as quickly, meaning that aero drag increases with the square of speed. If your weapon is a thin disk that displaces little air as it rotates this isn't too much of a problem, but if the weapon is a big and aerodynamically ugly beater bar you're going to run into a great big wall of resistance.
Energy storage in a spinner weapon is also exponential. If you do succeed in doubling the weapon speed you will store four times as much kinetic energy -- but the power needed to maintain the weapon spinning at double the speed will be more than doubled because of exponential aero drag.
Q: Hey, it’s the idiot lifter guy back again. What material would have the best kinetic energy storage? I saw some things about energy storage in riobotz and the archives. [Tampa, Florida]

A: [Mark J.] For a given size and shape, the Moment of Inertia of an object depends on mass -- so denser materials have greater kinetic energy storage. The densest metals are:

MetalDensity (grams/cm3)
None of these metals are commonly used in combat robot weapons.
  • There is more to a good spinner weapon than energy storage. A pound of osmium would cost you about $5800 and a spinner weapon made of that very brittle metal would shatter on impact. It makes more sense to use a more durable metal and make the weapon a bit thicker/longer/wider to compensate.
  • Look at build reports for successful spinner robots in the weight class you're building and see what's commonly used.
  • But first, finish building your antweight wedge. 'Ask Aaron' answers questions from builders.

Q: Hi Mark.

What are some of the advantages and drawbacks of multi-motors setups for HW Horizontal weapons? [New York, New York]

A: [Mark J.] Good engineering practice calls for a design solution to be as simple as possible. Abandoning the traditional single-motor solution to a spinner weapon adds to system complexity and increases the number of potential failure points in an already highly stressed system. If you're going to do that you'd better have very good reasons.

One common reason for resorting to multi-motor weapon drives is satisfying dimensional restrictions imposed by the robot design. The current incarnation of 'Son of Whyachi' abandoned the motors used in previous versions because they were too tall to fit into the desired very low profile of the robot chassis. Team Whyachi's final weapon design relies on a circle of eight(!) 'Mini Magmotors' driving a central spur gear. The ring of small motors allowed the height of the robot to be reduced by several critical inches.

Q: For my first robot, I was thinking of building a finger tech robotics kit and adding a lifter like the add on like Whiplash’s without the spinner powered by a weapon motor. Any good?

8 minutes later

Q: For my first robot, I was thinking of building a finger tech robotics kit and adding a lifter like the add on like Whiplash’s without the spinner powered by a weapon motor. Any good?

3 minutes later

Q: I’m sorry, I send a message about my lifter idea, where do I find it? I will look for this question in the archive, will that work? [Tampa, Florida]

A: [Mark J.] You appear to have mistaken 'Ask Aaron' for 'Instagram'. Should you submit another question here, please do take a moment to read the explanatory 'thank you for your question' page that appears after your question is submitted.

Since you're expecting an immediate response and you're paying nothing, perhaps you'd like to go to the Frequently Asked Questions and read section #8 -- which gives quite specific advice on the design for your first combat robot.

Q: For my post, I pick the second. Also, I did not mean to re send the exact same post eight minutes later. I know that makes me sound like an idiot, and.. I kind of am. Is there any major flaws with the lifter idea? Is it too complicated? I don’t want to make a wedge.

A: The major flaw with the 'lifter idea' is that you are a first-time builder. FAQ #8 refers you to this list of reasons why a first-time builder should not build a 'bot with an active weapon. That's my "good and free" answer. If you'd rather have "The answer the guy asking the question wants to hear" just click this button:

Q: Hello, it's Jacksonville lifter guy again! I was sitting around, minding my own business, when I suddenly got an idea: rather than having a huge plow that can lift up and down, what if I used a small lifting arm in-between two hinged wedges? Any advantages and disadvantages to having a small lifter like that rather than a wide plow? [Jacksonville, Illinois]

A: [Mark J.] It's kinda dangerous to let ideas sneak up on you like that. Stay vigilant. Some random lifter observations:

  • A narrow lifter arm as you describe can let an opponent 'fall off' to one side or the other. It's kinda like trying to eat peas with the flat side of a knife, except the peas have wheels and they're trying to drive off the knife.
  • You can get away with a narrow arm on a 4-bar lifter that is moving upward and toward your opponent (think 'BioHazard') but the combination of a short and narrow arm with a single pivot lifter that is pulling away from your opponent has a very limited number of real-life circumstances where it might be useful.
  • A wide plow lifter has the added advantage of 'getting out of its own way' as it rises, allowing your 'bot to move forward and keep the lifter under the edge of your opponent rather than pulling the lifter away. If either of the hinged wedges in your design is not 'under' your opponent, they will block and push them off the lifter as it rises. Bah!

Spring Powered Flipper Weapons

Pneumatic flipper weapons are awesome but their complexity can be troublesome, particularly in smaller combat robots. Flippers powered by mechanical energy stored in springs or elastic bands could offer attractive alternatives to pneumatics if designs that use electric gearmotors to trigger and reset spring flippers were better known to builders.

I've put together animations and discussion of four poorly known spring flipper designs and wrapped them up in a new webpage: the 'Choo-Choo' overrunning clutch winder, the constant-torque 'Snail Cam', the compact 'Slip Gear' ratchet, and the elegant and stable 'Servo Latch' reset.

Ask Aaron: Four Spring Flipper Mechanisms

Q: Hey, Mark. It's time I show the idea I've been throwing about. While I had admittedly planned on a FBS, I'm leaning more towards a lifter for simplicity and the fact I've grown to become a fan of the DUCK! method of using my face to break your fist. I made a post in regards to the lifter designs themselves here:

[Facebook permalink removed]

Please share any comments or critiques you have in regards to both designs! Thanks again! [Jacksonville, Illinois]

A: [Mark J.] I don't split comments across 'Ask Aaron' and other forums/venues. If you ask a question here I will comment here. If you post on the Facebook Combat Robotics group I may comment there. However, I will not comment here on an active discussion elsewhere, and I will not link to an active forum thread.

If you want my opinion, ask here. If you want a grab bag of forum comments, ask there.

Q: OK, Mark! RESET I have a question regarding a featherweight lifter.

I wanna note that I would've built a wedge as a featherweight, but EOH requires a weapon that's at least 6 pounds in order to qualify unless the design is approved with a weapon that's less than that.

I was planning on using a 182:1 gearbox and a Banebots 775 motor for the lifter. However, I've come across a big thing in regards to their designs. One of them is inspired by 'DUCK!', with a short, beefy lifter on the front while the other is based off of the UK heavyweight 'Shockwave', with a lifter that can rotate 360 degrees around itself. I'd assume that one version is better at doing something than another, I just wouldn't happen to know what that is.

  • What are the pros and cons to both designs?
  • What kinds of problems might be encountered with each weapon design?
Another thing I should mention is that I was thinking of adding a 1:1 gear connecting the lifter to the motor as a simple way to alleviate shock from directly hitting the lifter, or perhaps using a chain system. Would something like this work? If not, what is a way I can reduce the shock felt on the lifting arm, especially during a heavy collision with a spinner?

A: [Mark J.] How did you decide on a 182:1 gear ratio for your lifter before you settled on a lifter design? The gear ratio determines the torque available to the lifter, and the torque required for a simple lever-arm lifter depends on the length of the arm from axle to tip. Examples:

  • A featherweight lifter with a one-foot lifter arm will require one foot * 30 pounds = 30 foot-pounds of torque to offset the weight of a featherweight opponent and start to lift it. I recommend gearing for twice that level of torque to allow the lifter motor to quickly lift that amount of weight: 60 foot-pounds of torque. A 12 volt BaneBots 775 motor delivers 61 ounce-inches of stall torque at its rated voltage; 61 in-oz = 0.18 ft-lb. That makes the required gear reduction = (1 foot * 60 pounds) / 0.18 foot-pounds = 333:1.
  • Shortening the lifter arm length to half-a-foot reduces the torque requirement and required gear reduction by half: 30 foot-pounds, achievable with a 167:1 ratio gearbox.
  • Shortening the lifter arm farther to a 'Feather Duck!' range quarter-foot reduces the torque requirement and required gear reduction by a factor of four: 15 foot-pounds with an 83:1 ratio gearbox. Maybe even less because such a short lifter can really only hope to lift one end of your opponent.
Your next consideration is the torque rating of the gearbox. The maximum recommended torque rating of the BaneBots P60 gearbox is 35 foot-pounds. Given the shock loading a lifter gearbox may have to endure from weapon impacts, it is wise to stay well under that figure in your gearing. Many lifters use two gearmotors -- one on each side. This is not because a single gearmotor could not provide adequate power, but because a single gearbox might not survive the torque loading.

Shock Reduction

Your proposed 1:1 geartrain connecting the gearbox to the lifter will not reduce loading on the gearbox -- a shock load on the lifter will be passed on at a 1:1 ratio the box. You will often see additional gear reduction between a lifter and a gearbox, and that DOES reduce shock transmission! An external 4:1 reduction will increase the torque from the gearbox to the lifter by a factor of four, and will reduce the torque of shock transmitted back along the lifter toward the gearbox by the same factor.

Pros and Cons

  • A 'Shockwave' style long-arm lifter can lift your opponent higher and can also self-right your 'bot. The long lifter arms require more torque to operate and transmit greater shock from a spinner hit back to the gearbox.
  • A 'Duck!" style short-arm lifter can really only lift one end of your opponent off the floor and break their traction to make them easier to push. The short lifter arms are by nature stronger, require much less torque to be effective, and will transmit less spinner shock to the gearbox. If you're building a 'brick', this is your preferred approach.
You'll be interested in scrolling a couple posts down this archive to a question about 'Sandstorm' for more info on lifter calculations. That's a lot of info in a short package. Write back with specific questions if you need greater detail.

Q: Hey Mark. Let's talk theory.

I've been more or less dubbed the "horizontal drum" guy due to my choice of over-sized and thick horizontal weaponry that I tend to favor. While most builders in the ant and beetle class tend to use horizontal weapons with a thickness ranging between 3mm and 3/8", my weapons range between 3/4" and 1" in thickness. The basic idea is to increase the impact zone, much like a drum, instead of a smaller, more focused zone of impact.

My initial theory on this was that because the area of potential impact is increased, the energy in those hits would be spread out over a larger area and would actually be weaker, but covering more ground. A bit like throwing a stiff jab when you were born with hands that would make Andre the Giant blush.

However, I'm finding the reality to be a little different and I'm not sure why. It appears that the impacts are actually NOT weaker, but instead far more powerful than anticipated. It's as if the larger area of impact is increasing the percentage of energy transferred because it has more area to transfer TO.

But then again, I'm just a guy with a few crazy ideas and a garage full of tools. Do you have a little bit of insight about what might actually be happening here?

David Rush [Livermore, CA]

A: [Mark J.] OK, let's get theoretical. A spinner weapon needs a combination of things to land a powerful impact:

  1. good 'bite';
  2. high kinetic energy; and
  3. unyielding impact surfaces.
Assuming the same diameter, speed, and profile in comparing your 'drum' to a more conventional STD, I think we can exlain the advantages you have in impact.
  1. Bite - Your tall impactor sweeps a larger area on your opponent and has a better chance at running across and grabbing an exposed screw head, sharp edge, or hard wheel hub in a foamy tire than does a shorter-height impactor. That gives a significant advantage in 'bite'.
  2. Energy - Odium's 'drum' appears to be made of aluminum, but given that it's three to eight times as thick as a typical steel STD you've got at least as much kinetic energy storage potential as a thin disk and potentially a fair amount more.
  3. Unyielding - Have you ever seen a road sign that some frustrated hunter shot with a rifle? The energy went into making a nice clean puncture and transferred very little energy into the sign as a whole. Contrast that with a road sign hit by a shotgun -- it's in much worse shape because the energy was applied over a larger area and transferred to the sign material more effectively.
At the end of his stand-up act Gallagher smashes a watermelon with big flat mallet and sprays fruit mush over the first several rows of the audience. If he hit the watermelon with a drywall hammer he would just punch a hole in it in the same way the rifle bullet punctured the road sign. Spreading out the impact energy over a large area assures that the whole melon gets its share of the impact energy.

Granted, your opponent is not made of melon -- but the various materials from which they are made can and will absorb energy by deformation. If you're trying for localized damage by tearing into your opponent to grab or rip away material, a small and possibly sharpened impactor will serve that purpose. If you're trying to 'swat' your opponent and send it flying with as much energy transfer as you can manage, a large impactor area will minimize localized deformational energy dispersion and transfer more of the kinetic energy to your opponent as a whole.

Q: Hey, Mark! Back once again with another question. I noticed in a recent post that 'Sandstorm' used two individual gearboxes connected jointly in order to give its [lifter] weapon power. Is this, in any way, more effective than using one gearbox with more power? If so, why? I was thinking, for example, would there be any benefit from using two Banebots gearboxes with 129:1 ratios over one Banebots gearbox with a 182:1 ratio.

As an off-topic question, if using two motors has any advantage, would using two Banebots gearboxes with 64:1 ratios be a good option, or would they be too weak for a decent lifter? [Close to Champaign, Illinois]

A: [Mark J.] Don't confuse 'power' and 'torque', Champaign. Mechanical power is expressed as rotational force (torque) multiplied by its rotational speed (RPM). A reduction gearbox will increase output torque and decrease speed in equal measure -- there is no increase in 'power' regardless of the gear ratio.

Why use two gearboxes/motors rather than one gearbox with a motor twice as powerful? Electric lifter weapons require a great deal of torque, but there is a limit to how much torque a given gearbox can handle without destroying itself. BaneBots gives this warning for their P60 gearboxes in all ratios:


We recommend maximum torque not exceed 35 ft-lb for all P60 Series Gearboxes. It is possible to mount motors that will exceed this in higher gear reductions. Higher reduction gearboxes should be utilized primarily for speed reduction. Designs utilizing a P60 gearbox / motor combination that will exceed 35 ft-lb should include a method of limiting torque to prevent damage to the gearbox.

It's very easy to exceed a 35 ft-lb gearbox requirement, particularly when impact loads are added in. The lifter in Ellis Ware and Giles Ruscoe's heavyweight 'Sandstorm' starts with two BaneBots P80 gearboxes and adds on external spur gear reduction to take some of the torque load off the P80s. A pair of smaller gearboxes are often easier to pack into a low-profile 'bot than a bulkier single gearbox, plus twin drives offer redundancy should one motor fail.

The suitability of any gearbox or gearbox pair requires analysis of the loads that will be placed on the gearbox shaft(s) by your specific lifter design. I can't make a general comment about specific gearboxes without knowing much more about the lifter design. Several posts in this archive give examples of lifter analysis -- search here for "calculate how much torque" for a start.

Q: Hi Mark, Had a conceptual question about impactors on horizontal spinners. Traditionally, the inserts come to a triangle point with the hypotenuse facing bot. Is there any sense in making a vertical ramp impactor to get under opponents instead of a straight line to hit anywhere in the height of the weapon?

Many Thanks. [Pittsburgh, Pennsylvania]

A: [Mark J.] I'm confused by your question. I'm not sure why you'd want to use a horizontal spinner to try to 'get under' your opponent when the weapon is designed to tear them apart. If you want to get under them, build a wedge.

The impactors and inserts I typically see on horizontal spinner weapons are typically flat, hardened, vertical surfaces designed to transfer destructive impact energy against an opponent's hard surfaces. Some spinners have special blades with sharp leading edges to swap-in against softer-armored opponents, but hitting a hardened surface with a sharp blade blunts it immediately. See the comparison of 'Tombstone' bar weapons at right.

For a reader question (now in our Ants, Beetles, and Fairies archive) I asked builder Jamison Go why he sometimes replaces the single-tooth disk (STD) with a saw-edged impactor in his beetleweight horizontal spinner 'Silent Spring'. His answer:

Everyone is hype about single tooth blades for "bite" but why? What does bite actually do for you? It is heavier engagement on a piece of material which is arguably more energy dissipated just because it has nowhere to go.

What if the opponent has no such grabbable surfaces? Say for example, a robot made of rubber or foam? The traditional STD would be ineffective unless sharpened every match and even then its likely only good to the first few hits because it is THE singular wear point.

The saw-blade is for whittling opponents who have only compliant armor or soft things at the hitting surface. Instead of going for one big impact which would normally be absorbed, I flake material away at a high rate. What happens is I end up grabbing the same amount, but only after several milliseconds of tearing deep into them.

[My opponent was going to] use his over undercutter attachment which meant the only hitable surfaces were his wheels, hence the decision for that type of blade.

Q: Hi Mark! I was wondering if there were any robots with instead of a single drum, Had two or three disks next to each other to make more power from all three motors? Thanks! (Also go RunAmok) [Lynn, Massachusetts]

A: [Mark J.] Thanks for the 'Run Amok' shout-out, Massachusetts.

There have been several multi-spinner combat robots, but the spinners are usually located on opposite ends of the 'bot. A current example of that design is BattleBot 'Rotator', armed with large independent horizontal disks on front and back. The flaw in this design is that only one of the two disks can impact your opponent at a time, limiting the damage the system can inflict.

An alternative twin-spinner system design is employed by 2018 BattleBots competitor 'Double Dutch'. Their design features counter-rotating bar spinners placed above and below the robot body. The two bars are powered by a single electric motor. Although both weapon bars can attack an opponent at the same time, it's likely that one bar with strike before the other and throw the opponent clear before the second bar can impact. Almost certainly the two potential weapon impacts will strike different locations on the opponent, which will also limit their effectiveness.

A different double-spinner approach is seen in antweight 'Not So Free Hugs' which has twin saw blades on articulated arms. The idea here is not impact damage but the ability to trap and saw into soft wheels or plastic side armor. Here the ability to access different locations on the opponent is a benefit as it gives a greater chance to find something vulnerable to saw damage.

You'll note that all of these designs are horizontal spinners, and that none of them are terribly successful. The potential benefits of multiple vertical drums or disks are even more difficult to imagine. Why bother with three small independent disks and three small motors when you can have one larger drum and a large motor three times as powerful? A hit anywhere on the drum would transfer full impact energy to the opponent, whereas a less-than-perfectly-aligned hit from a triple disk might only transfer impact energy from one of the disks. Sorry, but I'm seeing added weapon complexity and weight with no real advantage. Simple robots win.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: I've had difficulty sourcing tool steel for my beetleweight bar spinner, so I've been looking into methods to harden mild steel. Have you heard of Robb Gunter's "Super Quench" method? It sounds like you can get a good facsimile of tool steel out of mild steel fairly easily (well, as easy as other home blacksmithing techniques anyhow).

Have you heard of this method, or do you know of any teams who have tried this? Would it be effective for a bar spinner? I think I'm going to give it a shot, unless I hear a good reason not to. [Mark from Vancouver - again]

A: [Mark J.] What do the things in this list have in common?

  • Apricots cure cancer.
  • Global warming is a hoax.
  • Chain letters are like free money.
  • Goat pheromones will make you irresistible to women.
  • Elvis is alive in a nuclear submarine under the polar ice cap.
  • Detroit suppressed a device that attaches to your car's air filter and doubles gas mileage.
  • Hillbilly metallurgy is awesome.
Give up? They're all things that you'd really like to believe are true but absolutely aren't.

There's a long list of desirable and required metal properties for spinner weapons, and if Billy Bob's mystical backyard hardening provided a reasonable mix of those properties you wouldn't have to search the back alleyways of the internet to find out about it. Don't waste your time.

Tool steel and abrasion resistant steel are fine spinner materials, but if you can't lay hands on them a nice bar of 'cheap and widely available' 4130 steel (AKA 'chromoly') is your go-to substitute. You can harden chromoly using standard practices and when you're done you'll know what you've got.

A little more about 'Super Quench'

The lower the carbon content of steel, the faster the cooling temperature drop has to be in order to create the crystalline structure needed to harden that alloy by heat treating. 'Mild steel' has a very low carbon content (0.05% to 0.30%) and is generally not hardened by heat treatment because the techniques needed to chill mild steel quickly enough to achieve barely significant hardening are simply not worth the effort. It's ever so much simpler and more productive to start with a higher carbon alloy.

So far so good, but then some backyard metal bangers dredged up work done by a metallurgical researcher who was able to add a little heat hardening to "unhardenable" mild 1018 alloy steel as kind of a parlor trick. The internet has built this 'super quench' mild steel up to mythical proportions, touting it as a substitute for tool steel. In truth, the stuff isn't even a match for common low-alloy steels.

Q: Hey Mark, question about insect-class pneumatics:

I understand that a larger cylinder bore equals more useable force, however, how do you calculate the limit of this, when the slower piston speed due to larger bore becomes a greater limitation than the additional force?

For example, say my valve has a flow rate (scfm) of 2.1469; I am thinking of switching from a cylinder with 5/8" bore to 3/4". Could you point me in the right direction, even external links on the math behind calculating this?

Thanks! [Utrecht, Netherlands]

A: [Mark J.] It's very difficult to model the speed of pneumatic systems, in part because of the interaction of multiple system elements on gas flow and in part because of the compressible nature of gasses. Ultimate force is easy, but actuator motion starts as the pressure first builds past the force preventing system movement and may be complete before the force even approaches full theoretical force.

You can find a discussion of factors effecting pneumatic system speed at the Machine Design: Pneumatic Speed page, and you can poke thru the Team Run Amok: Pneumatic Tricks page for tips on improving speed.

In wildly over-simplified terms both your speed and force are related to the cross-sectional area of the actuator bore: double the area = double the force and double the time to extend.

Your existing 5/8" cylinder has a p * (5/16)2 = 0.307 in2 cross-section area and your proposed 3/4" cylinder has a p * (3/8)2 = 0.442 in2 cross-section area.

Switching to the larger actuator should yield (0.442 / 0.307) = 144% of the original theoretical force, with full extension in 144% of the original time -- but that's not what you'll see in actual practice. There is no simple math to get real-world numbers.

If you're like most builders you picked your current components because they 'looked' about right given what you've seen in other 'bots. In general it's simply too difficult to quantify a combat robot pneumatic system design. From a practical standpoint, improvements to your system are best approached in an entirely experimental fashion: try something and see if it works.
Q: Hi Mark, I'm working on a hobbyweight horizontal spinner and wanted your opinion. There are spinners like 'Last Rites'/'Tombstone' that ping pong like a fbs after impact and others like 'Hazard' that barely turn. I looked in the forum and read up on Hazard's faux clutch and don't fully understand it. So really 3 questions:
  1. Is a clutch needed for a 12" bar spinner in this weight class?
  2. Does it make a difference if the bot is 4wd?
  3. If a clutch is needed, how do you make one?
I looked on McMaster Carr and they have clutches, but I do not know if they are suitable.

Thank you for your time and insight. [New York]

A: [Mark J.] There have been a lot of questions from builders about weapon clutches here at 'Ask Aaron'. A search of the Ask Aaron Robot Weapons archive shows 51 hits for 'clutch', including a few that discuss the odd pressure clutch used by 'Hazard'. You'll be interested in reading thru those earlier posts for additional depth on the subject, but I'll summarize important points here.

  • 'Hazard' had a weapon clutch, and recent versions of Ray Billing's big bar spinners also have weapon clutches. The presence or absence of a clutch isn't what accounts for the difference in their reaction on impact.
  • 'Tombstone' has its huge bar spinner way out in front of the 'bot. The point of impact is a good distance away from the robot's center of mass, which is not far ahead of the two drive wheels. That distance gives a lot of lever advantage to the reaction force from a weapon impact to spin the 'bot around. Combined that with the enormous kinetic energy of the monstrous weapon and you've got the reason for the 'hockey puck' reaction of Ray's 'bots on impact.
  • 'Hazard' had the axle for its spinner centered in the square formed by its drive wheels. The point of impact was much closer to the robot's center of mass, which was surrounded by tires that would have to be dragged sideways across the floor to spin the 'bot around. That makes for much less reaction to a weapon impact.
  • The purpose of weapon clutch is to protect the weapon drive components from damage from an abrupt deceleration when the weapon strikes. The weapon motor and belt/chain drive build up a lot of kinetic energy of their own and the shock-loading from a sudden stop can overstress shaft interfaces, bearings, and hubs. The larger the robot, the more of a problem this is, and I'd put a hobbyweight into the 'probably need' category. Even a little 'slip' in the system can go a long way toward saving the weapon drive.
  • The simplest and most common method of providing that little bit of 'slip' is to use a belt drive to transfer power from the motor to the weapon. A V-belt is common for large 'bots, and 'round' belts are common in sub-lights. An alternative is to use a widely available toothed 'timing belt' and pulleys, but to machine away all or most of the depth of the 'teeth' on one of the pulleys to allow slippage under high load.
  • Do Not Even Think about direct driving a 12" spinner with an unmodified brushless motor -- with or without a clutch. The side-loading from a weapon impact would immediately destroy the motor. There are a very few hobbyweight direct drive weapons, but they use custom fabricated hub motors with huge axles designed to take the load.
For what it's worth, I don't understand Hazard's clutch either. The weapon bar is squished in place on the shaft between two phenolic washers by a shaft collar. I've got a really good picture of the collar, but I can't figure out how you get enough 'squish' pressure from the collar to hold the weapon bar firmly enough. If anybody happens to run into builder Tony Buchignani do me a favor and ask him, wontcha?
Q: Hi Mark, it's drum spinner guy again. Here is my weapon design thus far. The length is fixed, the radius is flexible, but the larger the better obviously for energy storage. I fell it's overbuilt just because of its weight, but wanted your take. This is for a 15# robot.

Mostly, I want to know if I can use a thinner drum as it is currently 3/8 aluminum. I am trying to maximize bite so I was only planning on spinning it to 5000 rpm and have it store around 2000 joules while having a max bite between 12.5 and 25 mm. Any advice is, as always, much appreciated. [Pittsburgh]

A: [Mark J.] Looks like a classic drum -- no problems with what I can see, but your comments raise a couple points:

  • The photo of the drum attached to your earlier posts a bit down the page is from a 12-pound robot that fought more than ten years ago -- before LiPo batteries and brushless weapon motors raised weapon energy levels to the high levels seen in current competition. It spun much slower than you plan to spin your drum and faced opponents with much less damaging weapons than are seen today. Comparing your CAD to that photo and updating to current standards of combat, I don't believe your proposed drum is 'overbuilt' at all.
  • You say the drum will be built from aluminum -- but there's aluminum and then there's * A L U M I N U M *.
    • If you're making a drum of nasty, soft aluminum alloy like '3003': the drum is gonna get gashed/slashed/crunched and will need to be very thick to survive.
    • If making a drum of heat-tempered 'aircraft' alloy like '7075': the drum will be harder, stronger, and can be made with a thinner wall.
    The question of "How much thinner?" will be answered in combat.
  • If you're designing to get a realistic 25mm 'bite' you'll want an impactor tall enough to take advantage of that bite: about 25mm. You want to have the 'reach' to impact your target before it can penetrate far enough to impact your drum!

Q: How can you design a single impactor or snail style drum weapon and assure it is balanced? Every time I start to design one in Autodesk Inventor, I keep making it an egg beater or really asymmetrical, and doubt its balance. [Pittsburgh, Pennsylvania]

A: [Mark J.] The Autodesk Inventor CAD program can display an object's center of gravity. Hover over the CG to display its coordinates.

  • If the center of gravity is centered on the axis of rotation of your drum, it's balanced.
  • If not centered, move the axis or add/move some mass until it is.
Here's a video of builder Marcus Quintilian designing a Single Tooth Disk on Fusion 360 -- the process is similar in Inventor.

Q: Hi, single tooth drum guy again. What screws are recommended for attaching an impactor to a drum? It is bad design practice as the force on the screws is sheer force, but I do not see another way around it. Just looking at alternatives to a unidrum as it gets very complicated very quick.

A: You haven't mentioned the weight class of your 'bot, and impactor strength issues increase rapidly by weightclass. I'll need to be a bit general.

Impactor failure from shear at high weapon energy levels is what led to the development of unidrums. Machine screws are made for optimal tensile strength at the expense of shear resistance, and you REALLY can't expose screws to impactor shear forces anywhere above insect level weapons.

If you're going to attach drum impactors with screws you'll want to machine a recess into the drum such that the lateral impact force is transferred directly to the drum. The retaining screws are isolated from shear and serve only to pull the impactor down into the recess. See diagram and photo at right.

Q: Hi. I am building a 15 lb combat robot, and I have a 1700Kv Brushless Inrunner motor by Dr. Mad Thrust. I am running a 25v system with a reduction of 1:1.5 so the weapon will be spinning at around 28,000 rpm. Do you think this is too fast for the weapon? I have seen many beater bar robots spinning insanely fast, and the downfall of my last robot I believe was the weapon speed. I have attached my weapon so you can base your answer off of that as well. Thanks :) [Dublin, Ohio]

A: [Mark J.] Your drawing/CAD failed to 'attach' so I don't have the details of your weapon design. I'll have to be general in my comments.

  1. It's unlikely that your actual weapon speed will approach 28,000 RPM even with a very powerful weapon motor. The Kv 'speed constant' value is accurate for an unloaded motor. As you add load to the motor the speed drops, and the aerodynamic load on a spinning weapon increases with the square of speed. Spinning a weapon at 28K RPM requires four times the power needed to spin the same weapon at 14K RPM -- sixteen times the power needed at 7K RPM. Your weapon will fall farther off the calculated speed as aero drag increases, and will pull greater current in direct proportion to the increasing load.
  2. Read thru the Ask Aaron Spinner Weapon FAQ for considerations in evaluating spinner speed. Note in particular the section about drums/beaters going 'weapon-to-weapon' against similar designs.

Q: So I heard that 'Warrior Clan' or now called 'Warrior Dragon' used kinetic energy from their spinner. Is this true? If so how? By the way I'm a big fan of your website! [Turks and Caicos Islands]

A: [Mark J.] Team Whyachi's kinetic spinner/flipper has a long history, having first fought in 2009 at the non-televised 'BattleBots Professional Championship' as 'Warrior SKF'. There are multiple posts about Warrior SKF in this archive: start here.

Note Some sources incorrectly claim that 'Warrior SKF' is a rebuild of Team Whayachi's earlier rambot 'Warrior'. The name was recycled, but the two very different robots are structurally unrelated.

Q: Hey, Mark! Apologies for sending you a question right after my last bunch, but it wasn't until this morning that I actually remembered it. Okay, so I noticed 'Wrecks' uses a vertical flywheel that is much thinner than conventional flywheels.

What material did they use (I'd assume S7 steel, but I didn't know)? Other than less weight, is their any other advantage to having a thinner flywheel? Are there any notable disadvantages? [Champaign, Illinois]

A: [Mark J.] 'Wrecks' relies on gyroscopic precession generated by its vertical spinner (made of AR400 steel) for it's 'walking' motion. A larger diameter weapon has a greater mass moment of inertia (MoI) and generates greater precession force at the same RPM. Cutting the flywheel width in half allows for a 40% increase in diameter at the same mass, and doubles the MoI.


  • Thinner allows you to make the spinner larger in diameter for the same weight. The larger spinner makes it better at generating gyroscopic precession -- which makes 'Wrecks' a better walker.
  • More precession force is good for a precessional walker, but it causes problems for a wheeled robot. See this earlier post on 'gyrodancing' for details.
  • A thinner vertical spinning weapon is more vulnerable to damage from horizontal spinner strikes.

Q: About how large a brushless motor do I need for a beetle spinner weapon? [Cincinnati, Ohio]

A: [Mark J.] Specifics will depend on your design, but popular beetleweight spinner weapon motors run about 150 watts per pound of robot and about 6% of the total robot weight. For a beetle, that typically translates to outrunner motors in 28mm or 36mm diameters.

Q: Weapon BLDC motor: 6% of total mass; this rule applicable to FW and LW weight classes? [Paris, France]

A: [Mark J.] The guideline (not a rule) was specific for beetleweights, and extrapolating to larger robots is non-linear. The Square-Cube Law requires larger robots to devote a greater percentage of their mass to structural elements, leaving less for other components. As robot weight increases you find proportionally smaller spinner motors and those motors are pumping out less power per unit weight because they are also subject to square-cube issues for structure and heat dissipation. A heavyweight spinner motor might typically produce 50 watts per pound of robot at about 3% of the robot mass. Fitting this to a log curve gives the chart shown -- but it's only a guideline.

Current successful beetle spinners have brushless weapon motors that cluster around 6% of the robot weight -- but you can't run that backwards to say that any motor that is 6% of robot weight will make a good beetle spinner motor. Different BLDC motor versions of the same size and mass will have quite different performance figures. Combined with the '150 watts per pound' guideline it's a quick check to see if you're in the ballpark. If you're considering a beetle weapon motor that weighs 3% or 12% of your total weight allowance you'll want to make sure you have good reason to use it.

Q: Do you have any general suggestion for a good saw for a 6 lb vertical saw? [Reading, Pennsylvania]

A: [Mark J.] Weapon design doesn't start with the hardware. First consider the function and execution; what will the saw do and how do you plan to do it?

  • Will you attempt to cut into your opponent, or will you snag and toss?
  • Do you want to have a saw in a fixed mounting, or will it be on a moveable arm?
  • Is the saw your primary weapon, or is it secondary to a clamp or dustpan?
  • Are you building a great big weapon to shove around the arena, or a weapon balanced with a strong/fast chassis?
Tell me about your weapon design and I'll try to recommend some hardware.

Q: It will cut, not snag and toss. It will be on a fixed mounting. The saw is the primary weapon, and has a wedge leading up to it. The blade will be balanced with a strong and fast chassis. Can you recommend a weapon motor with this info too?

A: That's a tough combination. The blade will take hard impacts from front and side, which leaves out ceramic cutoff wheels. You'll have no control over the 'feed rate', so a toothed blade is likely to jam. Blades of a size suited to a primary Mantis weapon are also uncommon.

My choice would be a Dremel Saw-Max 3" multi-purpose carbide blade. It's metal, reasonably sized, and uses carbide grit instead of teeth. It will cut thru a variety of materials, and make pretty sparks against metal too tough to slice. I think it's probably your best option, given your weapon design.

Motor recommendation I hope you don't plan to direct-drive a saw blade off an unmodified outrunner and go crashing into things with it. If you're going direct-drive you're going to need very serious modifications, similar to the rework required for a motor-in-drum design.

Direct-drive or indirect, you'll want about 700 watts of motor power and 12K to 16K RPM blade speed. Something like the Turnigy Aerodrive SK3 3542-1000kv would be about right.

Q: I am the 6 lb saw guy and... That blade allegedly can’t cut plywood! I think the motor choice is good, and i’m going indirect drive.

A: It's true; if your opponent is made out of eight feet of 3/4" plywood you're screwed. There are much better blades for that purpose. But assuming that...

  • Your opponent is made out of more conventional combat robot materials; and
  • Your design requires that you have to chew on whatever part of your opponent gets jammed into your blade;
...your best overall choice IMHO is a metal blade with carbide grit. You can thank me later.
Q: Greetings! I asked a question here awhile ago about downforce/suction system ideas for a sumo bot. I never really got around to testing anything, because I've had near perfect success with the wedge I already had (funny how these things work).

Anyway, my new question is about a saw attachment for a beetleweight. I'm bolting a servo-powered arm to my weird modular beetle-thing, and I've got one rather important question left before I can complete this. Should I go brushless or brushed for a saw weapon motor? I've never used an active weapon more complex than a servo before, so I'm not sure where to start. I know that with a brushed motor I'll get more torque on start up, that seems like it'd be good to prevent stalling the blade. I also know that I could control it with just a simple relay, leaving the throttle channel free to control the arm. However, brushless is obviously lighter and faster, and the more obvious choice for conventional spinners. How should I approach this?


  • The 'saw' is a 4-inch diameter cut-off disc (couldn’t find a real sawblade).
  • Will have a 3s battery with enough capacity to run a spinner.
I have enough weight for either option. [Toronto, Canada]

A: [Mark J.] This is a very big step in complexity from a simple servo weapon, and there are a lot of pitfalls. A saw is very different from a conventional 'impact' spinner:

  • An impact weapon stores energy in the rotational inertia of the spinning mass and releases that energy in one burst when it hits the opponent, greatly multiplying the continuous power output of the weapon motor.
  • A saw weapon must continuously supply power to overcome the frictional drag of the cutting process. That requires MUCH greater power output than spinning up an impact weapon for a single hit.
You have no real option except brushless motor power. No brushed motor of reasonable size is going to have nearly enough power to operate a saw blade.

Some other design considerations:

  • A motor and saw blade on the end of a simple arm directly attached to a servo forms a lever that greatly multiplies force placed on the blade end and focuses that force on your servo. The servo isn't going to be able to exert enough force to apply cutting pressure on the blade, and the blade is going to kick back enough force to destroy the servo. At very least your saw arm should be strongly hinge-mounted to your chassis, and your servo should attach to the arm via a link and servo-saver to isolate it from side forces -- a simple 4-bar linkage.
  • Some event organizers (the smart ones) do not allow ceramic cut-off discs. They produce a cloud of abrasive dust in operation and have a tendency to shatter when subjected to side-loading from a moving target. Arena debris is bad enough without adding grit and shards.
  • You're not going to have enough power to effectively operate a 4" blade on a beetle. There are small, high-speed, fine-toothed metal blades available that would be much better for your purpose: like this Proxxon 2" 100-Tooth circular blade.
  • Since you made no reference to a belt drive system I assume you're planning direct-drive from the motor to the saw blade hub. You'll want to keep as much of the load created by the saw on the more strongly supported bearing nearest the motor mounting. Do not hang the whole assembly off the end of the motor farthest from the mount!
  • A brushless motor requires a brushless motor controller, but that controller can be assigned to a transmitter channel activated by a switch for simple on/off control -- just as your brushed motor relay could be switch controlled. Your 'throttle' stick can still be free to control the servo.
Saws are cool, but they're more of a show-off weapon than a competitive design. I'll strongly suggest that you consider another weapon design for your first spinner weapon. If you do build it, pack extra weapon motors and servos -- you're gonna need 'em.

Q: Thanks for the help so far! I am happy to take a leap in complexity if it results in a cooler bot, putting on a good show is my #1 priority.

It took some searching, but I was able to find small, fine-tooth saw blades in my area for a reasonable price. They're all listed as 'slitting saws', and seem to be a perfect choice for this. Good to know I don't need to use that cut-off disc.

I forgot to mention the belt system I was planning on using to drive the blade. I didn't even know direct drive was an acceptable option, but it's certainly one I'll consider if (when) weight gets too tight.

A: I'm not sure I'd call it 'acceptable', but direct drive weaponry is fairly common in insect class 'bots. A nice belt drive with a little 'slip' will repay itself in performance and motor longevity.

Q: For controlling a brushless motor via a switch, it's as simple as just plugging the ESC into the correct channel, right? That is a huge relief if so.

A: If you're running a single direction 'forward-only' controller, all you need to do is plug it into a switched auxiliary channel.

  • The 'off' setting on the switch sends the same signal to the controller as a throttle stick pulled all the way down.
  • The 'on' setting on the switch sends the same signal to the controller as a throttle stick pushed all the way up.

Q: Hi, I'm designing a mantis weight vertical spinner using a pair of fairly low kv outrunners (run at around 15v, 330kv) I had lying around and have been using your spinner calculator to figure out the kinetic energy of the weapon I had designed. I have settled on an 85mm diameter by 10mm thick steel single toothed disk as the weapon but am struggling with figuring out the right RPM to run it at.

I can get more than enough KE running the disk on a 1 to 1 ratio where it spins at about 5000 rpm, and I know crazy high rpm is not good, but looking around I have seen other small robots with crazy fast spinners and so was wondering if 5000 rpm is an OK speed to run at, or if the faster spinners know something I don't. I know this isn't a design service but I was hoping if you could tell me if I was in the right ball park. [New York, New York]

A: [Mark J.] I'll assume that you've read our Spinner Weapon Design FAQ and plugged your weapon design numbers into the 'Bite Calculator' therein. Your weapon speed is actually conservative for an STD insect spinner. The calculator says that with a 5 MPH closing rate you'll have more than an inch of 'bite' -- crazy good! If you're happy with the energy storage and spin-up times, you'll be fine. Have fun at 'Bot Blast'.

Q: I know this will vary greatly due to design, but for a 1lb drumbot what's a good amount of KE to shoot for? [Charlotte, North Carolina]

A: [Mark J.] "How much KE" is the first question at the top of our Spinner Weapon FAQ. Read the rest of that FAQ while you're there.

Q: Hello Mark!

Does LEM 130s make good weapon motors for a heavyweight overhead spinner?

and how's does their power compare to an ETEK-R? [a suspicious server in San Jose, California - at 4:46 AM local time]

A: [Mark J.] The LEM 130-95 or 95S are not generally used for heavyweight spinners. They are about 1/4th the weight and power of the ETEK-R (aka Motenergy ME0708), cost 3 times as much, and are considered by builders to be more fragile.

Ray Billings has tried all the big pancake motors in his spinners; his current choice for a balance of power, durability, and cost is the ETEK-R. If Ray likes it, I like it.

Q: Heyo! I had another quick question: I noticed in a video with the UK featherweight flipper Legion that they put the cables for their flipper on a small hinged arm. Why? Is there any sort of advantage when compared to putting it on a different part? [Champaign, Illinois]

A: [Mark J.] I'm unable to find photos or videos showing the weapon structure you're referencing, Champaign. Perhaps you'd be kind enough to share a link?

Q: Hey, it's me again! I found the video where I saw Legion's Flipper! You can see it at 1:58 when it lowers and you can see it again better at 2:04.

A: Legion's flipper arm is not attached to the pneumatic ram (visible in picture 1). I assume that the 'cable' (seen in picture 2) is an elastic bungee cord that passively pulls the flipper arm down to assist the ram's retraction. The 'small hinged arm' it wraps around is the bottom stop for the flipper arm -- it does not appear to be hinged.

The bungee must continue around the underside of the 'bot and anchor near the rear of the 'bot, allowing enough length to apply a retraction force for the flipper arm all the way down to the stop. Attaching the bungee far forward on the flipper arm gives it a leverage advantage to pull the ram downward and to hold it down against bouncing.

Q: Is there any history of horizontal spinner bots, large or small, using a slip clutch on the weapon bar to minimize stress on the motor/axle? It seems to me that *some* rotational slippage of the weapon would be acceptable if not even desirable, as during an impact the energy released is built up from the momentum of the spinning bar itself rather than the force being immediately supplied by the motor.

I was thinking of something maybe even like maybe a rubber tube that slides over the axle then a clamping hub is mounted around that, the rubber might provide some give. No ideas set in stone right now, just shopping around possible ideas. [Ballwin, Missouri]

A: [Mark J.] Some form of slip clutch is very common in all types of spinner weaponry, but it's rarely located at the weapon/shaft interface. Radial and axial impact loading at that location is much greater than the torsional loading on the axle -- great enough to destroy most clutch designs.

The most common way to get some impact slip protection for the motor is to use a round or v-belt drive to the weapon. The inclusion of a belt tension mechanism allows for adjustment of the amount of slippage: tight enough to keep the spin-up quick, loose enough to slip on a big impact.

Larger 'bots may use slip clutches from industrial equipment or a clutch of the builder's own design. I've used automotive clutch material squashed between jumbo washers with lock nuts and spring washers. Other builders have tried clamping hubs on finely polished shafts with collars above and below for location. Lots of designs are possible. A web search for 'torque limiter design' may give you some ideas.

Q: I think the torque limiter might be too complex (read: heavy) for my application (antweight). I guess that’s not really a question so much as a statement, so here’s my question, what sort of slip mechanism if any would scale down well to my weight class?

A: You did ask about "large or small" applications. If you'd specified 'antweight' I could have tailored a better response.

Insect class weapons typically isolate their motors from impact shock with belt drives. Custom pulleys used with round belts are effective at power transmission with a good slip response when the weapon is abruptly decelerated on impact. See the photo of 'Silent Spring' above. However, the small motors used in antweights are not generally sensitive to deceleration. Although they do not have much 'slip' capability, weapon drives with commercially available timing belts and toothed pulleys adequately protect antweight motors from impact shock.

If you're thinking about a direct drive weapon, the radial and axial impact loadings are a much greater concern than the torsional loading a slip clutch will deal with. Extreme reconstruction of the motor to include a much larger shaft and bearings is required for such an application. See this post in the Ask Aaron Robot Weapons archive for an overview of the preparation required. You may also search the Ask Aaron Ants, Beetles, and Fairies archive for "direct-drive" to find other posts on the topic.

Q: Heyo, guess who's back again! This time around, I had several questions regarding a post I made a while back about a featherweight FBS. I had a few more questions that came to mind as I came up with ideas for the design:
  1. I've noticed that there are usually two kinds of full-body spinners: those that are low and flat, like Capt Shrederator, and those that are big and square-ish, like Megabyte and Gigabyte. What pros and cons come with each of those designs or, more specifically, those shapes?
  2. Flat vs Angled sides: which is better to have and why?
  3. Why are "Tuna Can Spinners" like Mauler's Season 3.0 appearance not very common or popular anymore?
Thanks again for your help and advice! [I-74 near the Illinois-Indiana border]

A: [Mark J.] It will make more sense if I answer your questions in a different order than you asked them:

  • The tall and flat-rimmed 'tuna can' spinner design as used by the 'Mauler' series of robots encountered stability problems as energy storage levels rose to higher levels. 'Mauler 51-50' in particular suffered from instability caused by sensitivity of the shell to "polhode motion" that caused the entire 'bot to flip up on edge! With ever-higher levels of energy storage in rotary weaponry, the tall 'tuna can' is no longer a viable design.
  • To make a full-body spinner more stable you can reduce the height of the cylindrical shell and thicken the outer wall to make it more compact and less flexible while maintaining the same spinning mass. This is the approach the 'Shredderator' series takes -- but it is difficult to cram a weapon shell support and drive system into a short structure. Because of this difficulty many builders take a different approach...
  • Going to a domed/conical/slope-sided shell gives a more rigid and stable mass distribution while still allowing ample room for weapon motor(s) and a tall, strong, well supported weapon axle. 'Gigabyte' falls into this design class.

Q: Why are ramming spikes so bad? [Richmond, Virginia]

A: [Mark J.] What exactly do you expect a spike to accomplish? Unless your opponent is made of balloons, a pointed metal spike traveling at 'bot speed won't even scratch their armor.

Even worse, spikes help your opponent to beat you by giving their wedge a spot to slip under your bot and/or handing their spinner weapon an easy-to-grab target. If you're unlucky you'll get the spike stuck in a wooden arena bumper and be a sitting duck. Leave them off!

Q: Hi Mark.

Most of us have heard that if two vertical spinners go weapon to weapon, the faster tip speed of the two will usually win that exchange. However, I've recently heard a counter-argument that if one of those weapons has a much higher RPM, but slower tip speed (example: Drum vs Vertical disk) it might actually have an edge over the faster weapon. Is there any validity to this claim? Are there other exceptions to the "faster weapon wins" rule?

Thanks, David [Livermore, CA]

A: [Mark J.] I've heard that same theory. It's based on the statistical likelihood that the weapon with the higher RPM (but lower tip speed) may land the first impactor. While true, that impactor will land on a surface that is moving upward faster than it is, so the smaller weapon has no opportunity to dig in and 'throw' the larger weapon. The best that can be hoped for from such an impact is that the larger weapon may be 'bumped' up and back by the glancing blow to a curved surface with little bite.

Take a look at this post for a discussion of deciding factors when dissimilar verticals go head-to-head.

I think it's safe to say that tip speed wins with similar sized weapons but adding in design variables like ramps and major size variation clouds that advantage.

Example a large vertical spinner has a disadvantage to a small drum if the big vert has no ramp to elevate the smaller drum weapon up into the 'upsweep zone'. The big vert will likely strike the smaller drum without much upward vector to the impact. This results in an impact that will throw both 'bots back and away from each other. The smaller weaponed 'bot will likely come out of this type of confrontation better than the taller and less stable big vert. It may look as if the small drum won the impact, but it's actually just the big vert's unfortunate reaction to its own hit vector.

Q: I'm very curious as to what actually goes into hits as far as transferring energy into the other team's chassis without self destructing yourself. I have been playing with vertical spinners for the last few years and I still have alot to learn with this.

When I first joined [the team] our director essentially told us that a higher moment of inertia [MoI] is the key to the kingdom as far as making a weapon that hits hard. In the past few years I've played with making high moment of inertia weapons to try to hit hard but never quite getting the results I want. I only really seem to hit hard and 'out' and send way too much shock back into my own chassis and end up bending my weapon shaft and chassis considerably. I've seen several bots with tiny weapons like 'Knockout' and 'Disko' hit ridiculously hard and almost directly vertical. As far as throwing 'up' goes, they tend to roof the other teams quite often and never seem to wreck themselves too much.

Is there something simple I'm missing? Is there a way to graph bite, RPM, and moment of inertia to find the optimal setup? Is there any help with this in general? [Warren, Pennsylvania]

A: [Mark J.] We've discussed the elements of your questions previously, but they're scattered broadly around the 'Design and Construction' and 'Robot Weapons' archives. Let's see if I can pull those prior posts together to get your answer in one place.

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when your kinetic energy weapon strikes your opponent the impact on your weapon and its support structure is as great as the impact on your opponent. There isn't any escaping this 'kickback' force: your weapon has to be strong enough to withstand this impact. The advantage you have is that you know the place and direction of this force, while your opponent may have to take this force pretty much anywhere.

If your weapon and shaft are bending they simply aren't strong enough.

Hitting 'up' versus 'out' is a function of where in the weapon arc it strikes your opponent. A large diameter vertical weapon will tend to strike the opponent low in the upsweep zone and will hit more 'out' than 'up. This is why you see so many vertical spinners with a wedge/fork ramp leading up to the weapon. By elevating your opponent as they approach your weapon you will get more of an 'up' angle on your hit and as a bonus will often get greatly improved 'bite' by exposing a beautiful sharp edge at the underside edge of your opponent.

There is no magic equation to balance weapon speed, bite, and MoI because the optimal balance of RPM and MoI is entirely situational.

  • Against an opponent with nice sharp edges for your weapon to dig into you will want high weapon speed to build maximum energy storage because the importance of 'bite' and MoI are greatly reduced.
  • Conversely, facing a carefully smoothed opponent that presents only gently curved surfaces (like any of Russ Barrows' "Dark" series of robots) will require maximum 'bite' and a high MoI to compensate for the reduced weapon speed.
If you aren't getting good hits you'll need to examine what it is about the design of your opponents that is preventing your attack from being successful. Adjust your weapon and attack strategy to adapt to their design. Remember, a faster closing rate on your opponent increases 'bite', so don't be timid in your charge toward them.

If you haven't read thru our Spinner Weapon FAQ you will be interested in doing so. There are some more detailed expansions on the topics I've mentioned above. You may also wish to read thru section 6.5.2 of the RioBotz Combat Tutorial.

Q: So I was wondering how they make flames shoot out huge bursts of flame like Shamen did but there would not be a lot of room for lots of flame? ty! [Lynn, Massachusetts]

A: [Mark J.] We answer no flamethrower questions here at Ask Aaron. See Frequently Asked Questions #28.

Q: Looking at the previous questions and answers, what are your thoughts on bots like Brutality in the modern era Mark? Recently just watched a match of it with Last Rites back in the late 2000s, the wedge seemed to have made it under Ray quite easily. [San Jose, California]

A: [Mark J.] "Bot's like Brutality"... meaning top bar spinners? Top spinners (like tradional bar spinners and full-body spinners) are a design that is very effective if you get everything right. People remember the successes of specific examples of these designs ('Hazard', 'Last Rites', 'Ziggo'...) but overlook the myriad copies that failed miserably. None of these designs are as simple to get right as they appear.

More specifically to your question, I think the 2007 ComBots Cup fight between 'Last Rites' and 'Brutality' marked a turning point in weapon design. It became obvious that long and relatively thin weapon blades like Brutality's were subject to breakage from the 'modern era' hyper-energy thick and robust weaponry; compact fast-spinning single-toothed drums/drumettes like 'Touro' and big bars like Last Rites/Tombstone had a strong advantage.

If you specifically want to beat 'Last Rites/Tombstone' you would do well to take note of the design and tactics of the robot with the best record against Ray's big spinners: 'Original Sin' has won 6 of its last 7 matches vs. 'Last Rites' and is 12 of 19 overall against the big bar spinner. Simple robots win.

Q: Hello Mark.

Our team is currently designing a heavyweight bot with a Hazard/Brutality style overhead bar spinner. The bars will likely be between 75-90 lbs ranging from different materials depending on matches.

Right now we are looking to drive the weapon with four 3" long mags, or four 3" AmpFlow a28-400 motors, with Whyachi M3R2 right-angle gearboxes joining the two motors and two belt/chain drives to join the two pairs. This was designed so we could keep our bot chassis rather low to the ground for an overhead spinner, hopefully keeping most components in a 4-5" height constraint excluding the bar itself, instead of using the big and bulky ETEKs.

Are there any disadvantages or advantages to this setup other than parts costs? We have heard from builders that describe Ampflow motors as inconsistent and unreliable.

Are there any better alternatives to this? Two or three LEM-130s perhaps?

And are there any sufficient ways to synchronize the motors for these kind of setups? [New York, New York]

A: [Mark J.] Have you noticed that Team Whyachi isn't using their own gearboxes to power their weapons? Take a look at this recent post about the 2016 version of 'Son of Whyachi'. Their weapon drive solution meets your design goals with much less weight, complexity, and expense. Team Whyachi has been running a common spur gear weapon drive across multiple design revisions with good results.

AmpFlows/Magmotors become unreliable when loaded heavily for prolonged periods or overvolted on robot drive trains or lifter weapons. Their brushes are not up to sustained operation at very high amperage. They are quite reliable when running at nominal voltage with reasonable gear reduction in a spinner weapon.

All the 'cool kids' are running brushless weapon motors, but that's a high risk power solution. I consider brushed motors to be a very viable weapons drive option in heavy robots. If SoW can spin their huge weapon with eight Mini Mags I suspect that you could get away nicely with a bit less power -- like a couple LEM-130s.

I know it's a bit counterintuitive, but you really don't need to worry about motor synchronization with multiple similar motors driving a common load. The math is more than I want to write out on a nice spring day like this, but buy me a beer the next time I'm in New York and I'll explain. In the meantime, don't sweat it.

Q: Hey, Mark! Just had a quick question: I noticed a lot of eggbeaters don't have leading wedges and instead use flat fronts, such as 'Bigly' and 'Conker 3'. Is there an advantage to having a flat front on an eggbeater? Are there any drawbacks? [Champaign, Illinois]

A: [Mark J.] Take a closer look. Those 'flat-front' beaters actually slope back underneath -- kind of a 'negative wedge'. It is deliberate, and it's done to take advantage of the huge 'bite' available from a large diameter beater bar.

  • A fast-spinning mini-disc with small 'bite' has to play the 'lowest wedge' game to get under their opponent and find an edge to grab.
  • A beater can store the same level of energy as a disc of the same mass while spinning much slower. That gives it enough bite to be effective against flat surfaces.
The attack plan is to ride up the opponent's wedge and beat the stuffing out of whatever it finds up there. If there is no wedge, just hit whatever you run into. It often works quite well.

Disadvantage Going weapon-to-weapon against a smaller diameter drum with a faster tip speed is pretty much instant death.

Q: So I heard "Wrecks" uses his weapon to power his legs. How does this work? Ty! :) [M] [Lynn. Massachusetts]

A: [Mark J.] You don't have that quite right. 'Wrecks' is a 'precessional walker' -- its legs aren't powered at all, but the weapon still makes it walk. See this post in the Ask Aaron Design archive for a description of the principle.

Q: What weapon esc would be appropriate for a a28 150 ampflow motor running an ar400 8 inch diameter 2 inch thick disc at 24 volts with a 2:1 chain reduction? [Philadelphia, Pennsylvania]

A: [Mark J.] Calculating ESC requirements for a spinner weapon is much different than finding a suitable drive train ESC. There is a strong surge of current at startup that falls away to a low maintenance current level. Attempts to use current limiting ESCs or adding in a 'servo slower' to the signal line can severely reduce spin-up performance, so you should seek out an ESC with a high surge capacity.

The first step is to model the performance of the weapon system to find out how quickly the start-up current surge lasts. The A28-150 AmpFlow has a stated armature resistance of 64 mΩ, which would give a stall current of 375 amps -- but there is additional resistance in the battery/wiring/ESC. Adding in 20 mΩ (my default 'guess' factor) for those other circuit elements brings the real-world 'stall' current for the A28-150 to about 285 amps @ 24 volts.

Plugging 84 mΩ into the Run Amok Excel Spinner Spreadsheet along with the other motor and weapon details (thanks for providing those) gives a chart of spin-up performance and the current draw. It looks like you've done your homework and you have a good balance of energy storage and spin-up time. Current draw drops from 285 amps at hard start to about 100 amps at the one-second mark.

There's a good ESC match for this level of current and voltage. The Robot Power Vyper has a 120 amp continuous rating and a 'greater than' 250 amp peak current rating. It's good to 36 volts, and has over temp protection. Size is 3.125" x 2.875" x 1.375" with a 185 gram weight. I think that's your winner.

The only potential problem I see is possible stalling of your weapon. Replacing your chain drive with a very slightly slippy v-belt might prevent a full-scale meltdown if your weapon stalls in combat and you're too busy to tend to that immediately.

There are other options for control of your weapon motor. I mention them here for completeness:

  • I mentioned current limiting and adding a 'servo slower' to the receiver signal line to bring the peak current down for a mid-range ESC, but either of these options lengthen spin-up time and waste the big torque of the AmpFlow. You might as well just swap in a less powerful motor.
  • You could go overkill on the ESC with a Vex Pro Victor BB. That will certainly do the job, but your pocketbook will ache.
  • You might consider a power solenoid/contactor for simple on/off weapon control. 'Spinner god' Ray Billings doesn't like solenoids with high-performance AmpFlows, but your quick spin-up time gets you out of huge amp draw quickly, so the motor should survive the big power dump - particularly if you switch to a v-belt with a little slip.
  • And then there's the old trick with four-brush AmpFlows: split the wiring for the two brush pairs and power each set with a separate ESC. Yes, it's a little Mickey Mouse and you can fry the ESCs if you don't know what you're doing. You might just get away with splitting the brushes and running two VEX pro Talon SRX ESCs. The Vyper is a better option, but some builders like to be crazy/different.

Q: If the amperage rating on the ESC isn't 'real' then how do you find the 'true' amperage rating?

A: There are too many design, component, and construction variables to get a 'true' number from a physical examination of an ESC, and you certainly can't rely on any given manufacturer's advertised ratings due to huge inconsistencies in how that number is determined. Your choices are:

  • Precision Buy one, instrument it, and test to failure.
  • Reputation Consult with builders who have used that ESC in applications similar to the one you have planned.
  • YOLO Stuff one in and see what happens.
The last two options are the most popular. The 'Vyper' is widely used in combat - particularly in the UK - and has a good reputation. I would not hesitate to use it in your application.

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Q: Would the ampflow a28 400 running a small disc be appropriate for a lightweight? If so, would the RageBridge 2 be a good weapon ESC? [Arlington, Virginia]

A: Several comments:

  • I cannot make a weapon motor recommendation based on a weight class and a general description of the weapon type. We offer a collection of design tools so that builders may perform their own evaluations of combat robot system. These tools include two different programs that will model the performance of specific spinner weapon systems. You will do well to learn to use these tools.
  • You will also benefit from reading our Spinner Weapon FAQ to gain an understanding of the principles of spinner weapon design and performance.
  • The RageBridge 2 is a dual-channel brushed controller designed to control drive motors on opposite sides if a tank-steer robot. The special features it offers are intended to support drive train applications. Using it to run a single weapon motor would be a waste of its features.
  • The A28-400 AmpFlow motor is a big, heavy brushed motor -- less than ideal for a 'small disc' on a lightweight robot. Current weapon design favors much lighter brushless motors that are well suited to spinner weapon applications.
So... I'm gonna say 'no' and 'no'. Stop guessing and do your design homework.
Q: Hey, man! It's been quite a while since I last posted here, huh? Welp, I'm back with a bit of a featherweight conundrum. To give you the long story short [You call this 'short'?] I went to EOH [the Engineering Open House] at the U of I [University of Illinois] in March and watched the featherweight competition [Robobrawl] they had. After doing so, I've been thinking about getting a team together to compete for next year. However, there's a catch: I don't want to pitch anything at them, guns blazing: I want to have most, if not, all of the specs down and ready to present to a group of people as a document and a presentation (if needed) in order to heavily streamline work or maybe even get it done before the competition (seeing that the next competition isn't until March 2019, I can say I've got a bit of time on my hands). Not only that, but it will help our team get the money to participate both through iRobotics [website] (which gives a starting budget of $2500) and through local, smaller businesses that may want to help out.

My idea is to make a featherweight full-body spinner similar in shape to that of 'Ziggo' (the idea was that it would be themed around a land mine) or the only FBS that was a featherweight I could recall off of the top of my head: 'Badger' (from Australia).

There are five things I came here to ask, which I will lay out for you below:

First What would you recommend for weight distribution? Given that a FBS has the ability to add more weight to its weapon because the weapon is also the armor, what percentage should I try to aim for? Following along the 30-30-25-15 rule, it would result in the robot being able to use up to 55% of its weight towards the weapon and armor. However, that would mean the shell would weigh in at 16.5lbs, which I fear would be WAY too much for the robot. Using only 30% seems to small for a FBS because that means the shell weighs in at only 9lbs. The largest spinner at EOH had a bar made of S7 Steel that was 8" in diameter and weighted in at 8lbs, and since I assume a full-body spinner would be a bit larger than a bar, that 9lbs would be a bit on the weak side. 40%-45% would put it at 12 and 13.5lbs, respectively, and I feel that those numbers are better than the other ones mentioned. However, I still wanted to ask you in advance on what you thought I should do.
Second I was trying to calculate the minimum amount of energy storage the robot should be able to hold. Going off of your statement that for every Kg of mass, the robot should be able to store 60j of energy, I crunched the numbers for the approximate weight in Kg of the robot: 13.6, 13.63, and 13.7Kg, respectively. I found that the ABSOLUTE MINIMUM (without multiplying by 2, as you recommend) would be 816j and the ABSOLUTE MAXIMUM would be 1644j. Here is the question that I have about these numbers: if my robot is, for example, storing 1644j in its shell, will the amount of force itself affect driving capability or would it have more to do with the shape and mass of the robot? Also, do you think 1644j is too much or too little for a featherweight FBS?
Third I was wondering what material you'd recommend to use. From what I was told, the main building block of robots that compete at EOH is Aluminum of a wide variety of grades. I was thinking to use something along the lines of either 6061-T6 Aluminum, 5083-H131 Aluminum, or 7075-T6 Aluminum. Which grade of Aluminum do you think would be the best bet? If, by some chance, we were able to stretch our budget and were able to buy Titanium or had enough weight to go for Steel rather than Aluminum, which grade(s) would you recommend?
Fourth Do you think it would be better to have bolted or welded teeth? I know this would vary depending on what material was chosen for the shell, but I'm afraid of having teeth that constantly need bent back into shape. I know the newest iteration of Captain Shrederator uses bolted teeth, while Megabyte uses welded teeth. Under what circumstances should I use one or the other, and which one do you think I should go for?
Finally How should testing be done? I'd want to be able to test driving with the robot both with and without an active weapon, but I have no clue what kind of area would be suitable for a 30 pound FBS.

I know that that's a lot of info, and some of the info is missing because an exact size hasn't been chosen yet, but I hope you can help me out! As always, if there's anything else you would recommend I am always happy to hear your advice! Thanks again for the advice and all of your help! [Champaign, Illinois]

A: [Mark J.] New question length record - 847 words I think I can answer your questions more concisely.

The featherweight FBS that you should 'recall off the top of your head' and emulate should be:

Combat Robot Hall of Fame - Honorable Mention 2015

Tetanus / Triggo - Team Brain Damage owns the featherweight class in the northeast US. Full-body spinner 'Tetanus' went 21-2 with five titles before being updated with a stronger shell, renamed 'Triggo', and adding on a 20-6 record and three more titles to date.

Go to the Team Brain Damage website and read all three of the build logs for 'Tetanus'. Zac understands FBS design very well and his build logs will answer most of your questions. Update - the links to the 'Tetanus' build logs on Zac's website are down, and they are not available on the Internet Archive. I've written to Zac to see if he has that info available someplace. Stay tuned.
  1. Full Body Spinners commonly place 50% or more of their weight into the weapon shell and drive system. It isn't too much. If you're gonna go for a big weapon you should go REALLY BIG!
  2. FBS are all about energy storage. I would say that 1600 joules for a feather FBS is conservative. Keep the center of gravity low and you should have no driving problems.
  3. Shell material is not critical - 'Ziggo' used a modified cooking pot. For a feather I'd suggest making it easy on yourself and use steel; chromoly is tough, widely available, and inexpensive.
  4. At this energy level you can get by with welded teeth if using a steel shell. For an aluminum shell, recess and bolt steel teeth thru the shell to a backing plate.
  5. Safety first! Check with iRobotics to see if they make their Robobrawl arena or another facility available for testing -- they should.
  6. Zac's first FBS was unsuccessful. Most FBS have losing records -- they are not an easy style of 'bot to build. I don't recommend an FBS for your first move into heavier weight classes.
  7. Landmines are not amusing. They maim and kill innocents. Pick another theme.

Q: So I had a simple (or I think simple) idea of a bot that had a rotating spike. Is this a good design or does it need some work? By the way ty for wandering all of my questions! :) [Lynn, Massachusetts]

A: [Mark J.] I'm certain you have a clear idea in your mind of 'a bot with a rotating spike', but your description leaves too much to the imagination for me to share in your idea. Read: The Hamburger is Bad.

Q: Heres a cad of the idea. The spike is the spike that spins. Is there anything you would add to make it better? Thanks! :)

A: So... it's a spike... on the front of the robot... that spins like a drill bit.

I don't understand what you expect the spike to accomplish, and I don't understand why you think spinning the spike helps. The spike WILL help wedges to slip under your bot, the spike point IS highly vulnerable spinner bait, and a sharpened point - spinning or not - will do no damage to any 'bot armor.

My suggestion to make the 'bot better: remove the spike and rotating mechanism and use that weight to make a stronger wedge.

Here's the cheerleader button if you need to press it:

Q: Hi Mark,

I was doing calculations for our competition and was wondering about the relationship between bite and energy storage. What is the tipping point for bite to energy storage? For our 5 inch radius spinner we can spin it to 16,000 rpm with a bite of 5 mm at 12,000 joules stored. We can also spin it to 8,000 rpm with a bite of 11 mm with 3,500 joules stored. At what point is bite more important than energy storage, and in what situations? Does this change for bot designs and weight classes?

Many Thanks. [Pittsburgh, Pennsylvania]

A: [Mark J.] Take a look at the 'Rotational Speed' section of the Ask Aaron Spinner FAQ. The 'bite' required is highly situational, and it does depend on your weapon design, your opponent's design, and the influence of the strategy employed by both you and your opponent as it effects the true maximum closing rate of the 'bots. As it says there:

There is no single 'right balance' of bite and speed for a given weapon. The balance is situational:
  • Fighting a hard-surfaced opponent with no sharp edges calls for all the bite you can muster.
  • A drum head-to-head against another drum requires maximum speed and can dispense with bite.
  • Small arenas and close fighting call for big bite, while larger arenas and higher closing speeds need less.
  • When your opponent has only soft exposed surfaces it may be better to ignore 'bite' and switch to sharp 'shred'.
  • If you have an effective ramp to help your vertical spinner get a shot at your opponent's sharp front under-edge you need very little bite.
Stay flexible. Design for ample bite and adequate energy storage, keep some extra RPM available for special cases, and be prepared to throttle back your weapon if it's just 'skittering' across the hard surface of your opponent. Consider swapping in a sharp edge blade for 'soft' opponents.
You'll also want to read this post farther down in this archive that discusses design effects and special situations for 'bite' adjustment.
Q: Does the shape of the bar on a bar spinner make any difference? Thanks! :) [Lynn, Massachusetts]

A: [Mark J.] Yes, shape and size make a huge difference in spinner weapon performance. Read the Ask Aaron Spinner Weapon FAQ and then play with the Spinner Weapon Kinetic Energy Calculator. You might also be interested in the extended discussion I had with a builder designing a large bar spinner: Texas Bar Spinner.

'Original Sin'

Q: So you said simple robots win. What are some examples of simple robots that did well? Heres one I like... [Lynn, Massachusetts]

A: [Mark J.] 'Tombstone' is likely the simplest 'bot at the BattleBots reboot and it has the best record at those events -- but the producers of the show only accept complex robots with big weapons that generate a lot of destruction for the cameras. Truly simple robots aren't allowed to compete, so this can't really be considered 'open' competition.

The 250-pound version of 'Tombstone' does very well against the artificially complex robots at the new BattleBots: a 91% match win rate! However, Ray Billings' other bar-spinner robots don't have the same success rate against more competitive 'real world' combat robots.

NameWeightWin Rate
Tombstone250 lbs91%
Tombstone340 lbs64%
Last Rites220 lbs61%
The Mortician120 lbs60%

Why not as good? Because simple robots are tougher competition than complex showboat 'bots. Which heavyweight robot has the best record against 'Last Rites'? That would be an even simpler robot: the fearsome wedgebot 'Original Sin'.

  • 64 wins and 15 losses overall (81% win rate);
  • 12 wins and 7 losses vs. Last Rites (63% win rate);
  • An unmatched seven RoboGames heavyweight titles;
  • An unmatched four ComBots Cup championships; and
  • Full membership in the Combat Robot Hall of Fame.

    Simple 'bots win

Note - 'Tombstone' is actually a deceptively simple robot. The design appears to be straightforward, yet numerous builders have tried to copy Ray's bots with very little competitive success. There is more to the Team Hard Core 'bots than meets the eye.
Addendum - 'Original Sin' just added another championship at the 2018 RoboGames. That makes eight!

Q: What if a bot had a hammer but the blade could split in half and turn into a spinner? Would this be a good idea? What are some of the pros and cons about this? By the way thanks for answering my Bronco pneumatics question! Thank you! :) (M) [Lynn, Massachusetts]

A: [Mark J.] You may not have thought this through. Perhaps I don't fully understand your design, but it makes very little sense to me.

  1. A conventional hammerbot swings an arm with a weight on the end in a half-circle arc from the rear of the 'bot to impact (hopefully) your opponent out in front. If you miss your opponent, the hammer hits the arena floor. How exactly do you split that hammer arm in half and have enough clearance for the resulting weapon to spin without hitting the floor?
  2. Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

    Design Philosophy

    A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

  3. The three primary types of active weapons have very different design priorities to accomplish their tasks. Compromising the weapon drive system to bridge the needs of two weapon types will result in a weapon that is not good at either task.
    • A spinner stores kinetic energy in rotational inertia that is transferred to the opponent on impact. That energy is accumulated by the weapon as it spins up to speed over some period of time. More energy storage is generally better, but there is a trade-off between increasing rotational speed and the ability to obtain enough 'bite' to effectively transfer that energy.
    • A hammer also stores kinetic energy as it accelerates from its resting position toward impact, but it has much less time-distance to accumulate that energy. It's nigh impossible to get levels of energy storage in a hammer weapon comparable to a spinner weapon. Get as much as you can and hope its enough. Heavyweight hammerbot 'ßeta' manages about 7 joules per kilogram.
    • A flipper does not store kinetic energy to impact the opponent. The flipper mechanism applies force generated from stored potential energy directly to the opponent to accelerate them upward. Measuring the 'static' force of the system does not equate to the net force applied while the system is in motion.
  4. Finally, do not overestimate the importance of the weapon system.

Aaron's Wisdom  I've said this often but builders don't want to believe me:

The weapon may be the least important system on a combat robot.

If you're not winning matches it isn't because you have a poor weapon.

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Q: So I had an idea that I have no idea if it is good bad or dumb. A spinner that can change from vertical to horizontal. If this were to compete, what would its advantages and disadvantages be? Thanks!

A: Let me say this again:

Simple robots win Adding complexity increases the number of possible failure points and takes weight away from components that actually do contribute to a successful robot. Champion robots focus on a single approach and do it very well.

There have been many attempts at robots with multiple/interchangeable weapons, and very few have done well. You may be interested in reading up on 'Bombshell' -- a heavyweight robot designed around a modular interchangeable weapon set. 'Bombshell' was constructed by a large, well-financed, very experienced team -- this is not an undertaking for an inexperienced builder.

Build a simple robot, strong enough to hammer down a door. If a part breaks, make it stronger for the next fight. Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose.

Q: How does 'Bronco' make his pneumatic flipper so powerful? I heard pneumatics are normally slow and not that powerful. Thanks! :D (M) [Lynn, Massachusetts]

A: [Mark J.] You heard wrong. Inertia Labs is one of many teams on both sides of the Atlantic that have been building lightning quick and terrifying pneumatic flippers, hammers, and crushers for the last 20 years.

I'll warn you that performance like you see in 'Bronco' does not come from 'off-the-shelf' components. Most if not all of the pneumatic components in the twin Broncos (yes, there is a complete, ready to run, spare 'Bronco') are custom made at no small expense. Pneumatics are dangerous even if you know what you're doing. Don't try pneumatics on your first robot... or your second... and probably not your third.

Q: I intend to build an underweight beetleweight lifter. The mechanism at the moment is a 4 bar lifter powered by a servomotor. Judging by the T.I. tool, it appears that the front bar 'H' should be driven in a frontbar setup. Is it alright if I attach my servo to the rear bar 'F' though? [Paris, France]

A: [Mark J.] I understand your confusion.

Early versions of the T.i. Four-Bar Simulator were only capable of modeling lifter designs with powered front bars, and the diagrams all showed lifters with that layout. From version 2.0.6 onward the tool could model either front or rear powered bars, but the diagrams were not updated.

You may power either the front or rear bar, but in most cases it is preferable to power the shorter of the two 'upright' bars as that approach requires less torque. That's usually the rear bar.

Q: I'm presently designing a horizontal bar-spinner attachment for my fairyweight (150 gram) robot to better combat the vertical-disc weapons I'm beginning to see more frequently. I've run the numbers through the Run Amok spinning weapon calculator and they seem to line up reasonably well energy-to-weight wise, but want a second opinion seeing as this is the first time I've designed a serious spinning weapon. I have 55 grams of weight to make these modifications.

I'm presently powering it with an E-flite Park 250 motor:

Voltage Supplied: 7.4 volts
Speed: 16280 rpm
Kv: 2200 rpm/volt
Torque: 0.103 N·m
Ri: 250 milliohm
The bar specs are as follows:
101.6mm long x 20mm wide x 1mm thick Steel
Steel bar weighs 15.8 grams
Steel bar reaches 12 joules at 16,270 RPM in one second
The bar has no impactors or anything of note, it's just a solid rectangular chunk of metal. Is there any blatant rookie mistakes I'm making with the design of this blade? [Calgary, Alberta]

P.S: Love the Cheerleader button. I've definitely used quotes from it before when talking to prospective builders who don't want my advice

A: [Mark J.] I think you made a typo in your question as I get 21 joules rather than 12 joules, but your overall calculations are good. There are, however, a couple caveats:

  • The Run Amok Javascript Spinner Weapon KE Calculator makes a few simplifications in order to run on your laptop instead of a NASA supercomputer. The simplification in this case assumes spherical chickens in a vacuum -- that is, no aerodynamic drag on the weapon. At 16,270 RPM your weapon would have a tip speed very close to 200 MPH. The aero drag at that speed is well beyond what your adorable little outrunner motor could provide -- you're not gonna see that speed and your motor may bog down and pull a lot of amps trying.
  • You can get away with direct-drive on a small diameter fairy drum weapon at 16K RPM because the tip speed is reasonable and the impact will take place at a small radius from the motor shaft. The greater the weapon radius, the longer the lever arm the impact has to transfer bending force back to that 2mm weapon motor shaft. Take a look at this post in the Ask Aaron "Ants, Beetles, and Fairies" archive for a discussion of structural considerations on mounting motors for direct-drive weapons.
I like the 'horizontal vs. vertical' matchup, but a direct-drive horizontal fairy spinner has some serious design challenges.
Q: Hey, Aaron! I have a question this time regarding RPM and tip speed. For example, 'Hobgoblin' ran at 2500 RPM, but only had a tip speed of 90 MPH. At the same time, PP3D ran at 2500 RPM, but was much more powerful. Does RPM correlate to weapon [power], or does the weapon [power] correlate more with the weapon design and the motor used? [Decatur, Illinois]

A: [Mark J.] Many terms and measurements are applied to spinner weapons, but two measures are particularly important:

  • Moment of Inertia (MOI); and
  • Rotational speed (RPM).
Moment of Inertia is a measure of the resistance of the spinning mass to changes in speed. A weapon rotor with a high MOI will require more power to spin up to speed, but will deliver a more powerful 'hit' because it more forcefully resists attempts to slow or stop its rotation. MOI is calculated from the mass of the rotor and how far each bit of mass is located away from the center of rotation (weapon axle).

Weapon RPM should be obvious -- the faster something is moving, the greater the impact when something tries to stop it. What may not be obvious is that the relation between speed and impact strength is not linear; the kinetic (motion) energy of an object increases with the square of its speed. An object moving at 80 feet per second will strike with four times the impact of the same object moving at 40 feet per second.

Combining MOI and RPM gives the energy storage of a rotating mass, and the amount of energy stored by a weapon determines the amount of damage that weapon is capable of doing. Too much weapon speed can reduce the ability of the weapon to deliver a forceful blow because a very fast weapon may 'skitter' across the surface of your opponent without getting any 'bite' to transfer destructive force. Tip speed is generally only a factor when two similar spinner weapons attempt to strike 'weapon-to-weapon'.

The Ask Aaron Spinner Weapon FAQ covers all these topics in greater detail and offers links to calculators for MOI, bite, and tip speed. It also discusses proper motor selection for specific weapon designs. If you have an interest in spinner weapons I would suggest that you study this FAQ. An effective spinner weapon involves more than just getting something to spin.

Q: Hi Mark. Is there a practical way to determine a rough percentage of how much stored kinetic energy is actually being transferred to an opponent?

Thanks, David [Livermore, CA]

A: [Mark J.] I enjoy estimating energy transfer from the pitch and volume of my opponent's screams, but I suspect that you're looking for something more objective.

If your radio gear is set up for telemetry (a FlySky FS-i6, for example) you can add a compatible optical tachometer to read the RPM of your weapon from the transmitter. Convert the RPM of the weapon before and after a good 'hit' into stored energy levels; the difference in energy was transferred by the impact.

With a horizontal spinner a good part of that transferred energy may go into throwing your 'bot across the arena in reaction, but with a vertical spinner the greater portion of the energy should go to your opponent.

Q: Is there a calculator to determine if a bot will flip over based on the MOI of the weapon, the distance from the wheels to the weapon shaft, and the diameter of the wheels? Can you calculate how fast you would be able to turn based off the information from the spinner spreadsheet? Does overall width matter in stability of the weapon and in the turning ability of the bot - are wider bots more stable?

Many Thanks! [New Castle, Pennsylvania]

A: [Mark J.] Yes there is a calculator, but the variables used by the calculator are a little different. It will tell you how fast you can rotate without wheel lift, and if you know the weapon MOI you don't need the spinner spreadsheet. All the design factors are explained on the calculator page. So... where is this calculator? If your question starts with 'Is there a calculator...' your first stop should be the 'Combat Robot Design Tools from Team Run Amok' page:

...I realized that there was another tool that should be salvaged from the wreckage of the T.i. Combat Robotics site. Their 'Designing Around the Gyroscopic Effect' page has a full explanation of the forces at play when a vertical spinning weapon exerts a lifting force on a turning robot. That page is now safe here at

After walking you thru the math, the page offers a simple javascript calculator that will model the stability of specific robot designs to let you know how serious the gyro effect will be on your planned robot. If you're designing a big drum or vertical disk/bar spinner you'll want to make use of this calculator to avoid unpleasant surprises in the behavior of the finished robot.

Follow the link above to our design tools page and scroll most of the way down the page to find the link to the 'Designing Around the Gyroscopic Effect' page. I'd give you the direct link here, but you'll benefit from looking over the other tools on that page.
Q: I noticed SOW uses eight Mini-Mags for its weapon. Is there any advantage to using eight Mini-Mags over, say one ETek or two Rotomax 150cc's? [Decatur, Illinois]

A: [Mark J.] There are advantages, but they aren't obvious.

  1. Team Whyachi has learned the benefits of a low body profile for overhead spinners. At 4" height, the Mini-Mags are shorter than the options you mentioned, and easier to 'package'.
  2. Team Whyachi has a long history with brushed motors. With a short BattleBots build schedule I suspect they didn't want to take time to sort out a cluster of unfamiliar small brushless motors.

Q: I've read thru the Team DaVinci pneumatics guide, but I've seen pneumatic flipper 'bots that have much different layouts.

  1. Some large 'bots have something called a 'QEV'. What is it, why is it there, and how does it work?

  2. I read a comment in an on-line forum about a 'bot that failed to pass safety inspection because they couldn't de-pressurize the pneumatic system without causing the weapon to fire. Huh? How is that possible? Was there some advantage to setting the system up like that?

  3. I've seen video of a 'full pressure' UK beetleweight flipper. There are very few pneumatic components and they don't look anything like what I've seen in larger 'bots. How do these systems work, and where can I buy the parts? [Voices in my head]

A: [Mark J.] My answers to this set of questions kept getting longer and longer, so I moved them onto their own page in the 'Team Run Amok Tips and Tricks' collection: Tips and Tricks for Robot Pneumatic Weapon Systems.

Q: What is the purpose of 'True Grit'? Its weapon seems dumb. [Williamsport, Pennsylvania]

A: [Mark J.] With all of the 'Wedge Industries' designs to choose from, you wanna pick on 'True Grit'? Not 'Cone Army'? Not 'Spongetron Roundwheels'? Not 'Pizza Party'?

'True Grit' is a featherweight 'Sportsman' class 'bot, currently ranked #8 by Botrank. You can't judge sportsman weaponry by open class standards:

"Another goal of this [Sportsman] class is to eliminate the high energy destructive spinning weapons and to encourage robots dedicating their weight allowance to more complex and creative weapons that typically cannot survive in the traditional weight classes. Weapons that have portions that are exterior to the robot envelope and rotate more than 360 degrees are limited to rotating at tip speed of less than 21ft/sec or storing no more than 200 joules of energy."

A 200 joule impactor spinning weapon by itself isn't going to do much damage against 30-pound opponents, so Alex Horne designed a set of electric lifter forks to work with the toothless (Alex calls it 'infinite toothed') sandpaper drum to form a lifter/flipper system.

How well does it work? Well enough to have a winning record, but sportsman class is more about fun than cut-throat competition and Alex seems to have ample fun with this design.

Q: I think the spinner spreadsheet might not be correct in its current ratings. I simulated the beetle weapon from Weta with a NTM-3536 1400KV and it says I'm drawing 700 amps peak at 4s. I got the Ri value from the Hobbyking website and cross checked it against a similar hacker motor so I'm pretty sure it's correct (or at least close-22mOhm). I got the MOI from the CAD model of the drum after applying materials to the bodies (~180,000 g-mm^2). Commutation max is 1024 and power max is 25%. It says it spins up (95%) in 0.28s. [Ontario, Canada]

A: [Mark J.] Thank you for your concern. Allow me to explain the current calculations used by the spreadsheet.

Ohm's Law is used to calculate current flow thru a conductor of known resistance. When power is first applied to a permanent magnet DC motor, the wire coils act as simple resistors. Entering the values you have provided into the equation gives the theoretical current flow:

Current = Voltage / Resistance = 14.8 volts / 0.022 Ohms = 673 Amperes

Does the motor actually pull that many amps? No, because:

  1. It is unlikely that your battery can supply that much current;
  2. There is resistance in the circuit in addition to the motor coils; and
  3. The brushless motor controller limits current at low RPM to prevent huge start-up current peaks.
Once the motor starts to rotate the interaction of the rotating magnetic field with the wire coils generates an electro-magnetic backforce that adds effective resistance to the coils. The faster the motor rotates, the greater the backforce. By the time the motor reaches full no-load RPM, the current will have dropped to just a few amps.

The Run Amok spinner spreadsheet does not perfectly model a spinner weapon, but it offers a good estimate based on a reasonable amount of input data. Real-world data has validated its performance estimates.

Now... why are you spinning that tiny little spinner with that big motor?

Q: That's interesting and makes sizing an appropriate ESC a weird question. Is there a good way to figure it out other than go with trial and error or intuition (or copying what others do for similar motors)?

That big motor is actually the recommended motor for the Weta drum. If you're doing it anyways, you might as well overdo it I guess.

A: ESC selection is always a problem and the output from the Spinner Spreadsheet isn't going to be any help in this area. In truth, the specifications given for motor controllers are generally fictional and meaningless. One might assume that a '60 amp' motor controller would be capable of continuously sourcing 60 amps of current at whatever voltage the controller is rated. Attempting to pull that current thru a random Chinese ESC labeled 60 amps will quickly give you a puff of smoke and charred remains (for reasons given previously here at 'Ask Aaron'). Yet the 60 amp ESC that comes with Weta kits will survive a momentary surge many times that large.

Let other builders fry controllers with testing. See what's actually working in similar designs and start there.

Q: Hello Mark,

Thank you for providing such a useful online resource for robot builders. It can be a challenge to find robot combat specific content sometimes.

I am trying to design a horizontal bar spinner that can be made predominantly with basic tools, manufacturing processes, and off the shelf components where possible (ie. drill press, water-cutting) for cost, reparability, etc. This is very early on in the piece, not a lot of calculations have been done yet. Any dimensions below are just for the sake of the discussion/question.

I think I want to use a chain drive for the weapon, in large part because off the shelf components are much more readily available, but I have a couple of questions regarding the implementation.

  1. Would it be reasonable to attempt to mount a sprocket to a brushless motor in a similar style to the riobotz DIY friction clutch (p.142 of riobotz manual) using a stock mounting fixture (pic at right)? I'm not sure how else to go about it before biting the bullet and getting something machined for purpose.

  2. In a system similar to 'Suitcase Nuke', is there a rule of thumb for clearance around the weapon chain? Currently it's sitting around 2cm above the bar and 0.5 - 1 from the top plate.

  3. To mount the sprocket on the weapon side of things I intend bolt (/permanently mount) the 2024 T3 bar (20mm thick, 60mm wide) to a round 2024 piece (20mm thick, diameter same as bar width) which the sprocket will mount to. This 'hub' would be drilled/watercut small and reamed to size as one piece then have a bronze bushing pressed in. UHMW spacers above and below to keep bar vertically in place.

I think it will work but I was wondering if I could get some feedback on the idea. The appeal is that it should be very simple to make, and avoid needing to get specific parts machined. Any help/feedback would be greatly appreciated.

Cheers, Matt [Australia]

A: [Mark J.] Hi, Matt! You didn't mention the weight class you're building. I'll assume from given dimensions and popular classes in Aus that you're going featherweight.

  1. I'm a believer in Trantorque keyless hubs for functional and robust off-the-shelf hub solutions. They expand their outer diameter and decrease their inner diameter when tightened to create in interference fit to the shaft and sprocket/pulley bore simultaneously. This is much more secure than clamping the sprocket between compressing plates in the manner of a prop mount.

    If you use the pictured mount I'd suggest drilling off-axis thru the sprocket and hub to add a hardened pin for rotational locking and use the prop-nut purely to hold the sprocket onto the shaft.

    Incidentally, I've had no success with DIY friction clutches like those depicted in RioBotz. I used a similar design to torque limit the 'sidewheeler' blade weapons for Robot Wars heavyweight 'Run Away'. Keeping a constant torque slip setting proved unworkable -- it rapidly loosened in operation in spite of my best efforts. If you want to limit torque in your system I'd recommend a simple V-belt instead of a chain drive.

  2. There is very little off-plane movement to roller chain. I think your clearance values will be fine.
  3. So... a dead shaft with a bronze bushing pressed into the aluminum blade/hub with a sprocket on top. Sure -- that works. Leave a lubrication hole to add a squirt of oil to the bushing once in a while. I'd be tempted to use axial thrust needle bearings in addition to the UHMW spacers -- UHMW is slick and will flex a bit to absorb axial shock, but the loadings could be great enough to cause binding.
Overall, I think your design is practical and appealing. Keep me up to date on your progress.

Q: Thank you for the quick response and feedback. Yes, this robot is for the featherweight class and will be my first combat robot. As a follow up to each part;

  1. I'll look into the keyless hubs, they look like a good option if/when I give up on trying to friction limit the system. Just to clarify, I assume you mean to make a key way in the prop-pin and sprocket when you suggested the off-axis hole and pin. My original plan was to use a v-belt but I found it quite difficult to find appropriate pulleys that didn't require significant modification to use. I was just considering that it may be an option to use some steel rod + plate to make a stronger mount (similar in shape to a prop-mount) using the same mounting points. At least there are some options to explore, thank you.

    I am probably reading too much into this but your responses seem to indicate that friction limiting is nice to have but not as necessary as I am currently thinking. Might be a good idea just to buy a brushless motor and test it to see what it can deal with.

  2. Thank you.

  3. I'll look into some needle bearings, it's a good idea. When I eventually get to making this robot I'm happy to test both just spacers & spacers + needle bearings against a concrete wall (in a safe and controlled environment of course) to see how they compare and let you know the results.
Anyway, thank you for your feedback. I'll be sure to send some updates your way although I expect it to be a fairly slow process. Cheers, Matt.

A: My suggestion about an 'off-axis hole and pin' was a poor-man's keyway and key substitute. I'm unable to guess at the shaft diameter of your prop mount, and a small shaft made of soft material (aluminum) isn't a good candidate for a keyway. A steel pin pressed deeply into the aluminum base has a better chance of surviving without weakening the shaft -- see sketch.

The reason for torque limiting in a spinner weapon is not to reduce stress on spin-up -- that can be done thru your motor controller firmware. The torque limiting comes into play when your weapon is abruptly decelerated from hitting your opponent or the arena wall. The weapon is made to survive this deceleration, but your off-the-shelf weapon motor is another matter. The larger the motor and the faster it spins, the greater the risk of damage from impact shock loading.

Featherweight 'Suitcase Nuke' has no torque limiting in its weapon drive, and I suspect that your weapon can do without as well. Hobby brushless outrunners have a fairly small rotational inertia compared to the heavy armature of the larger brushed motor used to spin Nuke's weapon. You might want to pour an epoxy film around the magnets in the rotor (rotate it slowly as the epoxy sets) to 'battle harden' it against shock, but that's easier than a torque limiter.

Best luck!

Q: Hello! It's me again! I had a question after watching 'King Of Bots'. So, I'm sure you're aware of the Russian entry 'Stingray'. Well, I was wondering what you think about the design for the drum?

I know you said in an early post of mine that a beater bar is pretty ineffective for heavyweights. But, what about this design? Do you think it's a viable alternative to a circular drum? What would be some advantages to this shape other than weight reduction, and what would be some disadvantages? [Champaign, Illinois]

A: [Mark J.] What I said about 'beater bars' was: at heavyweight size they are not structurally sound. Given what happened to Stingray's drum at KoB I think we can agree that it had structural problems as well.

  • Disadvantages The weapon is too long for the diameter of its support shaft, and the cutout sections focus high loading at specific 'stress raiser' locations on that shaft. Cutting away from the outer radius both weakens the structure and removes mass from areas critical to maximizing energy storage. When removing material on a spinner weapon to save weight you should choose areas close to the center of rotation -- both to preserve strength and retain as much energy storage as possible.
  • Advantage It's relatively simple to machine.

Q: I hope this was not asked before but if I missed it, I could not find it.

How can I best determine the maximum ratio between spinner and body weight on the Robot? I have a concept and it does involve more than 50% of the robot mass to be actually spinning and I am concerned it will reduce handling by too much.

- Alex [Noord-Brabant, Netherlands]

A: [Mark J.] The archives have become so large that it can be a challenge to find a specific topic therein, but I do appreciate your effort.

  • The Gyroscopic Effects section of the Ask Aaron Spinning Weapon Design FAQ has a short discussion of the maneuverability issues associated with high energy storage spinner weapons.
  • Handling issues are much more severe in vertical spinners than in horizontal weapons, but they are also more predictable. The T.i. Combat Robotics Designing Around the Gyroscopic Effect page is invaluable for estimating gyroscopic problems in vertical spinners, and their calculator lets you modify design elements to reduce those effects.
  • There have been several horizontal shell spinners with more than 50% of their mass rotating, including some 'melty brain' spinners that twirl 100% of their mass. Turning motion is not an issue with horizontal spinners, but rotational stability is a serious concern. There are several articles in the Weapon Archive on this topic -- start with this post on Polhode Motion.
If you would be willing to share more specific details about your design I might be able to give more specific advice.
Date marker: January 2018
Q: Is the Saifu kit a drum or an eggbeater? [Beckley, West Virginia]

A: [Mark J.] People like to put things in neat and tidy categories, even when it doesn't matter. I'll answer your question as soon as you tell me whether a photon is a particle or a wave and whether a frog is a lizard or a fish.

Q: Hey, man! Back again! I was watching a few videos of Ol' Nightmare fighting, and I noticed that it kinda looks like the weapon bends a bit as it turns, almost like it's twisting the body. Watch the rumble between it, Witch Doctor, and Overhaul: there are several instances that show what I'm trying to explain to you. Why is it doing that? Is it supposed to do that? Am I just going plain insane and seeing things?!? [Champaign, Illinois]

A: [Mark J.] Yes, Nightmare's huge weapon does twist the chassis when the robot turns. This is the same gyroscopic force that causes the gyrodance you asked about in this earlier post (now in the 'Ask Aaron Robot Weapons' archive). Nightmare's minimal and weak chassis visibly twists a bit before the gyro force is great enough to lift the outside wheel off the arena floor. Physics.

Q: What is the difference between a crusher and a hammer [Arlington, Virginia]

A: [Mark J.] Same as the difference between a hammer and a vise:

  • A hammer hits things - quickly;
  • A vise squeezes things from two sides - slowly.

Q: What are the physics of average wedges beating most average spinners? [Fairfax, Virginia]

A: [Mark J.] Physics... Do you mean 'probability'?

The findings were essentially the same for both studies -- you may be surprised by the results...
Q: Thanks for the new brushless spreadsheet, it helps immensely in planning. Your calculations are based on the SimonK firmware, but how different is that from the default firmware most chipsets come with? Or is flashing to SimonK such an essential part of getting a bot ready for the arena that I shouldn't even think about using off the shelf firmware? Still learning the process, I'm in the midst of designing two beetleweight bots and I just learned about SimonK a couple of days ago (just when I thought I had all the requisite bases covered).

Is there a good SimonK tutorial you can recommend that explains the settings for weapon motors? All I've found for robotics is that you can set the ESC up with a reverse mode for drive motors (the Robert Cowan video you linked to in the FAQ), but I don't know what settings to tweak for a weapon motor. [Vancouver, Canada]

A: [Mark J.] I'm glad to hear that you're finding the new Brushed / Brushless Spinner Weapon Spreadsheet useful. The newly released version 19c includes input for the SimonK 'soft-start' parameters so that you can model the effects of these settings. All the un-sensored brushless ESC firmware flavors have a common approach to motor start-up and low-speed operation:

  1. There is a brief period where the ESC responds to R/C input and applies a little power while it sorts out the electrical responses it receives from the motor to make sure the motor has started rotating and is spinning in the right direction.
  2. Next, the firmware increases the power available a bit more and monitors the rising motor speed. This 'current restricted' period protects both the motor and the ESC from overcurrent until the Back EMF (video) rises high enough to restrict current on its own.
  3. Once this 'safe' speed is achieved, the firmware ends the current restriction and as much power as the R/C commands can flow to the motor.
Note The firmware 'restricts' current flow by reducing the current pulse width to a set percentage of full power. This is not the same as true 'current limiting' which cuts off current above a set amperage limit. You can still exceed the current capacity of the controller.

The nice thing about SimonK firmware is the large number of parameters that are user adjustable. If you know what you're doing you can match the firmware to the performance needs of your application. If you don't know what you're doing you can smoke both the motor and the ESC. Very, very few people know what they're doing.

Fortunately, brushless ESCs are widely available that have SimonK or BLHeli firmware already installed -- no flashing required. A brushless weapon is MUCH less demanding on firmware than is a brushless drive system, and the default settings for either of those firmware packages will work well with a reasonably designed spinner weapon. I highly recommend this approach. If you really want to tinker with setup, you can play with the parameters listed in this earlier post that determine when the current restriction is lifted -- at your own risk.

If you do want to dig deeper into setting SimonK parameters, get a fresh cup of coffee and read thru 'How to Be a Brushless Hipster'. Charles knows what he's doing.

Q: Okay, I read through Charles Guan's treatise on brushless drives. It made my head hurt, but I think I've absorbed most of the important information.

A: Charles excels at making heads hurt.

Q: The brushless motors I'm looking at as weapon motors (ACK-3510CP-630KV) list a max current of 22 amps and recommend a 40 amp ESC. I am intending to direct-drive the weapons. I know that's a risk and has downsides, but I see so many belts and chains come off on insect weights, and recently there have been several highly-successful insect weight bots with direct drive (Sgt. Cuddles, Weta2 kits, Margin of Safety).

A: 'Sgt. Cuddles' and 'Weta' kits have low moment of inertia weapons supported on both ends, and 'Margin of Safety' has a custom motor with a HUGE bearing mounted in a large aluminum plate to absorb impact. A little bird tells me that your design has a large moment of inertia weapon at one end of the stock motor and the single point of motor support at the other end. I'm gonna guess... three hits before the motor comes apart and that 4mm shaft bends.

1) Does using a larger ESC make any sense? Will it, for instance, let me push the drives to slog through that low-rpm phase faster without exploding the capacitors?

2) Is it equally practical (or, perhaps even better) to do as Charles does and purchase some really good capacitors to replace the ones on the ESC? I have pretty good experience with soldering and electronics, so I figure I can probably handle that. As far as I can tell, this is one of the big differences between say a 40 and a 60 amp ESC, and if I'm reading his blog right it's the capacitors that balance the surge of energy required as they ramp up to speed.

A: Better caps and more/better MOSFETs are seldom a bad idea, but I suspect that the weak link in your direct-drive high MOI weapon will be the extended time your motor will be pulling amps beyond rated capacity during spinup. With your current motor design you may need to extend the ESC current restriction to a higher RPM range to keep the motor from smoking.

Q: I know I'm probably getting way over my head as a newb, but for me the learning and experimenting aspect is one of the most exciting aspects of the sport.

A: I understand perfectly -- but efficient experimentation requires a baseline condition against which the results may be compared. I recommend starting with the stock set-up and finding your weak points. Team Juggerbot had a saying that should be better known: 'Damage is weakness leaving the robot'. Break it, then make it stronger.

Q: Also, I know it's early, but are you thinking about attending the Seattle Bot Bash this spring? I'm hoping to get there with at least one working bot. I'll be the guy watching as his 10 year-old daughter drives (see, this is why I want two bots).

Thanks again for all your help and for maintaining this fantastic resource!

A: You're very welcome. No promises about SBB -- best luck to you and your daughter.

Q: So I'm straining the motor too much, plus its 4mm shaft and bearings can't stand the rigors of direct-drive to the weapon for long. Clearly some kind of pulley system is necessary unless I'm going to build my own motor (some day, maybe, but not on my first bot). I think something with a slim motor up front with a very short belt between weapon and motor pulleys would probably be best (similar to Silent Spring). Then the question becomes, what kind of ratio do I need?

I hope I'm correct in assuming that the delay where the ESC is restricting current to the motor can be used as a proxy to guesstimate "motor strain". If so, what do you think would be a safe(ish) level of motor strain, measured by the amount of time it takes before the motor kicks into full power?

Thanks again, this is all immensely helpful!

A: You're on the wrong track, Vancouver. The proxy for strain on a permanent magnet DC motor is current consumption. While the ESC 'soft-start' function is active, the stress on the motor is greatly reduced. The newest version of the spreadsheet features plotting of amperage consumption -- let me show you a few graphs:

Vancouver sent me some specs and drawings for his weapon that he would prefer not to share. These graphs are based on that info.

1) Here's your direct-drive weapon as it stands:

Your weapon motor is highly stressed, pulling WAY above its 22 amp max rating for almost two seconds. It won't last long and neither will your ESC.

2) Here's your weapon with a 2:1 drive reduction:

Better. There's still a big power/stress spike when the soft-start function stops, but the total time your motor spends frying is greatly reduced.

We can reduce the peak current consumption by tinkering with the SimonK parameters to extend the soft-start RPM range.

3) Here's your weapon with a 2:1 reduction and the TIMING_RANGE3 parameter in the SimonK firmware reduced from '1024' to '256':

Now we're getting into the survivable range. Your spin-up time is down around 1 second, the peak current has been cut by half, and the blade speed is at a reasonable level to get some 'bite'. Your energy storage drops from 900 joules to about 225 joules, but that's entirely adequate for a beetleweight.

The need to tinker with SimonK firmware settings indicates to me that the selected motor is simply too small for so large a spinner bar. My recommendation is to either shrink the weapon or grow the motor. I'd shrink.

Hi, it's Vancouver.

Again, can't thank you enough for all the help you've provided. This is my go-to resource for bot building information and you've compiled an amazing collection of tools and advice over the years.

Here is the latest CAD of my beetle bar spinner. Pretty much a total redesign of the chassis. I hope it puts enough weight on the wheels for traction now. The blade... well I know it's a total beginner move, but I want a big weapon, so let's assume that'll stay.

The new motor I've found is an AX-4008CQ, stats: 600KV, 84 Ri, 24 (as far as I can tell from the pictures) magnets. I have it on a 1/4" timing belt going from a 30 tooth to a 40 tooth pulley for a 1.5:1 reduction. As far as I can see without your fancy new tool (any ETA on when it'll be available for the public btw?) it should handle the load without much difficulty. I've also found room (and weight) for a 60A ESC.

The bar is .254m long x .038m wide x 4.8mm steel.

Chassis is 10mm UHMW, electronics box will be covered in either garolite or lexan, weapon motor cover 2mm Aluminum.

Do you see any more major problems, or is it looking reasonably competitive now? [Vancouver, B.C.]

A: [Mark J.] Wait no longer, Vancouver -- the full release of the new Brushless/Brushed Spinner Spreadsheet is live! Details and download link. I think you'll like it.

I've learned not to argue with a man about the size of his weapon. I think you'll be fine. I'll point out that a 30 tooth pulley driving a 40 tooth pulley is a 1.33 to 1 reduction.

The only worry I have is with chassis flex. Your rails are all thick and well braced, but UHMW flexes a lot with impacts -- maybe enough to pop off a belt with a weapon that large. Do you have any weight left for a carbon fiber A-frame stiffener piece to sandwich in between the UHMW weapon supports? Garolite would probably do. If it seems a little 'flexy' when assembled keep that fix in mind.

Major Update to the Spinner Spreadsheet - Now Live!

The new Run Amok Spinner Weapon Spreadsheet is now live in full release!

  • I've integrated the brushless and brushed versions of the spreadsheet into a unified version. Radio buttons let you pick the familiar brushed calculations or the new 'soft start' modeling for brushless motors.
  • The output chart now includes current consumption for the weapon motor to assist in choosing the gear reduction ratio and SimonK soft-start settings.
  • A new 'SimonK Soft Start' parameter section allows modeling of brushless motor performance with modified soft-start settings. This feature can show how to reduce or delay an excessive current consumption peak.
  • A new layout places all the spinner rotor bits in one section, and the motor/controller inputs in another.
Thank you for your helpful comments on the beta release -- I hope you find the new version useful! You may download the new Spinner Weapon Excel Spreadsheet from the Ask Aaron Combat Robot Design Tools page.

Q: Hey there,
Just wondering if there is a good baseline for flipper force to weight ratio? I'm shopping for pneumatic cylinders for a 30 pounder I've designed (think Wheely Big Cheese with guarded wheels) and the rams im coming across have a force rating of like 450-500 pounds. This number seems to be too low to really throw an opponent with much authority. Is there a force number I should say "Ok it can't be less than this" or am I over thinking it? [Adrian, Michigan]

A: [Mark J.] 'Force' is useless in a flipper unless it comes with 'speed'. Force times speed equals power, and it is power that you need for a flipper. Here's how you get power...

There are four primary factors that must work together for an effective flipper:

  1. Ram force: calculated as gas pressure times the piston area;
  2. Ram extension: over how great a distance can the force be applied;
  3. Lifter geometry: the physical lifter mechanism usually decreases force and increases speed; and
  4. Gas flow efficiency: how quickly the gas can get into the cylinder to provide effective force.
That last factor is the critical factor in flipper effectiveness. Pressure regulators, hoses, ports, and valves must be selected to flow the gas as quickly as possible from the pressure tank thru the system and into the cylinder in order to extend the ram at maximum speed under the load of your opponent's mass.
  • A featherweight ram weapon that provides a maximum 500 pound force but which extends slowly because of poor gas flow will only be a 'lifter', but...
  • A smaller diameter ram delivering only 200 pounds of force may fill quickly enough from the same valving system to produce an effective 'flip'.
The calculations involved in modeling flipper performance are not trivial. Typically, flipper builders will simply obtain the pneumatic components with the greatest flow capacity available and hope for the best. If you wish to model your flipper design, take a look down at the bottom of our Combat Robot Design Tools page for links to the 'Team Da Vinci: Understanding Pneumatics' page and the 'Hassocks Hog: Pneumatic Flipper Spreadsheet'.

Q: What exact purpose (other than serving as a directional tool) does the arm of Megabyte serve? [Decatur, Illinois -- close to Champaign]

A: [Mark J.] The tube sticking up thru the center of 'Megbyte' serves as the dead shaft for the rotating shell, is bent to provide a directional cue, and aids self-righting by preventing the 'bot from flopping all the way onto its back.

Q: How does one go about adding safety switches and locks to full body spinners? [Champaign, Illinois]

A: Use a hollow tube for the weapon dead shaft, run the power wires up the inside of the tube, and mount the kill-switch at the top. The lock can be a carabiner run thru matching holes in the shell and chassis.

Q: Do you think putting a motor inside a drum for a heavyweight would be a good idea or a bad idea? '841' is a modified Weta kit, and the weapon inside the drum is really good, especially for making the robot more compact! However, I have a feeling that it wouldn't work so well with bigger bots... [Champaign, Illinois]

A: The high energy storage levels of modern robots combined with the 'scale factor' (FAQ #17) have rendered hollow drums generally unsuitable for large combat robots.

Q: Hi Mark - feedback on your beta brushless weapon calculator.

7800 kg/m3
I think that there is an error in the kinetic energy output, although I'm not sure how to correct it. I've attached a couple of screenshots showing the differences between the beta spreadsheet and the Team Cosmos KE calculator using a drum with the following specifications:

Your spreadsheet gives the correct weight of 35.38Kg, whereas the Team Cosmos calculator gives the weight as 31.48Kg. To eliminate the weight as the variable, I have changed the Team Cosmos calculator to show the correct weight.

In terms of kinetic energy at 50% (4271RPM), the Team Cosmos calculator shows 25,243 joules. The beta spreadsheet shows 132,045 joules. I'm not quite how much this is off by, but the numbers don't stack up.

Regards, Rob K [Trumpington, England]

A: [Mark J.] You've been caught in a dimension input difference between the calculators, Rob.

  • The Team Cosmos calculator asks for the diameter of the tube;
  • The Run Amok spreadsheet asks for the outer radius of the tube.

Plugging the input values from the Team Cosmos calculator screenshot you sent (a steel tube 250mm long and 100mm in radius with a 35mm wall) into the Run Amok spreadsheet gives essentially identical output: 35.38 kilos storing 25,306 joules @ 4271 RPM.

Thanks for checking with me and for testing the beta spreadsheet. The new Brushed / Brushless Spinner Spreadsheet is now in full release.

Q: Hey Mark! The new brushless spreadsheet looks good. How exactly do you figure out the speed where a SimonK ESC switches from startup into full power mode? [Philadelphia, Pennsylvania]

A: [Mark J.] Thanks. The power level and switchover process was a deep riddle. Configuration variables and their explanations on the SimonK Home Page turned out to be outdated or just plain wrong. With some help from brushless gurus Cosmin Gorgovan (who confirmed my suspicions in the firmware) and Charles Guan (who did his best to confuse me) I worked my way to the bottom:

1) In spite of what the documentation says, the default controller firmware settings will limit the power pulse length to 1/4th [PWR_MAX_RPM1] of the full power pulse length [MAX_POWER] until the motor speeds up enough to complete one 'commutation' in under 1024 µ-sec [TIMING_RANGE3]. That's [60 seconds per minute / 0.001024 seconds per commutation] = 58,594 commutations per minute. Great... but what's a commutation?

2) A hobby brushless motor (three phase) requires six electrical power switchings (commutations) to complete one 'electrical revolution'. That makes [58,594 commutations per minute / 6 commutations per e-revolution] = 9766 eRPM at switchover to full power. Interesting... now what's an eRPM?

3) An electrical revolution is the mechanical rotation of the motor required to move thru one pair of permanent magnet 'poles' and return to the electrical starting point in the commutation sequence. Switchover to full power happens at [9766 eRPM * 2 magnet poles per pair / number of magnet permanent magnet poles]. Awesome... how many poles does my motor have?

4) The motor specs will tell you how many poles a motor has; it's always an even number. A typical outrunner might have 14 poles, where a typical inrunner might have 4.

  • For a 14-pole outrunner motor: cutover to full power comes at [19532 / 14] = 1395 RPM.
  • For a 4-pole inrunner motor: cutover to full power comes at [19532 / 4] = 4883 RPM.

Commutation Speed to RPM Conversion
Commutation: µ-sec RPM:  
Magnets: poles

In general form: 20,000,000 / (commutation in µ-sec * magnetic poles) = RPM.

Q: Hi, I have seen somewhere that a robot used a banebot wheel for a spinning disk. What are the advantages and disadvantages of using a wheel for a spinner? [Great Falls, Montana]

A: [Mark J.] There have been a few 'toothless' spinner 'bots. Pictured is 15-pound 'Mad Cow Disease' that uses multiple Colson wheels as a spinning drum weapon. The robot has a 4-4 record at BotRank, so it can hold its own.

Since toothless spinners rely on contact friction rather than 'bite' they can be spun very fast to store big energy without penalty and they are effective against smooth surfaces that standard spinners just skitter across. They are also durable, inexpensive, and easy to replace.

Primary disadvantage: their hits are friction-limited, so they aren't effective at delivering really huge impacts. I suspect a high-speed toothless disk could be a very effective weapon in a fairyweight 'bot...

Q: This is regarding the selection of motors for weapon system.

I've noticed that selection of a brushed motor is simpler since the RPM and torque characteristics are linear. However, brushless motors do not follow this linear nature. I've also noticed that selection of a brushless weapon motor is primarily based on experience, brand and the stator diameter for a specific kv rating. Also the ESC plays a major role.

How do I select a BLDC for my drum with 0.01 kg·m2 moment of inertia, weighing 4.8 kg?

Can you recommend some BLDC motors with a kv rating in the range of 300-400. I've been looking at Alien Power System and Scorpion motors. I have smoked a Propdrive 5060 with 380kv running on 6s earlier. The ESC accompanying this was a Redbrick 200amp model. [Braunschweig, Germany]

A: [Mark J.] I'm told that there aren't many combat robot builders in Germany and that the German Roboteers Association doesn't know of anyone building a drum robot such as you describe. They'd like to get in touch with you.

While you've noticed quite a bit about brushless motors you may not have noticed that these very compact and lightweight motors do not like to be bogged-down in their lower RPM range. Hobby brushless motors are designed to spin very light propellers quickly into their upper RPM range before encountering significant loading.

I've tried modeling the performance of your weapon powered by the Propdrive 5060 380kv. I suspect that you're using the low-RPM outrunner to direct-drive the weapon drum -- a very poor idea for a weapon this large. The Propdrive has a maximum current rating of 90 amps, but my model shows the motor drawing nearly three times that amperage for the first three-quarters of the fairly long spin-up. I'm not surprised that you smoked it.

Featherweight drum weapons are typically powered by a belt reduction from a brushless inrunner motor. This design allows the motor to build RPM quickly for greater efficiency. Read thru the build report for featherweight 'Big Ripto' for the general layout. When in doubt, copy a successful example. 'Big Ripto' uses a KB 44-74 1500KV brushless inrunner weapon motor rated 2600 watts at a maximum 120 amps.

Q: Remember flail = fail? Well 'Hellachopper', if he was more reliable could win a lot of fights! How does flail equal fail if he succeded? [Fairfax. Virginia]

A: [Mark J.] That's very grand speculation about a robot that never fought. Come back and tell me about its success when it has a winning record. "If" is a fantasy:

If your aunt had bollocks, she'd be your uncle.

Q: I've thought about putting a Continuously Variable Transmission into a weapon system so that it can always have optimum gearing for combat. I never see any other bots with CVT's (probably for a good reason) and I want to know what you think of using a CVT in a weapon system? Thanks! [Nashville]

A: [Mark J.] Scroll down one post.

Q: Concerning spinners who don't need to worry much about tooth bite or are drums-fighting-drums and so have reason to be capable of reaching absurd RPMs: Could one use a changing ratio (shifting gearbox, pulley variator, etc.) to start with high torque to spin up to a decent speed quickly, then shift down the torque to up the speed and start a longer climb to a higher RPM?

Am I missing some physics something? Is the idea simply only worth anything in theory; it isn't possible to implement it to any effect IRL? [Troy, New York]

A: [Mark J.] Your physics are fine, but the weight required for a variable gearbox capable of reliably surviving combat loadings exceeds that of a more powerful motor that can attain the same time-to-speed performance. It's very tempting to add complexity to a combat robot, but simple robots win.

Q: Hi Mark, I'm currently building my first robot which is a beetleweight vertical spinner. I have several questions about my weapon I'm designing:

  1. I used your Spinner Weapon Kinetic Energy Calculator to determine that with the motor I have picked out the disk can store about 450 joules of energy after only 4 seconds. I would like to know on a scale of 1-10 (1 being extremely weak, 10 being overkill) what you think of my weapon?

  2. Using the same parameters I used for the Weapon Calculator, my robot has a maximum bite of only 4mm. I don't think this is nearly enough bite and I want to know what your opinion on how much bite is enough for a robot of my size (beetleweight)?

I absolutely love your website and I have learned many things just from reading questions. Thanks! [Nashville. Tennessee]

A: [Mark J.] I'm glad to hear from you, Nashville. Energy storage is one of several factors to consider in evaluating potential weapon performance. You need stored energy, but you also need to gather that energy quickly and deliver it to your opponent in an effective hit. A careful read of the Ask Aaron Spinner Weapon FAQ will answer most of your questions. Some specific comments:

  • You don't have four seconds to build energy in a small insect arena. Your opponent will be on top of you in a lightning sprint over a few feet -- you need to be ready.
  • You're right to worry about 'bite'. Most of your weapon impacts will be made in close quarters at low closing speed, and a weapon with poor bite will just 'skitter' across your opponent unless it can find a sharp edge to grab. There is no magic number but 4mm is very poor. As it says in the Spinner FAQ:

    Bite is good, and more bite is better.
  • The gyroscopic forces produced by a 450 joule beetle weapon are likely crippling to your mobility -- you'll be unable to turn rapidly to stalk/avoid your opponent. Read the T.i. Combat Robotics Gyroscopic Effects article and plug your design numbers into the gyro calculator at the bottom of that page to see how much trouble you're gonna have.
  • New builders find it hard to believe but the weapon may be the least important system on a combat robot. Drivetrain, radio set-up, general construction practice, reliability, and balance are all much more important. You might be surprised to learn what type of weaponry has the best win/loss record.
In short I don't know enough about your robot design to comment on how effective the whole robot may be. From what little you've told me my best recommendation is to gear down that spinner weapon for a quicker spin-up, better bite, and improved mobility.

Q: Hello Mark it's Nashville again.

I took your advice and changed the gear ratio for my weapon and entered the same values as before into the Kinetic Energy Calculator. Now the disk can spin up to maximum speed (about 200 joules) in only 2 seconds, but it still only has 5mm of bite. Is there any good way to increase bite without sacrificing kinetic energy or mass?

A: [Mark J.] You haven't shared details of your weapon except for energy level and spin-up time. Not knowing more about your design, I can't make intelligent suggestions on what you could change. The Hamburger is Bad. General notes:

  • For a given mass, larger diameter weapons spinning at lower speeds will have greater bite at the same energy storage level;
  • Reducing the number of impactors benefits bite -- which is why asymmetric single-tooth weapons have become so popular.

2. I am competing in the BotsIQ team for our high school and we would like to design a 'Ciclone-esque' bot but are having difficulty with one rule in particular:

"[The Switch must] be positioned in such a way that it can be operated without placing any body part in the path of any weapon system or other powered movable part of the Bot." conjunction with:

"The Master Switch (or Switches) shall be accessible without the need to lift or turn the operating Bot or to remove any cover or grill."

Any thoughts how one could place and securely fashion the switch and where it would be located? Many thanks as always. [New Castle, Pennsylvania]

A: Have you wondered why 'droop end' bar spinners are uncommon? Have you noticed that RioBotz retired 'Ciclone' with a disappointing record? There is a reason! Read the post on polhode motion in this archive -- droop bar spinners are unstable and will cause big trouble at high energy levels. Pick another design.

If you insist on going with this design, mount the spinner on a large-diameter hollow dead shaft, run the power wires up the inside of the dead shaft, and mount the kill-switch at the top.

Q: Hi again Mark, couldn't the facebook guy use iPhone's slow-motion record then count how many spins from the video to estimate RPM? It goes up to 240 fps so up to 14k RPM should be fine. [Bristol, England]

A: [Mark J.] Ah, you're referencing the oddball speed estimations post farther down in this archive. Slow motion video has two components: frame rate and shutter speed. Apple doesn't give out shutter speed info for its iPhone slow-motion mode, but even assuming a speedy 1/500th second shutter in good light you're still gonna have nothing but a blur at several thousand RPM. Without an accurate shutter speed you can't even estimate RPM from the blur length.

You can purchase a digital laser tachometer that's good up to 99,999 RPM for less than the price of new antweight hubs and you would 'know' rather the 'guess'. An enterprising builder could make lunch money renting one out at tournaments to resolve bets.

Q: Hi, I am entering a competition soon that has a 45 second spin down time. I am concerned that what I'm looking at with my weapon right now will not meet this spin down requirement, is there any way I can calculate this in advanced to be sure? Thanks, Aiden. [Bedminster, New Jersey]

A: [Mark J.] Sorry, Aiden -- we can calculate the energy stored in your weapon that must be dissipated, but there are too many variables in the mechanical/electrical/aero drag during spin-down to get a calculated time estimate. I can provide a few tips to decrease your spin-down:

  • If your weapon uses a speed controller: make sure the selected controller has 'dynamic braking' available and that it is turned on.
  • If your weapon motor is brushed and activated with a relay/solenoid: see the Ask Aaron Solenoid and Relay Guide for info on setting up a DPDT solenoid for dynamic braking.
  • If you have a reversing ESC: a VERY gentle application of reverse current toward the end of spin-down is useful.
  • A small servo can be used to provide mechanical friction by directly applying a pad to a belt/disc or by tightening a strap around a shaft/can/drum.
If all else fails and your design allows, you can drive the weapon into the arena wall to stop it quickly.
Q: Hi mark an old builder from [someplace]... I was off the work from some time... leave it... I was thinking about to build a wedge bot (actually tired of drum and disc in [someplace]... yah here are too much of them)...

I was thinking of solid triangular wedge than a metal sheet... 'cause I have seen wedge bot they get easily knocked out from weapons 'cause these wedges have less inertia... what if I put a strong hardened solid wedge to take weapon bites on it... I want your views? [Western Part, Someplace]

A: [Mark J.] It isn't a secret: counter drums and mini-disks with a THICK - HARDENED - STEEL - SCOOP

Q: Alright, here's a question: I am looking to determine the amount of force that an impact will bring from an overhead thwackbot when I have the following variables:

* The robot has 3-inch wheels that are .5 inches wide
* The robot's body is a 2-inch square with a width of 6 inches
* The swinging arm is 5.5 inches long and 1/4 inches wide
* The arm is 6 inches wide and 1/8 inches wide and holds 5, 3 1/4 inch nails (so it looks like a rake)
* The robot's overall weight is 1lb
* The speed is 5mph
* The robot can accelerate to top speed in about .5 seconds

Knowing this, how much force is my rake on a stick going to deliver? [Champaign, Illinois]

A: [Mark J.] You aren't catching on, Champaign.

  1. You're gonna have a very hard time trying to accelerate zero to 5 MPH in 0.5 second in a dusty, greasy combat arena while your thwack hammer (rake) is flopping back and forth.
  2. Assuming that you have magnet wheels and enough torque to accelerate that quickly, the attack strategy for an overhead thwack is to accelerate toward your opponent and slam full reverse on the motors at just the right instant to bring the hammer (rake) over to strike your opponent -- who will not be just sitting there waiting for you to pull off this tricky maneuver.
  3. What you effectively have here is an electric hammer (rake) with the available torque limited by robot traction. Electric hammers without this limitation aren't competitive, so you cannot expect a hammer with this handicap to be effective.
  4. Bullet point #1 in the Ask Aaron Mission Statement says:

    The Ask Aaron site exists to support builders of combat robots with information, design tools, and advice based on our robot competition experience. We are not a free engineering service, and we won't do your homework for you.

I'm not going to run the numbers for you because:

  • Bullet point #3 above entirely kills the possibility that the energy for this weapon is significant;
  • Bullet point #4 above notes that "we won't do your homework for you"; and
  • I'm tired of proving to you that the reason old combat designs aren't used anymore is because they don't work.
If you have time on your hands, the Ask Aaron Electric Hammer Spreadsheet (one of the 'design tools' mentioned in our Mission Statement) allows you to find the number for yourself. Use the combined torque of your two drive gear motors as your input, but limit the torque to traction breakaway. You won't like the number.

Now, go build a contemporary design!

Update This question sat in my mind until curiosity made me pull up the hammer spreadsheet and run the numbers. I found the weight of five 3.5" common nails (0.34 ounce times five), assumed that the rake arm was aluminum, and fitted a pair of FingerTech Silver Spark gearmotors @ 7.4 volts. With best case traction and a perfectly timed reversal, the rake stores a bit less than 4 joules of energy on impact. The rake couldn't crack open a peanut with 4 joules.

The question asked about 'force' rather than energy storage, but the translation from joules to impact force depends on the nature of the impact, which depends on the properties of the object struck. See Hyperphysics: Impact Force.

Q: Is there a calculator that helps you find the right balance of bite and speed for a spinning weapon? [Great Falls, Montana]

A: [Mark J.] There is no single 'right balance' of bite and speed for a given weapon. The balance is situational:

  • Fighting a hard-surfaced opponent with no sharp edges calls for all the bite you can muster.
  • A drum head-to-head against another drum requires maximum speed and can dispense with bite.
  • Small arenas and close fighting call for big bite; larger arenas and higher closing speeds need less.
  • When your opponent has only soft or resilient exposed surfaces it may be better to ignore 'bite' and switch to 'shred'.
  • If you have a ramp to help your vertical spinner get a shot at your opponent's sharp front under-edge you need very little bite.
Stay flexible. Design for ample bite and adequate energy storage, keep some extra RPM available for special cases, and be prepared to throttle back your weapon if it's just 'skittering' across the hard surface of your opponent. Consider swapping in a 'sharp' blade for 'soft' opponents.
Q: Hi, so when two vertical spinners hit weapon to weapon, from my understanding the one with higher tip speed wins. But what if they each have the same tip speed but one is symmetrical and the other asymmetrical, would the symmetrical spinner win? [Great Falls, Montana]

A: [Mark J.] The 'higher tip speed wins' rule assumes otherwise similar weapons. If there are significant differences in weapon size or design the advantage analysis gets complicated.

Tip speed offers an advantage because the faster tip can catch the opponent's drum tooth from behind and throw their 'bot upward while remaining safe from the same attack. Two identical drums with the same tip speed will interlace the teeth of their drum weapons and bounce off each other with harmless glancing impacts.

If the drums are not identical the advantage in a same-tip-speed battle goes to the weapon with the greater bite and tooth-depth:

  • The weapon with a longer tooth (or teeth) can reach in to strike the opponent's drum body while keeping its own drum body out of harm's way.
  • The weapon with greater bite has a better chance to strike the opposing weapon drum with better than a glancing blow.
In your scenario, the winner is likely the asymmetric (single-tooth) weapon.
Q: What would be the most appropriate motor for my robot idea to use? Yes, a flipper and axe (or more rather a hammer, or a plank of wood with a nail in it) as one weapon don't go together, but I want to have one single 180 degree weapon.

I also want a robot with a protective and defensive chassis, and a Tornado-esque 4 wheel drive even though very few recent successful robots in UK/Europe have more than two wheels these days and, 'Bronco' aside, successful US robots with more than 2 wheels today are also hard to name.

What is the best motor for the arm, and how should it be connected to the rest of the robot interface? [London, England]

A: [Mark J.] I believe you answered your own question when you wrote, "a flipper and an axe as one weapon don't go together." Several prior posts in 'Ask Aaron' have discussed the disparate requirements of the two weapons: explosive torque and acceleration thru a short arc for the flipper, and efficient acceleration to maximum speed at the end of a long swing for the axe.

There is no electric motor solution that can simultaneously cover both of these requirements regardless of the weight class for which you are designing. This is the answer to the question you failed to ask, "Why are there no robots like my design?"

Mechanical linkages for hammers and flippers are extensively discussed in the Ask Aaron archives. Start with this post about the electric hammer used by 'ßeta', and you may be interested in this archived post about an electrically wound snail-cam spring flipper.

Q: I'd like to find the actual speed of my spinner weapon but I have no measuring tools and no budget. Are there any oddball tricks? [Facebook]

A: [Mark J.] A few, but I can't vouch for any of them...

  • The eyeball test [Paul Gancitano] -- download one of many free stroboscope apps to your phone and put a daub of white paint on the side of the weapon tip. Spin up in a darkened room and find the fastest strobe speed that 'freezes' the white spot in place. Result may be revolutions per second (Hz) or direct RPM depending on the app. Note: I can't find a working app that is fast enough for our purpose.
  • The earball test [Derek Reihe] -- put the edge of a playing card in the path of the tooth and match the tone it makes to a known source (frequency generator app). Matching the tone (Hz) gives revolutions per second. Divide by two if two teeth hit the card on each revolution.
  • The tongue test [Anonymous] -- spin it up, spin it down, then quickly lick it to see what it tastes like:

  Egg Salad
  2k RPM
  4k RPM
  6k RPM
  8k RPM
10k RPM
12k RPM
14k RPM
Let it spin down some more.  

If you actually want to know how fast your weapon really spins you can buy a digital laser tachometer online for less than $20.

Q: Hi, I have a tooth on a antweight vertical spinner that is 5mm thick, 30mm wide and 13mm long. Would O1 tool steel be suitable for this use? [Great Falls, Montana]

A: [Mark J.] Given only that information I really can't make a specific comment, Montana.

  • No competent engineer would spec material before knowing a great deal more about the design than you have told me. You didn't mention how the tooth is supported, the weapon speed, or the weapon moment of inertia.
  • It's poor engineering practice to specify dimensions for a heavily stressed part and then try to find a material that will survive in that application. Material and design must work together to perform a specific function.
  • See Frequently Asked Questions #17 and the Hamburger is Bad.
I can say in general that tool steels are commonly used for weapon teeth, as are various grades of abrasion resistant steel. Shock resistant S7 tool steel is a common choice for its great toughness. Your O1 steel may be an adequate choice, depending on your design details. Remember that tool steel is only as good as the heat treating it receives. I'd suggest tempering O1 to no more than 60 HRC to retain toughness.
Q: Hi Mark! How would you calculate the gear ratio of a friction drive weapon system on a ring spinner? Would it be like the calculation of a v-belt pulley?

A: [Mark J.] Two design elements that don't work -- let's put them together! Yes, the reduction calculation is similar to a V-belt system:

Reduction = Inner Diameter of the Ring ÷ Diameter of Drive Wheel (to 1)

Q: Because of your dislike of the design as a whole (for legitimate reasons) I also have to ask (as I cannot find it in the archives): how would you calculate the gear ratio of a ring spinner using a ring and pinion gear style or simply inter-meshed gears? Many Thanks! [New Castle, PA]

A: That calculation is the same as for any system of two gears:

Reduction = Number of Teeth on the Driven Gear ÷ Number of Teeth on the Motor Gear (to 1)

Gear reduction diagrams and more detailed explanation at wikiHow.

Q: I was watching Battlebots and Robot Wars (again) and noticed that robots such as 'Ringmaster' and 'MR Speed Squared' could gyrodance. I saw it firsthand as well with the Antweight 'Spinook' in Peoria this year. I know any robot with a spinning weapon is capable of feeling some kind of gyroscopic force, but how exactly does this happen to horizontal spinners? Do you think there is a way to harness this power to make a gyrobot like Gyrobot? If there is, I'm going to assume it would be a better option to try to build a combat robot out of a potato and a rubber band. [Champaign, Illinois]

A: [Mark J.] I can post formulas and diagrams about gyroscopic forces that go on for pages, and in fact I have: search the Ask Aaron 'Robot Design' and 'Robot Weapons' archives for 'gyroscopic'. If you really want to understand gyro forces I strongly recommend that you purchase a small toy gyroscope and play around with it.

Any time you apply a force to change the direction that the axis of a spinning mass is pointing, gyroscopic resistance will attempt to redirect that force at a right angle to the original force direction.

  • A vertical spinner must deal with this force redirection every time it turns. A quick pivot with the weapon at full speed results in that familiar 'gyro dance'. Check the Total Insanity Gyro Effects page for the math behind the dance.
  • Horizontal spinners don't change the orientation of the spinner axis when they turn, so are easier to drive. In order to 'dance' a horizontal spinner has to be tilted, either from a reaction to a hit by their own weapon, from an opponent attack, or from instability in their weapon design.
  • Horizontal spinners do not work for a Gyrobot-style 'walker'. With a vertical spinner the gyro force that resists axis deflection causes the unsupported edge of the robot to rotate forward and 'walk', but with a horizontal spinner the gyro force only causes the front or back of the robot to 'lift' which provides no useful walking rotation.

Q: I've seen several Antweights ('Odium' from Bot Brawl 2017, 'Debacle' from TeamVelocity's YouTube Channel, and a green spinner in the Team Velocity video of the rumble at NG Conference 2017) that get thrown onto their backsides and manage to drive on their back ends. Is the force being made from the spinner essentially acting as a front-mounted propeller?

A: Horizontal spinners standing on their tails are stabilized by the gyroscopic forces of their weapons, not an aerodynamic 'propeller' effect. Think of it as a very slow gyro dance and then go back to my answer to your first question.

Q: I am exploring three options for a horizontal spinner. All three assume a Team Whyachi TWM3R gearbox with a blade storing up to 50,000 to 100,000 joules of energy. The vibration mounts have a 258 lbs max static compression load, 15072 lbs/in spring rate in compression, 86 lbs max shear load, 1102 lbs/in spring rate in shear.
  • Option one mounts the gearbox to the frame rigidly and the blade to the 1" TI 6AL-4V threaded spin shaft of the gearbox. This design will transfer a big portion of the hit back to the frame and gearbox, but minimal energy is wasted.
  • Option two mounts the gearbox to the frame with vibration mounts and the blade to the 1" TI 6AL-4V threaded spin shaft of the gearbox. This design will absorb part of the hit energy on the vibration mounts, thus wasting part of the hit energy.
  • Option three mounts the gearbox to the frame with vibration mounts, has a belt and pulley or sprocket and chain between the gearbox shaft and a 1" TU 6AL-4V live axle with the blade. This design is more complicated and heavier, but the pulley and belt provide a clutch to protect the gearbox.
My tendency is to go for option two as the least likely to break. Option one may be too efficient in not absorbing energy fed back to my bot when it hits. Option two is simple. Option three is used by multiple bots, but isn't simple, is heavier, has more components that could fail and it would seem like the energy fed back to the frame would be almost as much as option 1.

I am looking for very sturdy, simple and able to deliver a big hit.

Which option would you choose and why (or is there a better fourth option)?

Can you point me to any sources for the equations to calculate the feedback energy, vibration mount sizing, component failure points? I'm trying to figure out what the weakest link is in the weapons chain to see if I can strengthen it.

Thanks [Denton, Texas]

A: [Mark J.] For bar spinners in this energy range I defer in design expertise to Ray Billings and Team Hardcore. He has been working big bar spinners for many years and has uncovered all the weaknesses and engineering tweaks the hard way.

Combat robots have such unpredictable force vectors that the standard engineering calculations for strength are largely useless; the process is to build it, break it, then make it stronger next time. Ray has cycled thru this process countless times, and he gives away too many of his secrets in this Reddit Q&A session. Read it carefully and treat it as gospel.

To your designs: my suggestion is to scrap the twin-Magmotor/M3R2 combination. That 1" titanium shaft won't survive impacts for long at those energy levels. Ray kept upsizing his shaft diameters and materials until he settled on 1.5" 4340 chrome-moly hardened to 45rc. Take your needed gearing reduction in the pulleys/sprockets instead of a heavy and (at these energy levels) fragile gearbox.

I'd follow Ray's choice of a big pancake motor - he likes the Motoenergy ME0708 - isolated from the weapon shaft. He tried belt drives but found them unreliable, so for years Ray's big spinners used simple chain and sprocket drive from motor to weapon. Recently he as added a slip clutch in the drive to give the motors a bit of protection. Ray estimates that it costs him about $1000 per match in ruined components and general wear to fight his big spinners. Yes, that's per match, not per tournament. The slip clutch helps a little with expenses. With the motor isolated, no shock mounts are used.

Also of note is that Ray likes bushings rather than bearings for the weapon. There is a litle more drag, but they will survive the huge impact loads more reliably.

You may be interested in an extensive and detailed heavyweight bar-spinner design discussion I had with another Texas builder. I gave it a dedicated section in the archives: The Texas Bar Spinner Papers.

Q: The is the three options for horizontal spinner guy. I was doing some research with the Rio Botz tutorial on the issue and came across this picture. Looks like you are right about not trusting the Team Whyachi shaft. I'm going with a 1.75" Ti 6AL4V ELI shaft (including bearings and external shaft to hold the sleeve bearings 2.375") and a chain drive (ala Tombstone).

Thanks for the advice.

A: RioBotz bent that shaft in a middleweight spinner about ten years ago. I think your upgrade is wise, although I'd prefer a suitably tough steel in place of 6AL-4V titanium. Keep me up to date on your progress.

Q: For a pneumatic weapon what is a good benchmark power to aim for? I understand from a spinner perspective the you recommended [a minimum of] about 60 Joules per kilogram but how would this translate to a pneumatic flipper or hammer weapon? Thanks [Baltimore, Maryland]

A: [Mark J.] Your question involves an 'apples to oranges' comparison.

  • A spinning weapon stores kinetic energy in the rotational inertia of the weapon that is transfered to the opponent on impact. That energy is accumulated by the weapon as it spins up to speed over some period of time. More energy storage is generally better, but there is a trade-off between increasing rotational speed and the ability to obtain enough 'bite' to effectively transfer that energy.
  • A pneumatic hammer also stores kinetic energy as it accelerates from its resting position toward impact, but it has much less time-distance to accumulate that energy. It's nigh impossible to get levels of energy storage in a hammer weapon comparable to a spinner weapon. Get as much as you can and hope its enough. Heavyweight hammerbot 'ßeta' manages about 7 joules per kilogram.
  • A pneumatic flipper does not store kinetic energy to impact the opponent. The flipper mechanism applies force generated from stored potential energy directly to the opponent to accelerate them upward. Measuring the 'static' force of the system does not equate to the net force applied while the system is in motion. Calculating that force by modeling gas flow thru the regulator, valves, and actuator ports is difficult and is further complicated by the geometry of the flipper mechanism.
The traditional answer to "How much lifter speed and force is enough for a pneumatic flipper?" is "All you can get." If you do want to model some potential flipper configurations, the 'Hassocks Hog' team webpage features an Excel spreadsheet that attempts to do just that: A guide to designing a pneumatic flipper.
Q: Hi Mark. I was doing some research on ways to prevent bolts and screws from loosening during vibration or stress. Two common solutions I am aware of include nylon insert bolts/nuts and thread-locker. Generally speaking, is one method better than the other? Also can both be used simultaneously or will the thread-locker mess up the nylon? In my specific instance I am building a weapon arm with through holes at the ends that holds 2 S7 tool steel impactors approx. 1" by 1 3/8" which will be tapped and have bolts run into them. Many Thanks... [Pittsburgh]

A: [Mark J.] There is a good discussion of the benefits and drawbacks to threadlocker vs. nylon inserts at Briefly, threadlocker holds better than nylon IF properly applied to clean threads and allowed to fully set, while nylon inserts are foolproof and immediately useable. If there is a chance that I'll need to remove the bolt and replace it between matches I'll use nylon, otherwise a carefully applied threadlocker is my choice. Don't use both -- most threadlocker compounds don't mix with plastics.

Important You requested that I not use the drawing you sent -- but you may have noticed that there are no current robots running similar designs. There is a very good reason: your weapon is unstable. Take a look at the post about polhode motion in this archive and watch 'Secto' in this video. Your design is not identical but it has the same instability! Also the fastening method you propose for the impactors puts a huge shear load on the bolts, which is very poor engineering practice. The bolts will fail. I'd strongly suggest starting over on the weapon design.

Q: I was thinking of using a drum 6" in diameter (4" drum with 1" teeth on each side) and 6" wide. However, several people said that having a drum with square dimensions was a bad idea. But why? Is it a bad design choice, and if so, is there a way to counteract or reduce its flaw(s)? [Champaign, Illinois]

A: [Mark J.] I don't know who 'several people' are, but they were paying close attention in their physics classes. Take a look at this post about polhode motion in this archive.

Long rotating drums are pretty 'stable'. Very short rotating drums (disks) are stable as well. However, drums close to 'square' dimensions may have very similar moments of rotational inertia in all three possible rotational axis and become unstable -- possibly flipping the entire robot in unpredictable directions.

The flaw comes from the distribution of mass in the rotating drum and there is no 'fix' other than redistributing the mass into a less-square shape. The instability effects are less of a problem in a robot with a weapon at one end, as most drum weapons are, than a full-body or overhead spinner. I'd still be worried about possible instability and change the weapon dimensions.

Q: I have a question more about a certain segment in the Riobotz book. I was reading it and came across section 6.5.5 regarding Full-Body Drum Spinners. I get the part regarding how what they refer to as a "shell" drum spinner, but I don't get the next two parts. They read as follows:
  1. "If the drum was a cylinder mounted on rollers, it would be called a ring drum."

    What does that mean? The whole body is a ring shape inside with the drum rolling on top of it?

  2. "A full body drum, which would be like an overhead thwackbot but without a rod. The robot would use the power of its own two wheels to spin its entire chassis (and not just its armor) as if it were a big drum."

    Okay, I get that.

    "...The challenge would be to implement at each wheel an independent braking system that would allow the chassis to spin up without moving the bot around. After reaching full speed, the braking system would release, and the robot would be driven by slightly accelerating or braking each wheel motor."

    Um... What?

Can you help me with some kind of translation or breakdown? I have a feeling I'm overthinking it. [Champaign, Illinois]

A: [Mark J.] The RioBotz Combat Tutorial was written in Portuguese and loses a bit in translation. The descriptions are also a bit brief in places. To be fair, 'Ask Aaron' probably doesn't translate well into Portuguese.

  1. They're talking about a ring spinner like 'The Ringmaster'. The spinning 'drum' is a ring around the outside of the chassis. The ring is entirely supported by rollers around the chassis edge leaving the top of the robot chassis clear of any drum supports. The advantage being that inverted operation is possible if large wheels stick all the way thru the chassis or - like 'The Ringmaster' - an additional drive system is made for the top.
  2. A conventional 'thwackbot' cannot move when spinning -- it has to stop spinning to move like a normal wheeled robot. By carefully timing very brief independent decreases in the speed of the drive wheels while the 'bot spins, the rotation is disrupted in a way that moves the robot in a controlled direction rather than just spinning in place. This is called translational drift, 'melty brain', or 'tornado drive'. It's horribly complicated to implement and not terribly effective.
These designs have not proven successful in combat. They are 'show off' technology to impress other competitors with your mad building skills.
Q: Why do saws have less bite than discs with less teeth? And why do hyperspeed weapons not get bite? Yes, I did read the spinner weapon FAQ. It said the reason was longer insertion time. Over speed weapons should do the same thing as mid-speed weapons except faster. Bots like Saifu, Weta, Algos, and DDT all use high speed weapons and they have no problems. [Arlington, Virginia]

A: [Mark J.] I believe you misunderstand what is meant by weapon 'bite'. A faster weapon, or one with more teeth, allows less time for the opponent to be inserted into the arc of the spinning weapon before the next tooth rotates around to strike. The Spinner Weapon FAQ points readers to Section 6.3 in the RioBotz Combat Tutorial for an explanation of the relationship between weapon speed and bite, as well as the formulas for calculating bite depth. If the explanation in the FAQ is unclear to you, I suggest reading the RioBotz tutorial.

The robots you list do have effective weapons - it is possible to have an effective spinner that has poor bite if you compensate with other design elements.


  • 'Algos' uses a wedge to lift the opponent up and into the drum, exposing a sharp angle on the underside of the opponent that requires very little bite to grab and throw.
  • 'DDT' does not direct-drive the big weapon blade -- the large diameter disk gives a high tip speed at a lower RPM for better bite.
  • 'Weta' has a reduction belt drive to keep the RPM reasonable, plus offset impactors for adequate bite.
  • 'Saifu' is a direct-drive high-speed drum, but you don't need or want full RPM against all types of opponents -- you can and should throttle back for 'hard to hit' opponents.
Opponents with sharp edges to grab do not require good bite, but better bite allows effective attacks on flat or rounded surfaces.

Q: Does bite apply to hammers?

A: 'Bite' as described here applies only to continuously rotating 'spinner' weapons. The concept of 'bite' does not apply to 'strike on demand' axe or hammer weapons.

Q: Hello, Mark! Back with some questions: this time, these questions are regarding a heavyweight build. The robot would be heavily based off of Electric Boogaloo:
  1. My original idea was to use a spinner similar in shape to Deathroll's spinner (I even asked them if they could sell me a spare! Of course, the dimensions would probably change), but around 12" tall. However, I may be better off with using a 12" bar seeing as it will be easier to put together and get machined, especially since this would be my first heavyweight bot. Which would you recommend and why? Also, which is better for self-righting, like with Aftershock? I'd like to be able to do so with this design.

  2. I was thinking of mounting the bar or disc with two motors. Why? First, if one motor gives out, I got a spare. Second, I like symmetrical designs or designs with singular motors centered (motors to the side that make the bot look asymmetrical always bugged me for some reason lol). Of course, this would mean extra batteries and more money (which I probably would have if I used a bar rather than a custom disc). What do you think? Should I use two motors or one? What would the major perks of this, and what would be the biggest flaws or concerns be?

  3. Depending on the parts I use and disc's dimensions (as well as personal taste), I may be able to split the chassis entirely in half (EB had a small but reinforced portion in the back if I recall correctly, so it wasn't split in half). Only one problem: this adds onto the whole "expense" and "complexity" issues of having two motors (When I say "expense" and "complexity", I'd prefer not to spend over $7500 on the bot). I'd probably need two receivers, and coding would probably be a PITA. More importantly, it would be hard to keep it in one piece if it's being held together by one small piece of aluminum. What do you recommend as solutions?
As with all of your responses, your response and help will be greatly appreciated! [Champaign, Illinois]

A: [Mark J.] Having conquered the lower weight classes you have decided to move up to the heavyweight class?

  • How many combat robots have you built?
  • What prior experience have you in mechanical design and construction?
  • In how many tournaments have you fought?
  • How many battles have you won?
  • The last set of questions you asked had you attempting to turn metal salvaged from the scrapped drinking fountain at your school into an antweight -- how did that go?
Very seriously: You have no business building a heavyweight combat robot. If you're actually planning a heavyweight build you should reconsider. If this is simply an exercise in fantasy design, you should stop wasting my time. I will be pleased to assist you in building an insect or sub-light robot, and when you have learned the painful but relatively inexpensive lessons to be overcome in those weight classes I will assist you in moving up to a suitably heavier robot.

Your questions are well thought out and nicely presented. I'm not taking this position to be cruel, I'm attempting to do you a favor by giving you the best advice I can provide.

Update I've reflected on my reply to your questions. I still believe I gave you my best advice, but it does not sit well with me to leave your specific questions unanswered. What follows is as much for my benefit as yours:
  • Keep your design as simple as possible -- simple 'bots win. As you noted, a bar spinner is simpler and less expensive than an oddball custom single-toothed cutaway disk. It's also more durable.
  • A bar is not good for self-righting. Your best choice there is a full-circle disk like 'Aftershock'. An inverted circular disk in contact with the arena floor is still easy to spin, even if slowed by the impact that flipped the 'bot. When the tooth strikes the floor it has a chance of popping the 'bot back upright (if you're lucky).
  • One motor, please. Let it bug you. It's more efficient, less complex, less expensive, and - in spite of your hopes - more reliable. If one motor quits you probably took enough damage to take the whole weapon out.
  • A split chassis is a weak chassis. Dual receivers would be the least of your problems. It's very embarrassing to have your robot torn in half. One... solid... strong... box... chassis.
Still, do not build a heavyweight!

Q: For a heavyweight horizontal spinner weapon system, is the perm PMG 132 a better choice than the E-tek Motoenergy ME0909 PMDC motor? [Portsmouth, Ohio]

A: [Mark J.] The preferred brushed heavyweight weapon motor - for a balance of power and durability - is the Motoenergy ME0708.

Q: Hello, I have 2 questions. Bot info:
  • A 12-Pound Tombstone clone
  • Two wheels: 2.375" diameter, 0.5" width
  • Two RS395 Banebots motors
  • 4:1 P60 Banebots gearboxes
  • Hobby King ACK-5312CP 330KV weapon motor

1) I'm very new to this, could you double check my gearbox choice (or even motor choice if necessary)?

2) I will be using A123 LiFe batteries (3.2v, 1100 mah 30 amp discharge). If my calculations are correct (which I doubt) 1 of these batteries should be able to power the weapon and drive for 25.714 minutes.

How many batteries do you think I need? Thanks for the help. [Hicksville, New York]

A: [Mark J.] I can't see all your input fields in the image you sent from the Team Tentacle Torque Calculator, but it's clear that you have not entered the correct values for the RS-395 motor. Let's start over with the correct values:

The 20:1 ratio P60 gearboxes with the 2.375" wheels give the best performance from the RS-395 motors in a moderate size arena: around 5.5 MPH, reaching that speed in about 6.5 feet. That's ample speed and power to push around a big spinner weapon. The 26:1 gearboxes would be a little easier on the motors, but the 20:1 should be fine.

The RS-395 motors run well between 12 and 15 volts -- you'll need four LiFe cells wired in series to provide 12.8 volts. The drivetrain will use about half of the capacity of the 1100 mah cells in a 5 minute match.

Your weapon motor is a powerful choice for a hobbyweight. It operates in the 22 to 30 volt range, so you will want a separate battery to operate your weapon. That battery will require between 7 and 9 LiFe cells wired in series. You have not given me enough information about your weapon to calculate the battery capacity (mah) needed. Heavier, longer blades will use more battery power to spin up, and the reduction ratio between your weapon motor and blade will also impact current usage. Example, courtesy of the Team Run Amok Excel Spinner Spreadsheet:

A steel bar 15" long by 2" wide by 0.5" thick running a 2:1 reduction from the ACK-5312CP motor at 28.8 volts (9 LiFe cells) would spin to about 1300 joules energy storage in roughly three seconds. Assuming six spin-ups from a dead stop in a 5 minute match, total weapon current consumption would be about 520 mah.

So, you'd need one four-cell LiFe battery to run your drivetrain and one 7 to 9 cell LiFe battery to power your weapon. Combined, they should run your 'bot in combat for about 10 minutes on a full charge.

Are you sure you wouldn't like to build a nice wedge robot instead?

Q: Thanks for the quick response. My weapon's current plan is a 5 pound 12 inch diameter 1/2 inch thick S7 steel asymmetrical "disc", shaped somewhat like a teardrop. The thickness is to avoid vertical spinners from breaking the blade. None of this is set in stone but the 12 inch diameter, and somewhat the weight.

My fights will only be 3 minutes long. The bot does strategically not need to be fast - in fact it should be as slow as reasonable to save weight and power consumption. The 26:1 gearboxes look best to me to that effect.

I hope to have as little power left over after a fight as reasonable, perhaps only enough to last 4 minutes in total. That being said, I still think I can squeeze 12 LiFe A123-18650 batteries into my bot, in 2 groups of 4 and (with slight design alterations) 2 groups of 2. But of course the fewer needed, the better.

Funny you should mention wedgebots, as I eventually hope to build a bot to take down Original Sin! Anyway, do you have any blade alteration or battery number suggestions? Thank you.

A: A 12" diameter, 1/2" thick steel disk weighs more than 16 pounds, so your 'teardrop' design must have a LOT of material cut away from the full disk. A simple steel blade 12" long, 1/2" thick, and 2.875" wide weighs 5 pounds. A 5-pound weapon is heavy for a hobbyweight, so be careful with your weight calculations.

The number of cells I'm suggesting for your batteries are not there for current capacity, but simply to obtain the voltage needed for your two motor systems running at differing voltages. You can't run your drive motors at 30 volts, and running your weapon motor at 15 volts would drop it's power output from 1000 watts to 250 watts. 'Tombstone' has this same problem and solves it the same way that I suggest you do: two battery packs with differing voltages.

I'm a bit concerned about the ability of the cells you have selected to provide the start-up amperage needed by your monster weapon motor. The motor is rated 40 amps continuous, but can briefly draw as much as 140 amps under starting load and will draw over 70 amps for as much as two seconds while staining to pull that heavy weapon up to speed. Given that your cells are rated for 30 amps continuous draw, pulling more than twice that current may damage the weapon battery. You may want to reconsider your LiFe cell choice.

Q: Thanks for the advice. I'm happy to lock in 4 1100 mAh batteries (the yellow ones) for the drive power with 26:1 gearboxes, if you think that's not too many.

A: My earlier explanation was not sufficiently clear. A battery pack for a specific application must supply three things:

1) Adequate capacity to power the device for the required time (mAh).
2) The current needed by the device -- without damage to the battery (amps); and
3) A suitable voltage for the device (volts);
We have calculated that the drivetrain for your robot will use about 330 mAh of total current in a three-minute match, so your 1100 mAh cells have that well covered.

We have also calculated that the peak current draw of the drivetrain is about 10 amps, so your 30 amp continuous output cells also have that well covered.

That leaves voltage. To get adequate performance from your chosen drive motors you need a battery pack that will supply 12 to 15 volts. Each of your cells produces 3.2 volts:

One cell = 3.2 volts;
Two cells wired in series = 6.4 volts;
Three cells wired in series = 9.6 volts;
Four cells wired in series = 12.8 volts.
You require four cells wired in series to supply adequate voltage to your drive motors. Fewer will not do. You could get by with smaller cells, but you need four of them.

Q: Now for the weapon power. Apparently the bigger A123s (green) are back in stock. They have a very slightly lower voltage (3.2V) but a higher amperage(50A), plus a maximum impulse discharge of 120A.

However, as I understand it, the same number (7-9) of these green batteries would be needed for the weapon motor's voltage as of the yellow batteries. Since the greens weigh more and are slightly bigger, this is less ideal. LiPos are not allowed at my upcoming event. Is there another battery you recommend instead?

Not using 100% of my motor's capability seems ok to me, as it might help prevent breakage (I think). Obviously I want to use as much as possible, but I'm okay with using less. Perhaps not filling up the voltage requirement completely but satisfying the corresponding Amp requirement would do the trick. The green A123s might be better at that. I currently have 1-1.5 pounds to spare for batteries, and the fewer cells I can reasonably use the better. What do you think?

1 yellow = 0.088125 pounds
4 yellows = 0.3525 pounds
8 yellows = 0.705 pounds
12 yellows = 1.0575 pounds
1 green = 0.18125 pounds
4 greens = 0.725 pounds
8 greens = 1.45 pounds
I plan on ordering the parts this week so I can experiment with a prototype before I finalize the bot.

A: Your problem is that your weapon motor is, as I mentioned before, a powerful choice for a hobbyweight. It requires both high voltage and a high peak current capacity. Having a big weapon motor and not running it at full voltage is a serious waste of power. Power varies with the square of voltage, so dropping a couple of cells cuts your power by almost half:

9 cells = 920 watts
8 cells = 728 watts
7 cells = 558 watts
6 cells = 410 watts
With each drop in power your spinup time increases and your weapon power storage drops. That big weapon motor is causing you more problems than it solves. If you want to run at a lower voltage you'd be much better off with a smaller motor designed to run at that voltage.

If you ask around the on-line forums, someone is going to suggest that you build a 9-cell battery pack to power the weapon and 'tap' the pack at 4-cells to power the drivetrain. This is possible, but I cannot recommend it. You'll be drawing more mower from some of the cells than others, and the draw from that big weapon motor may 'brown out' the drive and electronics. Proceed on that path at your own risk.

I think I'd go find a more reasonably sized weapon motor that would run well at four or five cells, downsize the weapon rotor, and run the whole bot off a single battery pack.

Q: Can you make a permanent page for those series of questions about crazy weapon designs? I think you'd be doing a great service to dissuade new builders from doing something too crazy. [Waltham, Massachusetts]

A: [Mark J.] I generally assume that builders writing in to 'Ask Aaron' are interested in combat robots that will win matches, but an increasing segment of builders are interested in impressing audiences and other builders with 'show-off' designs. While our focus remains on supporting simple and effective designs, I also recognize that one man's crazy is another man's awesome.

The recent long series' of questions about... unusual... weaponry and design has found a place in the Ask Aaron Design and Construction archive and I'll keep trying to set reasonable expectations for any design thrown at me.

Q: In the weapon FAQ or something it says discs are better than bars. Why do bots like tombstone or last rites use bars if discs with carvings such as in Nightmare or DDT are better?

A: [Mark J.] A spinning disk stores more energy at a given RPM than does a spinning bar of the same mass and swept diameter. This is because the disk has more of its mass located farther from the axis of rotation. The Ask Aaron Spinner Weapon FAQ features a comparison of the energy storage capacity of four spinning weapon designs with the same mass and swept diameter: bar, disk, eggbeater, and hollow drum.

Better energy storage is a good thing, but it is not the only factor to consider when designing a spinning weapon.

  • Simple designs like bars and solid disks are much easier to construct than eggbeaters and drum weapons.
  • At large sizes and very high energy storage levels the durability of thin disks, eggbeaters, and hollow drums becomes a serious concern.
  • Some consideration must be given to the vulnerability of the weapon to attack by other weapon types. For example, the large edge area of a horizontal disk is vulnerable to a fast-spinning vertical drum weapon.

Q: And if vertical spinners supposedly generate more kinetic energy then why do people use horizontals? [Baltimore, Maryland]

A: Who said anything about verticals storing more energy? You didn't find that here. The orientation of the spinner has no effect on energy storage.

Q: Is there a rule of thumb for how much battery capacity to have for a spinning weapon? [Manassas, Virginia]

A: [Mark J.] Too many variables for a 'rule of thumb' -- how about a calculated solution? The Team Run Amok Spinner Excel Spreadsheet calculates the mass, moment of inertia, stored kinetic energy, tip speed, spin-up time, AND the approximate battery capacity requirement for your spinner weapon. You'll need Microsoft Excel to run the spreadsheet.

In general, the capacity needed for your weapon will be much less than the capacity needed for your drivetrain.

Q: Is there any advantage in hooking 2 brushless motors to a single spinning disk weapon?

A: [Mark J.] Two motors = twice the power -- but because each brushless motor relies on it's controller for commutation as well as current, each motor requires its own ESC. It's generally preferable to use a single larger motor with twice the power.

Q: Also, can I put a custom aluminium pulley around an outrunner brushless motor retained by a couple of set screws with plenty of loctite in a hobbyweight? [Quebec, Canada]

A: Set screws are best avoided for this purpose: they apply large point-pressure that may deform the rotor and are failure-prone even with a threadlocker. If you're making a custom pulley you should size the bore for an interference fit and press the rotor into place.

A word about threadlockers: some builders recommend applying Loctite to shafts and other sliding assemblies to secure them, but that's well outside the intended use of the standard blue or red Loctite. There are special formulations of green Loctite designed for holding loose-fitting components to shafts, but I can't recommend them for torque carrying applications in combat.

Q: I saw some of your old answers about 'Beta' and I thought I would investigate how much better 'Beta' is than just using regular gears, but I couldn't figure out if I was doing it right.

So anyways, what I was doing was putting some motor statistics into your hammer spreadsheet and figuring out the best gear ratio for maximum joules as a starting point. Then I doubled the gear reduction and wrote down the number of degrees to reach 60% speed and what the speed was. Then I reduced the ratio a little, and took the speed from the previous step as a starting point, and found how many more degrees it would take to reach 60% again from that point and repeated this out to 180 degrees.

I ended up with almost the same amount of power as without all the fancy gearing! Did I do something wrong? The only thing I could think of was maybe it is only worth it if the hammer is really heavy and the motor would really struggle or something. [Quebec, Canada]

A: [Mark J.] For readers unfamiliar with John Reid's 'beta', the heavyweight British hammerbot uses an intricate chain 'snail cam' (technical term: 'fusee') that decreases the motor reduction ratio as the hammer accelerates thru its arc in order to keep the motor close to peak output power. This previous post in the Ask Aaron Design and Construction archive discusses both the design theory and how to use the Team Run Amok Excel Hammer Spreadsheet to find a snail cam design solution.

You aren't doing anything wrong in your calculations; the approach you used is quite similar to the one I outline in the above referenced post. You are also correct that the snail cam results in only a small increase in weapon strike force -- I make it about 20%. There are, however, other benefits to the snail cam. In particular the lower starting load on the weapon motor increases motor efficiency and reduces current consumption, making life easier on the motor, battery, and motor controller. This is very important when your expensive motor is stressed very close to its limits! Think of it as an investment in longevity.

Q: I'm planning out a hobbyweight Tombstone-esque horizontal spinner. I was recommended to use an outrunner of 1000-1500 Watts for the weapon motor, and I found one that's 70mm (2.75591'') tall when oriented appropriately. Meanwhile, I found another outrunner that's not much heavier, but has over 2000 Watt power. The problem is, it is 83mm (3.26772'') tall.

The heights of these motors would be added to the thickness of the top and bottom plates, each .25'' (as of the current plan). That for the big motor makes a bot almost 4 inches tall. I'm finding it difficult to locate the dimensions of other hobbyweights, but I have a feeling that this is too tall, and I know a horizontal spinner should be short as possible. Mainly I worry that the required chassis would be too heavy, and my rough calculations put V1 above the ideal 3 pound range.

Any structural advice? Thanks. [New York City]

A: [Mark J.] Building a hobbyweight and you've never been to a live robot fight? Not recommended. I'll get to the structural advice, but I have a few more suggestions first...

  1. Don't let your choice of weapon motor dictate the design of the rest of your spinner weapon. Start with the required energy storage and available weight allowance, find weapon dimensions that will meet those goals at reasonable RPM and 'bite', and then select a motor that will spin your weapon to that RPM in a reasonable length of time. Adding more motor power to a correctly designed weapon will not make it more destructive -- it will only shorten the spin-up time. See the Ask Aaron Spinner Weapon FAQ.
  2. There are very short multicopter motors that deliver ample power for your hobbyweight weapon. Take a look at the 'Hobby King ACK-5312CP' as an example.
  3. Your proportions are off. The weapon system in a design such as yours should weigh about half the total robot weight, and a large diameter weapon will store much more energy than a smaller diameter weapon of the same weight. The spinning bar weapon in pictured hobbyweight 'Fiasco' is 16" long.
  4. Weapon drive can be 'outside' the chassis to reduce the chassis height/weight and improve strength.
  5. Structure: the entire top and bottom plates of the robot should be single pieces. Do not tack the weapon support onto the body of the robot and expect it to survive. The weapon support structure has to survive the same impact that the weapon deals to your opponent. 'Fiasco' uses 0.375" thick 7075 aircraft alloy aluminum for it's chassis, but solid 0.25" thick aircraft aluminum without the fancy cutouts is likely alright. Much more 'meat' around the weapon hub is needed for adequate support.
Don't go crazy with weapon motor power. 'Fiasco' had well less than 1000 watts, but its weapon was nearly as dangerous to itself as to its opponents.

I've edited the text of this next question to remove identifying details at the request of the builder. I've also trimmed some text to get to the question I think is important to share with 'Ask Aaron' readers.

Q: I'm having trouble with the design for my featherweight drum weapon.

My drum is 3-1/2" diameter by 4-3/4" long with two full-length steel impactors. Your Spinner Weapon Calculator shows that my drum has a moment of inertia of 0.0025 kg·m2 and that my 2400kv inrunner brushless motor (14.4 volts, 3:1 reduction) will spin it to about 1800 joules at 11,500 RPM. Drive is a pair of 1300 RPM brushed gearmotors with 5" diameter wheels spaced 13" on center. Plugging all these values into the T.i. Gyro Effects calculator gives a huge gyro lifting force of 57 Newtons on the raising wheel!

I know that featherweight spinners with more stored energy than my design are drivable, so why does my design have such a strong gyro lift? What can I do to reduce that lift besides making the chassis crazy wide? [location withheld]

A: [Mark J.] One of the problems with the weapon design calculators at 'Ask Aaron' is that they're a little too easy to use. It's a simple matter to plug numbers into the input boxes, pick the numbers out of the boxes at the bottom, and run with them. I think I've failed to draw the full picture of what those values mean and how to best use them for modeling a weapon. I'll work on that, but for now let's clear up your concerns.

Weapon speed: You aren't going to get your drum up to 11,500 RPM. Your motor's 2400 kv assumes zero load, but your drum will generate significant aerodynamic drag at high RPM. Aero drag increases with the cube of speed, so while the drag at 4000 RPM is modest, drag at 12,000 RPM is 33 = 27 times as great! This is one reason I recommend weapon designs with a high moment of inertia and modest speed. A wild guess: the weapon might spin to 8000 RPM with your current gearing before drag stops acceleration.

Gyro lift: Buried and inadequately highlighted in the text of the T.i. Robotics Gyro Effect page is a description of the conditions assumed for their calculation of gyro lift:

" fast the robot is rotating when you have one side of your drive in [full speed] forward, and one in [full speed] reverse..."

That's spinning in place at full speed! It's an unlikely scenario for a drumbot during combat, and I advise against setting up your R/C transmitter to even allow such a condition. Smooth control at speed in combat is best achieved by limiting the max turning rate at the transmitter. I discuss setting the turn rate in the 'Dual Rates' section of our Transmitter Programming Guide.

The best use of the T.i. Gyro Calculator is to play with the numbers to see how fast your design CAN turn before the gyro effect becomes a problem. My 'back of napkin' calculation shows that with an 8000 RPM weapon speed your design can rotate 90 degrees in 0.4 second without wheel lift -- more than fast enough.

I'll get to work on a new output box for the gyro calculator that will give a maximum spin rate without wheel lift.

UPDATE: I've completed my additions to the T.i. Gyro Effects calculator. The calculator now has two new output fields:
  • 'Max Robot Spin Rate': the time needed for the robot to make one full revolution at the calculated maximum rotation speed of the robot; and
  • 'Max Flat Turn Rate': the fastest the robot can make one full revolution without wheel lift.
Together these values allow the builder to evaluate how much the turn rate of the robot must be slowed to keep it flat on the arena floor, and whether the 'flat turn' rotation speed is adequate for combat. I think this will simplify use of the calculator allow builders to better interpret the output.

Q: Hi Mark, what is an EDF brushless motor, and is it a good idea to use them on lighter bots to power spinning weapons? [Sydney, Australia]

A: [Mark J.] There's nothing special about an 'EDF' motor; they're just brushless inrunners suitably sized to power an 'Electric Ducted Fan' in R/C 'jet' aircraft. In general an 'outrunner' style motor of similar power will spin at lower RPM and will be easier to adapt to a combat robot weapon in a light combat robot.

Q: What is tip speed? [Kansas City, Missouri]

A: [Mark J.] The outermost edge (tip) of a spinning weapon travels a circular path around the axis of rotation. With each rotation the edge of the weapon travels a distance that can be calculated by a simple formula:

Distance Traveled = 2 * Distance from the Tip to the Axis * Pi

If we then multiply this distance travelled in one revolution by the number of revolutions per second we obtain a distance travelled by the weapon tip in one second -- the tip speed.

Example: a simple spinning bar weapon has its impact tips located 250mm from the spin axis. If this weapon spins at 4000 RPM the tip speed of the weapon is:

Tip Speed = 2 * .250 meter * 3.1416 * (4000 / 60) = 105 meters per second = 235 MPH

Tip speed is often used as a 'brag number' by builders, but it is a critical number for drum spinners. When two drum spinners strike 'weapon-to-weapon' the weapon with the greater tip speed has an enormous advantage.

The Team Run Amok Spinner Weapon Kinetic Energy Calculator will calculate your weapon's tip speed, as well as the weapon mass, moment of inertia, stored kinetic energy, and spin-up time.

Q: Alright, I'm here with the last question for a while (to let other people ask questions) - I've been really interested in HUGE drum spinners. However, I'm needing to look for some inspiration for a design. I currently have a few "big drum spinner" designs in mind, like REDRUM, Fluffy 2, and Barber-Ous. However, I'm looking for some other large drum spinner designs as well. Note when I mean huge, I mean freakishly huge (larger than Yeti's). Also, I do know the pros and cons with drums that are much, much bigger than normal (I did do a bit of research already on that). Any names you can think of to look up and research in terms of HUGE drum spinners?

Thank you for responding to all of my questions! [North Carolina]

A: [Mark J.] I do recommend studying successful designs related to your own ideas, but it's also important to look into the challenges created by pushing design elements beyond conventional scale. For a 'freakishly huge' drum you'll want to explore the gyroscopic effects caused by trying to turn a robot with a big vertical spinner. Some robots' gyro-dance is so extreme as to render them useless in combat. I've reprinted a fairly recent post from the Ask Aaron Weapons Archive to give you a start:

Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator

Run a few huge drum designs thru the calculator to see if you can find something stable and effective.

Q: Hello (again). I am here (again) with a question! This one isn't about designs regarding my bot or scaling. My question this time involves flails. Simply put, what chain length to flail weight ratio is the best? Also, why has nobody ever mounted flails vertically, like on a drum or a flywheel? Finally, would a horizontal or vertical mounted set of flails do best? [Urbana, Illinois]

A: [Mark J.] If you run the numbers it is immediately obvious that the ideal chain length is 'zero'. A rotary weapon stores kinetic energy and delivers that energy in a single massive impact on your opponent. Flails are entirely ineffective at transference of stored kinetic energy in that manner.

You can't spell 'FLAIL' without 'FAIL'.

There has never been a successful flail robot -- either vertical (like 'Morrigan' from Near Chaos Robotics) or horizontal -- and I don't expect to ever see one. The only thing they do well is make noise. Avoid.

Q: Hello! I'm here with a new question. I was watching the newest season of Robot Wars (for the second time) and couldn't help but notice Hobgoblin being the only Beater Bar. Not only that, but I can't think of any other beater bots of that size or even close to that size. My question is this - why aren't there more beater bars in the big competitions? [Urbana, Illinois]

A: [Mark J.] A beater bar weapon is simple and very efficient at storing energy in a small 'bot (see the 'energy storage' section of the Ask Aaron Spinner Weapon FAQ) but as the size of the weapon increases the Square-Cube Law takes hold and the required cross section of the support arms grows rapidly in order to provide the strength required to survive impact. See also this post in the Ask Aaron Design and Construction archive from 'Buzzards Bay' about scaling up small 'bots to larger sizes.

Compare the weapon proportions of heavyweight 'Hobgoblin' (below left) to those of beetleweight 'Wave of Mutilation' (below right). The thickened support arms and impactor bars of Hobgoblin's weapon are necessary for strength in a heavyweight-scale beater, but they move mass toward the center of rotation and greatly reduce the energy storage efficiency of the design. A mini-disk or drum of the same weight and radius would be stronger and would store much more kinetic energy at heavyweight size -- which is why you don't see heavy class eggbeaters.

Incidentally, the RioBotz Combat Robot Tutorial section 2.2 spends three pages arguing that the square-cube law doesn't much apply to combat robots. Given that RioBotz tend to design large robots and then scale them down to lighter classes, the result of their ignoring the square-cube law is not disastrous -- but have you noticed that their larger 'bots do better than their smaller clones?

Q: Hi, my team is planning to build a [heavyweight] vertical spinner as shown in the image. The diameter is about 16 inches, and the total mass is about 43.358 Lbs. The bite is 2 inches. What would be the optimal RPM? [Buzzards Bay, Massachusetts]

A: [Mark J.] Notes:

  1. Listing weight to three decimal points implies precision much greater than 'about'. Let's round it off to 40 pounds, OK?
  2. 'Bite' is a theoretical calculated figure based on weapon speed, the number of impactors, and the closing rate of your robot and the target. It is unrelated to the shape or height of the impactor.
  3. See the Rotational Speed section of the Ask Aaron Spinner FAQ for a full explanation of bite and how to determine a reasonable RPM.
  4. There is no single 'optimal' RPM. Speed helps determine bite, and the needed bite depends as much on the design of your opponent as it does on your weapon. Again, this is explained the the Spinner FAQ.
  5. Those sharp 90 degree internal angles adjacent to the impactors are stress risers -- you'll want to smooth those out.
Read the whole Spinner FAQ while you're there -- it will save you from writing back four or five more times.

Then this missive from ChaosCorps arrived in my mailbox:

Hi Mark, figured I'd write in quickly on the post from the builder in Buzzards Bay, Mass.

Not sure if the disk image shown is the one he provided or not, but if it is I can explain a bit of what's going on with it, since it's the disk we used on Bombshell during BattleBots season 2. The disk itself is only part of our disk setup, the softer steel main body. The disk was designed to use S7 tooth inserts that key into the unusual cutouts and are retained with a high strength bolt. We also cut the disk on a waterjet which means those sharp corners actually did have a small radius that wasn't necessary for modeling the part. Between the disk being 2" thick and a softer steel which minimized the chances of a brittle failure it proved to be sufficient in our application.

As a side note, we released the full set of CAD files for the season 2 version of Bombshell.

Thanks, ChaosCorps! That image was provided by Buzzards Bay, and although I thought there was something familiar about it I failed to recognize its source. Knowing that the material is soft steel with impactors that drop into those cutouts makes much more sense. Made from hardened steel those sharp cutout angles would be trouble.

It's generous of you to post the CAD files; not many teams offer design details to help other builders. So... how fast do you spin it?

We run the vertical disk on Bombshell with two A28-150's on two series 4s lipo packs (29.6v nominal) with a 2.5:1 reduction. That puts the no-load speed at around 3,000rpm.

Thanks again! Buzzards Bay: you can watch the videos and see the effectiveness of this weapon at 3000 RPM.

Q: I am looking at building a 30 pound robot this year, and had a question for the lifter which will be around a foot long.

I was looking at using a banebots 256:1 or maybe more gearbox and a brushless motor for the lifter, but reading they say to not expose the gearbox to more than 35 ft-lbs. I'll gear the gearbox down more with a chain to the lifter arm so lifting 30 pounds should be fine, but if I run into the wall or the other robot or a weapon with the arm, how do I keep the gearbox from breaking on the impact when all that force goes back into it? [Quebec, Canada]

A: [Mark J.] Plug 'torque limiter' into your favorite search engine. There are a wide range of industrial devices that will limit torque to the range you seek. If you want to keep it simple (and cheap) a V-belt/pulley system in place of your proposed chain/sprocket can be set to slip as the torque approaches your 35 ft-lb limit.

Alternately, consider a lifter design similar that used in heavyweight 'Polar Vortex'. Their lifter mechanism presses down against the arena floor and is isolated from impact forces acting on the wedge.

Note: brushless motors have very poor low RPM torque compared to brushed motors Consider a brushed motor for your lifter.

Q: Hey there Mark,

I've been pondering a concept for a while and am wondering if it would even do anything different, or in fact, would create a new world of problems.

Many bots armed with spinning weapons use some sort of gear reduction in order to take a high RPM motor and bring it down a bit in order to increase torque and spin up time. But what if it were reversed? Instead of using a 1000kv motor with a 2:1 reduction, would anything change if we used a higher torque 500kv motor with 1:2 "reduction"?

David R. [Livermore, CA]

A: [Mark J.] Builders generally choose a weapon motor with the lowest RPM for a given motor weight and power so that the smallest gear reduction ratio possible is used. This assures that the reduction is efficient, using light and compact components. Consider:

  • Power is calculated as the product of RPM and torque. Amongst motors of similar size and weight, motors with lower kv factors will tend to have both lower RPM and lower torque, which equals lower total power. You can't make up for reduced power with gearing.
  • A motor with half the kv rating of another motor would need to produce twice the torque to output the same output power. The motive physics of electric motor design effectively say that isn't going to happen. You'd need to go to a larger and likely heavier motor to make that work, and weight is a precious commodity in combat robots.
  • Your math is a bit off. If you reduce motor RPM by 50% you need to decrease the gearing by a factor of [1 / 0.5 = 2] to maintain the same output RPM -- your example 2:1 reduction would become a 1:1 reduction rather than a 1:2.
Builders are already carefully selecting components to maximize the performance of their weapon systems for weight, power, and stored energy. You're not going to find significant improvement by further tinkering with motor kv ratings.
Q: I want to replace the brushed drillmotor in my hobbyweight bar spinner with a brushless motor. I'd like something with comparable RPM so I can use my existing 2:1 belt drive without ruining the 'bite'. In my last event I was beaten by another bar spinner with a much heavier bar and what I think was a Turnigy D2836 brushless motor. Could that motor work for me if I increase the bar mass and keep the original drive? [Rather Not Say]

A: [Mark J.] I suspect your opponent was running something larger than a Turnigy D2836. Motors that size are commonly used for beetleweight weapons and even some antweights. Power is comparable to the drillmotor you currently use, whereas a typical hobbyweight brushless weapon motor is well into the 1000 watt power output range.

The favored weapon motor in any given class changes frequently. I suggest that you check in at the 'Combat Robotics' group on Facebook for advice on the current 'best' motors and ESCs for your weight class.

Your current weapon speed is conservative for a sub-light spinner. If you're worried about bite, you can shorten and counter-weight one end of your blade to make your weapon a 'single-tooth' bar. The shorter end will never hit, giving you twice as much bite or allowing greater RPM. See the Ask Aaron Spinning Weapon FAQ for more on single-tooth weapons and bite.

Q: I have a robot idea which involves a 4 wheel drive and a weapon that is both an axe and a flipper at the same time (ie, it is a flipper which is rear hinged but able go 180 degrees). Which would be the best motor for the weapon, and which would be the best motor for a 4 wheel drive if it is a heavyweight? [Uxbridge, England]

A: [Mark J.] Axe and flipper go together like anchovies and ice cream. An axe head needs to be out at the end of a long handle so you can swing it up to speed, but you wouldn't deliberately design to lift the entire mass of your opponent way out at the end of a long arm. Consider:

  • The power profile of an axe weapon is very different from a flipper. An axe requires a drive that will accelerate the relatively light axe head on the end of a long arm to very high speed over the full span of the weapon throw, while a flipper requires enormous thrust to accelerate the entire mass of your opponent upward over the span of just a few inches. The same mechanism simply isn't going to do both jobs.

  • While an electric motor can drive the axe head on a long arm up to speed in 180 degrees of arc, truly huge torque is needed to lift (let alone flip) a 100 kg mass way out at the end of that same long arm -- enough torque to shatter a reasonably scaled gearbox. You can build an electric axe, but a decent flipper requires the explosive power release available from pneumatics.
As for the drive motors: selection depends on multiple factors about your design, pocketbook, skill level, and expectations that you have not shared with me. See Frequently Asked Questions #21. We gladly provide tools to assist builders in component selection but we won't do your homework for you.

Note: According to BotRank, there has never been a combat robot named 'Anchovy Ice Cream'. Help yourself.

Q: How to calculate spinup time for a weapon running on a brushless motor. I'm building a combat robot with an asymmetric single tooth disc running on two brushless motors. [Maharashtra, India]

A: [Mark J.] -- Why?

Your question has been previously answered here at 'Ask Aaron'. You are free to search for it -- should you find the determination to do so. Look here for a start: brushless motor power curve.

Q: Hey Mark can you comment on the vertical spinner of professor chaos. [Madhya Pradesh, India]

A: [Mark J.] -- Why?

You are welcome to search the archives for answers to your question. There are a multiple posts about 'Professor Chaos' in this archive. Search for 'Professor Chaos'.

Q: how does a spinneris attach on a ring spinner? i saw the ringmaster build photo... but how do it fit up there? [Quebec, Canada]

A: [Mark J.] The spinning ring on 'Ringmaster' is assembled in three parts. The impactor ring simply drops into position where it meshes with the drive gears. Two 'cowls' (dark grey in the photo) are then positioned to rest on the guide wheels above and below the impactor ring. The cowls are bolted to the top and bottom of the impactor ring, sandwiching the impactor in place and positioning it.

Q: But is there a better way to do it? like in a hobby or beetle?

A: Ring spinners aren't effective in any weight class. They have too little mass in the ring to be damaging and the structure is too complex to be reliable. People build them to show off their shop skills -- they are pretty!

The 'better way' is to build a conventional weapon.

Q: After some thought I think I want to make an undercutter type spinner either 12 or 30 pounds. I came up with the attached sketch, nothing is to scale, which is my question.

First does this make sense? I was thinking that making the part as close to the blade as strong as possible was the way to go, and tapered bearings are stronger than regular ones as long as the bolt is tightened right.

Second, how do you figure out the proper size of everything? I have no idea how big of a shaft I should be using, how big of bearings to use, how close I am to the blade cracking because I put too much stuff through it, ect. [Baton Rouge, Louisiana]

A: [Mark J.] Multiple suggestions:

  • Scroll down the page a few posts to find the undercutter layout question with an attached diagram. The two weapon bearings must be well separated to eliminate destructive 'moment loading'.
  • Tapered bearings are not commonly used for spinner weapons. The primary bearing loading when the weapon 'hits' is radial -- the lesser axial loading is accommodated by the freedom of the robot to move along that axis in response to the impact force. Standard ball or roller bearings will do nicely.
  • Find some 'build logs' for robots of your proposed style and weight. Like it says in the FAQ: "Look to see what other builders with similar designs are using and learn from their experience. If it breaks, make it stronger."
  • Grade 8 bolts are very strong in 'tension' -- stress applied along the length of the bolt -- but bolts in general are not designed to withstand lateral 'shear' forces of the type your design places on the weapon shaft. An undercutter in the sub-light weight classes should use a 'tougher' steel shaft that can absorb shock loading without breaking.
  • The hub and pulley must be attached to the shaft in a manner that prevents rotation. Set screws suck. Ideally the pulley, hub, and shaft are all broached for a keyway. Keyed shafts @ Robot Marketplace. Examples of keyways and other attachment options are available in the Ask Aaron archives.
  • The fewer holes you put on your weapon blade the better, and minimal localized stress on any hole that does exist is critical. The preferred blade hub design is a 'clamping' style that holds the blade in place without adding bolt holes thru the blade itself. There are examples of clamping hubs in the Ask Aaron archives.
Don't try to re-invent the wheel. Find examples of successful weapon mounting designs, study them, and base your design on what you have learned.
Q: I have desgined (not yet built) a beetle weight robot with a drum that lights up (with LEDs for entertainment purposes) but what if I spin my drum like so fast that it wont cause damage? I'm sure I'm going to get 'The Burger Is Bad' or 'Check the Archives'. [TCRM, Malaysia]

A: [Mark J.] The hamburger is good and you don't need to check the archives. Read thru the Ask Aaron Spinner FAQ. The 'Rotational Speed' section of that FAQ discusses the factors involved in getting the weapon to 'bite' into your opponent rather than just 'skitter' across them without doing any damage.

Read the whole Spinner FAQ while you're there -- it'll save you a lot of time and trouble.

Q: do you have a drawing on how an undercutter blade is fixed to a bot? i know u did for a fbs and a horizontal spinner. [Quebec, Canada]

A: [Mark J.] The mounting is pretty much like an FBS turned upside-down. The weapon shaft is rounded on the ends and usually sticks out both below the blade and above the chassis to allow the 'bot to invert.

Date marker: January 2017
Q: Why does RioBotz choose to have 13 radii and 18 sections [in the design of their integrated single-tooth 'snail drum' weapon]? [Dublin, Ohio]

A: [Mark J.] The selection of 18 initial sections was arbitrary -- it came from dividing the 360 degree polar coordinate plot into twenty-degree sections. That seemed to be a reasonable number of facets to machine into the drum in the final design. Two of the sections (40 degrees) were combined for the impact tooth at full radius and one section is the curved 'tooth notch' designed to reduce stress concentration that has no single radius. Then:

"After observing the nearly flat shape of the optimal solution in the regions between 220o and 320o... the algorithm [was] re-evaluated considering only 2 flat sides in such region. This new optimal solution is very similar to the previous one, but it is easier to machine due to the reduced number of facets." -- Drum Shape Design and Optimization Using Genetic Algorithms

The creation of just two facets ('k' and 'l') in the region from 220o and 320o reduced the number of radii to thirteen.

Q: Hi Aaron, could you tell me, for a spinning drum that stores 10,000J [30 Lbs class] what is the difference between a 10mm bite and a 20mm bite? Thanks. [Valle del Cauca, Colombia]

A: [Mark J.] Bite (what's bite?) is calculated as a maximum depth of opponent insertion into the arc of a spinning weapon at a given weapon RPM and forward velocity. You'll get that maximum bite rarely, just like 13 black only comes around rarely on a roulette wheel. Sometimes your luck will be very poor and you'll hit your opponent just as an impactor is facing them and get no bite at all! On average, you'll get half the max bite -- less as your attack speed drops.

  • Consider an opponent who was wise enough to avoid exposed sharp edges in their robot design. Given a nice sharp angle to attack you don't need a lot of bite, but if you're forced to attack a flat or gently curved surface you need all the bite you can get.
  • Greater bite also is a bonus when your attack velocity drops. A weapon with a lot of bite can still be effective in close quarters when you don't have a chance to back away and take a high-speed run. Watch some combat videos and pay attention to the speed at which most weapon hits are made.
What's the difference between 10mm and 20mm bite? A few more match wins. Bite is good -- more bite is better.

I've been writing quite a bit of JavaScript lately, so what's a little more? Take a look at the new 'Bite Calculator' in the Spinner Weapon FAQ.

Q: Why doesn't 'Witch Doctor' have gyroscopic forces acting on it? One side doesn't lift. [Reston, Virginia]

A: [Mark J.] See this post on gyroscopic forces a little farther down in this archive.

Q: Hey Mark,

How does the flipper on Lock jaw in Battlebots Season 2 harness the power of the springs? From what I could tell it was winched back but how was it able to fire then reset again? Additionally, could this method of flipping be utilized in all other weight classes as an alternative to pneumatics? Thanks in advance! [Straight Outta Facebook]

A: [Mark J.] Donald H. doesn't divulge much about his robot designs, and I can't see enough detail in the photos of 'Lock-Jaw' to understand the clutch mechanism. Fortunately there are builders who do share their spring-powered flipper designs:

  • Dale Hetherington has built pretty much every exotic robot design there is. Take a look at his very detailed build log for 'Dead Air'.
  • I'm personally very fond of the snail-cam spring loader for it's simplicity and adaptability to small weight classes.
That will get you a start.
Q: Hello Mark,

I've heard it said that Wrecks' vertical disk (30-35 lbs) has a much higher Moment of Inertia than Electric Boogaloo's vertical spinning weapon (~70 lbs). With the assumption that Wrecks is using a similar motor (big assumption), how is this possible?

-David R. [Livermore, CA]

A: [Mark J.] A little clarification:

The Moment of Inertia (MOI) is NOT a direct measure of how much energy a spinning weapon stores; the motors or speed of rotation have no bearing on the moment. MOI is a measure of the energy needed to change the rate at which the weapon is spinning. Its value depends on the mass of the weapon and (most importantly) on how that mass is distributed relative to the rotational axis.


  • A small chunk of matter is rotating around an axis at a given RPM at a distance of six inches. With each rotation the chunk of matter travels a distance of ( 2 * 6" * pi = ) approximately 37.7".
  • That same small chunk of matter is now rotating around an axis at the same RPM at a distance of 12 inches. With each rotation the chunk of matter travels a distance of ( 2 * 12" * pi = ) approximately 75.4".
  • The chunk of matter 12" from the axis must be travelling twice as fast as it did when only 6" from the axis in order to complete a revolution in the same length of time.
  • The kinetic energy of a moving object increases with the square of its velocity (K = ½mv2), the mass 12" away from the axis has four times as much kinetic energy as an equal mass 6" from the axis when rotating at the same RPM.
  • Doubling the distance of the mass from the rotational axis doubled the speed of the mass, which raised the energy stored by the spinner at any given speed by a factor of four (velocity2). This also increases the MOI of the spinner by a factor of four, even though the mass of the spinner has not increased.
Now, let's take a look at the spinner weapons on 'Electric Boogaloo' and 'Wrecks': 'Electric Boogaloo' has a bar spinner weapon - closer to 50 pounds than 70. A bar has a lot of its mass close to the rotation axis and relatively little mass far from the axis. The formula for the MOI of a rectangular bar spinner is:

MOI Bar = (mass/12) * (length2 + width2)

'Wrecks' has a large diameter spinner with most of the mass concentrated in a ring at the outer edge -- far away from the axis of rotation. The formula for a the MOI of a thick ring (discounting the supporting spokes) is:

MOI Ring = (mass/2) * (inner radius2 + outer radius2)

Let's use the new Run Amok JavaScript Spinner Weapon Calculator to compare the MOI of the two designs based on rough estimates of their sizes:

  • A steel bar similar to the spinner weapon used by 'Electric Boogaloo': 455 mm long by 160 mm wide by 40 mm thick weighs 22.7 kg and has an MOI of 0.440 kg·m2.
  • A steel ring similar to the spinner weapon used by 'Wrecks': 300 mm outer radius, 200 mm inner radius, and 12 mm thick weighs 14.7 kg and has an MOI of 0.956 kg·m2.
The ring weapon is less than 2/3 the mass of the bar weapon, yet has more than twice the Mass Moment of Inertia. That's a much more efficient use of weapon mass for energy storage.

Q: How do electric hammers not burn out [...just down the road from Ashburn, Virginia]

A: [Mark J.] If you aren't careful they do burn out.

The motor for an electrically powered hammer weapon needs to be powered off at either end of the weapon swing to avoid an extended 'stall' condition where the motor would consume damaging current levels. This can be done a couple of ways:

  • Most builders control the hammer with one of the spring-centered joysticks on the transmitter and release the stick at either end of the weapon swing. Just don't forget to release the stick!
  • Some R/C relay boards and a few ESCs (like the Talon SRX) have provisions for 'limit switches' to sense when the controlled device has reached the end of its travel and automatically shut off power in that direction. This allows an electric hammer to be actuated by a simple single-throw switch on the transmitter -- typically channel 5.

Q: What is the best motor to use for a horizontal spinner [Los Angeles, California]

A: [Mark J.] The Hamburger is Bad.

See Ask Aaron Spinner Weapon FAQ.

Q: Hello! I am a high school student that I building a new robot. I am building a drum spinner, and that spinner will be operated by a Brushless motor with the specs of:
  • Turns: 10T
  • Voltage: 12S Lipoly
  • RPM/V: 560kv
  • Motor Poles: 10
  • Internal resistance: 0.017 Ohm
  • Max Loading: 100A
  • Max Power: 4200W
I currently do not have the dimensions of weapon itself, but I am interested in knowing the "equation" in finding the speed of the weapon at full speed. I have the Wh of the battery, Volts, and most of the other specs. Also, where can I find the equations to calculate torque, and stall torque. [Dublin, Ohio?]

A: [Mark J.] What's wrong with this group of questions?

Given the syntax, grammar, and language structure, I have trouble believing that the author is from Ohio. Further, the motor and weapon design are unusual for a combat robot that might be constructed by an American high school student in the mid-west.

If the author is a high school student in Ohio and they are building a combat robot this size, they should have a local mentor to guide them in design, construction, and safety. The mentor should be providing the answers to questions like these.

Either way, I'm not comfortable answering your questions. The best I'm willing to do is to point you to this Wikipedia article on 'Motor Constants' and warn you that stall torque on brushless motors is much less than the calculated value due to the software in the motor controller limiting current at low motor speed.

Q: How to make 2 [pneumatic] cylinders work in sync? Use 2 buff tank and 2 valve or 1 buff tank with 1 valve [to] supply 2 cylinders? Thank you (just like Subzero) [Yunnan, China]

A: [Mark J.] I see that you've just asked the same question of Subzero's builders on their Facebook page. The better question might be, "Why use two pneumatic cylinders instead of just one of larger diameter?"

If I had to use two cylinders I'd want both to fill from a single gas supply thru a single valve to avoid any pressure imbalance. Why don't we wait a few hours and see what the builders say?

Subzero's builders did reply a few hours later:

Team Hammertime / Teamxd: That version was one supply tank with 2 rams, one valve and no buffer tank.

Q: What's the best & safest way for someone who's only done non-weaponed bots to do their first weaponed one? [New Jersey]

A: [Mark J.] I'm not sure how to respond to 'best' but I can offer some safety guidance. You didn't mention how large a robot you are interested in building or the type of active weapon you are considering, so I'll have to keep this general.

  1. Review your basic workshop safety practices:
    • Wear eye protection when there is danger of flying chips, abrasive dust, or irritants.
    • Remove rings and other jewelry before operating machinery.
    • Keep your workspace and the floor around it clean and un-cluttered.
    • Securely anchor materials being drilled, ground, or machined.
    • Cover and secure sharp edges and points in the work area when not in use. Use gloves where appropriate.
    • Do not wear loose clothing around power tools -- a work apron may be appropriate.
    • Monitor your lithium battery charging, and use a suitable charging container if needed.
    • If it's gonna be loud, wear hearing protection.
  2. Keep safety restraints on weapons that prevent them from operating unexpectedly until you are ready to test/use the weapon.
  3. Do not test or operate your weapon without containment suited to your weapon and robot.
  4. Be cautious of mechanism pinch-points: articulated levers, chains, hinged surfaces.
  5. Keep your design and expectations at a level appropriate for your building skills and resources.
  6. If you aren't sure you understand the safe operation of specialized equipment, fittings, or assemblies -- ask!

Q: Dear Aaron, which horizantal spinner has more effect on the other bot? And to you? Thx, [Google Fiber ISP]

A: [Mark J.] I'm not sure I understand your question. The base physics of a horizontal spinner are the same if it's a bar or disk -- or a top/mid/undercutter. The effectiveness depends on other factors such as energy storage, 'bite', and chassis stability. Suggest you read the Ask Aaron Spinner Weapon FAQ and then send in a more focused question.

Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator

Q: Hi again, I've stumbled upon a problem, the snail cam spring reloader [needs to stop] after one full rotation, I cannot seem to find a suitable solution to do so with a gear motor. Do you think a stepper motor is better for this or is there a way to make motor start and stop after one rotation and a push of one button if you will. [Bristol, UK]

A: [Mark J.] When you turn off the windshield wipers on an automobile have you noticed how they continue for the rest of the wipe stroke and then stop in the park position? That's what you're looking for and here's how you do it:

  • The circuit shown at right allows the gearmotor to run until the flipper arm (not shown) reaches the fully loaded position and presses down to open the 'interrupter' micro switch -- stopping the gearmotor.
  • A momentary closure of the normally open R/C switch re-starts the gearmotor long enough to fire the flipper, which closes the interrupter switch.
  • The gearmotor then continues to run thru the rest of the reload cycle to an automatic stop.
The positioning of the micro switch is simplified if you use a lever-style switch that can be bent to fine-tune the switch point. The micro and R/C switches must have enough current capacity to handle the gearmotor, and the micro switch must be wired 'normally closed' (NC). For larger robots the micro switch can trigger a relay with the capacity to handle the motor load.

Q: I was actually thinking to have snail cam consist of 2 shapes, 1) the main cam and 2) a smaller cam with sudden increase in radius where reloading needs to stop that way there is no need for finest tuning. [Bristol, UK]

A: You have lots of options on the interrupter. A micro switch is simple, but you can certainly use other sensor types: infrared emitter/detector, inductive proximity, magnetic... whatever you're comfortable with.

Q: I was wondering if you have heard of or made any progress on getting the T.i. 4 Bar Simulator ported to a newer version of Windows. Thank you for any information and providing a valuable service. [Kansas]

A: [Mark J.] The author of the T.i Four Bar Simulator wrote the code back in 2007 when he was still in school. He tells me that he's not sure where the source code is, but that it may or may not be on an old desktop which he believes is stored in his parent's attic. The best he could offer was to take a look in the attic the next time he went home for a visit. In my mind I picture a dust-covered computer leaning up against the holy grail and partially covered by a lost da Vinci manuscript.

I keep an old Windows XP desktop next to my 'Super Nintendo' console in a dark corner of my basement just so I can run 'Four Bar' and play 'Donkey Kong Country'. A similar set-up may be your best option as well.

Q: Hi there,
I've got an idea for a spinner robot where the rotating ring spins inside a circular chassis, and once it reaches a sufficiently high speed, two 'teeth' on the ring are extended (to protrude beyond the chassis) by centrifugal force. On impact, the ring is slowed, meaning the teeth retract again (using a spring), leaving the ring to spin up again without external resistance. This would mean the ring could spin up and inflict damage on an opponent, even if the opponent was continuously in contact, since the teeth would be shielded by the chassis until the ring was at full speed.

I haven't been able to find any examples of a robot which uses this system, so my question is, are there any, and if not, why not? I feel there must be some fundamental reason why robots don't use such a technique - do you have any suggestions?

Many thanks, M [Bournville, England]

A: [Mark J.] I know of no examples of such a weapon system in use, and I think I know why there are none.

Spinner weapon teeth take huge abuse. The entire force of the weapon is transferred thru them to the opponent. Typically they are made of hardened tool steel and set into well braced recesses in the weapon body where they are secured by the best quality bolts obtainable. Still, an impactor tooth's life is short. A good impact can and will shear them away.

Your proposed design places the impactors on pivots which would themselves bear great impact forces, as would the stops required to restrict the tooth's outward motion. Moving parts subject to high loading are bad. Adding more parts subjected to high loading is worse. Simple is good.

Consider the situation just after your weapon impacts your opponent. If your opponent is still there to restrict your weapon from spinning back up, then your weapon isn't doing its job. Your opponent should be flying away from you at great speed, unable to prevent your weapon from spinning back up.

I don't believe that the benefits of a retracting-tooth weapon would offset the added complexity and fragility. The mass of the circular chassis shield would be better put to use in the mass of the spinner and the weapon motor. Keep it simple; simple robots win.

I just got more curious than normal and ran a calculation on the force needed for a spring to hold an impactor in against the outward centrifugal acceleration on a typical weapon. At 3000 RPM an object 250 mm from the axis of rotation sees 2500 gravities!

Q: Hey, M's weapon sounds like 'Greenspan' that used a flywheel with free spinning hammers. [Dublin, Ohio]

A: [Mark J.] I don't think that's what 'M' has in mind. There were a lot of 'flail' and 'pivot hammer' weapons in the early days, but those designs have been replaced by fixed impactors that are better at transmitting the full energy stored in a flywheel as a single big hit on the opponent.

The 'M' weapon shields the flywheel from direct contact with the opponent which allows it to spin-up even if the 'bot is in contact with the opponent or another obstruction. The impactors are held inside the protective bumper until they approach full speed, and then they either slide or pivot outward beyond the bumper in a fashion that locks them against lateral movement. See my sketch of a (poor) pivoting impactor design at right.

These types of retractable impact teeth would not swing out of the way on impact in the way Greenspan's hammer did -- they would deliver an unyielding blow. That's good, but my objection is that the sliding or pivoting mechanism would be a weak point subject to failure. Complex is bad -- simple is good.

Comment: To back your statement on M's weapon, I remember there was a Beyblade battling top kit called "Wing Attacker" which had that very setup. It... wasn't very good. [Arden, North Carolina]

Reply: [Mark J.] I'm not sure that Beyblade performance transfers to combat robots, but someone might appreciate that data point.
Q: After doing the calculation for my featherweight horizontal spinner, i got 1484 joules of energy. Is it enough? [Quebec, Canada]

A: [Mark J.] "How much energy should my spinning weapon store?" is the first question in the Ask Aaron Spinner Weapon FAQ. You can read your answer there. I suggest that you read the rest of the FAQ as well.

Q: I saw in my beta question you don't like electric hammers, what about pneumatic? in my first look at the math, it seems like pneumatic hammers are pretty tame too. Unless I did the work math wrong, you'd need a gargantuan cylinder like chomps to even break 1 KiloJoule which even some nastier 3 pound spinners beat. [Dublin, Ohio]

A: [Mark J.] Since you didn't include your calculations I can't check them, but a pneumatic system can provide much greater force than an electric system of comparable weight.

  • Are there any electric flipper robots out there? There are lifters, but there are no flippers -- an electric motor/actuator/solenoid cannot provide anywhere near the explosive release of power available from a comparable pneumatic system.
  • Also consider the great complexity, expense, and effort expended by Team Hurtz to construct their electric hammer. It is a beautiful piece of work, but a comparable pneumatic weapon could be built from off-the-shelf components at a fraction of the cost.
I'm not a fan of hammers in general, but if you're going to build one it makes sense to use pneumatics. Assuming perfect gas flow, a 3" diameter pneumatic actuator at 250 psi provides 3.14 * (1.5^2) * 250 = 1,766 pounds of accelerating force. Try to match that with an electric motor. The trick is in getting that 'perfect' gas flow...
Q: I have a third grader trying to answer a science fair question about the effectiveness of vertical vs horizontal spinners. He's built a bot out of a Thames and Kosmos building kit but he can't get enough power out of the motor to get spinners to do any damage to a piece of styrofoam. Spinner just stalls when it hits the foam. That's our problem to deal with, but:
  1. Are there equations we can run to determine the answer to this question?

  2. Does the angular velocity of a spinner change if it's in the vertical position, if all other things are equal?

  3. Does gravity assist or impede?

My engineer-son is asking questions his English-major mother can't answer. [Raleigh, North Carolina]

A: [Mark J.] You want to run some angular momentum equations for your third grader's science fair project?!?! My third grade teacher was still trying to get us to stop counting on our fingers. I guess things have changed.

It was my third-grade son who led his biology-major father into this mayhem. I'll be pleased to assist as best I can.

  1. Read thru the Ask Aaron Spinner Weapon FAQ. The entire FAQ will prove instructive, but you may find particular interest in the large, friendly blue text box near the top of that page describes the principle of spinning flywheel weaponry:

    General Principle

    Spinning weapons are flywheels. They rely on rotational inertia to collect energy from a continuous power source (electric motor, internal combustion engine...) over time and store it as rotational kinetic energy. On impact, the flywheel releases the stored energy in a blow that far exceeds the energy directly available from the continuous source.

    From your description of your son's spinner stalling, it is apparent that it does not have adequate rotational inertia to store sufficient kinetic energy from the small motor powering it. You could use a more powerful motor, but as a display of physics it would be much more interesting to increase the rotational mass of the weapon and note the change in the performance of the spinner.

    Here are your equations: How to Calculate Rotational Kinetic Energy, and I think this explanation of Kinetic Energy and Mass Moment of Inertia in Combat Robot Weapons might fill in some of the gaps.

  2. If you look thru the equations referenced above you'll discover that nowhere in the calculation of momentum or velocity is there a mention of horizontal vs. vertical orientation; the energy of the spinner system is not changed by its orientation.

    However, going from a horizontal to a vertical orientation does effect the performance of the weapon in another way. When a spinner weapon impacts the opponent there is both an action on your opponent and a reaction on your 'bot.

    • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. Since your 'bot is supported by the arena surface, it does not move and most of the impact energy is transferred to your opponent.
    • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of the impact is split between moving the two 'bots in opposite directions. The desired transfer of damaging impact energy to your opponent is much less efficient.

  3. A balanced spinning mass is neither assisted nor impeded by gravity. In a vertical orientation the effect of gravity on the rising side of the mass is perfectly offset by the gravitational effect on the descending side. In a horizontal orientation nothing is rising or descending.
I threw a whole lot of information at you, but I think you can pick thru it to find answers that make some sense to you. New questions will arise -- write back as needed.
Q: I am in 150g weight competition, I was wondering if its possible to make a spring loaded spike/ram with ability to reload it, do you have any resources I could have a look at? [Bristol, England]

A: [Mark J.] Take a look at this archived post describing a spring-powered flipper reset by a rotating snail cam. With a little imagination it could be oriented to reload a spike, although a flipper is a more effective weapon.

Comment: Thank you, i was in fact going to do a flipper inspired but that very video, there is a lot of useful info in that other answer!

Q: "Melty Brain" robots do not count as having an active weapon under current Battle Bot rules. Have you ever seen a melty brain style robot that was paired with another weapon? I imagine that the robot's rotation could add even more energy to a hit from a spinner. [Westerville, Ohio]

A: [Mark J.] No, and it won't.

The concept of a 'melty brain' spinner is that the entire mass of the robot becomes a spinning weapon. Stealing mass and energy from this very efficient primary weapon and trying to add it back with a secondary weapon will do no better than break even on energy, and will add undesired complexity. Use all your weapon weight allowance on a single weapon. Simple robots win.

Q: Has there ever been a counter rotating vertical spinner in a robot? I'm thinking of two large disks parallel and close to each other on the same dead shaft. Only one would have teeth and they would have one or more small perpendicular drive wheels between them causing the counter spin. The reasoning behind building this would be to lessen gyroscopic effects. Thanks! [Minnesota]

A: [Mark J.] I know of two big 'bots that were designed to nullify the annoying gyroscopic forces associated with vertical spinners. Both used mechanically simpler solutions than your proposed co-axial counter-rotating disks:

  • Team Boilerbots built 'Counter Revolution' to compete at BattleBots. The twin counter-rotating vertical disks are not co-axial, but the counter rotation largely cancels the net gyro effects when the robot turns.
  • Richard Chandler campaigned superheavyweight 'Strike Terror' at BattleBots 4.0 and 5.0 with a vertical spinning weapon that was free to pivot in the longitudinal axis. This allowed the weapon to twist near-horizontal when turning without effecting the chassis, and then re-establish a vertical spin when turning ceased.
Neither 'bot was particularly successful. I'd recommend against adding the mechanical complexity your design requires. Simple 'bots win.

'Wedgemaster Wedge' writes in to remind us of 'CounterStryker' -- a 6-pound 'mantisweight' with counter rotating vertical disks built for Bot Bash:

Comment: Zac O built and documented this bot which is pretty close to what that dude wanted.

Thanks, Wedge.

Comment: Thanks Mark and Wedge that is exactly what I was thinking about. I found a video of CounterStryker fighting and it seemed to handle the turns well.

A: 'CounterStryker' has a good record: 2nd at Bot Bash '15 and 3rd at Bot Bash '16. I'm not a fan of friction drive for weapons -- Zac took care with the design and it works well in this insect class 'bot, but I wouldn't try it in a larger 'bot.

Q: How do horizontal spinners keep their weapons off of the frame? I know for example the most iconic horizontal spinner Last Rights/Tombstone has an adjustable height blade, meaning it isn't riding on the bottom frame. Is the friction of the bearing on the shaft enough to keep the blade from shifting during big collisions or is there something more to it that I am missing because that doesn't seem adequate? [Cleveland, Ohio]

A: [Mark J.] Typically the weapon hub and pulley/sprocket fill the entire space between the frame members. They ride against the inner bearing races or against 'thrust bearings' that take displacement loading during a 'hit'. The diagram shows a 'live shaft' arrangement -- in a 'dead shaft' design where the shaft does not rotate the bearings are incorporated into the weapon/pulley hub and the spacer is part of the hub assembly.

If the design allows additional space between the weapon bearings for blade height adjustment, tubular spacers (orange in the diagram at left) slide over the weapon shaft above and/or below the weapon hub to raise or lower its position.

Q: How does a horizontal full body spinner like 'Barber-ous' work? Is it a shell spinner like 'Ringmaster' on its side? [a server in California]

A: [Mark J.] First, a little terminology clarification:

  • Spiners are classified by the direction their impact, not by axis orientation. 'Barber-ous' is a vertical spinner and 'Ringmaster' is a horizontal spinner.
  • A 'shell spinner' has the entire exterior of the robot spinning. 'Ringmaster' is not a shell spinner -- it is an example of the rare and complex 'ring spinner' where only the outer edge of the body spins, leaving the wheels exposed at top and bottom to allow inverted operation.
Team Rotractor's original Barber-ous webpage (archived) has the worst build report I've ever seen, but I'm still grateful that it exists. The chassis photo at right came from another source. The shell and electricals have been removed and you can see that the layout is not nearly as complex as Ringmaster's:
  1. The weapon motor is mounted on the central chassis that also carries the batteries and electronics. The entire central chassis is concealed by the weapon shell when assembled.
  2. Non-rotating stub axles come off each end of the chassis. This version of 'Barber-ous' has worm-drive gearmotors bolted to the ends of the stub axles to power the drive wheels. The gearmotors are obscured in the photo by the wheels and hubs.
  3. Laying on the floor is one of the two large weapon hubs. With the gearmotor removed, the free-spinning hub slides onto the stub axle and a drive chain connects the hub sprocket to the weapon motor. The weapon hub on the far side is already in place. With the weapon hubs in place, the weapon cylinder slides on over the hubs and chassis and is bolted to the hubs. The drive-wheel assembly can then be re-mounted.
'Barber-ous' went thru many revisions and updates to the chassis and drive motors, but the weapon drive principle remained the same.

Q: What are the best motors currently for horizontal spinner type robots? Our weight limit is 85 pounds, and we're looking for something with relatively high torque and low spin-up time for the weapon bar. Also not sure whether to use brushed or brushless. [a server in Illinois...]

A: [Mark J.] The hamburger is bad. I cannot match a weapon motor to a weapon based only on the robot's weight class and weapon type. See the Ask Aaron Spinner Weapon FAQ to find out why and to learn what information is needed to calculate weapon performance.

If you don't know whether to use brushed or brushless, use brushed. Brushless adds a level of complexity and many pitfalls for builders unfamiliar with their quirks -- particularly in a heavier weight class such as this. Brushed is simple. Simple is good.

Now, it's possible that you just want me to take a blind guess and recommend an affordable and reliable old school motor that will make your entirely undescribed heavier-than-lightweight bar spin-up and look like a combat robot. If that's the case, Robot Marketplace has found a few of the classic EV Warrior motors. Run one at 24 volts thru a 2:1 pulley reduction to your bar weapon and you're probably in the ballpark.

What event runs an 85 pound weight limit? That's not a standard US weight class.

Q: Why can't 'Warrior Clan' launch bots into the ceiling like its previous form 'Warrior SKF' could? [Massachusetts]

A: [Mark J.] There are no significant changes to the 'bot, so the capacity is still there. I can only speculate that they have not had opponents that are particularly vulnerable to their flipper weapon. The SKF weapon is powerful but it does not have a great deal of vertical motion; their ideal opponent would have a low structural edge close to its center of mass.

Q: Sorry sir for posting a question but can you please clear my doubt " heavier the drum,Oppenent will find difficult to Topel you" is it true???? [Maharashtra, India]

A: [Mark J.] Stop asking about weapons and start building Indian arenas to contain the weapons you have.

Q: Hi There

First off, a huge thanks for keeping this site going, it's a hugely valuable source of information and by far one of the most comprehensive sites on combat robotics on the web. I'm currently designing a heavyweight (110Kg) robot after an extended break - I last built autonomous antweight/sumo bots in 2005.

My question is about spinners - namely, getting a large bar - al la Tombstone/Last Rites - up to speed in a respectable fashion. The current Robot Wars arena is 22m x 22m, and allows for around 2.5s of spin up time (on average) before first impact occurs.

Bar specs:
1300mm x 125mm x 30mm
Weight is roughly 38Kg

Motor Spec:
8500RPM, 42Nm Peak Torque (stall torque isn't stated, nor is the internal resistance, so I've used the peak torque figure in the Run Amok spinner calculator)

Results using a 4:1 gear ratio:
1339 RPM in 7.18 seconds, storing 53397 Joules.

Now, obviously this isn't [quick] enough - is this a case of me just not working out the stall torque correctly, or a case of choose a different motor? (If it is, which ones would you reccomend?)

Thanks [Fulwood, England]

A: [Mark J.] Welcome back to combat robotics, and thank you for the kind comment.

The Team Run Amok Spinner Spreadsheet - like any modeling software - is only as good as the data that goes into it. A motor with uncertain specs or a brushless motor with a non-linear torque curve results in questionable output, but in this case I don't think the motor specs are the problem.

If we conservatively assume that 'peak torque' is the same as 'stall torque' as you have done, the power numbers for the motor are still very impressive. Let's run a comparison by replacing your un-named motor in the spreadsheet with the weapon motor 'Tombstone' currently uses -- the mighty 'E-Tek-R' at 56 volts. We'll set the reduction to 2:1 to get comparable weapon RPM from the slow-spinning E-Tek:

E-Tek-R with 2:1 ratio: 1360 RPM in 6.04 seconds, storing 55,120 joules

Not a whole lot different from your results, so motor power isn't the problem. The real issue is that you're spinning up a big, heavy chunk of steel with a huge moment of inertia and the motor needs more help on low end torque to get a quicker spin-up. Let's try modeling your weapon with increased reduction ratios at meaningful time intervals:

Reductionjoules @ 2.5 secjoules @ 5.0 sec

Energy storage in the kilo-teens range is plenty to warmly welcome hard-charging opponents, and potential energy storage that takes more than 6 or 8 seconds to obtain is effectively useless for anything but 'showboat' hits on an already-beaten opponent. Dial in some additional reduction. It will put less stress on your motor and battery, and will give you a better balance of spin-up time to useable peak energy storage.

Q: Hello Mark! It's said that the flipper of Warrior Clan (Warrior SKF) is powered by its spinning ring, that's amazing! Can you show me how it works? [Guangdong, China]

A: [Mark J.] Team Whyachi's 'Warrior SKF' has been around for several years and we've had quite a few questions about its design and function. Search this archive for "Warrior SKF" and "dog clutch" to find those posts. You'll also discover what 'SKF' stands for.

The best explanation of how a flywheel flipper works is the well-illustrated build report for Dale Hetherington's 'Flip-O-Matic'. If flywheel flippers were a good idea you'd see lots of them; you don't see lots of them.

Q: I've been thinking about this one weapon design that I haven't really seen anywhere. Normally, pneumatic "poking" weapons tend to not be very good in terms of effectiveness, but I was thinking about taking this weapon design to a logical extreme by making the entire front the robot a heavy pneumatic battering ram/plow.

The idea would be to push the other robot at full speed towards an arena wall, much like the typical strategy with a simple wedge or rambox, but then use the pneumatic ram to shove the other robot into the wall with even greater force than simply slamming it into the wall under conventional drive power.

Coupled with a powerful drivetrain and a sturdy supporting structure, this could end up doing some damage (though not necessarily as much as a spinner) while also being a potential counter to spinners, essentially putting the paper back in rock paper scissors without violating the active weapon requirement. Your thoughts? [That one guy from Asheville, NC, who occasionally also posts from Chicago, IL, and has a battlekit drum spinner and a Fingertech Viper with a ridiculous and excessively long name]

A: [Mark J.] Newton's third law is working against this weapon:

For every action, there is an equal and opposite reaction.

You're pushing an opponent of equal mass across the arena. When you fire the weapon it will shove your opponent forward and shove you backward with equal force. The energy of the system of the two 'bots has a net gain of... (wait for it)... zero. The impact of the two-robot system impacting the wall is unchanged, and no additional damage is done to your opponent. Not much of a weapon. Better to spend the weight used on the weapon on extra drive power.

It is an active weapon, but it is not a 'damaging' weapon. It adds complexity and weight with no improvement in performance. I suspect that's why you've never seen such a weapon.

Q: I am looking for slipping clutch on a horizontal spinner. The speed is going to be about 8,000 RPM to 12,000 RPM and the torque is from 1.8 Nm to 4 Nm and I will be happy if I can use the clutch on a dead shaft kind of system. I am using RS 40 chain to spin up the spinner disk and it is for Lightweight Robot. So where can I buy the clutch suitable for this job and usually in combat robots what brand(s) are used and who supply them? [Kuala Lumpur, Malaysia]

A: [Mark J.] Torque limiting clutches like you describe are not off-the-shelf items. Commercially available clutches are bulky, heavy, expensive, and poorly matched to the speed and shock-loading found in robot combat applications. A very few builders construct their own slip clutches thru a trial and error approach -- a great deal of error.

The standard method to limit torque in robot spinner weapons is to use a v-belt drive and set the belt tension to slip at your required loading. Don't make it complicated if you don't have to.

Q: I have now 2 questions that I would like to ask:

1) Is there any tutorial on making slipping clutches, any guide or someone who can help me on this issue that you know of?

2) I checked couple of places for V belt design. The Rio tutorial is not very detailed on the V part section. So is there any reference for designing V belts systems and describing the types of the belts and things like that. I am not looking for a straight solution, I prefer to dig and read to understand what I am doing.

Also if I understood correctly, the torque limitation using the belt is going to cause a force on the shaft of the motor. Since I am planing to use an R/C Brushless (On this part I know what I am doing so please do not wipe it out of the question :) ) without a gearbox, doesn't the extra force slow down the motor significantly and reduce the lifespan of the motor? (I think it does and pretty sure about it, but I want to know your opinion too and how significant you think the difference is)

Thank you for helping many people including me.

A: Slip clutch design is a rare engineering specialty. I can point you to a NASA Tech Brief on Slip Clutches for an overview, but so few people are involved in the actual design of such equipment that there is no tutorial. As I said above, trial and error would be your instructor.

In contrast, there is a great deal of V-belt design help out on the web. Here's a good place to start: Machine Design: V-belt selection.

Chains, gears, and belt drive systems all create a side loading on the drive motor shaft. The side load places stress on the motor bearing but does not directly place a drag on motor output -- a lateral load is not 'work' and does not subtract from output power. A properly tensioned V-belt is quite efficient at energy transfer; it is better than 95% efficient in many cases. Given that the expected lifespan of a combat robot is measured in minutes, I believe there are more important design issues you should be tackling.

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

R/C brushless hobby motors have become the standard for combat robot spinner weapon power. While I consider their use in drivetrain systems to still be experimental, I certainly would not question your selection of a brushless motor for your weapon.

Q: Can you tell me which motor is best for cylindrical fly wheels like the Minotaur bot who compete in BattelBots Against Blacksmith [Azad Kashmir, Pakistan]

A: [Mark J.] I have several problems answering your question:

  • I don't think they build 250 pound robots in Pakistan, and you didn't tell me how heavy a 'bot you are building;
  • I have no idea what motors are available to you in Pakistan; and
  • I am reluctant to assist in weapon design for competitors in the sub-continent due to poor arena safety in the region.
I suggest you read the Ask Aaron Spinner Weapon FAQ for weapon design help and evaluate the motors available to you according to the guidelines given there.
Q: Hi, I have a hammer robot and I choose to directly attach the ram 3 inches below the fulcrum. The pivot point is also just a pin though an aluminum bar. The ram has a 6 inch throw so the hammer has a roughly 100 to 120 degree angle cocked. My question is, would it be worth it to add gearing to get a 180 degree swing and bearings for better efficiency or keep the rugged, less component design. Thank you for you help - Team Humphrey [West Virginia]

A: [Mark J.] The amount of 'work energy' available from your actuator is not increased by adding gearing to expand the range of motion, but a rack and pinion can increase efficiency in converting the linear motion of the actuator into the rotary motion of the hammer. However, careful attention to the hammer linkage geometry can keep the thrust vector favorable without the added weight and complexity of a gear system -- see example at right. I recommend keeping it simple.

Your pivot is a weak point in the structure of your weapon arm. You didn't share any details of your arm design, but you should be very cautious about enlarging the hole in the arm at this highly stressed point. Without knowing more about the design I can't make a recommendation on the benefits. If you've got a big, meaty chunk of aluminum around that pivot I'd recommend boring the hole just enough to press in an oilite bushing to avoid steel-on-aluminum purely for reliability. If well lubricated your simple pivot has minimal frictional loss, but the bushing will prevent spalling and wear that can lead to failure.

Q: Hi, again thank you for your suggestions on my hammer bot. For the new Battlebot show design I am trying to upgrade my old 120 pound bot design to the new 250 pound. So my question is 250 psi components instead of my old 150 psi components. I have found a 250 psi 3.25 inch bore 6 inch stroke cylinder. But a 24-48v 5-port solenoid and quick exhaust valves rated at 250 psi are very hard to find in McMaster Carr and Grainger. Do you have any suggestions? Thanks!

A: The old standard 5-port valves no longer deliver the performance expected from high-performance pneumatic weapons. The current standard uses individual high-flow solenoid assisted valves to pressurize and vent your actuator. The R/C controls are more complicated than the simple bang-bang switch control for the 5-port as you have to control each individual valve in the correct order, but it's worth the effort.

The preferred valve is the Burkert Type 5404. They're expensive, but if you're gonna play with the big boys you're gonna need big valves, big actuator ports, and a high-flow regulator.

Q: Hi, I used to compete in battlebot season 5, battle at the beach, RFL nationals etc. I had a middleweight robot Major Punishment it was a 150 psi pneumatic hammer sort of. I actually used 2 quick exhaust valves one was set up normal for exhausting co2 out but the other was arranged to exhaust my buffer tank into the firing side making a cheaper(smaller 5 port solenoid) lighter and high cv flow (3/4 inch quick exhaust). I may do the same at 250 unless you see a flaw? [West Virginia]

A: [Mark J.] Your use of the 'quick exhaust' valve to dump your buffer tank is functionally identical to the way Burkert valves (mentioned in your previous post) work. The retract on you hammer can use smaller valves or even a simple spring return. I still don't have a source for a workable 250 psi 5-port valve.

You mention CO2 -- BattleBots rules no longer allow CO2, so you'll have to go with High Pressure Air (HPA) or nitrogen.

Q: Also I used a Flail Medieval style spiked ball and chain which I think gives me advantages like hitting spinners and drums and more likely keeping my hammer head attached and not bent. It also separates me from some of the rebound force into my bot weapon and arm. My question is what do you think are the good and bad to my weapon? and my bot weapon design. Thankyou!

A: I'm familiar with 'Major Punishment'. Tough competitor with good maneuverability!

The small mass of the spiked ball makes for poor energy storage, and the chain separates the ball from the additional energy stored in the arm. I suspect those are reasons why chain-flails are no longer seen in robot combat.

Your points about durability and rebound are well taken, but you're losing a great deal of attack energy in trade. BattleBots is looking for competitors with weapons that can cause massive damage. The solid hammer weapon on 'Beta' claims an impact energy of 3000 joules from their 11 kg hammer, and you're going to have a great deal of difficulty matching that with a small flail weapon. Think bigger -- much bigger!

Aside: unless the ratings for ABC BattleBots second season pick up there may not be a third season. Consider a 220 pound main 'bot and a 30 pound 'assistant' so you'll have something in a standard weight class if the 250s evaporate.

Q: Hi again and thankyou for your good knowledge of many robot tech subjects. Do you know how and why Beta's hammer is so powerful, my previous thought was pneumatics was much more powerful for the weight? [West Virginia]

A: [Mark J.] 'Beta' does have a powerful electric hammer, but your belief that a pneumatic hammer can be more powerful for the weight is entirely correct. Go back and look at old video of 'The Judge', or run the power calculations and you can satisfy your beliefs. But 'Beta' is an effective competitor where no previous electric hammer has been. What has changed? Batteries have changed.

With a PMDC brushed motor: amperage equals torque and torque equals hammer power. In the past it simply wasn't realistic from a weight perspective to have a weapon battery pack that could deliver somewhere in the neighborhood of 1000 amps that the Briggs & Stratton E-Tek motor could turn into torque. Modern battery technology has made that entirely feasible, and electric hammer 'bots are now a competitive option -- particularly in a competition where pneumatic systems are limited to 250 psi.

Q: Dear Mark,in the new season of Battlebots lots of vertical spinning weaponed-bots are seemingly getting smaller to give weight allowances for better armour,like Poison Arrow and Witch Doctor.But from Witch Doctor's rather shocking loss to Red Devil in the round of 32 I think being small is not a really good idea,which makes them become perfect targets for clampbots to get a hold of,do you agree? [Chinese Forum]

A: [Mark J.] I've known Red Devil's builder Jerome Miles for many years. He is a fine young man, a great builder, and a talented driver. He also got very, very lucky in his fight against 'Witch Doctor'.

Improvements in battery and brushless motor technology have made it possible to shrink the mass and size of effective spinner weapons. Robots with these more advanced weapons are quick, maneuverable, and deadly efficient in deploying their weaponry. Any weapon system has weaknesses against specific counter attacks but the high-speed single tooth disks can certainly hold their own in a tournament. Don't form your opinion on the outcome of a single battle.

Q: How is T-minus's flipping device so effective, since the ram is nearly horizontal when actuated. Wouldn't this initially direct the majority of the force horizontally instead of vertically? I would have thought a flipper would become more powerful the closer to vertical the ram is oriented; how does the T-minus design allow such force upon actuation? I'm trying to see it in terms of the statics behind the design. Thanks! [Grand Rapids, Michigan]

A: [Mark J.] You're entirely correct; from a statics point of view the design of the 'T-Minus' flipper is terribly inefficient. To efficiently convert the linear action of the actuator to rotational motion of the single-pivot lifter, the actuator should pivot to remain perpendicular to the lifter arm motion. This was a primary design consideration for my heavyweight lifter 'The Gap'.

The complication is that all of the robot systems must work together, and concentrating on the efficiency of any single system leads to design compromises in other systems. Inertia Labs elected to concentrate on a well-armored compact and maneuverable low-profile design. That design required a 'lay-down' initial position for the actuator. The actuator never gets close to perpendicular alignment with the flipper arm motion and the force vectors are horribly inefficient.

So, back to your original question: how is T-Minus's flipper so effective? BRUTE FORCE! Inertia Labs made up for inefficiency with a big actuator, huge valves, large ports, and scary high gas pressure. If you have enough power you can get away with inefficiency, and the overall robot design is brilliant.

Q: I had an idea and I wanna know if it could work, i dont think I saw this design anywhere. Imagine a hammerbot a little like terrorhurtz or the one i send you videos a in aquestion below. But, the rack and pinion isnt connected directly to the shaft of the hammer. The pinion is on a dead shaft, bolted or weld on a sprocket. Above, we have a smaller sprocket, wich is bolted on the hammer. In theory, i could get more speed out of the same actuator, by gearing it with a ratio of 4:1, for example. Do you think it could work? It's not for any weight class in particular, just a design i had in my head that i thouh was worth sharing to you.

Thanks a lot for all you do for the combat robot community, you inspired me to build robot, you showed me it wasnt only reserved to pros :) [Quebec, Canada]

A: [Mark J.] I'm glad to see you're enjoying combat robotics and spending some time thinking about design improvements. In the gear train you describe, the 'pinion' on the dead shaft is called an idler gear. An idler gear has no effect on the gear ratio -- you would get the same gearing if your 'smaller sprocket' rode directly on the rack without the added complexity and weight.

About Gearing: Your pneumatic actuator can produce only a certain amount of power as defined by the cylinder bore, the gas pressure, and the rate at which the gas can flow thru the valves and ports into the cylinder. Power is a function of time and is described by the formula:

Power(t) = Force * Velocity

Gearing changes the ratio of force to velocity, but does not change power. You can 'gear down' to get greater force (torque in this case) and reduced speed, or you can 'gear up' to increase speed with reduced torque.

To be effective your hammer weapon must accelerate to as great a speed as possible in only half a revolution -- it is torque that creates that acceleration.

  • If you reduce the torque by gearing up too much the hammer will accelerate slowly and not achieve its best speed in the distance available.
  • If you reduce the speed by gearing down too much the hammer will accelerate quickly to a peak speed that is much lower than it might have if it were geared to use the full distance available.
What you're looking for is the gearing that provides the torque needed to accelerate the hammer to peak speed just as it impacts your opponent, maximizing the power from the actuator.

Q: ok, but if i use a chain instead of a gear? would it still nt affect the gearing of the hammer?

A: I don't see how the system you described could be implemented with a chain, but regardless... number of intermediate idler gears, idler wheels, or idler sprockets in a drivetrain will impact the overall gear reduction. The reduction ratio is calculated from only the sizes of the first and last elements in the sequence.

Q: Mark, why does 'Lucky' and 'Son of Ziggy' take a lot of time to make their weapons ready before they can use weapons again? That's a deadly drawback! [Jiangsu, China]

A: I haven't noticed any particular delay on weapon reset for 'Son of Ziggy' (video). I believe SOZ uses a spring powered return on the flipper and it takes just a moment for the high pressure gas vent from the pneumatic actuator so the spring can pull the weapon back down.

'Lucky' was rushed into battle before the flipper could be fully sorted and a lot of problems surfaced at BattleBots. Scroll down two posts to find a report.

Q: does some kind of rack attachment exist to put on a pneumatic ram? I would want something like the one terrorhurtz use. Thanks a lot :) [Quebec, Canada]

A: [Mark J.] The 'Terrorhurtz' weapon rack is all custom machine work. There are industrial pneumatic actuators that perform a similar function, but they're expensive and heavy [example]. Google: 'pneumatic rotary actuator'.

Q: how did john reid make the rack and pinion system? did he just put grooves on a longer shaft he then put in the pneumatic ram? it seems like it in the picture. or what about the robot in this video? or this video?

A: John Reid cut precision gear teeth into an extended length shaft on the pneumatic actuator. He has also provided a roller to support that shaft from the underside. I would think that the gear teeth would interfere with the front seal on the actuator, so there's more going on than is explained in the photos.

The robots in the other videos have their hammers driven by rack gears that have been attached to the pneumatic system. The attachment and support of the racks is critical, and the method used by the builders in the videos is not clearly shown.

British builders are famous for scrounging parts from scrapyards and repurposing them, so I suspect that those racks were scavenged from a discarded mechanical assembly. I would point out that neither of the bots in the videos are delivering what I would consider 'damaging' blows.

There are other designs for pneumatic axes: see this post for a discussion of the pneumatic weapons on 'SlamJob' and 'The Judge'.

Q: how does Ziggy's flipper seems so powerful compared to Lucky's? Aren't they built by the same guys? [Quebec, Canada]

A: [Mark J.] In my original answer to this question I attributed the reduced performance to BattleBots rules prohibiting the use of custom pneumatic components. It seems that I may be misreading the BattleBots Design Rules. I thought section 10 was quite clear on pneumatic components:

"There are no specific restrictions on the system design; however, the pneumatic system must use best practices and commercially available components that are rated for the operating pressures used."

I received a note from a reader in Massachusetts who was on-site at BattleBots 2016 and who offers a better explanation:

I have to disagree with you on the Lucky vs. Ziggy question. If Battlebots rules prohibited custom pneumatic components, how do you explain Bronco's black-box variable pressure system or Chomp's custom everything-except-the-tank? Also, having been at the event I know that there were custom components inside of Lucky, although not to the same degree as the other pneumatic bots.

You can feel free to ask Mark or Rob for the specific details, but the story is that the short time period they had for building prevented testing until the last minute. At that point, it was shown that the spring-retract and release valve system simply wouldn't do, and firing the system on full pressure (which was significantly greater than Ziggy's) would guarantee the arm being jammed or unable to cycle. Even operating at a greatly reduced pressure, the arm still got jammed repeatedly, and was only occasionally able to cycle back down.

There were other fundamental design issues in Lucky's flipper that kept it from being as potent as Ziggy's, but they're of a more mathematical nature and since I wasn't privy to the whole design process I can't really answer in good faith. Really, all of Lucky's issues came down to their status as a last-minute addition and the extremely short time that they actually had to build in.

Thanks, Massachusetts. I look forward to seeing what 'Lucky' can do with the bugs sorted.

Q: what kind of ICE engine people use to power spinner? i know that Icewave uses a fireman saw engine, but i dont seem to be able to find one anywhere (ebay, mcmaster carr). Is there other type can use? i know it might not be the most efficient way or the most simple way to power a spinner.. I just want to see if i can make one.. like i always love hammerbot, even if they are not really that efficient. [Quebec, Canada]

A: [Mark J.] Chainsaw engines are popular choices for ICE spinners -- light, powerful, and easy to obtain. Outputs around one horsepower for every 20 pounds of robot weight are about right. Check carefully with event organizers for rules specific to internal combustion engines at their event. Many events simply do not allow ICE. Current BattleBots rules (Rev. 2016.2):

Internal combustion engines are allowed, but with the following requirements:
  • The engine must use a self-starter that is activated by remote control.
  • Any electric fuel pumps must be able to be shut off by remote control.
  • If the engine uses a separate fuel tank, the tank and fuel line must be well protected.
  • The fuel tank must be vented (no pressurized tanks) with a vent system that will not continuously leak fuel if the bot is upside-down.

ICE weapons are temperamental, unreliable, and have a poor record in combat. They are most certainly not a sane choice for novice builders.

Q: I am building a hobbyweight with a small (~2 lb.) vertical spinning bar sticking out of the front wedge. I recently attended an event in which a couple of the other competitors were running Turnigy brushless motors for their belt-driven weapons (both of them did well). I am basically just trying to replace my heavy brushed motor with a lighter brushless motor while keeping the belt drive. What should I attach to a brushless motor (5mm shaft) to spin that 2 pound bar on my hobbyweight? [Albany, Oregon]

A: [Mark J.] I need more info:

  • Dimensions of your spinning bar (length, width, thickness);
  • Diameter of the pulley on the bar;
  • What brushed motor you are replacing, and at what voltage; and
  • Which Turnigy brushless motor you want to swap in.
I suspect you'll need a larger reduction ratio than you have with your brushed motor, and so will need a smaller motor pulley. Send me the info and I'll run the numbers.

Q: The steel bar is 5" x 3" x 1/2" (roughly, a couple of the corners are taken off a little bit). It is attached to a 3" pulley. I was running a Kawasaki 21.6V circular saw motor with a 7s lipo. I haven't yet selected which Turningy motor but was thinking something along the lines of the Turnigy XK3665-1200KV and running it with a 3s or 4s.

A: Hmmm... I have no clue about the power output of your circular saw motor, and I suspect you don't either. How did you decide on a 2" to 3" pulley ratio for the weapon?

The Turnigy XK3665-1200KV is an inrunner motor that would spin at close to 18,000 RPM on a 4-cell battery, but running it on 4 cells rather than its rated 7 cells reduces the output power by almost 70% [1 / (7 / 4)^2 = 33% of max power]. Pick a motor rated for the number of cells you want to use. For 4-cells something like the Turnigy Aerodrive SK3-3548-1050kv would be about right.

Your weapon bar is puny. At 8000 RPM (too fast) it stores less than 570 joules of energy. Consider adding thickness, increasing the diameter, or going to a full disk. Changing out the bar for a 6" diameter steel disk 1/2" thick bumps the 8000 RPM energy storage to nearly 1700 joules -- no longer puny.

Running a 1" diameter motor pulley to a 2" pulley on the weapon could work nicely for this set-up. Make sure the belt width is adequate to carry this amount of power.

Q: What are the benefits of an asymmetrical spinning blade versus a symmetrical one? [California]

A: [Mark J.] Briefly, you can spin your weapon twice as fast and store four times as much energy without losing critical weapon 'bite'.

From the Ask Aaron Spinner Weapon FAQ:

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read. It turns out that [weapon bite at a given weapon speed] depends on the spacing of the impactors and how fast your 'bot moves forward during an attack. You can effectively use greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to obtain if you have multiple impactors and a timid attack.

There are also multiple posts in this archive discussing single-tooth weapons. Search here for 'asymmetric'.

Q: hi do u have a way to calculate how much torque my lifter will produce? what gearing should i use for the gearboxe in a 30 lbs robot? [Quebec, Canada]

A: [Mark J.] You asked a question a couple months ago about chain driving a featherweight lifter. My reply to that question featured a link to the formulas needed to calculate torque for a simple lever arm lifter (not a 4-bar lifter). Suggest you re-read that post and follow the link.

You should gear the lifter motor so that maximum lifter load requires only about half of the motor's stall torque (torque overage factor = 2). That assures the fastest lifter speed when fully loaded. Here's an example:

  • Lifter arm length (pivot to tip): 1.0 feet
  • Maximum lifting weight: 30 pounds
  • Maximum torque at gearbox (ft-lb): 1.0 feet * 30 pounds = 30 lb-ft
  • Max Load at Gearbox (in-oz): 30 lb-ft * 192 = 5760 oz-in
  • Motor Stall Torque: 166 oz-in (BaneBots RS-775 18v @ 18v)
  • Torque Overage Factor: 2.0
  • Gear Ratio Required: (5760 / 166) * 2.0 = 69.4:1

In the example given, the BaneBots 64:1 P60 Gearbox would do nicely. Run your own design numbers thru the same process to get your ratio.

Note: although a torque overage factor of 'two' provides the fastest lift at maximum load and keeps motor loading reasonable, some builders prefer a smaller torque overage factor to give a faster lift when the lifter is only raising one end of the opponent rather than the entire 'bot. This places a greater load on the lifter motor, but is an option. As long as the torque overage factor exceeds 'one' the lifter will function without stalling.

Q: hi srry if im annoying.. but do the lifter have to be a straight bar for the math to work? because my idea was to use something similar to the one nyx has [pictured at right]. I have done very basic physic at school, since i am only 17 so pardon my ignorance.

A: I'm always pleased to get follow-on questions. I have to make assumptions about most questions to keep the answers short, so don't be hesitant to ask for clarification. Curiosity and persistence are virtues.

The math works for any single-pivot lifting arm: measure the arm length as a straight line perpendicular to the pivot axle -- from center of axle to far tip of arm. Bends and angles along the way don't count.

Note the big gears 'Nyx' uses to handle the large torque loads on this long lift arm. A small BaneBots gearbox used as the pivot is unlikely to survive torque forces so great as seen in this design.

Q: so. can i use like a 16:1 ratio gearboxe then use gears to acheve the right ration, i assume thats how nyx work. Im i right?

A: Yes. That will reduce the torque load on the BaneBots gearbox. Chains require less precise positioning than gears and are more forgiving of minor misalignment -- easier for a novice builder.

Q: do u have a diagram on how an horizontal spinner like tombstone or mechavore is mounted? [Quebec, Canada]

A: [Mark J.] There are two common methods to mount spinner weapons; 'Tombstone' uses one, and 'Mechavore' the other:

  • 'Tombstone' uses a non-rotating 'dead shaft' solidly attached to the chassis as a structural member. The weapon blade and drive sprocket are attached to a tubular hub with bearings on each end that ride on the shaft. You can see spare weapon blade/sprocket assemblies for 'Last Rites' ready to slide onto the shaft in the picture at right.
  • 'Mechavore' has its spinner weapon and drive sprocket attached directly to a rotating 'live shaft'. The shaft itself rides in bearings attached to the chassis.
There is a diagram of live and dead shaft designs with a discussion in the Ask Aaron Weapon Archive: Live Shaft vs. Dead Shaft.

Q: hello im the guy that asked the question about the horizontal spinner mounting option. Do i need to put something between the bushing and the frame under the spnner? it seems like the friction at this point will be very high. Im sorry if my english was not really good ; )

A: Vous parlez mieux l'anglais que moi le français. Je parle le français comme une vache espagnole.

I assume we're talking about a 'dead shaft' design? Some bearings come with 'extended' inner races that will space the weapon hub up away from the chassis to prevent rubbing. If the selected bearings do not have an extended inner race, a small washer placed on the shaft will rest against the race and rise the hub up for clearance. I left the washers out of the linked diagram for the sake of simplicity, but then I left out the chassis as well.

Q: Hi Aaron,
First of all thanks for making your Excel Spinner Spreadsheet, it's amazing! There's one thing I'm not sure about it though.

I've got a Scorpion HKIII-4020-890KV brushless motor with a 6S LiPo battery to power my weapon. My horizontal bar is made out of 4340 stell (around 7.83 g/cc) and has Lenght = 0,42m Width = 0,04m and thickness = 15mm so my moment of inertia is 0,29269 KgM^2 and the mass is roughly 2Kg

So if I use a 3:1 gear ratio and consider that the no load speed would be 80% of the KV times the voltage (24V) I will get at my maximum energy (4724 J) and speed (5411rpm) in less than a second (around 0,85s) Would you say that's correct? I think I've mess up the no load speed.

Thanks in advance! [Brazil]

A: [Mark J.] Brushless motors in combat robots remain more of a black art than a science, but I can assure you that you're not going to spin up that bar in 0.85 second with that motor.

The Team Run Amok Excel Spinner Spreadsheet works quite well to estimate the performance of spinner weapons with brushed motors. As warned (perhaps inadequately) in the current version of the spreadsheet, brushless motor startup torque depends a great deal on the brushless motor controller. Differing controller hardware, firmware, and user settings will greatly impact the low-speed torque of a brushless motor. In particular, a sensorless motor like the Scorpion HKIII-4020-890KV will not come close to the 'best case' torque calculated by the simple brushless torque estimator included with the Spinner Spreadsheet because a sensorless controller must take an educated guess at the stator position relative to the magnet array. Also, the motor would need to pull over 1800 amps at start-up to produce that 'best case' torque, and your controller most certainly isn't going to allow that much current even if your battery pack could supply it.

I'd de-rate the stall torque of the Scorpion motor by a factor of at least six -- call it 3.3 Newton-meters. Aerodynamic drag will slow total spin-up time as well as reducing top RPM. Assuming 20% speed loss due to aerodynamic drag, I 'eyeball' weapon speed around 5200 RPM with 4200 joules in something like 6 seconds. That's still very nice performance for a featherweight.

Now the bad news: if that motor is bogged down to continuous operation at 80% of free running RPM it may pull in excess of its 65 amp continuous current rating. So much depends on who the motor controller behaves in situations like this. Brushless motors do poorly when bogged down -- you might consider increasing the reduction ratio.

Again, that stall torque rating could vary a good deal from the estimate and the performance figures would vary with it.

Q: I have a couple more questions about the size of my bar and the axis that supports it. This is all for a featherweight horizontal spinner, and if the model from above is correct I have 157 J per pound of my class which is pretty nice. I'm thinking about using the bar I've talked before with an axis of 10mm, both of them made out of 4340 Stell at 34HRC. Do you think that's enough thickness for my axis? How would you simulate that in a CAE program? I have experience with PTC Creo Simulate academic edition, I'm just not sure how to model this.

A: See Frequently Asked Questions #17. 'Ask Aaron' is not a free engineering service -- and if it was, no competent engineer would spec that weapon shaft without knowing a great deal more about the support the shaft would be given in your design.

Q: Hey Mark,
Are bearings absolutely necessary for a spinning disc/bar weapon? I was looking at Team Nightmare's build of Backlash (1st version) and nothing was mentioned about bearings. It's kinda hard to tell in the pics, but it appears that the disc is attached to the pulley and they both spin on a dead shaft. Are there some type of bearings in there that I'm not seeing or is it possible to spin a weapon on nothing but a greased up shaft? [A few miles south of here, Oregon]

A: [Mark J.] Take another look at the weapon pulley in the 'Backlash' build report -- that's a sealed bearing at the hub. The loading at the hub of a high-energy spinning weapon far exceeds the capacity of a 'greased up shaft'. On a good hit it would spall and seize. You can (and a fair number of builders do) use oilite bronze bushings in place of ball/roller bearings to support weapon shafts. They have a bit more friction but can survive enormous shock loadings. Keep them well oiled.

Q: So if I wanted to use something like a needle roller bearing, I would put it on the dead shaft and then attach the weapon/pulley to the outside of the bearing? If so, what is the best attachment method? I know set screws aren't great but could they work for a 12 pounder?

A: Not a great idea to apply significant point force (like a set screw) to the outside race of a bearing. A needle bearing would typically be installed in the weapon/pulley as an interference press-fit with thrust bearings and spacers taking up any lateral motion on the shaft.

Alternately, flanged ball bearings can be simply inserted into the sides of the weapon/pulley assembly to be held in place by the structure supporting the dead shaft. Washers or tubular spacers can take up any excess clearance.

Q: I've been noticing that some spinners that use outrunners to mount the motor pulley to the can of the motor and not the to shaft. Best example I can think of is the Australian featherweight 'Decimator'. Other than reducing height, does this offer any other advantages? Any significant disadvantages with this? [A few miles north of here, Oregon]

A: [Mark J.] A design like Aussie 'Decimator' (photo) mounts the weapon pulley on the can and tucks the entire weapon drive inside the chassis without requiring an internal bracket to support the weapon motor. It's clean and simple, and it does save a bit of height while still protecting the weapon belt. Simple is good.

The drawback is that the pulley is now located at the far end of the motor from the motor mount. This violates the general engineering rule of providing support as near as possible to the point where lateral force is applied to a structure. Decimator's design places great mechanical loading on the motor's internal bearing support compared to mounting the pulley in the conventional manner. The short height of Decimator's weapon motor keeps the load from becoming too great -- stresses on a taller motor would be worse.

Q: How can I install a pulley on a brushless outrunner motor? [Quebec, Canada]

A: [Mark J.] Well, you could the obvious and slide a nice FingerTech pulley onto the output shaft of the outrunner and tighten down the Allen screw. That works. I'd file a flat spot on the shaft to give the Allen screw a better bite, and I'd put a drop of threadlocker on the screw to keep it from loosening.

Q: how could i give a blue, energetic look to my flamethrower like th one used in complete control? [Quebec, Canada]

A: [Mark J.] We don't discuss flame weapons here at Ask Aaron. See Frequently Asked Questions #28.

Q: We're competing in a junior high battlebot competition. 50 pounds maximum on robot. We're wanting to do a spinning weapon like tombstone. Any thoughts on motor, maybe a cog belt. and pulleys? [Amarillo, Texas]

A: [Mark J.] Start by reading the Ask Aaron Spinner Weapon FAQ. The FAQ outlines the key elements in spinner weapon design and points you to tools to match a specific weapon design to a suitable motor and weapon drive.

There are also a great many posts in this archive about weapon motor selection.

As for drive components, browse the Mechanical and Drive Components selection at Robot Marketplace.

I'm pleased to provide the tools needed to design your weapon, and when you have your design roughed out I'll be pleased to look it over and offer suggestions. I won't design your weapon for you, but I'll give you a start:

An AmpFlow F30-150 motor @ 24 volts spinning a 24" x 3" x 1" steel bar via a 4:1 belt drive reduction. The bar weighs 20 pounds, spins up to more than 1600 RPM in 4 seconds, and stores better than 4200 joules of kinetic energy at that speed.

Q: Is this Park 370 Outrunner EFL-370-1360 Brushless Motor powerful enough to spin a 2.5 lb steel bar (overhead spinner curved down like that in one of the designs in the riobotz book) or a fbs shell? Live shaft most likely but dead shaft is possible. Using Timing belts and a gear ratio of about 3.5:1 with and rpm of about 4000. I would like to know if it is strong enough to spin the weapon up to speed in a reasonable amount of time under combat conditions (being hit while attempting to spin up). 11.1v lipo.

Or should I use this Feigo inrunner motor?

Thank you. I know my questions could probably be answered in the excel spinner spreadsheet but I am not very good at using that particular tool. Using the lighter motor would be amazingly helpful for weight but I fear it is too small to spin that much mass effectively under combat conditions and will get burned out. [Pennsylvania]

A: [Mark J.] I'd suggest you start by reading the Ask Aaron Spinner Weapon FAQ, wherein you will find out why a spinner weapon cannot be evaluated based only on the weight and material of the spinner, and why the dimensions of the spinning mass are critical in the calculations.

It's also not reasonable for me to comment on adequate performance of a spinner weapon without knowing in what weight class the robot will compete. Is this for a mantisweight, hobbyweight, BotsIQ, or maybe something else?

Neither of the motors you ask about are suitable for your purpose:

  • The Park 370 has a power output around 100 watts -- suitable for an antweight class spinner, but way too small for you.
  • The Feigo does not list any specifications except RPM per volt. All I can say is that it spins way too fast for your design.
If you aren't willing to do the calculations to pick out a suitable weapon motor, look at successful robots in your weight class with designs like yours to see what weapon motors they use. That will get you close.

Important note: you don't see any successful examples of that 'overhead spinner curved down' design in actual combat because that particular rotor shape in unstable! See this post elsewhere in this archive. Pick another design.

Q: Do you have any thought on the PERM PMG 132 motor vs a Motenergy ME0708 or ME1003? This will be for use in spinning blade or drum robots.

I understand the performance differences but wonder if there are any strengths or weakness in either of them that would be specifically important to combat robots. [Orange County, California]

A: [Mark J.] I'm going to defer to Ray Billings on this one. Ray and Team Hardcore have a whole lot of experience torturing big pancake motors in heavyweight weapons. They've gone thru a BIG stack of original Eteks, PERM PMG-132s, and Motenergys. None of them hold up for long, but if you need big weapon power you have to live with short lifespans. Ray estimates it costs him about $1000 a match for equipment replacement -- mostly batteries and weapon motors.

His current choice - for reliability and power - is the Motenergy ME0708, also known as the 'Etek-R'. If Ray likes it, I like it.

Q: Would a alien power system c8080 brushless motor work for a heavyweight vertical spinner? If not, what brushless motor would you recommend for a heavyweight vertical spinner? [State of Tennessee Department of Education]

A: [Mark J.] It's poor design practice to start with a weapon motor and build the rest of the robot around it. Design your robot to do what you need it to do, then build to meet that specification. All of the components of the weapon (and the robot) must work together.

That said, a 5000 watt brushless weapon motor like the C8080 is underpowered by current heavyweight standards. Design a suitable heavyweight-class weapon (think 'Tombstone') and plug in the performance specs for different motors until total weapon performance meets your goals. There are heavyweights that use Alien Power weapon motors, but not the cute little C8080.

Q: Hello,
Why are all full-body spinners' drive systems so slow? I know they need good stability especially with the gyro effect, but why can't they be fast? And would making the top part spin the opposite direction as the bottom tooth part eliminate gyro and maybe even help with the "throwback" con of those spinners?
Thanks, Missouri

A: [Mark J.] Let's talk a bit about the 'gyro effect':

  • The gyro effect -- more correctly 'gyroscopic precession' (video) -- is a force acting at a right angle to a force that's acting to change the direction the axis of a rotating body is pointing.
  • The axis of a vertical spinner weapon points left/right in relation to the robot. Turning DOES change the direction that axis is pointing. A vertical spinner (like a drum spinner) DOES exhibit a 'gyro effect' when turning that can lift one side of the robot off the arena floor.
  • The axis of a horizontal spinner weapon points straight up/down in relation to the robot. Turning DOES NOT change the direction that axis is pointing. A horizontal spinner (like a full-body spinner - FBS) DOES NOT exhibit a 'gyro effect' when turning.
Horizontal spinners don't suffer from the adverse 'gyro effect', but they can have trouble with simple torque reaction. Newton's third law says that for every action there is an equal and opposite reaction, so when you apply torque to spin-up a big horizontal spinner, an equal torque is applied that attempts to spin the robot in the other direction. This force greatly diminishes once the weapon is up to speed, but at weapon start-up it can be quite troublesome. Now that we have the terminology sorted out, let's get back to your questions.

Not all FBS are slow, but most builders skimp on the drive system in order to put additional mass and power into the weapon system. Let's face it, a FBS is all about the weapon. The attack strategy doesn't depend on speed, and any side of the robot is as dangerous as any other so maneuverability isn't an important factor either. All you need to do is move toward your opponent and wait for contact. Adding a powerful drive system steals power from the weapon and doesn't significantly add to the effectiveness of the robot.

Splitting the shell into two counter-rotating components adds significant complexity and fragility to the weapon system and results in less energy storage in the part that actually hits your opponent. It could eliminate the torque reaction on spin-up, but the 'throwback' on impact is a consequence of Newton's third law and the horizontal impact vector. You can't cheat Newton.

Q: So the rotational energy from the counter-rotating top wouldn't stop the bot from spinning away from impact?

A: That's right -- Newtonian counter-reaction trumps counter-rotation every time.

Q: What I'm trying to figure out is how to not get beat up from super-low wedge spinner bots. If they come in slow enough to move the gyro they can get under you. Unless you have a fast drive you can't get where they can be hit.

A: Like I said, not all FBS are slow. If you're willing to trade off some weapon power for robot speed there's nothing in the design to stop you.

Q: How do you defeat a bot that has a knife edge wedge and titanium spinner on top of that?

A: People beat wedges all the time. Suggest you watch some video of matches with this type of opponent to see how it's done in whatever weight class you're building. Modify your design and strategy accordingly.

Q: Greetings:
I was wondering how effective the following weapons would be on a robot. Note that I've ordered these roughly from "least likely to work" to "most likely to work", and am only seriously considering building the last three.
  1. A vertical spinning weapon on a pneumatic piston
  2. A horizontal spinning weapon on a pneumatic piston
  3. A drill/'Niterider'-style weapon on a pneumatic piston
  4. A pneumatic piston on a horizontal spinning weapon
  5. A lifter that deposits the opponent onto a spinning weapon on top of the robot (if you need a better description I can link you to a picture.
  6. A horizontal spinning drum weapon
  7. A "face spinner" (think 'Invertibrat' from Season 4)
  8. A front-hinge flipper a la 'Firestorm' or 'Cassius', except the lifting arm is replaced by a blade to potentially damage the opponent (obviously this would have to be a high-powered pneumatic flipper)
And yes, I know your "efficient, simple, easy to use" mantra, I just wanted your opinion on this. [Portland State University]

A: [Mark J.] I don't consider any of the weapons to be practical, so let's talk about the designs purely from a conceptual standpoint.

  1. A spinner weapon gets much better 'bite' if the closing speed on the opponent is greater. In close quarters, having the ability to quickly thrust the spinner forward is conceptually valuable.
    Conceptual score: A
  2. Same benefit as #1, but horizontal spinners throw both the attacker and target in opposite directions. Less desirable.
    Conceptual score: B
  3. 'Niterider' had a 'disemboweler' spike intended to penetrate another robot then twirl some stiff wires around to chop up internals. Would work really well if combat robots were armored with cardboard and filled with eggs. Adding pneumatics won't help.
    Conceptual score: F
  4. Whaaatttt?? How does that accomplish anything except sudden imbalance?
    Conceptual score: F
  5. I really hate dual-weapons. Decide whether you want to flip 'em or shread 'em, then put all your weight allowance into a single effective weapon.
    Conceptual score: D
  6. OK, maybe this one is practical. The drum stores more energy than a bar or disk, so it has some promise. Keep the drum fairly short and with a large diameter and you might have something.
    Too practical for a conceptual score
  7. 'Invertabrat' was an unsuccessful 'flipper' robot that violated the 'two-weapon rule' and added an ineffectual milling head to the back of the robot. Spinning at full speed the milling head might possibly leave a nasty scratch on aluminum or plastic. Enlarged, one edge goes up (good) and the other edge goes down (bad). Hit with the wrong edge and YOU fly upward.
    Conceptual score: F
  8. Blades can't cut modern robot armor, and a sharp edge makes it easy for your opponent to slip off to one side or the other. If you wanna flip 'em, flip 'em. If you want a pneumatic pickaxe, build that. Points for visual appeal.
    Conceptual score: C

Q: Hi Mark! I have a quick question on drumbots. How is a drum usually mounted and supported on a dead shaft, such that it can handle direct impacts and keep the shaft from bending? [Midvale, Utah]

A: [Mark J.] Usually? Sturdy endplates, great big bearings and a big hardened steel shaft anchored securely to the chassis as close to the bearings as possible.

Q: Hi Mark, I'm following up on my drum spinner mounting question. What if the drum was split in half with the pulley mounted in between? In my case this would be necessary given the compactness of the bot and motor mounting restrictions. So the shaft would either be 1.5" thick titanium or 1.1" thick steel anchored on the far left and right sides of the bot, and the drum itself is about 2" thick with a 6.3" OD not including teeth. It just has that vulnerability in the middle where the drum is split to be able to mount the pulley in between. Is this a viable option if built as robust as possible? If not, another option could be using the drum itself as a pulley? (Assuming the gearing could still be made correct)

A: I really dislike splitting the drum right at it's point of greatest structural weakness. Adding additional bearings near the center to support the split would load the shaft at its weakest point -- a poor engineering choice.

Your drum is amply thick to machine a v-belt groove (or two) into the drum. That will weaken the drum a bit, but not nearly so much as splitting the drum to add a central pulley.

Q: Hi Mark. That sounds good. I was leaning towards a built-in V-groove as well. Tolerances will have to be very high on the machining though, since the plan is to have a single tooth and a counterweight spanning across the entire drum. So the V-groove will have to go through those two AND the drum, misalignment between the three could cause troubles. It should work out fine though.

I could also just split the tooth and the counterweight in the middle to allow the pulley to go around just the drum. Is there a disadvantage to splitting a tooth into two pieces? When the drum hits it can potentially put the entire load on just one tooth.

A: You haven't mentioned the length of your drum. Assuming that it's fairly long I'd split the impactor tooth and the counterweight. At the energy levels a large drum weapon generates even a tool steel impactor will elastically deform enough on impact to localize the loading, so there is little benefit to a long-span tooth.

Q: Drum guy here again, thanks so much for your help so far! The total drum length is 11", but it actually won't be split directly down the middle, based on the latest design... more like into a 3.5" piece and a 7.5" piece. By the way, just wanted to mention that with the current materials and dimensions the drum will generate just under 20KJ at 90% RPM.

A: That's a lot of energy for a weapon that size. An aluminum drum with the dimensions you give plus steel tooth & counterweight weighs about 33 pounds and needs to spin close to 8000 RPM to store that much energy. Balance is going to be critical. Best luck.

Q: Drum guy here again. Thanks for the input! You are exactly right about the total weight and RPM. I'm curious though, what dimensions did you estimate the steel tooth would be, and how much it extends out of the drum? I've been trying to balance how far it extends out vs how far it is supported in the drum, and have yet to determine the sweet spot.

A: I didn't assign specific tooth dimensions. I just added a bit of mass to the outside diameter of a drum with the dimensions you gave to bump up the weight and get the RPM for 20K joules to come out a round number. There is a formula to calculate the maximum useable tooth height for a specific weapon at a specific closing rate:

Tooth Height (inches) = Closing Speed (inches per second) * 60
Weapon RPM * Number of Impactors

The formula requires some assumptions on closing rate, but does at least provide some guidance. How deep to set it into the drum? Best engineering judgement.

Q: Is there any difference between a pneumatic setup for co2 and nitrogen use? [Third Tube from the Right, Internet]

A: [Mark J.] Yes. CO2 converts to a liquid form under pressure and is stored at about 850 PSI. The storage tank must be mounted to assure that only the gaseous CO2 from the top of the tank enters the pressure regulator and actuator. As CO2 changes from a liquid to a gas it gets VERY cold -- cold enough to 'freeze up' a regulator designed for other gasses, so you need to make sure your regulator is made for use with carbon dioxide.

Nitrogen can be stored at pressures up to 5000 PSI and does not convert to a liquid form under pressure -- you'll need a larger tank to hold the same quantity of gas as the liquid CO2 system, but you don't need to worry about tank orientation or the problem with extreme cold. The higher pressure requires that the storage tank and regulator both be rated appropriately.

See the 'What a gas!' section of the Team Da Vinci: Understanding Pneumatics page for more details, and read the whole page while you're there.

Q: I can't think of one, and there may not be one just because I haven't seen many bots with it; but is there any pros to having a non circular kinetic weapon? Like Tillah from Team Juggerbots' square drum? [Spring, Texas]

A: [Mark J.] Can't think of one, you say?
  • Bar spinners like 'Tombstone'?
  • Asymmetric mini-discs like 'Witch Doctor'?
  • Cutaway discs like the 2015 version of 'Nightmare'?
  • Snail drums like 'Touro Maximus' (image right)?
  • Monotooth drum hybrids like 'HyperShock'?
Plenty of examples, some of them requiring a great deal of engineering and construction effort to produce. As covered in the Ask Aaron Spinner FAQ the best energy storage does not come from these designs, but builders don't generally waste effort so it's a safe bet that there is an advantage to be gained. The advantage varies with the style of weapon:
  • Bar spinners are simple to construct, robust, and durable;
  • Asymmetric mini-discs are single-pieced, durable, and can spin at high RPM while retaining 'bite';
  • Cutaway discs are... Well, they're pretty;
  • Snail drums are durable and show that you have serious design and machine skills; and
  • Monotooth drum hybrids have the advantages of asymmetric mini-disks but can store more energy.
Q: I think I worded my question wrong lol. I meant are there any advantages to a polygonal kinetic weapon like a square (or cube like I guess) drum. Then again, there are eggbeaters that are apparently good

A: Like it says in the 'Energy Storage' section of Ask Aaron Spinner FAQ an ideal design places as much of the mass as far from the rotational axis as possible. A circular cross-section achieves this goal for a given diameter, while something like the weapon on 'Tillah' (photo at left) has a good portion of the mass located at less than the max diameter and loses energy storage efficiency. As I recall, Ron Ender found that big square steel tube in a scrap yard and just really liked the look of it. It does look awesome, and that counts for something.

Eggbeaters can store a great amount of kinetic energy for their mass because they place a lot of the material a long way from the rotational axis where it does the most good. They are a viable design choice for insect class robots but they don't scale up well to heavier weight classes; they're too fragile at larger sizes.

Q: Is making a counter-rotating saw blade a good idea at all? I did a little test on some aluminum with my dad's saw (counter-rotating). It didn't have any throwback, and it made a clean cut just touching it. Using a single blade, it had a LOT of throwback, made not very deep but nasty looking cuts, but wasn't good at cutting from the flat side. Any thoughts? [Dearborn, Michigan?]

A: [Mark J.] I know that the new batch of 'BattleBots' are all complex examples of 'machine porn', but that's only because the audition process for the show demands that type of design. If you want to build a robot to win matches in open competition you need to keep the design simple and robust.

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

So, no counter-rotating saws. In fact, no saws at all. Effective use of a saw requires that you immobilize your opponent, and your opponent has other ideas about that. Keep it simple.

Q: How can u self right using a 4 bar lifter? i saw this video on youtube and even after watching it i just cant figure it out..

P.S. i really love ur site, its the best out there and u gave me the inspiration the build bot thank u :) [Quebec, Canada]

A: [Mark J.] Thanks for the props, Quebec.

The video of antweight 'Pad Thai Doodle Ninja' self-righting is taken from an awful angle to actually see what's happening. I think you'll get a much better idea of the process by watching this video of 'BioHazard' self-righting. Getting a 4-bar lifter to flop back upright requires extensive pre-planning and a fair amount of tinkering. You'll notice small extension 'claws' on the back of PTDN's lifter that I'm sure were added to get the self-righting to work.

Charles Guan's 'Equals Zero' website has an archive for PTDN that includes the design requirements for getting a 4-bar to self-right:

"[Self-righting] is kind of tricky with 4-bar lifters. You really have to take into account the center of gravity of the bot, and the length and extension of the arm, in order to facilitate this. Generally, 4-bar lifter bots flop onto their backs and come to rest on the arm whenever it is then deployed, as the CG is too far forward, and no self-righting is possible. [The classic video of former Battlebots heavyweight Biohazard shows how a 4-bar can self right.]

Notice how [BioHazard's] center of gravity is far enough back that the bot hinges on its rear edge and does not come to rest on the arm. The arm’s retraction then keeps the CG within the line drawn between the arm’s contact point and the bot’s rear edge, and it gathers enough momentum to push back over. Making it able to do this meant making the arm extend all the way back across the bot. Notice also how Biohazard had a ‘tang’ at the very back of the arm, a part that sticks up – this aids in the maneuver by making the contact point with the ground further forward, so the ‘line’ is longer.

This goal meant that I was continually watching the bot’s center of gravity in autodesk Inventor, and also continually modifying the linkage to suit. The arm had to have a certain amount of extension to make sure the CG was in the right place, and that extension had to jive with everything else’s placement."

The full text, with some pictures, is way down at the bottom of the Equals Zero archive for Pad Thai Doodle Noodle. When you scroll down to the part of the archive where Charles is doing a brake job on his van you're about half-way there. Keep going. WAY DOWN. Seriously.
Q: Hi Mark,

First of all, I know. You heavily advise against ring-spinners. However, there is one that catches my eye due to what seems to be a pretty solid design. This upcoming season of ABC's Battlebots will feature The Ringmaster. A Single-tooth ring spinner with the weapon system being driven via gears rather than friction wheel. You can see all how it all works on their facebook page.

I'm heavily considering using a similar drive design in a lightweight ring-spinner that i'd like to build (not a ripoff, I have standards.)

Now, you've almost completely crushed my dreams of building a ring-spinner. But if I would be wasting my time by building one with this type of drive system and ring support, I would indeed like to know.

For knowledge-sake, lets assume that I built a lightweight clone of The Ringmaster. Good RPM, Quick Spin up, "perfectly" Counter-balanced weapon, and so on. What deal-breaking issues or concerns do you think I'd most likely run into?

Thank you sir, David R. (Livermore, CA)

Quick addendum

I know that I am speculating that The Ringmaster works well even though we haven't even seen it in action, and we probably won't get a good idea of that until the new season airs. In fact, just about ALL of my question involved speculation. So I understand that nothing from the scenario I gave you is completely concrete. So upon your response, I'll be sure to have a pinch of salt ready. Thanks :)

A: [Mark J.] 'Crusher of Dreams' -- maybe I should put that on my résumé?

Where to start...

  • First: run a Google search for Ringmaster's team leader 'Hal Rucker'.
  • Second: examine your budget and technical skillset to see if they match Hal's.
  • Third: no-spoilers, but check in on the historical success of ring spinners.
  • Fourth: consider the wisdom of the Team Run Amok Design Philosophy:

Design Philosophy

A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.

Here's the problem with shell and ring spinners in general: the impact vector is lateral, parallel to the floor, and uncomfortably close to the center of mass of the robot. You hit your opponent and Newtonian physics throws you off in the opposite direction like a high-powered hockey puck. They are nearly as dangerous to themselves as to their opponents. A ring spinner has the advantage of being able to operate when inverted at the cost of much greater mechanical complexity. An extended center pole of the type used by lightweight champion shell spinner 'Ziggo' is a much simpler solution to this problem.

If you want to build a ring spinner as an example of 'machine porn' go ahead and do it for that reason -- and yes, gear drive it. Just don't expect it to dominate.

Quick addendum

1999: people watch BattleBots, see some frat boys build a robot out of a beer keg, say to themselves 'I can do that!', hundreds of teams do just that and combat robotics as a popular hobby takes off.

2016: people watch BattleBots, see robots that appear to be built by NASA contractors under DARPA funding, say to themselves 'I can't do that!' and combat robotics as a popular hobby dies.

Just a theory.

Q: Hello Aaron:
[pneumatic] cylinder has two holes: inlet and vent. As we know the speed of cylinder depend on air pressure and flow, So I got a idea: if we make cylinder have two "Inlet holes" the flow of the cylinder will be larger, so the cylinder speed will be faster? [Yunnan, China]

A: [Mark J.] A pneumatic system has multiple restriction points: the pressure tank valve, the pressure regulator, the solenoid control valves, and the actuator cylinder ports. Typically the speed of the system is limited by the performance of the regulator and valves. Gas flow at the actuator port is typically quite good; improving the flow there will not provide noticeably greater speed.

Q: do u have a diagram on how is actuated an axe like on shunt? can u explain how the linkage works? thx [Quebec, Canada]

A: [Mark J.] 'Shunt' uses a complex pneumatic powered multi-bar linkage to provide a full 180 degrees of axe motion. You can see Shunt's designer explaining and demonstrating that linkage in this video.

The diagram at top right is from Chris Hannold's book "Combat Robot Weapons". It shows a much simpler overhead axe/hammer weapon linkage.

Additional linkage options and a full explanation of pneumatic systems can be found at Team Da Vinci: Understanding Pneumatics.

Q: hi axe guy again in the picture u put, is the hammer able to do a 180 degree rotation?

A: No -- see the small diagram at right. When the axe retracts the axe bracket runs into the pneumatic actuator, and when the axe extends the actuator shaft runs into the axe pivot axle. It's only good for about 90 degrees.

You can play with the design and get a little bit more, but a simple single-pivot design like this is limited to an efficient swing of about 120 degrees. That's why Shunt's designer went to the complex linkage to get 180 degrees.

Q: axe guy again, do u have any diagram or image of the multi-bar linkage u were talking about?

A: The only images I have of Shunt's axe linkage are in the video link I provided above. I've sketched up an approximation of the linkage (right) taken from the video. A couple of the links are fused, so this is functionally a 4-bar linkage.

Note: I cannot recommend that a novice builder attempt to duplicate the 'Shunt' linkage or the other full-range linkage diagrammed on the Da Vinci Pneumatics page. Overhead axe weaponry is not effective in modern robot combat.

Q: is there a way for a featherweight lifter to be actuate by a motor with gear and chain, without any sprocket hanging out of the bot and without having it be a 4 bar lifter?
how could i make it to be like a rear hinged flipper? i dont want to have something like sewer snake... more like dantomkia.. but not gas operated
thx :) [Quebec, Canada]

A: [Mark J.] A long-armed rear-hinged featherweight lifter requires enormous torque to lift an opponent out at the far end of the arm. A short-armed featherweight lifter like 'Nyx' (pictured) can get away with a small sprocket on the lifter axle, but each time the length of the lifter arm doubles so also does the torque required to operate the lifter double. By the time the arm reaches all the way back to the rear of the robot the torque needed raises to gearbox-shattering levels unless a much larger sprocket is used on the lifter axle.

You can run the torque calculations to see for yourself how much torque is needed and shop around for a gearbox that can survive the load and provide the needed reduction ratio -- but you'll find that a suitable unit is heavy, bulky, and expensive. Four-bar mechanisms are popular for electric lifters because the torque requirement for a long lifter arm is greatly reduced. Perhaps you should reconsider your design.

Q: I just had a thought about how I could make the drum for my 60lb robot. My original plan was to machine the drum and drum tooth as one single piece of tool steel, but I've recently been considering forging a quarter-inch square bar of folded tamahagane (Japanese "Jewel" Steel, the material used in Japanese knives and swords) and welding it to a tool steel tube. Do you think this would be worth trying or should I just stick to my original plan? Or is the hamburger bad?

To note, the drum is going to be a simple counterbalanced single-tooth design either way (basically, two teeth but one is on the inside of the drum rather than the outside). [Arden, North Carolina]

A: [Mark J.] There are good reasons why most drum weapons have bolt-in steel impactors setting in pockets machined into a thick aluminum tube:

  • Impactor teeth take a beating. No matter what material is used the critical leading edge gets rounded off, they crack, and they deform. Throwing the whole drum away when this happens is expensive. Bolting in a new impactor makes better sense.
  • Correctly welding tool steel is a huge pain.
  • Welds break.
Tamahagane is a high carbon 'bloom' steel. Its performance in knives and swords comes from forging higher and lower carbon layers of tamahagane together to provide both resilience and strength. Blades made in this way were state of the art... 400 years ago. Even if you were able to obtain the different grades of tamahagane and correctly forge the multi-layered block via the intensive and laborious process, the performance of modern 'shock resistant' tool steel is much superior for your purpose.

I'd suggest building a conventional drum for your first attempt and saving the fancy stuff 'til after you've seen the challenges first-hand.

About balancing a single-tooth drum: I'm a little worried by your description of "two teeth but one is on the inside of the drum rather than the outside". That's like having two kids on a teeter-totter and moving one of them in toward the center -- they won't balance unless the kid moved toward the center is heavier than the kid that stays out on the end. I'm sure you had this figured out, but the next reader might be confused.

Q: Does a flamethrower count as an active weapon? [Quebec, Canada]

A: [Mark J.] In general, an active weapon is any weapon whose operation is controlled by a radio channel separate from those used to control the robot drivetrain. If your flamethrower meets that criteria it is 'active', however there may be additional constraints imposed by the event organizer.
  • Many events forbid the use of flame weapons entirely.
  • Some events require that an active weapon be 'effective' - a condition I have never seen a combat robot flame weapon meet.
Check with the organizer of the event you plan to enter for an official ruling, and please be careful with flame weapons.

Q: What do you think is the best way to get an impact slip on a drum spinner? So far we have two different pulleys that we can chose from, one with very shallow teeth and the other with no teeth. We are going to use a timing belt, and one thing to consider is that our pulley is mounted directly on the motor while having no support, so it can't take a whole lot of load. [North Kansas City, Missouri]

A: [Mark J.] Slippy belt drives vary a bit with weight class.

  • Big 'bots use v-belts that are easy to adjust for the degree of slip you want.
  • Insect classes tend to use timing belts or round (o-ring) belts. The round belts absorb shock and are easier to set-up for slip.
  • The in-between classes run all sorts of belts. I've seen flat belts, regular and inside-out timing belts, small v-belts, chains -- everything.
I don't know the weight class of your robot, the mass of the drum spinner, or the power/speed of your weapon motor. In general, I like the idea of a shallow-toothed pulley with a timing belt for a sub-light weapon. Set it up with enough slack that it can slip without too much load on the motor pulley. Specifics depend on the details of your weapon construction.
Q: Good Evening, Mr. Mark Joerger

Could you explain why Stinger's Mace is able to blow down whenever Scott inverted the movement?
Thank you! [New Jersey]

A: [Mark J.] I could, and I have. Stinger's weapon design is a 'torque-reaction hammer'. There is an explanation of torque-reaction hammers down the page in this archive.

Q:Hello! Mark
  Why isn't Stinger's mace considered as movable weapon, which was every competitor must have in RW series 7?
  If Scott used the "pizza cutter" did it still count?
Thank you!
Leo [Maryland]

A: [Mark J.] I'm willing to answer questions about robot design that use a Robot Wars competitor as an example, but Ask Aaron does not answer questions specific to UK Robot Wars events or competitors. The Ask Aaron Frequently Asked Questions #37 explains:

Q: Why are you no longer accepting questions about UK Robot Wars events or competitors?

A: Mark J. here: for many years Aaron and I did our best to answer any and all questions on the broad topic of robot combat. A few years ago we were flooded with a large volume of trivial UK Robot Wars questions that were well outside the mission and focus of this website. 'Ask Aaron' is not a 'fanboy' site and we are not interested in investing our time in this area. We regret having to cut off all UK Robot Wars questions, but the fanboys proved themselves to be both persistent and quite rude.

Q: I'm curious why the "rule of thumb" for spinners uses energy storage and not momentum storage. Because the acceleration/deceleration frame is so short momentum is conserved and not energy. As a side benefit, builders may see improvement by not optimizing for speed so much (get better bite) I've seen lots of matches of drum spinners ineffectually rolling against the opponent. It would also mean you could reduce the speed and get much better spin up time.

As a sidenote, I asked about gyroscopes earlier and want to thank you for the help. I used some of the info in a school project and it went really well: autonomous rickshaw walker (video) [Ontario, Canada]

A: [Mark J.] The rickshaw walker video is great -- thanks for sending the link!

I've deleted an extended exchange between myself and 'Ontario' wherein I inadequately answered his question, needlessly confused the issue, was in part simply incorrect, and generally took us down the rabbit hole. I'm taking a mulligan on this one and replacing the confounding original exchange with a brief note and an external link that succinctly answers the question. I'm embarrassed, but I'm willing to admit that I screwed this one up. My apologies to Ontario.

Here's a link that gives an example of the differences between momentum and kinetic energy plus a 'thought experiment' that clearly shows why robot spinner weapon design is based on energy storage: Kinetic Energy is NOT Momentum.

If the link doesn't adequately answer your question, you may wish to pursue the question on a physics forum. Ask Aaron provides tools and assistance in the design of combat robots, but a deep diversion into physics theory is outside our scope.

Q: Thanks for the help, despite mistakes being made on both sides, it was a really interesting discussion to have and I still learned quite a bit (damn moment of inertia and angular velocity messing with units). You do great work and discussions like this are why I love this sport. Keep it up.

A: Thank you, Ontario. Next time we'll take the discussion off-line until we get an answer that can be posted.

Q: In Featherweight robots, would a hydraulic system for a horizontal crusher (which works by pinning the opponent and then crushing their undersides) be feasible without compromising drivetrain or the wedges ? [Jawa Barat, Indonesia]

A: [Mark J.] In any weight class it's difficult to build a weapon and chassis structurally strong enough to survive the forces necessary to pierce or crush an opponent. It requires serious compromises of all the other systems of your robot in favor of the weapon. Most 'crusher' weapons turn out to be nothing more than slow and unreliable 'clampers'.

Search this archive for "hydraulic crusher" to find several previous posts on this topic.

Q: Hi. Is it at all a good idea to have a drum spinner with an 1/4" steel plate mounted at 45 degrees for a tooth? The diameter of the spinner is 2", width is 5", and the bot weight is 15lbs. I don't like it, but my team thinks it's a good design. Is there a scientific way you could convince them to change it, and to what?

Also now we are thinking about having a sort of drum with the center cut out, so basically there's two spinners. That a good idea either? I've sent sketches of the drums.

Thanks [Missouri]

A: The drawings help -- it's clear what you're thinking about. I had , and I got the wrong idea from your description. I've added arrows showing the presumed direction of rotation to your drawing.

There are good reasons why drum weapons look the way they do. Very clever builders have been tweaking the design for twenty years, but you can't cheat physics. Most of that is covered in the Ask Aaron Spinning Weapon FAQ so I won't repeat it all here. Let's concentrate on the unusual aspects of your designs.

The 45 degree Impactor: the drum certainly looks tempting with those up-swept impactors, but there are reasons to use more conventional 'straight' impactors:

  • To stick out a given distance from the drum an angled tooth has to be longer (heavier) than a straight tooth. That means that a straight tooth could be made thicker (stronger) and still weigh the same as a longer angled tooth.
  • Angling the tooth exposes the 'face' of the tooth to possible impact. If the face impacts the opponent it will hit at an angle that will push the opponent away rather than digging in and launching. This is a particular problem when the weapon spins very fast and forward speed is low. If you do angle the tooth, you'll need to grind away the face so that the leading edge of the tooth will always strike the opponent -- never the face. Unfortunately, that may weaken the impact area.
  • The angled 'pocket' needed to seat the tooth into the drum is a much nastier bit of machining than the simple pocket needed for a straight tooth. It's a lot of extra work for no real advantage.
  • A seldom considered factor, the aerodynamic drag of a tooth sticking out of a drum running at a few thousand RPM is significant. Adding to that drag by tipping the tooth into the airstream is only going to increase that drag, slowing the drum speed, and reducing the energy storage of the weapon.
The design has little if any advantage and several very real problems. I'd ditch it.

The 'Dumbbell' Drum: another 'looks cool' design, but there are reasons to avoid it:

  • Leaving out the center of the drum reduces the mass and rotational inertia of the weapon. Not good. You can use that weight savings to add to the diameter of the remaining sections of the drum and gain some additional rotational inertia, but if you leave the diameter the same you're losing weapon capacity.
  • Two small drums on a single axle will place a great load on the smaller-diameter axle when one drum strikes the opponent and slows while the momentum of the other drum tries to keep spinning. That makes for a weak point in the design that is prone to failure.
Same story as the angled tooth -- you aren't gaining anything and you're taking on new troubles.

Q: Hello! Would you please tell me how to intensify the instant power of a pneumatic flipper? Will extra gas bottle work? Thank you! [Maryland]

A: It's all about gas flow. When you trigger the flipper you need the gas to flow as quickly as possible from the pressure tank into the actuator, but there are lots of things in your pneumatic system that can slow down that flow.

  • Pressure regulator - selection of a 'high flow' pressure regulator is critical. Look for a regulator with a highest flow coefficient (Cv) you can find.
  • Valves - like regulators, actuator valves have flow coefficients that indicate how freely gas flows thru them. Higher is better.
  • Actuator ports - your pneumatic actuator has 'ports' -- openings for the gas to enter and leave the cylinder. Larger ports equal greater gas flow.
  • Hoses and fittings - these convey pressurized gas between your components. If the hoses are small and the fittings are restrictive the gas flow suffers.
It's important to note that the most restrictive element of your pneumatic system will be the limiting factor in system performance. It does no good to have a super high flow regulator if the actuator valve can't keep up. If everything isn't flowing well, the system won't 'pop'.

Adding an additional or larger gas bottle will give you a larger number of system actuations, but will not improve the 'pop'.

If you're determined to add an additional tank, read up on 'buffer tanks' at the Team Da Vinci Pneumatics page. A buffer tank can help overcome a system 'bottleneck' caused by a poorly performing pressure regulator, but it won't help if your other components are the problem.

Date marker: March 2016
Q: hi, i am having problem in entering details for spinner calculation entry.drum size (outer dia 9.5 cm, inner dia=5 cm , length=20cm). 2 tooth (2.5 cm, thicness =1.5) [Delhi, India]

A: [Mark J.] -- Click here

Q: Hello sir.please direct me to the archive where it tells me the calculations required to make single tooth to find the center of mass.i also want to know if turnigy rotomax 150cc is a good weapon motor for a drum in 40kg catogary [Chennai, Tamil Nadu, India]

A: [Mark J.] Click here.

One hour later...

Q: Is first CIM motor ok for 15kg robowars to run 4kg drum 120mm dia.its for a safe event called evvaa cup [Chennai, Tamil Nadu, India]

A: The purported safety of a single Indian event is not relevant. Read the "click here" again -- particularly the last sentence:

Until there is a significant and universal change in Indian arena safety and/or rules to control dangerous weapons, I can no longer in good conscience accept questions from the region.

One day later...

Q: Hello sir pls answer my previously asked question. I promise its for a safe event and im a school student studying 8th grade and my parents wont allow me to play on unsafe arenas [Chennai, Tamil Nadu, India]

A: I'm genuinely sorry, but until rules are in place to make all Indian arenas safe my conscience prohibits me from answering any combat robot questions from builders competing in India. People are being seriously injured at Indian combat events and I will not be part of that continuing problem.

You are welcome to search thru the Ask Aaron archives and the Spinner Weapon FAQ for assistance in the design of your robot, but I will accept no new questions from you or your fellow competitors.

Five days later...

Q: Sir please give an idea for mounting a pulley on rs 775 and if i ratio the speed of 19500 rpm will i get my torque incresed and how to drive the drum with it.sir please help.i beg you. [Chennai, Tamil Nadu, India]

A: I admire persistence, but I will not dishonor the purpose and spirit of Ask Aaron by continuing to answer questions from a region with such horrible safety control at so many robot combat events. Do not ask me again.

The questions you ask have all been previously asked and answered here at Ask Aaron. The answers and tools you need are in the links I provided in your previous request. If you'd been searching the archives instead of begging me for answers, you'd already have the knowledge you seek.

Q: Hey Aaron.
I am trying to build a single teeth bot and I have managed to balance the weight now the problem is should I use an amp flow motor or the starter motor which has considerably high torque than ampflow and I will be connecting 2 starter motors the one which are used in cars .. And it would be really helpful if u could sent me the formulas for energy storage calculations for single teeth wepon .. The design is similar to that of witch doctor from international robowars plz help me out [Quantil Inc - Pasadena, California]

A: [Mark J.] Why would you seek advice from someone you believe to be so stupid as to fall for this sham? The sentence structure, abbreviations, grammar, punctuation, and use of starter motors is not California - you're a conniving Indian builder from Bangalore using a SoCal proxy to sneak a question in thru the back door.

You know full well why I don't answer questions from builders competing in India and you've read how strongly I feel about this issue. In spite of this you disrespect me, 'Ask Aaron', your countrymen, and the sport of combat robotics by attempting to gain personal advantage over your competitors thru deceit.

Stop trying to build more powerful weaponry and start trying to build safer combat arenas, Julab!

Q: I'm the beetle spinner builder again [from Oregon]. My design is coming along alright, right now I just need some assistance on some of the most critical aspects of the design: the weapon shaft, attachment method, and reduction method.

I was originally going to use a Fingertech Blade hub to mount my weapon to the weapon shaft, but I had two major problems with that:

1: The hubs are notorious for loosing up and coming off
2: The maximum drill out size for the bore is 1/4"

The first problem I can work with, as Fingertech are redoing the design entirely, but the second part is a big problem. My original weapon of 6"x2"x.125" 4130 steel has become 10"x2"x.25" 4130 steel, running off of a 2:1 ratio of my motor(Maximum RPM of the motor is 21460rpm). While max speed of the weapon will be theoretically about 10100rpm(A whopping 2000 joules of energy storage in a 3lb robot), I really don't expect more than the mid end of 8000rpm(The less insane but still very high energy storage of 1200 joules) due to air resistance. With that much energy being thrown around, I don't expect a 1/4" shaft of kind to survive the sheer force. I know to make it as thick as I can while still being in weight, but I'm wondering if a .375" grade 8 bolt is enough handle the forces, or should I go up to .5" shaft for the weapon? I know bigger is better, but I'm on a very tight weight budget with so much of the bot dedicated to the weapon, and anywhere that I can save as much weight as possible, the better (Then again, the weapon shaft may be the last place I want to steal weight from).

The second issue is getting the weapon to stay on. Because my shaft needs to be bigger to handle the forces required, I was looking at Servocity hubs(The .770" bolt pattern ones to be specific)and bolt on the weapon, but I'm pretty sure if the screws didn't shear themselves first, the hub would snap like a cracker. I was wondering if I could take some washers(Like NORD-LOCK washers) and clamp tightly down on the blade with a pair of shaft collars like Hazard did. If I have to get the weapon mount custom machined for me, what would be the absolute best way for it to stay on the hub, and the hub stay on the shaft, no matter what? I feel like a weapon that under performs is still usable, but a weapon that flies off is worse than useless.

A: [Mark J.] I've never been able to figure out how 'Hazard' managed to get enough clamping force from the shaft collars to adequately hold the weapon blade in place. Tony B. got it to work, but I can't tell you how he did it. I also can't recommend a keyed shaft for a bar spinner -- too much localized stress around the key. I think the optimum solution is a keyless lock bushing that will position and clamp your weapon bar to the shaft. Shop around a bit to find one that meets your design needs.

I do not recommend bolts - grade 8 or otherwise - for weapon shafts. Bolts are designed with the metallurgy to survive high tension loadings, not the shear loading you would expose them to as a weapon shaft. Bolts should always be protected from shear loads, either thru the design of the elements they are holding or by insertion of hardened pins that will take the loading before it is transferred to the bolts. Use a shaft suited to the type of load it will encounter.

You didn't ask about the wisdom of spinning a bar weapon that accounts for 45% of the robot's weight at 8500 RPM, but I think I should warn you of a few things.

1) You didn't mention what specific weapon motor you plan to use. I'll assume it's brushless. Brushless motors put out enormous power for their weight, but they do not like to be bogged down spinning up a heavy weapon connected via a small reduction ratio. Also, pushing against aerodynamic drag that drops the free RPM of the motor by a very conservative 15% will likely place a continuous load and amp draw on the motor outside its design parameters. Combined result - you're gonna melt your weapon motor, and quickly.

2) There is a design issue about weapon 'bite' that I didn't put in the Ask Aaron Spinner Weapon FAQ because it's a trigonometry issue that's difficult to explain, but I'll give it a shot. Let's call it 'bite angle':

The larger the radius of the weapon, the shallower its impact angle will be for a given bite depth.

You can see in the diagram that for any given amount of bite depth a larger diameter weapon will impact at a much shallower angle than a smaller diameter weapon -- causing a more 'glancing' blow and decreasing energy transfer into the target. A larger diameter weapon typically more than makes up for this by spinning at a slower speed while carrying the same energy as the smaller weapon that spins faster -- giving a greater bite depth.

Bottom line: high RPM hurts the effectiveness of a large diameter weapon more than it would hurt a smaller diameter weapon. The large weapon is good, but slow it down and you'll get a much better hit.

Q: Last thing is getting the energy from the motor to the weapon. I was thinking of the Fingertech timing belts, but I feel like the forces the weapon would create would snap the belt(I've seen so many Last Rites clones on Youtube fail because the belt/chain failed). I also feel like O-rings would also snap on start up, so they're out. Should I move up to the wider 1/4" MXL belts, go with XL belts, or should I go for chain and sprocket, like the ones found on Servocity(Assuming the above problems aren't a problem for them, which I have a good feeling that they might be)? Obviously, this risks the weapon motor and its associated electronics, but I want to make sure that if weapon motor/ESC does die, its after delivering a huge hit that KO's my opponent.

A: You haven't told me about your motor torque. Drive belts are rated by torque and RPM and you've only given me the last half of that info. All I can suggest is that you look at the drives used by weapons in heavier weight classes that transmit a comparable level of torque at comparable RPM.

Q: Also, one last thing: How do you tell when a weapon bar is on its last legs and should be replaced before it snaps in half?

A: 'Taint easy. Generally a bar looks just fine right up to when it snaps. You may get lucky and detect a small change in the sound it makes when you strike it and let it 'ring', but that isn't a reliable test. The 'real' method to detect early signs of trouble is a magnetic dye penetration test. It takes some time, it isn't cheap, and it's not a perfect indicator. My recommendation is to carry a spare blade and hope for the best.

Q: Beetle spinner guy again, thanks for the help. Those keyless bushings you linked to seem like they would be perfect method to mount my weapon. They're a bit expensive, but for the weapon hub I think I should spend the extra bucks.

I completely forgot that motor torque is kinda of a big deal in transmission selection. The motor is question is Turnigy D3536/5 1450KV motor at 14.8 volts, a fair bit bigger than what most beetles run. I'm also thinking that I should bump up the gear ratio of the weapon from 2:1 to 4:1. Even if its not Tombstone ratios of energy storage, 500 or so joules of storage should more than sufficient for a beetle. Plus, if I want more energy, I can always change my weapons shape.

A: I think those are fine decisions.

The keyless bushing is an under-appreciated hub option. Shop around a bit -- there are different manufacturers and different designs that might click with your ideas, and might save you a few bucks a well.

Yea, that motor's a 'fair bit bigger' than the standard beetle weapon motor alright. The little bugger pumps out close to a full horsepower! Start-up torque is always a question mark with sensorless brushless motors -- a lot depends on the controller software. I think we can safely guess at around 300 oz-in of torque. Yes, I think 500 joules at around 5000 RPM will be VERY impressive in a beetle. You'll have a quick spin-up, good bite, and moderate motor loading.

In commercial usage, the XL timing belts are used for up to about 160 oz-in of torque and 4000 RPM. Combat robots overstress everything, so I wouldn't hesitate to use an XL belt in this application. I'd suggest the 3/8" width and the largest pulleys that work for your design and selected ratio -- larger pulleys reduce the effective torque transmitted by the belt. Be extra careful with pulley alignment; at the speed you're running alignment is critical.

It's going on ten years since I wrote the Team Run Amok Spinner Weapon Excel Spreadsheet. There have been several upgrades over the years (battery capacity guidance, english/metric conversion calculator, brushless motor torque estimator...) but it needed an update in the recommendations for energy capacity and spin-up time, as well as new default specs for the 'example' weapon.

Version 1.6 is now available for download: Team Run Amok Spinner Weapon Excel Spreadsheet

Q: Hi, Mark. Team WhoopAss's two flippers, HexaDecimator and Hexy jr, seemed quite different from some orthodox US flippers, such as Intertia Labs' flippers in many aspects. Do you think the two flippers should be classified as orthodox US flippers? What do you think is the edge of the two flippers? [Jiangsu, China]

A: [Mark J.] The only unusual element in the design of the Team WhoopAss flippers is the forward placement of the flipper hinge. A conventional flipper design has the hinge point as far to the rear of the chassis and as high as possible in order to maintain a constant mechanical advantage for the pneumatic actuator throughout its range of motion. Placing the hinge so far forward reduces the effective power of the weapon, but does have the advantage of allowing the actuator to be placed low and flat in the chassis -- lowering the height of the robot.

As is so often the case, the weapon is not what made the Team WhoopAss robots successful. Although not nearly as powerful as the flippers from Inertia Labs, the relatively modest weapons could be employed effectively because the rest of the robot was well designed and well driven.

Q: I am currently building my first combat robot. It's a featherweight bar spinner. I just have a few questions about motor and gear selection after using the spinning weapon spreadsheet.

My favorite option so far is using a geared AmpFlow E30-150. This geared motor has a no-load RPM of 670 and a stall-torque of 360 in-lbs. I plan on using a single stage pulley around 2.5:1 to bring the RPM up to the 1600-1700 range. My weapon is going to be approximately 16 inches long and weigh about a quarter of the robot's weight. Using the spreadsheet, the weapon will spin up in about 2 seconds with a tip speed of around 80 mph and store 880 joules of energy.

Is this a good design for what I hope to be a hard-hitting featherweight? What would be an ideal RPM for this weapon/weight class? Are there any other geared motors that you know of which would be better for this application? Any advice is appreciated! [Albany, Oregon]

A: [Mark J.] Several suggestions:

  • The AmpFlow E30-150 is a fine weapon motor, but consider how you plan to use it. You're attaching it to an expensive, heavy, and power-wasting gearbox that takes the RPM down too low, and then you're running it thru a belt drive to speed it back up. Spinner weapons typically use a single belt drive reduction to drop the motor speed and increase the torque to the weapon. Cheaper, lighter, more efficient, and less to go wrong.

  • The AmpFlow is a brushed motor: durable, easy to control, high in torque, and difficult to screw up. Brushless motors are currently very popular for spinner drives -- they have more power per pound than brushed motors, but they are also less durable, tricky to control, low in torque, and very easy to screw up. As this is your first robot, I think the brushed AmpFlow is a fine choice for a weapon motor.

  • Your energy storage is too low for a modern feather bar spinner. Trash the gearbox and run a 2:1 reduction belt drive to your bar. That gives you 2800 RPM and 1750 joules in 4 seconds -- 770 joules in the first 1.3 seconds. Much better.

  • Your remaining problem is stuffing that E30-150 down low enough to get an effective spinner height -- or is this a vertical spinner?
Have some fun with it, tear up a few opponents, and learn a whole bunch. Remember: a combat robot is more than just a weapon. Don't put all your effort into the weapon at the expense of the rest of the 'bot.

Q: Hey Mark, thanks a lot for the advice. I'm happy to get to trash the gear box. I guess my only concern was the weapon storing too much energy for the size of the robot (based on the 19 J/lb. mentioned in the spreadsheet).

I plan on matching the motor with a Talon SRX ESC and angling my bar so that it hits low while the motor can be mounted up higher (if this configuration raises any red flags, please let me know).

Other than that, my main concern is this: I have a half inch cold-drawn steel (C1018) shaft with compatible mounted bearings. My ever-changing CAD model has the shaft length around 5-6 inches. Would this shaft be sufficient for my weapon setup? Or should I spend a little more on going up to 3/4" or 1"?

A: The Weapon Spreadsheet was written about ten years ago -- back when weapons were much 'kinder and gentler' than the current standard. Around 60 joules per pound is entirely in line, particularly given your relatively low weapon speed.

Angled bar spinners were once fairly common -- I recall Chris Hannold's 'Six Million Dollar Mouse' at Robot Wars Extreme Warriors. The design has no serious flaws and solves a number of design problems. I haven't seen one built in quite some time, and it may come as a surprise to your opponents.

The Talon SRX has tested very well in similar applications. It should be fine -- just don't try a high-speed reverse of the weapon when it's spinning!

Your 1/2" shaft is likely too small for a weapon with this much power and that great a distance between supports. I don't have the details of your weapon construction, but I'd spend some of that money you saved by scrapping the gearbox on a 3/4" shaft and bearings.

Q: Hi Aaron. I want to know what Ziggy`s pneumatic pressure use, the Store pressure and the Regulator pressure? Ziggy builder's website can not open [Yunnan, China]

A: [Mark J.] There are descriptions of Ziggy's weapon system elsewhere in this archive.

Quick summary:

  • Gas of choice: high pressure air or nitrogen
  • Storage pressure: 3000 psi (~200 bar)
  • Regulated pressure: unregulated [correction -- see below]
Testing and operating such a weapon system is  EXTREMELY DANGEROUS  -- Do Not Attempt to construct a similar weapon without extensive experience in pneumatics and mechanical design.

Q: Hi Mark, I noticed it in the recent post that the Ziggy use a unregulated system which means the pressure in actuator is 3000psi, same as the pressure in the tank. However, I believe the Robogames rules limit the pressure at 250psi, Ziggy seems violated the rules. Is there some reason or just I made a wrong judgment about the pressure in the actuator is same as it in the tank? [Guangdong, China]

A: [Mark J.] Hmmm...

I had assumed that 'Ziggy' received an 'event organizer exception' for a high-pressure system, as allowed under the rules. The standard photos of 'Ziggy don't show a pressure regulator, but just to make sure I went thru my photo archives for a picture from a different angle. Guess what I found - a pressure regulator! According to my sources, that is a PR-59 GO Regulator.

So, I stand corrected. It appears that 'Ziggy' ran a 250 psi regulated pneumatic system -- at least some of the time. That amount of pressure does not mesh with the team's claim that their weapon put out 14,000 pounds of flipping force. You just can't get that much out of their actuator on 250 psi. My warning about the danger of a powerful pneumatic weapon like this still holds.

Q: Mark,
I came across this video of a spring powered flipper and it inspired me. I have been wanting to build a flipper for a long time but my budget wouldn't allow anything bigger than a beetle and ant/beetle weight pneumatics aren't quite up to combat specs. My question is how would you determine the torque needed to turn the cam in order to load the flipper? Thanks [Cincinnati, Ohio]

A: [Mark J.] Oh my! The concept is sound, but the components and layout of the mechanism in the video are horribly designed. The motor torque requirement is constantly varying as the oddly shaped lifting cam rotates. The motor visibly slows at one torque peak. In order to minimize the motor torque needed to load the flipper:

  • The spiral 'snail cam' driven by the motor must have a continuous and gradual lift.
  • The spring should be given a straight pull -- none of that 'bend in the middle' crud that requires calculus to figure out the non-linear change in spring tension.
  • Assuming a straight spring pull, the profile of the spiral cam must be a parabolic spiral to correctly adjust the pull rate to the increasing load imposed by the spring as it is extended (see Hooke's law).
I've given you just a start on how the calculation might be done, but it's almost midnight and I have to get some sleep. Let me think about this for a day to see if I can sort out the equations.

Q: Mark,
Spring powered flipper guy again. Would this be a better layout for the components?

A: Yes! Much better -- a proper snail cam and a nice straight spring extension. I'm not certain that the cam profile is entirely correct to provide an even loading on the motor, but the general layout is very nice.

Q: Also would using conservation of energy be a good approximation (i.e. rotational work converted into spring energy)?

A: Your question beat my update to the prior post. Calculating the rotational work needed to arm the spring and backing into the required torque is the path I decided would be best. Assuming that you get a straight pull on the spring and get the spiral cam correctly profiled to even out the torque requirement to a constant level, we can derive the required torque from the rotational work formula:

Work = Torque * Angle Thru Which the Axle Rotates [in radians]

In our case the work is the extension of the spring, which takes place over the course of one revolution of the axle. The spring extension force is a linear function (Hooke's law), so we can calculate the average force required to extend the spring as follows:

Average Spring Force = (Spring Force at Start + Spring Force at End) / 2

That plugs into the basic linear work equation [Work = Force * Distance] like this:

Work = Average Spring Force * Distance Extended

Getting close now. One full rotation is 2𝜋 radians, so with a little algebra the rotational work formula transposes to our needed torque equation:

Torque = (Average Spring Force * Distance Extended) / 2𝜋

Example: if you're extending a spring with a rate of 200 ounces per inch from its rest state (zero force) to one inch of extension with a parabolic snail cam over one full rotation, the torque required will be:

Torque = (((0 oz + 200 oz) / 2) * 1 inch) / 2𝜋 = 100 oz-in / 6.283 = 15.9 oz-in

There are some losses due to angular inefficiencies and rubbing friction with the cam, and you will want the motor to deliver that level of torque at some reasonable motor speed to extend the spring quickly. I'd use a gearmotor with a stall torque about three times the formula result to avoid bogging and let the motor operate up near its horsepower peak.

Addendum: There is some discussion out in the forums about the amount of force required for an effective spring flipper. There are too many variables (force, stroke, geometry, angle, expectations, ect.) to give a good calculated result to that question. Further, the dynamics of a spring flipper are different than those for a pneumatic flipper and cannot be directly compared. I'd suggest mocking up the weapon geometry and trying differing spring rates on a proxy opponent to find a result you like.

Q: Sewer Snake [Hebei, China]

A: [Mark J.] Well, art is art, isn't it? Still, on the other hand, water is water. And east is east and west is west, and if you take cranberries and stew them like applesauce, they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know. 1

Q: I think this compatriot from Hebei don't understand how the weapon of Sewer Snake works. He asked this question in Baidu Tieba and it was our administrator that recommend Ask Aaron. He is still confused with it because you explained it in English. Would you please draw a design diagram to help this guy, Mark?

By the way,there will be a robot combat competion held in China. That is why there are so many Chinese that ask questions. I am afraid that it will be a competition between institutions of higher in China. It is said that it will be broadcast on TV in the third quarter next year to schedule. [Jiangsu, China]

A: There are two nicely detailed photos plus a full description of Sewer Snake's weapon written by the builder himself in this post farther down in this archive. I can't do much better than that!

I think the best thing I can do is repeat the advice I give to all first-time robot builders -- Keep it Simple! Adding a complex weapon system will multiply the number of things that can fail and take you out of the tournament. See Frequently Asked Questions #8.

I'd appreciate updates on the Chinese combat robot tournament -- could be interesting!

  1 Groucho Marx, 1930

Q: i am building a 15 pound robot for bots IQ and i am using a beater bar as my weapon. i would like to know what is the best way to make it a dead shaft [Manchester, Connecticut]

A: [Mark J.] If you want a non-rotating (dead) shaft for a rotary weapon the bearings must be mounted to the weapon itself rather than the chassis. A 'beater bar' weapon typically does not have a lot of room to mount a bearing without excessively weakening the structure of the bar.

A common solution is to use needle roller bearings that add very little to the diameter of the supported shaft. This is the design used in the beater for the beetleweight 'Weta' kits (photo at right). Additional detail on the Weta beater bar can be found here.

Q: Hey Mark, quick question:

When two horizontal spinners meet weapon to weapon, which one wins? [Woodburn, Oregon]

A: [Mark J.] Generally, the one with the greater tip speed has the advantage.

Tip Speed (MPH) = RPM * Pi * Weapon Diameter (inches) * 0.000947

Q: Hi, Aaron, I am from Chinese, I wonder the statistics of the motor of the Tombstone, and the speed of the weapon! [Heilongjiang, China]

A: [Mark J.] At ABC BattleBots season 1, Tombstone's weapon was powered by an Motenergy ME0708 Motor (also called 'ETEK-R') at 59 volts. The motor produces nearly 15,000 in-oz of torque at stall with a peak output just over 16 horsepower. Speed is around 4000 RPM. The bar weapon is driven by chain and sprocket without speed reduction.

Previous versions of heavyweight bar spinners from Hardcore Robotics have used the mighty 'Perm PMG-132' motor which produces close to 26 horsepower and 3000 RPM at 59 volts. Hardcore currently uses the ETEK-R because they have found it to be more durable in combat. Given the price of the PMG-132 it's entirely understandable that durability is a key consideration.

Q: Hi I'm participating in robo games though I'm from India can u pls tell me any energy calculation software or the calculations required to design the best drum in a 75kg category robot.what motors do u smudges [suggest?] the for drive and weapon [Tamil Nadu, India]

A: [Mark J.] I'm very puzzled by recent questions from India. Safety concerns prevent 'Ask Aaron' from accepting new questions from builders competing in Indian combat robot events (click here for details) -- but the questions being asked are ones that have been answered many, many times here.

If you're not willing to spend five minutes searching the Ask Aaron Archives and FAQs for answers to these very common questions, I don't think you have the right mind set to build a combat robot.

I'll make it really easy for you this time -- Start Here.

Q: What is the minimum energy for a drum for thrashing a 75kg robot?and what is the length of teeth I should use [Tamil Nadu, India]

A: [Mark J.] Two things:

  1. We request that you search the Ask Aaron Archives before submitting a question to see if it has already been answered. Weapon energy requirements and correct tooth length have been discussed here multiple times.

  2. As noted in the submittal box into which you typed your question, due to safety concerns Ask Aaron is not currently accepting questions from builders competing in Indian robot combat events -- click here.

Q: I've designed and built a featherweight combat robot with a drum weapon.

The drum is mild steel, 75mm diameter with a 12mm wall thickness and 160mm long. End plates are 10mm aluminum and the full-length live shaft is 22mm aluminum [alloy unknown]. The weapon motor is an Ampflow E30-400 with a gear ratio of 0.6:1. Drum energy storage is 1100 joules.

I'm concerned that my aluminum shaft will not be able to withstand the impact of the drum and that it will crack and break. Would you advise that I keep it the same or change it. I only have 300gms left to add in the robots weight.

Please reply soon as the event is in 3 days. [Eastern Hemisphere]

A: [Mark J.] Three days 'til your event? Since you don't know what alloy your weapon shaft is, we'll have to resort to testing.

  • Find a safe testing environment.
  • Spin up your weapon and charge at full speed into an immoveable object.
  • Spin back up and hit it again.
  • If the weapon survives you're good to go.
  • If the shaft bends/breaks you've got three days to repair and
I know builders who throw their new 'bots off the roof to see how well they survive a good impact. Better to break your bot now than in the tournament. I suspect that a 22 mm shaft is adequate for an 1100 joule weapon, but I don't know the details of how your shaft is supported. Test it and find out.

P.S. - You didn't ask, but your drum design needs work. Read thru this archive for pointers on drum design and discussions about 'bite' before you build your next robot.


A: [Mark J.] Click here.

Q: Why doesn't Manta's flipper work well? [Chevy Chase, Maryland]

A: [Mark J.] Are you talking about the 'Manta' with these achievements?

  • Fighting Robots European Championships 2013 Runner-Up
  • Fighting Robots UK Championships 2013 Runner-Up
  • Robot Wars Winter Tour 2013 Champion
  • 3rd in Fighting Robots UK Championships 2014
  • Robots Live! - Whitwick 2015 Winner
  • 4th in Robot Wars World Championships 2015
Seems like a fine robot to me. Nothing wrong with their flipper.
Q: Hi, Mark, I want to ensure that will it be safe to use a 1 MPa ram in a 7 MPa pneumatic system? If not, what kind of ram the competitors are using? I didn't find any ram is designed to hold such pressure. [Guangdong, China]

A: [Mark J.] 

Running 7 MPa pressure (70 Bar, 1015 PSI) with a 1 MPa rated ram turns it into a bomb -- metal shards embedded in the walls and in anyone so unwise to be present. Don't even consider such an action! Your 'competitors' are most certainly not using 1 MPa rams and valves at 7 MPa.

'Ask Aaron' does not offer advice on the construction of pneumatic systems exceeding 10 Bar pressure. 'Full pressure' pneumatic systems are extremely dangerous for inexperienced builders. Even with the correct components a small mistake can be fatal. There are components that can operate at 70 Bar, but for safety reasons I'm not going to provide info on where to find them. By the time you're experienced enough to know how to use them you won't have to ask me where to get them.

Read the Team Da Vinci pneumatics guide for information on combat robot pneumatic systems.

Q: Sir I want to know which motor is good for rotating an 2-3 kg weapon? [India, masked as a SoCal ISP]

A: [Mark J.] Click here.

Q: hello sir, i m looking forward towards a vertical spinner. I just want to know which is the better option to go for, smthing like "Electric bogaloo" or "the witch doctor". What are the +ve and -ve aspects of these bots??? please help me out. [India]

A: [Mark J.] Click here.

Q: In the archives drum weapons are mentioned numerous times but no where does it actually [show how to] attach the drum to the [live] weapon shaft. Could you explain that to me? [Greenville, Pennsylvania]

A: [Mark J.] Live shafts are not common for drum weapons. A non-rotating dead shaft can be made into a structural member of the chassis to strengthen the weapon mounting area. That's a major plus.

If you have a good reason to use a live shaft, the method of attaching the drum to the shaft is similar to attaching drive pulleys or wheel hubs to shafts:

  • Heavier 'bots with shafts large enough to be broached for a keyway can make good use of that technique to lock the drum and pulley to the shaft.
  • Smaller 'bots can attach the belt pulley to the drum with screws and use the pulley hub to clamp to the shaft. If using a set screw hub, be sure to file a flat on the shaft and use a threadlocking compound to keep the screw tight!
I think that in general you'll do better with a dead shaft.

Q: Second, how in an insect class could you make a weapon with a dead shaft? Every way I can think to configure a dead shaft requires an unusablely large weapon hub/reduction pulley to attach to.

A: What do you consider to be 'unusably large'? Take a look at the beater-bar weapon made for the 'Weta1' beetleweight kit. The weapon rides on small outer diameter needle bearings to reduce the size of the bearing support, and the belt pulley is attached directly to the weapon.

It's also possible to eliminate the belt drive and embed a small outrunner motor into the weapon drum with the entire drum/rotor assembly riding on a dead shaft. I'm not a big fan of direct-driving a spinner, but the new 'Saifu 2' antweight kit powers their compact spinner in this manner. Note that it does require some good machining skills to implement.

Q: How does a fbs [full body spinner] prevent itself from getting fillpped by a wedge in a small arena? [Pennsylvania]

A: [Mark J.] Often, it doesn't. If you look thru the 'Ask Aaron' archives you'll see that we repeatedly warn about big spinners in small arenas. The standard tactic for a wedge vs. spinner is for the wedge to 'box rush' the spinner and trap it against the arena wall before it can spin up to speed. It's quite effective. There are only two practical things a FBS can do to counter this:

  • design around massive weapon motor power to spin-up before your opponent can get to you; or
  • work up enough drive power and control to be able to dodge your opponent's charge while you spin-up.
Another worry: a good hit by a FBS can send it ricocheting off its opponent and into the arena walls, turning itself into a self-propelled runaway pinball. Under the best conditions a FBS is nearly as dangerous to itself as it is to its opponent, and this is greatly magnified in a small arena. Audiences and builders love full body spinners, but they're a real crapshoot to drive.
Q: To comment on the number of teeth of beetle fbs, would it not be better to have a lesser bite given the size of the arena and that you will most certainly be box rushed? Less teeth would make it easier to balance and when impacted the shell rpm would not drop as severely. If the impactors would be sharpened it would increase the bite into plastic or thin sheet metal. [Pennsylvania]

A: [Mark J.] I don't think you have a good understanding of 'bite'. As defined in section 6.3 of the RioBotz Combat Tutorial:

"The tooth bite is a distance that measures how much the tips/teeth of the spinner weapon will get into the opponent before hitting it."

There are numerous posts about 'bite' in this archive. Some highlights:

  • Fewer teeth give better 'bite'.
  • Lower rotational speed gives better 'bite'.
  • Higher closing speed on your opponent gives better 'bite'.
  • A single-tooth spinner has the best 'bite' but can be a challenge to balance.
  • The ability of sharpened impactors to 'dig in' to a soft or deformable surface is not the same thing as 'bite'.
  • If you have poor 'bite' at low RPM you'll have really awful 'bite' at high RPM and you never will get a good hit.
  • A weapon with good 'bite' will be effective at transferring destructive force to its opponent and will lose nearly all of its speed on impact. Less speed drop = less energy transfer = less damage to opponent = less desirable outcome.
  • If you're being 'box rushed' you're going to have great 'bite' due to the low rotation speed but little stored energy. You should gear and power the spinner to provide effective energy storage quickly while retaining good 'bite'. Evasive maneuvers while your spinner gets up to speed is a viable strategy.

Q: Hi there,

Regarding wedge design. I know that offensive wedge serve well as a spinner killer, but I would like to know if it will launch another wedge or pushers similarly? My wedge will be at an angle of 35 Deg from a initial angle of 15 Deg from the scooping tip , it will be moving at a speed of approx 2.5 m/s. Please note that I am building a sumo robot, not a combat robot. [Singapore]

A: [Mark J.] You're not going to get a 'launch' out of a shallow two-step wedge at that speed. Spinners provide most of the energy to launch themselves when they hit a wedge.

If you can get under your opponent's wedge, they may just be able to drive up and over your shallow wedge. Put a 'stop' barrier/lip at the top of the wedge to keep them from doing that!

Q: Hey Mark. I'm the beetle spinner guy, your answers have been extremely helpful, but I still have a few more questions:

1: There's been a lot of talk about wedges that can counter spinners, but what about the reverse? Whats a good weapon shape to counter wedges - specifically, a good weapon shape to counter the two big beetle brick kits, 'Trilobite' and 'D2'? If it wasn't important, then Ray wouldn't have three or four weapon bars to switch out for a given opponent.

2: On the same topic, whats a good blade shape for dealing with drums/egg-beaters('Grande Tambor' being the big one)? I know staying below or above the up-sweep zone is important, but other than that, whats a good shape for hitting drums where it hurts? [Molalla, Oregon]

A: [Mark J.] Ray Billings' mind works in mysterious ways, and I don't know what specific logic he uses to select a specific chunk of metal to beat up his opponents (photo of 'Last Rites' bars at right). He claims he has a method, but he could be using tarot cards for all I know.

You really don't have a lot of options for blade variation with your "huge horizontal" beetle bar weapon:

  • You can install a sharp-edged blade to cut into soft armor materials like UHMW or aluminum for better 'bite';

  • You can drop in a flat-edged blade against harder surfaces like steel or titanium to deliver maximum impact;

  • You can swap in a titanium blade to make pretty sparks against hard steel alloys just for show;

  • You can mount a lighter blade against a quick opponent in a small arena to get a faster spin-up time.

Bottom line: There isn't much you can do with swapping blades to go against specific weapon types. All you can do is to consider what surfaces your blade is actually going to impact on your opponent and go blunt or sharp, and then guess on how quickly you need the weapon to spin-up. Sparks are optional.

Q: is axe more effective or a hammer in robowar? [Bangalore, India]

A: [Mark J.] Neither is particularly effective. Of the two, I'd use a hammer -- axes can penetrate and get stuck in your opponent, which can cause all sorts of problems.

Q: Can you explain the gyroscopic effect on a bot??? [Pasadena, California]

A: Not if you put three question marks on it -- that makes it three times as hard. Let me try editing...

Q: Can you explain the gyroscopic effect on a bot?? [Pasadena, California]

A: Nope, still too hard...

Q: Can you explain the gyroscopic effect on a bot? [Pasadena, California]

A: Ahhh, that's got it! Much better.

[Mark J.] Now, are you asking for an explanation of:

  1. The gyroscopic effect that lifts one side of a vertical spinner robot when it turns; or

  2. The effect that gyroscopic precession robots like 'Gyrobot' and 'Wrecks' use to walk?

Technically it's the same effect, but the explanations and diagrams are different. I kinda hope it's the first choice because I had that diagram drawn and the answer largely complete before I considered the alternative question. I'll put that diagram in here just because it's kinda pretty and I'd hate to see it go to waste.

Write back and let me know which question you're asking.

['Pasadena' never wrote back -- anyone else curious about this?]

Q: i plan to do a 4 bar lifter and i want 2 know if i powered it by 2 linear actuator who can provide 150lb of maximal charge, does it mean i get 300lb of power out of them? [Quebec, Canada]

A: [Mark J.] Yes, but that doesn't mean that your lifter has a 300 pound capacity.

  • Two 150 pound force actuators give you a maximum 300 pounds of force input to the 4-bar system.

  • The actual output force of the 4-bar lifter will vary with the attachment point of the actuators and the geometry of the 4-bar mechanism. The actuators for the 4-bar lifter of heavyweight 'BioHazard' provide more than 2,800 pounds of force to lift a 220 pound opponent. You will benefit from reading the BioHazard Mechanical Design page.

  • The force analysis of a 4-bar system is complex.

  • Actuators slow down as the load on them increases. An actuator that has a speed of 4 inches per second and a 100 pound force rating will slow to half that speed when loaded with 50 pounds, a quarter of that speed at 75 pounds, and will come to a full stop at 100 pounds of load.

There are many posts about linear actuators and designing 4-bar lifters in this archive. I'd suggest reading them.

Side note: 'pounds' is not a measure of 'power'; 'pounds' and 'newtons' are measures of 'force'. If you're building combat robots you should learn the difference. Start here.

Q: Hey just wanna know i plan to do a heavyweight 4 bar lifter wich will looks very much like storm 2. My question is: the front ofthe robot will have a angle of 45 degree with the design i have in mind, i want to know if its too much to be able to push opponent? [Quebec, Canada]

A: [Mark J.] Well, 'Storm 2' had a 45 degree front wedge. It worked pretty well for them.

Q: i just wanna know if a 45 degree wedge will be able to ''deflect'' a spinner like defensive wedge do?

A: Take a look at successful 'spinner killer' wedges. They're all much shallower than 45 degrees.

Q: What is the best active weapon against a vertical sawblade? I know nonactive scoops and wedges do well (especially lifters) but for an active weapon would a faster spinning vertical spinner or a horizontal or full body spinner be more effective? I know that a great driver and a solid strategy with a nonactive will do extremely well but I want the match to be more exciting than a pushing match. [Panama City Beach, Florida]

A: [Mark J.] Several thoughts:

  • Team Run Amok offers advice to builders who want to win matches. It seems you already know how to do that.

  • There aren't many vertical saw spinners around. Why are you designing specifically to fight one?

  • Traditionally, vertical disk/saw spinners are vulnerable to horizontal spinners.

  • When two vertical spinners go weapon-to-weapon, the higher 'tip speed' wins. Tip speed is calculated as RPM times circumference.

  • A lifter is an active weapon.

In my book an ugly win is a whole lot better than an exciting loss. My advice is to keep it simple and 'build to win'.

Q: I am participating in IIT Guwahati robowar so for this, i am making a robot. In my design, for my weapon i am using a motor High Torque Robo war DC Motor 15000 RPM with model no RM0939 so is it wise to use this kind of motor. I will be using it for the rotating a drum with cutting wheels in it. [India]

A: [Mark J.] You've told me nothing about the weight of your robot or the size of your weapon, and the RoboMart website gives no specifications for the motor other than "15000 RPM". There is not enough information for me to answer your question -- the hamburger is bad.

Best I can tell, the motor is similar to the BaneBots RS-775, and it will require substantial gear reduction to be useable for either weapon or drive use. The statement on the RoboMart page that says the motors can be used with '69 mm or 87 mm wheels' is entirely misleading.

Guidance on spinning weapon design and motor selection is available here at Ask Aaron. Start with the Spinning Weapon FAQ.

Q: Is there a way to wire a brushless motor and your DC motors so that you can drive with one stick and control the rpm of the [weapon] motor with the other eliminating the need for rpm reduction? Is it wise to direct drive a weapon in this fashion? [Brooklyn, New York]

A: [Mark J.] Yes, that's a simple control option. No, it's not a good idea.

Speed reduction serves two purposes: it reduces speed and increases torque in equal proportions. Torque is what allows the weapon to run up to operating speed quickly. Without the torque multiplication offered by mechanical speed reduction weapon spin-up time greatly increases, as does current consumption, motor heating, and possibly a brushless motor issue known as 'cogging'.

You can compensate in small robot classes by using a much more powerful motor, but that costs weight and still leaves the high mechanical stresses on a motor directly connected to the weapon. My advice is to follow conventional design and belt drive your weapon with an appropriate speed reduction.

Q: I saw a post in the weapons archive about full body spinners and dead vs live shafts. After reading it I still am not sure what components are placed where for each. For a live shaft there needs to be a bearing on the bottom that the shaft goes into then a spacer to keep the pulley from moving up and down (I think, but am not sure how a spacer would do such a task), then the pulley (I am not sure how this connects to the shaft), then a top bearing and a weapon hub above that (again how does this connect to the shaft). [Duvall, Washington]

A: [Mark J.] The weapon and pulley hubs are ideally connected to the shaft by a keyed joint that locks rotational motion plus a pin or clamp that prevents the hub from sliding along the length of the shaft. A small robot might use just a clamping hub for this purpose. Do NOT attempt to use set-screw hubs for this application!

The spacer does not prevent just the pulley from moving up and down -- it prevents the entire pulley/shaft assembly from moving up and down because the pulley is locked to the shaft.

Q: For a dead shaft, why are 2 bearings used if the shaft does not spin? Could not the shaft just sit in the base, use a spacer, the pulley with a bearing in the center that just connects to the top (the pulley is the weapon hub). Sorry if this is confusing but well... I am confused. Thank you.

A: A single bearing is very good at absorbing a straight 'radial load', but an impact load applied at a point along the spinner shell not directly radial to the bearing creates a 'moment load' or 'torque' on the bearing that will destroy it. Using two bearings spaced well apart in an elongated hub redistributes the un-centered radial load into pure radial loads on the two bearings.

The dead shaft must also be firmly supported at two separated locations to help absorb the 'torque' placed on it by an impact -- once at the base and again as close to the hub as possible. The forces generated by a full-body spinner impact are as great on the support structure of your robot as they are on your opponent. Build strong!

Q: Do you know of any highly detailed (in pictures and text) build logs of full body spinners? For dead and live shafts? I read your response but still cannot picture the working solutions. Thank you again.

A: This is a very tough topic for the short answer format here at Ask Aaron. Some photos and drawings may help. There's a thread on the Robowars Australia forum that has several links to photo collections and a couple of build reports. I hope my short notes and the photos combine to get you that 'aha!' moment.

Q: hi aaron imagine a situation where two drum bots rotating at same rpm (say 5000rpm) but 1st bot having a drum dia 80mm,tooth height 50mm & drum length 300mm and other bot having 120mm dia, tooth height 35mm, and drum length is 200mm my question to is
(1) if the smaller drum bot is moving with 1m/s and bigger drum bot is sationary
(2) if the bigger drum bot is moving with 1m/s and smaller bot is stationary
(3) if both the bots are moving relative to each other

[Chhattisgarh, India]

A: [Mark J.] You defined two drum weapons and set up three scenarios, but you never actually got around to asking a question. Do you want to know which 'bot will have the advantage if those drumbots go 'head-to-head' in each of those situations?

  • Anytime two drumbots go 'head-to-head' the drum with the greater tooth tip-speed has the advantage. At 5000 RPM the smaller drum has a tip-speed of about 47 meters/sec and the larger drum has a tip speed of about 58 meters/sec. The larger drum has the advantage at any closing rate.

  • The calculation for weapon 'bite' depends on the closure rate of the two bots. It does not matter whether one or both of the robots are moving, it's simply the sum of their speeds relative to each other.

Q: Hi, lifter guy again. I downloaded 4 bar but I am not sure what values to input. You called it right, the picture is how I would like to design my lifter but I do not know how to input those values into a "4-bar" as pictured in the program. Thank you as always. [Bellevue, Washington]

A: [Mark J.] Bad timing... My home computers can't run the T.i. Combat Robot 4-Bar Calculator due to incompatible operating systems, and I won't be in my office to use the laptop I keep there that runs the calculator for 5 days. I can't immediately confirm what I'm about to suggest, but let me give it a shot:

  • The line segments in the diagram at right are labeled with letters corresponding to the inputs for the T.i. Calculator.
  • Segment 'J' has a zero length.
  • Length measurements should be in inches.
  • Weight should be in ounces -- 64 ounces if you plan on lifting the entire 4-pound weight of your opponent.
  • Calculations should be run as a 'Rear Bar' lifter.
The layout is somewhat unusual and the T.i. Calculator may balk because of the odd angles. If it won't run from the initial 'lowered' setting you can try inputting the 'A' angle for the elevated position and see what happens. Sorry I can't pre-check this for you.

Q: Hi mark...lifter guy again. Sorry to annoy you so much but I cannot get a single piece of useful information or any information infact out of 4-bar. Every time I attempt to "calculate" it says that I either need to change angle A or bars F and G are too short or too long. I tried for an hours adjusting the input values but with no success. Thanks for your help in these troubled times.

A: Troubled times indeed.

I had feared that the T.i. Four Bar Calculator might have trouble with this layout. A conventional 4-bar used in a lifter has the powered link pushing the 'G' bar forward to raise the mechanism. This design has the 'G' bar stationary and the powered link pushing the 'E' bar forward. That's backward logic to the calculator and it did balk on you.

Time for 'back of the envelope' calculations:

If you simply put a 3" long arm on the servo output shaft, how much torque would the servo require to provide 4 pounds of force at the end of that arm?

  • Four pounds = 16 ounces * 4 = 64 ounces
  • 64 ounces * 3 inches = 192 oz-in torque
The HS-5585 servo stalls at 236 oz-in torque, so it is capable of providing more than four pounds of force under these conditions. As long as you have a 4-bar design that does not require more torque than a simple bar lifter you'll be fine. How can we assure that?
  • Keep the 'F' bar shorter than the 'H' bar
  • Keep the angle between the 'H' and 'E' bars as close to 90 degrees as you can throughout the range of lifter motion.
If your layout looks more-or-less like the diagram you'll be fine. If your 'H' bar is 1.5" you'll want your 'F' bar to be at least 0.75" to get enough motion from the lifter. I can't provide you with a nice graph of torque requirement, but total lift speed when fully loaded will be somewhere in the 0.6 second range. Remember that you'll usually be lifting less than 4 pounds, because you're only lifting one end or side of your opponent -- but it's good to be prepared to lift the whole 4 pounds if the opportunity presents itself!

Q: Hi sir,what will be the best curve radius for the scoop to tackle out Indian drum bots my bots height is 100mm in 155lbs category [Mumbai, India]

A: [Mark J.] A 'spinner killer' scoop should ideally have a radius just a bit greater than the radius of the weapon it will face and should be mounted to match the curve of that weapon. It's better to have the radius too large than too small, so since you will face a variety of drum sizes you should design for the largest weapon you expect to encounter.

Q: Hi There, I am making a shell spinner. Ampflow A28-400 is used as weapon motor and A28-150 as driving motors. Contactor i am using is 586 White Rodgers solenoid. My question is how should i select RC Interface? [India]

A: [Mark J.] Selection of Electronic Speed Controllers (ESC) for your drive motors is covered in Frequently Asked Questions #21.

Selection of an R/C switch interface to control a weapon solenoid is covered in the Ask Aaron Solenoid FAQ.

Q: In your insect archive it is discussed that you need to "run numbers/do math" to calculate different aspects for lifters. How would one do this? Can this be done for 3 and 4 bar designs? This would be in a 4lb robot competition. Thank you. [Morris Plains, New Jersey]

A: [Mark J.] First, some nomenclature:

The internet is full of references to '3-bar' mechanisms, but this is a misnomer. What they're calling a '3-bar' is actually a '4-bar' in which they fail to count the base as the 4th bar. A true 3-bar mechanism forms a triangle and has zero degrees of motion freedom - it won't move at all. Lifters use a 4-bar mechanism. Don't spread the incorrect 3-bar usage.

The math is covered in multiple posts in the Ask Aaron archives:

  • Simple Lever: see this previous post in this archive for a discussion of calculating the torque requirements for a lifter motor attached to a simple lever. A more complete math analysis is available at the HyperPhysics website.

  • 4-Bar: there are more than a dozen posts on 4-bar lifters in this archive that include discussions of power options and design tools. Mentioned many times in those posts is the T.i. Combat Robotics 4-Bar Simulator -- a very useful tool to assist in designing 4-bar lifters. It will 'run the numbers' for you. You can find other tools with an internet search for '4-bar mechanism calculators'.

Q: Also how are typical [4-bar] servo lifters built? Thanks.

A: Plenty of links, diagrams, and photos for insect-class servo lifters in the Ants, Beetles, and Fairies archive.

Q: Do you have any good sources for building lifters so I don't constantly bombard you with questions? Can these sources please include common designs? Thanks. [Baden, Pennsylvania]

A: [Mark J.] For insect class lifters, a page search for 'lifter' in our Ants, Beetles, & Fairies archive returns more than 100 hits.

Larger robots? A page search for 'lifter' in this archive returns more than 200 hits! We've answered more than 5100 robot questions -- we are the good source.

Q: Hey Mark. I'm looking to mount a 3" Ampflow motor horizontally for use with a horizontal spinning weapon. I need a right-angle gearbox. Team Whyachi manufactures one (the TWM3R) but the obvious downside is the cost. Assuming I have access to a CNC mill, is this realistically something I can try to manufacture myself? I'm worried that controlling the exact positioning of my bevel gears will be difficult, and considering that the input rpm is in the realm of 10,000 rpm, I definitely don't want my gears to jam up when this thing gets spinning. Thanks! [Lansdale, Pennsylvania]

A: [Mark J.] I don't know the level of your machining skill so I can't comment on your likelihood of success. It is certainly within the capacity of a good machinist to produce a right-angle gearbox using bevel gears, but perhaps only after a few failed attempts. The box will take very large off-axis loading from weapon impacts, the gears must me positioned accurately, and material selection is critical. I'm going to predict that you will eventually regret your decision to not purchase the proven Whyachi gearbox.

Q: Hi Mark. My single tooth spinner spins at 3500 rpm. My drive motors run at 300 rpm with 20 cm dia tyres. So by calculating how much time it takes for one rotation of spinner and how much my bot moves forward in that time I get the tooth size. It's coming to more than 5cm!! Is it safe to keep that much tooth length? [Pune, Maharashtra, India]

A: [Mark J.] That is a very tall impactor! I'm assuming that this is a drum weapon. A taller tooth can put more strain on the junction between the tooth and drum, but there are a few things to consider...

  • The calculation provides the maximum tooth height that can be used under ideal conditions.

  • The proper speed to use in the calculation is the combined closing speed between your 'bot and your opponent. It they're charging toward you at the same speed you're charging at them, the closing speed is twice what you're calculating. You might be able to use a 10cm tooth!!!

  • Have you used the 'Acceleration Calculator' tab in the Tentacle Drivetrain Calculator to check the speed your 'bot can actually achieve in a charge half-way across the arena? That might be well less that the speed you think you have.

  • I've seen a lot of Indian robot combat videos and I don't recall ever seeing a high-speed charge attack with a drum weapon. What I generally see is two bots slowly closing to within about a foot of each other while their weapons spin up. Then one 'bot pops forward into the other. Closing speed is maybe 2 MPH. That will use MUCH less tooth height.
You can use a 5cm tall tooth if you like. I'd suggest making it quite broad at the base, and providing a strong anchorage into the drum. Personally, I'd make it about half that height. Even if you really do plan high-speed attacks and your drive motors can give you that acceleration, you'll still get good 'bite and have a stronger weapon.

Q: I ran some numbers on the [Run Amok Weapon] spreadsheet. My weapon spins up to 95% speed in 3.8 secs, making 4005 joules. Is this spin up and energy ok for small arenas like 20ft x 20ft 60 kg bot?

A: The energy level is adequate for a 60 Kg robot, but...

  • If you're going to be charging across the arena at full speed at the start trying to use that tall impact tooth, then it isn't a quick enough spin-up time.

  • If you're going to wait for the weapon to spin up while slowly closing on your opponent, then my argument for a shorter impact tooth makes more sense.

Q: Also, I am not getting S7 tool steel for teeth. Is EN24 with 54 hrc hardening ok? Or can you suggest the hardening. Will this material be tough and impact resistant for teeth. Thanks.

A: I've discussed EN24 steel for impact teeth previously -- see this post in the Ask Aaron Materials archive. If you decide to use EN24, I'd suggest making extra teeth and being prepared to replace them quickly when they blunt or break.

Q: Hi, Mark.

I've been playing around with your Spinner Spreadsheet, and I really would like to thank you and Team Run Amok for creating it. I've found it incredibly useful; I was doing all the calculations by hand before. I have a couple of questions: First, does the figure for the battery consumption include a safety factor to ensure the batteries won't drain before the match is over, or do I need to add a bit more to ensure I can make it through the full 3 minutes?

One more question, do you think you could create a similar spreadsheet for spinners powered by internal combustion engines? If that's too cumbersome of a project, could you direct me toward the correct calculations for determining the weapon kinetic energy vs. time data table, as well as the gas consumption in 3 minutes?

Thank you very much for your advice. [Austin, Texas]

A: [Mark J.] I'm always pleased to hear that builders find our modeling tools useful. I appreciate you taking time to comment on our Excel Spinner Spreadsheet.

The spinner battery calculations are 'conservative' in that they assume a worst-case energy consumption for each event. For example, they assume that a weapon 'hit' will bring the weapon to a full stop and it will need to spin back up to speed from that state. In an actual match it is unlikely that every hit would do that, so you can think of the difference as a 'safety factor'. That said, I like to add about 20% extra to the calculated battery consumption for both the weapon and the drive train for 'unexpected events'.

About ICE spinner calculations:

  • Permanent magnet direct current (PMDC) electric motors are a performance modeler's dream. They have a linear torque 'curve' and peak horsepower is always at 50% of unloaded RPM. That makes calculation of their power output versus a given load a relative snap.

  • The torque curve for an ICE is not at all linear and the power peak can move higher and lower in the RPM range depending on intake and exhaust cam timing variables and manifold design. The instantaneous torque available to spin the weapon is constantly varying in a non-linear fashion!

  • Two ICE each rated '10 peak horsepower' could provide VERY different spinner weapon performance due to differences in their torque curves. See this article on automotive ICE power curves.

  • An additional complication comes from the centrifugal clutch that ICE spinners use to engage power to the weapon. How quickly that clutch engages and at what RPM has a large impact on the performance of the weapon.

  • The above factors plus carburation settings and atmospheric conditions (temperature, humidity, barometric pressure) also have an impact on fuel consumption. The good news is that it's simpler to pour a little more gas into the tank than it is to cram a zillion more electrons into the battery.

In very general terms, compared to a PMDC motor of similar peak power an ICE will be slower in accelerating a weapon of given moment of inertia over the first half of the RPM range due to lower torque and clutch slippage. Acceleration from mid to top speed will be quicker due to greater torque in the upper half of the ICE power range. You can actually see this when watching an ICE spinner like 'Icewave' -- painfully slow initial acceleration with a big burst of speed as the engine reaches its high RPM power range.

As far as actually calculating ICE spinner performance, I'm afraid that you're on your own. Too many variables and non-linear calculations for me. I can't even give you a pointer to equations that might be of use. ICE spinners are much more of an art than a science.

Q: Hello mark . I have put my weapon values in the spinner spreadsheet for my 60 kg vertical spinner . it shows that it stores 4137 joules at 2.58 sec at 63% rpm (2250). total energy is 9406 joules, about 8 secs for complete spin rpm . will this be ok for a arena of 15ft x 15ft . or should I reduce the spin up time ? but then I will loose energy . the weight wih endcaps is 14.7 kg . Also I wanted to ask how broad the teeth should be .. not the projection out from the drum or the length along the drum , the breadth . thanks [Pune, Maharashtra, India]

A: [Mark J.] That's a VERY long spin-up time for so small an arena. It will put a VERY great load on your weapon motor. Increase the reduction ratio, lose some weapon mass, and/or increase motor power. You've got energy to spare.

Impactor breadth depends on the material from which it is made, the temper applied to that material, the support provided to the impactor, and the anticipated impact energy. Don't ask me to run those calculations for you -- I'm not running a free engineering service. Look at impactors on comparable spinners for guidance. Make it strong. If it breaks, make it stronger.

Q: What type of shaft is typically used for overhead spinners and fbs? How are they mounted? I know you had a diagram on here somewhere but I cannot find it again. [North Carolina]

A: [Mark J.] The diagram appears a couple times in this archive, but I'll reprint it for you here. A typical layout (as shown) is a live shaft supported by well separated bearings above and below the pulley/sprocket. If the pulley is pinned to the weapon shaft and tubular spacers are inserted to take up the extra space between the bearings, the pulley will locate the shaft.

Q: Is it possible to have a spinning weapon without the [top] bearing? For the design I had in mind the top and sides would spin with a stationary base. I thought it would be about a 5/8 live shaft with a bearing on the base, a spacer, the pulley, then hubs to attach the shaft to the top. Would this work?

A: Oh hell no!! That turns the weapon shaft into a lever that multiplies weapon impact reaction force by a factor of about ten to bend the shaft and/or destroy the bearing. Basic engineering practice is to place a support bearing as close as possible to the point of the applied force -- in this case that's the top bearing at the weapon hub. A second bearing is placed as far from the main bearing as possible to control shaft alignment and nullify off-axis forces with the advantage of a long lever arm. Your weapon will transmit HUGE loads back thru the shaft to your chassis and it must be VERY securely supported. Two bearings, as shown.

Q: Does the bearing have to be mounted to a stationary surface or can I attach it to the top with out building a box just to hold the bearing? Also could I put the top bearing under the pulley? Stationary or roof mounted? Thanks.

A: You need to build a chassis box to provide stationary support the top end of the shaft. Take a look at the photos of heavyweight spinner 'Megabyte' with its shell on and with the shell removed.. Its shaft runs all the way down thru the top of the chassis box and is supported firmly at both the base and top of the box.**

Yes, you can put both the pulley and weapon hub above the top bearing. 'Megabyte' does. It isn't quite as strong as putting it under the bearing, but many spinners do it for convenience.

** 'Megabyte' actually uses a non-rotating 'dead shaft' with a combined pulley/shell hub and well separated bearings that ride on the shaft -- a workable alternative to a live shaft -- but either way the shaft MUST be well supported by the chassis at two well separated points.

Q: Hi mark , my single tooth spinner is 20 cm in dia length 20cm and thickness of 10 mm . Now now how do I attach teeth to it ? As I have calculated, The max length of tooth I can keep I around is 5 cm (calculated from riobotz tutorial , of approach speed and weapon rpm).
I have only 1 cm of thickness ... so I have to make a slot and place it only?

also ... In arenas measuring 20ftx20ft ... how much spin up time should a spinner have (60 kg war) ? I read in the spinner sheet ... smaller the arena ... smaller the spin up time should be .... [Thane, Maharashtra, India]

A: [Mark J.] Impactor attachment is covered elsewhere in this archive [click here]. The slot does not have to be very deep, just enough to provide a little edge to support the impactor and keep it from shearing the attachment bolts.

A 5 cm tooth height?? That's a VERY tall tooth. Unless your robot is blindingly fast or your weapon spins very slowly, something's wrong. Re-do your calculations. If you get the same number, send me info on your robot speed and weapon RPM and I'll check it for you.

Spin-up time: estimate the time it will take your speediest opponent to cross the arena and hit you. Watch videos of prior tournaments if available. That's your spin-up time. Cut that time in half if your strategy is to charge at them at the start. You can gain a little time if your robot is nimble enough to dodge their first attack but you'd better be able to actually get out of their way, spin around, and position yourself to attack if you're counting on that strategy.

Q: Hello,

Question 1:
I sometimes see horizontal spinners and full body spinners experience sudden bouts of instability. I am not talking about wobbling after a big hit; I mean sudden and very rapid movement seemingly without a direct cause. 'Secto' does so a few times during this video as does 'Steel Shadow' in this match. Is this a question of high center of mass? poor spinner balance about the main vertical axis? some nuance of moment of inertia about a secondary axis? some resonant structural bending?

Question 2:
Do you know of any centered horizontal spinners, or full body spinners with an asymmetric single tooth design? If designed correctly is there any reason they would be more prone to instability? [California]

A: [Mark J.] You've opened up a can of upper-class level physics worms here. Take a deep breath.

  1. That sudden change in the axis of rotation of a spinning rigid body is a 'polhode motion'. The magnitude of the polhode and whether or not it is self-damping depends on the ratio of magnitudes of the moments of rotational inertia of the three principal axis of rotation for the rotating mass. Follow the polhode link, read the description, then watch this video and see if it starts to make sense. If it does, you have a future in classical mechanics.

    I suppose this does qualify as 'some nuance of moment of inertia about a secondary axis', and the problem can be compounded by 'some resonant structural bending' -- so you were on the right track. An FBS like 'Steel Shadow' can get into even more trouble when the edge of the shell makes contact with the floor and adds a new off-axis torque element to the mix.

    I'll leave it to you to track down the equations that predict the onset of troublesome instability (start here) but there is a certain 'look' to 'bots that have this problem. 'Steel Shadow' is tall and tapered, which places more mass low than high; common in unstable FBS. 'Secto' has a blade that is wider at the center than at the ends, which increases the rotational inertia of its unstable 'middle' axis; that spells trouble.

    In this video clip astronaut Don Pettit demonstrates stable and unstable modes for solid body rotation on the International Space Station. Using a hard cover textbook, he demonstrates that it will rotate stably about the longest and shortest axis, which represent the maximum and minimum movements of Inertia. Trying to rotate the book around an intermediate axis results in an unstable rotation in which the book appears to flip-flop while it rotates.

    That's about all the help I can give without writing a thesis on the topic. Try a web search for 'rotational stability' for additional tidbits.

  2. I don't know of any 'centered' single tooth spinners, but as long as you pay attention to the polhode issues discussed above there should be no instability problem. I've seen several 'offset' asymmetric spinners that are crosses between bars and disks that I'm pretty sure would be problematical if they were 'centered', but the offset design is more forgiving of instability issues.

Q: Can you explain the difference between a dead shaft and a live shaft in term of spinning weapons? Which one is easier to build and which one is more reliable? Thanks. [Kansas]

A: [Mark J.] First, the terminology:

  • A 'dead' shaft is fixed to the chassis and does not rotate. The weapon has bearings that ride on the shaft.

  • A 'live' shaft is fixed to the weapon and rotates. The shaft is supported by bearings on the chassis.

Which design is easier to construct and/or more reliable depends entirely on the type of spinner weapon you're building.

  • A 'thin' weapon like a disk or spinning bar can benefit from a live shaft that allows the support bearings to be spaced farther apart to better control off-axis loading. This also allows the pulley/sprocket to power the shaft from a more convenient location outside the bearing supports. See horizontal bar spinner 'Fiasco' as an example.

  • A 'thick' weapon like a drum can mount attached support bearings well apart within the weapon itself. This allows the dead shaft to become a fixed structural chassis member for greater strength and rigidity. The pulley/sprocket must be attached directly to the weapon as the shaft cannot be powered. Drum spinner 'Helios' is an example of good dead shaft design.

Q: For a design like Ice Cube (scoop robot) is it better to have the rear wheels extend over the rear of the robot or not? I ask this only because it seams that it makes his fully frontal attacks ineffective because he gets flipped over due to the large wheels hanging over the rear (Video). Without the wheels extending past the rear of his robot though he could get stuck on the back unable to move. Thank you for your time. [North Carolina]

A: [Mark J.] 'Ice Cube' is designed to allow contact between drive wheels and arena floor from nearly any orientation. To take it off its wheels you'd have to lift the tail and balance it up on the scoop. Designing to be 'always mobile' is certainly a valid tactic. As long as you have a drive wheel in contact you can influence the match. If you don't you're just a passenger.

Those instances in the video where driver 'Fuzzy' Mauldin flips the 'bot over with a 'wheelie' are entirely deliberate. Look closely and the only times he does it are when the 'bot is upside-down. Flipping up back upright with a quick power blip is a very quick method of restoring the correct forward/backward control orientation that is reversed when the 'bot is inverted. It's a strength to be able to do that -- not a weakness.

Q: I've seen some builders that are running direct [weapon] drive off a brushless out runner replace the shaft of the old motor to something thats longer and, or in some cases, larger in diameter that the old shaft was. How hard is it to modify a brushless outrunner to accept a longer and/or larger shaft? [Aumsville, Oregon]

A: [Mark J.] This video shows the process of replacing a typical outrunner shaft.

If you want a longer shaft, you'll need to make or find a shaft of the new length with the correct C-clip groove and flat. Difficulty level: easy if you have the proper equipment.

If you want a larger diameter shaft, you'll need the new shaft plus:

  • Shaft support bearings sized for the new shaft;
  • Machining the bearing seats for the larger bearings;
  • Precision boring the motor bell shaft hole for a proper press-fit; and
  • A new C-clip and washer to assemble the motor.

Difficulty level: senior machinist with a shop full of tools.

[Mark J.] I'm very pleased to have a guest commentary on the topic of brushless outrunner shaft replacement -- see the previous post in this archive. Mike Jeffries from Near Chaos Robotics has experience in replacing and modifying brushless outrunner shafts and offered to share his knowledge.

The spinner mini-drum weapon he describes for 'Algos' rides on a dead shaft and incorporates the 'ring of magnets' cut from an outrunner can. The stator bearings are removed and the bore is drilled out to slide over the dead shaft. This cross-section drawing of the weapon assembly may help you visualize this. I think Mike underestimates his builder chops!

For the most basic replacement option (same diameter shaft, better material) you can often do this with just a press of some sort. Care needs to be taken to avoid warping the motor can if you do this, but it's a very easy process if you can buy shafts that are the right diameter and temper. I've had decent luck doing garage heat treating and tempering, but for some of the tougher steels you need to temper the shafts at a temperature well above what most household ovens can reach so when possible buying pre-hard shafts is the better option. The added bonus there is you don't have to worry as much about the treating process warping the shafts as they should be in decent shape when you get them.

On the more complex side of things, upping the size of the shaft takes a bit more effort. On my 1lb bot Algos I replaced the 3mm live shaft with a 1/4" dead shaft. The process for doing this was as follows:

  • Remove bearings from stator
  • Drill out stator to accept new shaft diameter (I went for a snug slide fit)
  • Remove shaft from motor can via press or similar
  • Cut away most of the end of the motor can, taking care to leave the portion supporting the magnets
  • Press remaining portion of can into the weapon assembly
  • Reassemble motor with new dead shaft using small outer diameter shims to maintain intended weapon location

Assuming the weapon itself is made to fit the weapon motor, the process at least can be done without all that much equipment. I used a lathe to shave down the motor can, though a steady hand and a cutoff wheel could do the same. The portion of the assembly done with an arbor press could also be done with a vise in a pinch.

Thanks, Mike. I second Mike's suggestion that you find pre-hardened shafts if at all possible. I personally wouldn't try to use a vise instead of a press to insert the new shaft, and under no circumstance should you try to 'tap it in' with a hammer! I know how you guys think...

If you aren't familiar with the Near Chaos Robotics website, do yourself a favor and take a look.

Q: From recent photos I see Nightmare's spinning disc's shape changed a lot in order to compete in the all new Battlebots series. One thing I've been wondering is: does that change decrease the energy that blade would store greatly when spinning?

Thank you for taking your time to answer! [Chinese Forum]

A: Nightmare's old spinning disk was made from aluminum. The new 'butterfly' disc is 1/2" titanium -- denser than aluminum. By my rough calculations the new disc has at least as great a moment of inertia as the old disc, and it spins a bit faster. Jim Smentowski is a very experienced builder who knows exactly what he's doing.

Q: hi sir i had confusion with pulley motor pulley diameter 66 mm and my weapon pulley diameter is 44mm, .. whether its ratio is 0.68:1 or 1:1.45 which ratio is correct and how i have to calculate? [India]

A: [Mark J.] The reduction ratio is the relation between the diameter of the weapon pulley and the diameter of the motor pulley:

Ratio = driven pulley : driving pulley
44:66 = 2:3 = 0.67:1

I think you have the pulleys reversed for your purpose. If you're trying to increase the motor torque and get the weapon to spin more slowly than the motor, you want the larger pulley on the weapon and the smaller pulley on the motor. That would give you a 66:44 = 3:2 = 1.5:1 reduction ratio.

Q: hello sir..sorry to say but i read the whole solenoid section..i think only 1 DPDT solenoid can be used to control both forward and reverse direction of motor..
plzz sir suggest the connection diagram for DPDT solenoid control for both drive motors and for weapon.. thanxx alot.. [Chhattisgarh, India]

A: [Mark J.] Sorry to say, but your thinking is incomplete.

A single DPDT solenoid can provide forward/reverse directional control of a PMDC motor, but there is no 'center off' position with a DPDT solenoid. Your robot would be continuously 'on' in one direction or the other. To get the required forward/off/reverse control you need the circuits already diagramed in our Solenoid and Relay Guide.

Q: Is the drum weapon shouldn't be oversize? I think the larger drum will store more energy. In fact, the ICE powered big drum weapon bot 'REDRUM' always get disadvantage when fighting with horizontal spinners.(It only has few videos,mostly versus spinner) The drum didn't give out much damage and its gyroscopic forces makes 'REDRUM' difficult to drive.However, everyone can see 'Touro Maximus' with a small snail drum won the STEM and RG2015 champion.Is that because size of drum shouldn't be as large as possible or it is because of the successful snail drum design or it just because of 'REDRUM's problem itself? [Guangdong, China]

A: [Mark J.] A great many new builders share a mistaken belief that the success of a combat robot depends primarily on the design of the weapon. This is entirely false.

The weapon may be the least important system on a combat robot. If you're not winning matches it isn't because you have a poor weapon.
Any weapon is only as good as the robot that uses it, and placing too much design emphasis on the weapon will detract from the overall performance of the 'bot. See this post in the Design archive for a discussion, and this post on the design of the 'Touro Maximus' snail drum.

About drum weapon design:

  • A larger diameter drum weapon will store more energy than a smaller diameter drum of the same mass at the same speed -- see the Spinner Weapon FAQ for examples.

  • A larger diameter drum will also exert greater gyroscopic force on the robot that may cause control problems. Total Insanity Combat Robotics has a tutorial on weapon gyroscopic forces (archived) that fully explains the issue.

  • If your attack strategy requires high-speed rushing attacks and quick pivoting turns, a small diameter drum spinning at high speed as used by 'Touro Maximus' is the correct choice.

  • If your attack strategy allows a slow and cautious approach toward your opponent without high-rate turns (common in Indian robot combat) then a slower spinning large diameter drum weapon is ideal.

Different combat environments require different designs. Do not blindly emulate a weapon designed for a combat environment different from the one in which you will compete!


A: [Mark J.] Designing to maximize energy storage is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. Please read the FAQs before asking a question.


A: The relationship between weapon speed and bite is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. PLEASE read the FAQs before asking a question.


A: Nobody has asked for an explanation, but yes I can provide.

63% of Max Generating that pretty chart in the Run Amok Excel Spinner Weapon Spreadsheet that shows weapon speed over time requires a fixed point of known speed-time. The formula to calculate that 'time constant' gives an answer for 63% of max RPM. The 63% is a logarithmic thing that would take too long to explain here; ask your physics professor. Since I have to generate that time constant anyway, I include it in an output field as a clue to math boffins that I know what I'm doing.

Once I have the time constant, a simple formula converts it to the spin-up time for other percentages of maximum speed:

Time to Speed = Time Constant * loge(1 - (speed / max speed))

95% of Max Given mechanical and aerodynamic loss, the weapon will never reach 100% of the unloaded motor speed potential, so I cut off the calculation at an arbitrary 95%. In the real world, the weapon is unlikely to reach even that 95% figure, but it serves to point out that 100% will not be obtained.

Q: Theoretically, driving as fast as you can into the enemy increases the bite your spinning weapon will have. While I see drum-bots flying full speed across arenas, I am not aware of a shell spinner that drives even half so aggressively. Is there a particularly good reason for this? I ask because I want to size the thickness of my teeth, and my drive-train to be optimal for the speed that I will really operate the robot. [Palo Alto, California]

A: [Mark J.] First, I know I'm being picky but that's my prerogative:

  • Increasing bite with speed isn't theoretical, it's demonstrable fact.

  • The other 'bot isn't your enemy, it's your opponent. There's a difference.

So why don't Full Body Spinners (FBS) charge aggressively? Two things to consider:

  1. The large diameter of the spinning mass allows a FBS to store huge potential energy at a relatively slow spin speed. Slow spin speed will give good 'bite' even with a slow approach speed. This allows the designer to use a less powerful (slower) drivetrain and shift that mass into the weapon to increase the destructive potential even further.

    Small diameter drum weapons must spin at high RPM to match the energy storage of a FBS. The high weapon speed requires a high attack speed to match the 'bite' of a slower-spinning FBS.

  2. The result of an impact with a horizontal spinner is much different than the result of a vertical spinning drum impact:

    • The drum impact will impart an upward motion on the opponent, with an equal but opposite Newtonian reaction pressing the attacker downward. As this downward reaction is countered by support from the arena floor, the attacking drumbot is relatively unperturbed.

    • An impact from a horizontal FBS will impart a lateral (sideways) motion on the opponent, with an equal reaction throwing the attacking FBS laterally in the opposite direction. A high-speed attack that lands a glancing blow on the opponent can send a FBS caroming around the arena. Each time it strikes the wall the impact from the shell can add more speed and prolong the mayhem. A slower and more planned attack run can minimize the 'pinball' rebound issue and help keep the FBS under control.

General advice: be cautious with your FBS attacks or you can do more damage to yourself than to your opponent.

Q: i have seen most of the bot's teeth are not welded in the drum.....i know they are not strong enough to bear the impact instead of welding they uses some types of screw (may be Allen bolt) can you please explain which type of bolt they use and how they manage to fit in the drum....and are they strong enough to bear the impact [India]

A: [Mark J.] Impactors are typically set snuggly into a machined groove or pocket in the drum. The groove supports the base of the impactor and prevents the tooth from moving laterally. Bolts run radially thru the impactor and into tapped holes in the drum to hold the impactors in place. A very thin drum may use nuts or a nutstrip inside the drum to give enough thread depth for the bolts.

Do not attempt to simply bolt the teeth to the surface of the drum without providing a groove -- bolts are very strong in tension along their length but are not good at resisting 'shear forces' they would face if the tooth is not well supported.

The bolts themselves are usually coarse thread and as large a diameter as is practical for the impactor design. High strength 'grade 8' bolts are favored by some builders.

Strong enough? Depends on the materials and dimensions of your design. See Frequently Asked Questions #4.

Q: how to determine the tooth height of a drum drum is 160mm in dia and 20mm thick

A: Optimum tooth height does not depend on drum dimensions. Use the search box at the bottom of this page - immediately below the box where you entered your question - to search the Ask Aaron Archives for "tooth height" to find an explanation and an example calculation.

Q: sorry to say but sir i have seen most of the archives but i didn't get the topic of tooth height please help me [India]

A: You're building a combat robot but cannot search a website? Please allow me to spoon feed this to you:

Tooth Height Post in this Archive

There is also a complete discussion of tooth height in section 6.3 of the RioBotz Combat Tutorial.

Q: If thick walled drum weapons are not effective, why is the 'snail drum' on the heavyweight champion 'Touro Maximus' built to this design? [India]

A: [Mark J.] Team RioBotz know what they are doing. They fight in large arenas and their attack plan is to charge at high speed into their opponent -- much higher speed than Indian robots can use in their small arenas. If the attack speed is high, the drum can rotate at a higher speed and still retain good energy transfer ('bite') into the opponent. Double the attack speed allows double the weapon RPM -- and double the RPM gives four times the energy storage in the weapon!


  • A specific robot with an attack speed of 3 MPH finds that it must the limit weapon to 2000 RPM in order to maintain good weapon bite. At 2000 RPM its weapon can store 1500 joules of energy.

  • If the attack speed of the robot is increased to 6 MPH, the weapon RPM can be raised to 4000 RPM and still retain the same 'bite'. At 4000 RPM this same weapon can store 6000 joules of energy.

  • If the attack speed of the robot is increased to 12 MPH, the weapon RPM can be raised to 8000 RPM and still retain the same 'bite'. At 8000 RPM the weapon can store 24000 joules of energy.
So, at 4 times the attack speed, the weapon can store 16 times the energy and still have the same ability to bite into the opponent without 'skittering' off. When you have the capacity to store that much energy in the weapon, you can afford to decrease the storage capacity a bit to make the weapon more durable and better able to survive the massive hits it will deliver.

Recap: the snail drum on 'Touro Maximus' is a thick-walled small-diameter drum in order to better survive the ultra-high energy impacts it delivers because it spins at very high RPM. It is able to spin at such high RPM because 'Touro Maximus' has a very high attack speed, which gives the weapon good 'bite' even at such high RPM.

This works in arenas that are large enough to allow robots room to accelerate to high speed. In a small arena it's much better to concentrate on larger, thinner walled drums to store high energy at lower RPM due to their greater rotational inertia. Don't blindly copy the snail drum if you're going attack at low speed!

Q: i am having drum of 110dia (OD) and thickness is about 40mm and length is 20cms and using bearing caps at end i want to participate in a 60kg robowar dose it store enough energy at 3000 rpm [India]

A: [Mark J.] Have you been asleep in the back of the classroom?

There are at least four other very recent posts asking me to calculate the energy storage of drum weapons. My answer to all of them has been the same. Take a look at this post which makes the same error of data omission you have made.

Read the Ask Aaron Spinner FAQ. It will tell you how to calculate the energy your spinner weapon should store, and it gives you three options on how to calculate that figure.

Q: ...and also in your spreadsheet how can i give detail of tooth...thankyou

A: Ahhh! So you have been paying at least a little attention -- you know about the Run Amok Excel Spinner Weapon Spreadsheet. Good.

Impactor teeth and bars come in all shapes and sizes. Calculate the mass of the teeth (volume * density of tooth material) and increase the value for the drum radius until the mass is increased by the mass of the teeth. This will give you a good estimate of the contribution to energy storage the impact teeth will make.

Now, stop being lazy and run the numbers for your drum. You'll find that your drum is too small in diameter to store a reasonable amount of rotational energy. A larger radius and a thinner wall will greatly improve energy storage with the same mass.

Q: Thank you for i am making a new drum 15cm DIA and 25 cm length with thickness 35mm for 60kg weight limit ...but i am little bit confused about the diameter of dead shaft can you suggest me the dia of dead shaft

A: Is this a steel drum? Have you noticed that the drum will weigh nearly 25 kilos? That's a very heavy drum for a 60 kilo class robot, and it doesn't store a lot of energy for its weight. You may wish to work on the design a bit more. Here is your current design:

  • Diameter: 15cm
  • Wall Thickness: 35mm
  • Length: 25cm
  • Speed: 3000 RPM
  • Weight: 24.7 kilos
  • Energy Storage: 4420 joules

Reducing the wall thickness to 20mm and increasing the speed to 4000 RPM will significantly reduce weight and improve energy storage:

  • Diameter: 15cm
  • Wall Thickness: 20mm
  • Length: 25cm
  • Speed: 4000 RPM
  • Weight: 15.9 kilos
  • Energy Storage: 6074 joules

Better still, increase the diameter to 20cm, reduce the wall thickness to 11mm, and keep 4000 RPM:

  • Diameter: 20cm
  • Wall Thickness: 11mm
  • Length: 25cm
  • Speed: 4000 RPM
  • Weight: 12.7 kilos
  • Energy Storage: 10,063 joules

Thick walled, small diameter drums simply aren't effective at energy storage. Think bigger!

Back to your question -- the required diameter of the dead shaft depends on many factors:

  • shaft material
  • solid or hollow
  • energy storage capacity of the drum
  • distance from drum bearings to shaft support
  • strength of shaft mounting to chassis...

Assuming solid hardened steel alloy shaft and the drum design you've mentioned, 20mm to 25mm is a good starting point.

Q: in your spreadsheet it shows the weapon has 109 joules of energy is it sufficient for a drumbot of length 22cm, 11cm dia and 30mm thickness and it is hollowed so that a dead shaft is inserted and bearing at end plates....if the energy is not enough then what should i do so that it can stores more energy i am using ampflow e30-400 [India]

A: [Mark J.] The Ask Aaron Spinner Weapon FAQ answers all of these questions. Read it.

Q: hi sir
whether12V 5310 RPM "CIM" Brushed DC Motor
  • 2.5-inch CIM brushed DC motor
  • Stall torque: 2.42 N-m
  • No load RPM: 5,310
can be used for combat if it is shall i buy this to run my 4kg drum [Tamil Nadu, India]

A: [Mark J.] The CIM motor has been used for years in the FIRST robotics program, and is certainly capable of being used in combat. However, it weighs a lot (46 ounces) for the amount of power it produces (about 1/2 horsepower). Compare that to the RS-775 motor That weighs 12 ounces and produces 3/4 horsepower.

Should you use the CIM motor for your 4 kg drum weapon? You didn't tell ne enough about your weapon for me to answer that question (see the Spinner Weapon FAQ), but YOU can use the Team Run Amok Spinner Excel Spreadsheet to model the weapon performance with the CIM motor to see if it meets your needs. This is the same solution I offered to the two posts immediately below. I'm sensing a theme here. Maybe that spreadsheet is useful?


A: [Mark J.] See post immediately below.

If you are unable to use the Team Run Amok Spinner Excel Spreadsheet, you do have other options:

Q: my drumbot length is 20cms and OD is 100mm and ID is 75mm and dead shaft is inserted in drum and at the ends bearings is mounted how much is energy it is storing ar 3500 rpm [India]

A: [Mark J.] 'Ask Aaron' is not a free engineering service. We provide tools and information to assist combat robot builders in the design, construction, and operation of their machines. We don't do your design work for you.

I spent a good deal of time and effort to create the Team Run Amok Spinner Excel Spreadsheet to assist builders in modeling the performance of spinner weapon designs. The spreadsheet is mentioned prominently several times on this page, in the Frequently Asked Questions, and in this archive. A few questions farther down in this archive is a detailed example of the use of the spreadsheet in calculating the energy storage of a drumbot.

I will occasionally work thru the process of calculating some performance factors for a specific robot as an educational example. However, having already performed that exercise and having provided you with the tools required to calculate the energy storage capacity of your drum weapon, it does not serve the purpose of 'Ask Aaron' to perform that calculation for you.

I will point out that you failed to mention the material from which your drum is constructed. Without that information it is not possible to calculate the mass of the drum, and without the mass it is not possible to calculate the energy storage. I could not answer your question if I wanted to. Be so kind as to run the numbers yourself -- you might learn something.

Q: hi sir considering single teeth drum and snail drum two different drums which one will give best hit? [India]

A: [Mark J.] There is a previous post about the 'snail drum' -- search this archive for 'snail' to find that post.

The snail drum does not store more energy or deliver a better 'hit' than a single-tooth drum with comparable dimensions running at the same speed. The snail is tremendously difficult to design and construct, and the primary benefit appears to be the durability of the integrated impact tooth. The only real reason to build a snail drum is to impress your opponents with your machine shop skills. Avoid the temptation.

Q: I read through your page but could not understand clearly how to build a hollow drum.
Could you please tell me the processes involved and how to build the drum? [India]

A: [Mark J.] Most of the examples of drum construction in this archive come from the Team Cosmos site. Suggest you read the Team Cosmos build log for 'Solaris' for photos and a description of the process. There are build logs for other drumbots on their site as well.

Q: Hello,
I want to build a hollow drum of id- 25mm and od-70mm .
How am I supposed to build the drum ?
The length of the drum is 250mm.
What materials should I choose and how much energy will it store at 5700 rpm?
- Siddharth [India]

A: [Mark J.] You're approaching this backwards. You don't start with the weapon dimensions and speed -- how did you decide on those values? You start with the weapon requirements and work toward a combination of dimensions, speed, materials, and components that will meet those requirements. Here is the correct sequence of events in designing a spinning weapon:

  1. Determine the performance parameters needed to achieve an effective spinning weapon in your weight class. Consult the Ask Aaron Spinner Weapon FAQ for guidance.

  2. Download the Team Run Amok Spinner Excel Spreadsheet and model the performance of a trial weapon design to determine the weapon mass, energy storage, spin-up time, and battery power requirements of the weapon.

  3. Modify the design parameters of the weapon (dimensions, materials, motor specifications, drive reduction ratio...) and observe the effect of the modifications on the modeled performance of the weapon.

  4. When you have a design that provides a good combination of performance parameters, evaluate the design for construction practicality. There are many posts in this archive that discuss drum weapon construction. Read them.
I can tell you that your current design (if constructed from steel) would weigh 6.5 kilos and would store only 810 joules of energy at 5700 RPM. Minor changes to the weapon design could greatly improve your weapon performance. Start over.
Q: hi mark
making 2 teeth drum materials
(mild steel)
(teeth- en18)

od 150mm
id 20mm
shaft -15mm to 20mm

bearings- shall i go for needle or tapered one's??
and my drum rotates @5700rpm at 24v can you tell me the energy stored in it
whether high horse power motors plays the role in storing the goodenergy in a drum orelse a high RPM ed one's [India]

A: [Mark J.] You've designed your drum weapon without knowing how much energy it will store? That's a critical element in the design, not something you save to find out about after the design is fixed. The Team Run Amok Spinner Excel Spreadsheet can calculate the mass, moment of inertia, energy storage, and spinup times for rotating weapons given their dimensions, construction material, speed, and motor specifications. I strongly suggest that you learn to use this tool to assist in your weapon design.

Bearings: drum weapons typically use radial ball bearings. They are economical, tolerate a little misalignment, and are available in sizes and ratings adequate for the purpose.

Energy storage: you have failed to include the length of your drum, so I cannot tell you anything at all about your weapon -- even if I were in a mood to do your design work for you.

Horsepower vs. RPM: this has been discussed previously here at 'Ask Aaron'. Take a look at this post on drum power in this archive. You'll benefit from reading thru the Spinning Weapon FAQ as well -- you might learn something that you didn't even know you didn't know.

Q: length of the drum -150mm could u say me the energy storing capacity for it

A: [Mark J.] I'll do much better than that -- I'll show you how to use the Team Run Amok Spinner Excel Spreadsheet to calculate the mass, moment of inertia, energy storage, and spinup time for your weapon. You'll most certainly need to use the spreadsheet to evaluate new designs once you see the performance of the proposed weapon!

  • Download the spreadsheet on a computer with Microsoft Excel installed.

  • Select the 'Calculations' tab and fill in the specs for your motor. I've assumed that your 5700 RPM motor is an AmpFlow E30-150.

    • No-Load Speed: 5700 RPM
    • Stall Torque: 5 N-m
    • Gear Reduction: 1:1

  • Fill in the specs for your drum tube.

    • Material Density: 7800 kg/m2 [steel]
    • Length: 0.150 Meter
    • Outer Radius: 0.075 Meter [half of 150 mm]
    • Wall Thickness: 65.0 mm [(150 mm OD - 20 mm ID) / 2]

  • You didn't provide dimensions for your impactor teeth, so we'll assume they're small in comparison to the drum and disregard their mass.

The output from the spreadsheet will tell you that the drum is quite heavy, and that it does store a lot of energy. It will also tell you that the spinup time is ridiculously long -- it takes a one horsepower motor a very long time to store that much energy in a spinning weapon. I suspect you'll want to re-design your weapon to weigh less, spin slower, and store a more reasonable amount of energy. Now you have a tool to help you do that, so go use it!

Q: Hi Sir...I am thinking of making a combat robot with two weapons: Wedge and Vertical Spinner. How can I use motors to operate these two weapons? Thank you in advance ! [India]

A: [Mark J.] Both horizontal and vertical spinner weapons are typically driven by a belt and pulley system. See the photo of horizontal spinner 'Fiasco' at right for an example of a belt driven spinner weapon. There are many posts about weapon motor selection and calculation of the correct belt drive reduction ratio in this archive.

A wedge is a 'passive' weapon that does not require its own motor. A wedge relies on a powerful drive train in the robot to ram and lift the opponent and break their traction, allowing you to push them with reduced resistance. There are many, many posts in the Motors and Controllers archive on selecting drive motors and calculating the correct gear reduction for your robot drivetrain.

Q: Hi Sir..I am completely new to a combat robot making.....I wanna have flipper as a weapon in it...Would you please tell me how can we transmit power from cylinder to function the weapon?And also,what can be the effective orientation of cylinder [with respect to] weapon? Thanks in advance !!

A: [Mark J.] There are DOZENS of posts about pneumatic flipper weapons in this archive. Search for 'pneumatic'. A few notes:

  • The Team Da Vinci Robotics Understanding Pneumatics page has several animated examples of pneumatic weapon designs, and a thorough discussion of pneumatic components.

  • Many Indian robot competitions do not allow pneumatic weapons. Check with the specific events you plan to enter before you build your flipper robot to make certain that you will be allowed to compete.

  • Pneumatic flipper weapons place great loads on the robot chassis and can be extremely dangerous if improper components or techniques are used. I can not recommend that anyone 'completely new' to combat robots attempt to construct and implement a pneumatic weapon. Pick something much simpler for your first combat robot.

Q: Hi Mark Sir......can you please tell me which motor is good to rotate a 13kg drum and 22kg single tooth spinner ,ampflo A28-400 or A28-150? [Mumbai, India]

A: [Mark J.] I can't recommend a motor based on just the mass of the weapon. I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in spinner weapon design and motor selection.

Q: Just wanted to know the Mechanism SKF Warrior by Team Whyachi !!!
what kind of mechanism is that ?
They say it stores energy when it spins, and then uses it to flip the opponent. How is this energy stored, spring ?
Or is it something else ? [Mumbai, India]

A: [Mark J.] Previously answered.

Search this archive for multiple posts on 'Warrior SKF' and its Spin Kinetic Force (SKF) weapon.

Q: thinking of building a bot with horizontal sppiner fr 25kg catg. compitations.. whic ampflow shud i use considering the cost factor also [India]

A: [Mark J.] It's generally not a great plan to start by picking a motor and then 'back into' a weapon design to suit that motor. You'll have greater success if you start with a weapon design (type, dimensions, material) that is capable of storing the energy you require at a reasonable speed -- then select a motor that can spin up the weapon to the desired energy level quickly enough to meet your needs.

I used the Run Amok Spinner Excel Spreadsheet to generate this simple example:

  • A steel bar 400 mm long, 90 mm wide, and 25mm thick weighs 7 kilos. That's a reasonable weight for a 25 Kg class spinner weapon.

  • Spinning like a lawnmower blade, the bar will store about 1000 joules of energy at 1400 RPM. That's an acceptable amount of energy for a 25 Kg class weapon and a reasonable speed.

Now that we have a practical weapon design, we can look at weapon performance when powered by a specific motor.

  • An economical AmpFlow E30-150 motor running on 24 volts with a 4:1 reduction belt drive to the bar weapon will spin the bar to about 1400 RPM in less than 2.5 seconds. That should be adequate performance in even a small arena.

That's only one possible weapon that may or may not suit your overall design. It does show that it's possible to design a 25 Kg class weapon powered by an Ampflow E30-150 motor. A more powerful motor could spin a weapon with greater destructive potential -- the choice is yours.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for additional guidance in weapon design and motor selection.

Q: hey mark, can you tell me that how much energy (J)/kg would be sufficient for a drum weapon to attack a bot of 15 kg?- sam [New Delhi, India]

A: [Mark J.] According to Stroker McGurk's Law:

"If some is good, more is better, and too much is just enough."

I suggest that you read the Ask Aaron Spinner Weapon FAQ for a more precise answer to your question, Sam. The first three questions there are:

  1. How much energy should my spinning weapon store?

  2. How do I calculate the kinetic energy storage capacity of a spinning weapon?

  3. How quickly should my weapon spin-up to speed?

Read the whole thing while you're there -- it might save you from asking a few more questions in the future.

Q: which type of dc motor suits to rotate 7kg horizontal drum (outer diameter 160mm, thick 40mm , made by MS) ?
it is advisable to use E-bike brushless dc motor having capacity of 200w , 12v , 20-18 A , 2000rmp (torque = p*30/3.14*2000)=0.9Nm) ?
or any other? [Gujarat, India]

A: [Mark J.] First, you've misapplied the formula used to calculate the torque constant (Kt) of a motor in an attempt to calculate the actual torque your motor supplies, and you've incorrectly entered the actual RPM of the motor into that formula instead of the motor's speed constant (Kv). The correct formula for estimating the stall torque of a brushless motor is given in the instructions page of the Run Amok Excel Spinner Spreadsheet. While you have the spreadsheet open, spend some time learning how to use it and you can do your own evaluation of motors for powering up a spinning weapon of any size, shape, and material.

You haven't given me enough info about your E-bike motor to model its performance in spinning up your weapon, but I can tell you that in general a 200 watt motor isn't going to be enough for the weapon you describe. The AmpFlow E30-150 motor provides 750 watts of output power, and would be a marginal motor for your purpose. If I correctly understand your description of the weapon, the E30-150 is capable of spinning it to about 1300 RPM and 1500 joules of energy in about 4.5 seconds (3:1 belt reduction). That would make an adequate weapon for a 40 kg robot.

Suggest you read the Ask Aaron Spinner Weapon FAQ for additional information in designing your weapon and selecting a weapon motor.

Q: sir which weapon is more effective from a Chucker or a lifter in 60kg bots ??and plz suggest me a best mechanism for it! [Low Earth Orbit over India]

A: [Mark J.] The answer depends on your competition rules and the design of the combat arena.

  • 'Flipper' weapons are quite effective in arenas where there is enough room between the arena boundary and the protective screening to toss your opponent out of the arena for a win. Flippers require an effective pneumatic system to provide the speed and power needed to toss an opponent end-over-end thru the air, and I know that pneumatics are not allowed in many Indian competitions. Direct use of electric power simply won't do.

  • 'Lifter' weapons are effective in breaking your opponent's traction and possibly toppling a top-heavy opponent. Few Indian robots can function when inverted or simply tipped on their side. Either pneumatic or electric power can be used for an effective lifter, although an electric lifter will require great gear reduction in order to provide the torque needed to lift a heavy opponent.

The 'best' design for a lifter or flipper will depend on the tournament rules, your ability as a builder, the materials and components available to you, and integration with the rest of your robot design. Suggest you search thru this archive for many previous posts about lifters and flippers to see what design might be best for you.

Q: can you please give the specifications of the dc motor we need to use to rotate a 3kg cicular disc at 1000 rpm ? [India]

A: [Mark J.] The power needed to spin a 3 KG disc to 1000 RPM depends on:

  • How quickly you want the disc to reach 1000 RPM; and
  • The diameter of the disc.
The power to sustain the disc at 1000 RPM is negligible, so it all comes down to how quickly you want your disc to reach the required speed. A larger diameter disc will take longer to spin up to speed than a smaller disc of the same mass, but it will store more energy for a greater impact.


  • An aluminum disc 300 mm in diameter and 15 mm thick weighs 3 kg. It will store about 170 joules of energy at 1000 RPM. A 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 0.8 second -- very quick, but very little stored energy.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. The same 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 3.5 seconds -- much slower, but much better energy storage.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. An Ampflow E30-150 motor with 5.0 N-m stall torque and a no-load 5700 RPM speed geared down 5.7:1 will spin this disc up to 1000 RPM in about 1.7 seconds -- a good balance of speed and energy storage.
I will mention that most builders choose to spin their weapons faster than 1000 RPM. Doubling the speed to 2000 RPM will give four times the energy storage, but will require four times as long to spin up to full speed. You didn't mention the weight class of your robot, but a useful spinning weapon will require at least 40 joules of energy storage per kilo of robot weight. Plan accordingly.

Q: Hi Mark, what do you think of abrasive disks as cutting weapons? [India]

A: [Mark J.] NO CERAMIC CUTTING DISKS!!! It doesn't take much of an impact to shatter the disk, sending sharp shards at high speed in all directions. Given the average combat arena in India, that's a disaster. No sane event organizer would allow you to use such a weapon, plus they simply aren't effective at doing significant damage to your opponent.

If you are intent on using a cutting blade, an arrangement like that used by 'Gloomy' is your best option: an all-metal circular cutting saw attached to a movable arm that keeps the blade out of harm's way until the opponent is pinned and the blade can be put to good use. See: Gloomy vs. Hyperactive.

Q: ...and also about how much pressure would be needed to make a decent lifter in midweight class.

A: The power of a pneumatic ram is calculated by multiplying the gas pressure by the area of the piston. A lower pressure simply requires a larger diameter cylinder to compensate. I suggest that you read the Team Da Vinci Robotics Understanding Pneumatics page to get a good overview of combat pneumatic weapons and how to calculate the force of your lifter design.

A typical 'low pressure' pneumatic lifter will operate at 150 psi. Commercial pneumatic regulators, cylinders, and other components certified for this pressure are widely available and are relatively safe to work with.

Our own heavyweight lifter 'The Gap' uses a 150 psi pneumatic system to power a 3" diameter actuator. The actuator provides 1000 pounds of force, which translates to more than 400 pounds of lifting force out at the tip of the lifter arm.

Q: what is the best possible weapon against 'Tauro Maximus'?? [India]

A: [Mark J.] Let's look at the fight record. In US competition from 2009 thru 2014, heavyweight drum robot 'Tauro Maximus' lost 11 of its 32 fights:

  • One loss to 'DXTER' - a hinged wedge
  • One loss to 'Polar Vortex' - a lifting wedge
  • One loss to 'Last Rights' - a horizontal bar spinner
  • Two losses to 'Original Sin' - a pivoting wedge
  • Three losses to 'Great Pumpkin' - a simple wedge
  • Three losses to 'Sewer Snake' - a forked wedge/lifter
Ten of those eleven losses were to some form of wedge. A smooth low wedge or scoop surface gives no edge for a drum impactor to strike against, neutralizing the drum weapon.
Q: hi mark...i am having drum bot when i give a big hit to opponent my shaft bends inside. first i tried 20mm shaft in MS it bends. and then i used 25mm shaft at EN24 It also bends , what can i do at which mm and which material i have to use? [India]

A: [Mark J.] 'Ask Aaron' isn't an engineering service (see Frequently Asked Questions #17) but if I were spec'ing a weapon shaft I'd need a lot more information:

  • Drum weight?
  • Shaft length?
  • Weight class?
  • Peak drum speed?
  • Dimensions of the drum?
  • Live shaft or dead shaft?
  • Where exactly did the others bend?
  • What is the design of the shaft support?
I suspect that the problem isn't the diameter of the shaft or the material from which it is made -- a 25 mm EN24 shaft is difficult to bend, hardened or not. If I had to guess I'd say the problem is a flawed design in the support and bearing placement for the shaft, but I can't confirm that because you haven't told me anything about your design. The hamburger is bad.
Q: hi mark ... i am making a 10 kg war bot .. with a vertical disc ( very thick , like a fat drum ) of 4 kg . I was planning to use two small johnson motor ( the one on robotmarketplace ) , for the weapon geared down to 3000 rpm . will this much speed and torque be good enough? also is 4 kg god , or shall i increase the weight ? Each motor gives 0.36 HP . [India]

A: [Mark J.] Spinning weapons of a given mass can have very different performance characteristics depending on how that mass is distributed. Example - here are calculated energy storage values for three different hollow steel drum weapons with the same mass and length, but different diameters:

  • 15 cm long - 15 cm diameter - 14 mm wall thickness: mass is 7 kg and it stores 2900 joules of energy at 4000 RPM

  • 15 cm long - 20 cm diameter - 10 mm wall thickness: mass is 7 kg and it stores 5600 joules of energy at 4000 RPM

  • 15 cm long - 25 cm diameter -  8 mm wall thickness: mass is 7 kg and it stores 9200 joules of energy at 4000 RPM

For me to calculate the performance of your 'fat drum' weapon you'll need to give me more detail: material, length, diameter, and wall thickness if hollow. Tell me those things and I'll tell you if your weapon has enough energy and how long it will take to spin up.

Four kilos of weapon in a ten kilo 'bot is plenty -- if it's the right shape.

Q: I would like to add details ... the drum is 7.5cm dia , and 15 cm long solid mild steel . this is non- machined piece so not sure of weight. the pulley will be machined in the same piece and used with a dead shaft of 1.6 cm dia. I don't want to make it hollow , just enough required for the dead shaft to go in ... used with bearings . so will this setup be enough for 10 kg , and how much will be the energy stored?

A: A useful spinning weapon will require about 40 joules of energy storage per kilo of robot weight -- at a minimum. Using two 'Small Johnson' motors geared down to 3000 RPM, the Run Amok Excel Spinner Spreadsheet calculates:

  • A steel cylinder 7.5 cm in diameter and 15 cm in length with a 2 cm hole running thru the center will have a mass of 4.84 kilos (not counting impactor teeth). It will store 180 joules of energy at 3000 RPM and will spin up to speed in about 0.5 second. That's 18 joules per kilo of 'bot weight -- not nearly enough to be effective.
The problem isn't the mass of the drum. The problem is that the mass is concentrated very close to the rotational axis. Rotating mass stores more energy as it is placed farther away from the rotational axis. Let's run the numbers on some 4 kg weapons with different dimensions:
  • A steel disc 15 cm in diameter and 3 cm thick with a 2 cm hole in the center will have a mass of 4.06 kg. It will store 580 joules of energy at 3000 RPM and will spin up to speed in about 1.7 seconds. That's 58 joules per kilo of 'bot weight -- enough to be very effective.

  • A hollow steel tube 15 cm in diameter and 7.5 cm in length with a wall thickness of 16 mm will weigh 3.94 kg (not counting end plates -- just an example). It will store 890 joules of energy at 3000 RPM and will spin up to speed in about 2.7 seconds. That's 89 joules per kilo of 'bot weight -- a VERY dangerous weapon.
That's why drum weapons are usually hollow. Reconsider your 'solid drum' design, or at least increase the weapon diameter to make it a disc.

Q: Hey I am using a horizontal spinner I am confused which one is most important speed or bite .since I failed to a wedge bot even though I had enough speed and energy [India]

A: [Mark J.] Which is more important -- the front wheel on a bicycle or the rear wheel? You need both!

  • If you have no bite you have to rely on your opponent to make a mistake and leave a sharp edge exposed for your weapon to grab;
  • If you have no speed your stored energy suffers and even a good hit on your opponent does no damage.
The solution is to design a weapon that has high energy storage at a rotational speed slow enough to deliver great bite. Search this archive for more discussion on this topic.
Q: can a hollow pipe be used as a dead shaft for drum? [India]

A: [Mark J.] Can a wooden boat carry an elephant? Depends on the boat -- depends on the elephant. The hamburger is bad.

Q: I m making a drum bot having ID 120mm,so i will fit bearing housing to end caps as i cannot get 120mm OD bearing having smaller bore. my question is, will the bearing housing sustain impacts caused by drum? [India]

A: [Mark J.] Short answer - yes. The force of the impact will be borne by the impactor, the drum, the endplates, the bearings, the drum shaft, and the shaft supports. Each of these elements must be strong enough to withstand the full force of the weapon backlash. Don't skimp on the bearings!

Date marker: March 2015
Q: A builder on one of the forums claims that he spins his featherweight [30 pound class] drum weapon at 20,000 RPM and still gets bite. Is this possible? [Ohio]

A: [Mark J.] Usually, no way -- but I know the bot in question.

A conventional drumbot has the drum well out in front of the rest of the robot and rams the weapon directly into the opponent. In this particular robot the drum is set fairly well back and rather tall 'wedgelets' lead the opponent up and into the drum at a height close to the drum axis. This gives the drum a good chance to impact the sharp leading edge of the opponent and provides a favorable attack angle.

This isn't 'bite' in the conventional sense, but it does allow a very fast drum to connect and release a decent hit IF the driver can coax their opponent up the wedge.

Q: Hey mark.. I have seen Robo Bacon in antweight matches in robogames 13 and I have decided to do that robot weapon for 35 kg bot ... What kind of weapon is that? How it differs from double teethed drum? Which produces more impact on opponent? please prefer some material to do that weapon.... [India]

A: [Mark J.] How can you decide on a robot weapon before you even know if it has any advantages?

Team Uai!rrior's antweight 'Bacon' has a single-tooth spiral drum weapon, similar to hall-of-fame middleweight 'Professor Chaos'. Go read section 6.3 of the RioBotz Combat Tutorial for an explanation of the advantages of an asymmetric single-tooth spinner.

See this previous post for some notes and a diagram on spiral drum construction. The machining is difficult, and balancing the drum is a real challenge. A spiral drum is typically made of aluminum with steel inserts for balance and a hardened steel impactor.

Q: hey Mark., what are the merits of using chain drive and belt drive in LAST RITES ?? what would happen if we use belt drive instead of chain drive ., [India]

A: [Mark J.] The 'Last Rites' chain drive has been previously discussed. See this previous post.

Note: 'Last Rites' still has the chain drive, but now has a mechanical clutch mechanism to provide a little 'slip'.

Q: Hello, I am designing a 15lb horizontal spinner robot, with an uncanny resemblance to Hazard. I was trying to figure out a way to securely mount my 5.16lb, 24in bar across the top. I have visited Team Delta's website, and they seem to be using shaft collars both on the top and bottom. I am currently running a dual bearing support system, and a v-belt pulley driven by a beefy brushless motor. Any help would be appreciated! Thanks -- Erik [Winchester, Massachusetts]

A: [Mark J.] I wrote one answer to this question, briefly posted it, thought a little more, took the original post down, and started again. Let's see if this makes sense.

Hazard's 22 pound spinning blade is not directly fixed to the 1.5" diameter steel weapon shaft. It is squashed firmly between two shaft collars (see picture) that are tightened onto the shaft by machine screws that reduce the diameter of the collars for a clamping fit -- not held in place by set screws! Custom phenolic washers on either side of the blade form a mechanical clutch that allows the blade to slip on impact and limit the shock transmission back to the drivetrain (Hazard Build Report).

I'm not very keen on squishing a weapon blade between two collars and relying on friction to spin it up, but it worked very well for 'Hazard'. If you do decide to use shaft collars, I would strongly recommend clamping collars over set-screw collars, but I think I'd axially fix the blade by broaching the shaft and blade and fitting a key to lock rotation -- let your v-belt act as the slippy-clutch.

Q: if we want to give maximum impact of drum teeth to the oposition,then what should be the position of the skid,it should be closer to the drum or should be away. [India]

A: [Mark J.] Suggest you read section 6.6 of the RioBotz Combat Tutorial for a full treatment of weapon support and maximizing impact. Read the rest of the RioBotz tutorial while you're at it.

In general, your skid should be placed directly underneath the drum axle and be as stiff as possible.

Q: The power of robot rotary weapons is measured in joules, but joules doesn't mean much to me in real world terms. Can you give me an example of a 1000 joule impact with everyday objects? [West of the Pecos]

A: [Mark J.] Sure. A baseball bat hitting a major league home run is an impact of just about 1000 joules. Bear that in mind when you're designing and building your robot -- can you hit it as hard as you can with a baseball bat and have it bounce back for more?

Q: hi mark ,i was planning to build a 30 kg bot with vertical spinning bar of dimensions 20*15*5 cm (l*w*th) of mass around 7 kgs (mild steel) and i want to run it around 3000 rpm ,i was using an ampflow e30-400 to power my weapon and i want know the reduction ratio and energy storage of my weapon(i was unable to get through the spinner spread sheet)....and can i use ms for teeth ? [India]

A: [Mark J.] Lets start with the reduction ratio. From the AmpFlow website: the no-load speed of the E30-400 motor at 24 volts is 5700 RPM. Allowing for mechanical and aerodynamic drag on a spinning weapon will reduce that speed by about 15%, so let's call it 5000 RPM. You want a 3000 RPM weapon speed, so the formula is:

Motor Speed / Weapon Speed = 5000 / 3000 = 1.67 to 1 reduction.

A chunk of steel 20 cm by 15 cm by 5 cm isn't as much a bar as it is a square, and it will weigh closer to 12 kg than 7 kg. If you are removing material from that almost-a-square to reduce it's mass to 7 kg, I'd need details of the true shape of the rotating mass to calculate the energy storage.

IF the weapon is a simple 12 kg steel block of the dimensions you give, it will store about 3000 joules of energy at 3000 RPM. Spin-up time will be about 3 seconds.

Tooth material should be harder than the material you'll be hitting with the teeth. Mild steel isn't very hard. If your particular steel alloy can be hardened, I would consider at least surface hardening the teeth.

Q: hi mark ,iam the spinning bar guy again sorry i have given wrong info about it..... actually its dimensions are 20*12*4 cm (l*w*t) around 7.4 kg ,what will be the energy storage of my weapon ?and was that weapon is enough for my 30 kg bot ? OR i should chage my weapon dimensions ? thank you for your info about reduction ratio......

A: [Mark J.] That's a big difference. I'm not sure how you confused the dimensions that badly and sent me down the rabbit hole with the other numbers. The 'Ask Aaron' service is free, but that doesn't mean my time has no value. Please double check your numbers in the future.

A steel bar 20*12*4 cm spinning at 3000 RPM will store about 1700 joules of energy with a spinup time of about 2 seconds. That's better than 25 joules per pound of robot weight, which isn't bad for a bar spinner.

If your design allows, a longer and narrower bar of the same weight would store more energy. Examples: a 24*10*4 cm bar spinning at 3000 RPM will store about 2100 joules (2.5 second spinup), and a 28*8.5*4 cm bar will store more than 2600 joules (3 second spinup).

I'd strongly suggest that you learn to use the Run Amok Excel Spinner Spreadsheet so that you can examine many weapon designs and find the one that best fits your needs and your design parameters.

Q: Hello sir. I am new to this site. I would like to build a 30 kilo war robot with a drum weapon. Would a hollow drum or a solid drum be better? What kind of a material should I use? And how should I use this? [Maharashtra, India]

A: [Mark J.] Welcome to 'Ask Aaron'. I do ask that you do me the courtesy of reading thru the Frequently Asked Questions and the Recent Questions page, and then performing a keyword search thru the 'Ask Aaron Archives' to see if we've already answered your questions.

  • Your solid/hollow drum question is answered just a few questions below, and there is a great deal of additional information on drum construction elsewhere in this archive.

  • Your question on drum material is answered just below the answer to the hollow drum question, and again is discussed at other places in this archive.

  • Your question about how to build the weapon is -- you guessed it -- answered a bit further down this page and discussed in detail elsewhere in this archive.
Read thru these earlier questions and answers. If you have NEW questions, write back.
Q: Hello sir. Sorry to disturb you, I had go through your page for the last two days.

A: [Mark J.] No apology needed. I'm always pleased to receive robot questions, particularly from people who have taken time to read thru the website first!

Q: First: will you please tell me about energy storage capacity? What role does it play in the drumbot?

A: Rotational weapons in robot combat (drums, disks, blades) are all devices that accumulate and store the mechanical energy output from a motor over a period of time as kinetic energy. That stored kinetic energy will resist attempts to stop the rotation of the weapon. The greater the stored energy, the greater the potential impact the weapon will have on an object (like another robot) that comes into contact with an impact surface on the weapon. More energy = greater potential damage.

The energy storage of a spinning weapon depends on its rotational speed (RPM) and its 'moment of inertia' (MOI) about the center of rotation. The Run Amok Excel Spinner Spreadsheet will calculate the amount of kinetic energy stored by a spinning weapon based on RPM, material density, and the shape and dimensions of the spinning components.

See the Wikipedia article on Flywheel Physics for a taste of the math.

Q: Second: you talk about 'thickness' of the drum -- what do you mean by thickness?

A: Most drum weapons are hollow tubes with caps at each end to support the bearings. This type of design is used because hollow drums are MUCH more efficient (by mass) than solid cylinders of the same dimensions at storing kinetic energy. 'Thickness' refers to the thickness of the tube wall.

For example, a hollow drum may be 30 cm long and 20 cm in diameter with a wall thickness of 1 cm. A solid drum of the same length and diameter could be thought of as having a wall thickness of 10 cm -- all the way to the center!

Q: Third: in the Team Run Amok Spinner Spreadsheet, which calculations do I use for the drum bot?

A: The spreadsheet has sections for 'motor and drive', 'ring or tube', 'disk', and 'bar'. A drum weapon is a combination of a tube and two disk caps plus the motor and drive:

  • In the motor section, enter the no-load speed of the motor, the stall torque, and the drive reduction ratio between the motor and the weapon. The drive reduction ratio is typically the ratio of the motor and weapon pulley diameters. The 'no-load current' and 'voltage' are optional and only used in the battery capacity calculation.

  • In the tube section, enter the material density of the tube (from the handy chart at the top center of the sheet), the tube height ( = length in a horizontal drum), the tube radius (to the outer wall), and the tube wall thickness.

  • In the disk section, enter the material density and dimensions of one drum end cap -- but double the thickness to represent two caps.

  • Leave the bar section empty -- you have no bar element in a drum.
What about impact teeth? If the mass of the impact teeth is significant, you can manually calculate their mass and add a bit to the tube length dimension in the spreadsheet to increase it's mass by the mass of the teeth. That will give a good approximation.

Q: Which materials do you prefer for the drum and tooth in a drum with a length of 20cm and a diameter of 15cm?

A: Drums and end caps are typically made from aluminum tubing and plate -- 6061 alloy is commonly available and in suitable sizes and is easy to work with. Mild steel can also be used if it is more easily available.

Impact teeth are preferably made from tool steel. There are many posts about the suitability of various tool steel alloys in this archive.

Q: Where can I get those materials in Tamil Nadu, India?

A: I have no idea what materials are available in Tamil Nadu or elsewhere in India. I suggest that you join the 'Combat Robotics India' group on Facebook for support on local parts and materials.

Q: How do I make the end caps?

A: The end caps must be made with great precision or your drum will not be balanced. Typically they are turned on a metal lathe. Consult with a metal fabrication shop if you do not have the skills or equipment needed.

Q: mark lucky here tell me exactly which type of bearing i should use in the end rings of my drum [weapon]
drum diamensions :
od = 14 cm
length : 20 cm
weight : 14kg ( including tooth plate )
speed : 4500 to 5000 rpm
shaft dia : 3 cm
[Maharashtra, India]

A: [Mark J.] In general I'd use unmounted, shielded, pressed-steel cage, radial ball bearings with a 30 mm bore and about 16mm width -- something like these: shielded bearings.

Q: I want to use ampflow e-400 motor for my drum [weapon],but as it is not available [in India], can i use 2 e-150 motors, one on each side of d drum?rpm would be same but will d hp and torque increase? [Maharashtra, India]

A: [Mark J.] Yes, you can use a pair of AmpFlow E30-150 motors to power your drum. Performance comparison below -- and I'll add a third option:

  E30-400 (x1)
@ 24 volts
E30-150 (x2)
@ 24 volts
E30-150 (x1)
@ 36 volts
Price (US)$109$158$79
Weight5.9 lbs7.2 lbs3.6 lbs
Stall Torque1500 oz-in1420 oz-in1065 oz-in

As the table shows a pair of AmpFlow E30-150 motors will have performance quite similar to a single E30-400 motor, but at greater cost and weight. An attractive option is to overvolt a single E30-150 by 50% to 36 volts. I might not recommend overvolting the E30-150 this much for robot drive use because of the high and sustained torque loading imposed on drive motors, but for a weapon that spends much of its time at low loads it is an option that you might consider.

Q: Hi, this is Chaitanya again, from India [scroll down a few posts to see Chaitanya's previous post] thanks for your guidance. As you mentioned, you want ratings of pmdc motors to calculate battery , they are
Stall torque: 50 kg-cm
stall current : 54 amp
rpm: 260
diameter of wheel : 16 cm
as you told, we are thinking on drum of material EN41 metal, length 22 cm, wall thickness 20cm [you must mean millimeters], diameter [I hope you mean radius...] 6.5 cm (excluding teeth height of 1 cm). as you said, wall thickness is too much less for strength because we are inserting 1 cm of teeth height inside the drum. so please give some suggestions regarding drum design and teeth dimensions so that we can design in a better way? as you told, following are the metals available in India for teeth are SS304, SS316, HEHER D-2, HEHER D-3, HDS H-13, BN-24, BN-31, OHNS.
which one is best for teeth? Is there any need to harden teeth metal? if yes then up to what value it should be hardened in HRC ? Please guide us about drum teeth. We read all the riobotz combat pdf for height of teeth but calculations were nt match.
please help about required battery for drive motors for 5 min match.
Thanks. [India]

A: [Mark J.] Please re-check the pmdc motor ratings you provided. A 24 volt motor with a 54 amp stall current should produce much less than 50 kg-cm stall torque if ungeared, and should produce much more than 50 kg-cm stall torque if geared down to 260 RPM. You also failed to send me the gear reduction ratio of the gearbox. I need clarification on both of these. Might you send me a link to the motor and gearbox specs?

Of the options given, D2 tool steel (HEHER D-2) is probably the best for drum teeth. D2 tool steel can be hardened to HRC 58/60, but the drum design must support the teeth well as D2 has poor impact toughness and can shatter if left unsupported.

Tooth height depends on robot attack speed, drum RPM, and the arrangement of impactors on the drum:

  • With 260 RPM gearmotors and 16 cm wheels, your max attack speed is a bit under 5 MPH (200 cm/second).

  • Allowing for mechanical and aerodynamic loss, your max drum speed is about 5500 RPM (92 revs/second).

  • Assuming two full-width impactor bars on opposite sides of the drum, you have an impactor passing a fixed point on the radius every 5.4 milliseconds (1 / (92 revs/second * 2 impactors) = 0.0054 second).

  • At top speed your robot can move forward 1.08 cm in 5.4 milliseconds (200 cm/sec * 0.0054 sec = 1.08 cm).
So, your optimum impactor height for a full-speed ramming attack is 1.08 cm. You got lucky in your guess of correct height.

Note: I've seen a lot of Indian robot matches and I have never seen a full-throttle, top-speed ramming attack by a drum robot. Indian combat arenas tend to be pretty small, and combat generally consists of the robots maneuvering into position and easing their drum weapon into the other robot with a short, darting attack. This is less than ideal and does not use the full depth of the impactor.

You have not mentioned the impactor design you have in mind. Something like the full-length impactor bars used by 'Solaris' (Team Cosmos - pictured) should do well for you. The Solaris drum has hardened steel bar impactors set into shallow grooves milled onto the drum that locate the bars and take the shear forces off the countersunk machine screws that hold it in place. There are a couple of additional impact teeth at the ends of the Solaris drum, but these were added to correct a specific weakness in the robot and may be ignored for your application.

Q: hey, this is chaitanya again, so sorry about those mistakes, the corrections are as follows:
voltage: 24 volt
stall current:- 9 amp
gear box ratio:- 4.5:1

so can you now tell me about battery for drive , pushing power of bot etc. for any other info, refer previous question. as you mentioned in the answer of First question, please tell me how many batteries of 12 volt 9 amp can we use (for both drive and weapon) so that bot can work properly for 5 min match?

New drum dimensions are length 22 cm, radius 6.5 cm, thickness 22 mm , are this dimensions suitable for our robot of weight 66 kg? if not then please guide us. which metal can we use for drum between EN31 or EN24? Is there any need of hardening drum metal? if yes then up to what value in HRC? Can you please tell me about tooth depth also. (length from surface of drum ). which metal can we use for dead shaft and tell us its radius also.
thnxs for helping us.

A: [Mark J.] I think you have some more work to do:

Drive Motors: the spec numbers you've given for your PMDC drive motors are quite odd, but if you're sure they're correct I'll continue.

The motors are not adequate for your drivetrain. I recommend a bare minimum of 4 watts of drivetrain output power per pound of robot, and your four-motor drivetrain produces less than 2 watts per pound of robot. A typical robot in your weight class might have 10 or 20 watts of drive power per pound.

With 16 cm diameter wheels, your four drive motors combined do not provide enough torque for reasonable pushing power in a 66 kg combat robot. Maximum pushing power will be well under 30 kg -- less than half what a robot in your weight class might be expected to produce. Acceleration will be very poor, and the drivetrain will stall under moderate load risking damage to the motors. I STRONGLY recommend that you obtain more powerful motors.

There is little point in calculating battery requirements for these motors, as I'm rather certain that the motors would fail well before the battery is depleted. For what it's worth, it comes out to just under 2 amp hours of battery capacity to operate the drivetrain for 5 minutes with these motors.

Drum: your drum dimensions and materials can store a reasonable amount of energy for a robot in your weight class. I can't comment on its overall suitability for your robot, as the weapon design must integrate with the rest of your robot design -- about which you have told me very little. The drum itself appears to be fine.

The material used for the drum is far less critical than the material used for the impactors. Given your dimensions, any grade of mild steel would work well. No need for hardening on the drum material.

The calculations for tooth depth were included in my answers to your last questions -- please find your answer there. None of the drum changes have altered those calculations, although they may change when you select new drive motors.

Dead Shaft: as discussed in Frequently Asked Questions #4 and #17, I do not provide a free engineering service. I'm pleased to discuss materials and dimensions in general, but I cannot recommend specific materials and dimensions for specific applications. There are too many unknowns in your design for me to have faith in an analysis at that level. I can recommend that you research designs similar to yours and examine the materials and dimensions other builders have successfully used.

Order of magnitude estimate: 3/4" chromoly steel, very well supported as close to the drum bearings as practical. When in doubt go bigger.

Q: Mark,
What do you think is a reasonable/appropriate [weapon] speed for a drumbot? Because I have heard claims of speeds upwards of 20,000 rpm and that seems ridiculously fast. At that speed isn't there no time for the robot to achieve bite? [New Richmond, Ohio]

A: [Mark J.] It's certainly tempting to spin a weapon up to stupid fast revs. Double the speed, get four times the energy storage -- awesome!

The problem, as you note, is that the faster the weapon spins, the harder it is to get the weapon to 'bite' into your opponent and get a powerful hit. A weapon without bite will just skitter across a smooth surface and do no harm at all. If you have no bite you must rely on your opponent to make the mistake of offering a sharp edge to give your weapon something to grab.

How fast is too fast? Depends on the spacing of the impactors and your attack speed. You can get away with greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to come by if you have multiple impactors and a timid attack.

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read.

There is one excuse for a hyper-speed drum weapon: when two drums go 'head-to-head' and their weapons meet, the faster drum wins. So, if you are expecting to fight a lot of other drumbots you might want to be able to reach for a few thousand extra RPM to see if you can launch them. The rest of the time you'd be much better off to throttle the weapon back and charge hard.

Q: In a pneumatic system for flipper which valve should i use ? Should i use 5 ports or 3 ports ? [West Bengal, India]

A: [Mark J.] I suggest you read the Team Da Vinci: Understanding Pneumatics page for a full description of 3, 4, and 5 port solenoid valves and their applications, as well as general design information and illustrations of robot flipper systems. I also suggest that you read thru the many posts about pneumatic systems in this archive for additional design considerations.

Q: Hi, this is Chaitany, from India, we are willing to design a robot same as tauro. We are using amflo f30-150 motor having noload current 2.5 amp and stall current is 375 amps for hollow drum having length 16 cm , diameter 14 cm and wall thickness 27 mm. We get the energy 4205 joules at 4347 Rpm at 3.02 sec. robot weight is 66 kg. Dimensions of bot is 60*70*14 cm. So the my question is how can we mount the drum on the shaft if dead shaft system is used ? Can i use bearings at edge of drum? any suggestions regarding drum and energy? We cannot increase the diameter.

We are using 4 pmdc motors for motion having rating 24 volt 9 amp. We are using 12 volt 9 amps battery, 2 for weapon motor and two for pmdc [drive] motors. So any suggestions about battery? If these batteris are use, then till what time robot work properly??? Is there any arrangement except dead shaft system?, if yes then guide us plz. Can we use carbide metal as a teeth? Plz help us. thnxs. [India]

A: [Mark J.] First, thank you for sending such complete information about your robot weapon. That makes it much easier for me to answer your questions.

One correction: the stall current on the AmpFlow F30-150 motor is misprinted as 375 amps on the Robot Marketplace site. Calculating the stall current from the spec numbers on the AmpFlow website gives either 296 amps (Volts/Resistance = 24/0.081 = 296 amps) or 294 amps (Stall Torque/Torque Constant Kt = 1370/4.66 = 294 amps) -- so let's split the difference and call it 295 amps.

Mounting the drum on a dead shaft: check a few posts down in this archive and you'll find an answer to this same question asked a few days ago -- with a photo. Hollow drum weapons typically have machined 'end caps' that slip into the ends of the drums and are held in place by machine screws inserted thru the drum face into threaded holes in the caps. The end caps have a hole sized for a bearing to support the drum on a dead shaft.

Drum suggestions: why so short a drum? A 24 cm long steel drum 14 cm in diameter with a 16 mm wall thickness will weigh the same (12 kg) as your short drum and will store more energy (4750 joules) at the same RPM (4347). For maximum energy storage, make the drum as long as practical for your design and reduce the wall thickness to keep the weight the same.

Battery suggestions - Weapon: if you look a bit further down the page of the Run Amok Excel Spinner Spreadsheet that you apparently used to calculate your drum energy, just below the 'Results' box is another box labeled 'Battery'. This box reports that your weapon system might be expected to use perhaps 1.45 amp-hours of current in a typical 3 minute match. With a 9 amp-hour battery you have many times the power needed for your weapon.

Battery suggestions - Drive: to calculate the current consumption of the drive motors I need much more information about the motors -- their stall torque, stall amperage, gear reduction, RPM, and wheel diameter. With that info I can estimate their current consumption as well as the robot speed, acceleration, and pushing power. That said, I very seriously doubt that the motors will come anyplace close to completely draining a 9 amp-hour battery pack in a typical match.

Commonly, combat robots have a single battery pack that operates both the weapon and the drive system -- consider using only two 12 volt 9 amp-hour batteries for the whole robot rather than four. You may likely be able to reduce the size of the batteries even further, but I need that drive motor info to be able to tell you by how much.

Alternative to dead shaft: sure -- you can use a live shaft firmly affixed to the drum endplates and supported by bearings in the robot chassis. A live shaft is rarely used because a rigidly fixed dead shaft can be a stiffening member joining the chassis supports together, but if a rotating live shaft suits your design you can certainly use it.

Carbide impactor teeth?: no! Tungsten carbide is very hard, but it is also very brittle and can SHATTER on impact and send sharp shards flying at high speed. US and European events typically forbid carbide impactors for safety reasons -- don't use them!

Q: Hi. For AmpFlow A28-150 motor, what is requirement of battery, and what will be the requirement of amp at 24 volts (for drum motor). [Maharashtra, India]

A: [Mark J.] Briefly, the amperage requirement for any electric motor depends on the torque load placed upon that motor, and you've given me inadequate information to calculate that load.

To calculate the torque load on your motor you need to first calculate the moment of inertia of your drum. This requires specific information on the dimensions of your drum and the material from which it is made. The Team Run Amok Excel Spinner Spreadsheet can perform those calculations for you and estimate the battery capacity requirement for a match of given length.

The AmpFlow A28-150 motor at 24 volts can pull 385 amps at full stall, and the closer you can come to providing that much amperage the faster your drum will spin-up. If your weapon should stall against an immoveable object, your motor may attempt to pull that full amperage from the battery for as long as the weapon is stalled and may damage a battery pack that is not rated for that great a drain. There is no point to having a powerful weapon motor if you do not supply it with the full current it needs to perform properly.

I suggest you search for the many previous posts in this archive covering the use of the Spinner Spreadsheet to calculate moment of inertia and the battery requirements for your weapon.

Q: Mark, I am a little confused about capacity of single tooth snail design drum of 'Touro'.
I am thinking to try that design in a local event.
I know it is very difficult in manufacturing but I wanna try.
Please tell me what I should do. [Tamil Nadu, India]

A: [Mark J.] RioBotz designed the "snail drum" for 'Touro Maximus' using the MatLab Genetic Algorithm Toolbox software. You can find the academic paper describing the drum and the design process here: Drum Shape Design and Optimization Using Genetic Algorithms. I suspect that the drum represents an investment of hundreds of hours and thousands of dollars. It's a whole lot of work for very little gain in drum performance -- largely an academic exercise.

A single-tooth drum is a fine concept, but it is MUCH easier for a competent builder to implement a design using a counterweight balancing insert on the opposite side of the drum from the uni-tooth. The guys at RioBotz were just showing off their technical muscle in designing the snail drum. You should NOT try to duplicate that effort.

Q: I'm making a drum with 14cm O.D. and 15mm wall thickness, which makes the I.D. 11cm. Drum weight will be around 7kg
What kind of bearings should I use for rotation of drum on rod? If I use 11cm OD bearings they will have a bore of 80-90mm which will increase the size and weight of the rod. Please help. [Maharashtra, India]

A: [Mark J.] There's no need for a huge bearing. Drum weapons typically have machined 'end caps' held in each end of the drum by screws. The end caps have holes to mount bearings of suitable size for the supporting rod.

Q: Am using 3HP AmpFlow motor for my drum bot. Weight of drum is 7-8 kg and the length of drum is 14-15 cm. Please suggest me the drum diameter and thickness of drum which stores maximum energy. [Maharashtra, India]

A: [Mark J.] I've spent considerable effort to put together the Team Run Amok Excel Spinner Spreadsheet that allows builders to model the performance of spinning weapons, but there still seems to be confusion about the basic physics of drum design and moment of inertia. I'd suggest that you study up on the topic before continuing your design efforts -- it's more than I can teach here.

The variables that determine the energy that a spinning drum holds at a given RPM are: mass, material density, diameter, and length. You've specified mass and length, and I'll assume that you're using steel. With mass, length, and speed held constant the energy storage will increase with increasing diameter. Examples - for a bare steel tube (no end caps or impactors):

  • 15 cm long - 15 cm diameter - 14 mm thickness: mass is 7 kg and it stores 2900 joules of energy at 4000 RPM

  • 15 cm long - 20 cm diameter - 10 mm thickness: mass is 7 kg and it stores 5600 joules of energy at 4000 RPM

  • 15 cm long - 25 cm diameter -  8 mm thickness: mass is 7 kg and it stores 9200 joules of energy at 4000 RPM
If your primary design consideration is greatest energy storage, make the drum as large in diameter as is practical for your overall design. There are other elements in drum design that should be examined before you get too excited about maximum energy storage -- see recent posts in this archive on drum design.
Q: Hi Mark. If I want 5000 joules of stored energy at 5000 RPM, what should be drum's outer dia and thickness if mass is limited upto 13 kg?
I am confused about it. I can't exceed outer dia 15 cm. Please help me. [Tamil Nadu, India]

A: [Mark J.] There seems to be a lot of confusion about stored energy in rotating weapons. Stored energy in a cylinder rotating around its radius center is a function of:

  • Rotational Speed (RPM)
  • Drum Diameter
  • Drum Length
  • Drum Wall Thickness
  • Material Density

You've given me a desired output and only two of the five variables (rotational speed and diameter). By selecting values for material density and drum length, I can give a design solution for any wall thickness to meet your criteria. For example:

  • A steel cylinder 15 cm in diameter, 100 mm in length, and solid to the center will weigh 11.5 kilos and will store 5000 joules of energy at 5000 RPM.

  • A steel cylinder 15 cm in diameter, 300 mm in length, with a 6.7 mm thick wall will weigh 7.1 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 300 mm in length, with a 31 mm thick wall will weigh 9.6 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 2600 mm in length, with a 2 mm thick wall will weigh 6.7 kilos and will store 5000 joules of energy at 5000 RPM.

I suggest that you download the Team Run Amok Excel Spinner Spreadsheet to model the performance of various spinner dimensions and designs.

I also suggest that you browse thru this archive for several recent posts about drum weapon design.

It is quite odd to specify a weapon diameter, speed, and energy storage as starting parameters, and to then back into the other dimensions. In particular, specifying such a high rotational speed is detrimental to the overall performance of a spinner weapon. Were it my weapon, I would design it to maximize energy storage at the slowest possible speed -- something like:

  • A steel cylinder 15 cm in diameter, 380 mm in length, with a 10 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 3760 RPM.

A 5000 joule weapon does you no good at all if it's spinning too fast to have decent 'bite' and the ability to transfer that energy to your opponent in a single, huge impact. Slow it down a bit. A larger diameter drum would be able to store the same energy at an even slower speed -- example:

  • A steel cylinder 20 cm in diameter, 390 mm in length, with a 7 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2780 RPM.

Or even better:

  • A steel cylinder 25 cm in diameter, 360 mm in length, with a 6 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2175 RPM.

Don't compromise weapon performance with dimensional restrictions that you can avoid, and design for the lowest weapon speed that will store enough energy to be effective.

Q: Hi Mark, is there an equation to calculate the force in a pneumatic ram?

A: [Mark J.] Sure - in pounds, inches, and psi:

Force = π * (0.5 * Ram Bore)2 * Pressure

Q: And is there an equation to calculate the force needed to flip a robot? Thank You. [San Francisco, California]

A: Unfortunately, no -- it takes both force and speed to flip an opponent. A ram with a great deal of force that extends slowly will lift but not flip a given weight into the air. A flipper must be able to flow pressurized gas through the regulator, valves, and ports at a very high rate to maintain pneumatic force as the ram quickly extends. The additional variables make this a difficult analysis. Most builders just go for as much speed and power as they can get and hope for the best.

Q: hi, i m planning to make a razer bot of 50kg, so for that hydraulic weapon , i have purchased a double acting hydraulic cylinder of 1 ton with 35-40mm bore and 150mm stroke, so for that can u give me what should be that psi of hydraulic pump(which pump is suitable)? and for up & down motion im using 12v solenoid valve, is it okay? and pls tell which motor i used to drive hydraulic pump? [Gujarat, India]

A: [Mark J.] Several thoughts:

  1. Your questions are very basic -- I'm guessing that you have no prior experience with pneumatics.

  2. Many very knowledgeable teams have tried to build 'Razer' style hydraulic weapon robots, but 'Razer' remains the ONLY successful hydraulic robot. A hydraulic crusher is a very difficult weapon to get right.

  3. If you knew enough to safely build a hydraulic weapon system, you wouldn't need to ask me how to do it.
Sorry, but I don't think you have enough knowledge or experience with pneumatics to take on building a complex and potentially dangerous crushing weapon. I certainly can't give you that knowledge in a few paragraphs here -- pick a different weapon system.
Q: hi mark lucky here how should i join my drum's impactor with my drum because i have made drum composite of mild steel and aluminium from inside as aluminium will cause major damage if i use the bolts while impacting ? [Tamil Nadu, India]

A: [Mark J.] You haven't told me nearly enough about your weapon drum for me to recommend an impactor attachment method. For example:

  • How thick is the drum?
  • How much of that is steel?
  • What aluminum alloy is the rest?
  • How much does the drum weigh?
  • How fast do you plan to spin it?
  • What is the drum diameter?
  • What do you plan to use for an impactor?
If I guessed wrong about any of these things I could give you a poor recommendation!

Q: lucky again steel hollow cylinder with 5mm thick and aluminum solid inserted in it i am thinkin to run it at 4000 rpm and material for impactor is mild steel and aluminum 6001

A: A solid core drum? Not efficient at storing energy -- the farther from the spin axis the mass is placed, the greater the energy storage. But you asked about impactor attachment...

You can certainly bolt thru the steel cylinder into a solid aluminum core, but bolts are not good at handling the shearing forces the impactor will be encountering. Ideally the impactor should be prevented from shifting on impact by insetting it into a groove machined in the drum and holding it in the groove with bolts. That takes the shear loading away from the retaining bolts, but a groove may not be practical given your composite design.

I'd be tempted to bolt down the impactor, then run a weld along the trailing edge to take some of the shear force off the bolts.

Q: Hi, still i have some doubt in Drum energy storage concept , already i read spread sheet calculation i didn't under stand the concept .. (if the weapon is 14kg and run at 5000 rpm with 5000 joules energy storage means ,what is the size of the drum length and diameter?) [Puducherry, India]

A: [Mark J.] There are a great many posts about spinner weapon design in this archive -- suggest you spend some time reading here.

The Team Run Amok Excel Spinner Spreadsheet can model spinner weapons and answer the type of question you are asking, although it's unusual to design a weapon by setting speed, mass, and energy targets and 'backing into' the drum size. There are several things to consider:

  • General Rule 1: for a given mass, increasing the diameter of the drum and thinning the wall thickness increases the energy storage.

  • General Rule 2: for a given mass, increasing the length of the drum and thinning the wall thickness has negligible impact on the energy storage.

  • 5000 RPM is too fast to spin a drum this massive. The problems with spinning a weapon too fast have been discussed repeatedly in the archives. Half that speed might be about right.

  • Why have you chosen 5000 joules of energy as your energy target? A good weapon design has other parameters that are of equal or greater importance as stored energy.

  • You'll need to consider the performance of the weapon motor you plan to use. Is it capable of spinning the drum to speed quickly enough to be useful?
Given your original parameters, the Spinner Spreadsheet quickly shows one possible solution: a steel drum 120 mm in diameter, 450 mm in length, with 10 mm wall thickness and 8 mm thick endplates will weigh close to 14 kilos (I left a little mass for impact teeth) and have about 5000 joules energy at 5000 RPM.

By using other materials and drum thicknesses, you can find a great many diameter and length combinations that will give the same energy storage at the same mass.

Q: As here you told 10mm thickness and 4mm thick end plate means the steel 10mm is i think its very small thickness hw can i fit 4mm end plate? and the center hole also big if the 120mm diameter means i fix big size of bearing?

A: I do not specifically recommend the given dimensions -- they were just the first solution that came from the spreadsheet after a few minutes of playing with it. If you don't like some of the dimensions, change them and use the spreadsheet to tinker with the other dimensions until the energy numbers come back into line.

The endplates may be welded into place, or machine screws may be run in to afix the end plates to the drum. The endplates may also be reduced in diameter a bit, and inserted into the ends of the drum tube before affixing. Look for a solution that suits your construction preferences.

The endplates may be drilled for any size bearing you like. Pick a bearing that will handle the high stress placed on the weapon and the speed of the drum.

Q: read spinner sheet there is one option for motor and drive which motor they asked driven motor (or) weapon motor?

A: This is a weapon spreadsheet. The 'motor' is the weapon motor and the 'drive' is the weapon drivetrain -- typically a belt and pulleys.

Q: if i can use Ampflow A28 400 means what is the gear ratio?

A: I really don't like the idea of running the drum at 5000 RPM. Search for 'bite' in this archive for discussion on problems with running a spinner weapon too fast. The AmpFlow A28-400 at 24 volts spins very close to 5000 RPM, so if you really insist on spinning the drum that fast, a 1:1 gear ratio would be correct.

Q: For disc column: what is material density Kg/m3? I'm not understand m3.

A: The spreadsheet uses units of kilograms per cubic meter (kg/m3) for material density. The spreadsheet includes a table of densities for common robot materials:

  • Steel is 7800 kg/m3
  • Titanium is 4500 kg/m3
  • Aluminum is 2700 kg/m3, and so on.

Q: actually 5k rpm with a 120mm dia weapon would be a decent tip speed. [Parts Unknown]

A: [Mark J.] My objection to a 5k RPM weapon speed has nothing to do with tip speed. Look up 'bite' in this archive, or read section 6.3.1 of the RioBotz Tutorial.

A weapon with two symmetrical impactors spinning at 5000 RPM has an impactor passing by any point in its arc every 6 milliseconds. How much of your opponent's 'bot are you going to be able to shove into your weapon arc in 6 milliseconds? Given that the question comes from India where the standard attack method is to slowly approach your opponent and ease your weapon into them, all a 5K weapon will do is skitter off a smooth surface. You might get lucky and find an exposed sharp edge for the weapon to 'bite', but I'd hate to rely on that luck to hold for a whole tournament.

The only real use for a 5K RPM weapon is if you're going head-to-head against another drumbot. In those fights, the faster tip speed wins.

Q: who say you have to spin your drum at full throttle?

You ALWAYS design a spinner to run at full throttle. Spinner energy increases with the square of speed -- if you cut your running speed by 50% your energy storage drops by 75%. Design the weapon to be effective at full speed or you're wasting motor power and weapon efficiency.

All these spinner design topics are well covered in the archives -- do your homework and read the earlier posts before you fire off another flip question.

Q: mark lucky again in case of drum's weapon [impactor] which material i should prefer i am thinking about cast iron and ms or i have also option of steel what u say? [India]

A: [Mark J.] NOT CAST IRON! You want a 'tough' material for your impactors -- a material that can take high impact. Cast iron is fairly strong, but it is brittle and can shatter on a high-energy impact sending metal shards in all directions. Various steel alloys cover a wide range of strength and toughness. I don't know what types of steel are available to you, but even mild steel is preferable to cast iron.

Q: Hello. We've all seen videos of antweight full body spinners (FBS) colliding with another bot and then flying off the walls of the arena (ultimately ending upside down or outside of the arena). This is why generally [Ask Aaron] has advised to avoid building FBS at this weight class (even though it is so fun to build them!).

What do you think of mounting the spinner on a suspension between the spinner and chassis. When the spinner hits another bot it will recoil into this suspension, attenuating the impact over a longer period of time so the bot as a whole will not bounce back as far. I imagine that it will not throw the other bot as far either. The shell of the bot at least is still very rigid. it just has a soft linkage to the chassis. not sure if there is a benefit to doing this, what do you think? [San Diego, California]

A: [Mark J.] Your analysis is correct. Placing suspension between the horizontal spinner shell and the chassis could reduce the recoil to your chassis, but it would also reduce the effective impact of the weapon on your opponent. Effectively, it would help your opponent as much as it would help you! You might just as well reduce the power of the weapon - it would be simpler. I've gotta vote 'no' on this idea.

For maximum weapon impact in a horizontal spinner you want the structural path from the point where the weapon strikes to the center of mass of the robot to be as stiff as possible. Vertical spinners have a strong advantage in this regard, as the recoil path (downward) is eventually supported by the arena floor (and the planet beneath). Section 6.6 of the RioBotz Combat Tutorial discusses the importance of mounting stiffness in maximizing weapon impact -- a good read.

Q: hey mark lucky here i am thinking to combine my design so that it should be combination of spearbot and drum bot whats your view [Maharashtra, India]

A: [Mark J.] See Frequently Asked Questions #26. My advice is to place your entire weapon weight allowance into a single weapon. Dual weapon robots have a serious drawback: two weak weapons that cannot work together are not as good as a single powerful weapon. Add the weight of that spear to the drum weapon to increase its power.

Q: Hello Mark. After seeing Biohazard being torn up into pieces in Combots Cup 2005 I was wondering a question. Could 4-Bar Lifting arms still be competitive in Heavyweight Class? [Beijing, China]

A: [Mark J.] The high-energy spinner weapons in the US heavyweight class are brutally destructive. With judging leaning so heavily on damage and no real possibility of boosting an opponent out of a US arena for a 'knockout', I don't think an electric lifter is a competitive design in the heavier weight classes.

Q: Dear Mark,I think Overkill and Toe-Crusher's design is quite different from other thwackbot. Is it has any advantages or disadvantages? [Yangzhou, China]

A: [Mark J.] 'Overkill' and 'Toe Crusher' are not technically 'thwackbots', they're torque-reaction hammers. There is a description of how they function and the drawbacks of the design in this archive -- search here for 'reaction hammer'. A torque-reaction hammer has far too little power to be a contender in current competition.

Q: Hey Mark,

I am building a 1lb ant weight vertical spinner bot. What is the best way to tell if a motor is adequate for a vertical spinner? I know that you can calculate what the kinetic energy in a spinning weapon. However, the motor I have (and I am trying to figure out if it is adequate) is a Rimfire 370 and is rated to 1000kV, weighs 1.4 oz and has a Burst Watts of 165. So basically, is there a way to look at the critical specs of a brushless outrunner (maybe there is a range of kV?) and know if it would be a good motor? I also know that the forces of a weapon are largely dependent on the radius and mass of the weapon itself, but I can assume that I can judge if said motor makes sense before designing the impactor. Does that make sense? If not, let me know.

Thank you very much,
New York

A: [Mark J.] You're on the right track, New York. The load on the motor when spinning up a weapon is dependent on the 'moment of inertia' (MoI) of the spinner, which does depend on the spinner shape, dimensions, and mass distribution. The challenge is in matching the MoI of the weapon to the power of the motor to assure that the motor can spin the weapon up to speed quickly enough to be useful.

If you're looking for a quick estimate of the suitability of the motor for a spinner, you'll want to look at the 'kV' and 'burst wattage' ratings:

  • The 'kV' of a motor tells you how fast the motor will spin for each volt of electricity it receives. Multiply the kV by your battery voltage for the no-load RPM of the motor. This is important because you don't want a motor that has to be 'geared down' a lot to provide a (not too fast) useful speed for your spinner. The Rimfire 370 has a kV of 1000, so with a 7.4 volt battery it will have a no-load speed of about 7400 RPM, which can be made into a reasonable spinner speed with a belt reduction of around 2:1. That's good!

  • The 'burst wattage' is an indication of the power the motor can produce to accelerate the spinning mass up to speed. Twice the wattage will spin a given weapon blade up to speed twice as quickly. The energy storage of the spinning weapon is measured in 'joules', and for an ant about 30 joules is a reasonable amount of stored energy. A joule is equivalent to one watt of power applied over one second of time, so your proposed 165 watt motor could (at peak output) store 30 Joules (watt-seconds) of energy in a spinner weapon in less than 1/5th of a second (30 watt-seconds / 165 watts = 0.18 second). That's huge overkill!
Now, in actual operation the motor has to start from a standstill and generates only a fraction of it's rated output power until well up in the RPM range - but even estimating that the average output of the motor as it accelerates is half the peak output, this motor has WAY MORE than ample power for an ant spinner. The Rimfire 370 would be more than enough motor for a 3 pound 'beetleweight -- for an ant I'd go with a lighter motor and put the saved weight into a heavier spinner bar/disk/drum. Around 50 watts output per pound of 'bot is plenty!
Q: i want to make a drum of 8 kg with length 15cm and dia 13cm, what material should i use for drum and teeth, and what should be the thickness??[Maharashtra, India]

A: [Mark J.] That's a very small drum to weigh 8 kg. A solid aluminum drum that size only weighs 5.5 kilos, so I guess you'd best use a mild steel alloy.

A steel drum 15 cm long and 13 cm in diameter with a 15 mm thickness weighs 6.34 kg. Add two steel end plates 5mm thick and the total weight comes to about 7.4 kg. That leaves you 0.6 kg for your teeth. Energy storage works out to about 1800 joules at 3000 RPM.

Threading large, short bolts into the drum and leaving the heads exposed works reasonably well for teeth. They are easy to replace when damaged and give you flexibility in placement and size. If you have more conventional teeth in mind, a hardened tool-grade steel (like S7 alloy) is the preferred material.

I will mention that it is unusual to design a drum weapon starting with a specific size and mass. Changes to the dimensions can greatly impact the energy storage capacity of the drum for a given mass. Some time spent with the Team Run Amok Excel Spinner Spreadsheet may yield a large bonus in performance with small dimension changes. Example: increasing the drum diameter to 15 cm with a 12.5 mm thickness will increase the energy storage by 22% at the same weight.

Q: if i use same size pulley to rotate my 8kg drum from ampflow e30-400 how will be the loss and how much rpm will i get?

A: A V-belt has very high power transmission efficiency: up to 95%. There is very little power loss. Peak RPM will depend largely on the aerodynamic drag of the drum and teeth. As a guess, I'd say perhaps 5200 RPM @ 24 volts. Spin-up time will be about 4 seconds and energy storage close to 4000 joules.

Q: What's the name of the weapon used by Original Sin when fight with Last Rite? The yellow wedge can stop Last Rite's spinning bar immediatly. [Guangdong, China]

A: [Mark J.] You're greatly overstating the effectiveness of Original Sin's big yellow bar spinner trap. Team 'Late Night Racing' built the trap specifically to counter Last Rites' huge bar spinner, but as you can see in this video the defensive attributes of the device are not always effective.

The only real strategy when fighting 'Last Rites' is to hope they break down; sometimes you get lucky and they do!

Q: What mechanism do i use to lift the 4-bar lifter? I am using a 12V battery. The weight of my robot is around 40 kg. I am not allowed to use compressor. The dimension of my robot is 40*30. [Poona, Maharashtra, India]

A: [Mark J.] There are more than a dozen posts on 4-bar lifters in this archive that include power options and design tools. Start by reading thru those posts.

If you can't use pneumatics your only real power options are high reduction gearmotors directly driving the front or rear bar and linear actuators operating a bell crank attached to a bar. Both of these options are well discussed in the archive. The T.i. Combat Robotics 4-Bar Simulator is a very useful tool to assist in designing 4-bar lifters with electric power.

Q: Hey Mark! Last year I powered my robot's spinning weapon with a V-belt. Generally, I had no problems. I'm making some design changes this year including upping the weapon rpm, and I'm considering switching to a timing belt. I figured the wider contact area and teeth would boost my power trans efficiency. However, I still need the pulley to be able to 'slip' as I'm not using a clutch and don't want to wreck my motor during a hard stop when my weapon makes contact. Will using this style belt risk damage to my motor or will I have some flexibility as with the V-belt? Thanks for the help! [United States]

A: [Mark J.] The larger the robot, the more common it is to find a V-belt weapon drive.

  • A V-belt is simple to set up for slip -- loosen it a little and it slips more, tighten it a little and it slips less. A V-belt will also put up with a fair amount of misalignment, and the slippage is smooth.

  • A toothed timing belt is designed to not slip at all. It must be set up quite loose before it will slip at all, and you may need to set it up so loose that you risk the belt jumping off the pulley. Timing belts are often used in small robots simply because small V-belts are not available.
Both types of belt are very efficient at transmitting power. If you've had good results with the V-belt drive I'd recommend sticking with it.

There are many discussions of weapon belt drives in this archive that may give you additional help in your decision. Search here for 'timing belt'.

Q: Hi Mark i am from india making a bot of 60 kg. My weapon is a verical disk and it is bububling so any sugestion to make it stable? [India]

A: [Mark J.] I'm not sure what you mean by 'bububling', but if your disk spinner is unstable it's probably out of balance. I've previously posted the process for balancing a spinning bar or disk -- search this archive for 'How do I balance the weapon'.

The faster you spin a weapon the more critical weapon balance becomes. You haven't given me any info, but you may be attempting to spin your disk at an unrealistic speed. The more information you can provide, the better chance I have of giving you a useful answer.

Q: hello sir, can you suggest me some designs of counter robot against drum weapon robot??? bot weight class is 120lb... and please suggest me some designs of lifting and flipping mechanism also.... [India]

A: [Mark J.] It's disrespectful to ask me to invest my time to answer a question without first spending a bit of your time to see if the answer already exists here at Ask Aaron.

  • The 'counter drum' question has been answered multiple times -- most recently just a few posts below.

  • As to suggesting designs, I have stated (recently and often in the archives, and for a long time in the ) that we do not supply designs for robots or weapon systems. Ask Aaron is not a free engineering service. If you submit a design, I will offer a critique and perhaps provide some alternatives.
I suggest that you start by doing your homework and reading thru this archive for some design ideas.
Q: Hey, I am unable to [mount] my ampflow e 30-400 motor vertical on my base...those screw in the base of motor are [too] small to hold it...suggest some solution? [Tamil Nadu, India]

A: [Mark J.] The E30-400 motor (diagram) does not have the large mounting bolt holes found on some other AmpFlow motors. You haven't told me anything specific about your design, but I can offer some general suggestions.

  • You may remove the motor endplates, enlarge the mounting holes, and tap them for larger bolts.

  • A large motor like the E30-400 really must be mounted at or near both ends to spread the physical loading forces.

  • Clamp-style mounts do not rely on the endplate mounting holes and can be adapted to a wide range of support designs.

Q: Hi, I wanted to know how can we calculate power required for a flipper like Ziggy who throws away his opponents rather than mere inverting? For example how can we calculate how much energy the motor should provide or how much energy should a pneumatic actuator provide?

A: Mark J. here: there are a great many posts about flippers in this archive. I suggest you start by reading thru them.

A few significant points:

  • Forget about direct electric power for a flipper. Electric motors cannot realistically provide the explosive release of power needed to get the job done.

  • While it is possible to calculate the energy required to toss an opponent a given height, the formula assumes direct and instantaneous conversion of that energy at the center of mass of the opponent -- both highly unrealistic assumptions.

    Maximum Height Opponent Will Be Tossed = Weapon Joules / (Opponent Mass (kilos) * 9.8)

  • 'Power' is a combination of force and speed. A pneumatic flipper must have very efficient gas flow thru large valves, ports, regulators and connectors to get that combination. It isn't cheap, and it can be very dangerous.
A successful flipper requires knowledge, experience, trial-and-error, and a fair scoop of luck. Study the design of good flippers until you can identify the factors they all share.
It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok

Q: Hi Mark, What according to you will be a better option as an impact weapon tooth - S1 or H13 tool steel?? [Las Vegas, Nevada]

A: [Mark J.] Without knowing the specifics of your design, I would in general pick the S1 tool steel.

The S-series tool steels are grouped together for their primary trait of shock resistance -- valuable in an impact tooth. S1 tool steel is fairly hard as well, which will give it an advantage in surface durability and the abilility to effectively transmit impact.

The H-series tool steels are known for their ability to maintain strength at high temperatures -- not a primary consideration for an impact tooth. H13 tool steel is very 'tough' but not particularly hard. It is also expensive due to its high chromium content.

Q: How would you calculate the oz-in [of torque] necessary for a beetleweight 4-bar lifter? I saw this in the Archives:

Stall Torque (oz-in) = length of lifter arm (inches) * weight class (ounces) * 1.67

...but how would this be transferred to a 4-bar lifter? [Fulton, Maryland]

A: [Mark J.] You didn't dig thru the archives quite far enough. The formula you found is for a simple single-pivot lifing arm, and it cannot be transferred to a 4-bar lifter due to the many design variables inherent in that design.

As noted several times in this archive, 'Total Insanity Combat Robotics' has a 4-Bar Simulator tool to assist in designing 4-bar lifters with electric power. Read thru their page and download the simulator. Play around with the lifter design until you get a good lift path and a reasonable torque requirement.

Q: When I tried to download the Setup for the simulator, an error message came up that said that the file requires .NET framework 1.1.4322, but when I went to the website it said that it is no longer offered that package. I have a Windows 8.1 computer and I'm not sure the other available packages will work?

A: .NET Framework 1.1 is still available for download direct from Microsoft, but it is not compatable with Windows 8.1. Microsoft offers some suggestions for Windows 8 users trying to run older .NET framework programs that might help.

If you simply can't get it to work on your computer, find a friend with an older version of Windows and install on their machine.

The Total Insanity simulator is very well suited for robot applications, but a quick seach for "4-bar simulator" reveals some other programs are out there. You may wish to investigate them as well.

Q: hi mark i'm using an ampflow E30-400 motor to drive a horizontal spinner of(40*20*2.5)cm(l*b*h) and its of 10 kgs. i just wanna know how much energy will it be storing and its effectiveness against drum bots? weight class is of 60 kgs. [Khamgaon, Maharashtra, India]

A: [Mark J.] I'm getting many questions for which 'Ask Aaron' already has tools that can provide the answer. In this case, the Run Amok Excel Spinner Spreadsheet is the tool. Let me walk you thru the steps.

Starting at the 'Motor and Drive' section (specs from the AmpFlow website):

  • No-Load Speed - The unloaded AmpFlow E30-400 motor spins 5700 RPM @ 24 volts.
  • Stall Torque - 1500 in-oz converts to 10.6 N-m.
  • You haven't told me what Reduction Ratio you'll use, so we'll leave it at the default 4:1 for now.

The default spreadsheet example is for a spinning disk and you're building a bar spinner, so in the 'For a disk' section change the Thickness from 6.4 mm to 0.0 mm to zero out the disk mass.

In the 'For a bar' section:

  • Material Density - 7800 (for steel)
  • Length - 0.4 meter
  • Width - 0.2 meter
  • Thickness - 25 mm

That's it. You can read the results right off the spreadsheet. The calculations say your bar weighs 9 kg, not 10. It also looks like the default 4:1 reduction ratio is too high -- the spin-up time is very short and the energy storage is small. A reduction around 2.5:1 looks about right. Want to know the energy storage? Run the spreadsheet for yourself.

You haven't told me enough about the weapon for me to even guess at its effectiveness. A weapon's effectiveness depends as much on the details and construction quality as on its type and dimensions. One critical factor will be the weapon height -- if you're building a drum-killer you'll want to set the weapon as low as possible in an 'undercutter' position. Hitting a drum weapon high is a major error.

I can say that horizontal bar spinners are at a general disadvantage. When your weapon hits it will throw your opponent in one direction and toss your 'bot in the other. This weakens the impact and can send your 'bot bouncing off arena walls like a pinball. Something to consider...

Q: Hi Aaron.. in a face to face combat between a drum and horizontal disk spinner, (both bots invertible ones) both having same mass and rpm, can you tell me which one will be more effective and which will have more impact?? [Orissa, India]

A: [Mark J.] Several considerations...

  • As detailed in the question immediately below, the power of a spinner weapon depends on more than just the mass and RPM.
  • A robot's overall effectiveness depends on more than just the weapon.
  • Different weapon types seem to do better in specific weight classes.
Being very general, the vertical attack angle and more robust structure of a typical drum weapon is usually a winner in a rock/paper/scissors comparison against a horizontal spinner. An exception may be in the hobbyweight class, where a lot of top competitors are horizontal spinners. Not sure why.

A horizontal spinner has the potential for a greater 'impact' than a typical drum weapon, but the horizontal impact tends to throw both robots in opposite directions. You may bounce around the arena and do as much damage to yourself as to your opponent.

Q: Hi Mark...same guy with drum and horizontal disk question.... Err! I made a mistake...instead of horizontal disk it's a vertical spinning disk. The weapons' maximum dia. (outer dia. for drum) 6 inches. Weight classes: 30kg as well as 60kg. Considering both bots are strongly built with 2 teeth each, which one will win or at least be more effective over the other??

P.S.: I am very sorry for Aaron. I came to know of his demise few days back. And thank you for continuing this website. May God bless you!

A: I can't tell you which will win. As I mentioned above, a robot's effectiveness depends on more than just the weapon. All of the components must work together reliably or the 'weak link' will fail and spell disaster. It's a serious mistake to concentrate on the weapon system to the detriment of the rest of the 'bot.

As to which is more effective, take note that there are no successful big vertical spinners in the current combat scene. They have several problems:

  • A big vertical disk has maneuvering problems -- a quick turn tries to tip the whole 'bot on its side due to gyroscopic forces. Awkward!

  • A large diameter vertical disk exerts a lot of impact force back into its mounting structure. The bigger it is the greater the difficulty in controlling the impact forces.

  • A large vertical disk will tend to strike the opponent as the impact tooth is traveling predominantly 'forward' rather than 'up'. This can result in a lot of 'kick back' that throws you and your opponent away from each other, rather than tossing your opponent upward while you remain braced by the floor.
Now, if you're just talking about some intermediate drum/disk hybrid design like the narrow drum on 'Professor Chaos' rather than the true vertical disk on 'Nightmare', I'd say that in general I would prefer a larger diameter drum to a smaller one. For any given mass and speed, a larger diameter drum will have greater stored energy for a more powerful impact.
Q: Sir, I am making a bot. I have used a A28-400 motor for weapon to drive a 16 kg hollow drum using a pulley belt mechanism. Voltage provided is 24V. I have to use the weapon for about 5 mins. total no. of spin ups will be around 10-15. How many Amps will it discharge?

I have a confusion of buying a Lipo and want to know which battery should i opt for- should i use a 6S 5000mAh 65C-130C battery or a 6S 5000mAh 45C-90C battery. Which one will be efficient considering the cost too? [Mumbai, India]

A: [Mark J.] You've given me too little information to answer your questions.

The amount of energy storage in a spinning weapon depends on:

  • The mass of the weapon;
  • The speed of the weapon (RPM); and
  • The shape and dimensions of the weapon.
Without a full description of the weapon dimensions and the speed at which it spins, the energy needed to spin the weapon up to speed cannot be calculated. Everything else being the same, a long but small diameter drum will store MUCH less energy than a short but large diameter drum. Example -- a hollow steel drum with a 1/2" thick wall spinning at 2400 RPM:
  • a drum 4" in diameter and 24" long will weigh 16 kilos and will store 920 joules of energy; but
  • a drum 9" in diameter and 10" long will still weigh 16 kilos but will store 4800 joules of energy -- more than five times as much!
The Run Amok Excel Spinner Spreadsheet can perform all of the calculations needed to determine the energy storage of your spinner weapon, and can estimate the battery capacity requirement. I strongly suggest that you make use of this powerful tool.

As to your battery selection: the 5000mAh capacity you propose is MUCH larger than just your weapon might use -- I assume you will use it to power the robot drivetrain as well. You have given me no information about your drivetrain, so I cannot comment on the overall suitability of the battery.

Q: Hey wassup, I have a question about horizontal spinner. I am planning to place a 7 kg horizontal spinner with a drive motor of ampflow E30-400, the robot weight category is 40 kg. so please tell will that motor be enough to take on the opponent? If yes, tell me which drive is more effective: belt drive or chain drive...

A: [Mark J.] The motor and total weapon weight are OK for a weapon on a 40 kilo robot, but the effectiveness of the weapon depends as much on the dimensions and shape of the rotating mass as it does on the mass of the weapon and motor power. Suggest you search thru this archive for prior posts on calculating weapon effectiveness.

Some quick calculations assuming a simple steel bar spinner (600 mm X 80 mm X 18 mm = ~7 kg) and an 8:1 weapon motor gear reduction show good energy storage and acceptable spin-up speed: about 3000 joules in 3 seconds. That would do nicely.

Robots in this size range typically use a V-belt drive for their weapon drive. A V-belt can be set to slip if the weapon stalls, which can keep the drive motor from stalling. A stalled weapon motor draws high current which can heat-damage the motor.

Q: Hey,thanks for the reply. I am planning to use a horizontal bar of steel weighing 7 kg(400mm,100mm,22mm) which has 1800 joules in 2 second. Weight class is 25kg, and i am going to use an ampflow e30-400 motor for the drive at 3:1 reduction. My question is, will my spinner stop if it hits the opponent?

A: I see you've been making good use of the Run Amok Spinner Weapon Excel Spreadsheet. Is it a 40 or 25 kilo weight class? You've mentioned both.

It's very difficult to model the precise results of a spinner hit on a complex shape such as a combat robot -- particularly for a horizontal spinner. I think it is safe to say that a hit by an 1800 joule horizontal spinner weapon on a 25 kilo opponent will send both you and your opponent skittering away in opposite directions, and if the weapon is stopped, the impact should push it clear and allow it to resume spinning.

An instantaneous stoppage like this is not a problem, but you want to avoid a prolonged stall that could damage the battery, speed controller, and motor. That type of stall can happen if the 'bot is trapped against the arena wall, or anytime your weapon is held immobile with power on.

Strange and unpredictable things happen in robot combat, and I advise use of a slip-capable belt drive for spinner weapons to keep potential stall damage to a minimum.

Q: Hi Aaron! I want to build a flipper robot for featherweight. not for some particular event, but for interschool and inter university events. Can i use an air retract for lifting a 6-7kg robot?

I found this air retract kit, let me know if its okay for my use.

I am new to pneumatics, read all your answered questions already, excellent responses!

A: [Mark J.] Model aircraft air retracts are designed to raise and lower a few ounces of landing gear in a slow and realistic manner. The valves, ports, and connectors are all low-flow rate to keep the actuation rate slow -- you don't want to wildly snap aircraft landing gear up and down.

The power of the actuators is calculated by multiplying the bore area of the cylinder by the operating pressure of the system; neither of those are given in the description. Guessing at 15mm bore and 80 psi pressure, the maximum thrust of the actuator would be about 20 pounds. Given the geometry of the retract mechanism that I can see in the photos, the thrust would be reduced to maybe 2 pounds at the flipper.

Two pounds of slowly applied thrust is not going to do you any good at all in a 7 kilo 'flipper'. Suggest you seach thru the this archive for previous posts on pneumatic flippers for component sources and suggestions.

Date marker: January 2014
Q: If you are driving a lifting arm in the shape of an 'L' do you measure the torque necessary to lift your opponent on the diagonal from the hinge to the tip of your arm or what? By measure torque necessary I mean arm length inches multiplied by weight you're lifting. [Seattle]

A: For calculation purposes on a single-pivot lifting arm, measure the straight-line from the pivot point to the tip of the arm. That will give the torque needed to hold the arm against gravity when that imaginary line is horizontal. Add some additional torque to prevent the lifter motor from stalling.

Robot haiku:

Greater torque will let
The lifting arm maintain speed
Under heavy load.

Q: Hi Aaron, i have read in the archives that by using the timing belts instead of chain for a drum will reduce impact to the motor because the belt will slip during impact. Between a toothed belts & v-belt, which one is the better option? [Pulau Pinang, Malaysia]

A: A toothed timing belt is favored by builders for small combat robots -- insects mostly. They are well suited to the high RPM of small weapon motors, and the small timing belts and hubs are easier to find than small v-belts. The greater rotating mass and amperage consumption of weapon motors for larger robots favors the more predictable and adjustable slip threshold of v-belts.

Q: I already have the toothed belt components for my drum weapon. It is a 3" drum, 10mm thickness, with length of 20cm, and weight around 3 to 4 kg. This drum will be driven by the Amplfow A28-150 motor. Do you think i should just stick with the toothed belt or i should seriously consider the v-belt? Can you clarify what do you mean by "more predictable and adjustable slip threshold of v-belts"?

A: A toothed timing belt is designed to not slip. The belt drive can be made to slip if set up very loosely, but adjusting the point at which load causes slipping can be tricky and imprecise. Slipping can also be very hard on the belt and can place large loads on the weapon mountings.

A v-belt is much easier to adjust for the torque it will transmit before it starts to slip. A little looser, it slips sooner -- a little tighter and it slips later. Slipping is smoother and places no extra load on the bearings and mountings.

I'd go with the v-belt for your purpose.

Robot haiku:

V-belts work well
For the heavier robots.
Make sure they're aligned!

Q: Hi Aaron, its me again with a follow up on drum weapon toothed belt vs v-belt post. I would like to hear your advice on which method should be preferred to control my [AmpFlow A28-150] weapon motor.

I have 2 options, the first one is to control the weapon using a battleswitch and a 24v white rodgers solenoid. This option provide higher amp operation but its one way rotation and no control over the speed. The 2nd option is using an ESC, which offer speed control and fwd-reverse rotation...but the best ESC that i have for now is only rated at 40amp constant and 80amps peak (for few seconds). The robot is designed to be compatible with both setup and once the drum weapon is assembled, both setup will be put to the test. Please share your view. Thanks! [Pulau Pinang, Malaysia]

A: I wouldn't be comfortable controlling a weapon motor that can pull 385 amps at stall with an ESC that turns into smoke after a few seconds at 80 amps. You'd have to set the belt drive pretty loose to assure a max current of 80 amps with the weapon stalled.

You haven't mentioned how fast you plan to spin your 4kg drum, but I don't think you'll have enough gyroscopic interference with robot turning motion to wish that you could slow the weapon down. The only real advantage of the ESC would be to reverse drum rotation if the robot is inverted.

I think you'll be better off with the solenoid control.

Robot haiku:

Best 'bot attribute
Is reliability.
Use the solenoid.

Q: Dear Aaron, Anthony D here with a curiosity question relating to a difference in weapon performance involving 'Mangi'. What speed increase (percentage wise @ 180 degree swing) would Mangi receive if Al switched from a EV Warrior weapon motor (1.55 hp @24v - no longer available) to a A28-400 AmpFlow motor (4.5 hp @24v), and how many joules of energy would be availible compared to the current 78 which you calculated and deemed dim against its spinner counterparts?

A: Mark J. here: one might imagine that pumping three times the power into a weapon would result in three times the impact energy, but this is not the case for an electric powered hammer like 'Mangi'. Three times the acceleration can give three times the energy storage in the same time period -- but the faster hammer will traverse the 180 degree swing more quickly and not have the same time to build energy as the slower hammer.

Some quick modeling with the Run Amok Electric Hammer Excel Spreadsheet shows that applying three times the power to Mangi's hammer would increase the speed of the hammer weapon at the end of a 180 degree swing by only about 42%. Since kinetic energy increases with the square of speed (Ek = 1/2 MV2), the stored energy would increase by just about 100%: call it 160 joules. That assumes that the increased hammer acceleration would not flip Mangi over backward!

A featherweight class spinner might easily pack four or five times that much energy in their weapon, so although the additional power helps it still isn't comparable to a good spinner.

Robot haiku:

Quicker hammer gives
Less time to store joules.
Diminished returns.

Q: I'm the antsaw guy, I've decided to abandon the saw blade for a pseudo drum. The drum will made from a four inch length of half inch hexagonal bar with steel screws for weapon heads.

How efficient is a hexagonal bar in terms of energy storage, the only other bot I could find with such a weapon was Rumble Robotics' Quiver. Should I go with the route of the drum or go back to my saw blade design?

A: The problem isn't the hexagonal shape, it's the tiny diameter. In general, the smaller the diameter of the drum, the poorer the energy storage. Read up on Rotational Moment of Inertia.

  • Your mini-drum will weigh about 3 ounces but will store only 1/7th of a joule at 4000 rpm. That totally stinks!

  • A three-inch diameter circular saw blade the same weight will store more than 40 times that much energy at the same speed.
Your mini-drum won't store enough energy to help at all -- the weapon motor would have to do all the work. Make it bigger or go back to the saw.

Robot haiku:

Did you notice that
'Quiver' never won a match?
Don't copy failure.

Q: hey aaron, m working on [a 60 kg copy of] professor chao's middle weight bot. i have alloted 18 kg weight out of 60 to weapon & its transmission. m not geting how to draw geometry of weapon. although i have calculated dimension as follow: base circle radius 10 cm, thickness 5 cm & tooth 5 cm, from riobot tutorial.
please help me how to draw a cad drawin with this dimension.
also i have decided to use ampflow E30-400 motor for weapon & E30-150 for drive .is it ok if not plz guide me....
thank you. waiting 4 ur rply. [Pune, India]

A: Sorry, but Team Run Amok doesn't claim any expertise in CAD. We couldn't assist you in drawing even a simple drum design, let alone a complex design such as used on 'Professor Chaos'.

Although I admire the design and machine work that went into making PC's one-of-a-kind spiral single tooth drum weapon, I can't recommend that anyone attempt to copy it. The drum features inset counter-weights, a hardened impact tooth, and elaborate pocketing. I don't know what software package was used to develop the complex balanced design, but it certainly isn't anything I have access to.

Although the weapon has been successful, its complexity goes against our team design philosophy:

A combat robot is a tool for defeating other robots.
The best tools are simple, reliable, and easy to use.

There are multiple posts about single-tooth spinner weapon design in this archive that cover simpler designs.

Your choices for weapon and drive motors are both adequate, although they are considerably lower in power than the Ampflow A28-150 motors used for both weapon and drive in Professor Chaos. Don't expect the same level of performance with lesser motors.

Q: hey aaron, thanks for your guidance, m the same person who had ask you about chaoe's weapon few days before. but i got one question, instead of spiral profile if we use smiple disc and attach a square block(red block shown in previous answer) on both sides?
are there any drawbacks with respect to original chaoe's weapon? will it be as successful as original one? please give your opinion. i hope you hv got sufficient info.
waiting for your reply. thank you.

A: A single-tooth weapon has a considerable advantage over multi-tooth designs. Browse thru this archive for several discussions on this topic.

I can't tell you how successful your robot will be based on a broad description of the weapon disk. The success of a combat robot depends on much more than the design of the weapon. All the components and systems of a combat robot must work together. A single weak link will undermine the potential of any robot. 'Professor Chaos' does have a well-designed weapon system - but without excellence in the drive system, chassis, armor, driving and radio setup it would be just another 'bot. Don't emphasize weapon design to the detriment of the other systems.

Robot haiku:

It's our team motto:
'Complex design is easy,
But simple takes work.'

Q: sup Aaron. thinking of making a FBS that has some angled wing flaps on it so that when it spins, downforce is created. Has this been done before, do you have any pics, and did it work well? what kind of downforce can you get without compromising your spin speed too much? is the downforce irrelevant since when you impact someone, you stop spinning and lose the downforce at precisely the instant you need it the most?

THANKS - antweight frying pan fbs guy from about a year ago

A: Mark J. here: just when I think that every possible design and gizmo has been tried in combat robotics, somebody comes up with a new idea. No, I don't think this particular approach to downforce has been tried. I've seen many magnetic downforce designs, and a few vacuum fan designs that attempted to suck the robot down to the arena floor, but never rotating winglets to push an FBS shell downward.

The amount of downforce you might expect from small winglets is not going to be very great, and they will create significant drag. You can model the downforce and drag with the FoilSim III java applet at the NASA site to get some idea of the effectiveness of potential designs and some parameters on wing size and attack angle.

Some really quick results I got from FoilSim indicate about one pound of drag for every 2 pounds of downforce from stubby winglets -- and only about one pound of total downforce for a pair of 2.5" square winglets spinning at 80 MPH. That drag is going to take a big dollop of extra torque to spin, and the downforce is pretty puny. It doesn't look promising.

The question of losing downforce on impact is interesting. When you hit your opponent, your FBS shell will not often come to a complete stop, but it will certainly slow down and downforce will decrease. Given that the downforce numbers aren't looking all that good to start with, my best guess is that the design isn't all that workable.

Play around with FoilSim and see if your results match mine. Maybe a little more experimentation will get some better numbers.

UPDATE: I worked thru some additional designs in FoilSim. It looks like a simple upcurved plate is superior to a true airfoil for winglets this size -- superior stall resistance and a little better lift/drag ratio. Unfortunately, still unimpressive downforce.

Robot haiku:

Spin little winglets!
Stabilize my shell spinner!
Wait -- why the slow spin?

Q: So I got my first robot running about and now I want to take it up to the next step and add a spinner to it, and I wanted to ask some things:

I noticed that older ants and beetles had saw blades for their weapons. How come they fell out of style, and is it still viable to use a saw blade as an ant spinner?

A: You're right -- big spinning disks/saws are 'out' and smaller diameter drums are 'in'. Several reasons:

  • A vertically-mounted saw blade has to be relatively large in diameter to store enough energy to be an effective weapon. Spinning a weapon that large creates significant gyroscopic resistance to turning, which greatly reduces the maneuverability of the robot. A smaller diameter drum weapon can store the same energy with much less gyro turning resistance.

  • The impact angle of a large diameter weapon is small, which creates a 'kickback' effect that than throw the 'bot backward. A small diameter drum has a better chance at a larger 'upward' impact angle that throws the opponent in the air and creates less kickback.

  • The many-toothed saw blade can have serious difficulty getting 'bite' on a smooth surface of an opponent. There is very little time for a decent chunk of your opponent to enter the spin radius of the weapon beween the teeth -- the blade will often just 'skitter' along the surface rather than digging in for a good impact. A spinner with one or two teeth has a much better chance at a big hit.
Going head-to-head, a fast drum weapon has a superior chance of 'launching' the big saw weapon due to a combination of maneuverability, attack angle, and bite. I don't think I can recommend a saw weapon against the current drums.

Q: Also, with proper mounting, can a finger tech pulley double as a spinner hub?

A: The key words here are 'proper mounting'. On the discontinued 'VDD saw blade hubs', the screws that hold the blade to the hub did so entirely by compressing the blade to the hub with the screw heads -- the screw shafts do not pass thru holes in the blade. Standard screws are NOT well suited to withstand shear forces of the type they would be exposed to if used to bolt directly thru the blade into the hub. The hub junction is a VERY high stress location.

Getting the blade perfectly centered and then adequately locked in place on the FingerTech pulley would be a significant challenge. Don't bodge the hub! Several on-line machine shops could make a hub similar to the VDD hubs that would do the job correctly. I'd go for that.

Q: Thank you for your help. I do have another question:

Is it possible to cut off extra teeth from the saw blade with out compromising the strength of the blade?

A: Yes you can, and you don't have to cut off the whole length of the extra teeth. If you do the math on how fast the blade spins and how fast the 'bots move forward, you'll find that the realistic effective 'bite' for a typical antweight is about 3/16ths of an inch. Keep two teeth opposite each other and trim 3/16ths of an inch off the other teeth. You'll gain the extra bite and save most of the prime rotational mass at the outer edge of the blade.

Robot haiku:

Big saw blades are out.
Smaller drum weapons are in.
Do NOT bodge the hub!

Q: Hey Aaron,
I am working with a tutor on Solid Works here at Brown University. I am doing a summer program and I am going to attempt to build a 1lb drum bot in SW. My plan is to build it with a dead shaft and a belt drive. What are all of the components that go into just the spinner apparatus? I basically need to make a parts list. I understand that the shaft does not spin and you need the belt to spin directly to the drum. My plan is to make a drum spinner at Starbot potentially with their equipment. Optimally, I would do it for Thunderdome. Thanks Aaron!!!

A: The components vary a bit with design -- like whether the drum is hollow or solid, and the style of impactor. A general list for a hollow drum:

  • Drum tube
  • Drum impactors (variable number - depending on design)
  • End caps for drum tube (2)
  • Bearings for end caps (2)
  • Weapon shaft [non-rotating 'dead' shaft]
  • Drum belt pulley
  • Motor belt pulley
  • Belt
  • Machine screws to put it all together
Is that what you need?

Robot haiku:

A drum is tricky.
Perfect balance is a must.
Don't spin it TOO fast!

Q: aaron i want to know about son of whyachi robot which won battlebots 3.0 title.. what motors are used for its rotating top and the motor specifications so that i can make a similar bot for my college championship of 50 kg bots . [Andhra Pradesh, India]

A: We have discussed 'Son of Whyachi' several times previously. Search this archive for 'yamaha' to find a listing of the many weapon power options S.O.W. has used. The 'shufflebot' version that won BattleBots 3.0 used two overvolted 'Briggs & Stratton Etek motors' for weapon power. The final version used two 15 HP Yamaha go-cart gas engines to spin the big rotor.

A few things you should think about before copying a Team Whyachi design:

  • The team had a VERY large budget;

  • The team had unlimited access to a large and well equipped commercial machine shop with experienced machinists; and

  • I can't think of a single successful copy of any Team Whyachi design.

Robot haiku:

Please search the archives
Before you ask a question.
It saves us both time.

Q: Hi aaron I'm using a permanent magnet starter motor for rotating a 6kg circular ring and can I able to run this motor with 45amp car esc with burst current of 320amps which is available in hk and one more doubt can I able to run two motors with one esc??? [Maharashtra, India]

A: Mark J. here: in order to match an Electronic Speed Controller to a motor you MUST have accurate specs for the motor and the ESC. You must also have full details of the load to be placed on the motor. If these factors are unknown you're just taking a wild guess.

  • You've given me no specs for the motor. I'm not surprized, because specs for starter motors aren't generally available. It is possible to obtain good estimates of the motor specs, but it requires both equipment and time.

  • Specs for hobby R/C car ESCs are ***Wildly Exaggerated!*** In my experience, real-world ratings for hobby ESCs are maybe 1/8th the stated values -- see: Understanding MOSFET Current Ratings. The performance numbers given for hobby ESCs are typically based in the absolute power ratings of the MOSFET power chips at room temperature. If you actually try to pull that advertised amperage thru the hobby car ESC it will fail quickly and spectacularly. The specifications for industrial and robotics ESCs are based on actual testing and can generally be trusted.

  • The load placed on the motor by spinning a '6 kg ring' to speed depends greatly on the size and shape of that ring. The Team Run Amok Spinner Excel Spreadsheet can calculate the Moment of Inertia (MOI) of a ring of specified material given its diameter, height, and thickness. The spreadsheet can also calculate spin-up time, battery drain, and energy storage of the weapon.

  • It is possible to run two brushed motors from a single brushed ESC -- IF the combined amperage draw of the two motors is within the current capacity of the ESC. In this case, I very seriously doubt that you could run one starter motor on the proposed ESC. If you happen to be talking about this 45 amp car ESC, note that it has a maximum rating of 7.4 volts, which further adds to its problems.
My often-repeated advice is to use components that other builders have already proven to be suitable and effective in robot combat. If you use untested components, prepare for a great deal of experimentation and very frequent failure.

Q: Can I use a fan [to cool] that 45 amp car ESC for my starter motor to run without any problem or any other techniques to control the heat and to control the amps coming from that esc???

A: Let me try this again: you need three pieces of information to match an ESC to your purpose, and you have NONE of them.

  • You have no idea how many amps the motor might use, but it's a lot;

  • The available specs for that '45 amp' car ESC are entirely fictional;

  • You haven't done the math to calculate the load the weapon will place on the motor.
Motor power is a product of amperage and the voltage ('watts'). If you restrict the amperage, you restrict the motor power. To use that very small ESC you'd have to restrict the amperage by a whole lot, which would cripple the powerful starter motor.

A cooling fan might add 20% to the capacity of that ESC -- you need to add about 800%. If there was any workable method to use a $10 ESC to control a multi-horsepower motor, builders wouldn't be paying many times that amount for ESCs that can actually do the job. There isn't a shortcut here -- don't ask again.

Robot haiku:

Some unknown starter
Pulls who-knows-how-many amps?
Give a guy a break!
Hobby ESC.
Advertised ratings? A joke!
Use at your own risk.
Loading on motor
By a six kilogram ring
Depends on ring shape.

Q: How important is it to allow some slip between a blade and the motor, using a V-belt or clutch, in a horizontal bar spinner? Last Rites and Mortician use a chain, Fiasco uses a timing belt, and Keres has the blade directly on the motor shaft, and I cannot see a clutch in any of these. How do they keep the shocks of impact away from the motor? Is this less of a problem in smaller bots? If you turn off the motor when it stalls (opponent pushes you into a wall), wouldn't that prevent it from burning out? Thanks. [Vermont]

A: 'Last Rites' and 'The Mortician' have chain drives because builder Ray Billings is a madman. Either may go thru 2 or 3 weapon motors in a tournament, but Ray is willing to bear that expense in order to eliminate all slippage and get every last drop of power out of the weapons.

Tiny robots -- like fairyweight 'Keres' -- have less of a problem with stalled weapons because small motors naturally have a larger surface area to volume ratio (see: square-cube law), giving them a better chance to dissipate heat before they melt down. We have frequently discussed other problems that come with directly mounting a weapon blade to a motor shaft (too high a spin speed, lengthened spin-up time, high motor bearing load...) -- browse this archive.

Timing belts do provide a bit of impact shock absorbtion and, although less predictable in behavior than V-belts, they can and will jump-slip under heavy loading if set up properly. They are entirely suitable for sub-light robots, like hobbyweight 'Fiasco'.

You DO need to turn off the weapon motor when the weapon stalls, but you may have VERY little time to do this before the motor or ESC melts. A slip belt will buy you another heartbeat or two to save the weapon.

Is it possible to design a weapon that will survive full stall without worry? Yes, but some Ray Billings 'bot will kick its ass.

Robot haiku:

Slippage costs power,
But it can save your weapon.
Power has a price.

Q: I'm designing a full body spinner bot like zigo and diameter of the spinning weapon is 500mm and height is 200mm .The spinning material is stainless steel of semispherical shape and I'm going to attach a chain to that bot with one end connecting to the spinning weapon and other end is to the spherical bob.So that the bob rotates along with the spinner.The weight of the spinner is 10kg and weight of the chain is 2 kg and bob is 8kg. Total weight of the bot is 45kgs ,and i need to rotate the bob for around 3000-4000 RPM.But i dont know how to calculate MOI and energy that the spinning weapon contains ,and also what kind of motors I need to use whether ampflow or any other type motors. [India]

A: Mark J. here: full body spinners with impactors attached by chains were fairly common in the early history of combat robotics, but you don't see them anymore -- for good reasons:

  • In order to effectively transfer the energy stored in a spinning weapon to your opponent the entire spinning mass must strike as a single, unified, inflexible object. With your design, the only energy transferred will come from the spinning 'bob' and a bit from the chain. The energy stored in the FBS shell will be isolated from the impact and will only be used to assist in spinning the 'bob' back to speed. This is highly inefficient.

  • A single 'bob'??? The rotating mass will be wildly imbalanced and will shake violently before you can get it anywhere close to effective speed. Changing the design to two 'bobs' would re-balance the spinning mass, but the impact mass would be reduced by half as only one 'bob' would hit at a time. I strongly suggest you scrap the sphere on chain impactor design.

  • I'm puzzled -- why do you think you need to rotate the weapon at 3000-4000 RPM if you don't know how much energy the weapon will store at that speed? Pure guess??
There are many posts in this archive about calculating the Moment of Inertia (MOI) of spinner weapons. If you'd care to browse here for MOI, you'll learn that our Excel Spinner Spreadsheet will give you spinner weapon performance data for specific spinner designs and motor options, but it won't calculate the MOI for a 'semispherical shape'. You'll also find links to relevant internet information like the Paul Hills Spinning Weapon webpage and the rotational inertia section of the Hyperphysics site for formulas and explanations.

I'll give you a start: our Excel Spinner Spreadsheet calculates that a 10 kg cylindrical steel ring 200 mm high and 500 mm in diameter (it works out to about 4mm thick) has an MOI of approximately 0.6 KgM2 and will store about 3000 joules of energy at 1000 RPM. That's way more than enough for a 45 kilo 'bot -- you don't need the 'bob'. An AmpFlow F30-150 motor geared down 7:1 would spin the ring to speed in about 2.5 seconds.

Robot haiku:

Already answered
More than four-thousand questions.
Please read the archives.

Q: Is 20 amp ESC enough for a ampflow F30-150 with a gear ratio of 8:1 for rotating a 10kg iron dome at 900rpm with 2700-3000joules of energy??? If its not enough can u guide me how much amp ESC is required..........?

A: The AmpFlow F30-150 motor at 24 volts pulls a theoretical 294 amps at stall. For the first half-second of spin-up with your weapon the motor will pull in excess of 150 amps, and it will consume more than 40 amps for at least the first two seconds. A 20 amp ESC is nowhere near enough for this motor/weapon.

You will need a controller with surge current capacity of about 200 amps and at least 100 amps for more than a second. Something like the 'VEX Pro Victor Spin Controller' should be fine. I don't think I need to warn you to power-off QUICKLY if the weapon is stalled. A 'slipable' V-belt drive can help to avoid a stalled weapon meltdown.

Note: as stated, the figures I provided as a starting point in the earlier post were for a 10 kg steel ring at 1000 RPM, not a 10 kg iron dome at 875 RPM. The Team Run Amok Spinner Spreadsheet doesn't have the capacity to calculate MOI for a dome shape, but I can roughly estimate the MOI of your uniform dome at 0.48 KgM2, with about 1800 joules of stored energy at 875 RPM. That's still adequate for a robot of its size, and a little easier on the motor/ESC.

Robot haiku:

If you change the shape,
M.O.I. also changes.
Mass distribution!

Q: Hey aaron
what are the changes made by team riobots in touro2008&touro2013 & which motor touro max is used to power the drum [Pune, India]

A: I can see a number of external changes to the 2013 version of middleweight drumbot 'Touro', but team RioBotz has been slow to update their web information. I can't list specific changes.

Heavyweight 'Touro Maximus' uses a large Scorpion HK-5035-500KV brushless motor with a continuous power output of more than 7 horsepower(!) to drive the drum weapon. Reliability of the drum power system has been a problem.

Robot haiku:

A brushless motor
Puts out a lot of power.

Q: Hey Aaron,
I've been working on re-designing my 120 after it got mangled at Robogames. I had been using an Ampflow F30-150 to spin my horizontal weapon disc (it worked great). A problem I had in initial design was that in order to mount the motor vertically, I needed a fairly high-profile chassis (the pulley to the weapon disc sat on the shaft, about 6" high). I'm trying to make my next design more streamlined. I've been thinking about building a small right-angle gearbox so that my motor can mount horizontally and my output shaft would be more low profile. However, I know how bevel gears can be tricky - not to mention I'm not sure how well they would perform at speeds of 6000+ rpm.

Do you have any advice? Is this endeavor worth it? Or is there some economical gearbox I can buy online, and use a direct coupling to the motor?

Thanks for your help.

A: Your concerns are well founded. Bevel gears are inefficient at power transfer, and are sensitive to shock and misalignment. Off-the-shelf gearboxes are expensive, limited in speed and power inputs, and difficult to interface.

Your current belt drive is very efficient, tolerates shock loading, and can handle misalignment -- a great deal of misalignment. Consider a quarter-turn belt drive. You may run into some problems that will require some tweaks to the design, but I'd rather try a quarter-turn belt drive than a bevel gear system.

I can't directly comment on whether going to a horizontal weapon motor is worth the effort. If your weapon was working well, perhaps you shouldn't tamper with the design. Keep it simple.

Robot haiku:

Don't complicate things.
Change things that break or don't work.
If it works, keep it.

Q: hey Aaron, I wanted to know if I could use the Harbor freight 900 rpm motor effectively for my spinning disk ? it is a 6 kg 1cm thick spinning disk which I'll use as my main weapon ......will the motor be good enough ? [Maharashtra, India]

A: Mark J. here: the diameter of the weapon disk is critical in the computations. If the disk is aluminum, I can estimate the diameter of the disk at 50 cm to get a 1 cm thick disk up to 6 kg. I'll also assume this is for a 60 kg 'bot.

An effective spinner weapon should have at least 16 joules of energy per pound of the weight class -- in your case that's a little over 2000 joules. You also need to be able to spin the weapon up to speed before your opponent can sprint across the arena and ram your weapon to slow it and keep it from reaching dangerous energy levels.

The Team Run Amok Spinner Spreadsheet says that the HF drillmotor/gearbox can spin your weapon disk up to about 800 joules in six seconds. That's too long a spin-up time for a small arena and too little weapon energy for a 60 kilo robot. You'll need both more power and speed.

Aim to spin the weapon disk to about 1500 RPM, reaching 1000 RPM in the first 2 seconds. You'll need about 1.5 horsepower to do that, and the 18 volt HF drill motor is good for only about 0.5 horsepower.

Robot haiku:

Question, India:
If you lack heavy motors,
Why build heavy 'bots?

Q: hey Aaron its that 900 rpm HF guy again, actually its for a 25 kg bot .....the disk will be 12 cm in diameter and made of cast iron ......maybe 6 kg will be too heavy , so m planning to use 4 kg , 1 cm thickness .....what now ? .....can u suggest any better material ?

A: Your calculations are off someplace. A 12 cm diameter cast iron disk 1 cm thick has a mass of only 0.9 kilo. That's WAY too small to be an effective weapon. Note that our spinner spreadsheet asks for the radius of the disk in its calculations, not the diameter. A 12 cm radius cast iron disk 1 cm thick has a mass of 3.5 kilos -- let's go from there:

16 joules times 55 pounds = 880 joules of energy storage needed

You'll need to spin that disk up to 2600 RPM to get that much energy storage. The HF motor has enough power to spin this disk up to that speed in a reasonable amount of time, picking up nearly 400 joules in the first 2 seconds.

You'll need to scrap the HF gearbox and find another gearing solution, as the weapon would have barely 100 joules of energy at 900 RPM. If you want to keep the motor/gearbox combination, you'll need a much larger diameter disk. A 25 cm radius steel disk 0.4 cm thick would weigh about 6 kg and would store about 800 joules of energy at 900 RPM. Spin-up time would still be good, but that's a pretty big disk for a 25 kilo 'bot.

As to your material choice, cast iron is brittle -- it tends to shatter when hit hard. Most spinner discs are made from aluminum alloy with tough steel impact blades bolted on. All steel construction could work for your weapon.

Robot haiku:

Spinner must be tough.
If it shatters on impact
You will loose your match.

Q: hey aaron,
how actually the mechanism of breaker box works internally ? how to decide the pivot point for such kind of mechanism so as to make the robot stable ? [Maharashtra, India]

A: The mechanism has been previously discussed. Search this archive for 'resembles breaker box'.

To get the 360 degree rotation of the scoop mechanism the robot body should be short and the pivot should be near the center of the robot. Component crowding may force you to move the pivot a little toward one end.

Robot haiku:

Please search the archives
Before you ask a question.
It saves us both time.

Q: where can i get high power servos for making breaker box in weight class of 120 lb ?

A: Servos don't come that big. 'Breaker Box' uses two custom gear motors with a dedicated electronic speed controller to power the scoop mechanism, and you'd need something similar if you actually want to be able to lift your opponent with the scoop. That takes a LOT of torque!

Robot haiku:

Formula for lift:
Weight lifted times length of arm.
Measure in foot-pounds.

Q: hey Aaron,
I want to make a pneumatic flipper . for tank of the flipper which material u suggest so that weight is also less and it can hold the pressure too. And what did ZIGGY used in his robot ? .
in India Max pressure is 8 bar ... so if I store air in tank above 8 bar and pressure to cylinder is 8 bar .. is it possible ? [Pune, India]

A: The Team Da Vinci: Understanding Pneumatics page will answer your questions about tanks and about pressure regulators that reduce the high tank storage pressure down to a lower pressure for the valves and cylinder.

Heavyweight flipper 'Ziggy' runs a high-pressure air system at about 200 bar straight to the actuator -- wildly beyond the 8 bar India limit. Their pressure storage is in an aluminum SCUBA tank.

There are dozens and dozens of posts about pneumatics in this archive.

Robot haiku:

From Maharashtra,
So many robot questions.
What's going on there?

Q: Hey aaron...
I heard that [brushless] motors have a very high power is to weight ratio. I recently heard abt a [brushless] DC motor weighing 1kg and delivering 4 hp.. IS there a motor with that power is to weight ratio. ? [Pune, India]

A: Many high performance brushless model aircraft motors can meet or exceed a 4 horsepower per kilogram power to mass ratio. For example, the AXI 5345/16 weighs 995 grams and has a peak output of about 4.7 horsepower -- but only for very short periods of time.

The problem is that model aircraft motors are designed to function with a cooling airflow from the propeller and to operate at fairly constant high RPM. If allowed to drop down below 10,000 RPM at full throttle for more than a couple seconds, rapid heat build-up will destroy the AXI 5345/16. That drawback makes most brushless motors unsuitable for robot drivetrains, but useable **with caution** in light robot spinner weapons.

Robot haiku:

A great big hammer
Is of very little use
If it's made of glass.

Q: It is very difficult to reverse the direction of a [brushless] motor... so I was planning to use it in my drum weapon.. Are there any disadvantages of [brushless] motors over ampflow types motor(I have ampflow F30 150). and I am making a 60 kg bot . and also suggest me weight of the drum. [Pune, India]

A: It is not at all difficult to reverse a brushless motor, but most brushless motor controllers are made for model aircraft that have no need to reverse the motor and so do not have that function. For reasons given above, hobby brushless motors are generally unsuited for use in robot drivetrains anyhow.

Your AmpFlow F30-150 brushed motor weighs about twice as much as the AXI 5345/16 and puts out about half the horsepower for about half the price. The advantage the AmpFlow has is toughness and reliability. The AmpFlow can be bogged down and even briefly stalled without much ill effect, effortlessly surviving abuse that would very quickly melt the AXI. The two motors are designed for very different purposes; a racehorse can outrun a farm horse, but it would fail at plowing a field. Choose wisely.

There are many factors in optimum drum weapon design beside weight. Here's a quote from an answer to an earlier question from this archive that addresses one of those factors:

The amount of power stored in a rotating drum is a function of its rotational moment of inertia (MOI) and its speed of rotation (RPM). The MOI of the drum is a function of the mass of the drum and how that mass is distributed. Two objects with the same mass can have very different MOI: mass located farther from the axis of rotation contributes more to the MOI than does mass close to the axis. A short, large diameter drum has a larger MOI than a long, small diameter drum of the same mass.

That said, a typical drum 'bot has about 1/3 of the mass of the robot invested in the weapon system (drum, motor, belt, share of battery...)

I'd strongly recommend reading thru the Robot Weapons archive for information on drum weapon design, and on the use of brushless motors for weapons.

Robot haiku:

Light drum, heavy drum.
Rotational momentum
Is what really counts.

Q: what is the difference between inrunner and outrunner dc motor.. [Pune, India]

A: See #23. Read the rest of the FAQ while you're there.

Robot haiku:

Already answered
More than four thousand questions.
At least read the FAQ!

Q: hey aaron,
i want to make a drum as a spinning weapon in my robot (60kg) . And rotating part of my drum is of steel whose dimension are 40 mm internal radius and 65 mm outer radius and length is 200 mm weight 12.8 kg . i want to store 10 kj energy . I cant afford ampflow A series motor . And i am in search for cheap motor than ampflow . one of my friend suggested me for [brushless] DC motor on hobby king . Motor here are cheap with high power and low weight. can i use this motor for my drum ? . i know that the motor get heat up . so if in case i use this [brushless] dc motor then what precausion should i take ? . which [brushless] dc motor you will suggest ? . [Mumbai, India]

A: Many problems.

  • Your drum has too small a Moment of Inertia (MOI) to realistically store 10K joules of kinetic energy. It would spin at about 7300 RPM to store that much anergy, and that's too fast to have effective 'bite'. Search this archive to learn more about 'bite'. The good news is that you don't need to store 10K joules of energy to have an effective weapon on a 60 KG 'bot. Around 40 joules per kilo is adequate, so 2500 joules would do nicely for your 60 kg 'bot. Your weapon will store that much energy at about 3700 RPM -- a much more reasonable speed for a drum this size.

  • Running a hobby aircraft brushless motor on a large drum weapon is VERY tricky. They are designed to spin a light propeller up to speed very quickly and maintain it at high RPM. Asking that motor to bog down and spin a 10 kilogram drum up to speed is well outside the type of work it was built to do. I cannot find any successful robot as heavy as yours that uses a brushless aircraft weapon motor.

  • If you insist on using a brushless aircraft motor, you must make absolutely certain that it will NEVER bog or stall -- even for an instant. A brushless motor of an adequate size for your weapon might be rated for a maximum 80 amps, but if stalled might attempt to pull more than 1000 amps! It's unlikely that your battery could supply that many amps, but it's very likely that the amperage spike the battery could supply might instantly destroy the motor and speed controller, as well as causing severe damage to the battery.

  • The most practical method of assuring that the motor will not stall is to drive the weapon with a loose belt that will slip under load. The more the belt slips, the lower the torque and amp load on the motor -- and the longer your weapon takes to spin-up. You won't be able to use the full theoretical power of the motor.

  • Finding the right amount of slip in the belt is done by experiment:

    • Start with the belt very loose and test the weapon.
    • Tighten the belt a little and test some more.
    • Keep tightening the belt until your motor, speed controller, or battery blows up.
    • Replace the blown parts and back the tension off a little.

    This gets expensive really fast.

  • About Hobby King products (from an earlier post):

    Many robot builders use cheap parts from [Hobby King]. Feel free to use them if you like. I think they're probably fine for a little R/C project you're going to play with in the local park, but not for combat competition. Quality control is awful, shafts aren't hardened, weak magnets aren't glued in securely, specifications are questionable, documentation is unintelligible, and customer support doesn't exist. I'll pass.

    I cannot recommend a Hobby King motor to you in good conscience -- see #16. In this specific case I cannot recommend an aircraft brushless motor from any source for your application. Brushless motors won't be a bargain when you start burning thru them.

  • I'd suggest using an AmpFlow F30-150 or the heavier E30-400 for a weapon motor in this weightclass. Either has ample power to spin up a drum the size you're planning: with a 2:1 reduction belt drive the weapon will spin up in about 2 seconds and deliver around 2500 joules of stored energy.

    If you're worried about weight, substituting a 200mm long drum with a 75mm outer radius and a 12mm wall thickness would have the same moment of inertia (MOI) as your current drum but would weigh only 8.1 kilos, saving more than enough weight for you to use either AmpFlow motor without reducing performance.

Robot haiku:

The part where I said,
'Choose wisely' ment 'not brushless'.
Should have made that clear.

Q: HI, I want to build a featherweight robot with a pneumatic flipper. I can figure out how to make a working robot that moves. However, I want to add a flipper but don't know where to start. How do you operate the flipper with your robot's remote control?

A: Start with the Team Da Vinci: Understanding Pneumatics page, then read thru the dozens and dozens of posts about pneumatics in this archive.

Note: I don't recommend a flipper weapon for a first combat robot project. Pneumatics are complex and potentially dangerous. You'll have plenty of things to worry about with battery maintenance, drivetrain optimization, R/C system set-up, armor materials, traction issues, ESC mixing, radio interference, tournament procedures, repair problems, and driving practice.

Robot haiku:

The very last thing
A novice 'bot builder needs?
An active weapon.

Q: Hi.. I plan to build a drum spinner weapon i did browse the 'spinner weapon starter package' at [no longer available]. They have the package of Ampflow A28-400 to be controlled using One 586 24VDC Power Solenoid with RC Switch. I wonder whether is that all needed to control the spinner motor? My previous drum spinner weapon motor is controlled directly using a dc motor driver and i'm not familiar in using the that actually the better way to control it? previous drum spinner weapon is using the 18v dewalt drill motor..which was not a success because i attached the motor shaft to the drive belt pulley using only set screw. Therefore, i welcome a suggestion on what is the best way to attached the 1/2inch ampflow motor shaft with the drive pulley so that it will be able to withstand the slamming impact during the weapon operation.

thanks! looking forward to your response.

A: A solenoid is the equivalent of a big switch that simply turns the weapon motor on and off -- no speed control and no reversing. A solenoid is a reasonable solution for large weapon motors that can consume huge amperage at start-up. A big solenoid like the WR 586 can deliver all the amperage your batttery can supply and assure the fastest possible spin-up time, while an electronic speed controller may restrict the maximum amp surge and reduce motor torque. If your spinner design allows for all-or-nothing speed and does not need to reverse, simple solenoid control is effective, inexpensive, and reliable.

The A28-400 output shaft has a keyway groove machined along its length. Use a pulley with a matching keyway and you can insert a hardened steel 'key' that will prevent the pulley from rotating on the shaft.

Q: Hey, I'm using a W-R 24v 124-series SPDT solenoid to activate an AmpFlow motor for a weapon. My question is how do the limits of the switch I order need to compare to the capabilities of the solenoid? I guess I don't quite understand the relationship between them. For example, can I use a 10A Battleswitch ( to activate my solenoid? Does the 10A refer to the current needed to switch the solenoid, or do I need a 100+ A switch for the times I expect my motor to draw that much current?


A: Go take a look at the data sheet for that solenoid. The power consumption of the 24 volt coil that activates the solenoid is listed at 12 watts, and that's all the power your R/C switch has to handle: that's 0.5 amp at 24 volts. Any R/C switch with at least that rating will do fine -- even the little PicoSwitch should do just fine, if you use an antiparallel flyback diode -- the solenoid coil is an inductive load. You can use the 10A BattleSwitch without the diode -- it's more robust.

Q: hey!!
i have nt so much knowledge abt all this but i m going to make a robot around 15-16 kg. & going to use blade or solid cylinder as a weapon so which type of motor should i use?? give me total specification & how much watt should it contain?? i m thinking to use 12v or 24v motor with 4000 rpm but don't know how much watt & amp should it contain?? plz guide... [Pune, India]

A: I can't give you a specific weapon motor recommendation based on the limited information you have provided -- the details of the weapon (dimensions, material, style) are critical in motor selection. See #29.

Q: hey i want to build a small biohazard like robot car but i cant really understand the mechanism of it so can u help me how can i build it using things easy available at home.

A: Are you asking about the mechanism of the whole vehicle, or just the BioHazard style lifter? If you're asking about the whole vehicle, that's way too complex for me to provide a short answer. Read thru the FAQ for some design help.

The BioHazard lifter is called a '4-bar mechanism'. It is made from four rigid elements hinged together. By careful selection of the bar lengths and hinge points, you can create a compact device that can extend forward and upward to lift your opponent. There are MANY posts in this archive about the design of 4-bar lifters -- search for '4-bar'.

To operate the 4-bar mechanism, you must apply force to one of the bars. This is typically done with a fairly powerful gearmotor and a radio control interface. I'm not optimistic that you can find components like these around your house.

Q: Dear Aaron, can you show me the design of Ziggy's weapon?

A: Super-flipper 'Ziggy' combines a 4-bar mechanism to define and control the flipper path with a lightning fast high-pressure air pneumatic system. The best photo I have is at right -- click it for a larger image.

There are MANY articles discussing 4-bar mechanisms and pneumatic systems in this archive.

Q: I have a 600w (24v. & 25a. at peak) weapon motor and I am using two lead acid batteries to get an output of 24v. and around 60a. But the motor stops suddenly at times (mainly after impacts) on the battery supply whereas on the other hand when I use a direct power supply the motor does not stop at all how much ever load it is subjected to. I am really confused about this can you help me work out what could be the problem??

Just if you find this useful when the motor stops while working on lead acid batteries and if I remove and reconnect the motor's connections on the battery it again starts working until the next unpredictable shut off. [Mumbai, India]

A: I don't think the problem has anything directly to do with the batteries -- batteries don't just stop providing power and restart if you disconnect and reconnect them. It would help if you had mentioned what weapon motor controller you are using. A couple possibilities:

  • Your weapon motor draws 25 amps 'at peak', but can draw much more current when bogged down or stalled by a hit. You may be draining so much current that the battery voltage falls and causes the motor controller to 'drop out' and reset only when the power is removed and restored. That could explain improved performance when operating from the 'direct power supply'. The solution would be batteries with greater capacity.

  • I've seen brushed motors show similar trouble if the motor brushes are binding in their holders. An impact can bump the brushes and cause them to stick. Inspect the brushes and make sure they slide freely thru the brush holders.
Q: Greater capacity as in more amps. ??

A: Greater capacity as in able to supply more amperage without a voltage drop. I suspect that your lead-acid batteries aren't really able to deliver as much amperage as you think they can.

Mark J. here: you haven't told us anything about your weapon motor controller or your 'direct power supply'. It's entirely possible that your batteries are fine and that your motor controller is shutting down under high amp loading and must be reset with a power-down. Your 'direct power supply' may not be capable of enough amperage to cause this. You may need a higher capacity motor controller rather than batteries.

Aaron correctly addressed the problem given the information you've given us, but there is a lot you haven't shared.

Q: Hey Aaron. A while back I ordered a 14" diameter, 0.75" thick steel plate to use as a flywheel for a middleweight weapon. I found a very cheap supplier so I jumped on it. I intended on broaching a keyway, but the 1" diameter hole through the center I had ordered is machined very sloppy. It seems the only way to salvage the piece is to bring it to a machinist and have him re-bore the hole. To about 1.25" or so. Since I'm locked into a 1" diameter shaft design, I need to fit the plate with some sort of keyed bushing to convert the new bore for my 1" shaft.

Do you have any suggestions? I was thinking of using a QD bushing and high-strength bolts to attach it to the plate. I'm hesitant because, as per the QD design, they feature a tapered OD. I can't seem to find any straight-bore flanged bushings I can use. Maybe some sort of hub? Any advice would be appreciated! Regards, Flash.

A: Mark J. here: I won't give you the sermon about cheap robot parts...

I don't have important details about your design, but consider boring out the hole and fitting a Trantorque or B-LOC keyless bushing. With either, there are no mounting bolts to shear, no keyway required, and they transmit huge torque while surviving great abuse.

Q: How do you make an antweight beater weapon without machining it out of a single piece of material?

A: A beater bar takes a great deal of abuse and must be both durable and well balanced. I can't recommend a method other than machining in a single piece. Several on-line machine shops could make a beater to your specifications

Q: Hey, I'm building a FBS (60 pound) however using a belt-pulley system and a AmpFlow F30-150 Motor. Is this a good idea? And how can i plug that motor (mean esc) should i use one of these Robot Power speed controllers or buy this White Rodgers solenoid and using 2 x 3s battery in parallel to get the voltage into 22v for the motor at 35c (each) with 2.700mah or the 1.500mah.

Need suggestion cause i got only one green light on the project and need 2 to be convence on the equipment being use. PLZ help Thank You [San Juan, Puerto Rico]

A: You haven't told me enough about your design for me to tell you if it's a 'good idea' or not. Full Body Spinners (FBS) are not easy to design or build, and none of the current successful lightweights are FBS.

The AmpFlow F30-150 motor is a reasonable choice for a lightweight spinner. A solenoid like the White Rodgers 586 would certainly control the motor, but an 'electronic spin controller' designed for spinner weapons will offer you greater weapon control. There are multiple posts in the Ask Aaron archives about weapon solenoids.

The Run Amok Excel Spinner Spreadsheet can assist you with evaluating the critical design elements of your spinner (dimensions, weight, gearing, battery selection), but again a FBS is NOT a good design for a novice builder. If this is your first combat robot I would suggest a different design.

Q: Is not my first design but is mi first in lightweight (60lb) we compete in 15lb, 30lb and 120lb (already done again this year drum) but i always use a drum-like-bot and was challenged (by sponsor) so im in the quest to finish the detail and convince him to pay all the equipment. The inside is already made but i don't want to spend without getting suggestions. The design is like this drawing but with a pullley system and only 4 wheel to move and the motor are not like that. So any suggestion than alot!

A: A drawing of somebody else's 'bot that's sorta like what you have in mind isn't much help. I need dimensions and materials! I can give you a few general comments:
  • That AmpFlow F30-150 motor is a bit over 6' tall if you stand it on end. Stuffing that under the shell with a pulley drive will make for a pretty tall lightweight FBS. A tall shell will not be as robust as a shell of lesser height.

  • You want as much mass as possible in the spinning shell. A typical FBS will have about half the mass of the 'bot dedicated to the weapon. You say you have the 'inside' already built -- do you have enough weight allowance left for the shell?

  • As noted above, the Run Amok Excel Spinner Spreadsheet can help you model the energy and spin-up time of your weapon design.

  • How long you have to spin-up your weapon depends on the size of the arena. A quick run of the Spinner Spreadsheet tells me that a 600 mm diameter aluminum shell 5mm thick and 180 mm tall will require a 6:1 belt drive reduction from the F30-150 to get a spin-up to 3000 Joules in 3 seconds. That sounds about right for a meduim-size arena.

  • Full Body Spinners are not as effective as vertical drums. When a vertical drum hits, most of the energy goes into throwing your opponent into the air. When an FBS hits, the energy of impact is split between throwing your opponent in one direction and throwing you in the other direction.

  • The center bearing support structure takes a LOT of abuse, and you can't support the shaft as well as you can on a drum weapon. Brace it well and make it strong!

  • The shell must be perfectly balanced. Precision construction is critical.
A successful FBS isn't an easy build -- best luck.

Q: Hey there, I forgot to design in a brake system for my middleweight's spinning weapon. (Rookie mistake, I know!). I haven't seen too much info on motor brake systems.. Are there any techniques you can suggest? I'm going to be activating my weapon motor with a DPDT solenoid; As of now the weapon is only designed to operate in one direction, so the opposite throw of my solenoid is unused. Is there some mechanical device I can hook up to it? Perhaps something that contacts the shaft to slow it? Thanks for the help.

A: The most common spinner braking method is called dynamic braking, and it uses nothing more than the components you already have. Take a look at the diagram to see how to wire the DPDT solenoid. Connecting the motor leads together turns the motor into a generator and dissipates the rotational kinetic energy of the weapon by converting it back into electricity and then into heat as it passes thru the resistance of the motor armature.

Dynamic braking works best when the weapon is spinning fast. The braking effect lessens as the weapon slows, but it is a simple and effective way to reduce spin-down time. We can discuss mechanical braking systems if dynamic braking isn't enough, but try it first.

Q: Hi Aaron, is it possible to control a weapon (in this case a lifting arm) without an ESC? The arm doesn't need variable control, it just needs to spin the motor clockwise, counterclockwise, and have an off position. Is there any way to do that? [Boston, MA]

A: Scroll down to the next question for a diagram of a relay/solenoid control system that provides forward/off/reverse control of a brushed DC motor. There are several posts about the use of solenoids to control weapon motors in this archive. A solenoid is a reasonable alternative to an Electronic Speed Controller (ESC) if you need only on/off control -- but if you need forward/off/reverse control of the motor you will find that a solenoid control system is heavier, less reliable, and about the same cost as a speed controller of the same capacity.

Note: brushless motors cannot be controled by relays/solenoids. Brushless motors MUST have a brushless motor controller to operate.

[Cross-posted from the archive]

Q: Hi Aaron!! I want to know whether the twin stick RC system can be used in conjunction with relays to operate a robot. Thanks. [Mumbai, India]

A: Yes, but there are several drawbacks.
  • You will require a special R/C interface between the receiver and the relay to translate the R/C signal into an on/off current to control the each relay -- you can't just plug the relays into the receiver.

  • If this is a fairly large robot, the relays needed to control the high current levels the motors require are expensive, heavy, and bulky.

  • A standard tank-steer robot will require at least four relays and interfaces to provide forward/reverse/off/left/right control.

  • Relays do not provide speed control. A robot controled by relays will be difficult to maneuver precisely and will be frustrating to operate.
Although there are commercially available 'dual relay boards' with a built-in R/C interface that can be used to control a single motor forward/off/reverse, a dual channel electronic speed controller is more compact, lighter, more reliable, provides better control, and costs less than two relay boards of the same capacity.

Q: I'm building a horizontal disc spinner. I was planning on using a live shaft for the weapon. I was going to order some hubs that would give my disc more surface area and keyway length on the shaft, but I'm pretty sure the force of the set screws alone won't be enough to hold the disc's position (axially) on the shaft. Planned design includes a 30-lb disc on a 1-in diameter shaft (on a middleweight). What do you recommend? Perhaps a hardened steel pin through the hub and shaft? Or would you recommend switching to a dead shaft?

Thanks for your help!
Regards, Flash

A: Conventional spinner weapon design allows the live shaft to float and transfer axial load from the disc hub to the support bearings, so there is very little axial load between the disc and a live shaft.

I think a hardened pin would be overkill, and I don't like to drill a hole thru a stressed shaft if it isn't needed. If you want to make real sure the hub doesn't move, you could incorporate a Trantorque bushing into your hub design, but in this application I'd be tempted to just grind a small flat on the shaft and use the set screws -- with threadlocker, of course.

Mark J. here: if you just can't bear the thought of using set screws, the use of retaining rings is a viable option. I also like Aaron's suggested use of a Trantorque bushing.

The dead/live shaft decision depends on elements in your design that you haven't shared. In general, a live shaft places less stress on the bearings for a given chassis height and offers more design flexibility.

Q: Aaron, What is the best way to run an unregulated C02 set up? Lets say its for a 30lb bot.

Thanks, New York

A: Two things we won't discuss here at Ask Aaron:

  1. Flame weapons; and
  2. HPA/unregulated CO2 pneumatic weapons.
Both are dangerous, and neither can be adequately discussed in our short answer format. By the time you have enough experience as a builder to safely construct either, you won't need to ask us how to do it.
Q: Dear Mark/Aaron
  1. What are the factors influencing a "bite" in a drum bot?
  2. What kind of bots are immune to a drum bot?
  3. I have been thinking a lot about this scenario lately - suppose a bot is cubicle in shape and its height is well exceeding my drum's diameter and its surface is super finished. I don't think I will ever get a "bite" on such built bots. How will I tackle such kind of bots? please suggest.

A: Go read section 6.3 of the RioBotz Combat Tutorial for a full explanation of 'bite' in spinning weapons complete with diagrams, tables, and equations. We also have many posts about 'bite' in both this archive and in the archive.

I'm not sure that any design is 'immune' to a well designed drum bot, but 'spinner killer' scoop designs - like 'Breaker Box' - have very effective counter measures. In general: the fewer exposed edges a 'bot has, the better it can resist a typical drum design.

Even a vertical edge, like the edge of a cube, allows some chance of 'bite' for a drum weapon. The sharp angle gives a hard and sharp tooth a good chance of deforming the material and creating its own foothold. Conventional drum design calls for very hard, sharp impact teeth for the best chance of getting bite in difficult conditions.

I haven't seen this tried in a long time, but a no-impactor drum covered with a high-friction material can get at least some grip on even the smoothest and hardest surface.

Q: I know your not a fan of direct drive weapons because of to much rpms to get a good "bite", but what makes antweight Metroid's weapon so effective? Examples: Metroid vs Spark Plug and Kilobots XIII - Antweight Rumble.

A: Antweight 'Metroid' is a very effective combat robot, but don't place all the credit on the weapon. The robot is well constructed, the components all work well together, and it's quite well driven. That said, there are some design elements that do help the drum weapon make the most of the limited 'bite' it has:
  • The drum diameter is a bit larger than a typical ant drum. This gives the drum teeth a large 'upsweep' area to search for an edge for bite.

  • The drum teeth themselves are undercut and sharpened. If the edge of the tooth does get even a little bite, it can pull the opponent deeper in toward the drum for a more effective hit. It may be an illusion, but it also looks like the mounting holes for the teeth are drilled just a bit off-axis to open up the exposure of the leading edge of the teeth.

  • The drum teeth on opposite sides of the drum are offset: centered on one side and out toward the drum ends on the other. This allows a clear full-circle path for any given tooth to penetrate deeper in toward the opponent before contact.

  • I'm not sure exactly how fast the drum is spinning. It doesn't have that crazy high-pitched whine that some direct-drive weapons emit, so it may be spinning at a more reasonable speed than you might assume.
Watch the videos carefully and you'll notice that Metroid's weapon is not effective against smooth, flat surfaces -- the high-speed drum dictates the attack strategy. The weapon needs an exposed edge to obtain grip, and driver Dennis Beck is good at picking the right time to dart in and catch an opponent with their side or rear aspect exposed when hard edges are more likely to be available. If your weapon has bite, you have more attack options.

One last design note: 'Metroid' fights only at the Kilobots events in Saskaoon. The Kilobots arena has a steel floor, and Metroid takes advantage of this with a large neodymium magnet that keeps the robot well planted and provides excellent traction for those quick, darting attacks.

Q: Hey Aaron can I use Team Whyachi C1 Contactor as a weapon actuator. My weapon is a drum of M.M.I= 0.03176kgm^2 driven by an [AmpFlow] E30-400. You earlier mentioned to use this DPDT-24V 586 Series SPDT White-Rodgers Solenoid. The TW C1 Contactor is almost half the price of what you have mentioned. Please suggest if i can use TW C1 Contactor.

A: Mark J. here: the TW-C1 contactor has a couple of drawbacks:

  • Fragility: the TW-C1 contactor body is made of a brittle material and does not withstand shock well. These contactors have been reported to fracture when the robot is hit hard -- even if shock mounted. The big White-Rodgers solenoids have a metal body and are much more shock resistant.

  • Uncertain capacity: contactors almost always have lower current capacity on the Normally Open (NO) contacts than on the Normally Closed (NC) contacts. If you're using the contactor for single-direction on/off control this isn't an issue. However, since I know you're using two of the contactors for forward/off/reverse control, all contacts must be rated for the surge current capacity required to control the selected weapon motor.
Since the full specs for the NC contacts aren't given, I can't tell you if the TW-C1 will handle your weapon motor while providing forward/reverse control. Use it at your own risk.

Q: Dear Aaron/Mark can we use this dc solenoid- 24V 124 Series SPDT White-Rodgers Solenoid instead of the 24V 586 Series SPDT White-Rodgers Solenoid (for E30-400) which you have earlier mentioned? We are on a real tight budget. And is relay a wise choice for activating E30-150 in forward and reverse direction?

A: You can read the engineering specs for the 124 series solenoid as well as I can. At 24 volts, the rated inrush current for the NC contacts is 100 amps, and your selected motor can pull more than 250 amps at startup. That solenoid may survive long enough for your purpose, but I don't recommend stressing a component that far beyond its rating.

Solenoid control of a motor is a reasonable option for single direction weapons. However, for a reverseable weapon an electronic speed controller is typically more reliable, more compact, lighter, and provides better control for about the same cost. If you are on a very tight budget, you may be better off to redesign for single direction Weapon operation, or perhaps select a smaller weapon motor.

Q: Theoretical, How would I mount a motor inside of a pipe?

A: Theoretically, it would depend on the motor, the pipe, and why you want to mount it in a pipe.

  • If you're mounting a 'outrunner' style (spinning can) motor in a tight-fitting tube to directly power a spinning drum, a pair of flush-head machine screws passsing thru the tube and into tapped holes in the motor is a reasonable mounting method.

  • Ideally, a keyway should be machined into the motor can and the interior of the tube to lock the rotation of the motor and tube, allowing a machine screw to locate the motor laterally without being exposed to high axial torque loading. This is difficult work, and the motor shell may not be suitably thick to allow a keyway.

  • I've seen small outrunner motors epoxy-bonded to the interior of a tube as well -- but that makes the motor non-replaceable.

  • If you have some other application in mind, I'd need more information before I could recommend a method.
Note: direct-driving a spinning drum in this manner is not a great idea. It places great load on the motor bearings, spins the weapon too fast for practical use, and lengthens spin-up time.
Q: Hi Aaron, great site. I really wish I knew about it sooner.

I am getting back into the combat robotics game after having partially built more than one. I never got the chance to compete. However, my first design was a 12 lb horizontal disc spinner (friction driven). I am going to revive the idea to some extent, perhaps redesigning it from the ground up and bumping it up a weight class or two.

My main concern lately has been the presence of numerous scary vertical drum spinners. Of course, we all know that severe off-plane impacts have adverse effects on a horizontal weapon assembly. My original design incorporated ball bearings in the frame within thick aluminum blocks above and below the disc, which spun on a "live shaft." I am thinking about designing it around a dead shaft to improve the structural integrity of the entire frame, but fear that bearings mounted close to the disc will endure much greater stress in the event of a vertical impact, as the disc radius will certainly exceed the height of the frame (resulting in a "twist" between the inner and outer races of the bearings). Would a "live" shaft be more appropriate for this application, or is there a particular bearing or placement which will prevent their destruction and improve overall durability?

Also, if a dead shaft is the best bet for this design, what is the method of keeping the disc in place, that is, from sliding up and down the shaft?

A: The 'live shaft' design [where the weapon shaft rotates and is supported by bearings in the chassis] is more popular than the 'dead shaft' design [where the weapon shaft is stationary and it supports bearings in the weapon hub] largely because it spaces the bearings a bit away from the weapon and allows the drive pulley to be located outside the compact support frame.

With a friction drive there is no drive pulley, and the dead shaft can become a fixed structural member of the chassis to greatly improve the weapon support strength. The weapon hub can be extended vertically to move the bearings some distance away from the disc plane and improve twisting resistance. How much bearing spacing you can get will depend on your chassis design, but I'd say that 'more is better'. Use of a bearing type that can effectively resist both axial and thrust loading (like a tapered roller bearing) can greatly improve bearing strength in this type of application.

Locating a disc on a dead shaft is simply a matter of tubular spacers on the shaft that rest against the frame supports and the inner races of the bearings.

Note: there are good reasons why you don't see many friction drive weapons. An effective and reliable friction drive is difficult to implement, particularly in the heavier weight classes. Best luck.

Q: Dear Aaron I have a query regarding the new E series motors which ampflow has recently introduced. I will be using an E30-400 motor for powering a drum (mass moment of inertia 0.03176kgm^2). The drum should operate on full rpm as i switch it on and it should be reversible as well. As we are low on funds we won't be using any speed controllers. I was just concerned if this will damage the motors.

I will be using lipo batteries (4s 2750mAh 65~130c, two of these in series). As I am not using a speed controller what should my operating voltage be 22.2 or 29.6V or can i use a 4s and a 3s in series? [Bangalore, India]

A: Are you planning to use solenoids to control the weapon motor? To have reversing capability, you'll need two DPDT solenoids rated for at least 300 amp inrush current on all contacts (see diagram at right). Too small an amp rating and the solenoid contacts can weld themselves shut! The Normally Closed (NC) contacts on DPDT solenoids are typically rated for less current than the Normally Open (NO) contacts, so check the current ratings carefully. A good high-power DPDT solenoid isn't cheap, and you need two of them. You aren't going to save much (if any) money over a suitable speed controller.

The AmpFlow motors are well built, sturdy, and unlikely to be damaged by direct application of operating voltage. Be sure to properly break-in the motor by running it continuously for at least 20 minutes at reduced voltage (~12v). This will contour the brushes to the commutator and prevent damaging arcing at high start-up current loading.

The AmpFlow motor is entirely capable of dealing with overvolting to 30 volts as a weapon motor, although 22.2 volts should give you ample power. Overvolting will increase power and speed, but it will also increase amperage so don't overdo it. You can run a 4 cell and a 3 cell LiPo in series IF the cells in both batteries are identical -- same capacity, model, and manufacturer. Some manufacturers offer 7 cell LiPo batteries, but not many LiPo chargers can handle that large a pack.

Q: Thanks for your last reply Aaron. I have few more questions to bug you. My drum weapon has M.M.I= 0.03176kgm^2, r.p.m around 5000. The drum will be mounted on a dead shaft of diameter 1.5" supported on two roller contact bearings. Now I have two [three] questions:

1) Ideally there is no axial force applied on the drum i.e only radial force acts on the drum and the bearings. So should I consider this fact while selecting the bearings or should I look for a bearing capable of taking combination loads. Also what kind of bearings should I look for? as in roller, cylindrical, deep groove, taper, spherical etc? (I will make sure that the shaft doesn't get bend so self aligning bearings should be out of question)

A: Mark J. here: 'ideal' engineering conditions don't hold in combat robotics. Your drum bearings might be expected to experience only axial loading from your weapon's actions, but your opponent will have weaponry as well that may inflict large impact loads from unpredictable directions. My choice would be tapered roller bearings.

2) What should be the ideal distance between the two bearings (extreme ends of the drum or a little towards the inward of it)?

A: Force vectors work out best with the bearings at the extreme ends of the drum.

3) Lastly, can we use bearing mountings as motor mountings or will it create some heat dissipation issues? (am using a E30-400 for weapon and two E30-150 drive) Thanks in advance!

A: A pillow-block style mounting is strong, simple, commonly used, and should cause no heat issues for your AmpFlow motors.

Q: hello Aaron, I am building a drumbot powered by an ampflow e30-400 motor for its drum. Can u please suggest to me the best and cheapest batteries for this very motor. Drum weight is 15 kg and we would prevent the motor from stalling. [Maharashtra, India]

A: Mark J. here: the load on a spinner weapon motor depends on more than the mass of weapon -- it also depends on the diameter of the weapon and the placement of the mass. Everything else being equal, a larger diameter weapon will have greater 'rotational inertia', will place greater load on the weapon botor, will take longer to spin up to a given speed, and will store greater rotational energy at a given speed. You need to determine the rotational inertia of the weapon in order to determine a proper speed reduction between the motor and weapon, and you need both the rotational inertia and the speed reduction to calculate the load on the battery.

The Team Run Amok Spinner Excel Spreadsheet can calculate the rotational inertia of a drum weapon based on the dimensions of the weapon components and the material of which they are made. Adding in motor data will allow the spreadsheet to also calculate the weapon spin-up time and the energy storage of the weapon system. It will also estimate the total battery load of the weapon for a match.

The AmpFlow E30-400 is a large and amp-hungry motor. Assuming that you will run the motor at 24 volts (they can be over-volted), you will ideally need a battery that can deliver a peak 270 amps of current. Less current capacity will reduce the peak torque of the motor and will slow the weapon spin-up time. If you can't deliver that much current, you might be better off using a smaller weapon motor and saving weight and expense.

As to the 'best and cheapest' battery, you can have either 'the best' or 'the cheapest' -- but not both. A pair of locally sourced Sealed Lead Acid (SLA) batteries would be cheap and could deliver the required amperage, but they would be bulky and heavy. The 'best' choice might be something like the ThunderPower Pro Power 65C LiPoly battery -- capable of more than 290 amps of peak current while weighing just over 13 ounces. This level of power is more expensive and would require a charger designed specifically to hanle LiPoly batteries.

Run the rotational inertia calculations for your weapon drum, select a practical speed reduction, determine the battery amp-hour requirement of your weapon, then seek out a high peak-amp battery to suit that capacity need. Note that most combat robots run a single battery to power the weapon and drive motors.

Q: how to make a flame thrower robot [Chandigarh, India]

A: See #28.

Q: how to build a simple flame thrower robot explain [Chandigarh, India]

A: Persistant, aren't you?

As explained in #28, we will not discuss flame weapon construction here because we don't want novice builders hurting themselves. By the time you are an experienced enough builder to safely construct a flame weapon, you won't need to ask us how to do it.

Q: i am gonna participate in a local techfest ....i wanna kno ..which motors are the best for lifting mechanism?
or shall i use hydrauliccs?
moreover the cutters should have which motors? [Mumbai, Maharashtra, India]

A: You can't go to a doctor and ask, "I'm not feeling well. What medicine should I take?" Your doctor would need much more information before they could recommend a treatment.

Likewise, you haven't given me enough information to recommend specific weapon motors for your robot. I don't know how much the robots at your 'techfest' can weigh, what rules govern your weapon selection, or the details of the design you have in mind.

Read thru this archive for tips on weapon motor selection. It may give you some ideas for your robot.

Q: Hi Aaron, Your site has been most helpful. I just have a quick question, however to make the hamburger as good as possible, I shall give you full details of my design. I plan on making a hobbyweight vertical spinner and I was planning on using a 8"x.75" aluminum disk being spun at around 1500 rpm, with a single .25"x2"x3" steel impactor as a tooth with a counterweight on the other side. A Turnigy L5055A-400 motor will drive the disk (reduced with a 3:1 belt system) with a 3s lipo along with Turnigy Brushless ESC 60A w/ Reverse so that I can reverse it when I'm flipped.

The problem comes from when I am calculating the weight of the disk. The online metals weight calculator says that the disk will weigh 3.692 lbs while the spinner spreadsheet I downloaded here says that the disk 6.82 kilos! that's a huge difference. maybe I put the information into the spreadsheet wrong? any help is greatly appreciated. thanks [Hawaii]

A: The Team Run Amok Spinner Spreadsheet looks for the radius of the spinner disk for input. I suspect you entered the diameter. When I enter a 4" radius (0.102 meter) and a 3/4" thickness (19 mm), I get 3.77 pounds (1.71 kilos) for the disk -- the tooth and counterweight combined add up to about 0.44 pounds (0.2 kilos).

A few things you didn't ask about:

  • Spinning that weapon at 1500 RPM is not going to store a lot of energy by current hobbyweight standards: only about 120 Joules. Some beetleweight spinners have that much energy, but spinning a single impact tooth at 1500 RPM will give you excellent 'bite' that may make up for the low energy storage.

  • I'm not a fan of Hobby King motors. I don't believe that either their specs or performance are reliable. Given the specs HK provides for the Turnigy L5055A-400 motor, I suspect it has the power you need.

  • The specs on most inexpensive 'hobby' ESCs - like those sold by HK - are typically based on theoretical values that wildly overestimate real world capacity. The HK ESC you've chosen may be adequate for your purpose, or it may not. Best luck!

Q: Hi Aaron it's the hobby weight vertical disk spinner guy again. I knew I was doing something wrong. I know your not a big fan of hobbyking motors, but I have trouble picking out a correct sized motor. I looked around and saw that a few people use this motor. I picked the car esc because its reversible and it says it can handle 60 amps. I like the ability to reverse my weapon. I was thinking of doing a 2:1 reduction instead maybe to increase rpm. What do you think? Thanks for your time.

A: OK, let's talk a little about weapon motor selection:

  • You're considering running the L5055A motor down at the bottom of its 11.1 to 29.6 volt range. If the motor can produce the advertised 1400 output watts at 29.6 volts, you're only going to get about 200 watts output at 11.1 volts.

    Power increases/decreases with the square of voltage:

    • 11.1 / 29.6 = 37.5% of max voltage
    • (0.375)2 = 14% of max power
    • 14% of 1400 watts = about 200 watts.

  • You can get better than 200 output watts at 11.1 volts from a much smaller and lighter motor -- something like the Turnigy G25 will give more than 300 output watts at 11.1 volts, assuming the specs are correct.

The G25 with a 3:1 reduction will spin the weapon up to better than 250 Joules at more than 2000 RPM in about half the time the L5055A needs to spin up to 150 Joules with the same reduction. Lighter, cheaper, and more powerful -- I think the G25 is a better choice, if you want an HK weapon motor. Now, about ESCs:

  • If a real robot ESC (Vantec, Scorpion, Sabertooth...) says it can handle 60 amps of continuous power, you can put the ESC on a test bench and pull 60 amps thru it until your batteries go dead. If it says it can pull a peak 85 amps for ten seconds, you count on 85 amps for ten seconds.

  • If a 'hobby' ESC says it can handle 60 amps of continuous power, it means that the manufacturer looked up the absolute current values for the power chips used in the ESC. If you try to pull 60 amps thru it on a test bench you'll either quickly cut in the amp limiter or produce a big puff of smoke. If you try to pull a peak 385 amps(!!!) you'll get a blue flash and enough ozone to bleach your lungs.
Will the Turnigy Brushless 60A ESC handle your weapon? Maybe. A weapon motor pulls maximum amps very briefly on spin-up, and some ESCs have a 'soft start' feature that reduces current flow at start-up (and incidentally lengthens spin-up time). I can't tell you if it will work or not.
Q: Is there a good way to use the spinner spread sheet for melty brain spinners? Do you have any tips for an aspiring melty brain spinner builder? [Oregon]

A: You know why they call it 'Melty Brain', don't you? Getting one to work requires such intensity of thought and such enormous frustration that your brain actually melts! Well, maybe not 'actually', but it feels like it. My best advice is to lock away your sharp objects to keep you from hurting yourself, and keep a bucket of ice nearby to cool your skull.

Calculating the stored energy and spin-up time for a thwackbot/melty spinner is beyond the capacity of the Team Run Amok Spinner Spreadsheet. If you're intent on building such a design you'll have to do it 'seat of the pants'.

Q: With flippers and axes not scaling down well it seems that only rotary weapons (bars, eggbeaters) are the only way to cause damage in the smaller classes. Do you think this is correct?

A: Are you interested in causing damage, or in winning? Lifters work VERY well in lighter classes if you're interested in winning matches. See: What Weapons Win?

Q: Does a vertical spinner have an advantage over a horizontal one?

A: Yes, and no.

Advantage to the vertical spinner: when a spinner hits, there is both an action on your opponent and a reaction on your 'bot.

  • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. That's good, since your 'bot is supported by the arena surface and does not move it can deliver a more powerful impact.

  • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of your hit is split between moving the two 'bots in opposite directions. You may do as much harm to your own 'bot as to your opponent.

Advantage to the horizontal spinner: the spinning mass of the weapon exerts gyroscopic forces on the robot if the rotational axis is deflected.

  • When a 'bot with a spinning horizontal weapon turns, the axis of the weapon/gyro remains straight up/down. No gyroscopic force acts on the 'bot as a result of the turning motion and maneuverability is unaffected.

  • When a 'bot with a spinning vertical weapon turns, the axis of the weapon/gyro must also turn. This action exerts a gyroscopic counter-force which raises one side of the 'bot and reduces maneuverability.

Most builders prefer to live with or work around the maneuverability problems in order to gain the improved impact power of the vertical spinner.

Q: Dear Aaron, can you show me how Panzer MK3's and Panzer MK4's weapons work?

A: The later versions of Panzer's front plow were mounted on a 4-bar mechanism [diagram at right] that allowed the plow to move up and down without significant change in orientation. Dual pneumatic actuators powered the raising and lowering of the plow. Special pneumatic control valving allowed the plow to be positioned and held at any height within the elevation range for maximum effect, rather than simply slaming from one end of the range to the other.

Please note that the plow itself was not really Panzer's weapon. The formidable power and speed of Panzer itself was the weapon -- the plow was just the fist at the end of that powerful arm.

Q: How does one measure a spinning weapon's RPM?

A: Actually measuring weapon RPM is most simply done with an inexpensive laser photo tachometer.

Builders will often calculate weapon speed by taking the published 'free running' RPM of the weapon motor and dividing it by the weapon gear reduction. Example:

6000 RPM motor through a 3:1 belt reduction = 6000 / 3 = 2000 weapon RPM

It's quite unlikely that the weapon will actually spin that fast due to frictional losses, but it makes a good 'brag number'.

Q: How can a vertical spinner self-right with its weapon? Do you think 'Electric Boogaloo' can do that?

A: A tall vertical spinner weapon has a chance to strike the arena floor and 'pop' back upright. Lightweight 'Backlash' did this at BattleBots. I certainly wouldn't say that this is a reliable method, but you might get lucky.

If you watch the video of 'Electric Boogaloo' vs 'Sewer Snake' at RoboGames '12, you'll see EB get flipped by SS about 18 seconds in. EB's weapon does hit the arena floor as the 'bot comes down and it does pop the 'bot back onto its wheels. It happens so quickly that it could go un-noticed, but I think it counts.

Q: Hey Aaron, so I'm thinking about making a robot. I don't have a lot of money, so what weapon should I start out making? Is a motor spinning a hammer or a blade cheaper that a sping disk? Thanks.

A: If you read thru this archive you'll see we frequently and strongly recommend that a new builder's first robot should NOT have an active weapon. You'll have plenty of new things to worry about with battery maintenance, R/C system set-up, armor materials, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems.

You'll also find out that 'bots with passive weapons (wedges, bricks, dustpans...) are - on average - more successful than 'bots with active weapons (spinners, flippers...). Passive weapon robots win a greater percentage of their matches and have higher rankings than their active weapon counterparts. Here's the proof.

If you're interested in winning matches with your first robot, build a wedge.

Q: Dear Aaron, how does Team Velocity's, "Crushing defeat" work? I am under the impression that this is one of those complicated designs. The robot has an 0-2 record, however in the two fights, he was box rushed and pitted in his first ever fight, and then in the next fight the weapon could not work, so he thinks he might have been able to win if the crushing weapon worked. I saw the video of it crushing [more like piercing] aluminum [very thin] and it was pretty sweet!

Writing from Paris, but no one is awake yet! hehe... Thanks, New York

A: 'Crushing Defeat' a sa propre page sur le site Web de l'équipe Vélocité. Il y a une description très complète de l'arme électrique de perçage, une liste des composants et beaucoup de photos.

Les constructeurs ont toujours cru qu'ils auraient gagné si quelque chose était différente.

Q: Why did 'Hot Stuff' remove its flamethrower? What do you think of it?

A: Lightweight 'Hot Stuff' still has its flamethrower, but it wasn't working at RoboGames '12. Here's the story, straight from builder Jerome Miles:

"In my first match the two wires I have running out to my igniter for my flame thrower were cut and shorted out, causing an electrical fire in the bot when I tried to light the flame thrower, and damaging it so that it didn't work for the rest of the competition. Kinda a bummer, but next time I'll try to make that harder to happen. I think Hot Stuff is the first bot to put out its own fire, I cycled the grabber a few times and the Co2 put it right out!"

Jerome builds very cool robots. I met him more than ten years ago at Robot Wars and he's both a nice guy and a great builder.

'Hot Stuff' violates my simplicity rule for combat robots by combining lifter, clamp, and flamethrower weaponry, but Jerome has been building for a long time and has the experience and skills needed to pull off a complex design. Current record: 11 wins and six losses, with podium finishes at RoboGames '10 and '11 - HOT!

Q: I saw this video online of a flame thrower 1 lb bot decimating 1/16th inch polycarbonate (Lexan). The description of how to make the weapon is confusing:


1) Can you explain how, maybe with a diagram, this weapon works?
2) From the test in the video and your knowledge, how effective is this weapon? I know that flame throwers are usually for show, but this one seem legitimate.
3) how much would a decently effective version weigh?

Thank you very much, New York, writing from Paris

A: You're in Paris, and you're spending your time watching robot videos? Dude!!!

I'm not surprised that Team Misfit's description of how they made their flamethrower is confusing. Here is their description - direct from the team website - of how to build a rotating drum weapon:

How to build a rotating drum
1. Get some metal
2. Build some stuff
3. Done!
4. Profit!
Super awesome. Consider getting your advice elsewhere. In answer to your questions:
  1. We don't discuss flame weapon construction for the reasons given in #28.

  2. The video was posted by a member of Team Misfit in November of 2009 but I can find no record of any flame robot from Team Misfit ever competing and there is no mention of a flamebot on the team's website. That should tell you something about the practicality and effectiveness of the weapon.

  3. I've never seen a 'decently effective' flamethower weapon -- again see #28.

Q: Is there a rule of thumb for finding out how much PSI a flipper in any given weight class should use? I understand that diffferent configurations would use/need different amount of PSI but is there a 'safe' amount to use?

A: That's kinda like asking how hard you should hit someone in a fight. The 'rule of thumb' is to use as much pressure as the event allows.

That said, the operating pressure of a pneumatic system is only one of many elements in the performance of a flipper weapon system. The force and speed of the flipper will depend on:

  • gas pressure,
  • actuator bore,
  • gas flow rate, and
  • the geometry of the flipper.
I have seen successful heavyweight flippers operate with anywhere from 150 to 3500 psi. UK antweight flippers are restricted to 100 psi and still manage rather well.
Q: Hi there. I'm designing a lightweight robot with a spinning bar weapon. Currently I'm thinking about supporting it with a pair of tapered roller bearings, but I'm worried bad things will happen if the axle gets bent because they aren't designed to handle mis-aligned axles. (not that I'm planning on that, but there is always Murphy's law). I could use a pair of self-aligning ball bearings, but they won't handle the same amount of force as a comparably sized roller bearing (and a spherical tapered roller bearing is extremely expensive). What would you recommend?

A: I can tell that you're an experienced designer and that you've given this some thought. I agree with your analysis of the bearings and your concern about a bent shaft. My recommendation is to use the tapered roller bearings, keep the shaft short, support the shaft close to the force vector, use suitably hardened shaft material, and make the shaft so crazy large in diameter that it just can't bend. A few more ounces of weight for the extra-large diameter shaft and bearings is cheap insurance.

Q: I noticed that several flipper robots position their lifting mechanisms near the fulcrum of the flipper. Wouldn't it allow for more fliping power by pushing the flipper surface as far away from the fulcrum as possible to gain leverage? I understand that it must have some advantage because many succesful robots such as Firestorm use this configuration?

A: There are many considerations and compromises in designing a pneumatic flipper weapon. Placing the attachment point for the actuator near the lifter hinge of a simple 3-bar lifter does decrease the force available at the tip of the flipper, but can increase both the speed and range of travel. Clever selection of attachment points and flipper geometry can result in high force at the start of the flipper cycle that changes to greater speed as the flipper rises.

Other considerations include the desired profile of the robot, the bore diameter of the actuator, the gas pressure available, the flow rate of the control valves, the length of actuator motion, the angle at which the actuator joins with the flipper arm, and the location of the flipper hinge relative to the actuator hinge. Way too much to cover here, but section 6.10 of the Riobotz Combat Tutorial covers many basic flipper design elements.

Q: What [is] sewer snake's weapon?

Why sewer snake removed flamethrower for robogames 2012?

A: Heavyweight 'Sewer Snake' has several interchangeable weapons that attach to the front accessory bar: flamethrower, lifter, wedge, etc. Different weapons are used in different situations.

Sewer Snake had its flamethower at RG12 -- Sewer Snake vs. Ragin' Scotsman video.

Q: So why sewer snake didn't used flamethrower against last rites in RG2012? Did sewer snake team knew that LR reinforced it's armor to survive flame of sewer snake?

A: Flame weapons are not effective in combat -- no top-level combat robot would be damaged by a burst of flame. Fire is entirely for entertaining the audience, and I suspect the the weight saved by removing the flame thrower was put to use as additional armor to resist the brutal attack of 'Last Rites'.

Q: Wait, didn't sewer snake won by using flamethrower against LR in RG2011?

A: 'Sewer Snake' had both a lifting anti-spinner scoop and a flamethrower fitted for the Robogames 2011 championship match against 'Last Rites'. Watch the match and you'll see that 'Sewer Snake' won by superior drive power and use of the scoop -- the flamethrower did not contribute to the win.

Q: Scoop stopped spinner, but why LR started to smoke when flamethrowed at 3:11? Coincidence?

A: Mark J. here: by 3:11 in the video "Last Rites' had one functional drive motor, the spinner weapon was inoperative, and the robot was stuck on its side against the rail. Anything that 'Sewer Snake' did with the flamethrower at that point had no effect on the outcome of the match. I suspect that the smoke was from a blown motor, ESC, or battery pack that had given its all in keeping Team Hardcore in the fight.

I've written to Ray Billings and asked for his definitive word on the source of the smoke pouring from his 'bot. Sorry to bring back memories of a hard loss, Ray.

Update: Ray Billings wrote right back:

Hey Mark

Battery pack - had nothing to do with the flame weapon on SS. I was pushing the weapon system WAY harder than I should have the whole event, and was on my last (and worst condition) weapon motor. Weapon motor died, drawing a shit-ton of current, and the smoking pack was the result.

Thanks, Ray.
Q: What do you think would make the best lifter for a science olympiad sumo bot? and what [type of lifter] platform do you think would be the best to use it? Do you like the idea of the lifter? Thanks, New York

[Several earlier Q&A in this thread deleted].

A: Mark J. here: this thread got off to a confusing start. I'm gonna call 'reset' and start over now that I know which robots you ment to ask about and which event you're entering. Recap:

  • You want to know if there is any particular advantage to a flat 'lifting fork' as used by 'Vlad the Impaler' over an angled lifting wedge as used by 'Juggerbot 3.0'.

  • You will be building an insect-class robot for the Science Olympiad sumo competition with a servo-powered lifter.

  • The rules for the competition allow only the wheels (or treads) to touch the arena surface - nothing else. A couple thousandths of an inch is enough clearance.

First, I really dislike that 'no touch' rule. It effectively outlaws 2-wheel robots, and creates a 'how low can I get without touching' war. Worse, it can't be effectively enforced. A wedge or lifter may have a small clearance when sitting still for inspection, but may become a zero-clearance 'scraper' due to dynamic forces when the 'bot is in motion. How do you check clearance during the match? An un-enforceable rule is a bad rule.

As you know we don't compete in robot sumo, but I don't see any particular advantage to the flat fork versus the angled wedge as a lifter. Either might have an advantage in a particular situation, but you couldn't predict that going into the competition. A large lifter platform would likely be best -- you'd need to lift it up at the start and end of the match to meet the max dimension rule for this competition, but it might be worth considering. I don't know how the event officials would feel about a large lifter platform that might touch the floor when an opponent's weight was on it. I really don't like that no-touch rule!

Overall, I like the idea of a lifter, but I'm having a lot of trouble interpreting the intent of the event rules. Sorry I can't be more help.

Q: Aaron, when you say 'lifting platform' can you elaborate? Do you mean like a gear motor with an arm that travels over the bot in a semi circle, or an iron awe-styled lifter?

A: The 'lifter platform' is the part of the lifter arm that can effectively be inserted underneath the opponent. The term has nothing to do with the mechanism that powers the lifter or the lifter layout. A large lifter platform - like that used on 'Vlad the Impaler' - can be inserted far underneath an opponent before lifting and is very effective at breaking traction.

Q: Finally, do you know of any ant weight lifters? not just for sumo bots, but for combat robots too. Thanks, New York

A: Try this search to find antweight lifters at the Builders Database.

Q: What mechanism would you use for the flipper? Many bots cannot self right, but of course I would like to have the lightest possible solution as drive is also important. Thanks, New York

A: Let's keep our terminology straight:

  • Flipper: sends opponent flying thru the air. Usually pneumatic, not electric.
  • Lifter: raises part or all of the opponent off the arena floor. May tip opponent over. No flying thru the air.
Keep the lifter design simple. A pair of high-power servos actuating a lifter via short arms for best torque would be effective at breaking your opponent's traction.
Q: Is there a way to beat an undercutter?

A: There must be -- people do it all the time. Speed and a low scoop works well vs. an undercutter, and ramming 'bricks' can get the job done as well.

Q: And do you need 4 motors for four wheels? I think I'm going to use an undercutter and a spinning blade, but I don't believe that two wheels will balance the whole bot. Thanks!

A: You can use a chain (or belt) drive to power a second (or third) set of wheels with a single gearmotor. See the design on Nyx for an example of a chain-driven 4-wheeler. Four motors tends to be simpler.

Most undercutters are 2-wheel drive -- you need to get the wheels out of the way to let that big blade spin! The front of the robot slides along on the rounded end of the blade hub.

Q: Hey, Aaron, its the spinning undercutter and spinning hammer-thing girl. I was wondering, which one would be better? 1 spinning hammer? Or an undercutter?

A: Spinning hammer? You didn't tell me about your spinning hammer idea, so I can't really comment on it. Undercutters are popular and proven, with plenty of examples to learn from.

Whichever you pick, remember that a robot is much more than just a weapon. The craziest weapon is of no use if the rest of the robot cannot reliably wield that weapon. The chassis, motors, hubs, batteries, radio, and armor must all function together with the weapon and driver to make a successful combat robot.

Q: I have seen some wedges with a design that allows them to flip the opponent by driving into them. How could i implement this?

A: I can't figure out what design you're talking about. Can you offer a specific example, or point me to a video?

Q: I think the q about the flipping weapon with out flipping had to do with the angle of the front part, so the bot would effectivly drive vertically to the point it falls backwards. Just trying to help, but this q seemed interesting, so any thoughts on how you would do this/is it smart/what material would you use? [New York]

A: We've discussed wedges and scoops many times in this archive. I don't have anything specific to add.

Q: I have some weight left on my design, do you think it would be a worth while attempt to intergrate an electric hammer in the design to score agression points?

A: I can't comment on how the event you will compete at might judge aggression but, in general, aggression points aren't dependent on the type of weapon your robot carries. According to the RFL Judging Guidelines:

Scoring Aggression

  • Aggression scoring will be based on the relative amount of time each robot spends attacking the other.

  • Attacks do not have to be successful to count for aggression points, but a distinction will be made between chasing a fleeing opponent and randomly crashing around the arena.

  • Points will not be awarded for aggression if a robot is completely uncontrollable or unable to do more than turn in place, even if it is trying to attack.

  • Sitting still and waiting for your opponent to drive into your weapon does not count for aggression points, even if it is an amazingly destructive weapon.
A strict interpretation of those rules says that to gain maximum aggression points all you have to do is move continuously toward your opponent in an attempt to attack. That's it. It makes no difference what your weapon is as long as you're always on the attack. Some judges may tip the odd aggresion point to a bot that is more effective at pressing the attack 'in a clinch' as your hammer might be able to do, but I don't think you can count on it.

You might be better off to spend that extra weight allowance on better armor to prevent your opponent from inflicting even cosmetic damage rather than adding a weak weapon that won't get any damage points for you. Have a look at 'You be the Judge' and weigh your options carefully.

Q: The archived version of Derek Young's website is in shambles. So I was wondering if you had any clue as to how the robot 'Complete Control' was able to get so much torque and power out of its motor-driven lifting forks? And Do you think this weapon type could have any success in a heavyweight competition?

A: Second question first -- I think the current high-energy heavyweight spinners would rip away a clamping weapon like gift wrap on Christmas morning.

First question last -- the lifting aparatus on 'Complete Control' used both pneumatic and electric components. A pneumatic cylinder provides power to the clamping arm, and dual gearmotors have their combined torque multiplied by a chain-drive reduction system to power the lifting platform. You can see the large chain sprocket near the lifter pivot point. A chain drive is very effective as an add-on last stage to increase torque.

Date marker: May 2012
Q: I thought of an idea for a spinning bar. The bar would be in an X shape so it is more balanced and will hit more. Any reason not to?

A: Yes, a few reasons:

  • A straight single bar is entirely stable in rotation and requires no crossbar for balance;

  • Twin bars weighing the same as a single bar would be less strong than the single bar but would still need to survive the same impact stress; and most importantly,

  • You don't want a spinning weapon that hits 'more' for a bunch of little taps. What you want is a spinning weapon that hits 'once' and expends all of its stored energy in a massive impact that sends your opponent flying.
In a rotating weapon, two impact points are better than four -- and one is better still. Search thru the many posts about 'bite' in this archive for an explanation.

Q: In [the post above] you said one impact point is better than two. How could I implement this and still keep it balanced when the bar spins?

A: Previously discussed in this archive. Shorten one end of the bar about 1/4" and affix a counterweight near that end to restore balance. See section 6.3.2. of the RioBotz Combat Tutorial for an illustration. Read the rest of chapter 6 while you're there.

Q: Why did 'Silverback' use a slow hydraulic lifter when it could use a fast flipper?

A: Silverback did not use a hydraulic lifter - it used electric linear actuators. Electric actuators are simple to implement, reliable, easier to service, and more compact than a pneumatic flipper system. Electric actuators are slow, but a slow lifter can still be very effective.

Q: Aaron, what are your thoughts on Linear Servos? I just saw that other post. Are they good for an ant weight flipper? Do they make them that small? How do the compare to other competing products?

A: Linear servos are REALLY SLOW. They are WAY TOO SLOW for a flipper in any weight class. As mentioned in the above post, they are simple to install and easy to maintain in combat conditions -- no pressureized gasses to deal with or heavy hydraulic systems to service. They come in all sizes and can put out a fair amount of force, but did I mention that they are REALLY REALLY SLOW?

Q: Hello Aaron,
Im currently designing my first combat robot. Im planning to use a pneumatics system to "clamp" down on my enemy. Ive figured out the mechanics on what I need to make the arms clamp. My predicament is I dont know what parts I need to purchase to make the whole process of the piston (possibly the official name is the acutator) extending work. I read the DaVinci guide to pneumatics and that helped me understand how the process work, but im not exactly sure what I newd exactly to buy. Could you possibly give me a list of parts I would need to purchase and where I could get them. Thank you so much,
- Andrew

A: I'm sorry Andrew, but I can't tell you what parts you need. I know far too little about your design to even guess at the the amount of force your actuator needs to produce or the length of travel needed to operate your weapon. I don't even know the weight class of the robot you're building. You could start by looking for robots with designs similar to what you have planned and asking their builders about the components they use. I can tell you that there are no 'off the shelf' pneumatic components available that are suitable for insect class robots, so I hope you're building something larger.

By the way, we don't recommend active weapon systems in a 'first combat robot'. You'll have plenty to worry about getting the radio, drive, battery system, motor controller, chassis, and armor correct without adding in a complex weapon system. Reconsider.

Q: How do slow hydraulics, like on crusher-type robots, work?

A: Previously discussed. Seach this archive for 'hydraulic system'.

Q: How do flippers with CO2 work?.I'm new so sorry for asking so simple a question.I would be appreciate if you can explain it with a picture. [China]

A: Team Da Vinci: Understanding Pneumatics.

Q: Are there any advantages for front hinged flippers? It seems to me they push their opponents around more than anything else.

A: A front hinged flipper will, when combined with a ramming charge, toss an opponent in a low arc up and away. This is very useful in an arena where the opponent can be thrown out over a low barrier for a instant win. Such arenas are the prevailing design in the UK, where front hinged flippers have been quite popular.

Q: Do front hinged flippers have any advantage over back hinged flippers?

A: The two designs have different applications. Relative to the resting angle, a rear-hinge flipper will launch an opponent in a high and upward arc, and a front hinge flipper will launch the opponent in a lower arc to the front. Match the flipper design to the arena, your attack strategy, and the overall design of your robot.

Q: Why ziggy's flipper is so slow even it uses High pressure flipper? Are there is reason that ziggy's flipper is slow?

A: In what universe is Ziggy's flipper slow? In this dimension Ziggy's flipper is blink quick, crazy powerful, wicked effective, and recycles in a flash. The only time it's going to be slow is when it runs out of air.

The four-bar flipper mechanism on the top-ranked superheavyweight is very different from single-pivot flippers like 'Toro': it traverses a greater distance, and the acceleration of the opponent is in a more effective and controlled arc. There is also less 'self-flip' reaction due to the improved thrust path.

Q: sorry if it's answered, But how i can make Flipper Release all used air at once (Like toro or ziggy)

A: High pressure flippers commonly have separate high-flow valves for fill and exhaust on both extend and retract sides of the cylinder. With separate valves you can dump cylinder pressure as soon as the cylinder is fully extended.

Q: Are axes any better than hammers? Does it really matter? Can you tell me the pros and cons of each?

A: An axe or pick weapon has a chance to penetrate top armor by concentrating the impact force in a small area. The odds of actually hitting a critical component with a penetration is small, but the judges do like to see holes in your opponent.

The down side of a penetrating weapon is that it can (and fairly often does) get stuck in the gash, leaving you vulnerable. I have seen broad, shallow angle spikes that are designed to avoid deep penetration and the danger of getting stuck, but this also reduces the chance of a penetrating hit to a vulnerable target under the armor.

A good hammer blow makes a lot of noise, shakes your opponent's entire structure, and has a much lower chance of getting stuck. I like hammers in preference to axes or picks.

Q: How does [the weapon in] Inertia Labs' robot Butcher work?

A: The only information I have is from Inertia Lab's archived description of their pneumatic pulse motor superheavyweight robot 'The Butcher'. The complex robot had only two fights: 1 win, 1 loss.

Q: How would i go about making a jaw sort of mechanism? One that starts at the top then can clamp downwards, thanks!

A: Multiple designs may be used, depending on the size of the robot and the force required at the jaw. Chris Hannold's Combat Robot Weapons devotes some space to high-power clamping/crushing jaws. The diagram at right shows one very basic design which can use hydraulic, pneumatic, or electric linear actuator power. Insect class robots can use servos for moderate clamping force.

Q: Hi Aaron. Is Megabyte's shell belt driven or gear driven?

A: Megabyte's shell is belt driven by dual V-belt pulleys at a 4:1 reduction. If you overload a belt drive, it slips. If you overload a gear drive, it breaks.

Q: I am not expecting a definite answer for this, but could you give me an idea of the necessary speed for a successful full body spinner?

A: While you can spin a weapon either too slowly or too fast, speed itself isn't the critical factor for a spinner weapon of any type. Much more important are the amount of kinetic energy the weapon stores and the time it takes the weapon to spin up to that energy level. Yes, a given weapon will store more energy as it spins faster, but a weapon shell with a high moment of inertia will be much more effective at any given speed than a weapon shell with a low moment of inertia at the same speed.

An effective FBS weapon should have a minimum of 20 joules of stored energy per pound of the weight class it competes in. Some mega-spinners have more than 10 times that much energy! The weapon should be able to spin up to at least 10 joules per pound before an opponent can cross the arena and attempt to stop the weapon. There is plenty of help in evaluating the energy storage capacity of a spinner weapon in this archive.

Q: I am working on a full body spinner, I am planning on using wedges with a low ground clearance to flip the robots over. Do you have any thoughts?

A: Mark J. here: I like the simplicity of the approach -- it's very appealing to visualize the effect of such a weapon. However, a quick force vector analysis of the weapon indicates more push-back than lift. The drag created by the wedge as it accelerates the opponent upward will impart a LOT of lateral force. You're likely going to do more damage from impact than have success as a flipper, but that's not a bad outcome.

Spinner-flipper designs have been tried. Superheavyweight 'Phere' had a wedge-nose that led opponents up to a rotating body with a corkscrew lifting wedge. The robot had some success: 4 wins and 3 losses.

Reply: I'm the robot who had the question about a full body spinner with wedges. From what you said the design seems feasible, so I'll try it. Thanks for your input.

Response: Best luck. If you're not happy with the wedges you can always swap them out for blunt impactors.

Q: how do you make your wedge razor sharp? can i use sanding paper?

A: You didn't mention what your wedge is made of or how thick it is. For a thin wedge, you can rough shape the edge with a hand file and switch to a sanding block (sandpaper glued to a wood block) to finish the edge so that it is both sharp AND perfectly flat to the arena floor.

Note: in many arenas a sharp low wedge will catch on irregular floor seams and be far more trouble than it is worth. Check with competitors familliar with the specific arena before you go 'too low'. If it's an unknown or new arena, be prepared to adjust and 'unsharpen' your wedge on-site.

Q: Just wanted to ask, do you need a locking pin for a hinged wedge like Original Sin?

A: RFL rules state that a locking device is required for any 'moving' weapon "that can cause damage or injury". If a hinged wedge can pivot and potentially pinch or crush hands and fingers, it does need a locking device. All powered weapons require a locking device; moving passive weapons are a judgement call by the event organizer. A locking device can make transport both easier and safer, so I'd consider locking any hinged weapon in the heavier weight classes.

Q: How does Warrior SKF Work?

A: Previously discussed -- search this archive for "Warrior SKF" and for "dog clutch". See also Dale Hetherington's Flip-O-Matic for a details on construction of a flywheel flipper weapon.

Q: Hi Aaron, i found this hub in robot marketplace. can i use this for the hub of my [large steel hobbyweight] blade? my family have a weld, and both of the pulley and the timing belt is from a car spare parts.

A: No!!! Those hubs are about 3/4" in diameter and made to attach a 4mm gearmotor shaft to antweight wheels -- not a large 1000 joule blade to a weapon shaft. You haven't mentioned how large a weapon shaft you plan to use, but I hope it's closer to 12 mm than 4 mm. The design of this hub also places all the rotational load as a shear force on the screws -- undesireable for a weapon hub. Also note that your welder won't help here: these hubs are aluminum and you can't weld aluminum to steel.

If you skimp on the hub your blade will break free on impact and fly across the arena, which is VERY embarassing. How was your large pulley held on to the shaft that it drove on the engine? If you can duplicate that fastening for your weapon shaft you may be able to let the pulley do double-duty as both pulley and hub by bolting the weapon blade directly to the pulley.

Q: What are some of the advantages of a "Rotary Flipper" like that on Thrasher and Omega Force?

A: Rotary or flywheel flippers like Omega Force and Thrasher are extremely difficult to design and construct. They are typically built by very advanced builders who just want to show off their skills with an unusual type of weapon. Very few examples of such weapons exist and I cannot recommend that you attempt to construct one.

Q: What are some basic components to buy for a pneumatic spike, and where should I buy them?

A: Pneumatic weapon components are VERY well covered in this archive. Search here, and be sure to read the Team DaVinci Pneumatics page.

Q: After the reduction stage, how do i bolt the blade to the shaft which is connected to the second pulley? The diagrams in the archives doesn't help me at all.

After reading Hazard's build report, i noticed that you need a flange to bolt the blade, is there any off-shelf parts for the flange or should i use Emachineshop?

A: Yes, you need a hub to connect the shaft to a weapon blade. This hub will be exposed to a LOT of force on weapon impact, so don't try to get by with some weak cast metal hardware store flange. Weapon hubs are usually custom made.

Q: Hi Aaron, how do you put a horizontal blade after going through the reduction stage? i meant like, how do i place the shaft between the blade, the pulleys, and the bearings? and what bearing should i use?

A: Many possible layouts, depending on your design. There is a diagram of the basic layout in this archive -- search for "central shaft". The position of the blade and pulley can be reversed for a center-blade weapon like 'Fiasco', or the whole structure can be inverted for an undercutter design. Search builder websites for examples. Standard ball or roller bearings are typically used.

Note: usually the belt drive IS the reduction stage.

Q: Hi Aaron, this will be my first Horizontal spinner(actually my first combat robot). i just wan't to ask, i'll be using the BaneBots RS-550 motor paired with the P60 4:1 Gearbox. my blade will be 50cm X 3cm X 1cm. and I'll be making a hobbyweight. i have 2 questions:
  1. According to these infos, what ESC should i use(please no victors, i'm striving for program and wiring easiness, and probably i'm using a Sabertooth 12)
  2. Is the length, thickness, and the width of the blade enough for my bot?
my bot's size will be: 30 cm X 15 cm X 2 cm and i'll be using a dustpan with the blade on top.(kinda like hazard mixed with that darn dustpan!)

A: Good news first -- the size of your steel blade is good for a hobbyweight and the weapon as described will top out at more than 2500 joules, which is killer for a hobbyweight. That is the end of the good news, bad news follows.

  • That weapon is going to take a LONG time to spin up -- more than 6 seconds to get to 3000 RPM and better than 15 seconds to full speed. That won't do in a small arena.

  • The blade speed (4500+ RPM) is too great to allow the blade to effectively 'bite' into your opponent and inflict significant damage. A blade passes by every 6 milliseconds, which does not give much time to insert a piece of your opponent onto the damage radius.

  • The combined length of the RS-550 motor and 4:1 P60 gearbox is going to set your blade hub at least 4" off the arena floor. Are your opponents tall enough to be hit by such a weapon?

  • In spinning up from a standstill, the RS-550 motor will pull more than 40 amps for the first five seconds. That's way too much for way too long for the Sabertooth 12 ESC to handle. In the best case the ESC will cut amperage back and drastically slow the already too long spin-up time. In the worst case the ESC lets out a big puff of smoke and dies.
Suggestions: you need greater gear reduction for that motor/blade combination. An 8:1 reduction will give a peak energy of more than 700 joules (still good for a hobbyweight), a much more effective blade speed, a greatly improved spin-up time (300+ joules in 1.5 seconds), and reduced current load on your ESC. I don't know of a suitable 8:1 gearbox, but most spinners are belt-driven rather than gearbox-driven because belt drives generally hold up better to the high shock load at weapon impact. A belt drive would also reduce the blade height of your weapon.

I don't know what you have against Victor ESCs. They require no programming, have very simple wireing, and are even easier to set-up than the Sabertooth. The 'Victor 884' is both cost-effective and a good match for your weapon.

Of course, you can also use a solenoid to simply switch your brushed weapon motor on/off. The 120 Series White-Rodgers Solenoid is inexpensive and has more than enough capacity for your weapon.

About now you're starting to understand why I don't recommend active weapon systems for first-time builders.

Q: Hi Aaron, what brushless motors do you recomend for a weapon which is 30cm long X 2cm wide X with a thickness of 0.4 mm between two of these:
  • Axi 2826/08 Brushless Outrunner

  • EFL-25-870 Power 25 Brushless Outrunner Motor? [Indonesia]

A: See #29. Additionally:

  • You haven't told me what the weapon blade is made of, so I can't calculate the weapon mass and rotational inertia for the blade.

  • Is this for a hobbyweight? Assuming that the blade is made from steel, the blade weighs only 20 grams and would store only 9 joules of energy at 3000 RPM -- too little for even an effective antweight weapon!

  • A blade that thin will not hold up well to high-energy impact. Go thicker -- much thicker.

Q: The weapon will be either made from chromoly, mild steel or 6061-T6, but probably mild steel because it's the most available metal here.

Yup, this is for a hobbyweight.

How thick is thick? Is 1cm thick enough?

A: See #17. We have a great deal of information about the design of spinning weapons in this archive, and we have a versatile Excel spreadsheet tool to evaluate spinner weapon design. Make use of them.

The whole idea of a spinning weapon is to store a lot of kinetic energy in a heavy rotating mass and then unleash that energy on your opponent. A typical spinner devotes about 30% of the weight of the robot to the weapon system. You're going to need a WHOLE LOT more than a 20 gram blade for a hobbyweight.

Q: Hi Aaron, what's the size of the spinning bars in Run Away? And what it is made of? I want to use the same bar design as Run Aways bar for my horizontal bar.

A: 'Run Away' had twin 30" by 2.5" by .375" mild steel bars rotating at 1200 RPM. I should mention that the primary purpose of the weapon was to look good on TV, not to do a great deal of damage. Total energy was under 3000 joules -- about half the energy in effective heavyweight spinning weapons of the period.

Q: Hi Aaron. I just saw the match between 'Son of Whyachi' Vs 'Bio-hazard'. How could SoW possibly rip off Bio-Hazard's armor so easily? Comparing this with the match between 'Sewer Snake' vs 'Last Rites', Sewer Snake absorbs most hits without being ripped off. What was the difference here?

A: 'Son of Whyachi' outweighed 'BioHazard' by 100 pounds. Its weapon was eight feet across and powered by two enormous 15+ horsepower motors! It was a legal heavyweight only because it was technically a shufflebot 'walker' and was given additional weight allowance. They changed the rules before the next event to take the weight allowance away from shufflebots -- that wasn't what they had in mind for a 'walker'.

Q: Dear Aaron, compared to 'Son of Whyachi', how powerful are 'Last Rights', 'Megabyte', and 'Touro Maximus'? I saw the question above and I was just wondering how they stack up.

A: Mark J. here: I don't have enough information about weapon mass and speed on the 'bots you list to directly calculate their energy storage capacity. Comparison is further complicated because there were multiple versions of SoW with different motor configurations -- both electric and internal combustion.

Elsewhere in this archive you can find a post where I estimate the energy stored in Megabyte's shell at about 50,000 joules -- likely greater than the effective weapon energy for either 'Last Rites' or 'Toro Maximus'.

I've seen estimates of more than 100,000 joules for the version of 'Son of Whyachi' that won the BattleBots championship, but equally important is the ability to deliver that energy to an opponent. SoW's very large weapon diameter and relatively low spin speed allows for greater 'bite' and a more effective hit. Terrifying!

Q: What geared motor would you recommend for a antweight spinner weapon? And where can I find a 15mm weapon hub?

A: There aren't many small gearmotors with suitable power and gearing to make a decent spinner weapon. That's part of why most spinners use a belt drive reduction. Belt drives are also better able to put up with impact stresses. Without the details of your weapon design I can't make a specific recommendation, but you might consider the 'Beetle B04' Gearmotor [no longer available].

I get tired of answering "Where can I find..." questions with "Robot Marketplace" but it's almost always true: the VDD 15 mm weapon hub [no longer availble]. You'll need to drill the 0.126" shaft hole out to 4 mm to use it with the B04 gearmotor.

Q: The 15mm blade hub says discontinued.

A: The Team Think Tank products have been getting scarce. It looks like most all of their stuff is sold out and they aren't making any more.

I don't have another source for 'off-the-shelf' weapon hubs. You might ask around the on-line forums to see if anyone has a spare. Team Whyachi will machine a hub to your specs -- for a price.

Q: Hi Aaron. How to attach a spinning bar to the driveshaft? I mean can we weld it directly to the shaft? I am building a middleweight undercutter with spinning bar weight equal to 10kgs approx. [Prague]

A: Mark J. here: there is a great deal of stress on the union between a weapon bar and the driveshaft. A simple weld junction places all that stress on a very small area with a high risk of structural failure. A machined hub that bolts onto the blade and is fixed to the shaft with a keyway spreads out the stress loading and is the preferred method of fastening a rotating weapon to the shaft.


How about just a really tall robot that drops an Anvil type thing (looney tunes style) from a massive height to go for a one hit KO? Or an equally tall robot that just has a bunch of conveyer belt platforms to try to grab on and lift other robots to massive heights and drop them?

Has this type of thing been tried and is there video? I know these aren't realistic ideas for title-contenders, but they present some interesting engineering challenges in form of lightweight structuring and balance and would fun to see for the fly-wheel/clutch type guys who like doing things differently.

A: No, it hasn't been tried -- and for good reasons, Here are a few:

  • Combat arenas have ceilings that limit the height of your 'really tall' weapon.
  • Really tall robot with significant weight at top = top-heavy robot soon to be on its side.
  • Do the math and you'll see that a good spinner or pneumatic hammer has MANY TIMES the energy of a gravity weapon.
  • An object falling from a height of 9 feet takes three-quarters of a second to hit; your opponent isn't going to be where you were aiming when it does hit.
  • Any robot that can't survive having its opponent dropped on it from the top of the arena is a joke.
  • Ditto any robot that can't itself survive a drop from the top of the arena.
Take a trip to a live competition and watch the speed and violence of robot combat. The experience will wipe from your imagination 99% of the whacky weapon ideas you may have.

Q: Actually a super tall robot to drop things on other bots has been tried, Tower Of Power from season three of Battle bots had a guillotine like weapon.

It jumped off the Brooklyn bridge and last time I heard it owns a Lama Ranch in Saudi Arabia and has two girls a boy and an electric fan **

A: Sorry, but no. The six-foot tall middleweight 'Tower of Power' did compete at BattleBots 3.0, but its weapon wasn't a guillotine -- it was an extra-high lifter. Whatever it hoped to accomplish, it didn't.

** See #32

Q: is a sheet of aluminium enough for [the weapon bar on] a hobby weight horizontal spinner (Hazard-style)? especially when knowing that some of the contestants in my competition uses plastic as their armor.

A: A 'Hazard' style bar spinner stores energy in the mass of the spinning bar. If your 'bar' is just a thin strip of soft sheet aluminum it will:

  1. store very little energy;
  2. flex too much to impart much damage to your opponent; and
  3. do more damage to itself than any armor it hits.
There is a lot more to a successful spinner weapon than bolting a bar to a motor. Read through this archive for tips on proper design.
Q: Hi Aaron!! We were wondering about the design of flywheel flippers. Almost all of us know that the main drawback of such a flipper is it's clutch mechanism that has to transmit high torque, have low weight and it should [MUST!] withstand huge stresses. But can a fluid coupling/torque converter serve an alternative to this problem? Thanks.

A: Mark J. here: I understand the interest that combat robot builders have in unusual designs, but creative builders have been thinking about design parameters for almost two decades. If a design hasn't found acceptance by now it means that there are very real obstacles in the way of successful implementation.

A flywheel flipper needs a coupling that slips freely at high speed and can then be induced to stop slipping. A fluid coupling does not do this; it slips at low speeds and becomes more efficient at transmitting power as the input speed increases. This behavior makes it useless at abruptly transmitting power from a fast-spinning flywheel to a stationary load. The high drag on the rotating flywheel would continuously dissipate power and severely restrict energy storage. Use pneumatics.

Q: I know you're probably sick of questions about electric hammers and flippers, but I didn't see this one in the archive: could a linear motor working on the same principle as a railgun or coilgun be used to power an electric hammer or flipper?

Obviously a linear motor with comparable performance to high-pressure pneumatics would draw far more current than any batteries could supply... but what about if a capacitor bank was charged off the batteries over the course of a couple seconds (its function would be similar to a buffer tank in a pneumatic weapon)? I know that theoretically the minimum energy to raise a 100 kg opponent 3 meters in the air is about 3000 joules... even at 30% efficiency the weapon would reach 10,000 joules per shot...

How would this kind of weapon compare to a pneumatic flipper or hammer in terms of power to weight ratio?

A: Mark J. here: I claim no specific expertise in railgun physics, but I was quickly able to determine that real-world railguns have ridiculously low efficiency -- on the order of 0.1%. That takes your output power estimate down to about 35 joules. Use pneumatics.

Q: How do articulate a lifting spike like Vladiator? The only image I could find offers [no] answers.

A: Vladiator's spike has a perpendicular crossmember welded (a guess) in place. The crossmember is supported by bushings on the chassis for articulation.

'Vladiator' uses an... erm... unusual high-pressure air system with twin pneumatic clyinders and a steel cable that wraps around a pulley to pull up on the lifting spike for actuation. Similar weapons, such as the "up-ender" on 'Juggerbot 3', attach the pneumatic actuator to the weapon with a hinged mechanical link that allows pulling and pushing to raise and lower the lifter.

Q: hi Aaron, whats the better design for a flywheel? a ring like Hypno-Disc, or a bar like Last Rites?

A: That depends on what you mean by 'better'. For equal diameter, speed, and mass a hollow cylinder stores more energy than a disk, and a disk stores more energy than a bar. However, a thin-walled cylinder is more fragile than a bar and more difficult to construct and balance. Like many design elements in combat robotics, there are trade-offs.

This archive has a great deal of information on spinner design, and the Team Run Amok Excel Spinner Spreadsheet allows you to compare the energy storage capacity of flywheels with differing shapes, sizes, and mass.

For a more complete explanation of the physics of spinner weapons see Paul Hills' Spinning Disk Weapon page

Q: Hi Aaron, my 3" drum weapon featherweight robot didn't make it past the group stage of our local tournament but i was satisfied with the drum performance. The drum did an awesome job but it has a weak link which is the connection between the drive pulley & the deWalt shaft. Twice in the tournament, the connection of the drive pulley became loose due to heavy stall. FYI, i mount the pulley to the motor shaft with 2 M3 set screw (1 on top of shaft, 1 on the bottom...each set screw grip 1.5mm tapped hole on the shaft). Can you suggest the best possible way to strongly mount a pulley to the DeWalt shaft? Or as you suggested previosly, using a chain drive might be a suitable choice...can you provide further explanation on this? Thanks!

A: Mark J. here: wait a second... you only asked us which weapon motor to use 5 days ago, and you've already built your robot and had the competition?

Repeat after me: "Set Screws Suck!" This is an old adage in robot combat, and you've learned exactly why builders hate set screws. We've talked about this many times. Look up 'set screws suck' in the archive for a discussion and alternative methods.

If your set screws are simply loosening, you may gain some reliability by liberal use of a threadlocking compound like Loctite. That isn't the best fix, but you may not have the resources to implement another of the suggested solutions.

Switching to a chain drive won't help your situation -- you're just replacing one hub with another. The problem of fixing a hub to your shaft still exists, and the loading on a chain sprocket is higher than the loading on a belt pulley since the chain does not slip.

All of the questions you have asked and all of the problems you have experienced have been covered in the Ask Aaron archives. Do yourself a favor and take the time to read the archives before you procede with your robot career.

Q: Dear Aaron, how powerful are the jaws on Diesector (the latest one)? Are they like Razor in that they can crush or are the jaws just grippers? Also how do the hammers work and are they effective? (I am referring to the 2002 Diesector). Thank you.

A: DieSector's jaws were grippers - not crushers. The side hammers were electric, simply attached to NPC gearmotors. They caused little damage but were quite effective in demonstrating aggression when the 'bot was gripping its opponent or had them wedged into a corner. You don't have to crush your opponent to score points.

Q: Hi Aaron, after i watched Last Rites battle videos on Youtube, i saw that when faced with rammers, it's blades just stopped and doesn't spin anymore. how do i made my robot's spinner doesn't stop like that?

A: Mark J. here: we've discussed 'Last Rites' before. Please search the archives before asking a question.

Builder Ray Billings chose a chain drive to handle the very high torque loading between the motor and weapon bar in order to assure a quick, no-slippage weapon spin-up time. Running a chain drive transmits a great deal of shock back thru the weapon drive to the motor and makes it more likely that the drive will fail. Ray is willing to put up with some failures to the weapon in order to increase the power and efficiency of the weapon system -- it's a trade-off.

You can avoid this type of failure by running a 'slippable' belt drive to the weapon, but the slippage will degrade weapon performance.

Q: Hi Aaron, i'm trying to use your Excel Spinner Spreadsheet and when i fill the data for an Ampflow E-150 motor, i stuck on the stall torque value. By referring to this:, there is a peak torque data but [no] stall torque data. Is it similar? Please guide me on this. Thanks!

A: For a permanent magnet DC electric motor (like an Ampflow) peak torque comes at stall: peak torque = stall torque. The Ampflow E-150 really isn't much of a spinner motor -- far too heavy for the power it puts out.

Q: Hi Aaron, based on the calculation using your spinner worksheet i have managed to calculate the right balance for my spinner weapon using the Dewalt 18V Old Style Drill Motor. Before i proceed to use this motor, do you have any suggestion and any tips in using this motor for spinner weapon actuator? Do you know any other combat robot that use this motor to drive their spinner weapon and what is the result?

A: The DeWalt 18 volt drill motor is a veteran of countless robot applications. It's light, very powerful, and puts up with plenty of abuse. If your spinner isn't a success you can't blame the motor. Brushless motors have become more popular than brushed motors (like the DeWalt) for spinner drives, but I think the DeWalt can still be a good spinner motor at a great price. Offhand, I cannot think of a current robot spinner using a DeWalt motor -- but don't let that stop you!

You haven't told me much about your robot (weight class, mass of spinner, spinner style, spinner drive ratio, whether you will be using the DeWalt transmission or just the motor) so it's difficult to make specific comments. Some general comments:

  • DeWalt motors provide more power when spinning counter-clockwise, so design for that direction.
  • The motor can be 'safely' run at 24 volts for a 1/3 increase in both torque and RPM.
  • You may have some difficulty finding a belt drive suitable for both the high speed and high power this motor produces. A chain drive might be a suitable choice.
  • The gear on the DeWalt shaft is difficult to remove. A good quality gear-puller will make short work of it, but attempting to pry it off with a screwdriver is NOT recommended!
  • The motor shaft is short (7mm) -- allow for that in your design.
Dale's Homemade Robots has additional information on the DeWalt motors in robots.

Q: Hi Aaron, thanks for you previous reply. FYI, i'm building a featherweight battlebot with a 3" drum weapon (the drum should be larger but i want to build a low profile robot). The mass of the drum is approximately 3kg. The drive ratio for the weapon is 4.8:1 and i'm using belt & pulley for the power transmission. Based on the spinner spreadsheet calculation, i realized that i should use a larger drive ratio, but the options available for the pulley sizes are limited. A bigger ratio would cause the driven pulley to be larger than the diameter of my drum weapon. The 18V DeWalt motor will be run at 24v, controlled by Syren 25Amp ESC. The weapon unit is in mounting stages and will be tested tonight. I would like to know your prediction on how my weapon would work and later i will let you know the actual result. I hope its all well...

A: Mark J. here: the hardware sounds like a reasonable featherweight weapon. A larger, slower spinning drum of the same mass would be more effective, but you have to balance your design elements as best you can. At 5000+ RPM you'll have difficulty getting a good 'bite' on your opponent -- the weapon may 'skitter' across without digging in and doing heavy damage, but it could still be impressive.

Problem: the 25 amp Syren ESC is WAY too light for that DeWalt motor. That ESC can handle a maximum 45 amps for a couple seconds, but the DeWalt stall current is more than five times that great - and the motor continues to draw more than 45 amps well past 20,000 RPM. That's going to kick in current limiting immediately, which will greatly reduce torque and seriously slow your spin-up. You'll likely spend a good part of each match with the weapon completely shut-down waiting for the ESC to cool. Worst case you'll fry the weapon ESC in the first match. A big weapon motor does more harm than good if you don't have enough ESC capacity to support it properly.

Set up your belt drive loose to allow for plenty of slippage to avoid getting anywhere close to stalling the motor -- although this will further slow your spin-up time. I strongly recommend upgrading to the larger Sysren 50A or IFI VEX Pro Victor SC ESC. It's cheaper to upgrade than to replace an inadequate ESC multiple times.

Q: How should I mount a spinning full body shell on a beetle weigh robot?

A: A FBS shell takes a lot of abuse and must be very securely mounted to survive. Seach this archive for 'hobbyweight shell spinner' for a diagram and text.

Q: The full body shell would be a frying pan. How should I mount it directly onto the motor?

A: You'd need a hub to connect the shaft to the pan, but mounting the spinner shell directly to the motor is a really bad idea. Structurally inadequate, too much speed, too little torque. There is MUCH more to building a spinner than bolting some round object to a motor shaft. You'll also have real trouble centering and balancing that frying pan to spin at a few thousand RPM without shaking the robot apart.

Read thru this archive for tips on how to properly design and construct a full body spinner.

Q: Dear Aaron, I was reading up on fly wheel flippers, and you said that pneumatics are better. I agree with this, as it is pointless to make it more complicated than it needs to be. However, this has been done by Warrior SKF. Warrior used the fly wheel as a weapon. My questions are these:
1) I watched the videos of Warrior SKF, and it beat megabyte, which is as you know a top ranked full body spinner. However, how affective is this system of using the fly wheel as a weapon and have it power a flipper? How much damage does that wheel do? Finally, how powerful is the flipper and could it eject a a robot from the Combots arena and/or the Roaming Robots arena?

Also, I had was making concepts for a bot and I thought of this. What if you had a fly wheel or two smaller fly wheels as vertical spinners like nightmare or 259, and had it power a flipper. It would use two high speed, low torc motors attached to two wheels. The Idea is that when if it was fighting Roaming Robots bots like Iron Awe 5, the fly wheels could do damage and potentially make it fly out of the arena like a drum bot like Fluffy De Large. Since the UK bots have low armor, then it could do damage to the armor. I also was reading about the way that the UK guys deal with spinners. If it fought a bot with a deflector (which bot do you mean like?) I would use the flipper.

What are your thoughts on effectiveness, possibility, practicality and if it is realistic? Any other thoughts?

Also, does Roaming Robots allow spinners? Thank you.

A: My advice is to forget about flywheel flippers. As previously pointed out, 'Warrior SKF' was built by a VERY well financed team that enjoys building oddball designs ('Y-Pout', 'Red Square'). They have professional machinists and a deluxe machine shop to support these experiments. It is the only flywheel flipper -- if the design was practical and effective you'd see many more teams building them.

The design suffers from the same problem associated with any 'dual weapon' robot: neither weapon gets a full weight allowance and the performance of both weapons is reduced. The team does a very good job with the 'bot, but that speaks more to the experience and expertise of the team than the design concept.

'Warrior SKF' did once defeat 'Megabyte' (and twice lost). 'SJ's pneumatic hammer has defeated 'Megabyte' four times, but that does not mean that pneumatic hammers are a superior design. I figure that design accounts for about 20% of the effectiveness of a combat robot. The other 80% comes from construction, materials, and operation. Build something simple, build it well, and learn how to drive it.

Roaming Robots does allow spinners but, as mentioned previously, they have not proven effective in that style of competition. Many UK 'bots have bolt-on deflector scoops available if needed. You don't often see them in use, but if a spinner shows up the scoops come out.

Q: Dear Aaron, thank you very much for the advice on the flywheel. I was wondering if you elaborate on the response to these specific questions regarding the earlier q about Warrior SKF. How much damage does that fly wheel do? I know it is less but how much less? Similarly, could the flipper eject a robot from the Roaming Robots arena, or is it too weak?

Additionally, what weight category is Warrior SKF, what team built it, and if you had to guess, how much would it cost?

Thank you very much!!!

A: You're welcome, but I see you haven't taken my advice to 'forget about flywheel flippers'.

I don't have data on the size, speed, and mass of the flywheel 'Warrior SKF' uses to store energy, so I can't give a quantitative answer to how powerful the spinner and flipper are. Based on video of 'Warrior SKF' I can only say that the flipping power is not nearly comparable to the specialized high-pressure UK ejectors.

A web search for 'Warrior SKF' will very quickly lead you to the team responsible for this heavyweight robot. Don't ask me to be your search monkey. I estimate the cost of design and construction of this very exotic robot in the tens of thousands of dollars.

Q: Dear Aaron im almost done with my beetle saw bot for motorama 2012 and i need your advice some how my heavier saw blade seems to be shaking my robot apart when i fire it up and i don't know how to properly center it. p.s i have tried with a drill press and a not so steady eye. Thanks for your help.....from anthony... team warpz robotics

A: You haven't told me enough about your saw weapon for me to provide much help, Anthony. I can tell you that a precision blade hub of the correct size is critical. You can't 'eyeball' this. Is there not already a perfectly centered hole in your saw blade? How are you mounting the blade to the shaft? What are you drilling with the drill press? How fast are you trying to spin the blade?

If you do get a well-centered weapon hub and the blade is still unbalanced (it shouldn't be), search this archive for "How do I balance the weapon?"

Q: Dear Aaron, what is the best angle to have a flipper/ wedge to get under the other opponent?

A: If your goal is to get 'under' your opponent, the lower the angle the better.

Q: I saw the flipper q and have another question. Why does a bot like Iron awe have a steep front lip if he wants to get under his opponent? If you could please elaborate, that woud be great.

A: 'Iron Awe' isn't trying to get 'under' the opponent -- it's trying to launch the opponent away and out of the UK-style arena. If you get far under your opponent you just launch them straight up. Watch some UK flipper videos.

Q: Dear Aaron, if I went to a uk competition with a powerfull spinner, wouldn't that be a good Idea if I can make it so that it bends the wedges of the flippers?

A: It has been tried, but the UK boys know how to deal with spinners in their competitions. Like I said, flippers are better suited to an arena where it's possible to eject an opponent for a win, and that's how they build arenas in the UK. You really can't compare the US and UK competitions.

Q: How do the 'UK boys deal with spinners?'

A: In a non-enclosed arena a spinner is already almost as dangerous to itself as to its opponent. Newtonian action-reaction is likely to send the spinner careening off into danger. Many UK teams developed add-on scoops to herd and re-direct a spinner, launching it directly or slowing it enough for a well-timed flip out of the arena.

Q: Aaron, I had an idea about a 1lb electric hammer. Maybee I can use a Fingertech 50:1 or higher to pull on a pulley like a blade. I heard about Anthony and his design and how his sportsman created about the same force as a beetle blade which I'll try as a backup.If it is possible how would I best construct a hammer?

Thanks .Critique anything you like

A: If you've been reading thru the archives you know that I do not recommend electric hammers. Do the math and you'll discover that the destructive energy an electric hammer puts out is very small compared to a spinner weapon of comparable weight. The hammer weapon in Antony's 30-pound sportsman has approximately the same impact force as a spinner in a 3-pound beetleweight! You can get away with a puny weapon in the sportsman class, but don't try it in open competition!

My best advice is to pick a different weapon.

Q: If I use the same 64:1 in a beetleweight, would it be possible?You did mention that DID have the same joules as a spinner. And, since most beetles don't focus on top plates, it would be possible to at least dent it.

A: Anthony's sportsman hammer has a 16 ounce head on an 18" long arm and is powered by an RS-550 motor (8 ounces) with a P60 64:1 gearbox (10 ounces). Total weapon weight is close to three pounds and the dimensions are too large for a beetle. When you scale down to a reasonable size for a 3-pound robot, your weapon power will drop proportionally. Add to that the problem in positioning and triggering the weapon at the right time (a spinner just has to run up and touch you) and you're fighting an uphill battle.

Electric hammers have been tried many times. If they were workable you'd see successful builders using them in open competition. My best advice is to pick a different weapon.

Mark J. here: A builder pointed out to me that sportsman electric hammerbot 'Mangi' has won in open competition. Aaron has previously noted that Mangi's builder could win matches with a half-empty tub of margarine. When you have 20 years of combat robot experience you can build what you like and do well. Until then, avoid the electric hammer.

Q: Hi Aaron. I was designing a middle weight flipper and wanted to know the relative advantages/disadvantages (if any) of the four bar flipper mechanism over rear hinge flipper. It would be a pneumatic actuated flipper and I am confused as to how to go about it.Can you also tell me as to how to decide the dimensions and calculate the lifting capacity of the pneumatic actuator for the flipper. Thanks.

A: Mark J. here: a pneumatic flipper is the single most complex robot weapon to design, build, and operate. It is also potentially the most dangerous to the builder. High pressure pneumatics are unforgiving. There's no way I can tell you how to 'go about it' in a few paragraphs, but I can point you to some resources to get you started.

  • Start by reading thru this archive. There are many posts here about 4-bar mechanisms and single pivot flippers.

  • Read every word of the Team DaVinci Pneumatics page, then go back and read it again.

  • The T.i. Combat Robotics 4-Bar Simulator is a tool to assist in designing 4-bar lifters with electric power, but the elements of the tool that model the path of the mechanism tip are useful for pneumatic powered flippers as well.
A correctly designed 4-bar mechanism has the advanatge of moving forward and upward, moving toward your opponent instead of pulling away from them as a rear-pivot flipper does. Very handy.

Flippers are all about speed and power, and the calculations are a nightmare. The usual design approach is to just cram the biggest components you can into your chassis and hope for the best. Big actuators, the highest flow-rate regulators, buffer tanks, and big valves are all critical. Not cheap, not safe, and not for inexperienced builders.

Q: Hi! I have a question about a virticle spinner.I'm using an R/C to control my spinner. So far, I'll be using the following: An A-40-300 motor(24 volts, 3.8 horsepower, 3840 oz-in Torque, 340 amps max current, 84 % efficiency, and 4000 rmp), 2 3/8 pitch Type B Sprockets - 11 teeth, 5/8 inch bore, #35 roller chain, a V-tail mixer, and a 24V 5.0ah NiMh BattlePack.

My questions are: 1 Is the mixer I chose good? If not, what kind do you recomend?( I use battlekit's drive module driving method with two driving A28-4000 motors). 2 What kind of R/C do you recommend? 3 Should I use an electric speed controller, witch kind do you reccomend?

A: OK, I really hope you're just pulling my chain. Given the level of questions you're asking, you're in WAY over your head building a robot with a A40-300 powered spinner.

Start reading thru the Ask Aaron Archives.

  • The weapon archive has tons of weapon design info.

  • The archive has all the radio and mixer details you need.

  • The archive will get you set up with speed controllers.
After you've gone thru the archives, rethink the size of your first robot project. Consider going smaller.
Q: How much damage does the smashing of the scoop of 'Breaker Box' or 'Shazbot' do? Thanks

A: Almost none. The electric scoop motors don't pack enough power to do damage -- that isn't their purpose.

Q: What do you think of the following idea?

A pnuematic piston is placed at the end of a hammer arm. The piston points towards one end of the arc, while the other end has a spike. When firing, the piston (which would be resting near the ground) would activate, pushing off the ground firing the hammer. Then, a winch would push it back into position.

The idea is that by having the force as far away from the fulcrum as possible, the piston would deliver more power to the hammer than if it delivered the force from the fulcrum. In addition, the piston would act as both a hammer weight and a power source, possibly making the system more weight-efficient.

Unless my understanding of the physics is off, the one issue I can see is protecting the piston from the impact.

A: Mark J. here: bonus points for creativity, but you are confusing 'force' with 'impulse'.

A pneumatic cylinder acting directly at the head of the hammer has greater mechanical advantage on the mass and will accelerate it more quickly -- but will act on that mass over a shorter time period and thru a smaller arc of motion compared to the same cylinder acting closer to the pivot. It works out that the theoretical 'impulse' (the product of force and time) is the same, and it is impulse that accelerates the hammer.

However, since the cylinder in your design extends very quickly, you run into friction and gas-flow inefficiencies that will reduce the real-world impulse compared to an equivalent cylinder extending more slowly and operating over the entire arc of the hammer. This yields less hammer acceleration.

Bottom line: less impulse power, additional complexity, and increased fragility. I don't think that the additional mass in the hammer head will offset these disadvantages.

Q: How well does a linear actuator attached to a spike or barb at the end work for a 15lb and 120 lb robot?

A: About as well as a pretzel with a wad of gum on the end. Try again.

Q: just out of interest so I know how to work around it, why does a linear actuator not work?

A: Linear actuators are slow -- really slow. Your opponent isn't going to sit still while you roll up, position your weapon, and very slowly push a spike at them. If by some miracle they did sit still:

  • from a side attack, the spike will make contact and slowly push the two 'bots apart;

  • from a bottom attack, the spike will lift your opponent until they just fall off;

  • from a top attack, it will slowly lift your bot upward rather that penetrating the opponent's armor;

  • from a top attack with a bottom fork under your opponent to hold your robot down, the actuator will not have enough power to penetrate even modest top armor. If you don't believe this, set up a test in your workshop and measure how much power it takes to penetrate representative top armor in the weight class you choose.
Remember, your opponent is rushing at you with their own high-power destructive weapon. All a spinner has to do is touch you and you go into low orbit. If linear actuators worked, you would see them in common use in combat robots. They have been tried, they don't work, and there isn't a work around. Pick something else.

Comment: Dear Aaron, I would just like to thank you with the linear actuator question, that was a HUGE help.

Thanks, New York

Reply: You're welcome. I may give a very short answer where the situation is obvious to me, but don't hesitate to ask for clarification if you need it.

Q: Dear Aaron, I saw that you said that under cutters are able to get into the drum bots "upsweep." Where is the upsweep? How should I attack a drum bot with that in mind? How should I defend a drum bot if I am going to build one? Thank you!

A: What I said [in this archive] was: "An undercutter targets wheels, and can get in underneath the dangerous 'upsweep' zone of a drum weapon where the impactors have significant vertical motion."

I've been getting a lot of questions about the 'upsweep' zone. It's really simple: the upsweep zone of a drum (or other vertical spinner) is the area where the impactor(s) are sweeping mostly upward and at least a little forward. This is the area where the drum is most efficient in biting in and launching the opponent upward. Below this zone, the impactor movement is mostly forward and a hit will just push both bots back away from each other. Above this zone, there is no forward motion to 'bite in' and the impact is weak.

If you want to attack a vertical drum head-on, a good bet is to get in under the upsweep zone with an undercutter or a low scoop. If you have a drum, a low wedge of your own positioned to protect the area under the upsweep zone will defend against that type of attack.

Q: What is the vertical "upsweep" of a drum, and what is an egg beater?

A: An 'eggbeater' is a flat rectangular-frame spinning weapon, related in design and function to a spinning drum weapon. The metal rectangle is typically cut from a single plate of steel. An eggbeater is easier to construct and balance than a drum, but usually has a little less energy storage capacity for the same weight.

The 'upsweep' area of a drum (or eggbeater) is the portion of the circular rotation path where the impactor is moving predominantly upward and at least a little forward. This area is most effective in 'launching' the opponent. See the diagram in the post above.

Q: Aaron, looking at the question about how drumbots with faster weapon speeds will be able to get hits on drumbots with slower weapons [down farther in this archive], is this dependent on the weapon's tip speed, or the drum's rotational speed? I've been trying to figure it out on my own and I'm a bit stumped.

A: Mark J. here: if two drum weapons are going 'head-to-head', the weapon with greater 'tip' speed will be dominant. Tip speed is a function of weapon RPM and weapon diameter.

Note that the speed needed to prevail in this very specific situation is much greater than the optimum speed for a drum striking a non-rotating target. It may be better to employ some alternate technique when fighting another spinner rather than compromise the overall effectiveness of your weapon by dialing in too much speed. Think about the 'big picture'.

Q: Hi Aaron, questions on drum weapon.
  • A drum weapon with same number of joules stored but different in diameter, what there be any difference in the weapon effectiveness?

  • I have 2 gear ratio option for the drum weapon on 30lb bot, battling in 11' arena, with results as below:

    1. 1128 joules in 1.2s at 4846 rpm & 2564 joules at 7308 rpm
    2. 476 joules in 0.54s at 3150 rpm & 1083 joules at 4750 rpm

    Should i choose option 2 because the spin up time is faster, thus suitable for a small arena battle?

A: A larger diameter drum will be spinning at a lower speed than a small diameter drum with the same stored energy, and slower is good! More time between the passage of each impact bar means better 'bite' and therefore a better impact on your opponent.

In an 11 foot arena the distance from nose-to-nose on the robots is going to be very small -- like 7 or 8 feet. You're going to have VERY little time to spin up your weapon, so from that angle I'd suggest the second gearing option. I have seen spinners start a match with their weapon pointed away from their opponent to gain more spin-up time. Judges in the US don't like that, but I don't know how judges in Malaysia might react.

Note: spinning a drum weapon at 7000+ RPM is not a great idea. Even 4750 RPM is faster than I'd suggest. For better results, add more mass to the drum and spin it at a lower RPM.

Q: I remember seeing somewhere that you mentioned when two drumbots' weapons collide, the 'bot with the faster spinning weapon will win. [It's in this archive] I understand it has to do with bite, but how exactly does a faster weapon guarantee a launch for that 'bot? Do these rules apply for eggbeaters as well? And what effect does a weapon's moment of inertia or weight have to do with this? How much faster does one weapon need to be to guarantee a positive outcome? I know it's a multi-faceted and variable dependent question, but an explanation and play-by-play of what happens here (and maybe even a diagram?) would be incredibly helpful. Thanks so much for your help.

A: Mark J. here: I don't think a diagram is going to help. The situation is this: with two drum (or eggbeater) weapons facing each other, the impact bars on both weapons are sweaping upward thru the 'impact zone'. The impactor that is moving slower has absolutely no chance of catching up to the faster moving impactor, striking it from the underside, and launching the opponent. Speed rules here!

Given the typical high spin speed of drum weapons, the distance impact bars stick out, and effective close rate, it is the impact bars that are going to meet rather than a bar impacting a drum body. If an impact bar should happen to hit the smooth near-vertical face of the drum, it will get no effective bite and just knock both bots back a bit.

More speed differential will offer greater impact on the other weapon. The faster you can close on that up-sweeping impactor running away from your impactor, the greater the energy transfer to the opponent weapon. A small speed differential will not 'launch' the opponent, but it can at least give it a 'bump'

Q: Hi Aaron, i plan to build a wedge bot with a bar spinner on top just like hazard. I'm currently selecting the motor to drive the bar spinner and i'm experimenting with your spinner spreadsheet. Details of my bar spinner are:
  • Material : Steel
  • Length : 0.5m
  • Width : 0.08m
  • Thickness : 10mm
I pick the Amplow A28-150 motor details for a reference:
  • Torque : 1970 oz-in (i didn't find the stall torque in their description, can i actually use this?)
  • RPM: 6000
Then, i try your spinner spreadsheet with few gear ratio setting and come out with the results below. I didn't quite understand what is their different in relation to my bar spinner performance.

At 2:1 ratio

  • The weapon has 1313 joules in 0.76s at 1890rpm
  • At 2850rpm, it has 2985 joules of energy.
At 3:1 ratio
  • The weapon has 583 joules in 0.34s at 1260rpm
  • At 1900rpm, it has 1327 joules of energy.
and at 4:1 ratio
  • The weapon has 328 joules in 0.19s at 945rpm
  • At 1425rpm, it has 746 joules of energy.
Can you explain what all those difference will made in term of my bar spinner performance? and which gear setting above is the better?

I got this motor data from but there is no "no load current" listed so i can determine the power required. Do you know where can i get this details? Otherwise i won't be able to decide what motor controller to be used & how much battery capacity that should be reserved for my weapon,am i right?


A: I'm getting lots of questions from Malaysia -- just how much robot fighting goes on there??

Full specifications for the AmpFlow motors - including no-load current and peak (stall) torque - are available at the AmpFlow website. Please don't ask me for info you could find for yourself with a simple web search.

There are multiple examples of spinner design and discussion of the effects of different gear ratios on spinner performance in this archive. I'm not going to repeat that information here.

Which gearing is 'best' will depend on many factors you have not mentioned -- robot weight, arena size, opponent design, arena layout, robot speed... Again, this has been previously discussed in this archive. The tools and information you need are all here - start reading!

Q: Hi Aaron. I've seen in a video that Last Rite's weapon is chain driven. Isn't belt driven weapon more appropriate there, since the weapon has enormous power?

A: You have to understand builder Ray Billings and Hardcore Robotics. Ray does not believe in sublety, slippage, or second chances. He doesn't even believe in armor!

'Last Rites' has a huge weapon bar, an enormous weapon motor, and a solid chain drive connecting the two. There's so much kinetic energy stored in the weapon that bringing the bar spinner to an abrupt stop just isn't going to happen -- a slip drive to protect the weapon motor isn't needed.

Q: Do you have a diagram of a hydraulics system and how to transfer the force into [a useful] direction like (most notably) Razer and Jawbreaker's Revenge?

A: A hydraulic system works just like a pneumatic system, except instead of gas from a pressurized storage tank the hydraulic system uses liquid drawn from a reservoir that gets pressurized by a pump.

Most hydraulic 'crushers' use a 'teeter-totter' lever system where the cylinder pushes upward (or rearward) to move the crusher down. Both the lever arm and chassis must be VERY STRONG to survive the force generated by the hydraulic system.

Q: Can I use stainless steel for a beetleweight spinning bar? If not, what should I use? Alloy steel is proving a little bit hard to find in the correct shape.

A: You can use whatever you like . 'Stainless' is a whole familly of steel alloys with a wide range of physical properties -- some of the 'stainless' alloys would be better than others for a spinner bar, but a simple low-carbon alloy would be better. You're looking for a good balance of 'hard' and 'tough': most stainless is 'tough' but not particularly 'hard'.

There are many posts about various steel alloys in the archive.

Q: Steel guy again. Many sources of information extoll heat-treating your steel weapon first before sending it to the arena. I realized, however, that I have no idea how to heat treat an object, let alone have any of the tools needed. I know that you need more than a lighter or an oven, but thats about it.

Can I get away with not heat treating the mild steel weapon? Remember, I am having a hard time finding alloy steel in the right shape, so I don't have much choice in terms of material.

A: Mark J. here: heat treating is a specialty process best left to professionals. Check the yellow pages for 'heat treating' and hand your blade off to someone who has the equipment and experience to do it right. You'll need to know exactly what alloy your part is, so don't walk in with some unknown metal. Some alloys (all steels are alloys) repond very well to heat treating, and others not at all -- so know in advance what you've got!

Heat treating is not mandatory, but you don't want your weapon getting beat up worse than your opponent. The more energy you pump into the weapon, the greater the need for exotic metallurgy.

Q: I had an idea to mount a beetleweight spinner weapon to the shaft (Or, more of, pulley) Simply bolt the [spinner] to the pulley and use shaft collars to keep them in place. Will that work?

A: So, you have a pulley spinning at high speed on a bare shaft, located by shaft collars? Very poor practice -- how about putting a couple bearings or bushings in there! There is also a well known saying in robotics: "Set screws suck." Don't rely on a set screw anyplace where its failure would cause a problem.

Q: Hello Aaron. I was thinking of designing a spinner killer that is 15lbs that resembles breaker box and shazbot from team nightmare. How do their scoops work? Maybe you have a diagram or detailed pictures?

A: The motion of the scoops on Team Nightmare's 'Breaker Box', antweight 'Shazbot', and beetleweight 'Wallop' are all controlled by powerful gear motors mounted on the chassis where the scoop support frame enters the robot. 'Shazbot' and 'Wallop' use high power servos for the scoops, while 'Breaker Box' uses custom gearboxes mated to electric motors. Photos are available at the linked sites.

Q: Do you have any diagrams on the inner working of the scoop of breaker box and shaz bot? I am confused about the answer of how they work.

A: A top-view diagram of Shazbot's scoop lifter is at right. Servos are frame-mounted and hacked for continuous rotation. Servo outouts are attached to the scoop support arms. For larger 'bots, substitute gearmotors with suitable torque capacity.

Q: Does breaker box work the same way as shazbot? What is the difference in there diagrams?

A: Same thing, just bigger.

Q: Also, I wanted to add a weapon to it. What do you think about having a small hole in the front of the scoop so that a pneumatic spike can do damage? It would be one unit so the spike goes with the scoop so that when you lift the scoop you can still hit the with the spike Do you have any opinion on this design if it sounds good, unpractical, impossible, or any improvements or problems? Any remarks you have are fantastic...Thank you so much!

A: Poor idea. The strength of the lifter plows on 'Breaker Box' and 'Shazbot' come from their large range of motion -- a full 360 degrees of rotation. Hanging a pneumatic actuator and hoses off the back of the plow would restrict motion and greatly complicate the weapon system. Many robots have tried multiple-weapon systems, but simple weapons win.

Q: how should I make a pneumatic spike for a light robot? I was wondering if you had any diagrams of any pneumatic spikes for a 15 lbs robot... Thanks!

A: You shouldn't. Simple pneumatic spikes have been tried many times and they are not effective in robot combat. If you want to try anyway, read the Team DaVinci Pneumatics page for details on pneumatic weapon systems.

Q: Dear Aaron,

I am going to enter a bots IQ or bot bash or battle bots.... It seams to me that all the winners are vertical spinning drum weapons, or weapons that are extreamly similar. I would like to be original and make a robot that doesn't have a drum. What is the best way to counter a drum without running away? One of my plans was to make that resembles Hazard. How good would that do against a drum? I am talking about a 6 to 15 lbs robot.

Thank you so much, Aaron. This website is fantastic.

A: Glad you're enjoying the site!

Drum weapons are very popular because they work well against a wide range of other weapon types. It's like you could pick both 'rock' and 'paper' at the same time; 'scissors' wouldn't stand a chance.

'Hazard' style horizontal blade spinners are not a good match against a drum weapon -- the blade sets too high. Big under-cutter horizontal spinners like 'Totally Offensive' are a better bet, but they may be too common if you're trying to be 'original'.

The ultimate 'spinner killer' is a heavy scoop/lifter as seen on 'Breaker Box'. The design works against vertical and horizontal spinners -- just make that scoop STRONG!

Q: I love this website even more! The response was great about the drum bot. Thanks!

Do you have images of 'Totally Offensive?' What is an under cutter? Thanks.

I like the look of breaker box a lot. Is there a way to make a good drum-stopper, and still have the capability of doing damage? I would just love maybe a combination of a drum killer and a damage-weapon.

I am not trying to be pushy or anything...I don't want to insult the best site for robots. What about the design of 'village idiot' for the drum killer and damage? What type of bot is 'Village Idiot'?

Thank you so much. I really enjoy robot fighting. This website is my access.

A: Images and video of 'Totally Offensive' can be found at the Team Mad Overlord website. An 'undercutter' is a horizontal spinner weapon set very low down near the arena floor. An undercutter targets wheels, and can get in underneath the dangerous 'upsweep' zone of a drum weapon where the impactors have significant vertical motion.

Combining multiple offensive and/or defensive designs into a single robot has never proved successful. It is possible to have interchangeable attachments to suit different opponents, but multiple simultaneous weapons are a no-go. Pick a design and commit all of your weight allowance to that design.

The various versions 'Village Idiot' (9 wins, 4 losses) had twin vertical disc weapons. Early versions used circular saw blades, but later incarnations used milled aluminum discs with impactor 'teeth'. From a design standpoint this is a very minor variant on a drumbot.

Q: Hey Aaron, Which weapons died out do to there lack of effective power and ability? Thanks!!!

A: There is a fairly complete list of weapon types in the Wikipedia article on robot combat. All of them save for the spinners, wedges, and spinner-killer scoops are pretty much extinct.

Q: when you said wedges for the non-extinct weapons, does that include flippers?

A: No -- lifter wedges are fairly common, but flippers aren't competitive in the U.S. Flippers still dominate in the UK due to the different arena design and judging prevalent there.

Q: I have I question about a robot design a was considering. If you had an axe bot that would smash downwards like a normal axe, but then it could be switched into a spinning weapon. The hammer/axe would do this down position turn so the tip was flat/perpendicular to the axe . Then it would spin around and do damage that way. The second mode would essentially look like hazard with one blade. My thinking is that with this combo the enemy would stay away and be hit by the spin mode but if it came to close I could smash it with the axe. This might be mechanically accomplished by having to motors one for axe mode and the other for spinner. The engine would have a cam so only one motor would be necessary for both modes. Thanks!

A: You said you had a question, but it seems you never got around to asking it. As a general comment I'll point out that complex weapons add weight, reduce reliability, and win fewer matches than simple weapon systems. Team Run Amok's motto: "Complex design is easy - simple takes work."

Q: how does sewer snakes weapon work?

A: There is a description of Sewer Snake's weapon - with photos - in the this archive. Search for "Sewer Snake's unique weapon".

Q: Is there any particular reason no one's ever used an grinding disk for a weapon?

A: There have been grinders - lightweight 'Grunion' (1 win, 1 loss) fought at the BattleBots 2.0 with a grinding wheel weapon, and 'Zero' ran a milling cutter at Robotica 3. A grinding weapon has several drawbacks: ceramic grinders are fragile, there is small potential for damage, and it's extremely difficult (impossible?) to hold correct position and pressure on your opponent in a chaotic battle arena. I couldn't recommend it.

Q: I want to use C02 for my robot's flipper. I found what tank (A fire extinguisher) and a ram (A hydraulic cylinder) but I don't know what regulator I need. I know that I know that it needs to be a C02, but there are so many things to note like ones with solenoids, some with strange gages, its all confusing me. Could you at least help me to figure out what would be a good regulator, I can find the rest of the parts myself.

A: Mark J. here: I'll be pleased to help you select a regulator, but I need more information about your robot:

  • In what weight class will you be competing?

  • What are the dimensions (bore and stroke) and pressure rating of your actuator?

  • At what pressure do you want to run the actuator?

A couple of points:
  • Hydraulic cylinders are heavier than standard pneumatic cylinders and are generally used only if you intend to operate the pneumatic system at very high pressure -- such as unregulated CO2.

  • Are you sure you're looking for a regulator? The fact that you mention solenoids makes me suspicious that you're looking at multi-port valves and not regulators. Exactly what function do you want this device to perform?
Read thru the Team DaVinci Pneumatics page to get general information on pneumatic weapon systems and to make sure we're using the same vocabulary.

Q: I'm the flipper guy again, I just want to help out with the question:

  • The weight class is middleweight, but it's flexible enough to be a heavy if it's overweight by a fair margin.

  • The cylinder is a four inch stroke with a 1 1/2" bore, and can go well up to 2500 PSI.

  • 850 PSI.
I know that I could go on with out the regulator, but I REALLY don't want my cylinder to fail.

PS: I already read the DaVinci page many times. I wasn't going to get my head into this with out the know how on to do it.

A: Go back to the Team DaVinci page and read thru the 'What a gas!' section again. Compressed CO2 liquifies and self-regulates its pressure to about 850 PSI at room temperature. The pressure will drop as phase-change thermodynamic effects reduce the temperature of the CO2 remaining in the cylinder. You have no need for a regulator -- the pressure available to your actuator will not exceed 850 PSI.

Please be VERY CARFUL with high-pressure pneumatic systems! They are violent and dangerous. Handle with great care.

Q: Flipper guy once more, thank you for your help.

One last thing, is their any advice you can give with CO2 systems for optimal performance?

A: The flow performance of your valve system is critical for best performance. The flow capacity is listed as the 'Flow Coefficient' (Cv). Larger values for Cv flow gas faster and will give better performance. Don't scrimp on the valves!

Q: How do you articulate lifter forks like the ones on Vlad The Impaler?

A: Vlad had a very simple single-pivot lifter hinged low toward the back of the chassis. I don't have a photo of Vlad's actual mechanism, but it must have looked something like the sketch at right.

Date marker: May 2011
Q: Aaron, I have a theory about having an electric hammer. Just bear with me. Based on the rack and pinion concept of the judge. I was planning on using the rack and pinion method with a pulley connected in a closed circuit with the motor, kind of like the pulleys used to control multiple wheels with one motor only you replace the wheels with a single axe.

I also had a similar in my idea about the 3lb Razer design. I was going to make the chassis of mainly two 1/8 lexan middle pieces. [Brandon, the guy with all the craziest beetle or ant weight designs]

A: Albert Einstein is reported to have said that crazy is "doing the same thing over and over again and expecting different results." By that definition, expecting to win with an electric hammer or electric crusher is certainly crazy. Best luck.

Q: This is a snippet of conversation between the builders of two very powerful horizontal spinners. My question is whether this notion is accurate and what the physics behind it are: "I'll tell you how to beat my bot: with a blade that will cut in both directions [using a reversible ESC]. Then if you're spinning opposite of me because of your higher rpm, you'll cut into my shell." Does it really work like that? The other builder seemed skeptical.

A: Mark J. here: I'd really like to know who the two builders were.

The theory works well for drumbots. When two drumbots go 'head-to-head' they are (viewed from the sid I'm not so sure that you can count on the same effect with full body spinners that don't get support against newtonian reaction. I think having a blade that can cut from both sides is a good idea, but I think it will cut just as well if it's spinning faster or slower than the other shell.

Q: Looking at your archive gave me a interesting idea. In one of them you said it was a good idea to keep a wedge and a thwack separated, and would not work well together in one bot. That gave me the idea to literary keep them separate, and have one bot, a wedge and opponent in tow, smash into its teammate, a madly spinning thwack. I bet theres some bugs in my idea, but what do you think about it?

A: I like the idea, but let's take it one step further - have the thawack madly spin on top of the wedge as it drives around so the undersized wedge doesn't have to fight to shove the twice-as-heavy opponent over to the thwack. It'd look a lot like 'Hazard'. Oh wait, it'd be 'Hazard'.

Q: I've been using your Excel spinner spreadsheet, and it saves me tons of time, thank you!

I know I should avoid very general questions, but one thing I'd like to ask is "how fast is too fast?". My teammate and I have very different opinions about how fast our weapon should be. If it helps, the weapon is a 20 inch long spinning steel bar, 1.5'' wide and .25'' thick, on a 15lb combat robot. The motor we're using has 9Nm of torque, a max RPM of 30800 at 22.2V, and we're using a 4:1 gear ratio to get a spin-up time of about .54 seconds, but I understand that I should add about another second for the soft start feature. The power consumption is also reasonable, at 1.29 amp hours for six spin-ups during a 3 minute match).

I know that similar lawn-mower style robots like Brutality and Hazard have their bars spinning at about 2K rpm, but I also thought that a smaller weight class would make a big difference in terms of how fast our weapon can spin. We're not very concerned about bite, since the bar is very long, and we calculated there to be about 1/2" of bite if the weapon spins at 10K rpm with both bots approaching each other at 4MPH.

Personally, I think that 10K rpm is much too fast, and that something like 5K is much more reasonable. There is a lot of uncertainty with high speed, since aerodynamic-drag will be a huge factor with such a large bar. My friend's solution to that was to make the bar into a wing-shape, but I'm skeptical about the functionality that would have as a weapon.

My teammate's greatest fear is that the weapon will be relatively ineffective with low energies, and he has thus become what I call "Joules-obsessive". Since weight is tight, and increasing the speed by two increases the amount of joules four-fold**, he really wants to reduce the gear reduction for greater speed and maybe use two motors instead of one to make up for the loss in torque and spin-up time. With either configuration, we are well above the "27 Joules/pound of robot" guideline, but the 15lb class has become filled with many ultra--speed drumbots with very high weapon energies.

Please don't worry about us trying to create a robot that is "all weapon", because we still have a sturdy drivetrain and chassis in case all else fails. Please let me know if you think me, my teammate, (or both of us) are being absurd with our weapon plans.

A: Mark J. here: so, you're thinking about spinning a 20" long, 1.5" wide, .25" bar of steel at 10,000 RPM, storing 10,245 joules of energy in it, and slamming it into a 15 pound object - or worse, the arena wall? Have you considered what happens to the blade in an impact at that energy level? I think you get one good hit and the blade turns into a bow tie.

The laws of physics haven't changed since Hazard and Brutality were designed, and if higher RPM was the way to get better weapon performance you can bet that their designers would have taken full advantage of it. There is a balance to be struck between speed, energy storage, spin-up time, durability, and bite -- and the only way to find that balance is by trial and error. Look to robots that are successful and learn from their designs.

My recommendation is to not get greedy. I'd pick a smaller motor, put the weight savings into tip weights on the blade, and spin it at 3K RPM for about 1600 joules.

** Note: the statement "increasing the speed by two increases the amount of joules four-fold" is incorrect.

I'm sure the builder ment 'four times' instead of 'four-fold'. A 'fold' means to double; 'four fold' would double the output four times:

2 * 2 * 2 * 2 = 24 = 16 times the energy.

Kinetic energy increases with the square of speed, so doubling the speed gives = 22 = 4 times the energy, not four-fold.

Q: Thinking about a flywheel powered flipper, would it be possible to make an electric flywheel powered hammer?

A: Mark J. here: possible - yes. Simple - no. Reliable - unlikely. Successful - I think not.

We've discussed flywheel-powered flippers in the archives. Their mechanical weak point is the dog clutch that transfers the power from the flywheel to the weapon. The concept is like revving up a car engine and trying to jam it into gear without pushing the clutch in first. The components are not 'off the shelf', they are difficult to design and machine, and the mechanism is subject to huge stress.

Pneumatics are much better at this type of work, so why make it so hard on yourself?

Q: I thought of a way to get more power out of a hammer bot. The robot would have a hammerhead with a heavy ball in it. When the robot hits the ball would increase shock damage.

A: Mark J. here: sorry, but no. What you describe is a type of dead blow hammer. The passive internal mass strikes slightly later than the main hammer body and spreads out the impact while absorbing some of the rebound. The total energy of the impact is the same as a conventional hammer of equal mass, but the energy is released over a longer time period.

A dead blow hammer is used in situations where you wish to reduce damage to the surface you are striking -- just the opposite of what a combat robot is trying to do.

Q: I am trying to activate a 24v NPC motor for a weapon using a White Rodgers SPNO 24v solenoid and a Battleswitch. How Do I set this up? My Battleswitch is connected to my radio system and switches properly. How do I use the Battle Switch to power the Solenoid? I have one 24v Battery, can I use this battery to switch both the solenoid and power the motor? Thanks!

A: Follow the diagram at the right. Use the 'SC' and 'S2' connections on the BattleSwitch and connect the output from the BattleSwitch to a small 'coil' connection on the solenoid. The receiver energizes the BattleSwitch relay, the BattleSwitch sends a small current to the solenoid coil, and the solenoid switches the large current to the weapon motor.

If your battery has sufficient capacity, you can use it for both solenoid and motor. If the solenoid won't stay 'on' you need more battery.

Q: Dear aron I found out a way to possibly get more energy out of an elctric hammer. This idea would consist of a heavy fly wheel powerd by a motor (double supported) which would then turn a crank and a bar like the ones on realy old steam engines.....and this for some reason enables the bar to act like a jack hammer when it roatates with the energy stored in the roatating fly wheel. do you think this idea may work?. p.s this idea is very hard for me visualize verbily on forms please help. from anthony

A: I understand the crank and bar idea, Anthony. There was a version of the German robot 'Ansgar' that tried the design. If you watch the red speed bar in the animation, you can see the problem with the mechanical linkage: the output link reaches maximum speed and energy in the middle of each stroke, then slows and comes to a complete stop (zero energy) before reversing direction. That makes for very poor transmission of the energy from the flywheel to an impact on your opponent. Ansgar's weapon wasn't successful and there is little to be gained from repeating their design error.

Q: Why do so many drum belts snap? Is there a way to counteract this? Does using two belts help at all?

A: Timing belts have limits on the power they can transmit. Some builders either don't know those limits or pretend that the limits don't exist. See the RoyMech timing belt website for guidance.

Two belts may help, but it's difficult to get twin belts to share load equally. Proper design calls for a single belt with enough load capacity to survive the abuse.

Q: Dear aron I have given up on hammer robots and decided to build a beetle weight hybrid bar spinner like snow cone but with a different chasis desighn. would the bane bots 5.1 28mm gear box work with the six mm shaft if alowed to slip on imapact. from anthony

A: The BaneBots 28mm gearboxes aren't well suited to the high lateral loading you get from a weapon impact, and a slip clutch to absorb the radial impact on a spinner is not as easy to implement as it appears - we learned this from the weapon on 'Run Away'. After your experiments with the electric hammer, I'd suggest that you pick a more conventional and proven design for your next 'bot.

Q: I've been thinking about drumbot weapon designs lately. Why aren't there any asymmetrical drums (such as a disk with one tooth but balanced)? If counter weighted correctly, wouldn't it increase the "bite" of the drum / spinner since there are fewer teeth? I understand that there are gyroscopic forces to take into effect, but I'm just a little curious as to what would actually happen. Have you ever seen / tried to implement this kind of weapon?

A: Asymmetrical spinners are difficult to design and construct, but a good number of them do exist. The simplest form is a bar spinner with one end shortened and weighted for balance, but much more complicated forms are possible -- like the spiral rotor on middleweight 'Professor Chaos'.

Reducing the number of teeth does increase 'bite' as the time between passage of the rotor teeth is increased, allowing a bigger chunk of your opponent to enter the weapon's 'impact radius' before it takes a hit. The gyroscopic forces acting on the weapon are no different than those acting on a similar weapon with a symmetrical tooth distribution.

Q: Building on the idea of an asymmetrical drum such as Professor Chaos, would it be possible to make an asymmetrical "lawnmower" type horizontal bar spinner that is also asymmetrical and counterweighted? Is there any advantage to this? Has it been implemented before?

A: Mark J. here: read section 6.3 of the RioBotz Combat Tutorial for an explanation of 'bite' and the advantage of an asymmetric spinner. The tutorial includes an illustration of an asymmetric spinner bar.

I've seen counterweighted bar spinners, but I can't name a specific example.

Q: I'm thinking of making a 15lb drumbot, and i'm trying to figure out what motor would be good. I've been looking at a lot of specs, but it tends to be very hard to discern what I need.
  • Many have told me that high rpm speeds are whats best, but many 15lb robots with high rpms are extremely unstable, as when turning, they cannot keep their wheels on the ground.

  • Torque seems to be a major appeal, as it is the rotational power the robot can give, and with a lower rpm, it can still deliver power, and a large amount of force on each hit.

  • Some have told me rotational inertia / momentum is what makes the robots best, and i'm not entirely sure. This make sense, as it is the amount of force/momentum that is contained in the drum. This (seems to be) directly proportional to torque.

  • Power is a major spec on motors, but I'm not sure what to make of it. It seems to be unrelated to both torque and rpm, but the biggest motors have a ton of power, but lower torque and rpm.
Can you explain to me what all these specs mean?

A: Mark J. here: the answers to your questions might best be provided by a 4 semester-hour class at a good university, but I'll do my best to boil that down to a few paragraphs. It's gonna be a little sketchy.

Let's start by sorting out the relationship between motor torque, RPM, and power. Power is the product of torque and RPM:

Power = Torque * RPM

A high RPM motor can be 'geared down' to provide more torque at lower RPM, but there is no change to the amount of actual power produced. This all means that you want a motor with high power to spin up your weapon drum, as torque and RPM can be interchanged with proper gearing. Motor output power is usually expressed in either watts or horsepower: 1 horsepower = about 746 watts.

The power that the motor produces over some time period will be stored as rotational kinetic energy by your drum. Once the drum is spinning at top speed, the motor is simply maintaining that spin speed and is no longer a factor in weapon performance -- no additional energy is being stored in the drum. The stored energy is measured in joules: one watt of motor output power applied to spinning your drum for one second can create one joule of stored energy in the drum.

The amount of power stored in a rotating drum is a function of its rotational moment of inertia (MOI) and its speed of rotation (RPM). The MOI of the drum is a function of the mass of the drum and how that mass is distributed. Two objects with the same mass can have very different MOI: mass located farther from the axis of rotation contributes more to the MOI than does mass close to the axis. A short, large diameter drum has a larger MOI than a long, small diameter drum of the same mass.

The energy stored in a rotating object increases with the square of the rotational speed: spin it twice as fast and it holds four times as much energy. This is why many builders believe that very high RPM is the ticket to weapon superiority. However, storing energy is only half the requirement for a successful weapon; the weapon must also be able to effectively deliver that energy as impact to your opponent. Very high speed spinning weapons have difficulty getting 'bite' on their opponent and cannot get a reliable hit on a robot that has no sharp exposed edges. There are many discussions about 'bite' in this archive.

  • A motor with greater power can spin a given drum to its target speed more quickly than a motor with less power.

  • Once the weapon is up to speed, motor power plays no further role in the weapon effectiveness: the impact force comes from the energy stored in the drum.

  • A drum with larger MOI can store more energy at a given speed than a drum with smaller MOI.

  • A faster spinning drum stores more energy than a slower spinning drum, but will have more trouble transferring that energy to the opponent as an effective impact.

The approach I recommend is to spend some weight allowance on a large MOI weapon and belt drive it at a reasonable speed. This will give better general performance than a small MOI weapon spinning at very high speed.

There is a special case that does call for really high RPM: when two drum-spinners go 'weapon-to-weapon' the slower drum looses. I've seen drum spinners that cruise at a moderate weapon speed for most attacks, but have spare motor speed that their driver uses against other drums. Something to think about.

Q: You said in an earlier post that electric hammer weapons weren't effective in current competition. Does that mean that robots like frenZy are outdated by now? Because frenZy's hammer still looks like it has some oomph.

A: I really like 'frenZy', but it wasn't a very effective combat robot even back in the BattleBots days. Overall record: 7 wins and 10 losses, with zero wins in the last three events it entered.

FrenZy looked as if it was delivering powerful blows only because its small chassis would react wildly to each impact. It could make itself dance, but not its opponent.

Q: Would it be possible to make a rack and pinion flipper?

A: Mark J. here: sure -- but there isn't any advantage to it. A rack and pinion drive for a hammer weapon is useful because it can translate linear force from a pneumatic cylinder into uniform radial acceleration of a hammer across a large arc. A flipper acts across a much smaller arc and uniform force is generally not an issue. You're better off to avoid the weight, complexity, and power loss from the rack and pinion.

Search for "Jacha Little" in this archive for an animated diagram of a rack and pinion hammer mechanism.

Q: [Chinese Forum] As for electric hammer bots, I remember the very first version of Deadblow was equipped with an electric hammer during LB99 Battlebots event - and it went to the semifinal. Could that be considered as "Successful"?

A: You are mistaken. According to Grant Imahara's Team Deadblow website, Deadblow's weapon system at the 1999 Long Beach event was pneumatic:

"The weapon system for Deadblow 1999 was a 6.5 lb machined steel sledgehammer which swung 180 degrees. It was powered by a Bimba rotary pneumatic (air-powered) actuator with a working pressure of 150 psi."

In 2000 the rotary pneumatic actuator was replaced by a conventional linear pneumatic actuator. The hammer never had electric power.

Q: Do you think DeadBlow would have been just as effective if its hammer was electric?

A: 'Deadblow' did not rely on high weapon power, but on speedy and repetitive attacks. Even so, the '1999' version of the robot did not do well with a relatively slow and weak (but better than electric) pneumatic hammer. Deadblow's record in 1999 was an anemic 1 win, 2 losses, and 1 no-show bye. Success came only after the weapon power and speed were increased with a linear pneumatic actuator. An electric hammer would have continued to perform poorly.

Repeat after me: electric hammer weapons do not have enough power to be effective in current open competition.

Q: You said in earlier posts that you can't find a hammer bot with a winning record. But in the hall of fame The Judge had a great record and Team Hurtz robots have been sucssesful.

A: I think you're refering to a post where I say,

"I can't find an electric hammer with a winning record in any weight class..."

Team Hurtz has certainly had success with their pneumatic axe robots, but their electric hammer robot 'Beta' was not at all successful. 'The Judge' was also pneumatic. You can get an overhead hammer to work, but powering it with an electric motor is generally a poor idea.

Q: hey Aaron, are horizontal spinners or vertical spinners more controllable in the ant weight class? thanks.

A: Both have problems:

  • Vertical spinners in any weight class are difficult to control because their gyroscopic effect resists turning motion. A peizo gyro in the radio system can't compensate for this this problem - you can't cheat physics.

  • The axis of rotation on a horizontal spinner does not resist turning motions, but the counter-reaction to impact can send it careening away at high speed.
So, one doesn't want to turn and the other loses control on impact. Take your pick.

Q: As a follow-up to the (as of writing) most recent question, what about a disc mounted at a 45-degree angle? Would that give the benefits of both a horizontal & vertical spinner, or just the problems?

A: Mark J. here: a quick check of my trigonometry tables [cosine of 45 degrees = 0.7071] says that you'd still have 71% of the turning resistence problem with a 45 degree diagonal disc, and 71% of the lateral force vector on impact. You aren't gaining much on either problem, and you're adding a big dollop of new trouble.

Turning in one direction would be a little easier as the gyroscopic forces lift one wheel and 'flatten' the disc angle a bit, but turning in the other direction would tilt the disc more upright and add resistence.

I think you're better off shooting a big hole in one foot rather than a smaller hole in both feet; stay either flat or upright.

Q: Hi Aaron I am building a antweight robot, and it has a flipper. how to I attach the flipper so it can move? And I am also using pneumatics to power the flipper.

A: Where did you find pneumatic components small enough for an ant?

A simple single-pivot flipper will require a hinge at one end that attaches to the robot chassis, and additional hinges at each end of the pneumatic actuator where it attaches to the chassis and flipper arm. The design of the hinge is less important than making sure it is strong enough to survive the forces that will act upon it.

Q: I have an idea for a type of four bar lifter. It involves a bit of a rack an pinon system like what 'The Judge' uses, but only on the back (or front bar). Would I get any more force out of this, or am I over complicating things?

A: Mark J. here: the advantage of driving a pivoting arm with a rack and pinion is that a source of linear force (pneumatic actuator, linear actuator) can apply a constant rotational torque to the arm over a large range of motion. This is very useful in powering a single-pivot axe or hammer, but less useful in actuating an arm that has limited range - such as the front bar on a 4-bar lifter.

In a 4-bar lifter, the lifting force applied will be non-constant even if the force applied to either the front or back arm is constant. Whether or not you will get more force from a rack and pinion or by direct application of linear force to an arm will depend on the details of your 4-bar design. The T.i. Combat Robotics 4-Bar Simulator will let you experiment with torque requirements and lifting force with different 4-bar layouts. I don't have a tool to provide similar calculations for direct application of linear force.

Q: How do you figure out the best shape for spinner teeth or axe heads?

A: There may not be such a thing as a single 'best'. Different opponents will have different strengths and weaknesses. A sharp 'cutting' tooth that slices thru one opponent may jam and stick in another. Any design will be a compromise. Take a look at what's working against the types of robots you expect to face and be prepared to punt.

Q: Which is better for a hobbyweight, a detachable wedge or a wedge robot?

A: A wedge takes a lot of abuse. If your wedge is detachable you can make easy repairs/replacements/modifications. If your whole robot structure is the wedge it becomes more difficult to maintain. Let your opponent beat up something you can replace quickly in the pits -- not the basic structure of your machine.

Q: I am working on a silly weapon for the Robot Battles 30lb rule set (open platform sumo). How would you calculate the power required from a motor (brushed) to push a single cylinder piston stroke to move 200 in^3 of ambient pressure air into a bladder of equivalent volume when filled? Possibly the bladder would be under a lifting plate. Any commentary is welcome.

A: Mark J. here: lots of unspecified variables here! 'Power' is defined as the amount of work done in a specified time period, so:

  • The first variable is how much work is involved. That depends on how much weight is actually on the lifting plate (the whole 30 pounds, I'm guessing) and how high the plate will lift the weight when the bladder is filled.

  • The second variable is how fast you want the bladder to fill. With proper gearing, a very small motor could fill your bladder and lift the required weight to the required height, but it would happen very slowly.

  • A 'hidden' variable is the power lost to friction in the gear train and to the piston seals.
So, the higher and faster you want to lift the load, the more power you'll need.

If I were going to do this, I think I'd ditch the piston and cylinder design and use a small high-volume electric air inflator like you use to inflate a raft. I've seen versions that plug into car cigarette lighters - more compact and no heavy cylinder and gear train required.

Q: Is it possible to implement a four bar actuator upside down? The idea i have for a robot is so that an arm like biohazards can be used to push away robots with a wide bumper, instead of lifting them up.

A: You can orient a 4-bar mechanism any way you like: up, down, sideways, or someplace in-between. I don't like the idea of trying to 'push away' an opponent - you are just as likely to push yourself away from them as you are to push them away from you.

What will you have gained by increasing the distance between your opponent and the center of your robot? I don't think that counts toward aggression points, it's unlikely to do any damage, and your opponent gets just as many damage points for beating up your 'bumper' at the new location as they would if it had never moved.

The 4-bar lifter is a proven effective design. Lifting your opponent disrupts their attack, keeps their weapon away from you, and controls the match strategy. I think I'd stick with that.

Q: Have there ever been any featherweight 4 bar lifters? I'm currently thinking of making one and want to know if there is a good reason why I haven't seen any before.

A: There are several featherweight 4-bar lifters in the UK, and 'Defiant' won the 1997 US Robot Wars 'lightweight' class at a weight someplace close to 30 pounds. There are also successful examples in both lighter and heavier classes so I can't think of any design reason why it would be a bad idea.

Q: On a pneumatic cylinder, is there any drawback to widening the hole in the front of the cylinder if it is for a single action system?

A: Mark J. here: by 'hole in the front of the cylinder' I'm assuming you're referring to the gas port that would be used to retract the actuator in a double-acting system. I'll also assume you want to enlarge the port to speed up the actuator by allowing the 'exhaust' air to exit more quickly.

You can certainly enlarge the port, or add an additional port if you like. Consider some measures to protect against debris entering the cylinder, and be careful to retain enough strength in the end cap to keep things together. From a practical standpoint, don't expect a big increase in actuation speed. You didn't mention the size of your cylinder or the existing port but your speed gain is apt to be small.

Q: Aaron, I'm trying to build a beetleweight hammer. For the hammer, would it be suitable to use a brushless motor and pulleys or gears to effectively drive the hammer?

A: In spite of builder claims, I can't find an electric hammer with a winning record in any weight class - they just can't generate enough power. Search this archive for 'electric hammer' to find a recent discussion on the topic and a link to our electric hammer calculator spreadsheet.

Q: Mangi has been a dominate force in the 30lbs Sportsman class for the past few years.

A: Fair enough. 'Mangi' has a winning record, but I must point out:

  • The 'sportsman' class isn't the same as open class combat. A lot of things fly there that wouldn't work anywhere else;

  • A record of 11 wins and 9 losses isn't what I'd call 'dominant'; and

  • Mangi's builder has been in combat robotics for a very long time and could win matches with a half-empty tub of margarine.
If you must build an electric hammer, use a brushed motor. Brushless motors really don't like being stalled and a hammer weapon spends a fair amount of time with the motor stalled.
Q: Does Rat Amok's weapon count as an active weapon?

A: In general, any weapon that is controlled by its own radio channel is considered active. 'Rat Amok' has a spring-loaded bar that is held and released by a servo controlled clasp. Since the weapon servo has its own channel control, it is considered to be an 'active' weapon.

Q: What do you think about this idea?For a weapon the robot would have a saw blade rotating one way and another blade rotating the other way.Because they are moving oppisite ways kickback would be canceled.

A: The kickback would be cancelled only if both blades dug into your opponent equally and at the same time. That's a really big 'if'. I'd be willing to bet that almost all the time you'd have only one blade dig in and toss you off to one side or the other. With a single blade you at least know which way you'll be kicked! Having one blade dig in would also be really hard on whatever type of drive you had that spun the blades in opposite directions.

One thing that counter-rotating blades will cancel out is gyroscopic forces. Not a big deal on horizontal weapons, but with vertical blades it would improve mobility a great deal.

Final verdict: thumbs down. We like simple designs at Team Run Amok, and counter-rotating blades add more complexity than they would be worth.

Q: is there a way to calculate egg beaters on your spinning weapon spread sheet? thanks

A: Yes - this has been answered previously. Search this archive for "eggbeater weapon".

Q: hello, what is the equation I can use for finding out how often the tooth on a spinning disk comes by? thanks

Q: hi again i found the answer to my question before but what is a good speed for teeth to get a good bite in a ant weight
would 100 times a second be ok?

A: For the benefit of others, the formula is:

Time Between Weapon Teeth (seconds) = 1 / ((Number of Teeth * Weapon RPM) / 60)

Example: two teeth on opposite sides of a disk spinning at 3000 RPM = 1 / ((2*3000)/60) = 1/100 second between teeth passing.

I can't give you a specific time interval that's 'OK'. More time means better 'bite' and the ability to effectively impact a 'smoother' target, but more time means slower rotational speed and less energy. About 1/100th of a second is a fairly typical antweight tooth timing - you'll have some bite against edges and corners, but very little against smooth surfaces. You can always add a speed controller to your weapon motor and choose your speed to match your opponent.

Q: Hey Aaron

First of all, I would just like to say that I love your spinning weapon calculator. It is making things so much easier for our design phase, especially how it can calculate the spin-up time for a weapon and how much energy it will drain off your battery. I am having SolidWorks compute the moment of inertia for our weapon, and then I plug that value into the calculator. I did the calculations myself for our design binder, and let me say, it saves a load of time.

Anyways, what do you think would be a competitive amount of potential energy for a weapon in a 15 lb weight class? On the first sheet you say to aim for 16 J per lb of your weight class, which would be 16 J x 15 lbs = 240 J. This is WAY [lower] than what I am getting in the calculator, 3,000 J. Is this overkill? If it makes a difference, I am using a steel beater-bar weapon, with a 2.5" max radius, spinning at 15k rpm. The reason I say max radius is because it uses one tooth: the one side of the bar extends farther out than the other, and to keep it balanced the longer side has a larger channel cut out of it (again SolidWorks can compute the COG for me, and it tells me that the MOI is 0.0026 kg x m^2). So. Do you think this is overkill?

Thanks so much

A: Mark J. here: we're always happy to hear from builders making good use of our design aids.

At what point is the amount of kinetic energy overkill? Many current spinners have energy numbers similar to those you calculate for your own design, but those big numbers invariably come from spinning the weapon at very high speed.

Consider: at 15,000 RPM the longer bar on your beater makes one rotation every 0.004 second! You've got to stuff some part of your opponent's 'bot inside the swept radius of the weapon after the tooth goes past but before it comes back around. Unless your opponent has a very sharp protrusion that is well anchored to the chassis, and unless the closing speed of your 'bots is very great, you're going to have a very hard time doing that. A very high speed weapon will just 'skitter' across the opponent without damage, unless the opponent did a very poor job of smoothing exposed edges.

So 3000 joules isn't itself a problem, but the rotational speed at which you are storing that energy renders it ineffective. I'd say your weapon has too little rotational mass and you're spinning it too fast. Double the mass and slow it down to about 4000 RPM to store around 370 joules - you'll have a much more effective weapon.

Note that there is one situation where high weapon speed is needed: going weapon-to-weapon with another vertical spinner. The slower weapon loses in that matchup. Consider a speed controller for your weapon so you can 'crank it up' when there's a need of speed.

Q: What is your opinion on brushless melty's like 'Spinning Tortoise'? Do you see them becoming more popular as brushless motors and esc's have become absurdly cheap?

A: A melty brain spinner is a very complex and expensive design to implement. It requires electronics expertise, careful construction, and a great deal of development and testing time. A drop in price for the drive motors and ESCs is unlikely to greatly increase their popularity since those components are a very small part of the total robot budget.

Melty brain spinners are able to use brushless drive motors because they do no pushing and the drive spends most of its time just maintaining spin speed. I would still recommend use of premium quality motors, as they come with reliable specifications that are critical in obtaining optimum performance from the weapon design.

In spite of their status as a considerable tecnological achievement, I'm not much of a fan of the melty spinners. They aren't much fun to drive, they're very susceptable to 'spinner killer' scoop countermeasures, and they tend to 'hockey puck' themselves around the arena after a good hit. Exciting, but likely to do as much damage to themselves as their opponent.

Q: In my antweight spinner, I plan on using a 6061-T6 aluminum 1/2" tube with 0.083 walls as a dead shaft. I would use 2024 or 7075 aluminum, but the shaft needs to be welded so those are out of the picture. Do you think the shaft will hold up okay even though the aluminum is a little gummy while using sintered bronze bushings or needle roller bearings?

I can't use normal ball bearings with an inner race because there is not enough room for a hole that big. There is a place that sells 1/2" aluminum rod that is hard anodized to Rc 60 - would this have any effect when using the bronze or needle bearings, or will it just scrape off?

Finally, for needle bearings, do you need to use an inner race or could you use one without the inner race?

Thanks so much! You are so helpful.

A: I'm not sure how helpful I can be here. You haven't told me near enough for me to guess at how much load is going to be placed on that shaft. What is the mass of the spinner? How fast will it spin? How many joules of energy will it have? How many bearings will support the spinner on the dead shaft? How far apart will the bearings be? How far apart are the supports for the dead shaft? A competent engineer would require all that information at least, and as we point out in the #17: we aren't an engineering service.

I can say that in general I would not recommend an aluminum tube for a spinner dead shaft. Newton pointed out that for every action there is an equal and opposite reaction, and the point at which that violent reaction is transferred to your robot is the dead shaft. It has to absorb energy equivalent to the energy that will smack your opponent. Is your hollow shaft design up to that punishment? I'd recommend a well secured solid rod, at a minimum.

I also cannot recommend use of needle or ball bearings without an inner race for an aluminum shaft. The hard anodizing would help, but hardened steel rollers on aluminum is not a good match. I'd use bronze bushings for an aluminum shaft -- they stand up well to hard impact.

Q: If I want my [weapon] motor to be above the blade similar to 'Greenwave', what is the most reliable way of attaching the motor? I am thinking of having the motor mount clamping around the 1/2 inch aluminum [dead] shaft. Do you think it would be strong enough to stop the horizontal blade from moving upward from hits from drums and such?

A: Mark J. here: there are good reasons why you don't see many 'Green Wave' style spinners. The design places enormous stress on the dead shaft since only one end of the shaft is supported. Mounting the shaft to the chassis securely enough to withstand the impact force is a real problem. See the previous post on 'Green Wave' in this archive.

The motor mount will pretty much have to clamp onto the dead shaft since nothing else extends above the spinner! The design of the clamping mount will determine its strength but, since the whole shebang is likely to tear itself free from the chassis on the first good hit, you probably don't need to worry about the motor mount strength.

Q: Why are weapons that rely on electrocuting the enemy banned?

A: There are three reasons why specific types of weapons are banned:

  1. Safety: weapons must not be a risk to the drivers, crew, spectators, or the arena (explosives, projectiles...)

  2. Delay: weapons must not cause an unreasonable delay in the tournament to clean up after their use (liquids, adhesives...)

  3. Lack of entertainment: the audience came to see robots fight. Weapons that have no visable impact (radio jamming, electrical discharge...) or that interfere with audience visibility (smoke screen, high-intensity light...) defeat the whole purpose of having a robot fight.
Electric discharge weapons have a lot going against them. They are potentially quite dangerous because of the difficulty in telling if they are 'on' or 'safe'. They are also quite ineffective because the metal armor and chassis of a typical robot is an effective Faraday cage that would prevent electrical penetration. But the biggest problem is that there just wouldn't be much of a 'battle' for the audience to see: two robots touch, there's a spark, and the match is over. Boring!
Q: I just had an idea for a hammer weapon. This may be a bit hard to visualize...

Instead of having a hammerhead fixed onto a single spot on the hammer arm, it would be possible to have the hammer arm be a track that the hammerhead can move up and down on. It would be somewhat like an unpowered monorail. In addition, there is a weak spring that pulls the hammerhead towards the bottom of the hammer arm.

When the hammer is "at rest" the hammerhead is very close to the fulcrum/pivot/bottom of the shaft (Don't know a better word.) thanks to the spring. When the hammer is fired, the centrifugal force should overcome the spring and cause the hammerhead to move up to the top of the hammer arm. However, since the hammer head will still be close to the center, it will be much easier for the motor/piston to swing. By the time the swing is complete, the hammerhead will have moved to the top of the shaft and thus be in damaging position. After the swing is done, the spring draws the hammer back to the bottom of the hammer arm.

Problems could include protecting the delicate shaft. What do you think of the idea?

A: Mark J. here: your explanation is quite clear, but you can't gain energy by moving mass inward or outward within a rotating system. Yes, the hammer will be easier to start spinning with the mass closer to the shaft, but as the head mass moves outward the speed of the hammer as a whole will slow down to conserve the energy in the rotational system -- you don't gain any impact power. See the Hyperphysics notes on Angular Momentum.

However, if you get the point and rate at which the head moves just right you might gain a little efficiency by keeping the motor RPM in the optimum power zone. Search this archive for 'thought of this before' (really) to find a previous post about a sliding-weight spinner. For multiple reasons it would be better to work up a varying gear reduction drive system to acomplish the same effect for your hammer.

Q: Thinking about that virus bot idea [next post down]. There are a few more problems.
  1. It would count as a cluster/multi bot. If it got immobilised, the whole robot would be out. 50% of clusterbots rule and all that.

  2. If the robot was immobile like stated in the plan, then it wouldn't be moving under its own power, either giving points to the opponent due to the opponent pushing it around, or, if it fell off, would be counted out.

  3. To make it mobile would probably mean replacing the suction cups with electromagnets for convenience, and it probably wouldn't be practical getting powerful enough electromagnets in such a small robot.

  4. The only way to make a virusbot effective is to make it produce a lethal electric charge, and if I remember correctly, that's banned in pretty much every robot combat event/organisation.
As you said, however, it is a fun idea and I too would love to see it in action if ever possible.

A: I agree that there are a great many problems with the 'drilling virusbot' idea, but I don't agree with all of your points.

  • I don't think the virus would count as a multibot. A combat robot has to have mobility under the rules; since the virus doesn't I think I can argue that it doesn't qualify as a robot and therefore isn't part of a multibot. Call it a 'detachable weapon pod'.

  • Most current multibot rules require that 50% or more of a multibot by weight must be immobilised for a K.O. If the virus does count as a multibot but weighs less than the primary 'bot, you could lose it and still continue.

  • I can't recommend magnetic attraction for a virusbot. Too many common robot armor materials (aluminum, titanium, polycarbonate, stainless steel, garolite...) are non-magnetic.
You do remember correctly -- electric discharge weapons are banned by absolutely every robot combat organization.
Q: What do you think of this combat robot idea?

The robot has a slow moving electric "Hammer" that has a small claw on the end. Gripped in that claw is a second, immobile robot with suction pads and a drill bit that can be raised and lowered.

In combat, the larger robot would "hit" the enemy's flat top with its "hammer." This would do no damage, but it would be hopefully enough to make the smaller "virus" robot's suction cups stick. The larger robot than uses its claw to let go of the virusbot.

At this point, the Virus bot slowly drills a large hole into the enemy robot. It would be uninhibited, as few robots would have a way of removing it.

Depending on the virusbot's location, the enemy could take severe internal damage. [Oakland, CA]

A: I think it has too many 'dependings' and an unfortunate 'slowly'.

  • Many robots aren't going to provide any good spot for suction cups to stick;

  • If you do stick down you're unlikely to be in position to drill thru anything critical;

  • The drilling mechanism would be fairly heavy and complex (we like simple); and

  • While slowly being drilled, your opponent is rapidly beating the lockwashers out of your main robot.
Worst part: your opponents can defeat your suction virusbot by taping a sheet of paper to their top armor. Fun idea, but impractical.

Q: Are Diesector's jaws modified ''Jaws of Life''?

A: No. Builder Donald Hutson writes on the 'Diesector' webpage:

"At first, I wanted to use a "Phoenix" Jaws of Life but they were very heavy, slow, and require a 5000psi hydraulic pump... I ended up fabricating my own jaws using two heavy-duty MOTION (1000lb) actuators and some 3/8" steel plate... The idea of the jaws wasn't exactly to crush other robots, it was to create some thing that would be fast, reliable and gain control."

Q: Out of curiosity, exactly WHAT rules would ban a robot from using a plasma torch as a weapon? I know there must be at least 3.

Personally, I believe that the reason flamethrowers are ineffective is that they are overregulated. (In addition, the overregulation dashes my hopes of one day seeing a flamethrower bot explode.) My friend's homemade propane flamethrower is more effective than what I have seen in the robots.

A: There are a number of obstacles beyond rule infraction that would effectively prohibit use of a plasma torch in a combat robot. I don't know of any plasma cutter that will run off 48 volt battery power, electrical interference from a portable unit would fry your own electronics, and the need to 'strike an arc' and ground to the opponent would render the weapon useless against any non-conductive surface.

Topping the list for rules infractions would be section 12.4 of the RFL rules: "Heat and fire are forbidden as weapons." Granted, heat weapons may be allowed at the discretion of the event organizer, but no EO is going to allow a plasma torch loose in their arena.

Also likely to put the boot into a plasma torch is section 12.1.2 of the RFL rules, which prohibits "RF jamming equipment". Portable plasma torches typically throw enough RF interference to blot out the whole radio spectrum for a fair distance.

Terribly sorry that you find it difficult to enjoy robot combat without arena-filling explosions and balls of flame. If it were your arena, you'd think differently.

Q: What would be the best way to go about calculating the gearing needed for an electric hammer weapon?

A: Mark J. here: several comments:

  • Electric hammers are not effective. The difference between a correctly and incorrectly geared electric hammer is going to me minimal - neither will be impressive.

  • As pointed out in the [next post down], calculating optimum gearing for an electric flipper or electric hammer weapon is similar to calculating the gearing for a spinner weapon but greatly complicated by the need to include the rotational mass of the armature [and geartrain].

  • Data on the rotational mass of specific motor armatures is not available. A 'best guess' is about all you can reasonably do.

  • I have a modified version of the Run Amok Spinner spreadsheet that examines the early stages of weapon spin-up and approximates the action of an electric hammer. It is difficult to use, time consuming, and provides only approximate results.
My recommendation is to avoid the electric hammer. If you do procede, be prepared to experiment, and don't expect great results.

Addendum: OK, nobody believes me when I tell them that electric hammers don't have enough energy to be effective weapons. Maybe I can help you to prove it to yourselves. As noted above, I've been using a special version of the Run Amok Spinner spreadsheet modified to focus on the first full revolution that a spinner makes when climbing up to speed. The first half-revolution is the realm of the electric hammer weapon. I spent a few hours cleaning up this special version, adding in a few 'real world' performance adjustments, and creating a graphic output.

Like the Run Amok Spinner spreadsheet upon which it is based, the hammer spreadsheet requires Microsoft Excel to run. Run Amok Electric Hammer spreadsheet v1.2.

The spreadsheet is intended to model electric hammers with a 180 degree overhead swing that start and end in a horizontal position. Corrections are included for the effect of gravity on the hammer and for variance in torque with the speed of the electric motor. No correction is attepted for armature and gearbox inertia, so cases with unusually high or low gear reduction may yield faulty energy or actuation time estimates. Output includes guidance on correct gearing.

If you play around with the 'hammer' and 'spinner' spreadsheets, you will rapdily uncover the problem with an electric hammer: the energy output is only about 5% that of a spinner weapon of comparable mass using the same motor. A spinner has many revolutions and typically several seconds to store energy in the spinning mass, while an electric hammer has exactly half a revolution and a few milliseconds to gather energy from the motor. Electric hammers are not effective!

Q: Would an Ampflow A28-150 with a 4:1 ratio make a fast lifter/flipper for a 30lb robot? I'm trying to get a faster lift by using a bigger motor with less gearing.

A: Mark J. here: it's tempting to think that a bigger and more powerful electric motor would result in a big lift speed increase, but that isn't always the case. A lot of the low-range torque in a large motor can be used up accelerating their own substantial rotating armature mass. This is inconsequential in the hundreds of revolutions a spinner weapon makes to get up to speed, but for an axe or lifter it's another matter. Your proposed lifter motor would rotate only half a revolution to raise a simple lifter arm 45 degrees, so a good portion of the force that you would hope to go into lifting your opponent will be absorbed by the rotational mass of the motor itself.

You'd get a good lift with a reasonable length lifter arm, but I think you can forget about the weapon being a 'flipper'.

Q: I am trying to drive a spinning bar for a hobbyweight. The motor I want to use has a 1/8th shaft. The only good pulleys I can find that fit onto 1/8th inch shafts are all timing pulleys. I am looking for something that can slip, so that the motor doesn't stall whenever the spinning bar weapon hits something. I am afraid that if I can't find a belt/pulley setup that doesn't slip, I will face the dual threats of motor stalls and mechanical damage.

A: Mark J here: I'm more than a little concerned about driving a hobbyweight spinner from a 1/8" shaft. The small hub diameter timing pulleys that would fit that shaft won't handle enough torque for a decent hobbyweight weapon. Consider a different motor and/or a machined hub to connect the shaft to a properly sized pulley and belt - see the RoyMech website for guidance.

For general reference, timing belts can slip if set up very loose. I've seen several insect-class spinners running loose timing belt drives. Alternately, you can turn the belt inside-out and run the smooth side against the pulleys. Align the pulleys carefully and use a wide enough belt to handle the torque or you'll have real trouble keeping the belt on the pulleys!

Q: Any suggestions on building a hobbyweight crusher? I.E. What type of hydraulics should I use, how can i design my robot to do maximum damage to the opponent without tearing my own robot apart? Is it really worth all the time and effort building a hobbyweight crusher?

A: See #4. I think the probability of anyone building a successful hobbyweight crusher is very small.

The probability of someone who has to ask me how to do it building a successful hobbyweight crusher is indistinguishable from zero.

Q: Is it possible to use compressed gas as a alternative to Co2 for a flipping weapon?

A: Both possible and fairly common. Section 7.2 of the 2010 Robot Fighting League rules says:

"Pneumatic systems on board the robot must only employ non-flammable, nonreactive gases (CO2, Nitrogen and air are most common)."

CO2 is stored in compressed, liquefied form and gives more 'shots' for the size of the storage tank, but the entire pneumatic system gets very cold from the conversion of the CO2 from liquid to gas. High Pressure Air (HPA) or nitrogen can both be stored at higher pressures than CO2 and they avoid the 'chill' problem.

Q: I want to use a Small Johnson motor for a weapon drive in a 12 pounder. It will use a belt drive, so slippage can occur. How many amps should the controller be able to handle?

A: It depends on how much 'slippage' is in the belt drive. Formula:

Peak Amp Draw with Weapon Stalled = (1 - Percentage of Maximum RPM Allowed by Slippage) * Motor Stall Amps.

Example for a Small Johnson motor and a belt allowing the motor to run at 30% of max RPM with the weapon stalled:

Peak Amp Draw with Weapon Stalled = (1 - .30) * 91.8 = 64.3 amps.

That's peak amps. Hopefully you'll shut down the weapon quickly when stalled, so the 'continuous' amps can be considerably less.

Q: How do I figure out belt slippage, then?

A: Trial and error. Start too loose and tighten 'til you get close.

Q: Sorry to bug you on the belt slippage issue again, but I still don't get how exactly you can tighten the belt once you have mounted the motor and the two pulleys. Drilling mounting holes is hard enough as it is when it comes to motors, and you can only make so many holes in a piece of metal...

A: Off the top of my head: eccentric bearing mounts, an idler pulley, and elongate slots with adjustment shims (don't rely on just the mounting bolts to hold tension) all address your adjustment issue. As the belt wears, further adjustment will be needed.

Q: Does using two motors to drive a lifter arm reduce the amount of torque and gear reduction each motor needs? I saw that 'IO' used 2 motors to drive the lifter mechanism. I thought maybe the lift could be faster with two motors having less gear reduction.

A: Yes - more power (from two motors or a single more powerful motor) will allow for a faster lift, but selecting the optimum gear ratio is critical. A lifter that appears faster when operating under no-load may well bog down to a crawl or even stall completely when loaded down.

The trick in getting the correct gear reduction for best lifting speed is in understanding that a permanent magnet direct current (PMDC) electric motor generates its peak horsepower when loaded to produce half its stall torque:

  • If you load the motor more heavily (too little gear reduction) the motor bogs, pulls excessive amperage, and the lift speed decreases.

  • If you load the motor more lightly (too much gear reduction) the motor spins more freely, pulls fewer amps, but again the lift speed decreases.
You won't know exactly how much weight is going to be on your lifter in every situation: sometimes you'll be lifting just an edge of your opponent, and sometimes you'll have the entire weight of your opponent balanced on the lifter tip. We design for lifting the full weight of our opponent, but many builders design to lift only half that amount.