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Due to size constraints the Ask Aaron Robot Weapons archive is divided into two parts by date of post.
This archive holds posts from 2017 to present. Posts prior to 2017 are found here: Ask Aaron Robot Weapons - Part 2.
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.
You can now take a tour of posts in the Ask Aaron Archives that have been referenced to answer new questions -- let's call them Less Frequently Asked Questions. Click the 'Mystery Post Tour' button above to get started.
A Rubber Baseball Bat
Q: Bristol guy here. I was kinda disappointed after finding out how low the energy density of springs is.
Are there spinners that use tethered weights akin to weedwackers? It should allow you to bypass the dimensional limits, have higher moment of inertia, avoid kickback from impacting another bot and reduce gyro instability. Although, I'd imagine the counter weight would cause instability shortly after impact. [Bristol, England]
A: [Mark J.] There were a lot of tethered 'flail' and hinged 'pivot hammer' weapons in the early days of combat robotics: padlocks on chains, weights on cables, articulated bars, all sorts. The longer the flails, the more grief they caused for the unfortunate robots to which they were attached. Flail designs were quickly replaced by rigid spinners that proved enormously better at transmitting the energy stored in a spinning weapon as a single destructive strike on the opponent.
There are multiple 'flail' posts in this archive -- start with this one. If you're really set on building a flail spinner, perhaps you should talk with our creative support department.
Q: I've looked through the archives and elsewhere but I haven't found any serious designs implementing flails. If they are so bad it would be nice to see how exactly they fail (god knows there are plenty of designs using saws and flamethrowers).
"Flails swing out of the way after striking and transfer only a fraction of the energy stored in the spinning mass of the weapon", well it doesn't really matter what happens after impact as the other robot is hopefully flying far and away, the only difference I see is that only 50% of energy may be transferred as the counter mass doesn't strike the opponent as it does with solid spinners, however it is not clear if this counter mass would cause instability and fling your own robot or if it would stabilize and bring the weapon weight up to speed faster. Surely that 50% is worth the extra bite from much larger diameter and extra MoI?
A: Let me clear up that uncertainty you mention. Your double 'weedwacker' is a special case of a double pendulum system:
Then one of the impactor masses strikes a glancing blow (most are) on a target:
It's Not Gonna Melt
Q: I'm trying to use your Spinner Weapon Spreadsheet, but I think I'm missing an important detail. Based on the numbers, it seems my little motor is going to melt into oblivion as it draws considerably more current than it can handle.
The motor is rated for a maximum of 90 amps, or 2000 watts. My ESC claims it can supply 200 amps. I am using A123 cells which claim 50 amps constant and a 10 second peak of 120 amps. My batteries are shared by my drive, so I am unlikely to use all 120 for the weapon regardless. My question is...how do I get the spreadsheet to limit the current used by the weapon motor? I can play with the numbers and adjust the value for Ri. A value of 150 uses a maximum of 112 amps, but I'm not sure that will give me correct numbers for spin up either. This has a 4.3 second spin up, but it only uses the 112 amps for a brief moment which is likely not correct. Based on this, it seems the correct spin up time is somewhere between 0.4 and 4.3 seconds. Is there a way to narrow it down further? [Kansas/Missouri Border]
A: [Mark J.] The spreadsheet assumes that the internal resistance of the motor is the only resistance in the system, but the motor controller and battery also have internal resistances. These additional resistances become increasingly important as the resistance of the motor itself decreases -- and the 0.014 ohm resistance of your selected motor is pretty damned small. Let's add in an estimate for those other resistances.
Plugging that figure into the spreadsheet for the Ri produces a spinup time of 2.5 seconds with a current peak at 180 amps -- pushing above 120 amps for about 0.25 second. I think your weapon system will be fine.
Nasty guesstimated calculations like this have led to builders turning to alternate methods of selecting brushless weapon motors. The excerpt below from the Ask Aaron Brushless Motor Selection page provides motor selection guidelines based on motor mass and output per pound of weight class:
A: 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: 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.
Comment: Thanks for replying to my question about weapon spin up. That makes sense regarding the total resistance of all components. It sounds like I was at least on the right track for increasing the value of Ri, but I didn't have the rest of the numbers. I think a 2-3 second spin up will work nicely. Appreciate you taking your time to help.
Reply: You're very welcome. Your question gave me impetus to think thru overestimation of brushless motor power output. I've added a shortened version of this Q&A to the Ask Aaron Spinner Weapon FAQ in the 'Motor Selection' section. I should also add a note to the weapon spreadsheet itself...
Significant Digits
Q: In your Spinning Weapon FAQ it says,
"A steel bar 300 mm in length, 75 mm wide, and 23mm thick has a mass of 4 kg.
When I did the calculation for the rotational kinetic energy, I got around 699.08 joules, not 710. I have attached my work.
Could you please explain how you got 710 joules as the rotational kinetic energy? [Woburn Massachussets]
Energy storage at 2000 RPM: 710 joules"
Annoying Newtonian Reaction
![]() The latest roadblock: When I spin up the weapon, the whole bot tips forward and ends up upside down. The centre of mass of the whole bot is back, and it drives fine without the weapon, but if it's spun up quickly it flails! Do you know why, and how I could stop it? [Walthamstow, England] A: [Mark J.] The 'why' is easy.
"For every action there is an equal and opposite reaction."
The action of applying torque to spin-up your weapon in one direction results in an equal torque applied to your chassis that attempts to spin it in the other direction. In your case that opposite torque is strong enough to flip the chassis forward.
The 'how to stop it' is trickier.
![]() UK ants tend to have wood floors so magnets are out, but before messing with power curves I slapped some M4 bolts through the rear support bars which seems to have done the trick: Spinup Test (video). Reply: That's the perfect spot to add a little weight, and it looks like a little weight was all it took. I had imagined that the 'bot was flipping over hard and would need more aggressive correction so I'm glad the solution was that simple! Complex System = No Simple Answer
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A: [Mark J.] The impact of powering the rear bar ("crank") instead of the front bar ("rocker") in a 4-bar lifter sounds like a simple question, but the full answer takes up a couple chapters in a mechanical engineering textbook. I'll try to summarize:
2 Servo speeds are usually given as the time required for the unloaded servo to rotate 60 degrees. As the crank must rotate farther to raise the lifter than does the rocker, the lift speed for an unloaded lifter will be slower for a lifter with a servo powered crank than for a rocker powered by the same servo.
3 Powering the crank may or may not result in a slower actuation for a weight bearing lifter. Servos slow down from their unloaded speed in proportion to the load placed on them:
4 The torque required to power either the crank or rocker varies as the lift progresses. The screen grab below is from my Run Amok 4-Bar Lifter Spreadsheet -- a free Excel spreadsheet design aid. The spreadsheet is modeling a mechanism with a three-pound load on the lifter arm. The blue lines in the torque plots show the torque requirements for a powered crank and a powered rocker over the full lift for this specific design:
If the servo used in this specific design has a stall torque rating of 160 oz-in it will breeze thru the lift if powering the crank, but will strain at very slow speed if powering the rocker. A more powerful servo might result in a faster rocker powered lift than if used on the crank.
5 You may find value in reading thru the Ask Aaron Electric Lifter FAQ.
Gotta Get Down to Go Up
Q: I have been seriously thinking about getting into combat robots for a few months now so obviously I have been reading this website for a few months. I just became a father this year and I am so sorry to hear about Aaron! I wanted my first bot to be a vertical spinner but reading your website (FAQ #8) and some other forums (NO SPINNER) I settled on a 4 bar lifter. If it fails I still essentially have a wedge bot but before that I can try to do stuff actively. Anyway I have it all drawn up in CADD and I ran the 4-bar Calculator but ...
A: [Mark J.] You stopped reading FAQ #8 on 'first bot' a little early. After it says no spinner it continues with no lifter, no flamethrower, no crusher. Keep it simple....
Before I get to your questions, I spotted a couple errors in your spreadsheet inputs:
1) The problem you've encountered is not directly due to setting a negative value for the Rear Bar Base Rise. The geometry of your design causes the rear end of the Top Bar [Connector] to start its rise faster than the front end, which causes the front tip of Extension: R to briefly dip down below the base of the Front Bar [Rocker]. This negative relative height of the Extension tip breaks the calculation engine formulas and crashes the spreadsheet. The giveaway is the #NUM! Error that appears below the 'Front Bar Torque Chart':
3) Four-bar design is as much art as science. Keep playing with values 'til you get a workable design. A couple suggestions:
My Conscience Won't Let Me
![]() Propdrive 50-60 380kv Red brick 200 amps This was our previous weapon config. The redbrick burnt out every battle. How to solve this issue [Karnataka, India]
A: [Mark J.] A poem:
He fights in southern India Effectively Incalculable
Q: Hey, Mark! I know there are a lot of shell spinners that use hollow shafts for their weapons. I was wondering if there was some sort of rule of thumb when it came to determining the thickness of the shaft’s walls? I’m in the process of designing a shaft myself, which is for a 1lb robot and has a 1/8in hole. The shaft itself is currently planned to be machined from 7075 and attached to my robot with bolts and a mount built into one end of the shaft. I know a lot of robots in the 1lb class use 1/4in 18-8 stainless steel shoulder bolts and they hold up well, as such initial plans were to have a 1/4in OD on the shaft. However, I was worried with a wall thickness of only 1/16in if that would cause the shaft to easily bend or snap under load, especially while handling a weapon that’ll be 8oz or more.
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A: [Mark J.] Unfortunately, the number of design variables makes this type of engineering decision a great deal more complex than adopting a "Rule of Thumb". Where 'I' is the moment of inertia of the cross sectional area of the shaft, 'σb' is bending stress, and 'y' is the distance from support to farthest loading -- the bending moment 'M' to be survived by the shaft is:
A Tricky Balance
![]() I bought the FingerTech's "Viper Vertical Spinner Add-On" as a first weapon to use for an ant (rest of bot won't be viper kit though). My first thought is - what if I ordered custom teeth from a site like sendcutsend that were a little longer or thicker to add weight and weapon diameter, and possibly at different sizes (but same weight) that could provide an asymmetrical design? Is there anything I should keep in mind before playing around with this idea, or any way you would approach the exercise? Best, Neil [Tukwila. Washington]
A: [Mark J.] Thicker or (reasonably) longer teeth will not significantly increase the moment of inertia of this weapon system -- but it will significantly increase the stress on the tooth and the tooth mounting site. Increasing weapon diameter only adds energy storage to the extent the you move mass away from the center outward to occupy the increased span. Don't expect to gain much weapon performance from small changes to the teeth.
Going asymmetric has the potential to increase weapon "bite", which is a good thing -- but the process is a bit trickier than you might think...
This is like placing two kids on a teeter-totter but seating one kid 10% farther from the fulcrum point: they don't balance. To get the system back in balance you need the shorter/thicker tooth to weigh 10% more than the longer/thinner tooth to make up for being closer to the center of rotation.
See Also: What Weapons Win.
The Neglected Force
![]() For example, I’m considering upgrading my plastic ant horizontal from 6000rpm to 10000rpm. If the 10k rpm weapon can reliably survive its own power on impacts with a static object, and the 6k rpm weapon could survive impacts from an opponent’s vertical spinner, would getting hit vertically while using the 10k weapon be more likely to cause breakage than the 6k one would? My intuition says no, since spinning the weapon faster horizontally doesn’t significantly affect the load a weapon would take in the vertical axis on an opposing hit. To my understanding, in horizontal vs vertical impacts, it's either one or the other imparting their energy, not both at once. But I’d want a sanity check on that one first before going taking the rpm upgrade to competition. [Philadelphia, Pennsylvania]
A: [Mark J.] You're overlooking something.
A Bottomless Hole With No Rabbit
![]() If flow rate over an orifice is dependent on the pressure on both sides, why is flow rate in all the calculators and spreadsheets a single value and not a function with respect to pressure during piston extension? It seems to me that in a system with a proper buffer tank, as the pressure in the cylinder increases the flow rate should decrease, not remain constant. Can you help me understand this, and, additionally understand how I should go about calculating this single value? My expectation is that I'm misunderstanding how pressure dissipates in such a system, or perhaps misunderstanding how a buffer tank truly works, or trying to calculate something that doesn't actually matter because these cylinders extend so fast when built properly, but I'm interested to hear what you have to say on this. Our team is planning on using a buffer tank so this shouldn't be one of those cases of running a piston directly from the regulator's relatively low flow rate, and we're making sure to consult manufacturer websites to get an adequate flow coefficient (Cv) value on everything before buying parts. Thank you so much for your help and for maintaining so many resources, this has been an incalculable wealth of knowledge for me as I've gotten into this sport. [Corvallis, Oregon]
A: [Mark J.] Thank you for your kind complement on the various Team Run Amok resources.
Combat robotics is chaotic by nature. Broad engineering analysis has some value in a chaotic system, but the deeper you go into detail the farther down the rabbit hole you go.
Push it Through
Q: I'm looking for a way to quantify material strength and I'm hoping for some guidance. I'm playing around with the idea of a crusher robot and I'm trying to figure out how much force I need to pierce through different metals. I can look up a materials tensile strength and it usually gives me a value in PSI, but does that take into account the materials thickness? Obviously a smaller tip on the crusher means a higher force over a smaller area, but how would I calculate it? Sorry if this is a little out of the scope of this site; any help would be appreciated thanks. [Paso Robles, California]
A: [Mark J.] There are a number of different standard measures of the strength of a material exposed to different types of stress and strain. Tensile strength is a measure of resistance to a force attempting to pull the the material apart along its length. That's very different from resisting a piercing force. As you go thru the list of different measures of strength you will notice that none of them directly measure force required to pierce. This is because piercing resistance is dependent on design elements like thickness, support, and the ability of the structure to flex. A simple calculation is not going to give you a useful number.
My best suggestion here is to build a mock-up of a 'typical' robot structure that you might encounter with your crusher and subject it to destructive testing with a model of your piercing tool and a hydraulic press. Sometimes you just gotta go out and break stuff.
Dead or Live
Q: Hi, I was wondering how to make sure an axle on a vertical spinner does not come out the sides but rotates, well, vertically? [Seattle, Washington]
A: [Mark J.] There are two broad types of weapon shaft design -- see this archived post on dead shafts and live shafts for a drawing and explanation.
Insect class robots will typically use a 'dead' shaft, which allows the shaft to serve as a structural element to strengthen and stiffen the chassis. In this layout the spinner weapon and any drive sprocket or rotor are on a hub that rides on shaft bearings. If you want a 'live' shaft that spins with the weapon you may use shaft collars on the outside ends of the shaft to hold it in place, but as a very wise builder once said, "Set screws suck."
A Kinda Lumpy Disk
![]() Q1: How would I calculate the kinetic energy for this design using your calculator?
A: [Mark J.] The kinetic energy storage capacity of a given spinner weapon design depends on the speed of rotation and the Mass Moment of Inertia (MOI) of the rotor. The online Team Run Amok Spinner Weapon Kinetic Energy Calculator can determine the MOI of a rotor made up of simple geometric shapes: disks, bars, and tubes. Calculating the precise MOI of more complex shapes like your weapon is more difficult. Options:
Pumping those numbers into the kinetic energy calculator with an assumed speed of 8,000 RPM gives kinetic energy storage of...
A: An interference press fit requires a high precision drilled hole -- much tighter tolerances than you might get from a waterjet cut hole. If you have a correctly sized hole a it's not all that tough to tap or vise-press a needle roller bearing into place.
I know of builders that fix bearings into slide-fit holes with Loctite 640 retaining compound. This is not the same stuff as the common blue Loctite threadlocker. The Loctite is effective -- but don't get it in the bearing!
You might consider flanged oilite bushings instead of needle roller bearings. The flange keeps the bushing in place and the bronze bushing can absorb enormous impact loading. Correctly oiled, the frictional difference is negligible.
The Guide Says...
Q: Hi, Back again, but this time building a beetle! What brushless motor would you recommend for vertical or drum spinners of the 1.5KG weight class? Thanks! [York, England] A: [Mark J.] The Ask Aaron Combat Robot Brushless Motor Selection Guide will give you general parameters for weapon and drive motors for a given robot weight. For reasons provided in the guide, a brushless motor massing about 90 grams with a power output around 450 watts will do nicely for a typically sized beetle weapon. Examples: Selection of motor diameter/length will depend on available space in your design, and selection of motor Kv rating will depend on the weapon design and drive method. Since all you've told me is "beetle vert or drum" I can't narrow it down more than that for you. Out At The End
Q: Does the shape of the ending of the vertical spinners matter or is it best to keep it at a standard shape? A: [Mark J.] See the previous post in this archive about spinner impactor shape for guidance. Liquid to Gas
![]() My question is: how much extra tank storage should I allow for to compensate for that switch of gasses. If 16g of CO2 worked, how much more nitrogen would I need for the same volume of gas I get from the CO2? I’m out of my element on this and couldn’t find the info I needed to make that simple comparison simply online. [Ontario, Canada]
A: [Mark J.] Because carbon dioxide converts to a liquid at about 850 psi and 'boils off' to release more gas as the pressure drops, a much larger volume of gas can be stored in a given volume compared to nitrogen that stores in the gaseous form. A tank of nitrogen @ 5000 psi holds only 63% as much gas volume as the same tank filled with CO2 @ 850 psi.
See the "What a gas!" section of Team DaVinci's Understanding Pneumatics for details on CO2 expansion, temperature, and pressure.
Up And Over
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A: [Mark J.] What you are proposing for your weapon is not a hubmotor. A hubmotor is embedded inside the device it is powering, like the vertical spinner weapon on 'Algos', or the simple Fingertech clamping drum weapon. Your design hangs a large spinning mass off the far end of the weapon motor, where a weapon impact will act with a large lever advantage on the bearing tube support at the base of the motor. This effectively guarantees structural failure of the weapon motor. Reconsider
I'll also point out that you are relying on compression by the retaining nut to both hold the weapon drumette in place and prevent it from rotating relative to the motor. Better design would have the drumette indexed to the top of the motor with hardened pins, or have the motor shaft and drumette keyed to prevent rotation even if the nut is a bit loose -- but 'iffy' design of this type is common in insect class 'bots. Keep your fingers crossed and check tightness before every match.
Not Current Limiting
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A: [Mark J.] Decreasing the amperage rating of your small hobby-grade ESC will not improve the longevity of your weapon motor, but it may cause early ESC failure.
Unpredictable Impacts
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A: [Mark J.] The force vectors encountered by a combat robot weapon are unpredictable. Maybe you hit your opponent, maybe an immovable arena structure, maybe their weapon strikes your weapon with force outside your control -- all at unpredictable angles.
Learn what you can from the designs of successful robots with comparable weaponry. Build something that makes sense. If it breaks, make it stronger.
An Exponential Increase
Q: I couldn't decide on a weapon blade for my new 4wd vert 1-pound ant, so thanks to the magic of [online metal service] I made five. All of these blades have a 72mm cut diameter, are 0.250" AR500, and press fit into a .750" aluminum hex hub. Arranged from left to right, heaviest to lightest.
![]() ![]() The "S-Hook" is meant for countering horizontals; less material to get caught by their blade. I think the real standout here is the "Reaper" assuming it's strong enough to survive, but the big MoI number for the "Pendulum" is hard to ignore.
Thoughts? Feedback?
A: [Mark J.] I think you're underestimating the impact of spinning the asymmetric blade "twice as fast to make up for it". Rotational energy storage increases with the square of speed so twice as fast equals four times the energy storage -- that's an exponential increase in weapon power. The Run Amok Spinner Weapon Kinetic Energy Calculator gives these kinetic energy storage figures for each of your blades spinning at 8000 RPM:
In this comparison 'Asym' doesn't look like a great option, but taking advantage of the added bite 'Asym' has (What's bite?) and spinning it faster has dramatic results:
Ice Ice Baby
Q: Why are CO2 pneumatic systems banned at Battlebots now when it has been legal in pretty much any other competition including the early Battlebots era? [The Aether]
A: [Mark J.] There is a potential safety issue with carbon dioxide. At room temperature, CO2 gas converts to a compact liquid form at about 850 psi and is stored in the pressure tank in that form. As gas is vented from the tank, liquid CO2 boils off to replenish the gas. That phase change from liquid to gas draws a great deal of heat from the system, creating extreme cold temperatures that can clog vent lines with ice plugs created from atmospheric moisture. This can prevent gas pressure from being fully vented from the system at the end of a match. The unvented gas may cause unexpected motion in the weapon system.
See the What a gas! section of Team DaVinci's Understanding Pneumatics for details on CO2 expansion, temperature, and pressure.
Q: How is that different from a BattleBots-legal nitrogen pneumatic system? A: Nitrogen can be compressed and stored at pressures up to 5000 psi but does not convert to a compact liquid form at room temperature. As a result, nitrogen requires a larger tank to hold the same volume of gas. A tank of nitrogen @ 5000 psi holds only 63% as much gas as the same tank filled with CO2 @ 850 psi.
However, because there no phase conversion from liquid to gas there is much less temperature drop as the gas is drawn off and passed thru the pneumatic system. The problem with ice in the system is avoided.
Somebody Stop Me!
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A: [Mark J.] You may be surprised, Kansas City. I know multiple veteran teams that DO use hard endstops and DO manually cut power at the ends of travel. More parts equals more failure points, and a failed limit switch can disable a weapon just as effectively as having your opponent rip it off. Simple is good.
I asked the teams you mentioned for details on their weapon arms and got these responses:
Zachary Lytle writes: "[Skorpios] is far less complicated than you might expect. We believe the fewer things you have in the robot to break the better. So the arm is just clutched and it's Diana's job to turn the motor off before it fries."
Now that you have two thinner discs, spacing them apart has several advantages:
A: [Mark J.] The simple answer is that the weapon with the greater tip speed has the advantage. See this archived post for an explanation of the principle and the effects of design variables. There is also some guidance in the Spinner Weapon FAQ.
I decided my model would be a vertical spinner, with two bars spinning opposite each other. Since I wanted to keep it under 250lbs, ive been looking up what material to make the weapon bars out of, which brought me here. So I scrolled through looking to see if my question had been answered already (I didn't find an answer), but I did find a different question that asked about counter-rotating weapons. in your answer, you said that it works good on an insect bot, but that you wouldn't want to try it on a bigger bot. so ive got two questions:
2) humor me for this one, saying that all the science worked, what would be the best material to make the weapons out of?
[Kalamazoo, Michigan]
A: [Mark J.] Your overall design has a serious flaw, but since you're not going to pay attention to the science, build it, or care what it might cost, I'm going to limit my answers to only the questions you explicitly asked.
Please also note that 'CounterStryker' has impactor teeth only on one of the two disks -- the disk that will impact on an upward swing. The second disk is there only to reduce undesirable gyroscopic effects in vertical spinners. You really don't want to make a high-energy downward strike on your opponent... but you aren't interested in the science.
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A: [Mark J.] I'm not sure what you're asking. SOMETHING has to sense the right moment to fire your flipper mechanism and that something is by definition a 'sensor'. The Hamburger is Bad.
Typically it's the driver who watches and taps a transmitter button to activate an R/C switch that fires the flipper. If you're trying to fire automatically without electronics, it's possible to use an entirely mechanical sensor trigger like Dale Heatherington's Flip-O-Matic.
Q: I have been considering building a Melty Brain spinner for my next design. Before you freak out, I am well capable of managing the electronics, my question is more about the physics. Does a Melty Brain spinner offer a significant increase in kinetic energy out compared to a more traditional full body spinner? My gut says yes however i've seen robots like this hockey puck much more than shell spinners and it looks like more energy ends up wasted on impact.
A: [Mark J.] For the benefit of readers unfamiliar with the concept of the 'Melty Brain': Wikipedia article on Translational Drift.
It's important to draw a distinction between 'kinetic energy stored' and what you're calling 'kinetic energy out':
Q: Adding to the earlier Melty Brain question; do Meltys count as an active weapon? If I'm interpreting the SPARC ruleset correctly, they would classify a Melty Brain as a passive weapon. I'm only asking cause I've been to a couple tournaments before with an active weapon rule and I want to make sure I can still compete.
A: Although there is no functional difference between a Melty Full-Body Spinner and a conventional FBS, the guidance given by the current SPARC Robot Construction Specifications v1.3 is open to interpretation by Event Organizers. The rule often cited in the SPARC ruleset is in the 'Sportsman Class' section 4.2:
Other EOs interpret "movement of the bot" as translational motion across the arena. Because Meltys can translate while spinning but do not require translational motion to function, this interpretation includes Meltys as active weapons.
Bottom Line You'll need to ask the EOs for specific events at which you plan to compete if they consider Meltys to be 'active' weapons.
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A: [Mark J.] I see this overly simplified equation popping up on the 'net more and more often. The short answer is no, because a "400 watt" electric motor does not produce full output power over the entire speed range from zero RPM to maximum RPM as it spins up a rotary weapon.
The full answer is too long for an Ask Aaron post, so I've published a new page that provides a detailed explanation of brushless motor power output and the demands of spinning up a rotary weapon: Estimating Weapon Spin-up Time.
![]() I've been watching combat robotics for about 5 years now and I've finally decided to take the plunge and make my first combat robot. I was thinking of making a featherweight bar spinner, very cliche but in the same sort of vein as 'Suitcase Nuke' but instead of having a solid bar it would have a dense moveable mass probably using a spring to keep it in at low speed and then the centrifugal force would force it further out at higher speeds, theoretically meaning it can spin up faster but still hit hard at full speed. Has this sort of thing been done before or do you think that it would be too complicated for what it's worth? Matt (Different from the other Aussie Matt I've seen on here) [Melbourne, Australia]
A: [Mark J.] Hi Different Aussie Matt. Glad to have a new builder joining the ranks. Ask Aaron was started to answer questions from new builders and help them avoid common mistakes that discourage first-time builders. If your first build does not go well it may be your last build. Three comments:
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: Would a bar of 1/2" 6061 work for a Hobbyweight spinner? I would want to use steel however all I have to machine with are power hand tools. [Roseville, California]
A: [Mark J.] That's like asking if a spruce 2X4 to would work to build a house -- the answer depends on how you plan to use it. You're not telling me enough about how you plan to use your bar of 1/2" 6061 aluminum for me to comment on its suitability for your unstated design.
Typically I would direct a question like yours to two resources at Ask Aaron:
Q: Hi Mark, it seems like hydraulics are only used in heavyweights. I am wondering if you know of any lower weight class robots that make use of hydraulics? Maybe something that used parts from LESU/RC construction equipment? [Arlington, Virginia]
A: [Mark J.] Hydraulic robots are uncommon in any weight class because of weight, complexity, and fragility. Hobby grade hydraulic systems of suitable size for sub-light combat robots are designed to make scale model construction equipment operate in convincing and realistic slow motion (LESU Video). A LESU hydraulic actuator provides a maximum 110 pounds of force: control valves are too small, the pumps and actuators have very limited pressure capacity, and the system is both heavy and unsuited to combat shock loads.
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We have a 0.5 litre compressed air cylinder with 4000 psi the cylinder is fitted with a paintball regulator set for 160 psi, but the circuit has only 9 bar of pressure and for only 4 shots (despite this the cylinder remains at 3500 psi) are there more suitable regulators? (air flow and outlet pressure)
A: [Mark J.] Hello, Jean Pierre.
I went into the archives and found your prior questions to refresh my memory on your design. You're building a 20 KG pneumatic flipper, but I know little more about the pneumatic design you've settled on: ram size, valving, buffer tank, etc. You also did not give any information about the paintball regulator you have found to be unsuitable.
The most popular small pneumatic regulators used by US builders are paintball units from Palmer Pursuit. Palmer makes a wide range of high performance regulators adaptable to a variety of fittings and layouts. Leave them a note with your requirements and they will help you select a suitable model.
If low flow rate thru the regulator continues to be a problem, you may want to include a small 'buffer tank' as described on the Team DaVinci Understanding Pneumatics page to accumulate a reserve of low-pressure air.
Q: I had a question regarding belts drives for weapons: does belt width matter? For context, I have a 30lb weapon system consisting of 2 15-lb spinners for a 250lb robot and I'm debating (for each spinner) a single belt of either 1/2" or 3/4" in width. Although it seems reasonable to go "the bigger the better", would you find there to be any problems with the 1/2"? Let's just say the smaller size may allows for more compact spacing and such. [U.C. Irvine]
A: [Mark J.] You're not giving me much to work with, Irvine. Before I make a belt recommendation I consider four primary factors:
I'm also puzzled by the information you did include about your weapon system. Twin 15-pound spinners are very small for a 250-pound combat robot. A typical combat spinner robot might devote 30% of total mass to the weapon system, including the motor. I'm not sure what weapon design you might have in mind that would compensate for that light weight, but an unusual design could alter my belt recommendation.
Best advice: Find a successful robot with a similar weapon design and emulate their belt system. Weapon belt drives are an all-too-common failure point, so base your design on a proven concept.
![]() A: [Mark J.] I'm always happy to receive and post comments from builders, David. I'll add your comments to the article, but I do have a question for you: how do you hydrotest a leaky ram? A loose clearance ram with no seals and a vent doesn't seem to be a candidate for pressure testing. Q: Kelpie's ram actually has no holes and only leaks because I don't wrap the pressure fitting with Teflon tape. There's also a groove in the piston for an O-ring. During the hydrotest I add the tape and install the O-ring, but in its normal operation in the robot the O-ring and tape are removed. I also considered drilling and taping a hole in the bottom that could be plugged to make a seal, but just not using tape was easier. A: I don't consider your system to be a true 'leaky ram'. It could operate safely with a 3-way solenoid valve, but you've chosen to run with a poppet and an intentional fitting leak. It's the type of system described and classed 'safe and legal' in the last paragraph of the article:
Why even mention this system here? There is a safe and legal use for a 'leaky' system with a pneumatic ram and a poppet valve. If your flipper design allows for a gravity-powered retraction, you can replace the solenoid valve in a conventional system with a high-flow poppet valve to get an improved flipper action. You shut the poppet valve off as soon as the flip is complete, and a small 'leak' anywhere downstream from the valve will allow the ram to 'leak down' for a slow retract.
My negative comments about true leaky rams were in response to some sketchy UK ants I'd seen that had VERY loose ram pistons and were in no way pressure-testable. A valve malfunction in that type of system would be horribly dangerous.
I've added your comments to the article and re-written the description to better describe the type of system I find unacceptable. Thank you for pointing out the need to improve the article. ![]()
A: [Mark J.] No, your weapon does not spin up to 1000 joules of kinetic energy in two seconds. It would have helped if you had included the numbers you entered into the Run Amok Spinner Weapon Kinetic Energy Calculator so I could see where you went wrong. Here are my numbers...
According to the EndBots product website your weapon bar has the following specs:
Q: Seth Schaffer gave you a shout out on [social media site] for solving the weapon instability problem he had with his antweight "Mini Mulcher". What did you tell him? [Tacoma, Washington]
A: [Mark J.] Seth is just showing his appreciation for a very small pointer. Both Justin Marple and I remarked that the long, narrow, and heavy weapon blade on antweight 'Mini Mulcher' was a prime candidate for a bad case of 'polhode instability' that would explain Mini Mulcher's wild gyrations.
Q: I don't understand what caused Mini Mulcher's instability. The weapon was spinning on it's "maximal inertial axis" which should be stable. What went wrong?
A:
Yes, according to the intermediate axis theorem it was spinning on a stable axis and everything was fine... until there was a change in the stored energy level. A change in stored energy will induce a small polhode motion wobble.
Flashback Post from 2018:
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 RunAmok.tech.
Follow the link above to our design tools page and scroll most of the way down the page to find the link to the 'Total Insanity Gyroscopic Effect Calculator'. I'd give you the direct link here, but you'll benefit from looking over the other tools on that page.
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. ![]() I'm currently designing/building a 30lb ring spinner (not a good idea but i like a challenge) and I'm looking at ways to power the ring. Currently my plan is to use two brushless motors on a 3 to 1 ratio on the inside of the ring via a large HDPE ring gear. My main concern with this is that the impacts would damage the weapon motors or break teeth off the inside of the ring, so my plan is to use a ridged TPU [Thermoplastic Polyurethane]. My thinking is that the TPU teeth would flex and "skip" on a big hit, protecting the motors. Would this work or am I just going to rip apart the TPU? If this wouldn't work what would you suggest to protect the motors. I cannot use belts due to a lack of internal space (mostly rubbish design on my part). I could run the motors with wheels to drive the ring but my fear was that i wouldn't be able to get the torque required to spin the weapon. Here's some info that might be useful
A: [Mark J.] Well, you already know that a ring spinner is a bad idea -- that saves me some time.
The combination of "flex" and "gears" is a bad one. The teeth are large, but I doubt you'd have enough clearance for them to flex enough to 'skip'. I worry that the flexi TPU teeth would distort under drive torque loading and jam at the gear interface, causing huge drag. That would bog down the motors and create enough heat to soften the plastic, creating more trouble.
I'm not a fan of friction drive for just the reason you mention. Stick with HDPE and make the ring and pinions as tall as possible for strength. I assume the pinion gears are mounted on the motor cans? Support both ends of the weapon motors: extend or reverse the motor shafts and add bearing blocks to the far end. If the ring support is strong enough you should be OK.
Q: Hello Mark. First of all congratulations for your website which is very enriching for novice builders like me (we are French). My question: We are working on the design of a pneumatic combat robot (20 kgs), we have tested the chassis part and made the choice of HPA to actuate the jack. The best to control the valve is an ESC or a relay?
Thank you for your reply.
Q: Hello Mark,
For our flipper type robot we are considering a Burkett 5404 2 port 24v type valve associated with a Wasp motor controller. What do you think of this choice? And what connection for a double acting cylinder?
Thank you for your reply. Best regards, Jean Pierre
The 5404 is a simple 2-port open/close valve. It can open to allow gas to flow into your actuator but when it closes there is no place for the gas in the actuator to go -- you need a second 2-port valve to open and exhaust the gas from the actuator to allow it to retract -- see the diagram. If controlling both valves from a single controller you will want the pressure valve to be the 'normally closed' type and the exhaust valve to be 'normally open' so that an actuation signal will open the pressure valve and close the exhaust.
Two 2-port valves will suffice for a flipper if you can live with a gravity or spring-powered retract. If you want a powered retract you'd need more valves to operate the retract side of the actuator. By this point a quarter of the weight of your 'bot could be valves and hoses!
Take a look at our Tips and Tricks for Pneumatic Weapon Systems page for some alternative designs.
A: [Mark J.] Hello, Jean Pierre...
For your purposes you'll want a solenoid operated valve with an internal pilot. Valves with an external pilot are for systems that must be able to actuate even when there is no pressure for them to control -- never the case in a robot flipper.
All 5/2 (five port, two position) pneumatic valves have two output ports. In the non-energized state:
Q: How exactly does the 'Greedy Snake' lifter mechanism work? I’ve seen it used on bots like 'SlamMow', 'Claw Viper', and 'Barróg Doom' and kinda have an idea as to how the system works, but I feel like I don’t get it entirely despite being a seemingly simple setup. Are there any diagrams, pictures, or videos that best describe how it works? As always, thanks! [Social Media]
A: [Mark J.] The mechanism in question is often referred to as a single motor clamp lifter. Powered rotation of a spur gear (shown with a red dot) drives a clamping arm attached to a second spur gear downward until on object stops it's motion and prevents further rotation of its gear. Continued rotation of the powered spur gear will then rotate the entire gearbox and attached lifting platform around the axis of the powered gear. Reversing the rotation of the powered gear will lower the platform and release the clamp. In practice, there may be additional gears involved to provide additional gear reduction in order to gain the substantial torque needed for the lift. The animation has been reduced to only the critical components.
Note The clamping force is created by weight on the lifting platform. As the lift angle increases there is less weight pressing down on the platform: clamping force becomes essentially zero when the lifter is vertical.
Q: I know that there's no way a bot with the kind of weapon I'm asking can ever be actually effective, but just for curiosity sake, has there ever been a combat robot that you know of with a jackhammer-esque weapon? As in like a pneumatic/electric powered sharp tip that fires extremely fast. [Tangerang, Indonesia]
Q: I've been trying to figure out the cross-section drawing of the spinner weapon assembly on 'Algos' you linked in your introduction to Mike Jeffries' guest post in this archive. Can you walk me thru that drawing?
A: [Mark J.]
Yes, that's a very dense and 'busy' illustration that was pulled straight from a CAD cross-section view. I've taken a shot at stripping it down to the key elements:
Q: I'm having trouble relating the diagram of the 'Algos' drum to the outrunner motor before it was modified. Can you give me a sketch of the outrunner in the same format?
A: Sure. The only parts of the original outrunner that are used in the finished weapon are the motor stator and the rotor magnet ring.
The small ball bearings are removed from the stator, and the tube that held them is drilled out to accept the larger shoulder bolt dead shaft that will support the larger ball bearings for the drum weapon.
Q: Hey Guys: With tip speed limits being imposed on the higher classes and at times the lower ones I feel like super high speed robots that go fast for the purpose of going fast are very rare. What is the fastest weapon tip speed that has been hit in combat robotics, I assume that it would be an insect weight robot. Has anyone ever hit Mach 1? There would be no point in going this fast other than for the show and an impact with anything at this speed would certainly be robot suicide but damned if I dont want to hit a sonic boom.
Interested to hear your take.
About Sonic Booms - A 'sonic boom' is not a noise made by an object when it 'breaks' the sound barrier. Any object traveling thru air faster than the speed of sound creates a pressure wave in front of itself that travels along with that object. As the object passes your location that pressure wave and the low pressure zone immediately behind it strike you, creating the 'boom'. If you could achieve a weapon tip rotating at the speed of sound it would strike you with a small pressure wave on every rotation, resulting in a loud 'sonic siren' -- not a boom.
Q: I audibly went "ooooo" when I read the words "sonic siren", its too bad I can't seem to find a good video of a similar stationary sonic boom effect. I feel that losses at supersonic speed would be massive. I'm certain there is very little information on the kinda effects a 3ibs supersonic spinner would face however in your professional guess is there a ball park efficiency loss I should aim for? Does the AskAaron spinner calculator factor in for any efficiency losses already?
I've calculated that with a 60% efficiency loss I should be able to hit 767mph if I run a fat brush less motor at a 0.5:1 overdrive. The idea would be to use a disengagable brushed motor to get the spinner up to a speed before the brush less motor can take over.
The robot would be startlingly noncompetitive I'm sure but I gotta see what this sonic siren sounds like now! Hopefully it won't kill itself until after it hits the magic speed.
You provide a ridiculously large service to people in the combat community and I really do appreciate that. Cheers
A: I do admire enthusiasm, but as part of my 'service to the combat community' I am required to throw a few roadblocks in your path.
200 MPH = 800 watts
400 MPH = 6400 watts
800 MPH = 51,200 watts
Response: See if you can talk an event organizer into some special arena conditions. If you can pump the air pressure down to the equivalent of 60,000 feet and reduce the temperature to 216 Kelvin, air resistance is greatly reduced and Mach 1 drops to 573 knots -- but I suspect this only works with spherical chickens. Q: So, I'm a bit befuddled... what exactly is Torque Overage Factor? I've read the Lifter FAQ and the archives, but there's not really a description as to what it is or what exactly it represents, unless I'm missing something. [Mattoon, Illinois]
A: [Mark J.]
Take another look at the first two questions in the Electric Lifter FAQ:
and...
Q: Is the 1.67 multiplication to provide safety or is the number produced by the equation the actual minimum oz-in?
New! Electric Lifter FAQ
There's been increasing interest in electric lifter weapons in recent months. I think the interest may stem from designs that have appeared at BattleBots and the various Chinese events. I had previously edited together posts from 'Ask Aaron' into topic area FAQs for LiPoly Batteries and Spinner Weapons, so I decided to try that same approach with electric lifters. Let me know what you think...
Q: Hi Mark, I've had a robot weapon idea bouncing around in my head for a little while and I wanted to bounce it off someone else to figure out if it was conceptually sound or not.
The idea is to build an overhead bar spinner, but make the bar out of a material like styrene butadiene rubber, then make teeth out of steel that would go on the ends. The theory being that when the bar contacts another robot the teeth would hit like normal but the contact time is then increased due to the deforming of the rubber.
This would lead to a greater transfer of momentum so that opposing robot would get thrown harder and the shock load on the “floppy bar” robot would be reduced, right? Is that a valid strategy, or is there something I'm missing?
Thank you for your insight, Joe
In addition to being ineffective, flails require that they be positioned such that the extreme 'wobbly' deformations they experience on even a glancing impact don't send the impactors into contact with the robot itself. A robot with a flail in an overhead bar position is going to be at least as dangerous to itself as to its opponent. I'm gonna have to give this concept a strong 'nope'.
Q: I know this is an odd question, but do blades with curved edges tend to get better bite or any other benefits, or does it just sacrifice a lot? For context, I'm referring to a blade with a slight curved indent into one side, somewhat like a scimitar or other related weapon.
A: [Mark J.] 'Bite' is a very specific technical term when referring to spinner weapons. As defined in the Ask Aaron Spinner Weapon FAQ:
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The purpose of the weapon would be to create a nice barrier between their weaponry and our shell, while still either bludgeoning them with a weighted end on each chain, or potentially cutting into them with the chains. Do you think this would be a valid primary weapon?
A: [Mark J.] Weighted chain weapons are a type of 'flail'. The BattleBots Wiki has a category page devoted to robots armed with spinning flails with photos and links. Chain flails were fairly common weapons in the early days of combat robotics, but builders quickly learned that rigid weapons were much better than floppy weapons.
There are several 'flail' posts in the Ask Aaron Robot Weapons archive. In the box below I've re-printed an archived post that sums up their performance issues.
A: [Mark J.] If you perform a numeric analysis on flail weapon performance it's quite clear that the ideal chain length is 'zero'. An effective spinner weapon stores kinetic energy and delivers that energy in a single massive impact on your opponent. Flails swing out of the way after striking and transfer only a fraction of the energy stored in the spinning mass of the weapon -- it's like trying to hit a home run with a rubber baseball bat. Solidly mounted hard 'impactors' have proven to be much more effective weaponry.
There has never been a successful flail robot -- either vertical like 'Morrigan' from Near Chaos Robotics, or horizontal like 'Chains Addiction' -- and I don't expect to ever see one. The only thing they do well is make noise. Avoid.
Q: I believe you just saved my team a LOT of time, I appreciate it! -Flail idiot A: You're welcome. Don't be hard on yourselves for finding this design appealing; many teams have gone down this same blind alley. ![]()
'Flaming Wheel' had two interchangeable spinning shells:
The unusual design of the tall impactors does provide strong support that also keeps impact forces away from the cylindrical shell itself, but the impactors are placed in a position that is shrouded by the gradual taper of the top and bottom mounting plates. The impactor itself needs a clear shot at the opponent without any material in the rotation path that might tap the target out of the way. I don't know what the "bite" calculations for 'Flaming Wheel' look like, but there is nothing to gain by adding material that can get in the way of your impactor path.
Energy storage advantage? Minimal.
Q: This is something I’d like to tack onto the previous question, but I’ve heard people online saying “shell spinners are obsolete”. What do you think? Do you think that these designs have no place in the future? What other designs might one consider obsolete (besides hammers, but don’t let Al Kindle know I said that).
A: [Mark J.] I'm gonna pass on this discussion. I already get enough hate mail for discouraging builders with new ideas. If I start talking builders out of old ideas there won't be anything left.
"Prediction is very difficult, especially if it's about the future."
- Nils Bohr 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
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.
Maximum Lifting Weight: 2 lbs
Maximum Torque at Gearbox (ft-lb):
Max Load At Gearbox (in-oz): (.66 lb-ft x 192) 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 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:
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:
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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.
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.
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.
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.
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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.
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.
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.
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.
A: The conversation has wandered around a bit -- lets recap:
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?
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]
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.
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 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.
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.
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:
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:
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:
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?:
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:
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.
so...am 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.
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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. 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.
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.
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 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:
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!
A: [Mark J.] I've seen this done, Raleigh. It's not a horrible design, but you lose more than you gain:
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]
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.
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:
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 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.
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:
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...
OK, I've got it. The general equation for a parabolic sprial cam is... wait... wait... %#!@*&%$!!!
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:
0 degrees rotation
90 degrees rotation
180 degrees rotation
270 degrees rotation
360 degrees rotation
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:
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).
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 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: 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:
√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 :/
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:
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:
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.
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.
![]() ![]() ![]() 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,
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...
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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]
![]() 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.
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.
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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:
![]() ![]() 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. 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.
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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:
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:
![]() 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?
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?
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]
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:
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:
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.
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.
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:
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
![]() 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:
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]
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 and motor combination that will exceed 35 ft-lb should include a method of limiting torque to prevent damage to the gearbox.
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.
![]() ![]() 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:
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]
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.
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.
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?
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.
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.
Switching to the larger actuator should yield ![]()
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.
![]() 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:
![]()
A: [Mark J.] The Autodesk Inventor CAD program can display an object's center of gravity. Hover over the CG to display its coordinates.
![]() ![]()
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.
![]() ![]()
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.
![]() 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.
Advantage
![]() 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?
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...
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...
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? Info:
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:
Some other design considerations:
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.
![]() 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?
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.
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:
A: [Mark J.] It will make more sense if I answer your questions in a different order than you asked them:
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!
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]
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.
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.
There is no magic equation to balance weapon speed, bite, and MoI because the optimal balance of RPM and MoI is entirely situational.
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.
A: [Mark J.] We answer no flamethrower questions here at Ask Aaron. See Frequently Asked Questions #28.
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
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.
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]
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.
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.
Disadvantage Going weapon-to-weapon against a smaller diameter drum with a faster tip speed is pretty much instant death.
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.
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.
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:
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:
A: Several comments:
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:
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:
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.
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.
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.
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:
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.
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.
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'.
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.
A combat robot is a tool for defeating other robots. The best tools are simple, reliable, and easy to use.
Aaron's Wisdom I've said this often but builders don't want to believe me:
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:
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: 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.
Voltage Supplied: 7.4 volts
The bar specs are as follows:
Speed: 16280 rpm Kv: 2200 rpm/volt Torque: 0.103 N·m Ri: 250 milliohm
101.6mm long x 20mm wide x 1mm thick Steel
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]
Steel bar weighs 15.8 grams Steel bar reaches 12 joules at 16,270 RPM in one second
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:
A: [Mark J.] Many terms and measurements are applied to spinner weapons, but two measures are particularly important:
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'.
![]() 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]
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.
Q: I've read thru the Team DaVinci pneumatics guide, but I've seen pneumatic flipper 'bots that have much different layouts.
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. ![]()
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:
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
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.
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.
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.
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;
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.
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!
![]() 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.
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.
![]() 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:
Q: What are the physics of average wedges beating most average spinners? [Fairfax, Virginia]
A: [Mark J.] Physics... Do you mean 'probability'?
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:
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:
Hi, it's Vancouver.
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.
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:
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.
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]
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:
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.
In general form: 20,000,000 ÷ (commutation in µ-sec × magnetic poles) = RPM.
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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...
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]
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:
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:
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:
Q: Hello Mark it's Nashville again.
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:
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...in conjunction with:
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:
![]() 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 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'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
A: [Mark J.] You aren't catching on, Champaign.
I'm not going to run the numbers for you because:
Now, go build a contemporary design!
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:
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:
![]() 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...
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.
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?
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:
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]
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.
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.
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.
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.
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]
![]() 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:
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.
Examples:
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:
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A: [Mark J.] Having conquered the lower weight classes you have decided to move up to the heavyweight class?
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.
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. ![]()
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 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:
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:
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:
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?
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:
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.
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'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:
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.
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:
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 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. ![]() 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:
Run a few huge drum designs thru the calculator to see if you can find something stable and effective. ![]() 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. 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? ![]()
A: [Mark J.] Notes:
Then this missive from ChaosCorps arrived in my mailbox:
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?
Thanks again! Buzzards Bay: you can watch the videos and see the effectiveness of this weapon at 3000 RPM. ![]() 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:
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:
Note: According to BotRank, there has never been a combat robot named 'Anchovy Ice Cream'. Help yourself.
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.
A: [Mark J.] Multiple suggestions:
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]
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.
Due to size constraints the Ask Aaron Robot Weapons archive is divided into two parts by date of post.
This archive holds posts from 2017 and later. Posts prior to 2017 are found here: Ask Aaron Robot Weapons - Part 2.
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Copyright 2006, 2021 by Mark Joerger -- all rights reserved.
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