Questions and Answers about Combat Robotics
from Team Run Amok

This page is one of several archives of 'Ask Aaron' questions and answers categorized by topic. To see the most recent questions or to ask a new question, go to the

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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 prior to 2014. Posts from 2014 to present are found here: Ask Aaron Robot Weapons - Part 1.

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.

Date marker: December 31, 2013
Q: If you are driving a lifting arm in the shape of an 'L' do you measure the torque necessary to lift your opponent on the diagonal from the hinge to the tip of your arm or what? By measure torque necessary I mean arm length inches multiplied by weight you're lifting. [Seattle]

A: For calculation purposes on a single-pivot lifting arm, measure the straight-line from the pivot point to the tip of the arm. That will give the torque needed to hold the arm against gravity when that imaginary line is horizontal. Add some additional torque to prevent the lifter motor from stalling.

Robot haiku:

Greater torque will let
The lifting arm maintain speed
Under heavy load.

Q: Hi Aaron, i have read in the archives that by using the timing belts instead of chain for a drum will reduce impact to the motor because the belt will slip during impact. Between a toothed belts & v-belt, which one is the better option? [Pulau Pinang, Malaysia]

A: A toothed timing belt is favored by builders for small combat robots -- insects mostly. They are well suited to the high RPM of small weapon motors, and the small timing belts and hubs are easier to find than small v-belts. The greater rotating mass and amperage consumption of weapon motors for larger robots favors the more predictable and adjustable slip threshold of v-belts.

Q: I already have the toothed belt components for my drum weapon. It is a 3" drum, 10mm thickness, with length of 20cm, and weight around 3 to 4 kg. This drum will be driven by the Amplfow A28-150 motor. Do you think i should just stick with the toothed belt or i should seriously consider the v-belt? Can you clarify what do you mean by "more predictable and adjustable slip threshold of v-belts"?

A: A toothed timing belt is designed to not slip. The belt drive can be made to slip if set up very loosely, but adjusting the point at which load causes slipping can be tricky and imprecise. Slipping can also be very hard on the belt and can place large loads on the weapon mountings.

A v-belt is much easier to adjust for the torque it will transmit before it starts to slip. A little looser, it slips sooner -- a little tighter and it slips later. Slipping is smoother and places no extra load on the bearings and mountings.

I'd go with the v-belt for your purpose.

Robot haiku:

V-belts work well
For the heavier robots.
Make sure they're aligned!

Q: Hi Aaron, its me again with a follow up on drum weapon toothed belt vs v-belt post. I would like to hear your advice on which method should be preferred to control my [AmpFlow A28-150] weapon motor.

I have 2 options, the first one is to control the weapon using a battleswitch and a 24v white rodgers solenoid. This option provide higher amp operation but its one way rotation and no control over the speed. The 2nd option is using an ESC, which offer speed control and fwd-reverse rotation...but the best ESC that i have for now is only rated at 40amp constant and 80amps peak (for few seconds). The robot is designed to be compatible with both setup and once the drum weapon is assembled, both setup will be put to the test. Please share your view. Thanks! [Pulau Pinang, Malaysia]

A: I wouldn't be comfortable controlling a weapon motor that can pull 385 amps at stall with an ESC that turns into smoke after a few seconds at 80 amps. You'd have to set the belt drive pretty loose to assure a max current of 80 amps with the weapon stalled.

You haven't mentioned how fast you plan to spin your 4kg drum, but I don't think you'll have enough gyroscopic interference with robot turning motion to wish that you could slow the weapon down. The only real advantage of the ESC would be to reverse drum rotation if the robot is inverted.

I think you'll be better off with the solenoid control.

Robot haiku:

Best 'bot attribute
Is reliability.
Use the solenoid.

Q: Dear Aaron, Anthony D here with a curiosity question relating to a difference in weapon performance involving 'Mangi'. What speed increase (percentage wise @ 180 degree swing) would Mangi receive if Al switched from a EV Warrior weapon motor (1.55 hp @24v - no longer available) to a A28-400 AmpFlow motor (4.5 hp @24v), and how many joules of energy would be availible compared to the current 78 which you calculated and deemed dim against its spinner counterparts?

A: Mark J. here: one might imagine that pumping three times the power into a weapon would result in three times the impact energy, but this is not the case for an electric powered hammer like 'Mangi'. Three times the acceleration can give three times the energy storage in the same time period -- but the faster hammer will traverse the 180 degree swing more quickly and not have the same time to build energy as the slower hammer.

Some quick modeling with the Run Amok Electric Hammer Excel Spreadsheet shows that applying three times the power to Mangi's hammer would increase the speed of the hammer weapon at the end of a 180 degree swing by only about 42%. Since kinetic energy increases with the square of speed (Ek = 1/2 MV2), the stored energy would increase by just about 100%: call it 160 joules. That assumes that the increased hammer acceleration would not flip Mangi over backward!

A featherweight class spinner might easily pack four or five times that much energy in their weapon, so although the additional power helps it still isn't comparable to a good spinner.

Robot haiku:

Quicker hammer gives
Less time to store joules.
Diminished returns.

Q: I'm the antsaw guy, I've decided to abandon the saw blade for a pseudo drum. The drum will made from a four inch length of half inch hexagonal bar with steel screws for weapon heads.

How efficient is a hexagonal bar in terms of energy storage, the only other bot I could find with such a weapon was Rumble Robotics' Quiver. Should I go with the route of the drum or go back to my saw blade design?

A: The problem isn't the hexagonal shape, it's the tiny diameter. In general, the smaller the diameter of the drum, the poorer the energy storage. Read up on Rotational Moment of Inertia.

  • Your mini-drum will weigh about 3 ounces but will store only 1/7th of a joule at 4000 rpm. That totally stinks!

  • A three-inch diameter circular saw blade the same weight will store more than 40 times that much energy at the same speed.
Your mini-drum won't store enough energy to help at all -- the weapon motor would have to do all the work. Make it bigger or go back to the saw.

Robot haiku:

Did you notice that
'Quiver' never won a match?
Don't copy failure.

Q: hey aaron, m working on [a 60 kg copy of] professor chao's middle weight bot. i have alloted 18 kg weight out of 60 to weapon & its transmission. m not geting how to draw geometry of weapon. although i have calculated dimension as follow: base circle radius 10 cm, thickness 5 cm & tooth 5 cm, from riobot tutorial.
please help me how to draw a cad drawin with this dimension.
also i have decided to use ampflow E30-400 motor for weapon & E30-150 for drive .is it ok if not plz guide me....
thank you. waiting 4 ur rply. [Pune, India]

A: Sorry, but Team Run Amok doesn't claim any expertise in CAD. We couldn't assist you in drawing even a simple drum design, let alone a complex design such as used on 'Professor Chaos'.

Although I admire the design and machine work that went into making PC's one-of-a-kind spiral single tooth drum weapon, I can't recommend that anyone attempt to copy it. The drum features inset counter-weights, a hardened impact tooth, and elaborate pocketing. I don't know what software package was used to develop the complex balanced design, but it certainly isn't anything I have access to.

Although the weapon has been successful, its complexity goes against our team design philosophy:

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

There are multiple posts about single-tooth spinner weapon design in this archive that cover simpler designs.

Your choices for weapon and drive motors are both adequate, although they are considerably lower in power than the Ampflow A28-150 motors used for both weapon and drive in Professor Chaos. Don't expect the same level of performance with lesser motors.

Q: hey aaron, thanks for your guidance, m the same person who had ask you about chaoe's weapon few days before. but i got one question, instead of spiral profile if we use smiple disc and attach a square block(red block shown in previous answer) on both sides?
are there any drawbacks with respect to original chaoe's weapon? will it be as successful as original one? please give your opinion. i hope you hv got sufficient info.
waiting for your reply. thank you.

A: A single-tooth weapon has a considerable advantage over multi-tooth designs. Browse thru this archive for several discussions on this topic.

I can't tell you how successful your robot will be based on a broad description of the weapon disk. The success of a combat robot depends on much more than the design of the weapon. All the components and systems of a combat robot must work together. A single weak link will undermine the potential of any robot. 'Professor Chaos' does have a well-designed weapon system - but without excellence in the drive system, chassis, armor, driving and radio setup it would be just another 'bot. Don't emphasize weapon design to the detriment of the other systems.

Robot haiku:

It's our team motto:
'Complex design is easy,
But simple takes work.'

Q: sup Aaron. thinking of making a FBS that has some angled wing flaps on it so that when it spins, downforce is created. Has this been done before, do you have any pics, and did it work well? what kind of downforce can you get without compromising your spin speed too much? is the downforce irrelevant since when you impact someone, you stop spinning and lose the downforce at precisely the instant you need it the most?

THANKS - antweight frying pan fbs guy from about a year ago

A: Mark J. here: just when I think that every possible design and gizmo has been tried in combat robotics, somebody comes up with a new idea. No, I don't think this particular approach to downforce has been tried. I've seen many magnetic downforce designs, and a few vacuum fan designs that attempted to suck the robot down to the arena floor, but never rotating winglets to push an FBS shell downward.

The amount of downforce you might expect from small winglets is not going to be very great, and they will create significant drag. You can model the downforce and drag with the FoilSim III java applet at the NASA site to get some idea of the effectiveness of potential designs and some parameters on wing size and attack angle.

Some really quick results I got from FoilSim indicate about one pound of drag for every 2 pounds of downforce from stubby winglets -- and only about one pound of total downforce for a pair of 2.5" square winglets spinning at 80 MPH. That drag is going to take a big dollop of extra torque to spin, and the downforce is pretty puny. It doesn't look promising.

The question of losing downforce on impact is interesting. When you hit your opponent, your FBS shell will not often come to a complete stop, but it will certainly slow down and downforce will decrease. Given that the downforce numbers aren't looking all that good to start with, my best guess is that the design isn't all that workable.

Play around with FoilSim and see if your results match mine. Maybe a little more experimentation will get some better numbers.

UPDATE: I worked thru some additional designs in FoilSim. It looks like a simple upcurved plate is superior to a true airfoil for winglets this size -- superior stall resistance and a little better lift/drag ratio. Unfortunately, still unimpressive downforce.

Robot haiku:

Spin little winglets!
Stabilize my shell spinner!
Wait -- why the slow spin?

Q: So I got my first robot running about and now I want to take it up to the next step and add a spinner to it, and I wanted to ask some things:

I noticed that older ants and beetles had saw blades for their weapons. How come they fell out of style, and is it still viable to use a saw blade as an ant spinner?

A: You're right -- big spinning disks/saws are 'out' and smaller diameter drums are 'in'. Several reasons:

  • A vertically-mounted saw blade has to be relatively large in diameter to store enough energy to be an effective weapon. Spinning a weapon that large creates significant gyroscopic resistance to turning, which greatly reduces the maneuverability of the robot. A smaller diameter drum weapon can store the same energy with much less gyro turning resistance.

  • The impact angle of a large diameter weapon is small, which creates a 'kickback' effect that than throw the 'bot backward. A small diameter drum has a better chance at a larger 'upward' impact angle that throws the opponent in the air and creates less kickback.

  • The many-toothed saw blade can have serious difficulty getting 'bite' on a smooth surface of an opponent. There is very little time for a decent chunk of your opponent to enter the spin radius of the weapon beween the teeth -- the blade will often just 'skitter' along the surface rather than digging in for a good impact. A spinner with one or two teeth has a much better chance at a big hit.
Going head-to-head, a fast drum weapon has a superior chance of 'launching' the big saw weapon due to a combination of maneuverability, attack angle, and bite. I don't think I can recommend a saw weapon against the current drums.

Q: Also, with proper mounting, can a finger tech pulley double as a spinner hub?

A: The key words here are 'proper mounting'. On the discontinued 'VDD saw blade hubs', the screws that hold the blade to the hub did so entirely by compressing the blade to the hub with the screw heads -- the screw shafts do not pass thru holes in the blade. Standard screws are NOT well suited to withstand shear forces of the type they would be exposed to if used to bolt directly thru the blade into the hub. The hub junction is a VERY high stress location.

Getting the blade perfectly centered and then adequately locked in place on the FingerTech pulley would be a significant challenge. Don't bodge the hub! Several on-line machine shops could make a hub similar to the VDD hubs that would do the job correctly. I'd go for that.

Q: Thank you for your help. I do have another question:

Is it possible to cut off extra teeth from the saw blade with out compromising the strength of the blade?

A: Yes you can, and you don't have to cut off the whole length of the extra teeth. If you do the math on how fast the blade spins and how fast the 'bots move forward, you'll find that the realistic effective 'bite' for a typical antweight is about 3/16ths of an inch. Keep two teeth opposite each other and trim 3/16ths of an inch off the other teeth. You'll gain the extra bite and save most of the prime rotational mass at the outer edge of the blade.

Robot haiku:

Big saw blades are out.
Smaller drum weapons are in.
Do NOT bodge the hub!

Q: Hey Aaron,
I am working with a tutor on Solid Works here at Brown University. I am doing a summer program and I am going to attempt to build a 1lb drum bot in SW. My plan is to build it with a dead shaft and a belt drive. What are all of the components that go into just the spinner apparatus? I basically need to make a parts list. I understand that the shaft does not spin and you need the belt to spin directly to the drum. My plan is to make a drum spinner at Starbot potentially with their equipment. Optimally, I would do it for Thunderdome. Thanks Aaron!!!

A: The components vary a bit with design -- like whether the drum is hollow or solid, and the style of impactor. A general list for a hollow drum:

  • Drum tube
  • Drum impactors (variable number - depending on design)
  • End caps for drum tube (2)
  • Bearings for end caps (2)
  • Weapon shaft [non-rotating 'dead' shaft]
  • Drum belt pulley
  • Motor belt pulley
  • Belt
  • Machine screws to put it all together
Is that what you need?

Robot haiku:

A drum is tricky.
Perfect balance is a must.
Don't spin it TOO fast!

Q: aaron i want to know about son of whyachi robot which won battlebots 3.0 title.. what motors are used for its rotating top and the motor specifications so that i can make a similar bot for my college championship of 50 kg bots . [Andhra Pradesh, India]

A: We have discussed 'Son of Whyachi' several times previously. Search this archive for 'yamaha' to find a listing of the many weapon power options S.O.W. has used. The 'shufflebot' version that won BattleBots 3.0 used two overvolted 'Briggs & Stratton Etek motors' for weapon power. The final version used two 15 HP Yamaha go-cart gas engines to spin the big rotor.

A few things you should think about before copying a Team Whyachi design:

  • The team had a VERY large budget;

  • The team had unlimited access to a large and well equipped commercial machine shop with experienced machinists; and

  • I can't think of a single successful copy of any Team Whyachi design.

Robot haiku:

Please search the archives
Before you ask a question.
It saves us both time.

Q: Hi aaron I'm using a permanent magnet starter motor for rotating a 6kg circular ring and can I able to run this motor with 45amp car esc with burst current of 320amps which is available in hk and one more doubt can I able to run two motors with one esc??? [Maharashtra, India]

A: Mark J. here: in order to match an Electronic Speed Controller to a motor you MUST have accurate specs for the motor and the ESC. You must also have full details of the load to be placed on the motor. If these factors are unknown you're just taking a wild guess.

  • You've given me no specs for the motor. I'm not surprized, because specs for starter motors aren't generally available. It is possible to obtain good estimates of the motor specs, but it requires both equipment and time.

  • Specs for hobby R/C car ESCs are ***Wildly Exaggerated!*** In my experience, real-world ratings for hobby ESCs are maybe 1/8th the stated values -- see: Understanding MOSFET Current Ratings. The performance numbers given for hobby ESCs are typically based in the absolute power ratings of the MOSFET power chips at room temperature. If you actually try to pull that advertised amperage thru the hobby car ESC it will fail quickly and spectacularly. The specifications for industrial and robotics ESCs are based on actual testing and can generally be trusted.

  • The load placed on the motor by spinning a '6 kg ring' to speed depends greatly on the size and shape of that ring. The Team Run Amok Spinner Excel Spreadsheet can calculate the Moment of Inertia (MOI) of a ring of specified material given its diameter, height, and thickness. The spreadsheet can also calculate spin-up time, battery drain, and energy storage of the weapon.

  • It is possible to run two brushed motors from a single brushed ESC -- IF the combined amperage draw of the two motors is within the current capacity of the ESC. In this case, I very seriously doubt that you could run one starter motor on the proposed ESC. If you happen to be talking about this 45 amp car ESC, note that it has a maximum rating of 7.4 volts, which further adds to its problems.
My often-repeated advice is to use components that other builders have already proven to be suitable and effective in robot combat. If you use untested components, prepare for a great deal of experimentation and very frequent failure.

Q: Can I use a fan [to cool] that 45 amp car ESC for my starter motor to run without any problem or any other techniques to control the heat and to control the amps coming from that esc???

A: Let me try this again: you need three pieces of information to match an ESC to your purpose, and you have NONE of them.

  • You have no idea how many amps the motor might use, but it's a lot;

  • The available specs for that '45 amp' car ESC are entirely fictional;

  • You haven't done the math to calculate the load the weapon will place on the motor.
Motor power is a product of amperage and the voltage ('watts'). If you restrict the amperage, you restrict the motor power. To use that very small ESC you'd have to restrict the amperage by a whole lot, which would cripple the powerful starter motor.

A cooling fan might add 20% to the capacity of that ESC -- you need to add about 800%. If there was any workable method to use a $10 ESC to control a multi-horsepower motor, builders wouldn't be paying many times that amount for ESCs that can actually do the job. There isn't a shortcut here -- don't ask again.

Robot haiku:

Some unknown starter
Pulls who-knows-how-many amps?
Give a guy a break!
Hobby ESC.
Advertised ratings? A joke!
Use at your own risk.
Loading on motor
By a six kilogram ring
Depends on ring shape.

Q: How important is it to allow some slip between a blade and the motor, using a V-belt or clutch, in a horizontal bar spinner? Last Rites and Mortician use a chain, Fiasco uses a timing belt, and Keres has the blade directly on the motor shaft, and I cannot see a clutch in any of these. How do they keep the shocks of impact away from the motor? Is this less of a problem in smaller bots? If you turn off the motor when it stalls (opponent pushes you into a wall), wouldn't that prevent it from burning out? Thanks. [Vermont]

A: 'Last Rites' and 'The Mortician' have chain drives because builder Ray Billings is a madman. Either may go thru 2 or 3 weapon motors in a tournament, but Ray is willing to bear that expense in order to eliminate all slippage and get every last drop of power out of the weapons.

Tiny robots -- like fairyweight 'Keres' -- have less of a problem with stalled weapons because small motors naturally have a larger surface area to volume ratio (see: square-cube law), giving them a better chance to dissipate heat before they melt down. We have frequently discussed other problems that come with directly mounting a weapon blade to a motor shaft (too high a spin speed, lengthened spin-up time, high motor bearing load...) -- browse this archive.

Timing belts do provide a bit of impact shock absorbtion and, although less predictable in behavior than V-belts, they can and will jump-slip under heavy loading if set up properly. They are entirely suitable for sub-light robots, like hobbyweight 'Fiasco'.

You DO need to turn off the weapon motor when the weapon stalls, but you may have VERY little time to do this before the motor or ESC melts. A slip belt will buy you another heartbeat or two to save the weapon.

Is it possible to design a weapon that will survive full stall without worry? Yes, but some Ray Billings 'bot will kick its ass.

Robot haiku:

Slippage costs power,
But it can save your weapon.
Power has a price.

Q: I'm designing a full body spinner bot like zigo and diameter of the spinning weapon is 500mm and height is 200mm .The spinning material is stainless steel of semispherical shape and I'm going to attach a chain to that bot with one end connecting to the spinning weapon and other end is to the spherical bob.So that the bob rotates along with the spinner.The weight of the spinner is 10kg and weight of the chain is 2 kg and bob is 8kg. Total weight of the bot is 45kgs ,and i need to rotate the bob for around 3000-4000 RPM.But i dont know how to calculate MOI and energy that the spinning weapon contains ,and also what kind of motors I need to use whether ampflow or any other type motors. [India]

A: Mark J. here: full body spinners with impactors attached by chains were fairly common in the early history of combat robotics, but you don't see them anymore -- for good reasons:

  • In order to effectively transfer the energy stored in a spinning weapon to your opponent the entire spinning mass must strike as a single, unified, inflexible object. With your design, the only energy transferred will come from the spinning 'bob' and a bit from the chain. The energy stored in the FBS shell will be isolated from the impact and will only be used to assist in spinning the 'bob' back to speed. This is highly inefficient.

  • A single 'bob'??? The rotating mass will be wildly imbalanced and will shake violently before you can get it anywhere close to effective speed. Changing the design to two 'bobs' would re-balance the spinning mass, but the impact mass would be reduced by half as only one 'bob' would hit at a time. I strongly suggest you scrap the sphere on chain impactor design.

  • I'm puzzled -- why do you think you need to rotate the weapon at 3000-4000 RPM if you don't know how much energy the weapon will store at that speed? Pure guess??
There are many posts in this archive about calculating the Moment of Inertia (MOI) of spinner weapons. If you'd care to browse here for MOI, you'll learn that our Excel Spinner Spreadsheet will give you spinner weapon performance data for specific spinner designs and motor options, but it won't calculate the MOI for a 'semispherical shape'. You'll also find links to relevant internet information like the Paul Hills Spinning Weapon webpage and the rotational inertia section of the Hyperphysics site for formulas and explanations.

I'll give you a start: our Excel Spinner Spreadsheet calculates that a 10 kg cylindrical steel ring 200 mm high and 500 mm in diameter (it works out to about 4mm thick) has an MOI of approximately 0.6 KgM2 and will store about 3000 joules of energy at 1000 RPM. That's way more than enough for a 45 kilo 'bot -- you don't need the 'bob'. An AmpFlow F30-150 motor geared down 7:1 would spin the ring to speed in about 2.5 seconds.

Robot haiku:

Already answered
More than four-thousand questions.
Please read the archives.

Q: Is 20 amp ESC enough for a ampflow F30-150 with a gear ratio of 8:1 for rotating a 10kg iron dome at 900rpm with 2700-3000joules of energy??? If its not enough can u guide me how much amp ESC is required..........?

A: The AmpFlow F30-150 motor at 24 volts pulls a theoretical 294 amps at stall. For the first half-second of spin-up with your weapon the motor will pull in excess of 150 amps, and it will consume more than 40 amps for at least the first two seconds. A 20 amp ESC is nowhere near enough for this motor/weapon.

You will need a controller with surge current capacity of about 200 amps and at least 100 amps for more than a second. Something like the 'VEX Pro Victor Spin Controller' should be fine. I don't think I need to warn you to power-off QUICKLY if the weapon is stalled. A 'slipable' V-belt drive can help to avoid a stalled weapon meltdown.

Note: as stated, the figures I provided as a starting point in the earlier post were for a 10 kg steel ring at 1000 RPM, not a 10 kg iron dome at 875 RPM. The Team Run Amok Spinner Spreadsheet doesn't have the capacity to calculate MOI for a dome shape, but I can roughly estimate the MOI of your uniform dome at 0.48 KgM2, with about 1800 joules of stored energy at 875 RPM. That's still adequate for a robot of its size, and a little easier on the motor/ESC.

Robot haiku:

If you change the shape,
M.O.I. also changes.
Mass distribution!

Q: Hey aaron
what are the changes made by team riobots in touro2008&touro2013 & which motor touro max is used to power the drum [Pune, India]

A: I can see a number of external changes to the 2013 version of middleweight drumbot 'Touro', but team RioBotz has been slow to update their web information. I can't list specific changes.

Heavyweight 'Touro Maximus' uses a large Scorpion HK-5035-500KV brushless motor with a continuous power output of more than 7 horsepower(!) to drive the drum weapon. Reliability of the drum power system has been a problem.

Robot haiku:

A brushless motor
Puts out a lot of power.

Q: Hey Aaron,
I've been working on re-designing my 120 after it got mangled at Robogames. I had been using an Ampflow F30-150 to spin my horizontal weapon disc (it worked great). A problem I had in initial design was that in order to mount the motor vertically, I needed a fairly high-profile chassis (the pulley to the weapon disc sat on the shaft, about 6" high). I'm trying to make my next design more streamlined. I've been thinking about building a small right-angle gearbox so that my motor can mount horizontally and my output shaft would be more low profile. However, I know how bevel gears can be tricky - not to mention I'm not sure how well they would perform at speeds of 6000+ rpm.

Do you have any advice? Is this endeavor worth it? Or is there some economical gearbox I can buy online, and use a direct coupling to the motor?

Thanks for your help.

A: Your concerns are well founded. Bevel gears are inefficient at power transfer, and are sensitive to shock and misalignment. Off-the-shelf gearboxes are expensive, limited in speed and power inputs, and difficult to interface.

Your current belt drive is very efficient, tolerates shock loading, and can handle misalignment -- a great deal of misalignment. Consider a quarter-turn belt drive. You may run into some problems that will require some tweaks to the design, but I'd rather try a quarter-turn belt drive than a bevel gear system.

I can't directly comment on whether going to a horizontal weapon motor is worth the effort. If your weapon was working well, perhaps you shouldn't tamper with the design. Keep it simple.

Robot haiku:

Don't complicate things.
Change things that break or don't work.
If it works, keep it.

Q: hey Aaron, I wanted to know if I could use the Harbor freight 900 rpm motor effectively for my spinning disk ? it is a 6 kg 1cm thick spinning disk which I'll use as my main weapon ......will the motor be good enough ? [Maharashtra, India]

A: Mark J. here: the diameter of the weapon disk is critical in the computations. If the disk is aluminum, I can estimate the diameter of the disk at 50 cm to get a 1 cm thick disk up to 6 kg. I'll also assume this is for a 60 kg 'bot.

An effective spinner weapon should have at least 16 joules of energy per pound of the weight class -- in your case that's a little over 2000 joules. You also need to be able to spin the weapon up to speed before your opponent can sprint across the arena and ram your weapon to slow it and keep it from reaching dangerous energy levels.

The Team Run Amok Spinner Spreadsheet says that the HF drillmotor/gearbox can spin your weapon disk up to about 800 joules in six seconds. That's too long a spin-up time for a small arena and too little weapon energy for a 60 kilo robot. You'll need both more power and speed.

Aim to spin the weapon disk to about 1500 RPM, reaching 1000 RPM in the first 2 seconds. You'll need about 1.5 horsepower to do that, and the 18 volt HF drill motor is good for only about 0.5 horsepower.

Robot haiku:

Question, India:
If you lack heavy motors,
Why build heavy 'bots?

Q: hey Aaron its that 900 rpm HF guy again, actually its for a 25 kg bot .....the disk will be 12 cm in diameter and made of cast iron ......maybe 6 kg will be too heavy , so m planning to use 4 kg , 1 cm thickness .....what now ? .....can u suggest any better material ?

A: Your calculations are off someplace. A 12 cm diameter cast iron disk 1 cm thick has a mass of only 0.9 kilo. That's WAY too small to be an effective weapon. Note that our spinner spreadsheet asks for the radius of the disk in its calculations, not the diameter. A 12 cm radius cast iron disk 1 cm thick has a mass of 3.5 kilos -- let's go from there:

16 joules times 55 pounds = 880 joules of energy storage needed

You'll need to spin that disk up to 2600 RPM to get that much energy storage. The HF motor has enough power to spin this disk up to that speed in a reasonable amount of time, picking up nearly 400 joules in the first 2 seconds.

You'll need to scrap the HF gearbox and find another gearing solution, as the weapon would have barely 100 joules of energy at 900 RPM. If you want to keep the motor/gearbox combination, you'll need a much larger diameter disk. A 25 cm radius steel disk 0.4 cm thick would weigh about 6 kg and would store about 800 joules of energy at 900 RPM. Spin-up time would still be good, but that's a pretty big disk for a 25 kilo 'bot.

As to your material choice, cast iron is brittle -- it tends to shatter when hit hard. Most spinner discs are made from aluminum alloy with tough steel impact blades bolted on. All steel construction could work for your weapon.

Robot haiku:

Spinner must be tough.
If it shatters on impact
You will loose your match.

Q: hey aaron,
how actually the mechanism of breaker box works internally ? how to decide the pivot point for such kind of mechanism so as to make the robot stable ? [Maharashtra, India]

A: The mechanism has been previously discussed. Search this archive for 'resembles breaker box'.

To get the 360 degree rotation of the scoop mechanism the robot body should be short and the pivot should be near the center of the robot. Component crowding may force you to move the pivot a little toward one end.

Robot haiku:

Please search the archives
Before you ask a question.
It saves us both time.

Q: where can i get high power servos for making breaker box in weight class of 120 lb ?

A: Servos don't come that big. 'Breaker Box' uses two custom gear motors with a dedicated electronic speed controller to power the scoop mechanism, and you'd need something similar if you actually want to be able to lift your opponent with the scoop. That takes a LOT of torque!

Robot haiku:

Formula for lift:
Weight lifted times length of arm.
Measure in foot-pounds.

Q: hey Aaron,
I want to make a pneumatic flipper . for tank of the flipper which material u suggest so that weight is also less and it can hold the pressure too. And what did ZIGGY used in his robot ? .
in India Max pressure is 8 bar ... so if I store air in tank above 8 bar and pressure to cylinder is 8 bar .. is it possible ? [Pune, India]

A: The Team Da Vinci: Understanding Pneumatics page will answer your questions about tanks and about pressure regulators that reduce the high tank storage pressure down to a lower pressure for the valves and cylinder.

Heavyweight flipper 'Ziggy' runs a high-pressure air system at about 200 bar straight to the actuator -- wildly beyond the 8 bar India limit. Their pressure storage is in an aluminum SCUBA tank.

There are dozens and dozens of posts about pneumatics in this archive.

Robot haiku:

From Maharashtra,
So many robot questions.
What's going on there?

Q: Hey aaron...
I heard that [brushless] motors have a very high power is to weight ratio. I recently heard abt a [brushless] DC motor weighing 1kg and delivering 4 hp.. IS there a motor with that power is to weight ratio. ? [Pune, India]

A: Many high performance brushless model aircraft motors can meet or exceed a 4 horsepower per kilogram power to mass ratio. For example, the AXI 5345/16 weighs 995 grams and has a peak output of about 4.7 horsepower -- but only for very short periods of time.

The problem is that model aircraft motors are designed to function with a cooling airflow from the propeller and to operate at fairly constant high RPM. If allowed to drop down below 10,000 RPM at full throttle for more than a couple seconds, rapid heat build-up will destroy the AXI 5345/16. That drawback makes most brushless motors unsuitable for robot drivetrains, but useable **with caution** in light robot spinner weapons.

Robot haiku:

A great big hammer
Is of very little use
If it's made of glass.

Q: It is very difficult to reverse the direction of a [brushless] motor... so I was planning to use it in my drum weapon.. Are there any disadvantages of [brushless] motors over ampflow types motor(I have ampflow F30 150). and I am making a 60 kg bot . and also suggest me weight of the drum. [Pune, India]

A: It is not at all difficult to reverse a brushless motor, but most brushless motor controllers are made for model aircraft that have no need to reverse the motor and so do not have that function. For reasons given above, hobby brushless motors are generally unsuited for use in robot drivetrains anyhow.

Your AmpFlow F30-150 brushed motor weighs about twice as much as the AXI 5345/16 and puts out about half the horsepower for about half the price. The advantage the AmpFlow has is toughness and reliability. The AmpFlow can be bogged down and even briefly stalled without much ill effect, effortlessly surviving abuse that would very quickly melt the AXI. The two motors are designed for very different purposes; a racehorse can outrun a farm horse, but it would fail at plowing a field. Choose wisely.

There are many factors in optimum drum weapon design beside weight. Here's a quote from an answer to an earlier question from this archive that addresses one of those factors:

The amount of power stored in a rotating drum is a function of its rotational moment of inertia (MOI) and its speed of rotation (RPM). The MOI of the drum is a function of the mass of the drum and how that mass is distributed. Two objects with the same mass can have very different MOI: mass located farther from the axis of rotation contributes more to the MOI than does mass close to the axis. A short, large diameter drum has a larger MOI than a long, small diameter drum of the same mass.

That said, a typical drum 'bot has about 1/3 of the mass of the robot invested in the weapon system (drum, motor, belt, share of battery...)

I'd strongly recommend reading thru the Robot Weapons archive for information on drum weapon design, and on the use of brushless motors for weapons.

Robot haiku:

Light drum, heavy drum.
Rotational momentum
Is what really counts.

Q: what is the difference between inrunner and outrunner dc motor.. [Pune, India]

A: See #23. Read the rest of the FAQ while you're there.

Robot haiku:

Already answered
More than four thousand questions.
At least read the FAQ!

Q: hey aaron,
i want to make a drum as a spinning weapon in my robot (60kg) . And rotating part of my drum is of steel whose dimension are 40 mm internal radius and 65 mm outer radius and length is 200 mm weight 12.8 kg . i want to store 10 kj energy . I cant afford ampflow A series motor . And i am in search for cheap motor than ampflow . one of my friend suggested me for [brushless] DC motor on hobby king . Motor here are cheap with high power and low weight. can i use this motor for my drum ? . i know that the motor get heat up . so if in case i use this [brushless] dc motor then what precausion should i take ? . which [brushless] dc motor you will suggest ? . [Mumbai, India]

A: Many problems.

  • Your drum has too small a Moment of Inertia (MOI) to realistically store 10K joules of kinetic energy. It would spin at about 7300 RPM to store that much anergy, and that's too fast to have effective 'bite'. Search this archive to learn more about 'bite'. The good news is that you don't need to store 10K joules of energy to have an effective weapon on a 60 KG 'bot. Around 40 joules per kilo is adequate, so 2500 joules would do nicely for your 60 kg 'bot. Your weapon will store that much energy at about 3700 RPM -- a much more reasonable speed for a drum this size.

  • Running a hobby aircraft brushless motor on a large drum weapon is VERY tricky. They are designed to spin a light propeller up to speed very quickly and maintain it at high RPM. Asking that motor to bog down and spin a 10 kilogram drum up to speed is well outside the type of work it was built to do. I cannot find any successful robot as heavy as yours that uses a brushless aircraft weapon motor.

  • If you insist on using a brushless aircraft motor, you must make absolutely certain that it will NEVER bog or stall -- even for an instant. A brushless motor of an adequate size for your weapon might be rated for a maximum 80 amps, but if stalled might attempt to pull more than 1000 amps! It's unlikely that your battery could supply that many amps, but it's very likely that the amperage spike the battery could supply might instantly destroy the motor and speed controller, as well as causing severe damage to the battery.

  • The most practical method of assuring that the motor will not stall is to drive the weapon with a loose belt that will slip under load. The more the belt slips, the lower the torque and amp load on the motor -- and the longer your weapon takes to spin-up. You won't be able to use the full theoretical power of the motor.

  • Finding the right amount of slip in the belt is done by experiment:

    • Start with the belt very loose and test the weapon.
    • Tighten the belt a little and test some more.
    • Keep tightening the belt until your motor, speed controller, or battery blows up.
    • Replace the blown parts and back the tension off a little.

    This gets expensive really fast.

  • About Hobby King products (from an earlier post):

    Many robot builders use cheap parts from [Hobby King]. Feel free to use them if you like. I think they're probably fine for a little R/C project you're going to play with in the local park, but not for combat competition. Quality control is awful, shafts aren't hardened, weak magnets aren't glued in securely, specifications are questionable, documentation is unintelligible, and customer support doesn't exist. I'll pass.

    I cannot recommend a Hobby King motor to you in good conscience -- see #16. In this specific case I cannot recommend an aircraft brushless motor from any source for your application. Brushless motors won't be a bargain when you start burning thru them.

  • I'd suggest using an AmpFlow F30-150 or the heavier E30-400 for a weapon motor in this weightclass. Either has ample power to spin up a drum the size you're planning: with a 2:1 reduction belt drive the weapon will spin up in about 2 seconds and deliver around 2500 joules of stored energy.

    If you're worried about weight, substituting a 200mm long drum with a 75mm outer radius and a 12mm wall thickness would have the same moment of inertia (MOI) as your current drum but would weigh only 8.1 kilos, saving more than enough weight for you to use either AmpFlow motor without reducing performance.

Robot haiku:

The part where I said,
'Choose wisely' ment 'not brushless'.
Should have made that clear.

Q: HI, I want to build a featherweight robot with a pneumatic flipper. I can figure out how to make a working robot that moves. However, I want to add a flipper but don't know where to start. How do you operate the flipper with your robot's remote control?

A: Start with the Team Da Vinci: Understanding Pneumatics page, then read thru the dozens and dozens of posts about pneumatics in this archive.

Note: I don't recommend a flipper weapon for a first combat robot project. Pneumatics are complex and potentially dangerous. You'll have plenty of things to worry about with battery maintenance, drivetrain optimization, R/C system set-up, armor materials, traction issues, ESC mixing, radio interference, tournament procedures, repair problems, and driving practice.

Robot haiku:

The very last thing
A novice 'bot builder needs?
An active weapon.

Q: Hi.. I plan to build a drum spinner weapon i did browse the 'spinner weapon starter package' at [no longer available]. They have the package of Ampflow A28-400 to be controlled using One 586 24VDC Power Solenoid with RC Switch. I wonder whether is that all needed to control the spinner motor? My previous drum spinner weapon motor is controlled directly using a dc motor driver and i'm not familiar in using the that actually the better way to control it? previous drum spinner weapon is using the 18v dewalt drill motor..which was not a success because i attached the motor shaft to the drive belt pulley using only set screw. Therefore, i welcome a suggestion on what is the best way to attached the 1/2inch ampflow motor shaft with the drive pulley so that it will be able to withstand the slamming impact during the weapon operation.

thanks! looking forward to your response.

A: A solenoid is the equivalent of a big switch that simply turns the weapon motor on and off -- no speed control and no reversing. A solenoid is a reasonable solution for large weapon motors that can consume huge amperage at start-up. A big solenoid like the WR 586 can deliver all the amperage your batttery can supply and assure the fastest possible spin-up time, while an electronic speed controller may restrict the maximum amp surge and reduce motor torque. If your spinner design allows for all-or-nothing speed and does not need to reverse, simple solenoid control is effective, inexpensive, and reliable.

The A28-400 output shaft has a keyway groove machined along its length. Use a pulley with a matching keyway and you can insert a hardened steel 'key' that will prevent the pulley from rotating on the shaft.

Q: Hey, I'm using a W-R 24v 124-series SPDT solenoid to activate an AmpFlow motor for a weapon. My question is how do the limits of the switch I order need to compare to the capabilities of the solenoid? I guess I don't quite understand the relationship between them. For example, can I use a 10A Battleswitch ( to activate my solenoid? Does the 10A refer to the current needed to switch the solenoid, or do I need a 100+ A switch for the times I expect my motor to draw that much current?


A: Go take a look at the data sheet for that solenoid. The power consumption of the 24 volt coil that activates the solenoid is listed at 12 watts, and that's all the power your R/C switch has to handle: that's 0.5 amp at 24 volts. Any R/C switch with at least that rating will do fine -- even the little PicoSwitch should do just fine, if you use an antiparallel flyback diode -- the solenoid coil is an inductive load. You can use the 10A BattleSwitch without the diode -- it's more robust.

Q: hey!!
i have nt so much knowledge abt all this but i m going to make a robot around 15-16 kg. & going to use blade or solid cylinder as a weapon so which type of motor should i use?? give me total specification & how much watt should it contain?? i m thinking to use 12v or 24v motor with 4000 rpm but don't know how much watt & amp should it contain?? plz guide... [Pune, India]

A: I can't give you a specific weapon motor recommendation based on the limited information you have provided -- the details of the weapon (dimensions, material, style) are critical in motor selection. See #29.

Q: hey i want to build a small biohazard like robot car but i cant really understand the mechanism of it so can u help me how can i build it using things easy available at home.

A: Are you asking about the mechanism of the whole vehicle, or just the BioHazard style lifter? If you're asking about the whole vehicle, that's way too complex for me to provide a short answer. Read thru the FAQ for some design help.

The BioHazard lifter is called a '4-bar mechanism'. It is made from four rigid elements hinged together. By careful selection of the bar lengths and hinge points, you can create a compact device that can extend forward and upward to lift your opponent. There are MANY posts in this archive about the design of 4-bar lifters -- search for '4-bar'.

To operate the 4-bar mechanism, you must apply force to one of the bars. This is typically done with a fairly powerful gearmotor and a radio control interface. I'm not optimistic that you can find components like these around your house.

Q: Dear Aaron, can you show me the design of Ziggy's weapon?

A: Super-flipper 'Ziggy' combines a 4-bar mechanism to define and control the flipper path with a lightning fast high-pressure air pneumatic system. The best photo I have is at right -- click it for a larger image.

There are MANY articles discussing 4-bar mechanisms and pneumatic systems in this archive.

Q: I have a 600w (24v. & 25a. at peak) weapon motor and I am using two lead acid batteries to get an output of 24v. and around 60a. But the motor stops suddenly at times (mainly after impacts) on the battery supply whereas on the other hand when I use a direct power supply the motor does not stop at all how much ever load it is subjected to. I am really confused about this can you help me work out what could be the problem??

Just if you find this useful when the motor stops while working on lead acid batteries and if I remove and reconnect the motor's connections on the battery it again starts working until the next unpredictable shut off. [Mumbai, India]

A: I don't think the problem has anything directly to do with the batteries -- batteries don't just stop providing power and restart if you disconnect and reconnect them. It would help if you had mentioned what weapon motor controller you are using. A couple possibilities:

  • Your weapon motor draws 25 amps 'at peak', but can draw much more current when bogged down or stalled by a hit. You may be draining so much current that the battery voltage falls and causes the motor controller to 'drop out' and reset only when the power is removed and restored. That could explain improved performance when operating from the 'direct power supply'. The solution would be batteries with greater capacity.

  • I've seen brushed motors show similar trouble if the motor brushes are binding in their holders. An impact can bump the brushes and cause them to stick. Inspect the brushes and make sure they slide freely thru the brush holders.
Q: Greater capacity as in more amps. ??

A: Greater capacity as in able to supply more amperage without a voltage drop. I suspect that your lead-acid batteries aren't really able to deliver as much amperage as you think they can.

Mark J. here: you haven't told us anything about your weapon motor controller or your 'direct power supply'. It's entirely possible that your batteries are fine and that your motor controller is shutting down under high amp loading and must be reset with a power-down. Your 'direct power supply' may not be capable of enough amperage to cause this. You may need a higher capacity motor controller rather than batteries.

Aaron correctly addressed the problem given the information you've given us, but there is a lot you haven't shared.

Q: Hey Aaron. A while back I ordered a 14" diameter, 0.75" thick steel plate to use as a flywheel for a middleweight weapon. I found a very cheap supplier so I jumped on it. I intended on broaching a keyway, but the 1" diameter hole through the center I had ordered is machined very sloppy. It seems the only way to salvage the piece is to bring it to a machinist and have him re-bore the hole. To about 1.25" or so. Since I'm locked into a 1" diameter shaft design, I need to fit the plate with some sort of keyed bushing to convert the new bore for my 1" shaft.

Do you have any suggestions? I was thinking of using a QD bushing and high-strength bolts to attach it to the plate. I'm hesitant because, as per the QD design, they feature a tapered OD. I can't seem to find any straight-bore flanged bushings I can use. Maybe some sort of hub? Any advice would be appreciated! Regards, Flash.

A: Mark J. here: I won't give you the sermon about cheap robot parts...

I don't have important details about your design, but consider boring out the hole and fitting a Trantorque or B-LOC keyless bushing. With either, there are no mounting bolts to shear, no keyway required, and they transmit huge torque while surviving great abuse.

Q: How do you make an antweight beater weapon without machining it out of a single piece of material?

A: A beater bar takes a great deal of abuse and must be both durable and well balanced. I can't recommend a method other than machining in a single piece. Several on-line machine shops could make a beater to your specifications

Q: Hey, I'm building a FBS (60 pound) however using a belt-pulley system and a AmpFlow F30-150 Motor. Is this a good idea? And how can i plug that motor (mean esc) should i use one of these Robot Power speed controllers or buy this White Rodgers solenoid and using 2 x 3s battery in parallel to get the voltage into 22v for the motor at 35c (each) with 2.700mah or the 1.500mah.

Need suggestion cause i got only one green light on the project and need 2 to be convence on the equipment being use. PLZ help Thank You [San Juan, Puerto Rico]

A: You haven't told me enough about your design for me to tell you if it's a 'good idea' or not. Full Body Spinners (FBS) are not easy to design or build, and none of the current successful lightweights are FBS.

The AmpFlow F30-150 motor is a reasonable choice for a lightweight spinner. A solenoid like the White Rodgers 586 would certainly control the motor, but an 'electronic spin controller' designed for spinner weapons will offer you greater weapon control. There are multiple posts in the Ask Aaron archives about weapon solenoids.

The Run Amok Excel Spinner Spreadsheet can assist you with evaluating the critical design elements of your spinner (dimensions, weight, gearing, battery selection), but again a FBS is NOT a good design for a novice builder. If this is your first combat robot I would suggest a different design.

Q: Is not my first design but is mi first in lightweight (60lb) we compete in 15lb, 30lb and 120lb (already done again this year drum) but i always use a drum-like-bot and was challenged (by sponsor) so im in the quest to finish the detail and convince him to pay all the equipment. The inside is already made but i don't want to spend without getting suggestions. The design is like this drawing but with a pullley system and only 4 wheel to move and the motor are not like that. So any suggestion than alot!

A: A drawing of somebody else's 'bot that's sorta like what you have in mind isn't much help. I need dimensions and materials! I can give you a few general comments:
  • That AmpFlow F30-150 motor is a bit over 6' tall if you stand it on end. Stuffing that under the shell with a pulley drive will make for a pretty tall lightweight FBS. A tall shell will not be as robust as a shell of lesser height.

  • You want as much mass as possible in the spinning shell. A typical FBS will have about half the mass of the 'bot dedicated to the weapon. You say you have the 'inside' already built -- do you have enough weight allowance left for the shell?

  • As noted above, the Run Amok Excel Spinner Spreadsheet can help you model the energy and spin-up time of your weapon design.

  • How long you have to spin-up your weapon depends on the size of the arena. A quick run of the Spinner Spreadsheet tells me that a 600 mm diameter aluminum shell 5mm thick and 180 mm tall will require a 6:1 belt drive reduction from the F30-150 to get a spin-up to 3000 Joules in 3 seconds. That sounds about right for a meduim-size arena.

  • Full Body Spinners are not as effective as vertical drums. When a vertical drum hits, most of the energy goes into throwing your opponent into the air. When an FBS hits, the energy of impact is split between throwing your opponent in one direction and throwing you in the other direction.

  • The center bearing support structure takes a LOT of abuse, and you can't support the shaft as well as you can on a drum weapon. Brace it well and make it strong!

  • The shell must be perfectly balanced. Precision construction is critical.
A successful FBS isn't an easy build -- best luck.

Q: Hey there, I forgot to design in a brake system for my middleweight's spinning weapon. (Rookie mistake, I know!). I haven't seen too much info on motor brake systems.. Are there any techniques you can suggest? I'm going to be activating my weapon motor with a DPDT solenoid; As of now the weapon is only designed to operate in one direction, so the opposite throw of my solenoid is unused. Is there some mechanical device I can hook up to it? Perhaps something that contacts the shaft to slow it? Thanks for the help.

A: The most common spinner braking method is called dynamic braking, and it uses nothing more than the components you already have. Take a look at the diagram to see how to wire the DPDT solenoid. Connecting the motor leads together turns the motor into a generator and dissipates the rotational kinetic energy of the weapon by converting it back into electricity and then into heat as it passes thru the resistance of the motor armature.

Dynamic braking works best when the weapon is spinning fast. The braking effect lessens as the weapon slows, but it is a simple and effective way to reduce spin-down time. We can discuss mechanical braking systems if dynamic braking isn't enough, but try it first.

Q: Hi Aaron, is it possible to control a weapon (in this case a lifting arm) without an ESC? The arm doesn't need variable control, it just needs to spin the motor clockwise, counterclockwise, and have an off position. Is there any way to do that? [Boston, MA]

A: Scroll down to the next question for a diagram of a relay/solenoid control system that provides forward/off/reverse control of a brushed DC motor. There are several posts about the use of solenoids to control weapon motors in this archive. A solenoid is a reasonable alternative to an Electronic Speed Controller (ESC) if you need only on/off control -- but if you need forward/off/reverse control of the motor you will find that a solenoid control system is heavier, less reliable, and about the same cost as a speed controller of the same capacity.

Note: brushless motors cannot be controled by relays/solenoids. Brushless motors MUST have a brushless motor controller to operate.

[Cross-posted from the archive]

Q: Hi Aaron!! I want to know whether the twin stick RC system can be used in conjunction with relays to operate a robot. Thanks. [Mumbai, India]

A: Yes, but there are several drawbacks.
  • You will require a special R/C interface between the receiver and the relay to translate the R/C signal into an on/off current to control the each relay -- you can't just plug the relays into the receiver.

  • If this is a fairly large robot, the relays needed to control the high current levels the motors require are expensive, heavy, and bulky.

  • A standard tank-steer robot will require at least four relays and interfaces to provide forward/reverse/off/left/right control.

  • Relays do not provide speed control. A robot controled by relays will be difficult to maneuver precisely and will be frustrating to operate.
Although there are commercially available 'dual relay boards' with a built-in R/C interface that can be used to control a single motor forward/off/reverse, a dual channel electronic speed controller is more compact, lighter, more reliable, provides better control, and costs less than two relay boards of the same capacity.

Q: I'm building a horizontal disc spinner. I was planning on using a live shaft for the weapon. I was going to order some hubs that would give my disc more surface area and keyway length on the shaft, but I'm pretty sure the force of the set screws alone won't be enough to hold the disc's position (axially) on the shaft. Planned design includes a 30-lb disc on a 1-in diameter shaft (on a middleweight). What do you recommend? Perhaps a hardened steel pin through the hub and shaft? Or would you recommend switching to a dead shaft?

Thanks for your help!
Regards, Flash

A: Conventional spinner weapon design allows the live shaft to float and transfer axial load from the disc hub to the support bearings, so there is very little axial load between the disc and a live shaft.

I think a hardened pin would be overkill, and I don't like to drill a hole thru a stressed shaft if it isn't needed. If you want to make real sure the hub doesn't move, you could incorporate a Trantorque bushing into your hub design, but in this application I'd be tempted to just grind a small flat on the shaft and use the set screws -- with threadlocker, of course.

Mark J. here: if you just can't bear the thought of using set screws, the use of retaining rings is a viable option. I also like Aaron's suggested use of a Trantorque bushing.

The dead/live shaft decision depends on elements in your design that you haven't shared. In general, a live shaft places less stress on the bearings for a given chassis height and offers more design flexibility.

Q: Aaron, What is the best way to run an unregulated C02 set up? Lets say its for a 30lb bot.

Thanks, New York

A: Two things we won't discuss here at Ask Aaron:

  1. Flame weapons; and
  2. HPA/unregulated CO2 pneumatic weapons.
Both are dangerous, and neither can be adequately discussed in our short answer format. By the time you have enough experience as a builder to safely construct either, you won't need to ask us how to do it.
Q: Dear Mark/Aaron
  1. What are the factors influencing a "bite" in a drum bot?
  2. What kind of bots are immune to a drum bot?
  3. I have been thinking a lot about this scenario lately - suppose a bot is cubicle in shape and its height is well exceeding my drum's diameter and its surface is super finished. I don't think I will ever get a "bite" on such built bots. How will I tackle such kind of bots? please suggest.

A: Go read section 6.3 of the RioBotz Combat Tutorial for a full explanation of 'bite' in spinning weapons complete with diagrams, tables, and equations. We also have many posts about 'bite' in both this archive and in the archive.

I'm not sure that any design is 'immune' to a well designed drum bot, but 'spinner killer' scoop designs - like 'Breaker Box' - have very effective counter measures. In general: the fewer exposed edges a 'bot has, the better it can resist a typical drum design.

Even a vertical edge, like the edge of a cube, allows some chance of 'bite' for a drum weapon. The sharp angle gives a hard and sharp tooth a good chance of deforming the material and creating its own foothold. Conventional drum design calls for very hard, sharp impact teeth for the best chance of getting bite in difficult conditions.

I haven't seen this tried in a long time, but a no-impactor drum covered with a high-friction material can get at least some grip on even the smoothest and hardest surface.

Q: I know your not a fan of direct drive weapons because of to much rpms to get a good "bite", but what makes antweight Metroid's weapon so effective? Examples: Metroid vs Spark Plug and Kilobots XIII - Antweight Rumble.

A: Antweight 'Metroid' is a very effective combat robot, but don't place all the credit on the weapon. The robot is well constructed, the components all work well together, and it's quite well driven. That said, there are some design elements that do help the drum weapon make the most of the limited 'bite' it has:
  • The drum diameter is a bit larger than a typical ant drum. This gives the drum teeth a large 'upsweep' area to search for an edge for bite.

  • The drum teeth themselves are undercut and sharpened. If the edge of the tooth does get even a little bite, it can pull the opponent deeper in toward the drum for a more effective hit. It may be an illusion, but it also looks like the mounting holes for the teeth are drilled just a bit off-axis to open up the exposure of the leading edge of the teeth.

  • The drum teeth on opposite sides of the drum are offset: centered on one side and out toward the drum ends on the other. This allows a clear full-circle path for any given tooth to penetrate deeper in toward the opponent before contact.

  • I'm not sure exactly how fast the drum is spinning. It doesn't have that crazy high-pitched whine that some direct-drive weapons emit, so it may be spinning at a more reasonable speed than you might assume.
Watch the videos carefully and you'll notice that Metroid's weapon is not effective against smooth, flat surfaces -- the high-speed drum dictates the attack strategy. The weapon needs an exposed edge to obtain grip, and driver Dennis Beck is good at picking the right time to dart in and catch an opponent with their side or rear aspect exposed when hard edges are more likely to be available. If your weapon has bite, you have more attack options.

One last design note: 'Metroid' fights only at the Kilobots events in Saskaoon. The Kilobots arena has a steel floor, and Metroid takes advantage of this with a large neodymium magnet that keeps the robot well planted and provides excellent traction for those quick, darting attacks.

Q: Hey Aaron can I use Team Whyachi C1 Contactor as a weapon actuator. My weapon is a drum of M.M.I= 0.03176kgm^2 driven by an [AmpFlow] E30-400. You earlier mentioned to use this DPDT-24V 586 Series SPDT White-Rodgers Solenoid. The TW C1 Contactor is almost half the price of what you have mentioned. Please suggest if i can use TW C1 Contactor.

A: Mark J. here: the TW-C1 contactor has a couple of drawbacks:

  • Fragility: the TW-C1 contactor body is made of a brittle material and does not withstand shock well. These contactors have been reported to fracture when the robot is hit hard -- even if shock mounted. The big White-Rodgers solenoids have a metal body and are much more shock resistant.

  • Uncertain capacity: contactors almost always have lower current capacity on the Normally Open (NO) contacts than on the Normally Closed (NC) contacts. If you're using the contactor for single-direction on/off control this isn't an issue. However, since I know you're using two of the contactors for forward/off/reverse control, all contacts must be rated for the surge current capacity required to control the selected weapon motor.
Since the full specs for the NC contacts aren't given, I can't tell you if the TW-C1 will handle your weapon motor while providing forward/reverse control. Use it at your own risk.

Q: Dear Aaron/Mark can we use this dc solenoid- 24V 124 Series SPDT White-Rodgers Solenoid instead of the 24V 586 Series SPDT White-Rodgers Solenoid (for E30-400) which you have earlier mentioned? We are on a real tight budget. And is relay a wise choice for activating E30-150 in forward and reverse direction?

A: You can read the engineering specs for the 124 series solenoid as well as I can. At 24 volts, the rated inrush current for the NC contacts is 100 amps, and your selected motor can pull more than 250 amps at startup. That solenoid may survive long enough for your purpose, but I don't recommend stressing a component that far beyond its rating.

Solenoid control of a motor is a reasonable option for single direction weapons. However, for a reverseable weapon an electronic speed controller is typically more reliable, more compact, lighter, and provides better control for about the same cost. If you are on a very tight budget, you may be better off to redesign for single direction Weapon operation, or perhaps select a smaller weapon motor.

Q: Theoretical, How would I mount a motor inside of a pipe?

A: Theoretically, it would depend on the motor, the pipe, and why you want to mount it in a pipe.

  • If you're mounting a 'outrunner' style (spinning can) motor in a tight-fitting tube to directly power a spinning drum, a pair of flush-head machine screws passsing thru the tube and into tapped holes in the motor is a reasonable mounting method.

  • Ideally, a keyway should be machined into the motor can and the interior of the tube to lock the rotation of the motor and tube, allowing a machine screw to locate the motor laterally without being exposed to high axial torque loading. This is difficult work, and the motor shell may not be suitably thick to allow a keyway.

  • I've seen small outrunner motors epoxy-bonded to the interior of a tube as well -- but that makes the motor non-replaceable.

  • If you have some other application in mind, I'd need more information before I could recommend a method.
Note: direct-driving a spinning drum in this manner is not a great idea. It places great load on the motor bearings, spins the weapon too fast for practical use, and lengthens spin-up time.
Q: Hi Aaron, great site. I really wish I knew about it sooner.

I am getting back into the combat robotics game after having partially built more than one. I never got the chance to compete. However, my first design was a 12 lb horizontal disc spinner (friction driven). I am going to revive the idea to some extent, perhaps redesigning it from the ground up and bumping it up a weight class or two.

My main concern lately has been the presence of numerous scary vertical drum spinners. Of course, we all know that severe off-plane impacts have adverse effects on a horizontal weapon assembly. My original design incorporated ball bearings in the frame within thick aluminum blocks above and below the disc, which spun on a "live shaft." I am thinking about designing it around a dead shaft to improve the structural integrity of the entire frame, but fear that bearings mounted close to the disc will endure much greater stress in the event of a vertical impact, as the disc radius will certainly exceed the height of the frame (resulting in a "twist" between the inner and outer races of the bearings). Would a "live" shaft be more appropriate for this application, or is there a particular bearing or placement which will prevent their destruction and improve overall durability?

Also, if a dead shaft is the best bet for this design, what is the method of keeping the disc in place, that is, from sliding up and down the shaft?

A: The 'live shaft' design [where the weapon shaft rotates and is supported by bearings in the chassis] is more popular than the 'dead shaft' design [where the weapon shaft is stationary and it supports bearings in the weapon hub] largely because it spaces the bearings a bit away from the weapon and allows the drive pulley to be located outside the compact support frame.

With a friction drive there is no drive pulley, and the dead shaft can become a fixed structural member of the chassis to greatly improve the weapon support strength. The weapon hub can be extended vertically to move the bearings some distance away from the disc plane and improve twisting resistance. How much bearing spacing you can get will depend on your chassis design, but I'd say that 'more is better'. Use of a bearing type that can effectively resist both axial and thrust loading (like a tapered roller bearing) can greatly improve bearing strength in this type of application.

Locating a disc on a dead shaft is simply a matter of tubular spacers on the shaft that rest against the frame supports and the inner races of the bearings.

Note: there are good reasons why you don't see many friction drive weapons. An effective and reliable friction drive is difficult to implement, particularly in the heavier weight classes. Best luck.

Q: Dear Aaron I have a query regarding the new E series motors which ampflow has recently introduced. I will be using an E30-400 motor for powering a drum (mass moment of inertia 0.03176kgm^2). The drum should operate on full rpm as i switch it on and it should be reversible as well. As we are low on funds we won't be using any speed controllers. I was just concerned if this will damage the motors.

I will be using lipo batteries (4s 2750mAh 65~130c, two of these in series). As I am not using a speed controller what should my operating voltage be 22.2 or 29.6V or can i use a 4s and a 3s in series? [Bangalore, India]

A: Are you planning to use solenoids to control the weapon motor? To have reversing capability, you'll need two DPDT solenoids rated for at least 300 amp inrush current on all contacts (see diagram at right). Too small an amp rating and the solenoid contacts can weld themselves shut! The Normally Closed (NC) contacts on DPDT solenoids are typically rated for less current than the Normally Open (NO) contacts, so check the current ratings carefully. A good high-power DPDT solenoid isn't cheap, and you need two of them. You aren't going to save much (if any) money over a suitable speed controller.

The AmpFlow motors are well built, sturdy, and unlikely to be damaged by direct application of operating voltage. Be sure to properly break-in the motor by running it continuously for at least 20 minutes at reduced voltage (~12v). This will contour the brushes to the commutator and prevent damaging arcing at high start-up current loading.

The AmpFlow motor is entirely capable of dealing with overvolting to 30 volts as a weapon motor, although 22.2 volts should give you ample power. Overvolting will increase power and speed, but it will also increase amperage so don't overdo it. You can run a 4 cell and a 3 cell LiPo in series IF the cells in both batteries are identical -- same capacity, model, and manufacturer. Some manufacturers offer 7 cell LiPo batteries, but not many LiPo chargers can handle that large a pack.

Q: Thanks for your last reply Aaron. I have few more questions to bug you. My drum weapon has M.M.I= 0.03176kgm^2, r.p.m around 5000. The drum will be mounted on a dead shaft of diameter 1.5" supported on two roller contact bearings. Now I have two [three] questions:

1) Ideally there is no axial force applied on the drum i.e only radial force acts on the drum and the bearings. So should I consider this fact while selecting the bearings or should I look for a bearing capable of taking combination loads. Also what kind of bearings should I look for? as in roller, cylindrical, deep groove, taper, spherical etc? (I will make sure that the shaft doesn't get bend so self aligning bearings should be out of question)

A: Mark J. here: 'ideal' engineering conditions don't hold in combat robotics. Your drum bearings might be expected to experience only axial loading from your weapon's actions, but your opponent will have weaponry as well that may inflict large impact loads from unpredictable directions. My choice would be tapered roller bearings.

2) What should be the ideal distance between the two bearings (extreme ends of the drum or a little towards the inward of it)?

A: Force vectors work out best with the bearings at the extreme ends of the drum.

3) Lastly, can we use bearing mountings as motor mountings or will it create some heat dissipation issues? (am using a E30-400 for weapon and two E30-150 drive) Thanks in advance!

A: A pillow-block style mounting is strong, simple, commonly used, and should cause no heat issues for your AmpFlow motors.

Q: hello Aaron, I am building a drumbot powered by an ampflow e30-400 motor for its drum. Can u please suggest to me the best and cheapest batteries for this very motor. Drum weight is 15 kg and we would prevent the motor from stalling. [Maharashtra, India]

A: Mark J. here: the load on a spinner weapon motor depends on more than the mass of weapon -- it also depends on the diameter of the weapon and the placement of the mass. Everything else being equal, a larger diameter weapon will have greater 'rotational inertia', will place greater load on the weapon botor, will take longer to spin up to a given speed, and will store greater rotational energy at a given speed. You need to determine the rotational inertia of the weapon in order to determine a proper speed reduction between the motor and weapon, and you need both the rotational inertia and the speed reduction to calculate the load on the battery.

The Team Run Amok Spinner Excel Spreadsheet can calculate the rotational inertia of a drum weapon based on the dimensions of the weapon components and the material of which they are made. Adding in motor data will allow the spreadsheet to also calculate the weapon spin-up time and the energy storage of the weapon system. It will also estimate the total battery load of the weapon for a match.

The AmpFlow E30-400 is a large and amp-hungry motor. Assuming that you will run the motor at 24 volts (they can be over-volted), you will ideally need a battery that can deliver a peak 270 amps of current. Less current capacity will reduce the peak torque of the motor and will slow the weapon spin-up time. If you can't deliver that much current, you might be better off using a smaller weapon motor and saving weight and expense.

As to the 'best and cheapest' battery, you can have either 'the best' or 'the cheapest' -- but not both. A pair of locally sourced Sealed Lead Acid (SLA) batteries would be cheap and could deliver the required amperage, but they would be bulky and heavy. The 'best' choice might be something like the ThunderPower Pro Power 65C LiPoly battery -- capable of more than 290 amps of peak current while weighing just over 13 ounces. This level of power is more expensive and would require a charger designed specifically to hanle LiPoly batteries.

Run the rotational inertia calculations for your weapon drum, select a practical speed reduction, determine the battery amp-hour requirement of your weapon, then seek out a high peak-amp battery to suit that capacity need. Note that most combat robots run a single battery to power the weapon and drive motors.

Q: how to make a flame thrower robot [Chandigarh, India]

A: See #28.

Q: how to build a simple flame thrower robot explain [Chandigarh, India]

A: Persistant, aren't you?

As explained in #28, we will not discuss flame weapon construction here because we don't want novice builders hurting themselves. By the time you are an experienced enough builder to safely construct a flame weapon, you won't need to ask us how to do it.

Q: i am gonna participate in a local techfest ....i wanna kno ..which motors are the best for lifting mechanism?
or shall i use hydrauliccs?
moreover the cutters should have which motors? [Mumbai, Maharashtra, India]

A: You can't go to a doctor and ask, "I'm not feeling well. What medicine should I take?" Your doctor would need much more information before they could recommend a treatment.

Likewise, you haven't given me enough information to recommend specific weapon motors for your robot. I don't know how much the robots at your 'techfest' can weigh, what rules govern your weapon selection, or the details of the design you have in mind.

Read thru this archive for tips on weapon motor selection. It may give you some ideas for your robot.

Q: Hi Aaron, Your site has been most helpful. I just have a quick question, however to make the hamburger as good as possible, I shall give you full details of my design. I plan on making a hobbyweight vertical spinner and I was planning on using a 8"x.75" aluminum disk being spun at around 1500 rpm, with a single .25"x2"x3" steel impactor as a tooth with a counterweight on the other side. A Turnigy L5055A-400 motor will drive the disk (reduced with a 3:1 belt system) with a 3s lipo along with Turnigy Brushless ESC 60A w/ Reverse so that I can reverse it when I'm flipped.

The problem comes from when I am calculating the weight of the disk. The online metals weight calculator says that the disk will weigh 3.692 lbs while the spinner spreadsheet I downloaded here says that the disk 6.82 kilos! that's a huge difference. maybe I put the information into the spreadsheet wrong? any help is greatly appreciated. thanks [Hawaii]

A: The Team Run Amok Spinner Spreadsheet looks for the radius of the spinner disk for input. I suspect you entered the diameter. When I enter a 4" radius (0.102 meter) and a 3/4" thickness (19 mm), I get 3.77 pounds (1.71 kilos) for the disk -- the tooth and counterweight combined add up to about 0.44 pounds (0.2 kilos).

A few things you didn't ask about:

  • Spinning that weapon at 1500 RPM is not going to store a lot of energy by current hobbyweight standards: only about 120 Joules. Some beetleweight spinners have that much energy, but spinning a single impact tooth at 1500 RPM will give you excellent 'bite' that may make up for the low energy storage.

  • I'm not a fan of Hobby King motors. I don't believe that either their specs or performance are reliable. Given the specs HK provides for the Turnigy L5055A-400 motor, I suspect it has the power you need.

  • The specs on most inexpensive 'hobby' ESCs - like those sold by HK - are typically based on theoretical values that wildly overestimate real world capacity. The HK ESC you've chosen may be adequate for your purpose, or it may not. Best luck!

Q: Hi Aaron it's the hobby weight vertical disk spinner guy again. I knew I was doing something wrong. I know your not a big fan of hobbyking motors, but I have trouble picking out a correct sized motor. I looked around and saw that a few people use this motor. I picked the car esc because its reversible and it says it can handle 60 amps. I like the ability to reverse my weapon. I was thinking of doing a 2:1 reduction instead maybe to increase rpm. What do you think? Thanks for your time.

A: OK, let's talk a little about weapon motor selection:

  • You're considering running the L5055A motor down at the bottom of its 11.1 to 29.6 volt range. If the motor can produce the advertised 1400 output watts at 29.6 volts, you're only going to get about 200 watts output at 11.1 volts.

    Power increases/decreases with the square of voltage:

    • 11.1 ÷ 29.6 = 37.5% of max voltage
    • (0.375)2 = 14% of max power
    • 14% of 1400 watts = about 200 watts.

  • You can get better than 200 output watts at 11.1 volts from a much smaller and lighter motor -- something like the Turnigy G25 will give more than 300 output watts at 11.1 volts, assuming the specs are correct.

The G25 with a 3:1 reduction will spin the weapon up to better than 250 Joules at more than 2000 RPM in about half the time the L5055A needs to spin up to 150 Joules with the same reduction. Lighter, cheaper, and more powerful -- I think the G25 is a better choice, if you want an HK weapon motor. Now, about ESCs:

  • If a real robot ESC (Vantec, Scorpion, Sabertooth...) says it can handle 60 amps of continuous power, you can put the ESC on a test bench and pull 60 amps thru it until your batteries go dead. If it says it can pull a peak 85 amps for ten seconds, you count on 85 amps for ten seconds.

  • If a 'hobby' ESC says it can handle 60 amps of continuous power, it means that the manufacturer looked up the absolute current values for the power chips used in the ESC. If you try to pull 60 amps thru it on a test bench you'll either quickly cut in the amp limiter or produce a big puff of smoke. If you try to pull a peak 385 amps(!!!) you'll get a blue flash and enough ozone to bleach your lungs.
Will the Turnigy Brushless 60A ESC handle your weapon? Maybe. A weapon motor pulls maximum amps very briefly on spin-up, and some ESCs have a 'soft start' feature that reduces current flow at start-up (and incidentally lengthens spin-up time). I can't tell you if it will work or not.
Q: Is there a good way to use the spinner spread sheet for melty brain spinners? Do you have any tips for an aspiring melty brain spinner builder? [Oregon]

A: You know why they call it 'Melty Brain', don't you? Getting one to work requires such intensity of thought and such enormous frustration that your brain actually melts! Well, maybe not 'actually', but it feels like it. My best advice is to lock away your sharp objects to keep you from hurting yourself, and keep a bucket of ice nearby to cool your skull.

Calculating the stored energy and spin-up time for a thwackbot/melty spinner is beyond the capacity of the Team Run Amok Spinner Spreadsheet. If you're intent on building such a design you'll have to do it 'seat of the pants'.

Q: With flippers and axes not scaling down well it seems that only rotary weapons (bars, eggbeaters) are the only way to cause damage in the smaller classes. Do you think this is correct?

A: Are you interested in causing damage, or in winning? Lifters work VERY well in lighter classes if you're interested in winning matches. See: What Weapons Win?

Q: Does a vertical spinner have an advantage over a horizontal one?

A: Yes, and no.

Advantage to the vertical spinner: when a spinner hits, there is both an action on your opponent and a reaction on your 'bot.

  • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. That's good, since your 'bot is supported by the arena surface and does not move it can deliver a more powerful impact.

  • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of your hit is split between moving the two 'bots in opposite directions. You may do as much harm to your own 'bot as to your opponent.

Advantage to the horizontal spinner: the spinning mass of the weapon exerts gyroscopic forces on the robot if the rotational axis is deflected.

  • When a 'bot with a spinning horizontal weapon turns, the axis of the weapon/gyro remains straight up/down. No gyroscopic force acts on the 'bot as a result of the turning motion and maneuverability is unaffected.

  • When a 'bot with a spinning vertical weapon turns, the axis of the weapon/gyro must also turn. This action exerts a gyroscopic counter-force which raises one side of the 'bot and reduces maneuverability.

Most builders prefer to live with or work around the maneuverability problems in order to gain the improved impact power of the vertical spinner.

Q: Dear Aaron, can you show me how Panzer MK3's and Panzer MK4's weapons work?

A: The later versions of Panzer's front plow were mounted on a 4-bar mechanism [diagram at right] that allowed the plow to move up and down without significant change in orientation. Dual pneumatic actuators powered the raising and lowering of the plow. Special pneumatic control valving allowed the plow to be positioned and held at any height within the elevation range for maximum effect, rather than simply slaming from one end of the range to the other.

Please note that the plow itself was not really Panzer's weapon. The formidable power and speed of Panzer itself was the weapon -- the plow was just the fist at the end of that powerful arm.

Q: How does one measure a spinning weapon's RPM?

A: Actually measuring weapon RPM is most simply done with an inexpensive laser photo tachometer.

Builders will often calculate weapon speed by taking the published 'free running' RPM of the weapon motor and dividing it by the weapon gear reduction. Example:

6000 RPM motor through a 3:1 belt reduction = 6000 ÷ 3 = 2000 weapon RPM

It's quite unlikely that the weapon will actually spin that fast due to frictional losses, but it makes a good 'brag number'.

Q: How can a vertical spinner self-right with its weapon? Do you think 'Electric Boogaloo' can do that?

A: A tall vertical spinner weapon has a chance to strike the arena floor and 'pop' back upright. Lightweight 'Backlash' did this at BattleBots. I certainly wouldn't say that this is a reliable method, but you might get lucky.

If you watch the video of 'Electric Boogaloo' vs 'Sewer Snake' at RoboGames '12, you'll see EB get flipped by SS about 18 seconds in. EB's weapon does hit the arena floor as the 'bot comes down and it does pop the 'bot back onto its wheels. It happens so quickly that it could go un-noticed, but I think it counts.

Q: Hey Aaron, so I'm thinking about making a robot. I don't have a lot of money, so what weapon should I start out making? Is a motor spinning a hammer or a blade cheaper that a sping disk? Thanks.

A: If you read thru this archive you'll see we frequently and strongly recommend that a new builder's first robot should NOT have an active weapon. You'll have plenty of new things to worry about with battery maintenance, R/C system set-up, armor materials, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems.

You'll also find out that 'bots with passive weapons (wedges, bricks, dustpans...) are - on average - more successful than 'bots with active weapons (spinners, flippers...). Passive weapon robots win a greater percentage of their matches and have higher rankings than their active weapon counterparts. Here's the proof.

If you're interested in winning matches with your first robot, build a wedge.

Q: Dear Aaron, how does Team Velocity's, "Crushing defeat" work? I am under the impression that this is one of those complicated designs. The robot has an 0-2 record, however in the two fights, he was box rushed and pitted in his first ever fight, and then in the next fight the weapon could not work, so he thinks he might have been able to win if the crushing weapon worked. I saw the video of it crushing [more like piercing] aluminum [very thin] and it was pretty sweet!

Writing from Paris, but no one is awake yet! hehe... Thanks, New York

A: 'Crushing Defeat' a sa propre page sur le site Web de l'équipe Vélocité. Il y a une description très complète de l'arme électrique de perçage, une liste des composants et beaucoup de photos.

Les constructeurs ont toujours cru qu'ils auraient gagné si quelque chose était différente.

Q: Why did 'Hot Stuff' remove its flamethrower? What do you think of it?

A: Lightweight 'Hot Stuff' still has its flamethrower, but it wasn't working at RoboGames '12. Here's the story, straight from builder Jerome Miles:

"In my first match the two wires I have running out to my igniter for my flame thrower were cut and shorted out, causing an electrical fire in the bot when I tried to light the flame thrower, and damaging it so that it didn't work for the rest of the competition. Kinda a bummer, but next time I'll try to make that harder to happen. I think Hot Stuff is the first bot to put out its own fire, I cycled the grabber a few times and the Co2 put it right out!"

Jerome builds very cool robots. I met him more than ten years ago at Robot Wars and he's both a nice guy and a great builder.

'Hot Stuff' violates my simplicity rule for combat robots by combining lifter, clamp, and flamethrower weaponry, but Jerome has been building for a long time and has the experience and skills needed to pull off a complex design. Current record: 11 wins and six losses, with podium finishes at RoboGames '10 and '11 - HOT!

Q: I saw this video online of a flame thrower 1 lb bot decimating 1/16th inch polycarbonate (Lexan). The description of how to make the weapon is confusing:


1) Can you explain how, maybe with a diagram, this weapon works?
2) From the test in the video and your knowledge, how effective is this weapon? I know that flame throwers are usually for show, but this one seem legitimate.
3) how much would a decently effective version weigh?

Thank you very much, New York, writing from Paris

A: You're in Paris, and you're spending your time watching robot videos? Dude!!!

I'm not surprised that Team Misfit's description of how they made their flamethrower is confusing. Here is their description - direct from the team website - of how to build a rotating drum weapon:

How to build a rotating drum
1. Get some metal
2. Build some stuff
3. Done!
4. Profit!
Super awesome. Consider getting your advice elsewhere. In answer to your questions:
  1. We don't discuss flame weapon construction for the reasons given in #28.

  2. The video was posted by a member of Team Misfit in November of 2009 but I can find no record of any flame robot from Team Misfit ever competing and there is no mention of a flamebot on the team's website. That should tell you something about the practicality and effectiveness of the weapon.

  3. I've never seen a 'decently effective' flamethower weapon -- again see #28.

Q: Is there a rule of thumb for finding out how much PSI a flipper in any given weight class should use? I understand that diffferent configurations would use/need different amount of PSI but is there a 'safe' amount to use?

A: That's kinda like asking how hard you should hit someone in a fight. The 'rule of thumb' is to use as much pressure as the event allows.

That said, the operating pressure of a pneumatic system is only one of many elements in the performance of a flipper weapon system. The force and speed of the flipper will depend on:

  • gas pressure,
  • actuator bore,
  • gas flow rate, and
  • the geometry of the flipper.
I have seen successful heavyweight flippers operate with anywhere from 150 to 3500 psi. UK antweight flippers are restricted to 100 psi and still manage rather well.
Q: Hi there. I'm designing a lightweight robot with a spinning bar weapon. Currently I'm thinking about supporting it with a pair of tapered roller bearings, but I'm worried bad things will happen if the axle gets bent because they aren't designed to handle mis-aligned axles. (not that I'm planning on that, but there is always Murphy's law). I could use a pair of self-aligning ball bearings, but they won't handle the same amount of force as a comparably sized roller bearing (and a spherical tapered roller bearing is extremely expensive). What would you recommend?

A: I can tell that you're an experienced designer and that you've given this some thought. I agree with your analysis of the bearings and your concern about a bent shaft. My recommendation is to use the tapered roller bearings, keep the shaft short, support the shaft close to the force vector, use suitably hardened shaft material, and make the shaft so crazy large in diameter that it just can't bend. A few more ounces of weight for the extra-large diameter shaft and bearings is cheap insurance.

Q: I noticed that several flipper robots position their lifting mechanisms near the fulcrum of the flipper. Wouldn't it allow for more fliping power by pushing the flipper surface as far away from the fulcrum as possible to gain leverage? I understand that it must have some advantage because many succesful robots such as Firestorm use this configuration?

A: There are many considerations and compromises in designing a pneumatic flipper weapon. Placing the attachment point for the actuator near the lifter hinge of a simple 3-bar lifter does decrease the force available at the tip of the flipper, but can increase both the speed and range of travel. Clever selection of attachment points and flipper geometry can result in high force at the start of the flipper cycle that changes to greater speed as the flipper rises.

Other considerations include the desired profile of the robot, the bore diameter of the actuator, the gas pressure available, the flow rate of the control valves, the length of actuator motion, the angle at which the actuator joins with the flipper arm, and the location of the flipper hinge relative to the actuator hinge. Way too much to cover here, but section 6.10 of the Riobotz Combat Tutorial covers many basic flipper design elements.

Q: What [is] sewer snake's weapon?

Why sewer snake removed flamethrower for robogames 2012?

A: Heavyweight 'Sewer Snake' has several interchangeable weapons that attach to the front accessory bar: flamethrower, lifter, wedge, etc. Different weapons are used in different situations.

Sewer Snake had its flamethower at RG12 -- Sewer Snake vs. Ragin' Scotsman video.

Q: So why sewer snake didn't used flamethrower against last rites in RG2012? Did sewer snake team knew that LR reinforced it's armor to survive flame of sewer snake?

A: Flame weapons are not effective in combat -- no top-level combat robot would be damaged by a burst of flame. Fire is entirely for entertaining the audience, and I suspect the the weight saved by removing the flame thrower was put to use as additional armor to resist the brutal attack of 'Last Rites'.

Q: Wait, didn't sewer snake won by using flamethrower against LR in RG2011?

A: 'Sewer Snake' had both a lifting anti-spinner scoop and a flamethrower fitted for the Robogames 2011 championship match against 'Last Rites'. Watch the match and you'll see that 'Sewer Snake' won by superior drive power and use of the scoop -- the flamethrower did not contribute to the win.

Q: Scoop stopped spinner, but why LR started to smoke when flamethrowed at 3:11? Coincidence?

A: Mark J. here: by 3:11 in the video "Last Rites' had one functional drive motor, the spinner weapon was inoperative, and the robot was stuck on its side against the rail. Anything that 'Sewer Snake' did with the flamethrower at that point had no effect on the outcome of the match. I suspect that the smoke was from a blown motor, ESC, or battery pack that had given its all in keeping Team Hardcore in the fight.

I've written to Ray Billings and asked for his definitive word on the source of the smoke pouring from his 'bot. Sorry to bring back memories of a hard loss, Ray.

Update: Ray Billings wrote right back:

Hey Mark

Battery pack - had nothing to do with the flame weapon on SS. I was pushing the weapon system WAY harder than I should have the whole event, and was on my last (and worst condition) weapon motor. Weapon motor died, drawing a shit-ton of current, and the smoking pack was the result.

Thanks, Ray.
Q: What do you think would make the best lifter for a science olympiad sumo bot? and what [type of lifter] platform do you think would be the best to use it? Do you like the idea of the lifter? Thanks, New York

[Several earlier Q&A in this thread deleted].

A: Mark J. here: this thread got off to a confusing start. I'm gonna call 'reset' and start over now that I know which robots you ment to ask about and which event you're entering. Recap:

  • You want to know if there is any particular advantage to a flat 'lifting fork' as used by 'Vlad the Impaler' over an angled lifting wedge as used by 'Juggerbot 3.0'.

  • You will be building an insect-class robot for the Science Olympiad sumo competition with a servo-powered lifter.

  • The rules for the competition allow only the wheels (or treads) to touch the arena surface - nothing else. A couple thousandths of an inch is enough clearance.

First, I really dislike that 'no touch' rule. It effectively outlaws 2-wheel robots, and creates a 'how low can I get without touching' war. Worse, it can't be effectively enforced. A wedge or lifter may have a small clearance when sitting still for inspection, but may become a zero-clearance 'scraper' due to dynamic forces when the 'bot is in motion. How do you check clearance during the match? An un-enforceable rule is a bad rule.

As you know we don't compete in robot sumo, but I don't see any particular advantage to the flat fork versus the angled wedge as a lifter. Either might have an advantage in a particular situation, but you couldn't predict that going into the competition. A large lifter platform would likely be best -- you'd need to lift it up at the start and end of the match to meet the max dimension rule for this competition, but it might be worth considering. I don't know how the event officials would feel about a large lifter platform that might touch the floor when an opponent's weight was on it. I really don't like that no-touch rule!

Overall, I like the idea of a lifter, but I'm having a lot of trouble interpreting the intent of the event rules. Sorry I can't be more help.

Q: Aaron, when you say 'lifting platform' can you elaborate? Do you mean like a gear motor with an arm that travels over the bot in a semi circle, or an iron awe-styled lifter?

A: The 'lifter platform' is the part of the lifter arm that can effectively be inserted underneath the opponent. The term has nothing to do with the mechanism that powers the lifter or the lifter layout. A large lifter platform - like that used on 'Vlad the Impaler' - can be inserted far underneath an opponent before lifting and is very effective at breaking traction.

Q: Finally, do you know of any ant weight lifters? not just for sumo bots, but for combat robots too. Thanks, New York

A: Try this search to find antweight lifters at the Builders Database.

Q: What mechanism would you use for the flipper? Many bots cannot self right, but of course I would like to have the lightest possible solution as drive is also important. Thanks, New York

A: Let's keep our terminology straight:

  • Flipper: sends opponent flying thru the air. Usually pneumatic, not electric.
  • Lifter: raises part or all of the opponent off the arena floor. May tip opponent over. No flying thru the air.
Keep the lifter design simple. A pair of high-power servos actuating a lifter via short arms for best torque would be effective at breaking your opponent's traction.
Q: Is there a way to beat an undercutter?

A: There must be -- people do it all the time. Speed and a low scoop works well vs. an undercutter, and ramming 'bricks' can get the job done as well.

Q: And do you need 4 motors for four wheels? I think I'm going to use an undercutter and a spinning blade, but I don't believe that two wheels will balance the whole bot. Thanks!

A: You can use a chain (or belt) drive to power a second (or third) set of wheels with a single gearmotor. See the design on Nyx for an example of a chain-driven 4-wheeler. Four motors tends to be simpler.

Most undercutters are 2-wheel drive -- you need to get the wheels out of the way to let that big blade spin! The front of the robot slides along on the rounded end of the blade hub.

Q: Hey, Aaron, its the spinning undercutter and spinning hammer-thing girl. I was wondering, which one would be better? 1 spinning hammer? Or an undercutter?

A: Spinning hammer? You didn't tell me about your spinning hammer idea, so I can't really comment on it. Undercutters are popular and proven, with plenty of examples to learn from.

Whichever you pick, remember that a robot is much more than just a weapon. The craziest weapon is of no use if the rest of the robot cannot reliably wield that weapon. The chassis, motors, hubs, batteries, radio, and armor must all function together with the weapon and driver to make a successful combat robot.

Q: I have seen some wedges with a design that allows them to flip the opponent by driving into them. How could i implement this?

A: I can't figure out what design you're talking about. Can you offer a specific example, or point me to a video?

Q: I think the q about the flipping weapon with out flipping had to do with the angle of the front part, so the bot would effectivly drive vertically to the point it falls backwards. Just trying to help, but this q seemed interesting, so any thoughts on how you would do this/is it smart/what material would you use? [New York]

A: We've discussed wedges and scoops many times in this archive. I don't have anything specific to add.

Q: I have some weight left on my design, do you think it would be a worth while attempt to intergrate an electric hammer in the design to score agression points?

A: I can't comment on how the event you will compete at might judge aggression but, in general, aggression points aren't dependent on the type of weapon your robot carries. According to the RFL Judging Guidelines:

Scoring Aggression

  • Aggression scoring will be based on the relative amount of time each robot spends attacking the other.

  • Attacks do not have to be successful to count for aggression points, but a distinction will be made between chasing a fleeing opponent and randomly crashing around the arena.

  • Points will not be awarded for aggression if a robot is completely uncontrollable or unable to do more than turn in place, even if it is trying to attack.

  • Sitting still and waiting for your opponent to drive into your weapon does not count for aggression points, even if it is an amazingly destructive weapon.
A strict interpretation of those rules says that to gain maximum aggression points all you have to do is move continuously toward your opponent in an attempt to attack. That's it. It makes no difference what your weapon is as long as you're always on the attack. Some judges may tip the odd aggresion point to a bot that is more effective at pressing the attack 'in a clinch' as your hammer might be able to do, but I don't think you can count on it.

You might be better off to spend that extra weight allowance on better armor to prevent your opponent from inflicting even cosmetic damage rather than adding a weak weapon that won't get any damage points for you. Have a look at 'You be the Judge' and weigh your options carefully.

Q: The archived version of Derek Young's website is in shambles. So I was wondering if you had any clue as to how the robot 'Complete Control' was able to get so much torque and power out of its motor-driven lifting forks? And Do you think this weapon type could have any success in a heavyweight competition?

A: Second question first -- I think the current high-energy heavyweight spinners would rip away a clamping weapon like gift wrap on Christmas morning.

First question last -- the lifting aparatus on 'Complete Control' used both pneumatic and electric components. A pneumatic cylinder provides power to the clamping arm, and dual gearmotors have their combined torque multiplied by a chain-drive reduction system to power the lifting platform. You can see the large chain sprocket near the lifter pivot point. A chain drive is very effective as an add-on last stage to increase torque.

Date marker: May 2012
Q: I thought of an idea for a spinning bar. The bar would be in an X shape so it is more balanced and will hit more. Any reason not to?

A: Yes, a few reasons:

  • A straight single bar is entirely stable in rotation and requires no crossbar for balance;

  • Twin bars weighing the same as a single bar would be less strong than the single bar but would still need to survive the same impact stress; and most importantly,

  • You don't want a spinning weapon that hits 'more' for a bunch of little taps. What you want is a spinning weapon that hits 'once' and expends all of its stored energy in a massive impact that sends your opponent flying.
In a rotating weapon, two impact points are better than four -- and one is better still. Search thru the many posts about 'bite' in this archive for an explanation.

Q: In [the post above] you said one impact point is better than two. How could I implement this and still keep it balanced when the bar spins?

A: Previously discussed in this archive. Shorten one end of the bar about 1/4" and affix a counterweight near that end to restore balance. See section 6.3.2. of the RioBotz Combat Tutorial for an illustration. Read the rest of chapter 6 while you're there.

Q: Why did 'Silverback' use a slow hydraulic lifter when it could use a fast flipper?

A: Silverback did not use a hydraulic lifter - it used electric linear actuators. Electric actuators are simple to implement, reliable, easier to service, and more compact than a pneumatic flipper system. Electric actuators are slow, but a slow lifter can still be very effective.

Q: Aaron, what are your thoughts on Linear Servos? I just saw that other post. Are they good for an ant weight flipper? Do they make them that small? How do the compare to other competing products?

A: Linear servos are REALLY SLOW. They are WAY TOO SLOW for a flipper in any weight class. As mentioned in the above post, they are simple to install and easy to maintain in combat conditions -- no pressureized gasses to deal with or heavy hydraulic systems to service. They come in all sizes and can put out a fair amount of force, but did I mention that they are REALLY REALLY SLOW?

Q: Hello Aaron,
Im currently designing my first combat robot. Im planning to use a pneumatics system to "clamp" down on my enemy. Ive figured out the mechanics on what I need to make the arms clamp. My predicament is I dont know what parts I need to purchase to make the whole process of the piston (possibly the official name is the acutator) extending work. I read the DaVinci guide to pneumatics and that helped me understand how the process work, but im not exactly sure what I newd exactly to buy. Could you possibly give me a list of parts I would need to purchase and where I could get them. Thank you so much,
- Andrew

A: I'm sorry Andrew, but I can't tell you what parts you need. I know far too little about your design to even guess at the the amount of force your actuator needs to produce or the length of travel needed to operate your weapon. I don't even know the weight class of the robot you're building. You could start by looking for robots with designs similar to what you have planned and asking their builders about the components they use. I can tell you that there are no 'off the shelf' pneumatic components available that are suitable for insect class robots, so I hope you're building something larger.

By the way, we don't recommend active weapon systems in a 'first combat robot'. You'll have plenty to worry about getting the radio, drive, battery system, motor controller, chassis, and armor correct without adding in a complex weapon system. Reconsider.

Q: How do slow hydraulics, like on crusher-type robots, work?

A: Previously discussed. Seach this archive for 'hydraulic system'.

Q: How do flippers with CO2 work?.I'm new so sorry for asking so simple a question.I would be appreciate if you can explain it with a picture. [China]

A: Team Da Vinci: Understanding Pneumatics.

Q: Are there any advantages for front hinged flippers? It seems to me they push their opponents around more than anything else.

A: A front hinged flipper will, when combined with a ramming charge, toss an opponent in a low arc up and away. This is very useful in an arena where the opponent can be thrown out over a low barrier for a instant win. Such arenas are the prevailing design in the UK, where front hinged flippers have been quite popular.

Q: Do front hinged flippers have any advantage over back hinged flippers?

A: The two designs have different applications. Relative to the resting angle, a rear-hinge flipper will launch an opponent in a high and upward arc, and a front hinge flipper will launch the opponent in a lower arc to the front. Match the flipper design to the arena, your attack strategy, and the overall design of your robot.

Q: Why ziggy's flipper is so slow even it uses High pressure flipper? Are there is reason that ziggy's flipper is slow?

A: In what universe is Ziggy's flipper slow? In this dimension Ziggy's flipper is blink quick, crazy powerful, wicked effective, and recycles in a flash. The only time it's going to be slow is when it runs out of air.

The four-bar flipper mechanism on the top-ranked superheavyweight is very different from single-pivot flippers like 'Toro': it traverses a greater distance, and the acceleration of the opponent is in a more effective and controlled arc. There is also less 'self-flip' reaction due to the improved thrust path.

Q: sorry if it's answered, But how i can make Flipper Release all used air at once (Like toro or ziggy)

A: High pressure flippers commonly have separate high-flow valves for fill and exhaust on both extend and retract sides of the cylinder. With separate valves you can dump cylinder pressure as soon as the cylinder is fully extended.

Q: Are axes any better than hammers? Does it really matter? Can you tell me the pros and cons of each?

A: An axe or pick weapon has a chance to penetrate top armor by concentrating the impact force in a small area. The odds of actually hitting a critical component with a penetration is small, but the judges do like to see holes in your opponent.

The down side of a penetrating weapon is that it can (and fairly often does) get stuck in the gash, leaving you vulnerable. I have seen broad, shallow angle spikes that are designed to avoid deep penetration and the danger of getting stuck, but this also reduces the chance of a penetrating hit to a vulnerable target under the armor.

A good hammer blow makes a lot of noise, shakes your opponent's entire structure, and has a much lower chance of getting stuck. I like hammers in preference to axes or picks.

Q: How does [the weapon in] Inertia Labs' robot Butcher work?

A: The only information I have is from Inertia Lab's archived description of their pneumatic pulse motor superheavyweight robot 'The Butcher'. The complex robot had only two fights: 1 win, 1 loss.

Q: How would i go about making a jaw sort of mechanism? One that starts at the top then can clamp downwards, thanks!

A: Multiple designs may be used, depending on the size of the robot and the force required at the jaw. Chris Hannold's Combat Robot Weapons devotes some space to high-power clamping/crushing jaws. The diagram at right shows one very basic design which can use hydraulic, pneumatic, or electric linear actuator power. Insect class robots can use servos for moderate clamping force.

Q: Hi Aaron. Is Megabyte's shell belt driven or gear driven?

A: Megabyte's shell is belt driven by dual V-belt pulleys at a 4:1 reduction. If you overload a belt drive, it slips. If you overload a gear drive, it breaks.

Q: I am not expecting a definite answer for this, but could you give me an idea of the necessary speed for a successful full body spinner?

A: While you can spin a weapon either too slowly or too fast, speed itself isn't the critical factor for a spinner weapon of any type. Much more important are the amount of kinetic energy the weapon stores and the time it takes the weapon to spin up to that energy level. Yes, a given weapon will store more energy as it spins faster, but a weapon shell with a high moment of inertia will be much more effective at any given speed than a weapon shell with a low moment of inertia at the same speed.

An effective FBS weapon should have a minimum of 20 joules of stored energy per pound of the weight class it competes in. Some mega-spinners have more than 10 times that much energy! The weapon should be able to spin up to at least 10 joules per pound before an opponent can cross the arena and attempt to stop the weapon. There is plenty of help in evaluating the energy storage capacity of a spinner weapon in this archive.

Q: I am working on a full body spinner, I am planning on using wedges with a low ground clearance to flip the robots over. Do you have any thoughts?

A: Mark J. here: I like the simplicity of the approach -- it's very appealing to visualize the effect of such a weapon. However, a quick force vector analysis of the weapon indicates more push-back than lift. The drag created by the wedge as it accelerates the opponent upward will impart a LOT of lateral force. You're likely going to do more damage from impact than have success as a flipper, but that's not a bad outcome.

Spinner-flipper designs have been tried. Superheavyweight 'Phere' had a wedge-nose that led opponents up to a rotating body with a corkscrew lifting wedge. The robot had some success: 4 wins and 3 losses.

Reply: I'm the robot who had the question about a full body spinner with wedges. From what you said the design seems feasible, so I'll try it. Thanks for your input.

Response: Best luck. If you're not happy with the wedges you can always swap them out for blunt impactors.

Q: how do you make your wedge razor sharp? can i use sanding paper?

A: You didn't mention what your wedge is made of or how thick it is. For a thin wedge, you can rough shape the edge with a hand file and switch to a sanding block (sandpaper glued to a wood block) to finish the edge so that it is both sharp AND perfectly flat to the arena floor.

Note: in many arenas a sharp low wedge will catch on irregular floor seams and be far more trouble than it is worth. Check with competitors familliar with the specific arena before you go 'too low'. If it's an unknown or new arena, be prepared to adjust and 'unsharpen' your wedge on-site.

Q: Just wanted to ask, do you need a locking pin for a hinged wedge like Original Sin?

A: RFL rules state that a locking device is required for any 'moving' weapon "that can cause damage or injury". If a hinged wedge can pivot and potentially pinch or crush hands and fingers, it does need a locking device. All powered weapons require a locking device; moving passive weapons are a judgement call by the event organizer. A locking device can make transport both easier and safer, so I'd consider locking any hinged weapon in the heavier weight classes.

Q: How does Warrior SKF Work?

A: Previously discussed -- search this archive for "Warrior SKF" and for "dog clutch". See also Dale Hetherington's Flip-O-Matic for a details on construction of a flywheel flipper weapon.

Q: Hi Aaron, i found this hub in robot marketplace. can i use this for the hub of my [large steel hobbyweight] blade? my family have a weld, and both of the pulley and the timing belt is from a car spare parts.

A: No!!! Those hubs are about 3/4" in diameter and made to attach a 4mm gearmotor shaft to antweight wheels -- not a large 1000 joule blade to a weapon shaft. You haven't mentioned how large a weapon shaft you plan to use, but I hope it's closer to 12 mm than 4 mm. The design of this hub also places all the rotational load as a shear force on the screws -- undesireable for a weapon hub. Also note that your welder won't help here: these hubs are aluminum and you can't weld aluminum to steel.

If you skimp on the hub your blade will break free on impact and fly across the arena, which is VERY embarassing. How was your large pulley held on to the shaft that it drove on the engine? If you can duplicate that fastening for your weapon shaft you may be able to let the pulley do double-duty as both pulley and hub by bolting the weapon blade directly to the pulley.

Q: What are some of the advantages of a "Rotary Flipper" like that on Thrasher and Omega Force?

A: Rotary or flywheel flippers like Omega Force and Thrasher are extremely difficult to design and construct. They are typically built by very advanced builders who just want to show off their skills with an unusual type of weapon. Very few examples of such weapons exist and I cannot recommend that you attempt to construct one.

Q: What are some basic components to buy for a pneumatic spike, and where should I buy them?

A: Pneumatic weapon components are VERY well covered in this archive. Search here, and be sure to read the Team DaVinci Pneumatics page.

Q: After the reduction stage, how do i bolt the blade to the shaft which is connected to the second pulley? The diagrams in the archives doesn't help me at all.

After reading Hazard's build report, i noticed that you need a flange to bolt the blade, is there any off-shelf parts for the flange or should i use Emachineshop?

A: Yes, you need a hub to connect the shaft to a weapon blade. This hub will be exposed to a LOT of force on weapon impact, so don't try to get by with some weak cast metal hardware store flange. Weapon hubs are usually custom made.

Q: Hi Aaron, how do you put a horizontal blade after going through the reduction stage? i meant like, how do i place the shaft between the blade, the pulleys, and the bearings? and what bearing should i use?

A: Many possible layouts, depending on your design. There is a diagram of the basic layout in this archive -- search for "central shaft". The position of the blade and pulley can be reversed for a center-blade weapon like 'Fiasco', or the whole structure can be inverted for an undercutter design. Search builder websites for examples. Standard ball or roller bearings are typically used.

Note: usually the belt drive IS the reduction stage.

Q: Hi Aaron, this will be my first Horizontal spinner(actually my first combat robot). i just wan't to ask, i'll be using the BaneBots RS-550 motor paired with the P60 4:1 Gearbox. my blade will be 50cm X 3cm X 1cm. and I'll be making a hobbyweight. i have 2 questions:
  1. According to these infos, what ESC should i use(please no victors, i'm striving for program and wiring easiness, and probably i'm using a Sabertooth 12)
  2. Is the length, thickness, and the width of the blade enough for my bot?
my bot's size will be: 30 cm X 15 cm X 2 cm and i'll be using a dustpan with the blade on top.(kinda like hazard mixed with that darn dustpan!)

A: Good news first -- the size of your steel blade is good for a hobbyweight and the weapon as described will top out at more than 2500 joules, which is killer for a hobbyweight. That is the end of the good news, bad news follows.

  • That weapon is going to take a LONG time to spin up -- more than 6 seconds to get to 3000 RPM and better than 15 seconds to full speed. That won't do in a small arena.

  • The blade speed (4500+ RPM) is too great to allow the blade to effectively 'bite' into your opponent and inflict significant damage. A blade passes by every 6 milliseconds, which does not give much time to insert a piece of your opponent onto the damage radius.

  • The combined length of the RS-550 motor and 4:1 P60 gearbox is going to set your blade hub at least 4" off the arena floor. Are your opponents tall enough to be hit by such a weapon?

  • In spinning up from a standstill, the RS-550 motor will pull more than 40 amps for the first five seconds. That's way too much for way too long for the Sabertooth 12 ESC to handle. In the best case the ESC will cut amperage back and drastically slow the already too long spin-up time. In the worst case the ESC lets out a big puff of smoke and dies.
Suggestions: you need greater gear reduction for that motor/blade combination. An 8:1 reduction will give a peak energy of more than 700 joules (still good for a hobbyweight), a much more effective blade speed, a greatly improved spin-up time (300+ joules in 1.5 seconds), and reduced current load on your ESC. I don't know of a suitable 8:1 gearbox, but most spinners are belt-driven rather than gearbox-driven because belt drives generally hold up better to the high shock load at weapon impact. A belt drive would also reduce the blade height of your weapon.

I don't know what you have against Victor ESCs. They require no programming, have very simple wireing, and are even easier to set-up than the Sabertooth. The 'Victor 884' is both cost-effective and a good match for your weapon.

Of course, you can also use a solenoid to simply switch your brushed weapon motor on/off. The 120 Series White-Rodgers Solenoid is inexpensive and has more than enough capacity for your weapon.

About now you're starting to understand why I don't recommend active weapon systems for first-time builders.

Q: Hi Aaron, what brushless motors do you recomend for a weapon which is 30cm long X 2cm wide X with a thickness of 0.4 mm between two of these:
  • Axi 2826/08 Brushless Outrunner

  • EFL-25-870 Power 25 Brushless Outrunner Motor? [Indonesia]

A: See #29. Additionally:

  • You haven't told me what the weapon blade is made of, so I can't calculate the weapon mass and rotational inertia for the blade.

  • Is this for a hobbyweight? Assuming that the blade is made from steel, the blade weighs only 20 grams and would store only 9 joules of energy at 3000 RPM -- too little for even an effective antweight weapon!

  • A blade that thin will not hold up well to high-energy impact. Go thicker -- much thicker.

Q: The weapon will be either made from chromoly, mild steel or 6061-T6, but probably mild steel because it's the most available metal here.

Yup, this is for a hobbyweight.

How thick is thick? Is 1cm thick enough?

A: See #17. We have a great deal of information about the design of spinning weapons in this archive, and we have a versatile Excel spreadsheet tool to evaluate spinner weapon design. Make use of them.

The whole idea of a spinning weapon is to store a lot of kinetic energy in a heavy rotating mass and then unleash that energy on your opponent. A typical spinner devotes about 30% of the weight of the robot to the weapon system. You're going to need a WHOLE LOT more than a 20 gram blade for a hobbyweight.

Q: Hi Aaron, what's the size of the spinning bars in Run Away? And what it is made of? I want to use the same bar design as Run Aways bar for my horizontal bar.

A: 'Run Away' had twin 30" by 2.5" by .375" mild steel bars rotating at 1200 RPM. I should mention that the primary purpose of the weapon was to look good on TV, not to do a great deal of damage. Total energy was under 3000 joules -- about half the energy in effective heavyweight spinning weapons of the period.

Q: Hi Aaron. I just saw the match between 'Son of Whyachi' Vs 'Bio-hazard'. How could SoW possibly rip off Bio-Hazard's armor so easily? Comparing this with the match between 'Sewer Snake' vs 'Last Rites', Sewer Snake absorbs most hits without being ripped off. What was the difference here?

A: 'Son of Whyachi' outweighed 'BioHazard' by 100 pounds. Its weapon was eight feet across and powered by two enormous 15+ horsepower motors! It was a legal heavyweight only because it was technically a shufflebot 'walker' and was given additional weight allowance. They changed the rules before the next event to take the weight allowance away from shufflebots -- that wasn't what they had in mind for a 'walker'.

Q: Dear Aaron, compared to 'Son of Whyachi', how powerful are 'Last Rights', 'Megabyte', and 'Touro Maximus'? I saw the question above and I was just wondering how they stack up.

A: Mark J. here: I don't have enough information about weapon mass and speed on the 'bots you list to directly calculate their energy storage capacity. Comparison is further complicated because there were multiple versions of SoW with different motor configurations -- both electric and internal combustion.

Elsewhere in this archive you can find a post where I estimate the energy stored in Megabyte's shell at about 50,000 joules -- likely greater than the effective weapon energy for either 'Last Rites' or 'Toro Maximus'.

I've seen estimates of more than 100,000 joules for the version of 'Son of Whyachi' that won the BattleBots championship, but equally important is the ability to deliver that energy to an opponent. SoW's very large weapon diameter and relatively low spin speed allows for greater 'bite' and a more effective hit. Terrifying!

Q: What geared motor would you recommend for a antweight spinner weapon? And where can I find a 15mm weapon hub?

A: There aren't many small gearmotors with suitable power and gearing to make a decent spinner weapon. That's part of why most spinners use a belt drive reduction. Belt drives are also better able to put up with impact stresses. Without the details of your weapon design I can't make a specific recommendation, but you might consider the 'Beetle B04' Gearmotor [no longer available].

I get tired of answering "Where can I find..." questions with "Robot Marketplace" but it's almost always true: the VDD 15 mm weapon hub [no longer availble]. You'll need to drill the 0.126" shaft hole out to 4 mm to use it with the B04 gearmotor.

Q: The 15mm blade hub says discontinued.

A: The Team Think Tank products have been getting scarce. It looks like most all of their stuff is sold out and they aren't making any more.

I don't have another source for 'off-the-shelf' weapon hubs. You might ask around the on-line forums to see if anyone has a spare. Team Whyachi will machine a hub to your specs -- for a price.

Q: Hi Aaron. How to attach a spinning bar to the driveshaft? I mean can we weld it directly to the shaft? I am building a middleweight undercutter with spinning bar weight equal to 10kgs approx. [Prague]

A: Mark J. here: there is a great deal of stress on the union between a weapon bar and the driveshaft. A simple weld junction places all that stress on a very small area with a high risk of structural failure. A machined hub that bolts onto the blade and is fixed to the shaft with a keyway spreads out the stress loading and is the preferred method of fastening a rotating weapon to the shaft.


How about just a really tall robot that drops an Anvil type thing (looney tunes style) from a massive height to go for a one hit KO? Or an equally tall robot that just has a bunch of conveyer belt platforms to try to grab on and lift other robots to massive heights and drop them?

Has this type of thing been tried and is there video? I know these aren't realistic ideas for title-contenders, but they present some interesting engineering challenges in form of lightweight structuring and balance and would fun to see for the fly-wheel/clutch type guys who like doing things differently.

A: No, it hasn't been tried -- and for good reasons, Here are a few:

  • Combat arenas have ceilings that limit the height of your 'really tall' weapon.
  • Really tall robot with significant weight at top = top-heavy robot soon to be on its side.
  • Do the math and you'll see that a good spinner or pneumatic hammer has MANY TIMES the energy of a gravity weapon.
  • An object falling from a height of 9 feet takes three-quarters of a second to hit; your opponent isn't going to be where you were aiming when it does hit.
  • Any robot that can't survive having its opponent dropped on it from the top of the arena is a joke.
  • Ditto any robot that can't itself survive a drop from the top of the arena.
Take a trip to a live competition and watch the speed and violence of robot combat. The experience will wipe from your imagination 99% of the whacky weapon ideas you may have.

Q: Actually a super tall robot to drop things on other bots has been tried, Tower Of Power from season three of Battle bots had a guillotine like weapon.

It jumped off the Brooklyn bridge and last time I heard it owns a Lama Ranch in Saudi Arabia and has two girls a boy and an electric fan **

A: Sorry, but no. The six-foot tall middleweight 'Tower of Power' did compete at BattleBots 3.0, but its weapon wasn't a guillotine -- it was an extra-high lifter. Whatever it hoped to accomplish, it didn't.

** See #32

Q: is a sheet of aluminium enough for [the weapon bar on] a hobby weight horizontal spinner (Hazard-style)? especially when knowing that some of the contestants in my competition uses plastic as their armor.

A: A 'Hazard' style bar spinner stores energy in the mass of the spinning bar. If your 'bar' is just a thin strip of soft sheet aluminum it will:

  1. store very little energy;
  2. flex too much to impart much damage to your opponent; and
  3. do more damage to itself than any armor it hits.
There is a lot more to a successful spinner weapon than bolting a bar to a motor. Read through this archive for tips on proper design.
Q: Hi Aaron!! We were wondering about the design of flywheel flippers. Almost all of us know that the main drawback of such a flipper is it's clutch mechanism that has to transmit high torque, have low weight and it should [MUST!] withstand huge stresses. But can a fluid coupling/torque converter serve an alternative to this problem? Thanks.

A: Mark J. here: I understand the interest that combat robot builders have in unusual designs, but creative builders have been thinking about design parameters for almost two decades. If a design hasn't found acceptance by now it means that there are very real obstacles in the way of successful implementation.

A flywheel flipper needs a coupling that slips freely at high speed and can then be induced to stop slipping. A fluid coupling does not do this; it slips at low speeds and becomes more efficient at transmitting power as the input speed increases. This behavior makes it useless at abruptly transmitting power from a fast-spinning flywheel to a stationary load. The high drag on the rotating flywheel would continuously dissipate power and severely restrict energy storage. Use pneumatics.

Q: I know you're probably sick of questions about electric hammers and flippers, but I didn't see this one in the archive: could a linear motor working on the same principle as a railgun or coilgun be used to power an electric hammer or flipper?

Obviously a linear motor with comparable performance to high-pressure pneumatics would draw far more current than any batteries could supply... but what about if a capacitor bank was charged off the batteries over the course of a couple seconds (its function would be similar to a buffer tank in a pneumatic weapon)? I know that theoretically the minimum energy to raise a 100 kg opponent 3 meters in the air is about 3000 joules... even at 30% efficiency the weapon would reach 10,000 joules per shot...

How would this kind of weapon compare to a pneumatic flipper or hammer in terms of power to weight ratio?

A: Mark J. here: I claim no specific expertise in railgun physics, but I was quickly able to determine that real-world railguns have ridiculously low efficiency -- on the order of 0.1%. That takes your output power estimate down to about 35 joules. Use pneumatics.

Q: How do articulate a lifting spike like Vladiator? The only image I could find offers [no] answers.

A: Vladiator's spike has a perpendicular crossmember welded (a guess) in place. The crossmember is supported by bushings on the chassis for articulation.

'Vladiator' uses an... erm... unusual high-pressure air system with twin pneumatic clyinders and a steel cable that wraps around a pulley to pull up on the lifting spike for actuation. Similar weapons, such as the "up-ender" on 'Juggerbot 3', attach the pneumatic actuator to the weapon with a hinged mechanical link that allows pulling and pushing to raise and lower the lifter.

Q: hi Aaron, whats the better design for a flywheel? a ring like Hypno-Disc, or a bar like Last Rites?

A: That depends on what you mean by 'better'. For equal diameter, speed, and mass a hollow cylinder stores more energy than a disk, and a disk stores more energy than a bar. However, a thin-walled cylinder is more fragile than a bar and more difficult to construct and balance. Like many design elements in combat robotics, there are trade-offs.

This archive has a great deal of information on spinner design, and the Team Run Amok Excel Spinner Spreadsheet allows you to compare the energy storage capacity of flywheels with differing shapes, sizes, and mass.

For a more complete explanation of the physics of spinner weapons see Paul Hills' Spinning Disk Weapon page

Q: Hi Aaron, my 3" drum weapon featherweight robot didn't make it past the group stage of our local tournament but i was satisfied with the drum performance. The drum did an awesome job but it has a weak link which is the connection between the drive pulley & the deWalt shaft. Twice in the tournament, the connection of the drive pulley became loose due to heavy stall. FYI, i mount the pulley to the motor shaft with 2 M3 set screw (1 on top of shaft, 1 on the bottom...each set screw grip 1.5mm tapped hole on the shaft). Can you suggest the best possible way to strongly mount a pulley to the DeWalt shaft? Or as you suggested previosly, using a chain drive might be a suitable choice...can you provide further explanation on this? Thanks!

A: Mark J. here: wait a second... you only asked us which weapon motor to use 5 days ago, and you've already built your robot and had the competition?

Repeat after me: "Set Screws Suck!" This is an old adage in robot combat, and you've learned exactly why builders hate set screws. We've talked about this many times. Look up 'set screws suck' in the archive for a discussion and alternative methods.

If your set screws are simply loosening, you may gain some reliability by liberal use of a threadlocking compound like Loctite. That isn't the best fix, but you may not have the resources to implement another of the suggested solutions.

Switching to a chain drive won't help your situation -- you're just replacing one hub with another. The problem of fixing a hub to your shaft still exists, and the loading on a chain sprocket is higher than the loading on a belt pulley since the chain does not slip.

All of the questions you have asked and all of the problems you have experienced have been covered in the Ask Aaron archives. Do yourself a favor and take the time to read the archives before you procede with your robot career.

Q: Dear Aaron, how powerful are the jaws on Diesector (the latest one)? Are they like Razor in that they can crush or are the jaws just grippers? Also how do the hammers work and are they effective? (I am referring to the 2002 Diesector). Thank you.

A: DieSector's jaws were grippers - not crushers. The side hammers were electric, simply attached to NPC gearmotors. They caused little damage but were quite effective in demonstrating aggression when the 'bot was gripping its opponent or had them wedged into a corner. You don't have to crush your opponent to score points.

Q: Hi Aaron, after i watched Last Rites battle videos on Youtube, i saw that when faced with rammers, it's blades just stopped and doesn't spin anymore. how do i made my robot's spinner doesn't stop like that?

A: Mark J. here: we've discussed 'Last Rites' before. Please search the archives before asking a question.

Builder Ray Billings chose a chain drive to handle the very high torque loading between the motor and weapon bar in order to assure a quick, no-slippage weapon spin-up time. Running a chain drive transmits a great deal of shock back thru the weapon drive to the motor and makes it more likely that the drive will fail. Ray is willing to put up with some failures to the weapon in order to increase the power and efficiency of the weapon system -- it's a trade-off.

You can avoid this type of failure by running a 'slippable' belt drive to the weapon, but the slippage will degrade weapon performance.

Q: Hi Aaron, i'm trying to use your Excel Spinner Spreadsheet and when i fill the data for an Ampflow E-150 motor, i stuck on the stall torque value. By referring to this:, there is a peak torque data but [no] stall torque data. Is it similar? Please guide me on this. Thanks!

A: For a permanent magnet DC electric motor (like an Ampflow) peak torque comes at stall: peak torque = stall torque. The Ampflow E-150 really isn't much of a spinner motor -- far too heavy for the power it puts out.

Q: Hi Aaron, based on the calculation using your spinner worksheet i have managed to calculate the right balance for my spinner weapon using the Dewalt 18V Old Style Drill Motor. Before i proceed to use this motor, do you have any suggestion and any tips in using this motor for spinner weapon actuator? Do you know any other combat robot that use this motor to drive their spinner weapon and what is the result?

A: The DeWalt 18 volt drill motor is a veteran of countless robot applications. It's light, very powerful, and puts up with plenty of abuse. If your spinner isn't a success you can't blame the motor. Brushless motors have become more popular than brushed motors (like the DeWalt) for spinner drives, but I think the DeWalt can still be a good spinner motor at a great price. Offhand, I cannot think of a current robot spinner using a DeWalt motor -- but don't let that stop you!

You haven't told me much about your robot (weight class, mass of spinner, spinner style, spinner drive ratio, whether you will be using the DeWalt transmission or just the motor) so it's difficult to make specific comments. Some general comments:

  • DeWalt motors provide more power when spinning counter-clockwise, so design for that direction.
  • The motor can be 'safely' run at 24 volts for a 1/3 increase in both torque and RPM.
  • You may have some difficulty finding a belt drive suitable for both the high speed and high power this motor produces. A chain drive might be a suitable choice.
  • The gear on the DeWalt shaft is difficult to remove. A good quality gear-puller will make short work of it, but attempting to pry it off with a screwdriver is NOT recommended!
  • The motor shaft is short (7mm) -- allow for that in your design.
Dale's Homemade Robots has additional information on the DeWalt motors in robots.

Q: Hi Aaron, thanks for you previous reply. FYI, i'm building a featherweight battlebot with a 3" drum weapon (the drum should be larger but i want to build a low profile robot). The mass of the drum is approximately 3kg. The drive ratio for the weapon is 4.8:1 and i'm using belt & pulley for the power transmission. Based on the spinner spreadsheet calculation, i realized that i should use a larger drive ratio, but the options available for the pulley sizes are limited. A bigger ratio would cause the driven pulley to be larger than the diameter of my drum weapon. The 18V DeWalt motor will be run at 24v, controlled by Syren 25Amp ESC. The weapon unit is in mounting stages and will be tested tonight. I would like to know your prediction on how my weapon would work and later i will let you know the actual result. I hope its all well...

A: Mark J. here: the hardware sounds like a reasonable featherweight weapon. A larger, slower spinning drum of the same mass would be more effective, but you have to balance your design elements as best you can. At 5000+ RPM you'll have difficulty getting a good 'bite' on your opponent -- the weapon may 'skitter' across without digging in and doing heavy damage, but it could still be impressive.

Problem: the 25 amp Syren ESC is WAY too light for that DeWalt motor. That ESC can handle a maximum 45 amps for a couple seconds, but the DeWalt stall current is more than five times that great - and the motor continues to draw more than 45 amps well past 20,000 RPM. That's going to kick in current limiting immediately, which will greatly reduce torque and seriously slow your spin-up. You'll likely spend a good part of each match with the weapon completely shut-down waiting for the ESC to cool. Worst case you'll fry the weapon ESC in the first match. A big weapon motor does more harm than good if you don't have enough ESC capacity to support it properly.

Set up your belt drive loose to allow for plenty of slippage to avoid getting anywhere close to stalling the motor -- although this will further slow your spin-up time. I strongly recommend upgrading to the larger Sysren 50A or IFI VEX Pro Victor SC ESC. It's cheaper to upgrade than to replace an inadequate ESC multiple times.

Q: How should I mount a spinning full body shell on a beetle weigh robot?

A: A FBS shell takes a lot of abuse and must be very securely mounted to survive. Seach this archive for 'hobbyweight shell spinner' for a diagram and text.

Q: The full body shell would be a frying pan. How should I mount it directly onto the motor?

A: You'd need a hub to connect the shaft to the pan, but mounting the spinner shell directly to the motor is a really bad idea. Structurally inadequate, too much speed, too little torque. There is MUCH more to building a spinner than bolting some round object to a motor shaft. You'll also have real trouble centering and balancing that frying pan to spin at a few thousand RPM without shaking the robot apart.

Read thru this archive for tips on how to properly design and construct a full body spinner.

Q: Dear Aaron, I was reading up on fly wheel flippers, and you said that pneumatics are better. I agree with this, as it is pointless to make it more complicated than it needs to be. However, this has been done by Warrior SKF. Warrior used the fly wheel as a weapon. My questions are these:
1) I watched the videos of Warrior SKF, and it beat megabyte, which is as you know a top ranked full body spinner. However, how affective is this system of using the fly wheel as a weapon and have it power a flipper? How much damage does that wheel do? Finally, how powerful is the flipper and could it eject a a robot from the Combots arena and/or the Roaming Robots arena?

Also, I had was making concepts for a bot and I thought of this. What if you had a fly wheel or two smaller fly wheels as vertical spinners like nightmare or 259, and had it power a flipper. It would use two high speed, low torc motors attached to two wheels. The Idea is that when if it was fighting Roaming Robots bots like Iron Awe 5, the fly wheels could do damage and potentially make it fly out of the arena like a drum bot like Fluffy De Large. Since the UK bots have low armor, then it could do damage to the armor. I also was reading about the way that the UK guys deal with spinners. If it fought a bot with a deflector (which bot do you mean like?) I would use the flipper.

What are your thoughts on effectiveness, possibility, practicality and if it is realistic? Any other thoughts?

Also, does Roaming Robots allow spinners? Thank you.

A: My advice is to forget about flywheel flippers. As previously pointed out, 'Warrior SKF' was built by a VERY well financed team that enjoys building oddball designs ('Y-Pout', 'Red Square'). They have professional machinists and a deluxe machine shop to support these experiments. It is the only flywheel flipper -- if the design was practical and effective you'd see many more teams building them.

The design suffers from the same problem associated with any 'dual weapon' robot: neither weapon gets a full weight allowance and the performance of both weapons is reduced. The team does a very good job with the 'bot, but that speaks more to the experience and expertise of the team than the design concept.

'Warrior SKF' did once defeat 'Megabyte' (and twice lost). 'SJ's pneumatic hammer has defeated 'Megabyte' four times, but that does not mean that pneumatic hammers are a superior design. I figure that design accounts for about 20% of the effectiveness of a combat robot. The other 80% comes from construction, materials, and operation. Build something simple, build it well, and learn how to drive it.

Roaming Robots does allow spinners but, as mentioned previously, they have not proven effective in that style of competition. Many UK 'bots have bolt-on deflector scoops available if needed. You don't often see them in use, but if a spinner shows up the scoops come out.

Q: Dear Aaron, thank you very much for the advice on the flywheel. I was wondering if you elaborate on the response to these specific questions regarding the earlier q about Warrior SKF. How much damage does that fly wheel do? I know it is less but how much less? Similarly, could the flipper eject a robot from the Roaming Robots arena, or is it too weak?

Additionally, what weight category is Warrior SKF, what team built it, and if you had to guess, how much would it cost?

Thank you very much!!!

A: You're welcome, but I see you haven't taken my advice to 'forget about flywheel flippers'.

I don't have data on the size, speed, and mass of the flywheel 'Warrior SKF' uses to store energy, so I can't give a quantitative answer to how powerful the spinner and flipper are. Based on video of 'Warrior SKF' I can only say that the flipping power is not nearly comparable to the specialized high-pressure UK ejectors.

A web search for 'Warrior SKF' will very quickly lead you to the team responsible for this heavyweight robot. Don't ask me to be your search monkey. I estimate the cost of design and construction of this very exotic robot in the tens of thousands of dollars.

Q: Dear Aaron im almost done with my beetle saw bot for motorama 2012 and i need your advice some how my heavier saw blade seems to be shaking my robot apart when i fire it up and i don't know how to properly center it. p.s i have tried with a drill press and a not so steady eye. Thanks for your help.....from anthony... team warpz robotics

A: You haven't told me enough about your saw weapon for me to provide much help, Anthony. I can tell you that a precision blade hub of the correct size is critical. You can't 'eyeball' this. Is there not already a perfectly centered hole in your saw blade? How are you mounting the blade to the shaft? What are you drilling with the drill press? How fast are you trying to spin the blade?

If you do get a well-centered weapon hub and the blade is still unbalanced (it shouldn't be), search this archive for "How do I balance the weapon?"

Q: Dear Aaron, what is the best angle to have a flipper/ wedge to get under the other opponent?

A: If your goal is to get 'under' your opponent, the lower the angle the better.

Q: I saw the flipper q and have another question. Why does a bot like Iron awe have a steep front lip if he wants to get under his opponent? If you could please elaborate, that woud be great.

A: 'Iron Awe' isn't trying to get 'under' the opponent -- it's trying to launch the opponent away and out of the UK-style arena. If you get far under your opponent you just launch them straight up. Watch some UK flipper videos.

Q: Dear Aaron, if I went to a uk competition with a powerfull spinner, wouldn't that be a good Idea if I can make it so that it bends the wedges of the flippers?

A: It has been tried, but the UK boys know how to deal with spinners in their competitions. Like I said, flippers are better suited to an arena where it's possible to eject an opponent for a win, and that's how they build arenas in the UK. You really can't compare the US and UK competitions.

Q: How do the 'UK boys deal with spinners?'

A: In a non-enclosed arena a spinner is already almost as dangerous to itself as to its opponent. Newtonian action-reaction is likely to send the spinner careening off into danger. Many UK teams developed add-on scoops to herd and re-direct a spinner, launching it directly or slowing it enough for a well-timed flip out of the arena.

Q: Aaron, I had an idea about a 1lb electric hammer. Maybee I can use a Fingertech 50:1 or higher to pull on a pulley like a blade. I heard about Anthony and his design and how his sportsman created about the same force as a beetle blade which I'll try as a backup.If it is possible how would I best construct a hammer?

Thanks .Critique anything you like

A: If you've been reading thru the archives you know that I do not recommend electric hammers. Do the math and you'll discover that the destructive energy an electric hammer puts out is very small compared to a spinner weapon of comparable weight. The hammer weapon in Antony's 30-pound sportsman has approximately the same impact force as a spinner in a 3-pound beetleweight! You can get away with a puny weapon in the sportsman class, but don't try it in open competition!

My best advice is to pick a different weapon.

Q: If I use the same 64:1 in a beetleweight, would it be possible?You did mention that DID have the same joules as a spinner. And, since most beetles don't focus on top plates, it would be possible to at least dent it.

A: Anthony's sportsman hammer has a 16 ounce head on an 18" long arm and is powered by an RS-550 motor (8 ounces) with a P60 64:1 gearbox (10 ounces). Total weapon weight is close to three pounds and the dimensions are too large for a beetle. When you scale down to a reasonable size for a 3-pound robot, your weapon power will drop proportionally. Add to that the problem in positioning and triggering the weapon at the right time (a spinner just has to run up and touch you) and you're fighting an uphill battle.

Electric hammers have been tried many times. If they were workable you'd see successful builders using them in open competition. My best advice is to pick a different weapon.

Mark J. here: A builder pointed out to me that sportsman electric hammerbot 'Mangi' has won in open competition. Aaron has previously noted that Mangi's builder could win matches with a half-empty tub of margarine. When you have 20 years of combat robot experience you can build what you like and do well. Until then, avoid the electric hammer.

Q: Hi Aaron. I was designing a middle weight flipper and wanted to know the relative advantages/disadvantages (if any) of the four bar flipper mechanism over rear hinge flipper. It would be a pneumatic actuated flipper and I am confused as to how to go about it.Can you also tell me as to how to decide the dimensions and calculate the lifting capacity of the pneumatic actuator for the flipper. Thanks.

A: Mark J. here: a pneumatic flipper is the single most complex robot weapon to design, build, and operate. It is also potentially the most dangerous to the builder. High pressure pneumatics are unforgiving. There's no way I can tell you how to 'go about it' in a few paragraphs, but I can point you to some resources to get you started.

  • Start by reading thru this archive. There are many posts here about 4-bar mechanisms and single pivot flippers.

  • Read every word of the Team DaVinci Pneumatics page, then go back and read it again.

  • The T.i. Combat Robotics 4-Bar Simulator is a tool to assist in designing 4-bar lifters with electric power, but the elements of the tool that model the path of the mechanism tip are useful for pneumatic powered flippers as well.
A correctly designed 4-bar mechanism has the advanatge of moving forward and upward, moving toward your opponent instead of pulling away from them as a rear-pivot flipper does. Very handy.

Flippers are all about speed and power, and the calculations are a nightmare. The usual design approach is to just cram the biggest components you can into your chassis and hope for the best. Big actuators, the highest flow-rate regulators, buffer tanks, and big valves are all critical. Not cheap, not safe, and not for inexperienced builders.

Q: Hi! I have a question about a virticle spinner.I'm using an R/C to control my spinner. So far, I'll be using the following: An A-40-300 motor(24 volts, 3.8 horsepower, 3840 oz-in Torque, 340 amps max current, 84 % efficiency, and 4000 rmp), 2 3/8 pitch Type B Sprockets - 11 teeth, 5/8 inch bore, #35 roller chain, a V-tail mixer, and a 24V 5.0ah NiMh BattlePack.

My questions are: 1 Is the mixer I chose good? If not, what kind do you recomend?( I use battlekit's drive module driving method with two driving A28-4000 motors). 2 What kind of R/C do you recommend? 3 Should I use an electric speed controller, witch kind do you reccomend?

A: OK, I really hope you're just pulling my chain. Given the level of questions you're asking, you're in WAY over your head building a robot with a A40-300 powered spinner.

Start reading thru the Ask Aaron Archives.

  • The weapon archive has tons of weapon design info.

  • The archive has all the radio and mixer details you need.

  • The archive will get you set up with speed controllers.
After you've gone thru the archives, rethink the size of your first robot project. Consider going smaller.
Q: How much damage does the smashing of the scoop of 'Breaker Box' or 'Shazbot' do? Thanks

A: Almost none. The electric scoop motors don't pack enough power to do damage -- that isn't their purpose.

Q: What do you think of the following idea?

A pnuematic piston is placed at the end of a hammer arm. The piston points towards one end of the arc, while the other end has a spike. When firing, the piston (which would be resting near the ground) would activate, pushing off the ground firing the hammer. Then, a winch would push it back into position.

The idea is that by having the force as far away from the fulcrum as possible, the piston would deliver more power to the hammer than if it delivered the force from the fulcrum. In addition, the piston would act as both a hammer weight and a power source, possibly making the system more weight-efficient.

Unless my understanding of the physics is off, the one issue I can see is protecting the piston from the impact.

A: Mark J. here: bonus points for creativity, but you are confusing 'force' with 'impulse'.

A pneumatic cylinder acting directly at the head of the hammer has greater mechanical advantage on the mass and will accelerate it more quickly -- but will act on that mass over a shorter time period and thru a smaller arc of motion compared to the same cylinder acting closer to the pivot. It works out that the theoretical 'impulse' (the product of force and time) is the same, and it is impulse that accelerates the hammer.

However, since the cylinder in your design extends very quickly, you run into friction and gas-flow inefficiencies that will reduce the real-world impulse compared to an equivalent cylinder extending more slowly and operating over the entire arc of the hammer. This yields less hammer acceleration.

Bottom line: less impulse power, additional complexity, and increased fragility. I don't think that the additional mass in the hammer head will offset these disadvantages.

Q: How well does a linear actuator attached to a spike or barb at the end work for a 15lb and 120 lb robot?

A: About as well as a pretzel with a wad of gum on the end. Try again.

Q: just out of interest so I know how to work around it, why does a linear actuator not work?

A: Linear actuators are slow -- really slow. Your opponent isn't going to sit still while you roll up, position your weapon, and very slowly push a spike at them. If by some miracle they did sit still:

  • from a side attack, the spike will make contact and slowly push the two 'bots apart;

  • from a bottom attack, the spike will lift your opponent until they just fall off;

  • from a top attack, it will slowly lift your bot upward rather that penetrating the opponent's armor;

  • from a top attack with a bottom fork under your opponent to hold your robot down, the actuator will not have enough power to penetrate even modest top armor. If you don't believe this, set up a test in your workshop and measure how much power it takes to penetrate representative top armor in the weight class you choose.
Remember, your opponent is rushing at you with their own high-power destructive weapon. All a spinner has to do is touch you and you go into low orbit. If linear actuators worked, you would see them in common use in combat robots. They have been tried, they don't work, and there isn't a work around. Pick something else.

Comment: Dear Aaron, I would just like to thank you with the linear actuator question, that was a HUGE help.

Thanks, New York

Reply: You're welcome. I may give a very short answer where the situation is obvious to me, but don't hesitate to ask for clarification if you need it.

Q: Dear Aaron, I saw that you said that under cutters are able to get into the drum bots "upsweep." Where is the upsweep? How should I attack a drum bot with that in mind? How should I defend a drum bot if I am going to build one? Thank you!

A: What I said [in this archive] was: "An undercutter targets wheels, and can get in underneath the dangerous 'upsweep' zone of a drum weapon where the impactors have significant vertical motion."

I've been getting a lot of questions about the 'upsweep' zone. It's really simple: the upsweep zone of a drum (or other vertical spinner) is the area where the impactor(s) are sweeping mostly upward and at least a little forward. This is the area where the drum is most efficient in biting in and launching the opponent upward. Below this zone, the impactor movement is mostly forward and a hit will just push both bots back away from each other. Above this zone, there is no forward motion to 'bite in' and the impact is weak.

If you want to attack a vertical drum head-on, a good bet is to get in under the upsweep zone with an undercutter or a low scoop. If you have a drum, a low wedge of your own positioned to protect the area under the upsweep zone will defend against that type of attack.

Q: What is the vertical "upsweep" of a drum, and what is an egg beater?

A: An 'eggbeater' is a flat rectangular-frame spinning weapon, related in design and function to a spinning drum weapon. The metal rectangle is typically cut from a single plate of steel. An eggbeater is easier to construct and balance than a drum, but usually has a little less energy storage capacity for the same weight.

The 'upsweep' area of a drum (or eggbeater) is the portion of the circular rotation path where the impactor is moving predominantly upward and at least a little forward. This area is most effective in 'launching' the opponent. See the diagram in the post above.

Q: Aaron, looking at the question about how drumbots with faster weapon speeds will be able to get hits on drumbots with slower weapons [down farther in this archive], is this dependent on the weapon's tip speed, or the drum's rotational speed? I've been trying to figure it out on my own and I'm a bit stumped.

A: Mark J. here: if two drum weapons are going 'head-to-head', the weapon with greater 'tip' speed will be dominant. Tip speed is a function of weapon RPM and weapon diameter.

Note that the speed needed to prevail in this very specific situation is much greater than the optimum speed for a drum striking a non-rotating target. It may be better to employ some alternate technique when fighting another spinner rather than compromise the overall effectiveness of your weapon by dialing in too much speed. Think about the 'big picture'.

Q: Hi Aaron, questions on drum weapon.
  • A drum weapon with same number of joules stored but different in diameter, what there be any difference in the weapon effectiveness?

  • I have 2 gear ratio option for the drum weapon on 30lb bot, battling in 11' arena, with results as below:

    1. 1128 joules in 1.2s at 4846 rpm & 2564 joules at 7308 rpm
    2. 476 joules in 0.54s at 3150 rpm & 1083 joules at 4750 rpm

    Should i choose option 2 because the spin up time is faster, thus suitable for a small arena battle?

A: A larger diameter drum will be spinning at a lower speed than a small diameter drum with the same stored energy, and slower is good! More time between the passage of each impact bar means better 'bite' and therefore a better impact on your opponent.

In an 11 foot arena the distance from nose-to-nose on the robots is going to be very small -- like 7 or 8 feet. You're going to have VERY little time to spin up your weapon, so from that angle I'd suggest the second gearing option. I have seen spinners start a match with their weapon pointed away from their opponent to gain more spin-up time. Judges in the US don't like that, but I don't know how judges in Malaysia might react.

Note: spinning a drum weapon at 7000+ RPM is not a great idea. Even 4750 RPM is faster than I'd suggest. For better results, add more mass to the drum and spin it at a lower RPM.

Q: I remember seeing somewhere that you mentioned when two drumbots' weapons collide, the 'bot with the faster spinning weapon will win. [It's in this archive] I understand it has to do with bite, but how exactly does a faster weapon guarantee a launch for that 'bot? Do these rules apply for eggbeaters as well? And what effect does a weapon's moment of inertia or weight have to do with this? How much faster does one weapon need to be to guarantee a positive outcome? I know it's a multi-faceted and variable dependent question, but an explanation and play-by-play of what happens here (and maybe even a diagram?) would be incredibly helpful. Thanks so much for your help.

A: Mark J. here: I don't think a diagram is going to help. The situation is this: with two drum (or eggbeater) weapons facing each other, the impact bars on both weapons are sweaping upward thru the 'impact zone'. The impactor that is moving slower has absolutely no chance of catching up to the faster moving impactor, striking it from the underside, and launching the opponent. Speed rules here!

Given the typical high spin speed of drum weapons, the distance impact bars stick out, and effective close rate, it is the impact bars that are going to meet rather than a bar impacting a drum body. If an impact bar should happen to hit the smooth near-vertical face of the drum, it will get no effective bite and just knock both bots back a bit.

More speed differential will offer greater impact on the other weapon. The faster you can close on that up-sweeping impactor running away from your impactor, the greater the energy transfer to the opponent weapon. A small speed differential will not 'launch' the opponent, but it can at least give it a 'bump'

Q: Hi Aaron, i plan to build a wedge bot with a bar spinner on top just like hazard. I'm currently selecting the motor to drive the bar spinner and i'm experimenting with your spinner spreadsheet. Details of my bar spinner are:
  • Material : Steel
  • Length : 0.5m
  • Width : 0.08m
  • Thickness : 10mm
I pick the Amplow A28-150 motor details for a reference:
  • Torque : 1970 oz-in (i didn't find the stall torque in their description, can i actually use this?)
  • RPM: 6000
Then, i try your spinner spreadsheet with few gear ratio setting and come out with the results below. I didn't quite understand what is their different in relation to my bar spinner performance.

At 2:1 ratio

  • The weapon has 1313 joules in 0.76s at 1890rpm
  • At 2850rpm, it has 2985 joules of energy.
At 3:1 ratio
  • The weapon has 583 joules in 0.34s at 1260rpm
  • At 1900rpm, it has 1327 joules of energy.
and at 4:1 ratio
  • The weapon has 328 joules in 0.19s at 945rpm
  • At 1425rpm, it has 746 joules of energy.
Can you explain what all those difference will made in term of my bar spinner performance? and which gear setting above is the better?

I got this motor data from but there is no "no load current" listed so i can determine the power required. Do you know where can i get this details? Otherwise i won't be able to decide what motor controller to be used & how much battery capacity that should be reserved for my weapon,am i right?


A: I'm getting lots of questions from Malaysia -- just how much robot fighting goes on there??

Full specifications for the AmpFlow motors - including no-load current and peak (stall) torque - are available at the AmpFlow website. Please don't ask me for info you could find for yourself with a simple web search.

There are multiple examples of spinner design and discussion of the effects of different gear ratios on spinner performance in this archive. I'm not going to repeat that information here.

Which gearing is 'best' will depend on many factors you have not mentioned -- robot weight, arena size, opponent design, arena layout, robot speed... Again, this has been previously discussed in this archive. The tools and information you need are all here - start reading!

Q: Hi Aaron. I've seen in a video that Last Rite's weapon is chain driven. Isn't belt driven weapon more appropriate there, since the weapon has enormous power?

A: You have to understand builder Ray Billings and Hardcore Robotics. Ray does not believe in sublety, slippage, or second chances. He doesn't even believe in armor!

'Last Rites' has a huge weapon bar, an enormous weapon motor, and a solid chain drive connecting the two. There's so much kinetic energy stored in the weapon that bringing the bar spinner to an abrupt stop just isn't going to happen -- a slip drive to protect the weapon motor isn't needed.

Q: Do you have a diagram of a hydraulics system and how to transfer the force into [a useful] direction like (most notably) Razer and Jawbreaker's Revenge?

A: A hydraulic system works just like a pneumatic system, except instead of gas from a pressurized storage tank the hydraulic system uses liquid drawn from a reservoir that gets pressurized by a pump.

Most hydraulic 'crushers' use a 'teeter-totter' lever system where the cylinder pushes upward (or rearward) to move the crusher down. Both the lever arm and chassis must be VERY STRONG to survive the force generated by the hydraulic system.

Q: Can I use stainless steel for a beetleweight spinning bar? If not, what should I use? Alloy steel is proving a little bit hard to find in the correct shape.

A: You can use whatever you like . 'Stainless' is a whole familly of steel alloys with a wide range of physical properties -- some of the 'stainless' alloys would be better than others for a spinner bar, but a simple low-carbon alloy would be better. You're looking for a good balance of 'hard' and 'tough': most stainless is 'tough' but not particularly 'hard'.

There are many posts about various steel alloys in the archive.

Q: Steel guy again. Many sources of information extoll heat-treating your steel weapon first before sending it to the arena. I realized, however, that I have no idea how to heat treat an object, let alone have any of the tools needed. I know that you need more than a lighter or an oven, but thats about it.

Can I get away with not heat treating the mild steel weapon? Remember, I am having a hard time finding alloy steel in the right shape, so I don't have much choice in terms of material.

A: Mark J. here: heat treating is a specialty process best left to professionals. Check the yellow pages for 'heat treating' and hand your blade off to someone who has the equipment and experience to do it right. You'll need to know exactly what alloy your part is, so don't walk in with some unknown metal. Some alloys (all steels are alloys) repond very well to heat treating, and others not at all -- so know in advance what you've got!

Heat treating is not mandatory, but you don't want your weapon getting beat up worse than your opponent. The more energy you pump into the weapon, the greater the need for exotic metallurgy.

Q: I had an idea to mount a beetleweight spinner weapon to the shaft (Or, more of, pulley) Simply bolt the [spinner] to the pulley and use shaft collars to keep them in place. Will that work?

A: So, you have a pulley spinning at high speed on a bare shaft, located by shaft collars? Very poor practice -- how about putting a couple bearings or bushings in there! There is also a well known saying in robotics: "Set screws suck." Don't rely on a set screw anyplace where its failure would cause a problem.

Q: Hello Aaron. I was thinking of designing a spinner killer that is 15lbs that resembles breaker box and shazbot from team nightmare. How do their scoops work? Maybe you have a diagram or detailed pictures?

A: The motion of the scoops on Team Nightmare's 'Breaker Box', antweight 'Shazbot', and beetleweight 'Wallop' are all controlled by powerful gear motors mounted on the chassis where the scoop support frame enters the robot. 'Shazbot' and 'Wallop' use high power servos for the scoops, while 'Breaker Box' uses custom gearboxes mated to electric motors. Photos are available at the linked sites.

Q: Do you have any diagrams on the inner working of the scoop of breaker box and shaz bot? I am confused about the answer of how they work.

A: A top-view diagram of Shazbot's scoop lifter is at right. Servos are frame-mounted and hacked for continuous rotation. Servo outouts are attached to the scoop support arms. For larger 'bots, substitute gearmotors with suitable torque capacity.

Q: Does breaker box work the same way as shazbot? What is the difference in there diagrams?

A: Same thing, just bigger.

Q: Also, I wanted to add a weapon to it. What do you think about having a small hole in the front of the scoop so that a pneumatic spike can do damage? It would be one unit so the spike goes with the scoop so that when you lift the scoop you can still hit the with the spike Do you have any opinion on this design if it sounds good, unpractical, impossible, or any improvements or problems? Any remarks you have are fantastic...Thank you so much!

A: Poor idea. The strength of the lifter plows on 'Breaker Box' and 'Shazbot' come from their large range of motion -- a full 360 degrees of rotation. Hanging a pneumatic actuator and hoses off the back of the plow would restrict motion and greatly complicate the weapon system. Many robots have tried multiple-weapon systems, but simple weapons win.

Q: how should I make a pneumatic spike for a light robot? I was wondering if you had any diagrams of any pneumatic spikes for a 15 lbs robot... Thanks!

A: You shouldn't. Simple pneumatic spikes have been tried many times and they are not effective in robot combat. If you want to try anyway, read the Team DaVinci Pneumatics page for details on pneumatic weapon systems.

Q: Dear Aaron,

I am going to enter a bots IQ or bot bash or battle bots.... It seams to me that all the winners are vertical spinning drum weapons, or weapons that are extreamly similar. I would like to be original and make a robot that doesn't have a drum. What is the best way to counter a drum without running away? One of my plans was to make that resembles Hazard. How good would that do against a drum? I am talking about a 6 to 15 lbs robot.

Thank you so much, Aaron. This website is fantastic.

A: Glad you're enjoying the site!

Drum weapons are very popular because they work well against a wide range of other weapon types. It's like you could pick both 'rock' and 'paper' at the same time; 'scissors' wouldn't stand a chance.

'Hazard' style horizontal blade spinners are not a good match against a drum weapon -- the blade sets too high. Big under-cutter horizontal spinners like 'Totally Offensive' are a better bet, but they may be too common if you're trying to be 'original'.

The ultimate 'spinner killer' is a heavy scoop/lifter as seen on 'Breaker Box'. The design works against vertical and horizontal spinners -- just make that scoop STRONG!

Q: I love this website even more! The response was great about the drum bot. Thanks!

Do you have images of 'Totally Offensive?' What is an under cutter? Thanks.

I like the look of breaker box a lot. Is there a way to make a good drum-stopper, and still have the capability of doing damage? I would just love maybe a combination of a drum killer and a damage-weapon.

I am not trying to be pushy or anything...I don't want to insult the best site for robots. What about the design of 'village idiot' for the drum killer and damage? What type of bot is 'Village Idiot'?

Thank you so much. I really enjoy robot fighting. This website is my access.

A: Images and video of 'Totally Offensive' can be found at the Team Mad Overlord website. An 'undercutter' is a horizontal spinner weapon set very low down near the arena floor. An undercutter targets wheels, and can get in underneath the dangerous 'upsweep' zone of a drum weapon where the impactors have significant vertical motion.

Combining multiple offensive and/or defensive designs into a single robot has never proved successful. It is possible to have interchangeable attachments to suit different opponents, but multiple simultaneous weapons are a no-go. Pick a design and commit all of your weight allowance to that design.

The various versions 'Village Idiot' (9 wins, 4 losses) had twin vertical disc weapons. Early versions used circular saw blades, but later incarnations used milled aluminum discs with impactor 'teeth'. From a design standpoint this is a very minor variant on a drumbot.

Q: Hey Aaron, Which weapons died out do to there lack of effective power and ability? Thanks!!!

A: There is a fairly complete list of weapon types in the Wikipedia article on robot combat. All of them save for the spinners, wedges, and spinner-killer scoops are pretty much extinct.

Q: when you said wedges for the non-extinct weapons, does that include flippers?

A: No -- lifter wedges are fairly common, but flippers aren't competitive in the U.S. Flippers still dominate in the UK due to the different arena design and judging prevalent there.

Q: I have I question about a robot design a was considering. If you had an axe bot that would smash downwards like a normal axe, but then it could be switched into a spinning weapon. The hammer/axe would do this down position turn so the tip was flat/perpendicular to the axe . Then it would spin around and do damage that way. The second mode would essentially look like hazard with one blade. My thinking is that with this combo the enemy would stay away and be hit by the spin mode but if it came to close I could smash it with the axe. This might be mechanically accomplished by having to motors one for axe mode and the other for spinner. The engine would have a cam so only one motor would be necessary for both modes. Thanks!

A: You said you had a question, but it seems you never got around to asking it. As a general comment I'll point out that complex weapons add weight, reduce reliability, and win fewer matches than simple weapon systems. Team Run Amok's motto: "Complex design is easy - simple takes work."

Q: how does sewer snakes weapon work?

A: There is a description of Sewer Snake's weapon - with photos - in the this archive. Search for "Sewer Snake's unique weapon".

Q: Is there any particular reason no one's ever used an grinding disk for a weapon?

A: There have been grinders - lightweight 'Grunion' (1 win, 1 loss) fought at the BattleBots 2.0 with a grinding wheel weapon, and 'Zero' ran a milling cutter at Robotica 3. A grinding weapon has several drawbacks: ceramic grinders are fragile, there is small potential for damage, and it's extremely difficult (impossible?) to hold correct position and pressure on your opponent in a chaotic battle arena. I couldn't recommend it.

Q: I want to use C02 for my robot's flipper. I found what tank (A fire extinguisher) and a ram (A hydraulic cylinder) but I don't know what regulator I need. I know that I know that it needs to be a C02, but there are so many things to note like ones with solenoids, some with strange gages, its all confusing me. Could you at least help me to figure out what would be a good regulator, I can find the rest of the parts myself.

A: Mark J. here: I'll be pleased to help you select a regulator, but I need more information about your robot:

  • In what weight class will you be competing?

  • What are the dimensions (bore and stroke) and pressure rating of your actuator?

  • At what pressure do you want to run the actuator?

A couple of points:
  • Hydraulic cylinders are heavier than standard pneumatic cylinders and are generally used only if you intend to operate the pneumatic system at very high pressure -- such as unregulated CO2.

  • Are you sure you're looking for a regulator? The fact that you mention solenoids makes me suspicious that you're looking at multi-port valves and not regulators. Exactly what function do you want this device to perform?
Read thru the Team DaVinci Pneumatics page to get general information on pneumatic weapon systems and to make sure we're using the same vocabulary.

Q: I'm the flipper guy again, I just want to help out with the question:

  • The weight class is middleweight, but it's flexible enough to be a heavy if it's overweight by a fair margin.

  • The cylinder is a four inch stroke with a 1 1/2" bore, and can go well up to 2500 PSI.

  • 850 PSI.
I know that I could go on with out the regulator, but I REALLY don't want my cylinder to fail.

PS: I already read the DaVinci page many times. I wasn't going to get my head into this with out the know how on to do it.

A: Go back to the Team DaVinci page and read thru the 'What a gas!' section again. Compressed CO2 liquifies and self-regulates its pressure to about 850 PSI at room temperature. The pressure will drop as phase-change thermodynamic effects reduce the temperature of the CO2 remaining in the cylinder. You have no need for a regulator -- the pressure available to your actuator will not exceed 850 PSI.

Please be VERY CARFUL with high-pressure pneumatic systems! They are violent and dangerous. Handle with great care.

Q: Flipper guy once more, thank you for your help.

One last thing, is their any advice you can give with CO2 systems for optimal performance?

A: The flow performance of your valve system is critical for best performance. The flow capacity is listed as the 'Flow Coefficient' (Cv). Larger values for Cv flow gas faster and will give better performance. Don't scrimp on the valves!

Q: How do you articulate lifter forks like the ones on Vlad The Impaler?

A: Vlad had a very simple single-pivot lifter hinged low toward the back of the chassis. I don't have a photo of Vlad's actual mechanism, but it must have looked something like the sketch at right.

Date marker: May 2011
Q: Aaron, I have a theory about having an electric hammer. Just bear with me. Based on the rack and pinion concept of the judge. I was planning on using the rack and pinion method with a pulley connected in a closed circuit with the motor, kind of like the pulleys used to control multiple wheels with one motor only you replace the wheels with a single axe.

I also had a similar in my idea about the 3lb Razer design. I was going to make the chassis of mainly two 1/8 lexan middle pieces. [Brandon, the guy with all the craziest beetle or ant weight designs]

A: Albert Einstein is reported to have said that crazy is "doing the same thing over and over again and expecting different results." By that definition, expecting to win with an electric hammer or electric crusher is certainly crazy. Best luck.

Q: This is a snippet of conversation between the builders of two very powerful horizontal spinners. My question is whether this notion is accurate and what the physics behind it are: "I'll tell you how to beat my bot: with a blade that will cut in both directions [using a reversible ESC]. Then if you're spinning opposite of me because of your higher rpm, you'll cut into my shell." Does it really work like that? The other builder seemed skeptical.

A: Mark J. here: I'd really like to know who the two builders were.

The theory works well for drumbots. When two drumbots go 'head-to-head' they are (viewed from the sid I'm not so sure that you can count on the same effect with full body spinners that don't get support against newtonian reaction. I think having a blade that can cut from both sides is a good idea, but I think it will cut just as well if it's spinning faster or slower than the other shell.

Q: Looking at your archive gave me a interesting idea. In one of them you said it was a good idea to keep a wedge and a thwack separated, and would not work well together in one bot. That gave me the idea to literary keep them separate, and have one bot, a wedge and opponent in tow, smash into its teammate, a madly spinning thwack. I bet theres some bugs in my idea, but what do you think about it?

A: I like the idea, but let's take it one step further - have the thawack madly spin on top of the wedge as it drives around so the undersized wedge doesn't have to fight to shove the twice-as-heavy opponent over to the thwack. It'd look a lot like 'Hazard'. Oh wait, it'd be 'Hazard'.

Q: I've been using your Excel spinner spreadsheet, and it saves me tons of time, thank you!

I know I should avoid very general questions, but one thing I'd like to ask is "how fast is too fast?". My teammate and I have very different opinions about how fast our weapon should be. If it helps, the weapon is a 20 inch long spinning steel bar, 1.5'' wide and .25'' thick, on a 15lb combat robot. The motor we're using has 9Nm of torque, a max RPM of 30800 at 22.2V, and we're using a 4:1 gear ratio to get a spin-up time of about .54 seconds, but I understand that I should add about another second for the soft start feature. The power consumption is also reasonable, at 1.29 amp hours for six spin-ups during a 3 minute match).

I know that similar lawn-mower style robots like Brutality and Hazard have their bars spinning at about 2K rpm, but I also thought that a smaller weight class would make a big difference in terms of how fast our weapon can spin. We're not very concerned about bite, since the bar is very long, and we calculated there to be about 1/2" of bite if the weapon spins at 10K rpm with both bots approaching each other at 4MPH.

Personally, I think that 10K rpm is much too fast, and that something like 5K is much more reasonable. There is a lot of uncertainty with high speed, since aerodynamic-drag will be a huge factor with such a large bar. My friend's solution to that was to make the bar into a wing-shape, but I'm skeptical about the functionality that would have as a weapon.

My teammate's greatest fear is that the weapon will be relatively ineffective with low energies, and he has thus become what I call "Joules-obsessive". Since weight is tight, and increasing the speed by two increases the amount of joules four-fold**, he really wants to reduce the gear reduction for greater speed and maybe use two motors instead of one to make up for the loss in torque and spin-up time. With either configuration, we are well above the "27 Joules/pound of robot" guideline, but the 15lb class has become filled with many ultra--speed drumbots with very high weapon energies.

Please don't worry about us trying to create a robot that is "all weapon", because we still have a sturdy drivetrain and chassis in case all else fails. Please let me know if you think me, my teammate, (or both of us) are being absurd with our weapon plans.

A: Mark J. here: so, you're thinking about spinning a 20" long, 1.5" wide, .25" bar of steel at 10,000 RPM, storing 10,245 joules of energy in it, and slamming it into a 15 pound object - or worse, the arena wall? Have you considered what happens to the blade in an impact at that energy level? I think you get one good hit and the blade turns into a bow tie.

The laws of physics haven't changed since Hazard and Brutality were designed, and if higher RPM was the way to get better weapon performance you can bet that their designers would have taken full advantage of it. There is a balance to be struck between speed, energy storage, spin-up time, durability, and bite -- and the only way to find that balance is by trial and error. Look to robots that are successful and learn from their designs.

My recommendation is to not get greedy. I'd pick a smaller motor, put the weight savings into tip weights on the blade, and spin it at 3K RPM for about 1600 joules.

** Note: the statement "increasing the speed by two increases the amount of joules four-fold" is incorrect.

I'm sure the builder ment 'four times' instead of 'four-fold'. A 'fold' means to double; 'four fold' would double the output four times:

2 × 2 × 2 × 2 = 24 = 16 times the energy.

Kinetic energy increases with the square of speed, so doubling the speed gives = 22 = 4 times the energy, not four-fold.

Q: Thinking about a flywheel powered flipper, would it be possible to make an electric flywheel powered hammer?

A: Mark J. here: possible - yes. Simple - no. Reliable - unlikely. Successful - I think not.

We've discussed flywheel-powered flippers in the archives. Their mechanical weak point is the dog clutch that transfers the power from the flywheel to the weapon. The concept is like revving up a car engine and trying to jam it into gear without pushing the clutch in first. The components are not 'off the shelf', they are difficult to design and machine, and the mechanism is subject to huge stress.

Pneumatics are much better at this type of work, so why make it so hard on yourself?

Q: I thought of a way to get more power out of a hammer bot. The robot would have a hammerhead with a heavy ball in it. When the robot hits the ball would increase shock damage.

A: Mark J. here: sorry, but no. What you describe is a type of dead blow hammer. The passive internal mass strikes slightly later than the main hammer body and spreads out the impact while absorbing some of the rebound. The total energy of the impact is the same as a conventional hammer of equal mass, but the energy is released over a longer time period.

A dead blow hammer is used in situations where you wish to reduce damage to the surface you are striking -- just the opposite of what a combat robot is trying to do.

Q: I am trying to activate a 24v NPC motor for a weapon using a White Rodgers SPNO 24v solenoid and a Battleswitch. How Do I set this up? My Battleswitch is connected to my radio system and switches properly. How do I use the Battle Switch to power the Solenoid? I have one 24v Battery, can I use this battery to switch both the solenoid and power the motor? Thanks!

A: Follow the diagram at the right. Use the 'SC' and 'S2' connections on the BattleSwitch and connect the output from the BattleSwitch to a small 'coil' connection on the solenoid. The receiver energizes the BattleSwitch relay, the BattleSwitch sends a small current to the solenoid coil, and the solenoid switches the large current to the weapon motor.

If your battery has sufficient capacity, you can use it for both solenoid and motor. If the solenoid won't stay 'on' you need more battery.

Q: Dear aron I found out a way to possibly get more energy out of an elctric hammer. This idea would consist of a heavy fly wheel powerd by a motor (double supported) which would then turn a crank and a bar like the ones on realy old steam engines.....and this for some reason enables the bar to act like a jack hammer when it roatates with the energy stored in the roatating fly wheel. do you think this idea may work?. p.s this idea is very hard for me visualize verbily on forms please help. from anthony

A: I understand the crank and bar idea, Anthony. There was a version of the German robot 'Ansgar' that tried the design. If you watch the red speed bar in the animation, you can see the problem with the mechanical linkage: the output link reaches maximum speed and energy in the middle of each stroke, then slows and comes to a complete stop (zero energy) before reversing direction. That makes for very poor transmission of the energy from the flywheel to an impact on your opponent. Ansgar's weapon wasn't successful and there is little to be gained from repeating their design error.

Q: Why do so many drum belts snap? Is there a way to counteract this? Does using two belts help at all?

A: Timing belts have limits on the power they can transmit. Some builders either don't know those limits or pretend that the limits don't exist. See the RoyMech timing belt website for guidance.

Two belts may help, but it's difficult to get twin belts to share load equally. Proper design calls for a single belt with enough load capacity to survive the abuse.

Q: Dear aron I have given up on hammer robots and decided to build a beetle weight hybrid bar spinner like snow cone but with a different chasis desighn. would the bane bots 5.1 28mm gear box work with the six mm shaft if alowed to slip on imapact. from anthony

A: The BaneBots 28mm gearboxes aren't well suited to the high lateral loading you get from a weapon impact, and a slip clutch to absorb the radial impact on a spinner is not as easy to implement as it appears - we learned this from the weapon on 'Run Away'. After your experiments with the electric hammer, I'd suggest that you pick a more conventional and proven design for your next 'bot.

Q: I've been thinking about drumbot weapon designs lately. Why aren't there any asymmetrical drums (such as a disk with one tooth but balanced)? If counter weighted correctly, wouldn't it increase the "bite" of the drum / spinner since there are fewer teeth? I understand that there are gyroscopic forces to take into effect, but I'm just a little curious as to what would actually happen. Have you ever seen / tried to implement this kind of weapon?

A: Asymmetrical spinners are difficult to design and construct, but a good number of them do exist. The simplest form is a bar spinner with one end shortened and weighted for balance, but much more complicated forms are possible -- like the spiral rotor on middleweight 'Professor Chaos'.

Reducing the number of teeth does increase 'bite' as the time between passage of the rotor teeth is increased, allowing a bigger chunk of your opponent to enter the weapon's 'impact radius' before it takes a hit. The gyroscopic forces acting on the weapon are no different than those acting on a similar weapon with a symmetrical tooth distribution.

Q: Building on the idea of an asymmetrical drum such as Professor Chaos, would it be possible to make an asymmetrical "lawnmower" type horizontal bar spinner that is also asymmetrical and counterweighted? Is there any advantage to this? Has it been implemented before?

A: Mark J. here: read section 6.3 of the RioBotz Combat Tutorial for an explanation of 'bite' and the advantage of an asymmetric spinner. The tutorial includes an illustration of an asymmetric spinner bar.

I've seen counterweighted bar spinners, but I can't name a specific example.

Q: I'm thinking of making a 15lb drumbot, and i'm trying to figure out what motor would be good. I've been looking at a lot of specs, but it tends to be very hard to discern what I need.
  • Many have told me that high rpm speeds are whats best, but many 15lb robots with high rpms are extremely unstable, as when turning, they cannot keep their wheels on the ground.

  • Torque seems to be a major appeal, as it is the rotational power the robot can give, and with a lower rpm, it can still deliver power, and a large amount of force on each hit.

  • Some have told me rotational inertia / momentum is what makes the robots best, and i'm not entirely sure. This make sense, as it is the amount of force/momentum that is contained in the drum. This (seems to be) directly proportional to torque.

  • Power is a major spec on motors, but I'm not sure what to make of it. It seems to be unrelated to both torque and rpm, but the biggest motors have a ton of power, but lower torque and rpm.
Can you explain to me what all these specs mean?

A: Mark J. here: the answers to your questions might best be provided by a 4 semester-hour class at a good university, but I'll do my best to boil that down to a few paragraphs. It's gonna be a little sketchy.

Let's start by sorting out the relationship between motor torque, RPM, and power. Power is the product of torque and RPM:

Power = Torque × RPM

A high RPM motor can be 'geared down' to provide more torque at lower RPM, but there is no change to the amount of actual power produced. This all means that you want a motor with high power to spin up your weapon drum, as torque and RPM can be interchanged with proper gearing. Motor output power is usually expressed in either watts or horsepower: 1 horsepower = about 746 watts.

The power that the motor produces over some time period will be stored as rotational kinetic energy by your drum. Once the drum is spinning at top speed, the motor is simply maintaining that spin speed and is no longer a factor in weapon performance -- no additional energy is being stored in the drum. The stored energy is measured in joules: one watt of motor output power applied to spinning your drum for one second can create one joule of stored energy in the drum.

The amount of power stored in a rotating drum is a function of its rotational moment of inertia (MOI) and its speed of rotation (RPM). The MOI of the drum is a function of the mass of the drum and how that mass is distributed. Two objects with the same mass can have very different MOI: mass located farther from the axis of rotation contributes more to the MOI than does mass close to the axis. A short, large diameter drum has a larger MOI than a long, small diameter drum of the same mass.

The energy stored in a rotating object increases with the square of the rotational speed: spin it twice as fast and it holds four times as much energy. This is why many builders believe that very high RPM is the ticket to weapon superiority. However, storing energy is only half the requirement for a successful weapon; the weapon must also be able to effectively deliver that energy as impact to your opponent. Very high speed spinning weapons have difficulty getting 'bite' on their opponent and cannot get a reliable hit on a robot that has no sharp exposed edges. There are many discussions about 'bite' in this archive.

  • A motor with greater power can spin a given drum to its target speed more quickly than a motor with less power.

  • Once the weapon is up to speed, motor power plays no further role in the weapon effectiveness: the impact force comes from the energy stored in the drum.

  • A drum with larger MOI can store more energy at a given speed than a drum with smaller MOI.

  • A faster spinning drum stores more energy than a slower spinning drum, but will have more trouble transferring that energy to the opponent as an effective impact.

The approach I recommend is to spend some weight allowance on a large MOI weapon and belt drive it at a reasonable speed. This will give better general performance than a small MOI weapon spinning at very high speed.

There is a special case that does call for really high RPM: when two drum-spinners go 'weapon-to-weapon' the slower drum looses. I've seen drum spinners that cruise at a moderate weapon speed for most attacks, but have spare motor speed that their driver uses against other drums. Something to think about.

Q: You said in an earlier post that electric hammer weapons weren't effective in current competition. Does that mean that robots like frenZy are outdated by now? Because frenZy's hammer still looks like it has some oomph.

A: I really like 'frenZy', but it wasn't a very effective combat robot even back in the BattleBots days. Overall record: 7 wins and 10 losses, with zero wins in the last three events it entered.

FrenZy looked as if it was delivering powerful blows only because its small chassis would react wildly to each impact. It could make itself dance, but not its opponent.

Q: Would it be possible to make a rack and pinion flipper?

A: Mark J. here: sure -- but there isn't any advantage to it. A rack and pinion drive for a hammer weapon is useful because it can translate linear force from a pneumatic cylinder into uniform radial acceleration of a hammer across a large arc. A flipper acts across a much smaller arc and uniform force is generally not an issue. You're better off to avoid the weight, complexity, and power loss from the rack and pinion.

Search for "Jacha Little" in this archive for an animated diagram of a rack and pinion hammer mechanism.

Q: [Chinese Forum] As for electric hammer bots, I remember the very first version of Deadblow was equipped with an electric hammer during LB99 Battlebots event - and it went to the semifinal. Could that be considered as "Successful"?

A: You are mistaken. According to Grant Imahara's Team Deadblow website, Deadblow's weapon system at the 1999 Long Beach event was pneumatic:

"The weapon system for Deadblow 1999 was a 6.5 lb machined steel sledgehammer which swung 180 degrees. It was powered by a Bimba rotary pneumatic (air-powered) actuator with a working pressure of 150 psi."

In 2000 the rotary pneumatic actuator was replaced by a conventional linear pneumatic actuator. The hammer never had electric power.

Q: Do you think DeadBlow would have been just as effective if its hammer was electric?

A: 'Deadblow' did not rely on high weapon power, but on speedy and repetitive attacks. Even so, the '1999' version of the robot did not do well with a relatively slow and weak (but better than electric) pneumatic hammer. Deadblow's record in 1999 was an anemic 1 win, 2 losses, and 1 no-show bye. Success came only after the weapon power and speed were increased with a linear pneumatic actuator. An electric hammer would have continued to perform poorly.

Repeat after me: electric hammer weapons do not have enough power to be effective in current open competition.

Q: You said in earlier posts that you can't find a hammer bot with a winning record. But in the hall of fame The Judge had a great record and Team Hurtz robots have been sucssesful.

A: I think you're refering to a post where I say,

"I can't find an electric hammer with a winning record in any weight class..."

Team Hurtz has certainly had success with their pneumatic axe robots, but their electric hammer robot 'Beta' was not at all successful. 'The Judge' was also pneumatic. You can get an overhead hammer to work, but powering it with an electric motor is generally a poor idea.

Q: hey Aaron, are horizontal spinners or vertical spinners more controllable in the ant weight class? thanks.

A: Both have problems:

  • Vertical spinners in any weight class are difficult to control because their gyroscopic effect resists turning motion. A peizo gyro in the radio system can't compensate for this this problem - you can't cheat physics.

  • The axis of rotation on a horizontal spinner does not resist turning motions, but the counter-reaction to impact can send it careening away at high speed.
So, one doesn't want to turn and the other loses control on impact. Take your pick.

Q: As a follow-up to the (as of writing) most recent question, what about a disc mounted at a 45-degree angle? Would that give the benefits of both a horizontal & vertical spinner, or just the problems?

A: Mark J. here: a quick check of my trigonometry tables [cosine of 45 degrees = 0.7071] says that you'd still have 71% of the turning resistence problem with a 45 degree diagonal disc, and 71% of the lateral force vector on impact. You aren't gaining much on either problem, and you're adding a big dollop of new trouble.

Turning in one direction would be a little easier as the gyroscopic forces lift one wheel and 'flatten' the disc angle a bit, but turning in the other direction would tilt the disc more upright and add resistence.

I think you're better off shooting a big hole in one foot rather than a smaller hole in both feet; stay either flat or upright.

Q: Hi Aaron I am building a antweight robot, and it has a flipper. how to I attach the flipper so it can move? And I am also using pneumatics to power the flipper.

A: Where did you find pneumatic components small enough for an ant?

A simple single-pivot flipper will require a hinge at one end that attaches to the robot chassis, and additional hinges at each end of the pneumatic actuator where it attaches to the chassis and flipper arm. The design of the hinge is less important than making sure it is strong enough to survive the forces that will act upon it.

Q: I have an idea for a type of four bar lifter. It involves a bit of a rack an pinon system like what 'The Judge' uses, but only on the back (or front bar). Would I get any more force out of this, or am I over complicating things?

A: Mark J. here: the advantage of driving a pivoting arm with a rack and pinion is that a source of linear force (pneumatic actuator, linear actuator) can apply a constant rotational torque to the arm over a large range of motion. This is very useful in powering a single-pivot axe or hammer, but less useful in actuating an arm that has limited range - such as the front bar on a 4-bar lifter.

In a 4-bar lifter, the lifting force applied will be non-constant even if the force applied to either the front or back arm is constant. Whether or not you will get more force from a rack and pinion or by direct application of linear force to an arm will depend on the details of your 4-bar design. The T.i. Combat Robotics 4-Bar Simulator will let you experiment with torque requirements and lifting force with different 4-bar layouts. I don't have a tool to provide similar calculations for direct application of linear force.

Q: How do you figure out the best shape for spinner teeth or axe heads?

A: There may not be such a thing as a single 'best'. Different opponents will have different strengths and weaknesses. A sharp 'cutting' tooth that slices thru one opponent may jam and stick in another. Any design will be a compromise. Take a look at what's working against the types of robots you expect to face and be prepared to punt.

Q: Which is better for a hobbyweight, a detachable wedge or a wedge robot?

A: A wedge takes a lot of abuse. If your wedge is detachable you can make easy repairs/replacements/modifications. If your whole robot structure is the wedge it becomes more difficult to maintain. Let your opponent beat up something you can replace quickly in the pits -- not the basic structure of your machine.

Q: I am working on a silly weapon for the Robot Battles 30lb rule set (open platform sumo). How would you calculate the power required from a motor (brushed) to push a single cylinder piston stroke to move 200 in^3 of ambient pressure air into a bladder of equivalent volume when filled? Possibly the bladder would be under a lifting plate. Any commentary is welcome.

A: Mark J. here: lots of unspecified variables here! 'Power' is defined as the amount of work done in a specified time period, so:

  • The first variable is how much work is involved. That depends on how much weight is actually on the lifting plate (the whole 30 pounds, I'm guessing) and how high the plate will lift the weight when the bladder is filled.

  • The second variable is how fast you want the bladder to fill. With proper gearing, a very small motor could fill your bladder and lift the required weight to the required height, but it would happen very slowly.

  • A 'hidden' variable is the power lost to friction in the gear train and to the piston seals.
So, the higher and faster you want to lift the load, the more power you'll need.

If I were going to do this, I think I'd ditch the piston and cylinder design and use a small high-volume electric air inflator like you use to inflate a raft. I've seen versions that plug into car cigarette lighters - more compact and no heavy cylinder and gear train required.

Q: Is it possible to implement a four bar actuator upside down? The idea i have for a robot is so that an arm like biohazards can be used to push away robots with a wide bumper, instead of lifting them up.

A: You can orient a 4-bar mechanism any way you like: up, down, sideways, or someplace in-between. I don't like the idea of trying to 'push away' an opponent - you are just as likely to push yourself away from them as you are to push them away from you.

What will you have gained by increasing the distance between your opponent and the center of your robot? I don't think that counts toward aggression points, it's unlikely to do any damage, and your opponent gets just as many damage points for beating up your 'bumper' at the new location as they would if it had never moved.

The 4-bar lifter is a proven effective design. Lifting your opponent disrupts their attack, keeps their weapon away from you, and controls the match strategy. I think I'd stick with that.

Q: Have there ever been any featherweight 4 bar lifters? I'm currently thinking of making one and want to know if there is a good reason why I haven't seen any before.

A: There are several featherweight 4-bar lifters in the UK, and 'Defiant' won the 1997 US Robot Wars 'lightweight' class at a weight someplace close to 30 pounds. There are also successful examples in both lighter and heavier classes so I can't think of any design reason why it would be a bad idea.

Q: On a pneumatic cylinder, is there any drawback to widening the hole in the front of the cylinder if it is for a single action system?

A: Mark J. here: by 'hole in the front of the cylinder' I'm assuming you're referring to the gas port that would be used to retract the actuator in a double-acting system. I'll also assume you want to enlarge the port to speed up the actuator by allowing the 'exhaust' air to exit more quickly.

You can certainly enlarge the port, or add an additional port if you like. Consider some measures to protect against debris entering the cylinder, and be careful to retain enough strength in the end cap to keep things together. From a practical standpoint, don't expect a big increase in actuation speed. You didn't mention the size of your cylinder or the existing port but your speed gain is apt to be small.

Q: Aaron, I'm trying to build a beetleweight hammer. For the hammer, would it be suitable to use a brushless motor and pulleys or gears to effectively drive the hammer?

A: In spite of builder claims, I can't find an electric hammer with a winning record in any weight class - they just can't generate enough power. Search this archive for 'electric hammer' to find a recent discussion on the topic and a link to our electric hammer calculator spreadsheet.

Q: Mangi has been a dominate force in the 30lbs Sportsman class for the past few years.

A: Fair enough. 'Mangi' has a winning record, but I must point out:

  • The 'sportsman' class isn't the same as open class combat. A lot of things fly there that wouldn't work anywhere else;

  • A record of 11 wins and 9 losses isn't what I'd call 'dominant'; and

  • Mangi's builder has been in combat robotics for a very long time and could win matches with a half-empty tub of margarine.
If you must build an electric hammer, use a brushed motor. Brushless motors really don't like being stalled and a hammer weapon spends a fair amount of time with the motor stalled.
Q: Does Rat Amok's weapon count as an active weapon?

A: In general, any weapon that is controlled by its own radio channel is considered active. 'Rat Amok' has a spring-loaded bar that is held and released by a servo controlled clasp. Since the weapon servo has its own channel control, it is considered to be an 'active' weapon.

Q: What do you think about this idea?For a weapon the robot would have a saw blade rotating one way and another blade rotating the other way.Because they are moving oppisite ways kickback would be canceled.

A: The kickback would be cancelled only if both blades dug into your opponent equally and at the same time. That's a really big 'if'. I'd be willing to bet that almost all the time you'd have only one blade dig in and toss you off to one side or the other. With a single blade you at least know which way you'll be kicked! Having one blade dig in would also be really hard on whatever type of drive you had that spun the blades in opposite directions.

One thing that counter-rotating blades will cancel out is gyroscopic forces. Not a big deal on horizontal weapons, but with vertical blades it would improve mobility a great deal.

Final verdict: thumbs down. We like simple designs at Team Run Amok, and counter-rotating blades add more complexity than they would be worth.

Q: is there a way to calculate egg beaters on your spinning weapon spread sheet? thanks

A: Yes - this has been answered previously. Search this archive for "eggbeater weapon".

Q: hello, what is the equation I can use for finding out how often the tooth on a spinning disk comes by? thanks

Q: hi again i found the answer to my question before but what is a good speed for teeth to get a good bite in a ant weight
would 100 times a second be ok?

A: For the benefit of others, the formula is:

Time Between Weapon Teeth (seconds) = 1 ÷ ((Number of Teeth × Weapon RPM) ÷ 60)

Example: two teeth on opposite sides of a disk spinning at 3000 RPM = 1 ÷ ((2*3000)/60) = 1/100 second between teeth passing.

I can't give you a specific time interval that's 'OK'. More time means better 'bite' and the ability to effectively impact a 'smoother' target, but more time means slower rotational speed and less energy. About 1/100th of a second is a fairly typical antweight tooth timing - you'll have some bite against edges and corners, but very little against smooth surfaces. You can always add a speed controller to your weapon motor and choose your speed to match your opponent.

Q: Hey Aaron

First of all, I would just like to say that I love your spinning weapon calculator. It is making things so much easier for our design phase, especially how it can calculate the spin-up time for a weapon and how much energy it will drain off your battery. I am having SolidWorks compute the moment of inertia for our weapon, and then I plug that value into the calculator. I did the calculations myself for our design binder, and let me say, it saves a load of time.

Anyways, what do you think would be a competitive amount of potential energy for a weapon in a 15 lb weight class? On the first sheet you say to aim for 16 J per lb of your weight class, which would be 16 J x 15 lbs = 240 J. This is WAY [lower] than what I am getting in the calculator, 3,000 J. Is this overkill? If it makes a difference, I am using a steel beater-bar weapon, with a 2.5" max radius, spinning at 15k rpm. The reason I say max radius is because it uses one tooth: the one side of the bar extends farther out than the other, and to keep it balanced the longer side has a larger channel cut out of it (again SolidWorks can compute the COG for me, and it tells me that the MOI is 0.0026 kg x m^2). So. Do you think this is overkill?

Thanks so much

A: Mark J. here: we're always happy to hear from builders making good use of our design aids.

At what point is the amount of kinetic energy overkill? Many current spinners have energy numbers similar to those you calculate for your own design, but those big numbers invariably come from spinning the weapon at very high speed.

Consider: at 15,000 RPM the longer bar on your beater makes one rotation every 0.004 second! You've got to stuff some part of your opponent's 'bot inside the swept radius of the weapon after the tooth goes past but before it comes back around. Unless your opponent has a very sharp protrusion that is well anchored to the chassis, and unless the closing speed of your 'bots is very great, you're going to have a very hard time doing that. A very high speed weapon will just 'skitter' across the opponent without damage, unless the opponent did a very poor job of smoothing exposed edges.

So 3000 joules isn't itself a problem, but the rotational speed at which you are storing that energy renders it ineffective. I'd say your weapon has too little rotational mass and you're spinning it too fast. Double the mass and slow it down to about 4000 RPM to store around 370 joules - you'll have a much more effective weapon.

Note that there is one situation where high weapon speed is needed: going weapon-to-weapon with another vertical spinner. The slower weapon loses in that matchup. Consider a speed controller for your weapon so you can 'crank it up' when there's a need of speed.

Q: What is your opinion on brushless melty's like 'Spinning Tortoise'? Do you see them becoming more popular as brushless motors and esc's have become absurdly cheap?

A: A melty brain spinner is a very complex and expensive design to implement. It requires electronics expertise, careful construction, and a great deal of development and testing time. A drop in price for the drive motors and ESCs is unlikely to greatly increase their popularity since those components are a very small part of the total robot budget.

Melty brain spinners are able to use brushless drive motors because they do no pushing and the drive spends most of its time just maintaining spin speed. I would still recommend use of premium quality motors, as they come with reliable specifications that are critical in obtaining optimum performance from the weapon design.

In spite of their status as a considerable tecnological achievement, I'm not much of a fan of the melty spinners. They aren't much fun to drive, they're very susceptable to 'spinner killer' scoop countermeasures, and they tend to 'hockey puck' themselves around the arena after a good hit. Exciting, but likely to do as much damage to themselves as their opponent.

Q: In my antweight spinner, I plan on using a 6061-T6 aluminum 1/2" tube with 0.083 walls as a dead shaft. I would use 2024 or 7075 aluminum, but the shaft needs to be welded so those are out of the picture. Do you think the shaft will hold up okay even though the aluminum is a little gummy while using sintered bronze bushings or needle roller bearings?

I can't use normal ball bearings with an inner race because there is not enough room for a hole that big. There is a place that sells 1/2" aluminum rod that is hard anodized to Rc 60 - would this have any effect when using the bronze or needle bearings, or will it just scrape off?

Finally, for needle bearings, do you need to use an inner race or could you use one without the inner race?

Thanks so much! You are so helpful.

A: I'm not sure how helpful I can be here. You haven't told me near enough for me to guess at how much load is going to be placed on that shaft. What is the mass of the spinner? How fast will it spin? How many joules of energy will it have? How many bearings will support the spinner on the dead shaft? How far apart will the bearings be? How far apart are the supports for the dead shaft? A competent engineer would require all that information at least, and as we point out in the #17: we aren't an engineering service.

I can say that in general I would not recommend an aluminum tube for a spinner dead shaft. Newton pointed out that for every action there is an equal and opposite reaction, and the point at which that violent reaction is transferred to your robot is the dead shaft. It has to absorb energy equivalent to the energy that will smack your opponent. Is your hollow shaft design up to that punishment? I'd recommend a well secured solid rod, at a minimum.

I also cannot recommend use of needle or ball bearings without an inner race for an aluminum shaft. The hard anodizing would help, but hardened steel rollers on aluminum is not a good match. I'd use bronze bushings for an aluminum shaft -- they stand up well to hard impact.

Q: If I want my [weapon] motor to be above the blade similar to 'Greenwave', what is the most reliable way of attaching the motor? I am thinking of having the motor mount clamping around the 1/2 inch aluminum [dead] shaft. Do you think it would be strong enough to stop the horizontal blade from moving upward from hits from drums and such?

A: Mark J. here: there are good reasons why you don't see many 'Green Wave' style spinners. The design places enormous stress on the dead shaft since only one end of the shaft is supported. Mounting the shaft to the chassis securely enough to withstand the impact force is a real problem. See the previous post on 'Green Wave' in this archive.

The motor mount will pretty much have to clamp onto the dead shaft since nothing else extends above the spinner! The design of the clamping mount will determine its strength but, since the whole shebang is likely to tear itself free from the chassis on the first good hit, you probably don't need to worry about the motor mount strength.

Q: Why are weapons that rely on electrocuting the enemy banned?

A: There are three reasons why specific types of weapons are banned:

  1. Safety: weapons must not be a risk to the drivers, crew, spectators, or the arena (explosives, projectiles...)

  2. Delay: weapons must not cause an unreasonable delay in the tournament to clean up after their use (liquids, adhesives...)

  3. Lack of entertainment: the audience came to see robots fight. Weapons that have no visable impact (radio jamming, electrical discharge...) or that interfere with audience visibility (smoke screen, high-intensity light...) defeat the whole purpose of having a robot fight.
Electric discharge weapons have a lot going against them. They are potentially quite dangerous because of the difficulty in telling if they are 'on' or 'safe'. They are also quite ineffective because the metal armor and chassis of a typical robot is an effective Faraday cage that would prevent electrical penetration. But the biggest problem is that there just wouldn't be much of a 'battle' for the audience to see: two robots touch, there's a spark, and the match is over. Boring!
Q: I just had an idea for a hammer weapon. This may be a bit hard to visualize...

Instead of having a hammerhead fixed onto a single spot on the hammer arm, it would be possible to have the hammer arm be a track that the hammerhead can move up and down on. It would be somewhat like an unpowered monorail. In addition, there is a weak spring that pulls the hammerhead towards the bottom of the hammer arm.

When the hammer is "at rest" the hammerhead is very close to the fulcrum/pivot/bottom of the shaft (Don't know a better word.) thanks to the spring. When the hammer is fired, the centrifugal force should overcome the spring and cause the hammerhead to move up to the top of the hammer arm. However, since the hammer head will still be close to the center, it will be much easier for the motor/piston to swing. By the time the swing is complete, the hammerhead will have moved to the top of the shaft and thus be in damaging position. After the swing is done, the spring draws the hammer back to the bottom of the hammer arm.

Problems could include protecting the delicate shaft. What do you think of the idea?

A: Mark J. here: your explanation is quite clear, but you can't gain energy by moving mass inward or outward within a rotating system. Yes, the hammer will be easier to start spinning with the mass closer to the shaft, but as the head mass moves outward the speed of the hammer as a whole will slow down to conserve the energy in the rotational system -- you don't gain any impact power. See the Hyperphysics notes on Angular Momentum.

However, if you get the point and rate at which the head moves just right you might gain a little efficiency by keeping the motor RPM in the optimum power zone. Search this archive for 'thought of this before' (really) to find a previous post about a sliding-weight spinner. For multiple reasons it would be better to work up a varying gear reduction drive system to acomplish the same effect for your hammer.

Q: Thinking about that virus bot idea [next post down]. There are a few more problems.
  1. It would count as a cluster/multi bot. If it got immobilised, the whole robot would be out. 50% of clusterbots rule and all that.

  2. If the robot was immobile like stated in the plan, then it wouldn't be moving under its own power, either giving points to the opponent due to the opponent pushing it around, or, if it fell off, would be counted out.

  3. To make it mobile would probably mean replacing the suction cups with electromagnets for convenience, and it probably wouldn't be practical getting powerful enough electromagnets in such a small robot.

  4. The only way to make a virusbot effective is to make it produce a lethal electric charge, and if I remember correctly, that's banned in pretty much every robot combat event/organisation.
As you said, however, it is a fun idea and I too would love to see it in action if ever possible.

A: I agree that there are a great many problems with the 'drilling virusbot' idea, but I don't agree with all of your points.

  • I don't think the virus would count as a multibot. A combat robot has to have mobility under the rules; since the virus doesn't I think I can argue that it doesn't qualify as a robot and therefore isn't part of a multibot. Call it a 'detachable weapon pod'.

  • Most current multibot rules require that 50% or more of a multibot by weight must be immobilised for a K.O. If the virus does count as a multibot but weighs less than the primary 'bot, you could lose it and still continue.

  • I can't recommend magnetic attraction for a virusbot. Too many common robot armor materials (aluminum, titanium, polycarbonate, stainless steel, garolite...) are non-magnetic.
You do remember correctly -- electric discharge weapons are banned by absolutely every robot combat organization.
Q: What do you think of this combat robot idea?

The robot has a slow moving electric "Hammer" that has a small claw on the end. Gripped in that claw is a second, immobile robot with suction pads and a drill bit that can be raised and lowered.

In combat, the larger robot would "hit" the enemy's flat top with its "hammer." This would do no damage, but it would be hopefully enough to make the smaller "virus" robot's suction cups stick. The larger robot than uses its claw to let go of the virusbot.

At this point, the Virus bot slowly drills a large hole into the enemy robot. It would be uninhibited, as few robots would have a way of removing it.

Depending on the virusbot's location, the enemy could take severe internal damage. [Oakland, CA]

A: I think it has too many 'dependings' and an unfortunate 'slowly'.

  • Many robots aren't going to provide any good spot for suction cups to stick;

  • If you do stick down you're unlikely to be in position to drill thru anything critical;

  • The drilling mechanism would be fairly heavy and complex (we like simple); and

  • While slowly being drilled, your opponent is rapidly beating the lockwashers out of your main robot.
Worst part: your opponents can defeat your suction virusbot by taping a sheet of paper to their top armor. Fun idea, but impractical.

Q: Are Diesector's jaws modified ''Jaws of Life''?

A: No. Builder Donald Hutson writes on the 'Diesector' webpage:

"At first, I wanted to use a "Phoenix" Jaws of Life but they were very heavy, slow, and require a 5000psi hydraulic pump... I ended up fabricating my own jaws using two heavy-duty MOTION (1000lb) actuators and some 3/8" steel plate... The idea of the jaws wasn't exactly to crush other robots, it was to create some thing that would be fast, reliable and gain control."

Q: Out of curiosity, exactly WHAT rules would ban a robot from using a plasma torch as a weapon? I know there must be at least 3.

Personally, I believe that the reason flamethrowers are ineffective is that they are overregulated. (In addition, the overregulation dashes my hopes of one day seeing a flamethrower bot explode.) My friend's homemade propane flamethrower is more effective than what I have seen in the robots.

A: There are a number of obstacles beyond rule infraction that would effectively prohibit use of a plasma torch in a combat robot. I don't know of any plasma cutter that will run off 48 volt battery power, electrical interference from a portable unit would fry your own electronics, and the need to 'strike an arc' and ground to the opponent would render the weapon useless against any non-conductive surface.

Topping the list for rules infractions would be section 12.4 of the RFL rules: "Heat and fire are forbidden as weapons." Granted, heat weapons may be allowed at the discretion of the event organizer, but no EO is going to allow a plasma torch loose in their arena.

Also likely to put the boot into a plasma torch is section 12.1.2 of the RFL rules, which prohibits "RF jamming equipment". Portable plasma torches typically throw enough RF interference to blot out the whole radio spectrum for a fair distance.

Terribly sorry that you find it difficult to enjoy robot combat without arena-filling explosions and balls of flame. If it were your arena, you'd think differently.

Q: What would be the best way to go about calculating the gearing needed for an electric hammer weapon?

A: Mark J. here: several comments:

  • Electric hammers are not effective. The difference between a correctly and incorrectly geared electric hammer is going to me minimal - neither will be impressive.

  • As pointed out in the [next post down], calculating optimum gearing for an electric flipper or electric hammer weapon is similar to calculating the gearing for a spinner weapon but greatly complicated by the need to include the rotational mass of the armature [and geartrain].

  • Data on the rotational mass of specific motor armatures is not available. A 'best guess' is about all you can reasonably do.

  • I have a modified version of the Run Amok Spinner spreadsheet that examines the early stages of weapon spin-up and approximates the action of an electric hammer. It is difficult to use, time consuming, and provides only approximate results.
My recommendation is to avoid the electric hammer. If you do procede, be prepared to experiment, and don't expect great results.

Addendum: OK, nobody believes me when I tell them that electric hammers don't have enough energy to be effective weapons. Maybe I can help you to prove it to yourselves. As noted above, I've been using a special version of the Run Amok Spinner spreadsheet modified to focus on the first full revolution that a spinner makes when climbing up to speed. The first half-revolution is the realm of the electric hammer weapon. I spent a few hours cleaning up this special version, adding in a few 'real world' performance adjustments, and creating a graphic output.

Like the Run Amok Spinner spreadsheet upon which it is based, the hammer spreadsheet requires Microsoft Excel to run. Run Amok Electric Hammer spreadsheet v1.2.

The spreadsheet is intended to model electric hammers with a 180 degree overhead swing that start and end in a horizontal position. Corrections are included for the effect of gravity on the hammer and for variance in torque with the speed of the electric motor. No correction is attepted for armature and gearbox inertia, so cases with unusually high or low gear reduction may yield faulty energy or actuation time estimates. Output includes guidance on correct gearing.

If you play around with the 'hammer' and 'spinner' spreadsheets, you will rapdily uncover the problem with an electric hammer: the energy output is only about 5% that of a spinner weapon of comparable mass using the same motor. A spinner has many revolutions and typically several seconds to store energy in the spinning mass, while an electric hammer has exactly half a revolution and a few milliseconds to gather energy from the motor. Electric hammers are not effective!

Q: Would an Ampflow A28-150 with a 4:1 ratio make a fast lifter/flipper for a 30lb robot? I'm trying to get a faster lift by using a bigger motor with less gearing.

A: Mark J. here: it's tempting to think that a bigger and more powerful electric motor would result in a big lift speed increase, but that isn't always the case. A lot of the low-range torque in a large motor can be used up accelerating their own substantial rotating armature mass. This is inconsequential in the hundreds of revolutions a spinner weapon makes to get up to speed, but for an axe or lifter it's another matter. Your proposed lifter motor would rotate only half a revolution to raise a simple lifter arm 45 degrees, so a good portion of the force that you would hope to go into lifting your opponent will be absorbed by the rotational mass of the motor itself.

You'd get a good lift with a reasonable length lifter arm, but I think you can forget about the weapon being a 'flipper'.

Q: I am trying to drive a spinning bar for a hobbyweight. The motor I want to use has a 1/8th shaft. The only good pulleys I can find that fit onto 1/8th inch shafts are all timing pulleys. I am looking for something that can slip, so that the motor doesn't stall whenever the spinning bar weapon hits something. I am afraid that if I can't find a belt/pulley setup that doesn't slip, I will face the dual threats of motor stalls and mechanical damage.

A: Mark J here: I'm more than a little concerned about driving a hobbyweight spinner from a 1/8" shaft. The small hub diameter timing pulleys that would fit that shaft won't handle enough torque for a decent hobbyweight weapon. Consider a different motor and/or a machined hub to connect the shaft to a properly sized pulley and belt - see the RoyMech website for guidance.

For general reference, timing belts can slip if set up very loose. I've seen several insect-class spinners running loose timing belt drives. Alternately, you can turn the belt inside-out and run the smooth side against the pulleys. Align the pulleys carefully and use a wide enough belt to handle the torque or you'll have real trouble keeping the belt on the pulleys!

Q: Any suggestions on building a hobbyweight crusher? I.E. What type of hydraulics should I use, how can i design my robot to do maximum damage to the opponent without tearing my own robot apart? Is it really worth all the time and effort building a hobbyweight crusher?

A: See #4. I think the probability of anyone building a successful hobbyweight crusher is very small.

The probability of someone who has to ask me 'how to do it' building a successful hobbyweight crusher is indistinguishable from zero.

Q: Is it possible to use compressed gas as a alternative to Co2 for a flipping weapon?

A: Both possible and fairly common. Section 7.2 of the 2010 Robot Fighting League rules says:

"Pneumatic systems on board the robot must only employ non-flammable, nonreactive gases (CO2, Nitrogen and air are most common)."

CO2 is stored in compressed, liquefied form and gives more 'shots' for the size of the storage tank, but the entire pneumatic system gets very cold from the conversion of the CO2 from liquid to gas. High Pressure Air (HPA) or nitrogen can both be stored at higher pressures than CO2 and they avoid the 'chill' problem.

Q: I want to use a Small Johnson motor for a weapon drive in a 12 pounder. It will use a belt drive, so slippage can occur. How many amps should the controller be able to handle?

A: It depends on how much 'slippage' is in the belt drive. Formula:

Peak Amp Draw with Weapon Stalled = (1 - Percentage of Maximum RPM Allowed by Slippage) × Motor Stall Amps.

Example for a Small Johnson motor and a belt allowing the motor to run at 30% of max RPM with the weapon stalled:

Peak Amp Draw with Weapon Stalled = (1 - .30) × 91.8 = 64.3 amps.

That's peak amps. Hopefully you'll shut down the weapon quickly when stalled, so the 'continuous' amps can be considerably less.

Q: How do I figure out belt slippage, then?

A: Trial and error. Start too loose and tighten 'til you get close.

Q: Sorry to bug you on the belt slippage issue again, but I still don't get how exactly you can tighten the belt once you have mounted the motor and the two pulleys. Drilling mounting holes is hard enough as it is when it comes to motors, and you can only make so many holes in a piece of metal...

A: Off the top of my head: eccentric bearing mounts, an idler pulley, and elongate slots with adjustment shims (don't rely on just the mounting bolts to hold tension) all address your adjustment issue. As the belt wears, further adjustment will be needed.

Q: Does using two motors to drive a lifter arm reduce the amount of torque and gear reduction each motor needs? I saw that 'IO' used 2 motors to drive the lifter mechanism. I thought maybe the lift could be faster with two motors having less gear reduction.

A: Yes - more power (from two motors or a single more powerful motor) will allow for a faster lift, but selecting the optimum gear ratio is critical. A lifter that appears faster when operating under no-load may well bog down to a crawl or even stall completely when loaded down.

The trick in getting the correct gear reduction for best lifting speed is in understanding that a permanent magnet direct current (PMDC) electric motor generates its peak horsepower when loaded to produce half its stall torque:

  • If you load the motor more heavily (too little gear reduction) the motor bogs, pulls excessive amperage, and the lift speed decreases.

  • If you load the motor more lightly (too much gear reduction) the motor spins more freely, pulls fewer amps, but again the lift speed decreases.
You won't know exactly how much weight is going to be on your lifter in every situation: sometimes you'll be lifting just an edge of your opponent, and sometimes you'll have the entire weight of your opponent balanced on the lifter tip. We design for lifting the full weight of our opponent, but many builders design to lift only half that amount.

Mark J here: Aaron is entirely correct, but maybe I can give you a little more help.

Let me re-write the lifter gearing equation from further down in this archive to make it easier to calculate the gear reduction needed for best lift speed at a given lift weight:

Optimum Lifter Gear Reduction = (Arm Length [inches] × Lift Weight [ounces] × 2) ÷ Motor Stall Torque [oz-in]

Example for an RS-550 motor (91.6 oz-in stall torque) driving a simple 8 inch arm to lift 30 pounds (480 ounces):

Optimum Lifter Gear Reduction = (8 × 480 × 2) ÷ 91.6 = 84 [to 1]

To calculate for two RS-550 motors, just double the torque.

Q: Thanks so much for the help with the RS-550 lifter info Aaron and Mark.

A: Happy to help.

Q: Would the PDX256 - 256:1 Gearmotor be a good choice to power a simple single-pivot 6 or 8 inch lifting arm for a 30lb robot? Would the IFI VEX Pro Victor HV-36V be a good choice of controller for it? If not is there a better way to control the arm?

A: It's kinda overkill, but the PDX256 would certainly get the job done. That gearmotor will deliver enough power to lift 90 pounds at the end of a simple 8" arm using only half of its rated stall torque. I don't think I'd stress the gearbox with that much weight, but it should certainly be tough enough for a featherweight lifter.

The IFI VEX Pro Victor HV-36V is also overkill. Lifting 30 pounds on the end of an 8" arm, the PDX256 gearmotor will pull only about 25 amps at 12 volts, well within the capacity of the less expensive IFI VEX Pro Victor 884.

Q: Could you tell me the formula for calculating the amount a motor will lift? If I could get away with a smaller motor, that would be great!

A: We've given a formula previously, but I think I can make it a little clearer. For a simple lever arm lifter:

Lift Capacity [pounds] = (Motor Stall Torque [oz-in] × Gear Reduction) ÷ (Arm Length [inches] × 16 oz/lb × 'TAF')

Peak Amps @ Lift Capacity = Motor Stall Current [amps] ÷ 'TAF'

Peak Amps @ X Pounds Lift = Peak Amps @ Lift Capacity × (X pounds ÷ Lift Capacity)

What is 'TAF' in the equations? The more heavilly a motor is loaded, the slower it runs. If loaded to its full stall torque rating, it will not move at all. The 'Torque Allowance Factor' at the end of the formula de-rates the max load to reduce current draw and allow a reasonable lift speed. I use a very conservative allowance of 2 for my robots. Some builders leave the allowance off entirely (= 1). You'll be OK with a 'TAF' around 1.5.

Examples for your PDX256 gearmotor:

Lift Capacity = (91.6 oz-in × 256) ÷ (8 inches × 16 oz/lb × 2) = 91.6 pounds

Peak Amps @ Lift Capacity = 148 amps ÷ 2 = 74 amps

Peak Amps @ 30 Pound Lift = 74 amps × (30 lb ÷ 90 lb) = 24.7 amps

Q: How did Mechavore get such a powerful saw?

A: Proper design fundamentals: powerful motor, correct drivetrain, well-designed cutting head. It didn't hurt to have a lot of opponents with weak side armor.

Q: I know you aren't a big fan of electric hammers, but humor me.

I want to use a PDX16 or the PDX26 motor to power a hammer weapon. (No, I will not switch to a different weapon.) It will drive a belt connected to the hammer, so that it can slip and not fry the controller on the first hit. Now for the question:

Can you think of a way in which I could reach my goal of at least 40 joules of force? I know you hate these general questions, but I am able to pretty much go with any arm length length/hammer weapon, and I have no freakin clue how to calculate joules. I just know they are important.

If nessicary, I can use two motors to power hammer.

A: Mark J here: I'm curious - if you aren't sure what a joule is, how did you decide that 40 was a good number?

Scroll down the page to the next question -- it's from a builder of a sportsman class electric hammer 'bot that uses the RS-550 motor you're interested in. Calculating weapon power from a specific motor for an electric hammer is not simple, but my solution for his weapon generates someplace close to 60 joules. I think you'll need more gear reduction than the PDX26 gearbox provides -- add some in your belt drive stage?

Q: dear aaron it seems that i don't have the time or money to build a crusher so i looked into designing a sportsman class hammer bot. is it possible to use any of the p60 gearboxes and motors to get the power and speed that i desire? can you please help me find a ratio that is suitable? p.s the chassis is built and im using a 12 volt sealed lead acid battery rated at 5AH. please help..................... from anthony

A: Hi, Anthony! I haven't heard from you for a while. Are you asking about powering the robot drivetrain or the weapon with the P60 gearbox?

For a drivetrain, the BaneBots P60 gearbox attached to the RS-550 motor would provide ample drive power and speed for a 30-pound robot. The RS-550 will operate quite well on 12 volts. The gear ratio will depend on the wheel size and the size of the arena. The 25.92:1 gearbox with 4" wheels would be about right for a 16 foot arena.

For the weapon, I don't think you're going to be happy with an electric drive. I'd need more information about the length and weight of the hammer (and a fair amount of time) to do the calculations, but he electric hammers I've looked at for other people's designs have all shown poor performance.

Q: hi aaron its anthony. the hammer arm on my robot will be 18 inches long and the hammer head will be 1 pound. the hammer arm will be made out of 3mm thick T-6 aluminum square tubing. thank you for your help i realy appreciate it.

A: Mark J here: an electrically powered hammer is difficult to model mathematically. The inertia of the motor armature and associated gearbox elements plays a large roll in the acceleration of the hammer. A model which fails to account for drivetrain inertia provides deceptively high performance for high gear reduction ratios.

I made some modifications to my copy of the Run Amok Spinner spreadsheet to better model the very early stages of weapon acceleration for your hammer weapon. The best numbers for the RS-550 motor and P-60 gearbox came with the 64:1 gear ratio. I'm not entirely confident of the numbers, but I calculate an actuation time of about 0.25 second and around 60 joules of energy at impact. That's very poor energy delivery compared to a featherweight spinner that could conservatively pack 600 joules, so don't expect to do much damage.

Q: dear aaron thanks for your help! im now registered and preparing to go to the franklin institute 2010. now im thinking of future upgrades for my bot. the upgrade i was thinking about was istead of using a speed 540 size motor i was thinking about using an ampflow A28 400 motor geard to 2.1 and having a one pound hammer head with the same 18 inch long 3mm thick aluminum square tubing atached with strong ball bearings and on the main shaft how much power or jouls will this have compared to a feather weight spinner?

A: You'll have a much better idea of what type of upgrades your bot will benefit from after the tournament, Anthony. You might be very surprised to find out that your weapon isn't your top priority.

Calculating energy for an electric motor powered hammer weapon is not easy -- those of you who may think it is are overlooking several factors. My adaptation of the spinner spreadsheet works fairly well for hammers powered thru large gear reductions, but with big motors and small gear reductions it works very poorly. I'd need to know the rotational inertia of the AmpFlow A28-400 armature to even get a start on the calculations and I just don't have that info.

Write back after the tournament and we'll figure out what upgrades your robot really needs.

Q: dear aron i just got back franklin institute 2010 and my robot came in last place. some how my weapon motor burnt out when it was tested. this acident was my fualt for not getting upgraded spectrum transmitter equipment. [The RS-550 motor with a] 64:1 gear ratio was not very powerful and so i took apart a 12 volt cordless drill and hooked the hammer up to it and it still was not impressive. you said that electric power sucks. do you have any ideas on how to better accellerate the hammer arm. should i go with a low pressure phumatics system mabe 300psi i now have more team members pitching in so i can spend close to another grand on it. i know that i'm new to phumatics but i still want to give it a try. if you agree on this can you rcommend a sysytem for me with listed compatible component's thanks for your help. and your site is awsome!.........from anthony..builder of warpz

A: Sorry to hear that things went badly for you at the tournament. I'm puzzled about how radio problems caused your weapon motor to fail, but you seem to have a working theory. I'm more interested in how the chassis and drivetrain performed. It would be a waste of time, money, and effort to concentrate on a more powerful weapon if the other systems aren't sorted out yet.

As we said in our earlier response, a small electric motor like the RS-550 is not going to deliver enough power for a hammer weapon to do much damage to a reasonably armored opponent. Your weapon has about as much kinetic energy as a good beetleweight spinner. But since you have chosen to enter the featherweight 'sportsman' class where high-powered weaponry is not allowed, it may not be a bad match.

I suggest that you continue development of your electric hammer and the other basic systems on your robot. If you can't get the kinks worked out of a relatively simple electric weapon you really shouldn't dive into the added complexity and danger of pneumatic valving systems, actuators, regulators, and pressure tanks. Get your robot to work before you make it more complicated.

Q: I am an avid robot builder, and for my next 'bot I'm going to make a flipper like T-minus's. Is pneumatics the best way to go?

A: Yes. Read thru this archive for reasons and alternatives.

Q: I am trying to make a 30lb lifter robot similar to Sewer Snake. Would an old style Dewalt 18 volt motor and gearbox work for the lifter motor?

A: It wouldn't be my first choice; it has way too much speed and only marginal torque for the purpose. Locked in 'low' gear the 18v DeWalt @ 24 volts spins up near 500 RPM, which would make a directly connected lifter difficult to control/position. The torque would be barely adequate for a short (maybe 6") 'Sewer Snake' style lifter wedge in a featherweight -- but it would pull a huge bucket of amps when bogged down or stalled.

I'd go with less motor and a higher reduction gearbox to slow the actuation speed and keep the amperage draw reasonable.

Q: Do you know how THz's axe work? I'm asking because I'm building my third 'bot, one with a functional weapon.

A: 'TerrorHurtz' has a pneumatic axe actuated thru a rack and pinion, much like the hammer on 'The Judge'. Search this archive for 'Jacha Little' to see an animation of the layout and for 'TerrorHurtz' for additional notes.


A: A common solution is to thread bolts into the drum and use the exposed heads as impactors. You can start with the bolts holding down your shift keys.

Q: What would you say is a good weapon for begginner builders?

A: If you read thru the archives you'll find many posts where we recommend that first-time builders concentrate on the basics of chassis, radio, and drivetrain and save active weapons for a later robot. Build a nice wedge for your first robot - you'll do much better with that than trying to get an active weapon right. Follow the link in #26 for more info.

Q: This may sound like a stupid question, but I'm building a invertible wedge 'bot like Rammstein and I can't find a pick for the weapon, (sort of what rammstein has) but I can't find one at Home Depot, or Lowes, or other stores like that. Do you kow where I can find one online?

Also, my brother also built a combat robot, but his is a VS (veritcle spinner) and even though mines more simple, his has already broken down after one fight, and has lots of repairs to do. I entered mine last year without a weapon, and it one quite a few matchs. Just goes to show that no weapon can [sometimes] be better than a big weapon, eh?

A: You neglected to tell me your weight class and how large a 'pick' you seek. I think you're searching with the wrong terminology. Try a search for "steel spike" and see if that turns up something suitable. Alternately, you can order some tool steel rod in the diameter you want and grind a point on one end. Another good option would be a cold steel chisel, which you should be able to find locally.

Yes, passive weapons are often better than more complex weaponry.

Q: If I recall correctly, Mauler was highly anticipated in Battlebots and everyone expected it to do really well. Instead, Mauler became known for flipping itself over and "breakdancing." What did Mauler do wrong?

A: There were many versions of 'Mauler' -- the first being 'South Bay Mauler' which appeared at the 1994 Robot Wars event. It was 'Mauler 5150' that performed the famous instability flip at BattleBots 4.0. The 'standard' explanation of the cause of the flip is that the shell was not properly balanced -- but that's not correct!

'Mauler 5150' violated a key law of rotational stability: of the three natural axis around which a rigid symmetrical body may rotate, the axis with the greatest and the least rotational inertia are stable, but the third axis with intermediate inertia is not! The spinner shell had two heavy impact hammers on opposite sides of the shell that made spinning on edge with the hammers to the outside the axis with the greatest rotational momentum! Distributing the impact mass more evenly around the perimeter of the spinner shell would have prevented this problem. A web search for 'rotational stability' will provide more detail on the physics involved if you're interested.

Q: Does using the rear-bar of the four-bar linkage required less power to lift the opponent as compared to uusing the front bar?

A: No, but don't confuse power and force. Power is the product of force and distance over time. Moving the rear bar will take less force, but the greater angular distance involved in moving the rear bar offsets the saving in force and ends up requiring the same amount of actual power as moving the longer front bar.

If you are using a linear actuator, it's usually more mechanically efficient to power the longer front bar because of the smaller change in angle it undergoes as the lifter rises. If you are using a gearmotor attached to the lower pivot point, the reduced rotational force (torque) required to move the shorter rear bar may be the attractive option.

Q: How does the weapon of 'zion' works?

A: At BattleBots 4.0 and 5.0 middleweight 'Zion' had a single pivot pneumatic lifter with the actuator attached to a lever descending from the point of the high mounted pivot. There is a photo of Zion and a brief discussion of its design in the archive.

Q: Does son of whiaci use one or two of the brrigs and Stratton Etek motors?

A: 'Son of Whyachi' has evolved thru several different rotor power configurations. The most recent version used two 15 hp Yamaha gasoline kart engines. The previous version used three Eteks. The BattleBots champion 'shufflebot' version used two Eteks.

Q: How do you set up a clutch system for a weapon? The only clutches I have ever seen are the 'VEX clutches', so I am very clueless on this topic.

A: We're talking about torque limiting slip clutches that decouple the motor from high loads near stall. Combat robots generally don't use them. This isn't the science fair, it's combat and you want the motor to deliver all the torque available even at the risk of melting. A fast finger on the shut-off switch and faith in the design is all the protection many weapon systems have.

The exception is for spinner weapons. Spinners often use a belt drive that can slip under high torque loading on impact. This is particularly useful with high performance brushless motors that just won't tolerate stalling.

On a related topic: combat robots don't fight with fuses in the power system. If you blow a fuse it's an instant loss and the robot becomes a chew toy for your opponent. Get every last gram of fight out of your machine. If you have to lose, go down fighting and smoking!

Q: How does this idea sound? Incorporating aspects of both an overhead hammerbot and a horizontal spinner, it would have a "hammer" like any other hammerbot, but it is mounted sideways in a way so that it can be swung across the diameter of the robot. The idea is based on 'Tazbot'. However, 'Tazbot', while a cool robot, fought more like a lifter than a hybrid thwack. My idea does away with the lifting arm part.

When the robot is close enough to its enemy, it would swing the hammer from around and hit the enemy's side... and then swing the other way and hit the enemy with an even stronger hit. (The weapon would have nearly twice as large an arc to build up speed the 2nd time.)

The advantages I can think of are as follows:

  • Unlike an ordinary hammerbot, the hammer would occasionally have an over 180 degree swing arc.
  • It would be easier to score a hit, as you don't need to line up attacks.
  • Unlike a true horizontal spinner, however, it doesn't bounce away from its opponent, allowing it to get more hits.
In addition, the simple fact that it is a new idea is a plus in itself. What do you think of the idea?

A: So - a horizontal, electric, full-circle hammer? The disadvantages I can think of are as follows:

  • Weak impact. Even given that 180 degree plus arc, an electric hammer weapon just won't build up much energy compared to a pneumatic hammer or an electric spinner gaining energy over hundreds of revolutions.

  • The lack of 'bounce away' would largely be due to that weak impact. If you did manage a high-energy impact you'd get a reaction similar to a horizontal spinner.

  • Judging criteria are different now than they were in the BattleBots days. You no longer get credit for impacts that don't damage, and judges unfortunately give zero points for 'new' ideas.
On the plus side, it sounds like it would be fun to drive. I really don't like to discourage builders from trying new things, but it's really hard to come up with a good idea that hasn't already been tried in the 16 year history of the sport. That is one of the factors that has lead to the decline of interest in combat robotics. We need new areas of competition where builders can express their creativity
Q: I want to make a horizontal spinner over the center of the bot, but I don't know the internal structure like which and how many motors are needed and how to get speed+torque with help of gears. Please guide?

A: You'll want to download the Run Amok Spinner Excel spreadsheet. The spreadsheet can take the dimensions of your spinner weapon and the materials it is made from and evaluate the speed, energy, and spin-up time of that weapon with any motor for which you can get specifications at any reduction ratio you choose. There are many posts about use of the spreadsheet in this archive. You'll also find help here with weapon drivetrains.

Q: Has any flamethrower robot ever...blown up? I have seen them catch fire upon losing, but that's it.

A: Event organizers take safety very seriously. A flamethower design with any real chance of blowing up would not be allowed to compete. It hasn't happened and it isn't going to happen.

Q: I want to create a robot that has a hammer, but I want hooks on the other side of it, to grab hold of robots behind it. Is it possible to build a hammer like that?

A: Possible sure, but I see a few of problems:

  • I don't know what you expect to grab onto with those hooks. Seems like a low-probability attack.

  • Hammers are designed for speed to give the greatest energy on impact. A lifter is usually designed for power, particularly if you've got something 'hooked' onto the end so that you're not going to be able to 'flip' it. Those two design requirements don't mix well.

  • The 'hammer' end needs a good bit of hangover on one end of the 'bot to get in a good hit. Your 'hook' end would need a good hangover on the other end to stand a chance of grabbing something on your opponent. With overhang on both ends, the wheelbase won't be able to well support either the lifting or the hammering -- the robot will be 'tippy'.
I'd suggest concentrating on a good single-function weapon.

Q: i meant the hook as a crushing one not a lifter

A: A crushing hook? To 'grab hold of robots behind it' and crush them??? Nah... stick with a single function weapon.

Q: Where can I find a good "drum" for a beetlweight drum weapon?

A: Drums aren't found, they're made. Read thru this archive for info on the construction of spinning cylinders. A well-balanced cylinder is not easy to construct -- you may want to consider an eggbeater-style spinner.

Q: I know this is gonna drive you absolutely nuts, but the little voice is telling me to go against your wisdom and build a hammerbot.....that is beetleweight....that uses an electric motor. So, assuming that it has about 5 joules, which of the following would it lose to?
  1. A top notch beetleweight
  2. An above average one.
  3. An average one.
  4. A below average one.
  5. A crappy one.
If it has a chance against a #4, I will be happy. (I am used to losing more often than not.)

A: I've said this before 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.

Pick whatever weapon will make you happy and go have a good time!

Q: Hi Aaron.

Well, we've been always talking about "Energy" stored in a weapon - for successful/effective combat weapons. On the other hand other thing which I am interested in knowing is "How much Impact force comes into play when we have a known amount of energy?". Well if we can calculate that, we can know the amount of stress the material has to handle without loosing it's functionality. I know that the loads and forces in this sport are wildly unpredictable. But still, we can't use mere judgment to design an effective weapon. Also, we cannot always afford to do design and then try it out all the time. It consumes a lot, a lot and a lot of time and wastes Money - which I think is one important parameter.

Moreover we've to choose a material which best matches with the required properties,how should one do that? I mean find out the right material..??

What are the other parameters other than forces and power should one take into account while designing a (spinning)weapon?

Are there any advances in the sport (regarding design) over the years?? Or people use the old conventional methods?

Can u point out any good links where from I can get more information regarding this matter and the sport?
Hoping for some quick replies.

A: Mark J here: your 'hoping for some quick replies' forces me to remind you of the well-known 'project triangle': fast, good, cheap - pick any two. Since the advice you get here is as cheap as it comes and you want it quick, that means it's not gonna be very good

You've obviously read our archives, as I recognize some of our phrases in your questions. The truth of combat robot design is that the forces are wildly unpredictable. We can calculate the energy available in a weapon, but the actual force delivered is dependent on a great many factors in a complex environment. Trying to calculate the forces and vectors involved will both drive you mad and result in values that are no better than the multitude of assumptions you had to make in order to even start the process.

It is true that one cannot use 'mere judgement' and that trial and error is wasteful of time and money. Fortunately, the efforts of hundreds of builders expending their time and money on thousands of designs that have come before us can be used to good effect. We very often advise builders to 'examine successful robots of design similar to your own' when considering materials, dimensions, and design details. In fairness, examining unsuccessful robots to learn from their mistakes can be equally educational. Learn from the successes and errors of other builders.

A critical factor in designing a spinning weapon is the ability to transfer the stored energy effectively to the opponent. A huge reserve of stored energy is without value if it cannot be effectively applied. This can result in opposing design requirements: high spin rate to store maximum energy and a long period between impactor passage at the weapon circumference to allow time to insert a large piece of your opponent into the 'danger zone'. A very small number of impactors (like 'one') can help here, as can a large diameter weapon with a high moment of rotational inertia to store large amounts of energy at relatively low RPM -- but the large diameter creates other problems. Combat robot design is FULL of compromises.

I'm going to lump your last two questions together and give you a couple of links that address new developments and an analytical approach to robot/weapon design:

RioBotz Combat Tutorial

Paul Hills' Spinning Weapon Physics

Q: I had a question about the Run Amok Spinner Excel spreadsheet that you have....

I have a brushless motor with the following stats: 750kv, 24 volts, 35mOhm internal resistance. I rounded the result down to 8Nm stall torque.

I'm planning on using this in a beetleweight spinner and using the spreadsheet I put .1 kilos of steel at the tips of a 1 foot bar of aluminum (also .1 kilo, .25" by .75").

The thing that is bothering me, though, is that according to the spreadsheet this gives me 1350 joules and over 10,000 rpm in less than half a second. I'm pretty sure this much energy would destroy not only my opponent, but me and the smaller arena that these could compete in...

So I guess my question is "What am I doing wrong on this spreadsheet?". Maybe I'm using too big of a motor, but it fits well within my weight (well, not after building a chassis to withstand that energy!) Thanks.

A: Mark J here: making a few assumptions about your gearing (direct drive) and the dimensions of your steel tips, I get 2042 joules at 11340 RPM in 0.68 seconds for your proposed weapon. The steel tips should be calculated as a 'ring' equal to their actual mass with outer radius and 'thickness' corresponding to the outside radius of the bar and their length. That probably accounts for the differences in our calculations. You have, however, made a different and very common mistake:

The Run Amok Spinner Spreadsheet is a valuable evaluation tool, but it is not a design tool. The spreadsheet will provide a good estimate of how much energy the weapon will store and how quickly it will store that energy. It will not tell you if the design being evaluated will be a good weapon.

The common mistake in spinner design is to spin the weapon too fast. It's very tempting -- the energy stored by the weapon increases with the square of speed, so four times the speed gives you sixteen times the energy! The problem is that increased speed creates new problems that rapidly offset the increased energy. Example: at full speed your bar spinner will have a blade tip passing a point on the weapon circumference every 1.7 milliseconds! How much of your opponent do you think you can stuff into the rotating blade in that length of time? Unless your opponent is all sharp edges, the answer is 'not enough'. Huge stored energy won't do you any good if you can't deliver it effectively to your opponent.

You've got too much motor, too much tip speed and not enough tip weight. A quick calculation with double the tip weight, a 3:1 belt speed reduction and the same motor give you a weapon that spins to 395 Joules at 3780 RPM in 0.13 second -- much more workable. Try a smaller motor and add the saved weight back into the steel blade tips.

Q: I did realize that this many RPM would be too high, and as you have done I geared it down and messed with different weights, I did not assume your spreadsheet giving me high energy storage would give me a good weapon.

I originally had .7 lbs into my spinning weapon, but even lowering to a more usable 3000 rpm with gearing down gave me scary results, similar to what you got. I estimated between 50-100 joules would be more than enough, I'm not sure if I could build a chassis with 1 lb that would survive that.

In fact the higher RPMs could be usable, if in a different way--depending on the final design.

I was thinking about gearing a vertical spinner with about 6-8 inch diameter weapon at about .5 lb. I'd have the weapon mounted close to the floor with spring suspension on my axles so that when a big hit happened the force would be transferred through the mounts to the floor -- is this viable?

I was also wondering if anyone has tried using a second, smaller flywheel next to their main weapon spinning in the opposite direction to counteract the gyro-forces? I realize this would take out some weight, however it could be made to be a true cylinder with little supporting it (since it wouldn't take large hits) and therefore weigh less...

If this would work, would it happen that when I hit someone the weapon would, for a moment, slow down enough to change they gyro forces to flip me?

A second opinion would be nice! Thanks

A: Aha! A reasonable and thoughtful approach to weapon design. Please consider my previous warnings to be for the benefit of the less experienced designers who may read this discussion.

I agree that 50 to 100 joules is plenty of energy for a beetleweight spinner. Careful design can provide a light chassis that can handle the resulting impact force quite well, but you are entirely correct to be wary of high energy levels.

There is a special case where spinners can make use of really high RPM. When two drum-spinners go 'weapon-to-weapon' the slower drum looses. I have seen drum spinners that cruise at a moderate weapon speed for most attacks, but have spare motor speed that the driver uses against other drums. This is probably not useful for bar spinners.

Spring suspension: I think not. Read thru section 6.6.5 of the RioBotz Combat Tutorial. They emphasize stiffness in the 'force path' between the weapon and the floor and make good points about this approach. I don't agree with RioBotz on all topics, but they have put a lot of thought into a wide range of design issues.

Counter-rotating gyroscopic masses: I haven't seen this used on a combat robot, but your physics thinking is correct -- two coaxial masses spinning in opposite directions can cancel each other's gyroscopic forces (see: discusion on coaxial helicopter rotors). This will make turning much less of a problem for a vertical spinner as there will be no appreciable gyroscopic precession attepting to push the robot over onto its side. Unfortunately the stabilizing effect of the spinning weapon is also removed, rendering your robot more succeptable to tipping from outside forces. Calculating the relative speeds of two unequally sized counter rotating masses to precisely cancel each other is not trivial, but there are equations in section 6.15 of the RioBotz Tutorial that you may find useful.

Don't worry about the gyroscopic effects from slowing down the weapon with a hit. Slowing one of the two counter-rotating spinning masses would return some gyroscopic stability, not create instability.

Q: I read through the Riobotz tutorial (which has a ton of information) and I understand that the path needs to be as stiff as possible to transfer the most energy to my opponent.

Back to the suspension idea - The springs holding the wheels in place would provide just enough force to keep the robot ~1/16" off the ground. This being said, when being compressed further hardly any energy would be lost and the second the chassis hit the ground it would provide all the stiffness needed, as per Riobotz.

Having this set-up would possibly increase mobility (over designs such as Altitude which just slides on the ground) but would probably add too much complexity when obviously a steel ball with a rounded end gets the job done!

A: Thanks for the more detailed explanation of your suspension. You may need quite a bit more spring pressure than you think to keep the chassis from 'digging in' during maneuvers -- particularly since the weapon is generating gyroscopic precessional side-forces! I'm a big fan of keeping things simple and would probably stick with ball casters or teflon skids.

Q: What's your take on using a UHMW [polyethylene] rod for a spinner bar? I recently ordered a 24" chunk of 3/4" diameter and this stuff is light, and impact resistance is it's specialty right?

Would using a 2' (or slightly less) piece of this with steel impact ends bolted on stand up in a 3lber? (my next project after this 30lb).

It seems like if you got that spinning you'd throw anything pretty far, but I'd rather not throw half my bar with the other 'bot. (I'm not sure if it's THAT impact resistant) Maybe I'll hook it up and test it...

A: No, I don't like the idea at all. UHMW is resistant to damage from impact because it's very flexible. You want the bar to be extremely stiff to impart as much of the stored energy as possible to your opponent rather than flex on impact to dissipate that energy. A rod spinner is quite inefficient at energy storage to start with, so you can't afford to waste energy with a squishy impact. It'd be like hitting a ball with a foam rubber bat: it wouldn't break but you're not gonna hit a home run with it.

I've seen UHMW used for the body of a disk spinner where there was less impact flexing, but even there I'd suggest something stiffer!

Q: Some more thinking has made me realize that a spinner bearing 1/3 of my weight is probably not the best route, so the rod may be shortened some (1 ft or so, with the steel impact ends), but more importantly I had another question, do current spinners incorporate any sort of slip-clutch mechanism ?

It seems like it would make sense to put one between the drive gear and the actual bar in order to put less shock on the motor (if left in full throttle during an impact) as well as providing a way to lessen the force that would fling my robot across the arena (in the case of a horizontal bar).

Any input?

A: Mark J here: the preferred drive for a spinner is a belt and pulley system which incorporates a natural 'slip' capacity. The need for weapon driveline slip decreases with the weight of the weapon, but it isn't a bad idea even in the insect weight classes. The shock on the high-reving motor will be large whether you have the throttle on or off at impact - just don't leave the motor on if the weapon gets jammed.

A slip clutch isn't going to lessen the newtonian action/reaction 'kickback' on impact -- that's applied directly to the fulcrum point of the spinner axle supports with a vector opposite to that imparted to your opponent. Your proposed flexible spinner rod would reduce the kickback, but would also (as Aaron pointed out) reduce the impact on your opponent.

I suggest you read thru this archive for spinner weapon design tips.

Q: I'm planning on starting my own beetle (hackbot) and I was wondering, what all can be used to keep the oponent on top of the bot? I'm hoping to set up a back hinged lifter and I thought about using sandpaper or tape, but would tape be an entanglement device?

A: The '2010 RFL rules' define 'entanglement devices' as:

"...weapons or defenses that can reasonably be expected to stop drive train and/or weapon motion by being wrapped around rotating parts."

Strips of tape that could wrap around rotating parts are out, but a firm sticky surface is technically allowable. A specific event organizer may or may not agree. I'm not sure what you're going to do with your opponent stuck to the top of your 'bot that's going to score any points. A mechanical 'clampbot' might be a better choice, and is certainly legal.

Q: Back hinge flipper guy again, the reason I thought of that was so there was no way an oponent could come off before I flip it, and using something quick and cheaply replaced.

A: Your first question said you were building a lifter. Has it turned into a flipper now? I don't think you want a sticky flipper -- you need to get a flipper as far 'under' your opponent as you can, not stop them before they are in position.

Q: I'm currently designing a 4 bar lifter [for a featherweight] and I'm running into issues with how to make it move!

I have a pair of 100 lb force 2" stroke linear actuators that I'm going to use to power it. I want to use the full 2" stroke to take advantage of the built in limit switches, which makes it impossible to use a rack and pinion set up (my robot has 2" height).

The solution I came up with is to push on the rear, smaller bar, straight toward the front of my robot. The problem I'm seeing here is that most of the force of the actuators is wasted pushing against the supports while only a small portion goes to rotating the rear bar (and thus lifting), at least in the beginning. Once it's up the bar becomes more vertical and more of the force is actually usable. (This would be easier to explain with a picture).

So I guess my question is "What would be a better way to use the 200 lbs combined force and 2" stroke to lift another 30 lb robot without using a rack and pinion?"

If I can get the power transmission more efficient then I could possibly purchase the same actuators but with a higher speed and less force.

A: Mark J here: it sounds like you made a decision about the height of your robot and now you're trying to design a lifter assembly with specific performance requirements to fit that height. It's not good design process to compromise the performance of a critical system by forcing it into an arbitrary dimensional envelope. I don't believe it's possible to fit those actuators into an efficient mechanical linkage in a 2" space. Let's look at some options.

A rack and pinion drive would not be desireable even if it did meet your dimensional requirements. The torque requirement for a powered bar on a 4-bar lifter varies with its position -- high near the 'flat' collapsed position and typically decreasing as the lifter rises. A rack and pinion would provide equal torque across the entire range of motion, which would not be efficient.

The geometry of your current solution is even worse. For the reason you point out, the actuators would provide very little rotational force with the lifter in the 'flat' position where the requirement is greatest. That's simply not going to work.

We (very) often advise builders to examine successful robots of similar design to gleen ideas. If only there was an extremely successful, very low profile robot with a 4-bar lifter powered by electric actuators... Hey, what about the most successful combat robot of all time: 'BioHazard'? Did you happen to wonder how Carlo beat this problem?

About half way down the BioHazard mechanical design page is a section called 'How does BioHazard's arm work?' Very briefly, the solution is to use a bellcrank that offsets the lever arms against which the actuators press by 90 degrees relative to the bar. This allows maximum rotational torque exactly where the lifter needs it and then transitions to greater speed advantage as the lifter rises. There are multiple pictures of the mechanism on the mechanical design page, and more photos on other pages at the site. I recommend that every robot builder read every word on every page at Carlo's site.

Do note that it takes a great deal of force to actuate a compact 4-bar lifter even with an efficient design. The actuators in 'BioHazard' produce more than 2800 pounds of force over a 3.5" stroke to lift a 220 pound robot. Scaling that down to a 30-pound featherweight takes you close to 400 pound of force for similar performance. Check your design calculations carefully or you may find yourself with an underpowered lifter.

Q: Thank you for your help.

I had read through Carlo's website a few times, and studied his pictures (including that one) but I couldn't make out exactly what he did until now, when you pointed it out. I had thought about doing the very same, although I haven't quite worked out how to make it fit.

As for the scaling down, I'm powering the rear bar which is much shorter than the front, which means the torque reduction will be significantly less than on Biohazard where the bar looks to be about 5-7 times as long as the one the actuators push on.

I will go back to my perpendicular bar design and figure out how to make it fit -- thanks.

A: Glad to help. The photos on Carlo's site are not great, so I'm releived that you were able to figure it out. I've made a drawing for those not so familliar with the issue.

About powering the 'short' bar: yes, the rear bar will require less torque, but it also has to move farther. To get it to move farther you'll have to reduce the length of the bellcrank. Reducing the length of the bellcrank increases the 'push' required to move the bar. If you run the numbers you'll find that it makes no difference which bar you power with your linear actuators -- either one will require the same amount of force to fully extend using the full actuator stroke. You may also get into trouble with actuator angles if the short bar needs to move thru more than 90 degrees.

Q: Hi Aaron, just wanted to say that you have helped me enormously in the past two years, but for my new hobbyweight, I have yet another question: Would A2 tool steel make for a good weapon bar? Robot is a horiz. spinner, blade will be 1/4" thick and spin at approx. 6000RPM. Thank You

A: Mark J here: from both Aaron and myself, you're welcome. We enjoy answering your questions.

In general I can say that I don't recommend any tool steel alloy for an entire spinning weapon. Tool steels are hard and inelastic, which makes them a fine choice for impact teeth or inserts at the ends of bar weapons. The problem is that tool steels are not tough and resilient; they tend to shatter when placed in tensile stress. A bar spinner weapon experiences a large tensile load on impact. I'd suggest you make the bar out of a tough aluminum alloy (or titanium if you have the budget) and save the tool steel for bolt-on impact surfaces. Don't forget to have them hardened!

Now before somebody writes to mention that Ray Billings uses a tool steel bar on superheavyweight 'Tombstone' without any trouble, I'd like to point out that a very large chunk of the entire robot's weight is in that massive bar. I think you might be able to build a bar that thick out of glass and not have it break.

Q: i was thinking of making my third robot a drum i can make a balanced one because my best friend works at briggs and Stratton and he has agreed to help, i looked at the motor that the guy below wants to use, the park EFL-370-1080 i played around with that in the spinner spread sheet and i figured with a 3 inch long 4 inch diameter and 1.5mm thick titanium it would use a 6.4mm bar as a end plate i could get it to weigh about 4.5 oz i would use a 2.1 gear ratio and get a spin up time of .54 second at 59 joules at 5881 rpm and one impact tooth i was wondering if 1.5mm is thick enough for it and does this sound like a good design or should i go with something else? thanks

A: That's a huge drum for an antweight! Recheck your calculations. I show the drum itself weighing more than 5.5 ounces without the impactor or end plates. I'd recommend full circle endplates to better support the ends of the drum and keep it from distorting.

I really can't tell you if 1.5 mm is 'thick enough'. Strength depends on design details as well as quality of construction, plus it's impractical to predict the loads the drum might need to endure. I will say that 1.5 mm sounds thin given the large size. If I wanted to build an ant weapon that large I think I'd go with an 'eggbeater' design for greater strength and simplicity of construction.

Q: hi i was wondering if 4 mm aluminum is thick enough for a 4.5" blade?

A: Mark J here: that's not an unreasonable question and I'm not picking on you, but I've had a rant building up for a while and today is the day I pop the cork. Many of the design discussions we get into go like this:

Q: Is a 1/4 pound hamburger good?

A: That depends on the hamburger. I need more information.

Q: I told you, it's a 1/4 pound hamburger. It has a bun.

A: Do you generally like hamburgers? How old is this hamburger? Where did the hamburger come from? What is on the hamburger? Are you going to eat this hamburger or do something else with it? How hungry are you? Does it come with fries and a drink? What color is the meat? Is there fuzz growing on it? Is it hot and juicy or frozen solid? Are other people eating this same type of hamburger?

Q: Just tell me if it's good.

A: I can't tell you if it's good. You'd better leave it alone.

Q: I don't believe you.

A: Suit yourself.

Q: The hamburger was bad. You should have told me it was bad. You suck.

A: Have a nice day.

I'll assume that this is the same 4.5" vertical disk antweight weapon spinning at 6000 RPM discussed in recent questions about Park brushless motors.

  • Which aluminum alloy? Aircraft grade alloys like 7075-T6 can have more than six times the tensile and yield strength of common 1000 series alloys.

  • What does your hub design look like? The hub is a high-stress area and a poorly designed hub will create a weak point that will require thicker disk material.

  • Is there a separate impact tooth? Aluminum is not a good choice for an impactor.

  • How is the impactor attached to the disk? The attachment for a separate tooth needs to spread the impact force over a reasonably large area on the disk -- not a single point.
Assuming that you have a high-strength aluminum alloy with a strong hub design and a well-attached impactor tooth, 4 mm thickness should be fine for a 4.5" antweight disk weapon spinning at 6000 RPM. Please note that 'fine' does not mean 'indestructable'.

Q: Has there ever been a robot with fruit as a weapon?

A: I don't recall any fruit -- but lightweight 'SPS3' carried a carrot on a stick at the 1996 US Robot Wars.

Q: Would you suggest using the Park 370 Outrunner EFL-370-1360 Brushless Motor or the Park 370 Outrunner EFL-370-1080 Brushless Motor for a vertical disk? I will be running it at 11.1 volts and my disk will weigh 4 oz. It's 4.5in in diameter and spinning about 6000 rpm, but if that's too fast what would be about the right speed? Oh, and it only has one tooth. Thanks.

A: Go with the EFL-370-1080. It's less powerfull than the 1360, but more reliable (less likely to melt) and still overkill for an ant spinner. Geared down 2:1 at 11.1 volts and assuming a solid disk, the spin-up time is well under half a second with peak stored energy around 33 Joules at 6000 RPM. The spin rate is a little high, but with a single impact tooth I think you'll be fine. It sounds like a good match of motor and weapon design to me.

Q: Thanks for the help with the Park question. It will come in handy! What is the [stall torque] on the Park EFL-370-1080? I know is 0.73 [N-m] on the [EFL-370-1360] but I could not find the Ri [internal resistance] for the 1080. Thanks.

A: I calculate an estimated 0.78 N-m stall torque for the EFL-370-1360 at 11.1 volts (100 mOhm Ri) and 0.52 N-m for the EFL-370-1080 at the same voltage (190 mOhm Ri).

Q: Do you know why "Z" was aloud to compete [at the 1997 US Robot Wars event] with the shot gun mechanic spike?

A: The 'safety requirements' section of the 1997 US Robot Wars rules states:

"All explosives, corrosives, flames, pyrotechnics are prohibited." But the team argued successfully that their device was safe since it was of commercial origin and the combustion was contained -- kinda like internal combustion engines which were allowed. Things were a little 'looser' in the early days.
Q: I wanna make a combat robot for the first time ,I have some idea but I dont know which one is more effective,for example I think about pneumatic method but I dont know anything about it features ,could you plz let me know where can I find some information especially for pneumatic method .

A: I can't recommend pneumatics for a new builder -- it's best to keep your first robot very simple. You'll have enough things to worry about with battery maintenance, R/C system set-up, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems. You really don't need to add to that with pneumatic valving systems, pressure tanks, weapons channels, digital switches, complex attack maneuvers, and additional pit checklist items.

There is plenty of information on pneumatic systems in this archive, and I have pointed many builders to the Team Da Vinci Understanding Pneumatics page for a good explanation of combat robot pneumatic systems.

Q: Is pneumatics recommended to power a flipper on a 3kg sumo robot??

A: We don't compete in sumo, but the last time I checked the rules a sumo 'bot has to be 'harmless'. I don't think that any flipper weapon is going to qualify as harmless. Check with your event organizer -- some don't even allow slow lifters.

Q: Were there any robots in Robotica with conveyor belts?

A: Conveyor belts?? No. Why would there be?

Q: Conveyor belts are a dynamic weapon, even though they are ineffective. What was the most sucessful Battlebots robot that had one, if there were any?

A: I don't know of any combat robots that used anything that I could describe as a 'conveyor belt' weapon.

Q: How would I build a three pound hammer robot with out any pneumatics?

A: Scan down a couple questions to the post on non-pneumatic power for a 15 pound flipper robot.

Q: Why do you think Team Plumb crazy changed the bracket on sewer snake to a wedge?

A: I'm not sure what you're talking about. 'Sewer Snake' does have interchangeable devices that can be fitted to the front bracket mount depending on their opponent. You can contact TPC thru their website and get a more specific answer as to when and why they change them.

Q: I understand that building a flipper requires a large amount of energy to be released in a short amount of time. The pneumatics rules in my 15 lb competition are very restrictive (150 psi maximum anywhere in the robot at any time) so I've been investigating alternate methods to release said energy. I was wondering what your thoughts were on the following three possibilities:
  • Flywheel based – heavy flywheel (accelerated by brushless motor) and clutch mechanism to quickly impart lots of torque in a very short amount of time

  • Spring based – Spring is compressed by a winder mechanism, released. Serves as a linear actuator to replace a pneumatic piston

  • Electromagnet based – Put a powerful electromagnet in the base of a cylinder. Attach the plunger to a neodymium disk magnet that sits directly on the electromagnet at rest. When the electromagnet is turned on, it repulses the disk magnet, causing the cylinder to actuate. Do you think this is even possible?
Thanks so much for donating time to this website!

A: Mark J. here: I like builders who are willing to think outside the box. Whether you come up with something useful or not, it's a good habit to get into. Your ideas all have merit, but each has a serious design drawback as well.

Flywheel based flippers have been on the minds of designers for quite some time. The main problem is a lightweight and reliable clutch mechanism capable of dealing with that large amount of torque. It is possible -- Team Whyachi built the only successful implementation of a kinetic flipper I know of: 'Warrior SKF'. See the previous post in this archive, and seach YouTube for a video.

Spring based weapons have a similar problem. A lightweight and reliable release mechanism is difficult to design. Again, there have been examples of robots using spring powered weapons: the overhead axes of 'Son of Smashy' and 'No Apologies' come to mind. The design challenge here is less of a problem than the spinning disk clutch and would be my choice of your three designs to actually implement.

Your electromagnet solution runs afoul of the inverse square law. As the magnets move farther apart the force decreases with the square of the distance. You just aren't going to get enough power out of the design to justify the weight. No examples of such a design exist -- for good reason.

Don't give up entirely on pneumatics. A clever design can do a lot with 150 psi. Volume can make up for pressure!

Q: Alternative flipper design guy again. I thought a bit more, and figured that the neodymium magnet would stick to electromagnet's iron core, so replacing it with a second electromagnet would fix that problem and make for a more powerful actuation. What do you think?

A: The iron core is not a problem. Once energized, the magnetic dipoles in the iron re-align and the attraction of the magnet is replaced by repulsion (assuming correct polarity). You actually don't even need a core -- the electromagnetic force is produced by the electrons traveling thru the wire. The core is only present to direct the field and can be removed if the coil is properly designed. 'Coreless' electric motors have no iron armature and work just fine. Regardless, the inverse square law still applies and still defeats this design idea.

Q: Alternative flipper design guy, yet again. I developed the spring powered actuator idea a little more... check out the pdf. What do you think?

A: Double points for clever! I have not seen this design previously. Your design problem considerations are good. Note that the solenoid will have a large side-loading when the spring is under compression -- pulling it in against that side loading may be a problem!

I still think that pneumatics could be effective in this competition. Have you considered a on-board air pump to top off that 150 psi storage tank between shots?

Q: Who was the first clamp bot? was it SOB?

A: Hard to define exactly who the first clampbot was -- but it certainly wasn't 'SOB' who first fought in early 2003. Middleweight clampbot 'Complete Control' fought at BattleBots 2.0 in November of 2000 and is a good candidate for 'first', although there were earlier robots that might argue for primative 'clampbot' status.

Q: who was the first flipper? was it flip?

A: You're going to get us into an arguement about the first flipper -- it's hard to draw the line between a fast lifter and a slow flipper. 'Vlad the Impaler' had a functional pneumatic lifter/flipper weapon at the 1996 Robot Wars that succeded in flipping 'Punjar' for a match win. It gets my vote.

Q: ...and first spinner? (any kind)

A: Several spinners appeared at the first Robot Wars in 1994, including 'South Bay Mauler', 'Pain Mower', and 'The Master'. I don't think there is any clear indication of who was 'first'.

Q: Someone says that a spinner's performance only relies on the RPM of its engine and mass of the weapon but by no means horsepower, is he right?

A: Mark J. here: unless this un-named someone has a very strange definition of 'performance', no.

Horsepower is the product of RPM and torque. If only RPM was important then torque would be of no value, which is simply not the case. Torque determines weapon spin-up time. Imagine trying to spin-up an 80 pound bar weapon with a direct drive 60,000 RPM inrunner brushless airplane motor. You could flip the weapon motor on, go out for lunch, come back, and it would still be trying to spin up to speed. You can play with various motor torque values for a fixed weapon design in the Team Run Amok Spinner Excel spreadsheet and see what happens to spin-up time.

Perhaps the un-named someone ment to imply that once the weapon is up to speed the weapon impact is no longer related to the power of the motor that is maintaining the weapon at speed. That is more or less correct.

Q: Do you have a suggestion for a better material for my bots lifting spike? My bot is Shish-Kabot it is a 30LBS sportsman class bot. It preformed great untill the spike got ripped off in the finals [at Motorama 2010]. The spike it currently has is a 15 inch long 3/8 inch diamiter steel rod. It is mounted with a machine screw running through the center of the spike at the 5 inch mark.

A: Mark J. here: I found photos of your 'bot at the GameTechMods forum.

A part failure can be attributed to either material or design -- sometimes both. Your design places a lot of stress at the pivot point. That point in the rod has been weakened by drilling a pivot hole, the machine screw looks small for the application, and the screw runs thru a hole very close to the front edge of an un-reinforced block of UHMW polyethylene. That's a lot of weakness right at the point of highest stress.

I need more information on the nature of the spike failure:

  • What actually failed: the spike, the UHMW mount, or the fastening hardware?

  • If the spike failed, did it fail at the pivot point? Did other parts fail as well?

  • What circumstances caused the failure: a lateral impact to the spike, straight-on wall collision, bad landing from a flip, etc.

  • A photo of the damaged parts would help.
With very little to go on, I suspect that the spike material itself is not the problem. Assuming that you want to retain the basic design and knowing only that the spike was 'ripped off' I might suggest the following changes, as you find appropriate:
  • Replace the steel rod with a length of flat steel bar the same thickness as the current rod but taller (maybe 3/8" thick by 3/4" tall). 4130 chromoly alloy steel is widely available and would be a good choice. The bar can be tapered at the ends and lightening holes can be drilled along its length to reduce weight.

  • Use a larger diameter bolt for the pivot - 1/4" at least. The wider lifter bar will accommodate a larger hole without excessive weakening.

  • Move the pivot hole in the UHMW further back to allow for greater material thickness to the front. Alternately, make new mounting blocks that bulge outward to the front to provide greater thickness. Another option, screw a reinforcing plate to the front of each mounting bracket.

Again, I have to guess at the actual cause of your lifter failure. Write back with more information if my assumptions are wrong.

Q: Shish-Kabot builder here. The steel rod failed at the piviot point. Not the screw. It failed because it was under a pnumatic fliper when it fired it's flipper when under me. YouTube video of fight where the lifter failed.

A: That all makes sense. The pivot hole drilled thru the rod removes a considerable cross-sectional portion of the steel at that highest-stress point. The small machine screw was the next weak point -- if the rod had not failed I suspect that the screw would have.

If you want to keep the 'Vladiator' look to the spike, you could gain considerable strength by welding a 3/8" thick steel tab to the rod and drilling the pivot hole thru the tab instead of the rod. I still suggest a larger bolt for the pivot point.

Comment: Shish-Kabot builder again. Thanks for the advice/input.

Q: Does [Team Blackroot heavyweight] 'SJ' still use pnumatics in the lifter arm/claw or has it been changed to a motor of some kind?

Any details would be appreciated.

A: Team Blackroot abandoned pneumatics for their current version 'SJ' design. The weapon arm is chain driven (photo at right) from an electric power source, but I have no specific information about the motor or remainder of the lifter drivetrain. Team Blackroot dropped off the internet a couple of years ago and I have no current contact information for them.

Q: Hi Aaron. I am building a 4-bar flipper in a 10 kg robot category. The flipper is to be powered electrically.

First, what type of drive should be used for the flipper either chain drive or gear drive? Chain drive is easy as I have chains and sprockets but i'll have to manufacture small gears and then use them for a gear drive. So, which drive would be good: chain or gear? Which one will give good torque and efficiency? Will I need one or two motors for the flipper if one motor has ratings 12V, 5A, 1440 rpm and 10 kg-cm torque?

Secondly will the flipper work well in this 10 kg category? Can you give me some links that deal with flipper mechanism in detail? Thank you.

A: Chain drive vs. gear drive: I'm a fan of chains. Chains and sprockets are easy to obtain, mechanically efficient, durable, can handle large torque loading, and are tolerant of alignment variance. Their only real drawback is that they take up a fair amount of space. A gear-reduction first stage (as in 'gearmotor') would reduce the complexity of a multi-stage chain drive.

About the motor: I'll assume that the torque figure you give is 'stall torque' and the amps are a continuous rating; a 1440 RPM 12 volt motor will not produce that much torque on only 5 amps. Stall current is probably around 120 amps. I ran a very quick analysis of a typical 4-bar design for your weight class and came up with an operational torque requirement of about 300 kg-cm when powering the rear bar (more to power the front bar). A 30:1 gear reduction with one 1400 RPM, 10 kg-cm torque motor would give you the needed torque and a loaded (half RPM) lift speed of about 1 second. I think that should do just fine.

You'll need to settle on a specific 4-bar design and calculate the actual torque needed for that layout. The T.i. Combat Robotics 4-Bar Simulator is an invaluable tool in this process. Four-bar design is as much an art as a science. Keep trying designs on the simulator until you find something that works for your robot.

An electric powered lifter will be too slow to accurately be called a 'flipper', but if your lifter is correctly designed, constructed, and integrated into the other robot systems, it has the capability of being quite effective. Lifters are statistically the most effective weapons in the sub-light weight classes, but they are also usually built by experienced teams.

I have no good links that deal with the mechanical design of electric lifters. The BioHazard Mechanical Design page has detailed photos of their lifter, but their weapon is powered by linear actuators rather than a gear or chain drive. The T.i. Combat Robotics 4-Bar Simulator page gives many good design tips, but without drivetrain details. There are also several posts in this archive about 4-bar design elements.

Q: how can an axe and flipper work together?

A: Poorly. I'm not a fan of robot designs with two active weapons -- try to think of a successful example. You're better off using the weight allowance to build a single powerful weapon than splitting the weight into two weak weapons. The axe/flipper combination is particularly awkward: one weapon trying to throw the opponent upward and another that works best when the opponent is well-supported on the floor. Not recommended.

< Q: [Chinese Forum] Some people said that The Matador's self-flipping is "by no means fortuitous" because it has flipped itself twice in Battlebots 5.0. I know Matador [was] experimental but I want to assure: does The Matador's flipping issue is "by no means fortuitous"?

A: Combat robots with extremely high-powered weapons are often nearly as dangerous to themselves as to their opponents. Horizontal spinners may careen across tha arena as a reaction to delivering a hit. Tuna can spinners may go 'tippy-top' and flip themselves over without hitting anything. 'The Matador' had such a powerful flipper that it could flip itself if it 'missed' a shot at an opponent. No way is that good!

Q: [Chinese Forum] Looks like Wipe Out's eletric lifting arm is obviously faster and more powerful than other electric arms. Does Team Plumb Crazy use some special technology in it?

A: No secrets or special technology, just a whole lot of input power. Lightweight 'Wipe Out' has a AmpFlow motor driving the lifter thru a very robust reduction gearbox and chain drive. Lots of power in, lots of power out.

Q: How does middleweight Dead blow's hammer work?

A: 'Deadblow' used a rotary pneumatic actuator to power its hammer. See the Team Deadblow website.

Q: Hi Aaron in the forum below you have said that MEGABYTE's shell was 45 kg in weight and spun at 1000 RPM approximately. Can you tell me the motor ratings like torque, current etc. for such kind of motor? Also what sort of batteries are used for such huge amount of power? Can you please specify the Ampere-Hours and weight of these batteries? [Raj]

A: Megabyte's weapon is powered by a Briggs & Stratton Etek motor producing about 15 horsepower at 48 volts. Stall current is about 700 amps with motor stall torque close to 90 N-m. The motor is geared 4:1 to the shell. A pit photo of 'Megabyte' shows four 24 volt 3.6 amp-hour BattlePack batteries supplying power to both drive and weapon. Each pack weighs 4 pounds.

Q: If megabyte uses 4 [24 volt] batteries of 3.6 a-h [each], and if the stall current is above 700A, how can the battery last for 3 minutes?

A: Mark J. here: stall current in a weapon motor is encountered only instantaneously as the weapon first starts spin-up. Note that the large peak level of current will only be seen if the battery/ESC can actually provide 700 amps. As RPM rises, current decreases until the system reaches maximum weapon RPM where the Etek may draw as little as 6 amps. Each time the weapon 'hits', energy is depleated and the motor will draw higher amperage as needed to replenish that energy -- but a restart from a full stop is uncommon.

The Team Run Amok Spinner Excel spreadsheet features a battery capacity calculator that will estimate the battery capacity required for a given weapon, motor, match length, and number of spin-ups. Using this calculator, it appears that well less than half of the capacity of Magabyte's 48 volt, 7.2 amp-hour battery capacity is used by the weapon in a typical match.

Q: would a 5" .125" thick titanium blade work good for an ant or could I go lower on the thickness?

A: Mark J. here: I can't isolate one piece of the weapon and tell you how well your whole weapon system will work. Each piece must be in balance with the other elements of the weapon system, and the weapon system itself must be in balance with the rest of the robot. I suggest you start with the Team Run Amok Spinner Excel spreadsheet to examine the interaction of the blade, drive, and motor.

Q: Hi, I have a large vertical titanium disc for my beetleweight that spins at around 3700 rpm. The drum spinners these days spin at over 10,000 rpm, so what can I do to prevent being thrown across the arena every time we go weapon to weapon?

A: There is more than RPM in the equation:

  • a 2" drum spinning at 10,000 RPM has an impactor velocity of about 90 feet per second;

  • a 6" disc spinning at 3700 RPM has an impactor velocity of almost 100 feet per second, has better 'bite', and most likely much greater potential energy than the small drum.
Looks to me like you have the advantage. If you get thrown across the arena it isn't because your weapon isn't spinning fast enough.
Q: Hi I am building an ant weight with a 8 in. By 1/2 in. by 5 mm bar made of steel I am using the park 370, u have said before that u want to have a top speed of 3000-4000 rpm but could I run my bar at higher speeds and still get a good [bite]? if so what would be my top speed of it? Thanks.

A: Mark J. here: there is no magic number for spinner RPM, but the faster it spins the less bite it's going to have. At 4000 RPM, one of your two bar tips will pass by every 7.5 milliseconds -- it's going to be difficult to insert much of your opponent's 'bot into the 'damage zone' with any higher speed!.

As mentioned below (read down three questions), it is possible to increase the time between impactor passes by decreasing the number of impactors. In the case of a bar spinner, you'd need to shorten one leg of the bar (half an inch should do) and add weight to that shortened leg to keep the bar balanced. With a single impactor you can double the RPM of the weapon and still get the same 'bite' as a weapon with two impactors.

Q: Then what about the mantisweight Chaos Theory? It has a [96 toothed] 14 inch saw blade, and apparently it can catch the other robot very easily. [Watch Chaos Theory vs Zillion] Why is this?

A: If you are fortunate enough to fight a robot with exposed sharp edges sticking out you don't need much 'bite' to catch that edge. Watch Chaos Theory vs Mystery Box and see the big multi-toothed blade skitter off the flat wedge because it has so little bite. With fewer teeth it would have done much better.

Q: Hi Aaron. What was the amount of Kinetic Energy stored in heavyweight MEGABYTE's spinning shell? Is it more effective weapon than a spinning Horizontal bar? in which case is the 'BITE' more? By which method were the blades attached to the spinning shell?

A: Mark J. here: Robotic Death Company has not released enough specific information for me to calculate the stored kinetic energy of Megabyte's weapon. A rough guess at speed (1000 RPM), mass (45 kilos), dimensions, and mass distribution puts the energy around 50,000 joules.

A spinning cylindrical body with the same mass and diameter of a spinning bar will store more energy, which makes it a potentially more effective weapon. A simple spinning bar the same weight and diameter as Megabyte's shell would store only about 40% of the energy at the same speed. The actual weapon effectiveness is dependent on a variety of design factors.

'Bite' is independent of the shape of the energy storing body. It is primarily a function of the time interval between passage of the weapon impactors, which is dependent on the weapon RPM and the number of impactors. A weapon with a lot of 'bite' has a longer interval between impactor passage, which allows an opponent approaching at a given speed to penetrate farther into the weapon's destructive radius. You may have noticed some spinner weapon designs that are counterweighted to allow a single impactor point for greater bite.

Robotic Death Company bolts the impactor blades onto their spinners.

Q: How do I make a bi directional limit switch? I need it to stop the lifter on my robot.

A: I assume you're using a gearmotor with an ESC to control your lifter, and not a servo? There is a good explanation of limit switches at The third circuit example (the one with two switches and two diodes) is a bi-directional circuit -- your ESC would substitute for the battery. Position the contact switches at the minimum and maximum lift points and you're good.

Relay boards used to control linear actuators often have built-in limit switch inputs. I use the Team Delta R/C dual ended switch to control the lifter on Zpatula to keep things simple.

Q: HI, I have built a horizontal overhead spinner which weighs nearly 7 kgs and revolves at nearly 2600 rpm. Is ther any modification available in this to deal with a wedge, because whenever i hit a wedge my bot tends to flip in the opposite direction.

A: Hard to say much without at least a photo -- I don't even know what weight class the robot is. I do understand the basic problem: the force vectors of an impact with an overhead spinner are all above the center of gravity of the robot. A weapon impact tries to flip the robot over backward. The farther above the center of gravity the impact site is, the greater the flipping force. Hitting a wedge simply aggravates the problem. Is this a bar spinner? You could try bending the ends of the bar downward a couple of inches to move the impact point lower and closer to the level of your robot's CG.

Q: Aaron, we have built a horizontal spinner like Son Of Whyachi, but instead of three rods we have managed to build two rod system. We have taken a leaf spring and bent it across its longitudinal axis so that when rotated it sweeps a hollow cylindrical surface around the robot chassis. It is a rigid rotor which spins at about 2500 RPM driven via belt and pulley. Speed reduction is 2. Motor spins at around 5200 RPM.

The main problem we are facing is the STABILITY of our robot. It topples over at very low speed for eg 1700 RPM, not gaining our top speed of the rotor 2500 RPM. We have balanced the rotor statically and dyanmically, but still the robot is unstable. It loses contact with the floor first and then it turns,swivels etc. Are there any effects such as gyroscopic etc. which come into play? Please suggest suitable methods for increasing the stabiltiy of our robot. Our bot weighs total 39kg ,chassis weighs 15kg, weapon weighs 8-9 kg approx. Also our C.G of the robot is not in the centre of chassis but shifted towards the rear side due to weapon motor weight which is mounted on the rear side.

You can see the robot video on youtube. The bot who wasn't flipped o'er is ours. It's urgent pls. Any help will be appreciated greatly. Thanx in advance.

A: Mark J. here: we've seen this type of instability before. It pops up in full-body spinners and designs like yours. A spinning object - such as your rotor - will attempt to orient its axis in a manner which will achieve the greatest rotational stability. If you spin a hard boiled egg on its side, it will change the axis of rotation and spin on its end. See the explanation of the Tippe Top for the related physics.

In your particular case, I believe the rotational mass is unstable and is attempting to reorient its axis to correct this -- turning your robot over in the process. I've given some thought to your problem and then did a little experimenting. Of the three natural axis around which a rigid body may be rotated, only two are stable. Your weapon is spinning along the third, unstable axis! Search the web for 'rotational stability' for more detail on this phenomenon. The three-armed rotor on 'Son of Whyachi' was stable in this particular rotational axis but the model I made of your two-legged weapon is not.

Why does your rotor become unstable at relatively low RPM when others like it do not? Check for any 'flex' in the weapon rotor -- although not responsible for the instability, flexing may create trouble at a lower speed than would otherwise be seen. That leaf spring you're using may be distorting and causing the onset of the problem.

P.S. - About the video - are you insane? Standing two feet away from an operational spinner in combat, holding a control cable up out of the way??? Somebody is going to get a chunk of robot embedded in their head! Cut it out!!!

Q: hi Mark sir. i asked a question regarding stability of our robot. u suggested that it's a sort of tippe-top phenomenon. i wanted to ask whether decreasing the rotor speed from 2000RPM TO 1000RPM will increase the stability of robot?

also did Son Of whyachi had rotor speed of only 950RPM? is that sufficient to damage the other robot? our rotor has dia. of 75cm. thank you.

A: Decreasing the speed of the rotor would not remove the instability, but would reduce the magnitude of the forces generated and make it more manageable. Unfortunately, reducing the speed of the rotor by half will decrease the rotational energy of the weapon by 75%. Not good!

The rotor on 'Son of Whyachi' did spin at 950 RPM, but there is more to the energy equation than RPM. The rotor was five feet in diameter which translates to a tip velocity of 175 MPH and way more than 50,000 joules of stored energy. A very rough estimate of the energy of your weapon spinning at 1000 RPM: 2750 joules. I'd suggest correcting the rotor instability and keeping the RPM up for maximum impact.

Q: Are there ways to make a crushing claw other than hydraulics?

A: Sure -- but hydraulics are the simplest. A reliable crushing weapon is very difficult to design and implement, and there have been very few successful examples.

Q: In previous posts you said to gear the Speed 300 motor at 2:1 for a 4 oz drum that is 2" in diameter. I am building an ant weight drum using two(2) Speed 300 motors with a 4 oz drum that is 3.3" in diameter and 2 1/2" long. I am curious what you think I should gear it at to gain the most success.

A: Mark J. here: that previous recommendation was a ballpark estimate based on quite a few assumptions -- I didn't have all the information needed for exact calculations, so recommended a gear reduction of 'about' 2:1. I have the same lack of information about your design.

Running what I know of your design plus a few assumptions thru the Team Run Amok Spinner Spreadsheet I get a good balance of spin-up time (0.65 seconds to 10 Joules at 3125 RPM), peak kinetic energy (23 Joules at 4700 RPM in 2.0 seconds), and tip speed with a gear reduction of 2.5:1. I'd suggest starting there.

Q: I got the dimensions wrong in my last question so here are the details for my drum it is 2" long, 2.375" in diameter, the wall thickness is .085" with two .5" and four .25" holes in it thanks.

A: And it is made out of what?

Q: sorry my drum thing was wrong again it would weigh to much so I decided to use a aluminum drum 3 in. long, 2 in. in diameter with a wall thickness of 3.2 mm, and I went to my grandmas to do the calculations but they did not come out right with a gear reduction of 2.5 to 1 using the drum i just described and two speed 300 motors I got a spin up time of 2000 seconds at 4500 rpm producing 140000 joules I think I got the torque wrong i had .39 newton-meters, I am 12 and my parents are always gone working so they cant help but you are very helpful Thanks allot (:

A: I think you're using the wrong units on the spinner spreadsheet - maybe meters instead of inches - and your torque is too large. I'm also getting the feeling that you're in over your head on this. If this is your first robot I'd strongly suggest building a passive wedge or brick to learn the important primary systems of drivetrain, radio, chassis and armor. Without strong basics even the wildest weapon will be useless.

Q: I already made two bricks and fought them each once and I have already made a atocad drawing for my bot and bought all the parts except for the timing belts and pulleys. I know I can get a balenced drum because my grandpa is a shop teacher at a high school that has all the meterials and mechines, and I tried the spinner thing again and I still got the same thing what is the n-m on two speed 300 motors, thanks?

A: As you wish: two Speed 300 motors at 7.2 volts produce a combined 0.06 N-M of torque. No-load speed is 12,400 RPM. Material density is 2760 kg.m3. Three inches is 0.076 meters height. Two inch diameter is 1 inch outer radius = 0.025 meter outer radius. Thickness is 3.2 mm. You haven't told me about the end plates for the drum, so I'm leaving them out.

Pumping these numbers into the spreadsheet gives a cylinder mass of 0.10 kilos = 3.5 ounces. With a 2:1 reduction, the drum will spin to 5 Joules of energy in 0.29 seconds and reach 10 Joules at 5890 RPM. I don't think that's enough energy, and the drum RPM is getting very high. I'd suggest making the drum larger in diameter -- more like the original dimensions you provided.

Q: Hi Aaron, my team and I are building a robot similar to tornado mer and we were wondering what kind of tip-speed your robot's weapon was doing, and the length of the bar you used.

A: 'Tornado Mer' is not one of our robots. It was built by Team Van Cleve from Prior Lake, Minnesota. I have no specific information about its weapon.

If you're asking about the bar spinner weapon on 'Run Away', it should not be used as a model for your weapon. We added the showy but largely cosmetic spinner weapon just to please the Robot Wars crowd. It wan't a serious attempt at a damaging spinner.

There is plenty of spinner design information in this archinve, including links to our kinetic energy calculator spreadsheet.

Q: Hello, I am thinking of building a hobbyweight crusher similar to razer, is there a formula to find out how many lbs. of force is required to pierce 1/8 in AL/Lexan or 1/16 AL/lexan? I am thinking of using linear actuators. Thanks

A: Mark J. here: too many variables here for a simple formula. The force required will depend on the cross section and shape of your piercing beak, the particular aluminum alloy, any heat treatment, and how well the piece is supported. Lexan is particularly difficult to pierce as it will deflect a great deal before failing.

I suggest you experiment with a mock-up of your piercing beak and a lever that will allow you to place a measured amount of force on a piece of test material. I believe you will find that it takes a great deal of force to pierce even relatively thin armor. I've never seen an effective piercer that used linear actuators.

Q: I am thinking of building a hobbyweight shell spinner and saw in one of your posts that it shouldn't be direct drive. How should it be then? I have seen that some builders have their weapon motors horizontal. How? Thanks

A: The standard method of driving a spinning shell is to create a new well-supported central shaft for the spinning shell with a pulley and belt drive to the motor. The drawing shows a well supported shaft for a bar spinner -- the same design will work for a shell.

I have only rarely seen horizontally oriented motor shafts with shell spinners. It is possible to lay the motor down and drive the shell with a friction-drive wheel attached to the motor shaft and rubbing against the inside top of the shell (not recommended). It is also possible to run the motor output thru a right-angle gear drive and then to a belt and pulley system, but this adds weight and mechanical losses. Keep it simple.

Q: Should a bulky flipper with pneumatics, like The Gap and Ripper, or a large flipper powered mainly by CO2 and high pressure, like Robochicken, be used?

A: Mark J. here: I'm not sure I undertsand your question. The design of the flippers in 'The Gap' and 'Ripper' are very efficient because they place the lifting force directly in-line with the direction of flipper travel. This does cause the pneumatic cylinders protrude at an exposed angle. More compact designes like 'Robochicken' are able to conceal the pneumatic cylinders within the robot body, but the force vectors are not as favorable and flipping force is compromised.

Whichever design is used, it must be integrated into the overall design of the robot. You can't treat the various systems of a combat robot independently -- they have to work together.

Q: i have built bots before for a 15 lb weight class for the azbbiq, i am looking to build a fly wheel bot this year and im looking for a solid weapon design. can you help? any ideas?

A: Start reading thru our archives, and take note of our Team Run Amok Spinner Spreadsheet which allows energy and spinup analysis of proposed spinner weapon designs. When you get to specific questions, write back.

Q: Does a squat cylinder a dome or a flattened cone fbs work best?

A: Pound-for-pound, a squat cylinder shape can store more rotational energy than a dome or cone of the same dimensions. More of the mass is located farther out from the axis. Many other factors go into the design of a successfull Full-Body Spinner, but a cylinder is a good place to start.

Q: An electic axe always doesn't have as much power as a pneumatical one,but Beta seems to be an exception for it's much more powerful than Terrorhurtz,why is that?

A: Mark J. here: electric axes generally do have less power than a good pneumatic axe. The axe on 'Beta' is an exception. The weapon is powered by a monster Etec motor running at 60 volts thru a complex geartrain that provides a variable ratio drive to better accelerate the weapon head. The reaction torque produced by the weapon is so great that electromagnets are needed to hold the chassis down to a steel arena floor.

The machining skills and expense to construct such a weapon are beyond the reach of most teams. I should also point out that the robot was a combat failure -- unable to compete because the reaction forces pulled up the arena floor. It was an interesting (and expensive) experiment, but I would suggest sticking with pneumatics.

Q: How can you get a crushing beak to exert piercing force?

A: Hydraulics are the usual approach. Suggest you get a copy of Chris Hannold's "Combat Robot Weapons" for a overview of design issues and solutions.

Q: Should I lubricate the weapons of my robot with Silicon Spray if the weapons are a crusher?

A: What, the outside of the crushing beak part? It won't make any real difference in it's ability to pierce, but a little lubrication may help prevent it from getting stuck.

Q: How does an overhead blade like Proppelor Head's cause damage, when there are few sharp edges?

A: The same way a hammer causes damage: blunt force impact. Sharp edges on a spinner weapon can cut in and 'stick' in the gash, which can be a problem. Blunt is more reliable.

Q: Hi, Ive built a few bots before so I have a bit of experience. I would like to build a 12 hobbyweight shell spinner with direct drive for the weapon. I know I shouldnt do direct drive but I am going to try. My question, I cant seem to find a decent motor, something strong enough to spin up a 3 pound shell and It has to have a large shaft...? at the robot marketplace they have a motor but the shaft of the motor is only.315 so thats to thin...

A: Mark J. here: you're right -- you shouldn't direct drive a shell spinner. You've also discovered one very good reason why not -- suitably sized motors do not have large enough shafts to support the load of an FBS shell.

If the motor you're talking about is the AME blower motor, you should also know that the motor is badly underpowered for it's weight: maybe 75 watts output at 12 volts. Really pitiful.

I can go on with reasons why a direct drive shell spinner will end in tears: slow spin-up, too high a spin speed, amp-sucking start-ups... you get the idea. Don't do it.

Q: why is mettal cord somtimes used as antweight disk belts? would'nt it slip?

A: I haven't seen metal cord used for weapon belts, but a little slippage in a weapon drive is not a bad thing -- it can keep you from stalling the motor.

Q: Hey, what was G.O.R.T.s weapon? it looks like a flipper and a cone with a flipper seems like a good idea

A: Superheavyweight 'G.O.R.T.' had an ineffective electrically powered spike. It never won a match. Pattern your robot after something more successful.

Q: Hey, which system can a Hobbyweight use to build lifting forks?

A: Hey, I hope we don't go thru every weight class like this. There are available pnumatic components small enough to use in a hobbyweight. See Pop Goes the Monkey for an example. Electric gearmotors are useful lifter power sources in this class as well.

Q: Why can Pyromancer always get underneath its oppenents while several of them have a hinged wedge just like Pyromancer?

A: Playing the 'lowest wedge' game is more complicated than just hinging your wedge. 'Pyromancer' has razor sharp leading edges to its twin wedges that are honed to lay perfectly flat on the arena floor. The twin wedges also help -- most arena floors are uneven, so one side of a single hinged wedge is likely to be raised off the floor just a bit by the other side.

Q: I was wondering if you could help me out on starting the pneumatics system for a 120 lb robot. My team was thinking of a trapezoid shaped robot that had a fork lift type arm attached at one end. Do you think you could give me some insight?

A: Start by reading the Team Da Vinci Understanding Pneumatics page. You'll also want to read thru the previous posts in this archive. Write back with specific questions.

Q: How many joules of energy does Hazard's weapon produce at full speed? Thanks alot, he is my favorite spinner too.

A: I've never heard Tony B. claim a figure for the energy stored in Hazard's weapon. I don't have the specific information about the weapon do so an accurate calculation, but I'm going to guess around 3000 joules.

Q: Is 'Megabyte's shell really made of Lexan? Don't you want as much weight on the shell for an FBS? I guess the blades must weigh a lot then...

A: No -- some versions of 'Megabyte' had a protective Lexan cover on the robot body under the shell, but the shell itself is titanium. From its size and weight (100+ pounds) I'm estimating the shell material is at least 1/2" thick.

Q: would 1/8" titanium work well for a heavyweight FBS?

A: It depends entirely on your design. See the question #17. In general, I'd say 1/8" titanium is VERY thin for a heavyweight.

Q: Hi, what type of motor does megabyte use for it's FBS, Thanks

A: Robotic Death Company reports that 'Megabyte' used the Briggs & Stratton Etek Motor at 48 volts for the weapon and four 24-volt DeWalt motors for the drive train. There is reference to a 'souped up' Etec being used in 'Rambyte', but no details are given.

Q: Hi Aaron, do you know what weapon middleweight "Fatal Error" has? It looks like a drum and a flipper combined.

A: A small drum on one end, a weak flipper on the other. A fine example of two poor weapons not equaling one good one. Record: zero wins, two losses.

Q: Do you think ziggo's way of converting energy, the vertical blade is beter than ways like typoon 2 and moebius, the horizontal blade?

A: I don't think I understand the question. All three of the robots you mention are full-body spinners: 'Ziggo' was a dome, 'Typhoon 2' was a cone, and 'Moebius' was a squat cylinder. They all sprouted assorted impact blades. 'Ziggo' also had the exposed ends of a square-tube beam sticking out thru the dome. I believe this is a structural issue rather than an attempt to improve on energy conversion -- I don't see any particular advantage.

Q: Sorry for not explaining my question well. What I ment was, do you think Ziggo's way of mounting his impact blade so the sharp side stands up instead of it sticking out horizontaly causes more damage?

A: I don't think it makes much difference. I suspect that it was just more convenient for the shell design to mount the impact blade as an end-cap on the structural tube. Note that spinner impact blades are generally not 'sharp' -- a sharp blade tends to cut into armor and wedge itself stuck. A blunt blade will rip without as much risk of sticking.

Q: Would a spinning disk on an arm like the origanal dead metal work in todays competitions?

A: There's another post about 'Dead Metal's weapon a little further down this archive. The weapon was designed for cutting rather than blunt impact, and was used only after a competitor was already weakened and 'docile'. I don't believe that a similar weapon would be useable in open combat.

Q: what does ziggy and toro use on there flippers

A: There is plenty of information on both 'Ziggy' and 'Toro' in this archive, probably including the answer to whatever question it is that you're trying to ask. If you're asking about the gas they use to power their flippers: 'Toro' uses CO2 and 'Ziggy' uses High Pressure Air (HPA) at around 3000 PSI.

Q: Would a spring powered spike be a good/decent weapon?

A: There are many questions and answers about spring powered weapons in this archive. Quite a few builders have tried them, but only one has been really successful. Pneumatics are a simpler, more powerful, and more popular solution.

Q: [Chinese Forum] Aaron, in your last question you said that there was only one bot using spring loaded system really succeeded - which is that one? I guess Derek Young's 'Son Of Smashy', wasn't that? Could you analyze why had SOS been that successful?

A: Yes, but no points for correct answers to your own questions Derek's success with 'Son of Smashy' had less to do with the spring power than it did with being the first powerful overhead axe middleweight to turn up at BattleBots (Long Beach, 1999). The judges were impressed by Derek's aggression and ability to do some damage. When overhead axes became less of a novelty you needed more than spring power to impress the judges and pneumatic power took over.

Q: If one had a spinning weapon powered by a motor geared down 3:1, could they attach a flywheel to the motor shaft and make the blade lighter by the same weight as the flywheel in order to increase kinetic energy without increasing weight or blade rpm?

A: Mark J. here: you've been thinking pretty hard about this. The short answer is 'yes', but there are problems in implementing such a design.

  • The kinetic energy of a rotating object depends on mass, speed, and the shape of the object. The amount of KE lost from the blade would depend on where the mass was removed as well as how much mass was removed. The kinetic energy of the added flywheel is also dependent on it's shape as well as it's mass. The Team Run Amok Spinner Spreadsheet will help you calculate the rotational inertia of the blade and the proposed flywheel.

  • The energy stored in the added flywheel would need to be transmitted thru the gearing system to the blade at the instant of impact. That will place a great deal of extra stress on the gearing system. If a belt-drive is being used, the stored energy from the flywheel could be lost to slippage. If a chain or gear system is used, you risk breaking the drive. The added weight needed to reinforce the weapon drive could cost more weight than you'll save.

Q: I have an interesting idea. I was thinking of a dustpan like the one on S.O.B., but instead of a saw, a vertical spinner, going downwards. The point is too remove the other bots contact from the floor, negating the lift caused by the spinner hitting downwards. Or will it just tear itself apart?

A: I get it -- if you have a dustpan under your opponent you can strike downward without flipping your own robot upward. Not bad, but a downward striking rotary blade will also strike 'inward', jamming the blade and your opponent into the dustpan and against your own 'bot. You are correct that there would be large forces applied to your own robot structure as well.

Q: how about a four sided wedge with a blade in the center? so if you go up any side, you get shreded. Kind of like an arena hazard on wheels.

A: Relying on an opponent's actions to cause their own damage is not a winning strategy. You're also in real trouble if your opponent can get under your wedge - how do you apply your weapon then? Better to put your weapon in a position where you have direct control of its application.

I'm not trying to shoot down your ideas, but a good number of very creative builders have been thinking and experimenting with combat robot design for 15 years now. I'm convinced that every design that you or I can come up with has already been considered and possibly built by some competitor by now. The time for big new designs in combat robots is long past. Successful combat robots are based on quality design and construction.

Q: could I do my "moving arena hazard" Idea of having a blade in the top center with wedges on the sides, but have the blade on pneumatics, so that it raises up from inside the bot when my opponent is on the top?

A: That either makes for a very tall 'bot or a very small blade, and I don't see what advantage you get by raising and lowering that blade. You could miss your opponent completely as they drive over. Better to put the blade somewhere where you can show aggression by pointing it at somebody and driving into them.

Q: Would it work if I put a spinning blade on the end of a four bar lifting arm above a wedge, so I could shred then flip?

A: Two problems:

  • When an impact spinner hits, there is enormous force placed on the weapon mount. Putting all that force out at the end of a lifting arm is going to give that force a lever to help it tear apart your lifter mounts. Robot Wars housebot 'Dead Metal' had a spinning blade mounted on a four-bar aparatus, but the circular blade had very small teeth designed for cutting -- not for impact.

  • I'm not a fan of multi-weapon designs. Each of the weapons will have less weight allowance and therefore less power. If your spinning bar is strong enough, you won't need to do any flipping and vice-versa. There have been plenty of multi-weapon robots, but no champions.

Q: Hi. I am building an ant spinning drum. I want to use a 4 oz. drum. Would the Radio Shack 9VDC Micro Super High Speed Motor work? It runs at 24,000 rpm with 36 g:cm stall torque. I would probably gear it down to 4 or 5 to 1. Thanks, your web site has helped me learn a lot.

A: That Radio Shack motor is a very small, inexpensive, general purpose, low-power unit designed to run a small cooling fan or the like. It has a lot of speed, but not nearly enough torque for an effective antweight spinner weapon. It would take a very long time (6 or 8 seconds) to spin the weapon up to speed, and you don't have much spin-up time in a small antweight arena.

Staying with an inexpensive brushed motor, I'd suggest a 'Speed 300' geared around 2:1 for a 4 ounce drum about 2" in diameter. These motors were used to great effect in the VDD antweight kits to power their spinning weapons.

Q: Would the Speed 300 motor work the best or could I get something better? I dont mind paying for it.

A: There's always 'something better', but exactly what that might be depends on your design and your skill as a builder. It takes more than power to make an effective weapon, and too much power can be worse than too little.

The Axi 2208/20 brushless motor would be a 'top end' powerplant for an antweight spinner, but it would require a brushless motor controller, a more robust weapon drive system, and a higher level of precision all around. For a first spinner, I'm going to suggest sticking with the Speed 300 - it'll be plenty.

Q: [Chinese Forum] Does 'Code:Black's weapon really have a top speed about 7000 rpm?! I heard this from the Big Bang Robotics site. If that's true, how did Team Logicom do that?

A: No secrets to the weapon in Code:Black. With a 12 HP internal combustion weapon motor spinning a low-mass undercutter blade in a lightweight robot you can spin the weapon pretty much as fast as you like. However, weapon speed is only one part of the spinner success formula. There is plenty of information about spinner design in this archive.

Q: Can you tell me the 'common' engine power of a spinner? Is 'Warhead's 22 horsepower too high or '[name deleted]'s 7 horsepower too low?

A: Mark J. here: horsepower is only part of the spinner equation. The idea is to store kinetic energy in the rotating mass of the weapon and release that stored kinetic energy on impact. My own rule: an effective spinner will have a minimum 20 joules of stored kinetic energy per pound of weight class and will be able to spin up to at least half that level before an opponent can attack them.

There are many trade-offs in spinner design: speed versus 'bite', kinetic energy versus maneuverability, horsepower versus rotational mass, optimum mass placement versus durability, etc. Read thru this archive for plenty of discussion on spinner design.

Opinion: Warhead's weapon was awesome to watch, but the gyroscopic effect of the spinning mass made maneuverability a real issue. Too much weapon, I think. With a different design that 22 horsepower might be more useable - or it might become a real disaster.

That other robot, who's name has been recently banned from the page, had no control issues and ample weapon power. I don't have enough info to calculate the weapon's energy storage, but it was certainly plenty. Given a choice between the two robots, I'd rather drive [name deleted] than Warhead.

Q: [Chinese Forum] How much power does Beta's hammer actually has? What do you think of it?

A: Beta's hammer has a claimed 3000 joules of energy. The robot was an interesting experiment, but usless without a suitable arena in which to fight.

Q: [Chinese Forum] A question about Mechavore for Mark J.: you said before that Mechavore's ICE weapon power was about 7hp (actually 6.5 hp,wasn't it?), but is this the actual power of 4.0 and 5.0 version? I know Mechavore equipped two ICEs for that blade since 4.0......

A: Mark J. here: the 7 HP I cited (I like to round up) was the version used at BB 4.0. The earlier version had about half that power. I don't know of any motor upgrade for BB season 5. Their best result came in BB 3.0 with the less-powerful engine.

Q: Who is the smallest / most compact pneumatic flipper in the world? Is it the hobbyweight 'Hexy Macro'?

A: Not even close. I know of at least one 150 gram UK antweight with a pneumatic flipper and there are plenty of 1 pound US antweight flippers.

Q: [Chinese Forum] How do KHZ and THZ's pneumatic axes work? Do they have the same mechanical design as the hammer on 'The Judge'? All of them have 180 degree range of motion.

A: Terrorhurtz uses a true rack and pinion weapon drive similar in concept to the drive on 'The Judge' but without the chain and sprocket bodge. The design on Killerhurtz was a much more complicated system involving lever arms, gears, and chains. It worked, but the rack and pinion is superior.

Q: The person who has troubled you with a dozen of questions in one day has a strong belief that 'Warhead' can't improve itself by using more sharp teeth. That's why he has been asking so many questions about Warhead within these days. So what do you think of this question: can Warhead improve itself by using teeth like Mechavore's? If yes, why he didn't do so in BB 5.0? If no, what is your reason?

A: Building a combat robot is roughly equal parts engineering skill, past experience, and 'that looks about right'. Sometimes an 'improvement' in one aspect of a robot can lead to unexpected consequences in other areas. I suspect that the style and position of Warhead's impact teeth were chosen to avoid stalling the weapon. A more aggressive tooth design might have caused trouble -- Team Razer is known to do extensive testing.

Q: [Chinese Forum] Someone in our forum said the blades for Spining Weapons MUST BE 'SHARP'. Otherwise those Spining Weapons could be stopped/disabled easily before their opponents get smashed. And he also thought this is the main cause of Warhead's loss in the BB 5.0 quarter finals. But I think Spining Hammers are quite good and they are pretty reliable, too. In my opinion the reliability of Spinners is not related to whether their blades are 'sharp' or not. Who is right between the two of us?

A: Whether the impact surfaces of a spinning weapon are sharp or blunt is a function of the intent of the weapon. If the weapon is designed to penetrate, the impact area should be sharp - but it risks becoming stuck in the gash or possibly in the arena itself. If the intent is to deliver a smashing blow, a blunt impact zone is easier to maintain, stronger, and less prone to damage.

Q: [Chinese Forum] For Mark J.: you said Warhead "had more than three times the power of MechaVore's weapon." Have you calculated it in detail? I think a gas engines' maximum velocity is pretty hard to measure.

A: Mark J. here: My comment was based on engine power - 'Mechavore' claimed 7 horsepower and 'Warhead' claimed 22. I don't have enough information on the weapon from either robot to calculate kinetic energy storage. You are correct that the non-linearity of the torque curve for an internal combustion engine makes weapon calculations difficult - but not impossible.

Q: is a braking system really important for those really fast spinner weapons?

A: Most tournament rulesets require a maximum 'spin-down' time for rotary weapons. The tournament director does not want to hold up the tournament while a weapon takes five minutes to stop spinning. If your weapon won't come to a full stop in the alloted time (currently 60 seconds for BattleBots and RFL) you'll be disqualified - so yes, very important!

Q: If you had a spinner with no brake system for stopping your weapon at the end of a match, could you just hit it against the arena walls to stop it? It dosen't specify in the RFL rules.

A: Nope. The RFL ruleset does specify. Section 10.2 says:

"Spinning weapons must come to a full stop within 60 seconds of the power being removed using a self-contained braking system."

Use of the arena wall does not qualify as 'self-contained'.

Q: Are there really big difference in the damaging effect between an aluminum blade and a hardened tool steel blade?

A: Tool steel is at least 50% harder than even the best 7075-T6 aluminum. When it comes to turning energy into damage, there is no subtitute for harness. An aluminum weapon blade could do more damage to itself than to the opponent. Plus, aluminum doesn't make those pretty (judge influencing?) sparks when it hits.

Q: You say "An aluminum weapon blade could do more damage to itself than to the opponent", but why does 'Mechavore' keep using aluminum blades instead of hardened tool steel blades? Weight issues, or just Robert L.'s mistake?

A: I think we have a difference in terminology. 'MechaVore' did not have an aluminum blade, it had an aluminum disk with large hardened steel impact teeth. This is entirely standard for rotating disk weapons. The aluminum alloy provides a high strengh to weight ratio for the body of the weapon and and the steel teeth provide hardness at the impact site to do the damage.

Q: Why did 'Diesector' change its hammers' shape in BattleBots 5.0?

A: Diesector had several upgrades for BB 5.0: new jaws, new batteries, new tires, and new hammers. I suspect that the hammer change was largely cosmetic -- the new hammers looked much more aggressive than the previous design and better matched the look of the whole robot. Diesector's hammers never did a great deal of damage, but they helped in the 'aggression' scoring.

Q: And what is Warhead's spinning dome's height-adjustability used for?

A: The adjustable weapon height allowed the team to 'aim' the weapon at critical parts of the opponent. Low target, low spinner. High target, high spinner.

Q: How come no one talks about 'Subzero'? It had a pretty powerful flipper!

A: Middleweight 'SubZero' did indeed have a powerful flipper and a very good 26 win, 10 loss record with three tournament wins. I suspect that little is said about the 'bot because the flipper design is entirely standard. However, there have been comments about the legality of the components used in the flipper. The pneumatic cylinder was rated 250 psi by the manufacturer, but was being operated at 850 psi with only minimal modification. I'm puzzled about how they got this grenade thru tech inspection.

Q: Aaron, is there a commercially avialable joint or bar that is extremly flexible that I could put on my gas engine powered weapon to prevent internal engine damage or weapon damage? Thanks.

A: Flexible how? There are slip clutches available that will prevent a large torque load from being transmitted along the shaft. There are universal joints that will allow axial flex, but I don't think you want your weapon flopping around at the end of the shaft like that. The best option to isolate the weapon from the motor is to use a v-belt drive that will also allow a suitable speed reduction/torque increase. You really don't want to direct drive a weapon from an internal combustion engine.

Q: When being compared with each other, what is the advantages and disadvantages in these three weapons: spinning bar, spinning disc, and spining dome? [Chinese Forum]

A: Mark J. here: for a given weight and diameter the three shapes will store different amounts of kinetic energy at a given speed. Examples, all weighing 10 kilos and spinning at 1200 RPM:

  • a bar 1 meter long and by 0.1 meter wide stores 6882 Joules of kinetic energy;
  • a disk 1 meter in diameter stores 9927 Joules of kinetic energy;
  • a cylinder 1 meter in diameter by 0.3 meter tall stores 19,728 Joules of kinetic energy.
A spinning dome would have an energy capacity in between the disk and cylinder, but my design software doesn't calculate spherical sections.

Of these designs, the bar is the simplest to construct and is extremely durable. The cylinder is difficult to construct, tricky to balance, and much less durable. The disk falls in between.

Other trade-offs go into designing a spinning weapon - I could fill a lengthy chapter in a robot design book - but the essential consideration is a balance between efficiency, durability, and simplicity.

Q: In my opinion, there are three top axes in Battlebots SHW class: 'Hammertime', 'The Judge', and 'No Apologies'. Do you think so? What do you think of them? Are they successful? Are they all powered by CO2? [Chinese Forum]

A: Again, I can't get excited about ranking combat robots by weapon/weight/tournament.

  • 'Hammertime' used high-pressure nitrogen regulated to about 200 PSI to power it's weapon.

  • 'No Apologies' had a spring powered axe/spike that was reset by an electric winch. It also had a pair of lifting arms.

  • 'The Judge' was CO2 powered.
You can judge for yourself if they are 'successful'.
Q: What do you think of those monster flippers like 'The Matador' and 'Gravity'?

A: In arenas where it's possible to win by throwing an opponent out of the arena, a monster flipper is a viable design choice. In a fully enclosed arena, they don't carry much benefit - they are more of a novelty.

Q: But what caused 'Toro' and 'T-Minus' got Giant Nuts? They're really powerful and I think they are belonged to 'Monster Flippers',too. Aren't they? [Chinese Forum]

A: Mark J. here: Inertia Labs was one of the premiere teams in Combat robotics. They were experienced, skilled, well funded, and loved innovation. Importantly, they knew how to learn from their combat experience and improve a good robot over time to make it a great robot (they also knew when to give up on a poor idea). I'm sure they could succeed in building a winning robot with any type of weapon they might choose.

Q: How does the spinning ring weapon on the robot 'Spinzone' work?

A: See the description of 'Ringmaster' further down in this archive.

Q: In our Chinese Forum there is a common idea that 'SOW', 'Warhead', 'Mechavore', 'MOE', and 'Surgeon General' are the top 5 spinners ever seen in BattleBots. Do you agree with us or have other choice?

A: I guess you're giving a lot of points for destruction. In my book, top spinners win championships.

  • How can you not include 'Ziggo' (13-3, 2 championships)?

  • How about 'Hazard' (17-1, 3 championships)?

  • 'Minion' qualifies as a spinner (12-4, 2 championships).

  • and the often overlooked 'Backlash' (13-4, 1 championship) probably belongs in the mix.

Son of Whyachi has a championship, but his win over 'Shaka' in the first round of that tournament (BB 3.0) was controversial. Then there's the whole 'shufflebot' issue: after being reclassified as a SHW, SOW could only manage a 1-2 record.

I think my 5th robot would be 'Shovelhead'. His record at BattleBots was only 3-3, but he has since gone on to win 5 tournaments and amass an impressive 39-15 record.

Q: I asked you a question about top 5 spinner but I made a mistake: not in all Battlebots, just in the heavyweight of Battlebots.

A: That becomes a very narrow category. I'm not a big enough fan of spinners to try to sort out which amongst a lackluster group of robots are the 'top heavyweight spinners at BattleBots'. Outside of 'SOW' and 'Warhead' the heavyweight spinners just weren't all that impressive - IMHO.

Q: What caused 'Hexy D's success? It doesn't has so much power in that flipper.

A: There is a great deal more to a robot than the power of its weapon. 'Hexadecimator' (21-9) was a well constructed robot that did everything well: quick, controllable, powerful, and well driven. When you have that combination going you can use a pointy stick for a weapon and still win matches.

Q: How does 'The Butcher's CO2 powered spining blade work?

A: I've never seen inside. I could only speculate.

Q: Is Voltronic's weapon hydraulic? How much power does it has? [Chinese Forum]

A: Some builders share a lot of information and some don't. The only details I have on Voltarc/Voltronic are that the lifter is electric and the main drive motors are made by Leeson.

Q: Which is better between Gas Powered Spinner and Electric Powered Spinner? It seems that Gas Powered Spinners have less reliability. [Chinese Forum]

A: Reliability is an issue with internal combustion engine (ICE) spinners. A few teams have that licked.

Electric weapons are simpler and more reliable, while ICE gives the potential for greater power at lower weight (and higher noise level).

Q: Why are drum weapons on heavyweights not as successful as those on smaller robots? [Chinese Forum]

A: Some designs work better in a specific size range. That's why there are no eagle-sized mosquitoes or mouse-sized elephants.

Q: Is there any robots using a electric drill as their weapons?

A: It's been tried, with very poor results. It's way too slow and does way too little damage. Think about the pressure and time needed to drill a hole in armor material. If your opponent is sitting still long enough for you to do that, he's already lost the match.

Q: Do you think 'Warhead' is strong enough to be a BB champion? Why are 'MechaVore', 'Surgeon General' and 'MOE's blades made of other materials but 'Warhead' does not? [Chinese Forum]

A: I was quite surprised when 'Warhead' was beaten by 'Overkill' at BattleBots 5.0. I thought it was going all the way.

There are many approaches to building a spinner weapon. Some builders like the simplicity of a single-piece bar, while others attempt to gain an advantage through complex design with several different materials. Warhead's aluminum dome covers their powerful internal combustion engine and acts as armor as well as adding to the rotational mass of the weapon. It was easiest to machine the dome from a single chunk of aluminum and add on the impact teeth.

Q: Is 'Warhead's blade made of hardened steel? [Chinese Forum]

A: Team Razor wasn't very keen on sharing construction details about their 'bots. I do know that the weapon dome on Warhead was aluminum, and it would certainly make sense to use a tool-steel alloy for the blades.

Q: In the Combat Robot Hall of Fame, why do you say that Warhead had "the most frightening internal combustion powered heavyweight weapon ever seen at BattleBots"? I think Mechavore is more awesome.

A: Mark J. here: you're entitled to your opinion on 'awesome', but 'Warhead' is an 'Honorable Mention' in the Hall of Fame and 'MechaVore' is still waiting outside.

Warhead had more than three times the power of MechaVore's weapon, and the display of gyroscopic forces put on when Warhead fully spun-up has never been matched. I'm frightened just thinking about it! Suggest that you go watch a few videos of Warhead and see if your opinion changes.

Q: Can Warhead's tail be used as a lifter?

A: No, it's too weak. It was only intended as a brace in the self-righting sequence.

Q: Hey, Aaron. What makes a wedge-bot so effective?

A: Simplicity. No moving parts, low weapon weight requirement, and an attack that develops quickly and works well against many types of robot.

Q: How is Razer's wedge so low? Everytime I see an opponent with a wedge Razer's is lower.

A: Razer has an adjustable 'torsion bar' suspension that allows very precise positioning of the front wedge 'snout'. The information I have is unclear about whether the suspension is adjustable remotely.

Q: How does Complete Control's Weapon Work?

A: Complete Control was the original 'clampbot'. Starting with a conventional pneumatic lifter, Derek added a pneumatic arm on the lifter platform itself that clamped downward to hold the opponent in place. Once clamped, the entire platform lifted upward with the opponent in a firm grip. Awesome!

Q: Hey Aaron! Love the site, it has helped me a lot!

I built my first 12 pound wedge bot, yet to compete, and I am wondering about adding a weapon. With the 1-1/2 pound steel wedge my robot only weighs 10 pounds 13.5 oz. I was thinking of making another lighter wedge and putting some type of lifting mechanism using a servo or linear actuator. I know both of those are slow but it does not need to be that fast. Any advice? Thanks, Daniel.

A: Glad to hear that you found the advice here helpful, Daniel.

My advice is to enter a competition with the robot as it is. You may discover a critical use for that extra 18 ounces of weight allowance. If you're pleased with the performance of your chassis and drive train you can think seriously about modifications to give you some additional offense.

Electric lifters are effective in the sub-light classes, and judges seem to like them. You are right to say that they do not need to be fast -- they do not need to lift very high either. Even a simple servo-powered hinged wedge can be very useful and can add to the versatility of the robot, but give the robot a shake-down in combat before you start modifications.

Q: Where can I find the RPM of 'Mechavore', 'SOW', 'Warhead', 'MOE' and 'Surgeon General'?

A: What... the weapon RPM? I'd start by looking at their websites. See #30 for help on finding archived websites for teams that aren't on the web anymore. Many teams consider this type of data 'top secret' and do not make it available.

There is much more to a rotary weapon than just RPM. Serch this archive for other design criteria.

Q: I hear that Team Whyachi's 'Warrior SKF' stores it's flipping power in the rear spining shell. How does it work?

A: Builders have been working on flywheel powered flippers for quite some time. There is a discussion thread at the RoboWars Australia Forum. The problem with such a design is building a clutch mechanism that can withstand the brutal force instantaneously transferred from the flywheel to the flipper without tearing itself apart.

I have not seen Warrior SKF up close, but unless you can match the budget and resources available at Team Whyachi I would suggest you try a more conventional approach.

Q: How come 'The Matador' from Inertia Labs didn't do well? It was the most powerful flipper out there.

A: It takes more than a powerful weapon to make a champion robot. I say this all the time: a successful combat robot has all of it's components and systems working together in balance.

A super-weapon on a chassis that doesn't work well to use that weapon is not going to perform well. A well designed chassis with a poorly set-up radio system will have a very rough time. A robot that's hard to repair and maintain won't get far in a tough tournament. The weapon is probably the least important system on a combat robot.

By the way, I think a 4 win 2 loss record is pretty successful. They were stopped at BattleBots 5.0 only by the barbaric 'Warhead'.

Q: Why did you say The Matador had a "next generation" flipper? What was so different about it?

A: Inertia Labs refered to it as a new generation. Here's what they had to say on their website: "The Matador is the third generation of our flipping robots. Optimizing everything from our previous designs and squeezing three times the flipping force of Toro into only two thirds the weight. The Matador exerts over 20,000 pounds of flipping force and can throw a robot in its weight class over 12 feet in the air."

Q: What weapon motor and ESC would you recommend for a beetleweight horizontal spinner?

A: Mark J. here: a well designed combat robot is not just a set of components bolted together. The design of each system must be made with consideration to the rest of the machine. Recommending a weapon drive without knowing anything about the design for the rest of the robot would be folly.

If I'm given a weapon design I can comment on potential flaws, but I have to assume the designer has an overall plan that will incorporate the proposed weapon into a well balanced robot.

Read down thru this archive a little to find an example of integrating a weapon into a beetleweight design.

See also #29.

Q: Is there any way to detect horizontal bar contact electrically, and shut off the blade?

A: Why would you want to? On impact your opponent and your 'bot will be thrown clear of each other and your blade can start to spin back up.

I can't think of any simple circuitry to accomplish an electrically detected contact shut-down.

Q: How do you use a dead shaft for a insect class horizontal bar spinner?

A: A dead shaft is fixed in place and does not rotate with the weapon. The weapon and the sprocket/pulley are fixed together and have bearings that allow them to rotate on the dead shaft as a unit. This eliminates the problems associated with trying to secure the weapon to a small diameter shaft. Search this archive for more tips on dead shafts.

Q: Could you coat the edge of a spinning blade with magnesium flakes or something else so that it would spark on contact? Would it help you with the judges?

A: A steel striker bar will make very pretty sparks when hitting titanium armor. I suppose you could add a titanium or magnesium insert to your steel blade to make sparks when attacking steel armor. Aluminum and plastic armor are too soft to make sparks no matter what you hit them with, and nobody would believe sparks off plastic anyhow.

Experienced judges will not be impressed by sparks, but they could help get the audience behind you.

Q: Hey guys, it's Dave B. again - formerly with the questions about greasing my robot's armor and building a "sliding mass" spinner. Thanks for the great info on my previous questions. Still speaking (for the moment) in terms of antweight design limits, here's my latest idea, I'd like to know what you think:

My plan is to (eventually) build a vertical disc spinner, and mount two rotating teeth on the outer edge of the disc. The teeth will be vaguely in the shape of a capital letter 'L'. My idea is to swivel mount the teeth so that the short leg of the 'L' is normally pointed out Rather than grabbing on impact, the teeth would rotate 90 degrees or so to expose the long arm of the tooth. The teeth would be spring-tensioned to return to original position when the disc slows significantly, such as after a big hit. The idea is to get the opponent well within the range of the long part of the tooth, then flip it outward by means of the smaller arm of the tooth in order to get a really deep impact. I figure this would allow me to run my spinner at a bit higher speed than I normally would, since I wouldn't need to worry about the weapon skittering over the armor of the other 'bot.

Do you envision any problems with implementing or using this design? Do you know if anyone has used a similar idea? Do you have suggestions on materials to build the disc and teeth out of? I don't plan to go too crazy with the weapon RPM as I know my robot's weapon mount won't be indestructable. Thanks again for all the help, guys!

A: Mark J. here: I don't like the idea of putting the entire stress of impact on a hinged tooth. Even an antweight spinner will transmit a big slug of energy thru the impact tooth and I think it's best to have that solidly anchored.

Suppose only one of the teeth gets rotated - you have a serious imbalance problem that would require shutting down the weapon to reset. Worse, one of the hinges gets tweeked and the tooth is stuck either in or out.

A 4" diameter disk rotating at 3000 RPM has more than 500 g's of centrifugal acceleration acting at the edge (calculator). The 'long arm' of your tooth would need to be held in place very firmly to keep from 'rotating out' prematurely. I can't think of a simple design to hold it in place, let it swing out when triggered by an impact on the short arm, and return it to position at lower RPM.

I have seen spinners with 'free swinging' impact bars that are held outward only by centrifugal action. The idea was that the bar could deliver a good blow, then rotate out of the way to prevent the weapon from stopping. This produced a series of rapid impacts that was impressive to the judges, if not as damaging as a single big impact.

Insect class rotating weapons are typically single piece steel to take great abuse. Larger disks are often aluminum with tool-steel teeth, but I've seen large disks made of steel, titanium, end even UHMW Polyethylene.

Q: I'm considering building a spinner once I have some more build experience. It will likely be an antweight or beetle, vertical disc or bar design. Would there be an advantage to building the weapon with sliding weights that would be kept close to the center when the weapon is stationary or slow, but would slide out to the outer circumference once the weapon attains sufficient speed?

I'd likely use springs or rubber bands to return the weights to center after a big hit. I should specify that the weights will not be the actual teeth on the weapon, those will be solidly mounted in place. Intuitively, this seems like it would allow a quicker spin-up time, but give the weapon some advantages in terms of stored energy once the weapon was going fast enough to cause the weights to shift.

I have to believe someone would have thought of this before, and the fact that I don't see this kind of thing in common use makes me think it's somehow a bad idea. What do you think?

A: Mark J. here: first, I like your plan to put off building a spinner until you have more experience. Many builders aim to dominate the sport with their first project. That usually ends in frustration. Compliments on your patience.

Some builders reading this answer will scoff, but your idea has theoretical merit. To understand why requires a short lesson in the characteristics of Permanent Magnet Direct Current (PMDC) motors.

Take a look at the chart. A PMDC motor generates maximum torque at stall, and that torque falls off linearly to zero at maximum RPM. Power is the product of RPM and torque, and that product is greatest at 50% of the maximum RPM.

In a conventional spinner weapon, the motor strains against the large rotational mass of the weapon to slowly reach the 'fat' part of the power curve, where it really pumps energy into the weapon. Quickly, the RPMs move beyond the peak power zone and the motor again struggles to add more energy with decreasing torque and power.

Your 'sliding weight' design achieves an effect similar to a continuously variable transmission. The weapon initially has a smaller Moment of Inertia (MOI), allowing it to accelerate more quickly into the peak power zone of the motor output curve. If the sliding weights are designed to start moving outward as the motor approached its output peak and completed their transit shortly after the motor power peak, the motor will spend a greater percentage of the spin-up time near peak power output and would average greater power output which would give a shorter spin-up time.

So, on paper the idea works. How much benefit there is to be gained depends on the amount of mass you are able to shift and how accurately your spring loading system controls the weight progression. The drawback: increased complexity on a component that will take a lot of abuse. It might be simpler to just pop for a more powerful weapon motor; simplicity has merit.

Q: What is the max speed I could take a 25cm x 3cm x 4mm steel bar to before it starts to get too dragged?

A: Mark J. here: aerodynamic and mechanical drag are the limiting factors on absolute top speed of a rotating weapon for a given amount of power; add more power and you can get more speed. However, if you spin your weapon too fast it won't be able to catch and 'dig in' to your opponent. It will just 'skitter' over the surface of their armor. Also extremely important in an insect class robot is spin-up time. The best weapon takes all these factors into consideration. Play around with the parameters in the Team Run Amok Spinning Weapon Excel Spreadsheet to put everything in balance.

Q: How many joules can a beetleweight bar spinner realistically dish out?

A: See the above question. Joules alone are not the answer. You see builders claiming astronomical numbers of joules, but a well-balanced design is going to win more matches.

Q: How much weight is usually alloted to a robot's weapon system - something like 'Totally Offensive'?

A: 'Totally Offensive' is an extreme example -- blade and motor alone make up more than 1/3 of the entire robot weight. Add in the weapon supports, battery, geabox, and controller and I'd guess at around 60% of the robot weight devoted to the weapon. A more typtical weight allotment might be 35%.

Q: I played around with the spreadsheet a bit and came up with two beetleweight configurations:

  1. A 0.4x0.04x0.004m (The obscene length represents weight concentrated towards the ends of a 25cm blade) steel bar, running on a Axi 2820/10 at 4:1. Giving a 0.41 second spin-up. With max RPM and joules of 3600 and 500 respectively weighing in around 24 ounces.

  2. A 0.2x0.04x0.004m steel blade on an Axi 2808/20 at 3:1, giving 0.17 second spin-up, and max stats of 4900 RPM and 85 joules.
Which of these is more realistic? What is that perfect balance you just talked about?

A: I get very different numbers for your weapon configurations, so I started over.

First, don't extend the length of a spinning bar to reflect extra mass out at the ends. That will overestimate the moment of inertia. The spreadsheet gets kinda clunky when trying to get this right, but here's how you do it:

  • Enter the actual length, width, and thickness of the main bar -- 0.25 meter by 0.04 meter by 4mm = 0.31 kilo.

  • Calculate the mass of the additional material -- say two chunks of steel each 0.025 meter by 0.04 meter by 12mm = 0.19 kilo.

  • Distribute the additional mass as if it were a ring out at the end of the bar, playing with the height to get the mass correct -- outer radius 0.125 meter (the outer edge of the bar), thickness 25mm (the length of each piece of additional mass), a height of 0.0014 meter gives you 0.19 kilo.
With the Axi 2820/10 at 12 volts and a 4:1 reduction, this correct mass distribution gives a spin-up of 0.16 second to 117 joules and 2268 RPM. The brute power of this motor will likely top out near 3200 RPM and over 220 joules. Calculating for the 0.4 meter bar gives an incorrect estimate of .26 second to 191 joules at 2268 RPM.

Most brushless motor controllers have a 'soft start' function the feeds power to the motor more slowly than just switching it on. This adds considerably to spin-up time. I'd add half a second to any calculated spin-up time for a brushless motor to allow for this function.

The 2808/20 motor powering the 20 centimeter blade at 12 volts and a 3:1 reduction gives a calculated spin-up of 0.25 second to 67 joules at 3755 RPM. Again, I'd add half a second to the spin-up time for the real world.

Bars have poor aerodynamics compared to disks or drums, so you aren't likely to get anywhere near the 95% maximum speed the spreadsheet provides. That's just a 'brag number' at best. I'd guess closer to 80% of maximum RPM with a bar and a reasonably sized motor.

I'd go with the Axi 2808/20 for a beetle. Think about adding more mass to the blade and increasing the reduction ratio. An 8mm thick blade with a 4:1 reduction gives a calculated 0.28 second spin-up to 76 joules at 2816 RPM. It will realisticly top-out around 125 joules at a reasonable 3600 RPM.

One thing to consider if you do build a mega-powered spinner with a huge motor: what happens when the blade hits the arena wall? The enoromous kickback might very well rip your weapon free from the chassis and send the remains of your 'bot flying across the arena. Keep it real. Much more than 100 joules for a beetle is way out there.

As for that 'perfect balance', I wish I had a formula for it. Many of the design factors are in conflict with one another:

  • A slower spinning blade does a better job of transmitting the stored energy to the opponent, but a faster spinning blade stores more energy.
  • A heavier blade stores more energy, but takes weight allowance from other systems.
  • A smaller reduction ratio gives more peak energy, but slows spin-up and taxes the motor.
Everything needs to be in balance, but that balance is hard to find. As I often say, look at what other builders are doing that works. If nobody is using an Axi 2820 for a beetle weapon motor, there's probably a good reason.

Q: What are you using for stats on the Axi 2808/20, because when I run it a 4:1 it gives a spin-up RPM of 2426, not 2816

A: Per the Axi website the Axi 2808/20 puts out 1490 RPM per volt and has a terminal resistance of 105 mohm. At 12 volts that's 17,880 RPM. Divided by 4 gives 4470 RPM. Take 63% for the spin-up calculation and that gives 2816 RPM.

Q: Do you need a motor controller to power an Axi, or can you just use a relay?

A: Axi motors are brushless. Brushless motors require a motor controller to operate - they won't run without one.

Q: Does anyone run hydraulics? If so where can I find a small DC electric hydraulic pump?

A: Hydraulics are rare but they are used, usually in heavyweight robots because industrial products are too large and heavy for lighter robot classes. Pumps and actuators sized for smaller robots are available, but they tend to be low-power and expensive -- see recent posts in this archive for a source.

Q: Hello Aaron, I looked at the site and saw that I could build a four bar lifter with a linear actuator. Where would you put it on the lifter?, Could I have a diagram? Thanks.

A: Suggest you take a look at the BioHazard Mechanical Design page. There are several photos and explanatory text covering their use of linear actuators to power a 4-bar lifter. Briefly, the actuators are attached to the front lifter bars via a bellcrank. The bellcrank allows the actuators to generate maximum force when the mechanism has the least mechanical advantage in the lift. Great design!

Q: Dear Aaron, I want to build a 30 pound flipper robot. What would be the basic parts? Could you recommend parts or part suppliers? I looked thru your website and you mention linear actuators - which would be easier because I am a beginner in flippers. I looked at a website that sells some linear actuators but it would be better to have a fast one. Could you recommend a website? Thanks, Dan.

A: Electric linear actuators work fine for a lifter, but are way too slow for a flipper. You need pneumatics for true flipper speed. There are several posts about pneumatic system parts and links to design help in this archive.

You have quite a bit of design work to do before I can recommend a specific components. Will the liting mechanism be a 4-bar mechanism like BioHazard, a single-pivot unit like Silverback, or some alternative design? How high do you want to lift?

If you do decide to go with a linear actuator I'd suggest a supplier who knows the needs of robot builders. Have a look at the actuators offered by Team Delta and Trossen Robotics.

Q: You said 1/2 inch titanium is one preferred armor for heavyweights. How much energy is required to pierce it with a pickaxe or a pointed ram?

A: Mark J. here: very hard to calculate because of so many variables: differing armor mounting systems, alloy type, heat treatments, pick cross-sectional area, attack angle, etc. I've never seen even 1/4" titanium pierced by such weapons. I'm told that a 30.06 armor-piercing round will penetrate 7/16" 'hardened titanium alloy'. Such a round carries close to 4000 Joules of energy.

Newtonian physics (action/reaction) effectively prohibits rams or overhead axes from developing enough power to do the job.

Q: Are internal combustion rams or hammers allowed (diesel/gasoline/propane)?

A: Linear internal combustion actuators are not allowed under section 7.2 of the current 'RFL Standard Extensible Rule Set'. Rotary internal combustion engines (ICE) are allowed for weapon power at the discretion of the event organizer.

Q: Were there any successful crusher robots besides 'Razer'?

A: That depends on what you consider to be a 'crusher' and what you call 'successful'. There were plenty of self-defined 'crushers' at Robot Wars -- off the top of my head:

  • 'Big Nipper'
  • 'Cerberus'
  • 'Chompalot'
  • 'Crusher'
  • 'Crushtacean'
  • 'Kan Opener'
  • 'Mantis'
  • 'Ming 3'
  • 'Pincer'
  • 'R.O.C.S.'
  • 'Snake Bite'
  • 'Tiberius'
  • 'Tough as Nails'
  • and even one version of 'Suicidal Tendencies'.
Not many of them could do any effective piercing or crushing -- most were simply clampers. The best tournament results I can recall from a crusher other than 'Razer' came from horizontal crushers. 'Tough as Nails' won 'Heat M' at the Seventh Wars, and 'Kan Opener' won the 'Annihilator' competition at both Extreme Series 2 and the Seventh Wars.
Q: Hey Aaron, I just finished my first 30 pound wedge bot. It's proabably not very competitive and I want to build a robot with a weapon. I'm not sure what design: a spinner like 'Terminal Velocity' or a spinner like 'Nightmare'. Either way the design should not require welding, and not a drum spinner because I can not make the drum for it. Any advice for a 30 pound robot design?

A: Take a read thru this archive and you'll find that I'm not a big fan of spinners. The truth is that wedges are more sucessful than spinners -- see my dad's analysis of tournament results. My advice is to refine your wedge and get it competitive before you move on to an active weapon.

Q: I'd like to build a spinning drum weapon with screws as teeth. What's the best and most accurate way to drill holes in the aluminum cylinder that's being used for the drum? I have access to a drill press.

A: Mark J. here: holes for the screw teeth? Use that drill press. Set-up a cradle for the drum that will position the perpendicular axis of the drum directly below the axis of the drill bit and still allows it to rotate and slide left/right to line up the position of the hole. Mark off the hole positions and drill away.

Q: Hi! I am only 10 and I've loved robotic combat all my life. Could you tell me some advantages and disadvantages for weapons, taking affordability into account?

A: I was only 9 at 'Robotica' and I remember how much I loved Robot Combat back then. Good to have you here!

The arguement over which weapon is 'best' has been going on since the first robot fight. The topic has been addressed in books and in the on-line forums.

This really is too big a topic to discuss fully here, but you'd probably be interested in the results of my dad's study of results from combat robot tournaments. The study shows that, on average, robots with active weapons (spinning blades, drums, lifters) actually do less well than robots with passive weapons (wedges, spikes, rammers). Particulary for your first robot, I'd recommend keeping it simple and avoiding any type of active weapon.

If you are determined to use an active weapon, the data from the weapon study indicates that lifters are the most successful active weapon style -- at least in small robots. A servo-powered lifter is very easy and inexpensive to implement in an antweight robot.

Q: Hi Aaron, great site. In your archives you mention that the main difference between a drum and a beater is the inertia each can wield. Does that that mean there are no advantages in the structure or shape when it comes to a hit?

A: With a weapon rotating at operational speed there is no advantage to the eggbeater structure on impact. The weapon is rotating so fast that penetration of the opponent into the weapon before impact is a small fraction of an inch. A shallow impact bar on a drum is every bit as effective as the eggbeater bar.

There are several posts about eggbeaters and drums in this archive that deal with construction, rotational speed, and impact bars. The question you reference came in after the other posts and asked if the two designs didn't basically do the same thing, and my answer generated a good deal of off-line discussion.

To clarify my original statement, a drum weapon the same weight and diameter as an eggbeater will have greater rotational inertia. Someone pointed out that an eggbeater can be built to a larger diameter and have the same mass as a smaller drum and therefore have greater rotational inertia. This is true, but it just points out the trade-offs and complexity of weapon design. Generally speaking, an eggbeater is simpler to build but a drum is more effective.

Q: What would be some pros and cons for an axe weapon and a spinner weapon?

A: Suggest you find a copy of Combat Robot Weapons by Chris Hannold for a good discussion on weapon pros and cons. Very briefly:

  • Overhead axe weapons can attack the usually weaker top armor of an opponent but must accelerate the weapon to high energy levels in very little time.

  • Spinners can more slowly build up kinetic energy in the rotational mass of their weapon for a devastating impact, but must deal with gyroscopic forces and the large 'kickback' from their own weapon.

Q: Dear Aaron, I've found a mini hydraulic sytem at The system has almost 40 pounds of force. Would this be a good system for builders who want to make a crushing weapon, or would a cordless drill and gearbox hooked up to a worm gear or screw and conected to a claw be better. Please help. [Anthony]

A: That mini hydraulic system is expensive! A pump, cylinder, valve and assorted lines and fittings looks like about $700, and they don't give you the weight of the system or the volume output of the pump.

Crushing weapons require a great deal of force and very strong chassis, The crushing pincer on Robot Wars heavyweight champion 'Razer' had 18,000 pounds of force -- 100 times it's own weight! So, 40 to 60 pounds of hydraulic force is not going to make much of a crushing weapon and neither is a geared-down drill motor.

Q: Would it be an effective crushing system on an antweight? Would I be able to multiply that 60 pounds of force with a lever like 'Razer' did when there cylinder force was 3 tons and now the tip of the claw has 9 tons? Sorry for bombarding you with questions -- I just really like this sport. I can't help it!!!

A: If I didn't like questions, I wouldn't have this site, Anthony!

There are a couple problems I see with using this system in an antweight:

  • Although the component weights are not given, the dimensions are. The pump alone is 6" long and 1.5" in diameter. That looks like a RS-385 motor powering it. I'd guess the pump alone might weigh half a pound. You wouldn't have enough weight allowance left to build a robot strong enough to withstand the force of it's own weapon.

  • Gardentrucking supplies components for builders of scale trucks and construction equipment. These builders are after realistic lifelike performance from the hydraulic system, and in this case lifelike means 'slow'. Adding a force-multiplying lever system to would slow this down even more: three times the force means one-third the speed.
In short, the gardentrucking mini hydraulic system is too heavy and too slow to be the power source for an effective antweight crushing weapon.
Q: Should I buy a motor having high torque or high RPM for a spinner weapon??

A: Either will have to be geared down to perform well with most spinner designs, but it's generally easier to work with a high torque motor. This is why the high-torque 'outrunner' style brushless motors are popular for spinner power. High RPM means high gear reduction and potential problems keeping drive belts in place on small drive sprockets.

Q: What is the best motor rating for a spinner weapon? My weapon weighs 3 kg and bot weight is 10 kg.

A: See #29 A good place to start with a weapon around that size would be the brushless Axi 4130/16 or the DeWalt 18 volt drillmotor.

Q: I've built a spinner weapon similar to 'Son of Whyachi'. It weighs 7 lbs. The total bot weight is 22 lbs. What is the ideal motor rating for my weapon? The maximum voltage I can use is 24 volts.

A: More power is better for a spinner motor, so 'ideal' is as much power as you can reasonably get. The Axi motor I referenced above would be a good choice. Geared down about 3:1 and running at 24 volts it would give excellent performance. Use the Team Run Amok Spinning Weapon Excel Spreadsheet to check the performance of your specific weapon with various motors.

Q: Hi. My beetle horizontal mid-cutter had its bar mounted to a 1/4" live shaft by an M Type Shaftloc. At a recent event I had a weapon to weapon collision with a drum. The nut on the Shaftloc was ripped off, stripping the threads and making the entire thing useless.

At Motorama I'm going to run it as an undercutter. What other similar components could I use for mounting the weapon to the shaft? I could use the Shaftloc again since an undercutter won't be taking the same kind of vertical hits, but I'd rather use something more proven and reliable. Thanks.

A: Mark J. here: you don't have a lot of options for securing a hub to a live shaft that small. Hobbyweight mid-cutter 'Fiasco' uses a custom hub that clamps the blade in place and is held to the shaft with four set screws. I'm not a fan of set screws! Given the options, your current solution may be the best choice; it sounds like that hit you took would have broken something else if the Shaftloc hadn't failed.

The most reliable method of securing a hub or other item to a round live shaft is to broach a keyway into the shaft and the hub and inserting a key to prevent rotation. This isn't practical for a 1/4" shaft, and neither is cross-drilling a hole thru the shaft and hub and inserting a hardened pin. I recommend using a dead shaft in insect class spinners for just this reason.

Q: Hey Aaron, is 0.07" thick 6Al-4V titainium thick enough for a beetle wedge? The wedge is securly mounted with 6 bolts.

A: The thickness needed for a wedge depends on the unsupported length of the material, the angle of the wedge, the type of the support it receives, and the punishment to which it will be exposed. There's no way I can even make a guess about the minimum thickness you'll need for your specific design. Follow my rule for armor: make it as thick as you can and still make weight.

Q: Are wedges cool? I have been making ants and beetles for 3 years. I want to make a beetle wedge but it seems that people say wedges are boring and lame.

A: Cool is building what you want to build. If other builders don't like what you build, beat the bolts out of their robots and smile while you're doing it.

Q: I want to use a 12 ounce spinning blade on a beetle weight spinner. What sort of motor or motor/gearhead would be good?

A: Selection of a motor and drivetrain for a spinning weapon needs more input than just the weight of the blade. I'd suggest reading thru the many posts about spinning weapons in this archive and the archive for a start. If you have access to Excel, you can download our Team Run Amok Spinning Weapon Excel Spreadsheet to help you calculate the performance of your weapon design with various motor options. Something like an Axi 2808 brushless motor with a belt drive reduction around 4:1 would be a good starting point.

Q: I want the blade to be a 15cm by 3cm by 1cm 10 ounce steel bar for my beetle weight robot. Is this going to be too powerful and what sort of motor/gearhead should I use? I looked at the Axi 2808 but I can't find gears or pulley small enough to fit on the shaft, can you suggest a place?

A: A steel blade that size weighs closer to 12 ounces than 10, so I'll assume a thickness of 8mm to cut the weight down to 10 ounces. With a 4:1 reduction and an Axi 2808/20 running on a 3-cell LiPo battery, the weapon will spin up very quickly to about 4000 RPM and pack nearly 50 Joules of energy -- plenty for a beetle, but certainly not 'too powerful'.

4mm shaft diameter is just a little over 5/32" You can drill out 1/8" bore timing belt pulleys to 4mm. Robot Marketplace sells 1/8" bore MXL timing pulleys that would work well for this purpose. Belt drive is prefered over gears for a spinning weapon.

Q: Thanks Aaron. One more thing, I keep trying to calculate stall torque of a brushless motor using your spread sheet and I keep coming up with numbers like 1.41N-m which I know is obscene. What am I doing wrong?

A: Maybe nothing. Brushless motors can pump out short bursts of really huge power. Just make sure you get the correct numbers for RPM/volt and internal resistance. Plugging numbers for the Axi 2808/20 into the Run Amok Spinner Spreadsheet (1490 RPM/volt, an internal resistance of 105 mOhm, and 11.1 volts) I get an estimated 0.68 N-m stall torque.

Q: Hey Aaron, I have 7 ounces left on my beetle. What is the most effective weapon I can put on it? I don't want a spinner. The beetle is a fast and powerfull wedge but I want some type of weapon. Thanks.

A: An effective weapon is integrated into the structure of the robot. The time to think about a weapon is at the start of the design process, not after the 'bot is built and you discover that have a little weight allowance left.

I'd spend the extra weight on armor, but if you're determined to tack on a weapon you'll need to take a look at your design and component layout and think about what type of weapon would make sense. Consider if there is room for a servo-powered lifter and if that would make sense in your design.

Q: Hi again -- I had the 7oz beetle weapon question. Is it possable to build an effective beetle spinner with 7 oz? Thank you!

A: I'm gonna say no. You'd want at least 5 ounces of rotating mass for an effective beetle spinning weapon. That doesn't leave much for motor, drivetrain, and support structure.

You really need to decide what your robot is going to be in the design phase and build the robot to meet that goal. If you just keep adding things on 'til you top out on weight you're going to have a robot that tries to do too many things and ends up being good at nothing. Use that extra seven ounces of weight allowance to make the robot better at what it already does well.

Q: Hey Aaron, your site is cool. I'm making a beetle and I was wondering if a weapon like on Messin with Sasquatch would be effective? What motor does it use for the weapon? Thanks.

A: Glad you like the site!

'Messin with Sasquatch' used a hole saw spun by (I'm guessing) a drill motor. This type of weapon has been tried quite a few times in various weight classes and has never proven effective.

Think about how you use a hole saw, how much pressure you have to apply, and how long it takes to cut thru a stationary object. Then think about trying to cut into another moving robot with the pressure a beetleweight could apply. The best you could do is to scratch up a little Lexan.

Pick a robot with a better record than 0-1 to emulate.

Q: So if a powered hole saw like on 'Messin with Sasquatch' isn't effective, then what's the coolest most effective weapon I can make on a beetle? I don't have enough weight to make an effective spinner.

A: 'Cool' and 'effective' are often two very different things in robot combat. Lifters win a higher percentage of matches than anything else in the beetle weightclass, but most people don't consider them to be 'cool'. Spinners do very poorly overall, but builders think they are 'cool' so they keep building lots of them. Read thru this archive and take a look at our analysis of weapon success.

Date marker: January 2009
Q: I plan on making a beetleweight robot using a VDD1 blade directly powered by a GWS brushless motor (2205/15T GWBLM001A) on a 3s LiPo battery. These were my results using your spinning weapons Excel spreadsheet:

Total Moment of Inertia: 0.00012 KgM2
Mass 0.08 Kilos

Weapon will spin to: 14,336 RPM [ 63% Max ]
with: 140 Joules energy
in: 0.23 seconds.

At: 21,617 RPM [ 95% Max ]
the weapon has: 319 Joules energy.

I know you recommend against direct drive weapon motors, but I will be using a Robot Marketplace blade hub, and the brushless motor will be directly mounted to .9 aluminum. There will be another piece of aluminum on the other side of the motor to support the shaft to prevent bending.

A: Mark J. here: did you wonder why other builders don't do this? Several problems:

  • I think you've miscalculated the stall torque of the GWS brushless motor. I can't find a quoted figure for internal resistance, but from size and performance parameters I estimate 170 mΩ. This gives a stall-torque estimate of 0.29 N-m, not the 1.30 N-m you were apparently using. This stretches the calculated spin-up time to 1.02 seconds.

  • The spinner spreadsheet was designed for weapons spinning much slower than what you propose. It does not take into account aerodynamic drag, and a 110 mm diameter saw blade with big teeth spinning that fast is gonna have a whole lot of drag. That will stretch spin-up time and reduce top RPM.

  • The ESC you pick to use with the motor may limit amperage to the motor via a 'soft start' feature. This will reduce the maximum torque and even further stretch spin-up time.

  • Driving the 'bot is gonna ba a real problem. A blade spinning that fast will generate sufficient gyroscopic force to make turning the robot very difficult.

  • At speed, you'll have less than 0.00025 second between passing blade tips. That's not going to 'dig in' to your opponent, it'll just skitter over the surface.

I've saved the best for last:
  • That blade has a maximum speed rating of 13,890 RPM. An impact at 20,000 RPM is likely to disintegrate the blade, throwing sharp chunks of steel in a deadly arc at over 300 feet per second. That'll destroy your 'bot, mess up the arena, and potentially cause serious injury to anyone unfortunate enough to be standing nearby. No sane event organizer will let you compete.
Seriously -- don't direct drive the saw blade. Spinners are geared down for a variety of reasons. The weapon will be much more effective at about 1/4 the speed you propose, and it probably won't kill anyone.

Q: I asked about connecting a GWS motor directly to a VDD1 blade, and you suggested gearing it down. I was wondering if the timing pullies from Robot Marketplace are a good choice? I am having trouble understanding what all of the things like pitch and flange diameter mean. I also don't know how to connect the the blade to the outer peices of aluminum. Thanks for your help!

A: The timing belt and pulley solution will work well, but the simplest and possibly least expensive solution would be to to use the 'VDD Polycarbonate Gearbox Kit' [no longer available].

Your GWS motor is small enough to fit right in place, the weapon shaft is attached with widely spaced bearings to support the load, and a set of gears are included for the reduction drive. The pinion gear will have to be drilled out to fit onto the 3 mm shaft on your GWS motor. It will save you from building a weapon shaft support and from figuring out the correct belt size and sprocket diameters.

Q: Hi Aaron, I have another question about my gws brushless motor, and VDD1 blade setup. Would an alternative solution to using a gearbox be leaving it direct drive, but not passing 1/3 throttle? I left all of the numbers for my motor the same, except I changed the rpm to 1/3 of what I had originally. I got these numbers, and they seemed okay:

Total Moment of Inertia: 0.00012 KgM2
Mass 0.08 Kilos

Weapon will spin to: 4,779 RPM [ 63% Max ]
with: 16 Joules energy
in: 0.34 seconds.

At: 7206 RPM [ 95% Max ]
the weapon has: 35 Joules energy.

What do you think?

A: You're still using the wrong numbers to calculate torque and spin-up time. On the 'Instructions' page of the spreadsheet enter '170' as the Ri (internal resistance) value, and '2160' as the Kv (rpm/volt) value. A drop in throttle will effect both RPM and torque -- if you're going to only use 1/3 throttle, reduce the Voltage to '3.7'. The calculated stall torque value now drops to 0.10 N-m.

With these changes, I get a 63% spin-up time of 1.03 seconds, with the weapon hitting 35 Joules in about 2.5 seconds.

I know that setting up a gear reduction drive for a spinning weapon is somewhat difficult, but if you look around at your competitors you'll notice that they all do it even though it adds weight and complexity. There is more to a good spinning weapon than just Joules of stored energy. The best approach has proven to be a gear reduction. This reduces load on the motor, helps to isolate it from impact shock, increases available torque to carry the weapon past initial impact, and reduces peak amperage draw from the battery/ESC.

Trust the other builders that have done the experimental work on this. Use a gear or belt reduction.

Extra thought: for a beetle, you might consider mounting two blades on the hub. A few drops of epoxy between the blades would hold them in alignment, and the doubled mass would double the weapon energy. Spin-up time would still be good if you use a gear reduction.

Q: How are the angled sides on some Full Body Spinners made?

A: Mark J. here: different builders use different techniques. Hall of fame member 'Ziggo' used a pre-made 20" diameter wok from a kitchen supply store for the spinner body. You can read a write-up of Ziggo's construction at the Team Ziggo website.

Other techniques include use of a sheet metal roller to create a cone that can be clipped to the proper height, and machining the entire shell from a single block of metal.

Q: I saw a bot on a video that had a vertical spinner that had no cutting tools on the disc. It was used to flip the other bot. How would I set up a bot like this?

A: That's the standard use of vertical spinners like 'Nightmare' -- a couple of impact 'knockers' are bolted opposite each other on the edge of the spinning disk to strike the opponent. When a circular saw blade is used for the disk, a blade with large teeth is used to catch and throw the other 'bot. This design is particularly popular in antweight 'bots that fight in arenas where an instant win can be scored by tossing your opponent off the arena platform.

Browse this archive for tips on spinning weapons.

Q: How did the dome spinner bot 'Ziggo' have so much power? Is this possible in a beetle weight?

A: 'Ziggo' was the product of a lot of hard work and experimentation. Builder Jonathan Ridder started building combat robots in 1995. His first attempts were not terribly successful, but he kept at it and learned from his experiences.

There isn't any real secret to the power of Ziggo's spinner. Quoting from the Team Ziggy website:

"Team Ziggy's robots all have one thing in common - maximum power in minimum volume. Combat robots require large amounts of power to accelerate quickly and have truly spectacular weapons. By making the robot as small as possible, you can use stronger armor and give less of a target to your opponents." Ziggo's weapon is powered by two Aveox 1415-4Y motors running at 36 volts. That's a huge amount of power, and power in means power out. There is a lot more to building a successful spinner than just power, but power is a good start.

Insect class full-body spinners have a serious problem. As I have pointed out often, insect arenas are small and there is very little time to spin the weapon up to effective speed before your opponent is on top of you. Build for the situation in which you will be fighting.

Q: Which sportsman electric hammer bot has more weapon Joules: 'Mangi' or 'Helios Sport'? Which one do you like better?

A: Mark J. here: I don't have enough information about 'Mangi' to make even a rough calculation on weapon energy. I've written to Mangi's builder for additional info. As a pure guess I'd say 'Helios Sport' hits a little harder, but 'Mangi' is quicker. They did have one head-to-head fight won by 'Helios Sport'.

UPDATE: I gathered enough info to estimate the performance of both wearpons. According to my calculations, 'Mangi' with 78 Joules is harder hitting than 'Helios Sport' at 50 Joules.

Q: Mark, you said you had an Excel spreadsheet for electric hammers. Would it be possible for you to e-mail me this? I'm working on a sportsman class hammer for Robot Battles and NERC. Thanks.

A: Mark J. here: the spreadsheet I have is just a few basic kinematic formulas strung together with a couple 'best guess' fudge factors. I put it together to get some order of magnitude comparisons between electric and pneumatic hammers. It has no documentation, is useable only over a small range of values without recalibration, and it relies on some questionable physics shortcuts. I don't think it would be of any use at all as a design aid.

A full-blown hammer simulator would be kind of a big deal. The motor torque continually changes with increasing speed, there is no good way to calculate the inertia of the armature from available data, and the effect of gravity on the hammer changes non-linearly all along the arc.

My best advice, as has often been repeated on the Ask Aaron webpage, is to use the construction examples provided by other builders as a starting point. In your case I'd carefully examine Helios Sport from Team Cosmos. Best luck!

Q: Has anyone used a C02 powered engine for a spinning weapon before?

A: Yes, 'The Butcher' from Inertia Labs had a massive pneumatic powered spinning weapon which was not very successful. Pound for pound, electric batteries store much more energy than compressed gas cylinders. Electric motors are also very efficient (80%) compared to compressed gas engines (15%). Pneumatics are great for quick bursts of power, but not efficient for continuous motion.

Q: Would a beetle hammer bot be effective? I'm using a speed 400 motor geared 25:1. The arm is 12" and hammerhead is 4oz with a sharpened tip.

Q: How much weight will the hammer head need to be able to do damage?

Q: Would the hammer get more damage if I increased the arm to 18" and lightened the hammer head to 3oz?

A: Mark J. here: assuming a 180 degree hammer arc and taking a guess at torque losses to armature acceleration:

  • the Speed 400 motor geared 25:1 will provide about 5 G's acceleration to a 4 ounce weight on a 12" arm;

  • the hammer head will complete a 180 degree arc in about 0.2 second;

  • the velocity of the hammer head at the end will be about 31 feet per second;

  • the kinetic energy of the hammer head at impact will be around 5 Joules.
For comparison, the pneumatic axe on heavyweight 'TerrorHurtz' generates 2300 Joules, more than 10 Joules per pound of weightclass. Electric motors are not nearly as good as pneumatic systems for producing large bursts of energy.

It's important to remember that the hammer is just a storage device for power from the motor -- you can't get more energy out of the weapon than you put in!

Weapon energy is a product of the acceleration provided by the motor and the length of time that acceleration is applied. Changes to the gear ratio, length of the arm, or mass of the hammer will change the weapon acceleration, but will be offset by the change in time available before impact. As long as these design elements remain anywhere near reasonable you'll still get about the same weapon energy output. To get more energy output, you'll need more motor power input.

Q: Is 5 joules strong for a beetle hammer?

A: A strong hammer runs close to 10 Joules per pound of weight class -- about 30 joules for a beetle. Electric hammers just can't put out enough power.

Q: How many joules did 'Helios Sport's hammer have?

A: Mark J. here: 'Helios Sport' is a 30 pound 'sportsman class' robot with a 3 pound hammer on the end of an 18" arm powered by a DeWalt 14.4 volt motor at 18 volts. That works out to about 50 Joules -- comparable to a 5 Joule hammer on a beetle. You can see from the video of 'Helios Sport' vs. 'Bounty Hunter' that the hammer isn't effective.

Q: I have the T.i. Combat Robotics 4-Bar Simulator. Is the weight on there the force of the lifter?

A: Mark J. here: not quite. The 'weight' input is the constant force pressing down on the end of your lifter. The motor torque needed to offset that force is shown as the green line in the output chart. Note that the torque needed varies with the position of the lifter, but the maximum torque available from your gearmotor is constant. This means that the maximum lift force will vary with lifter position.

Also, the motor torque shown in the graph is the amount needed to just offset the weight on the lifter. Permanent magnet electric motors produce maximum torque at stall (zero speed), so a gearmotor with the torque shown in the graph could hold position against that much weight at that position but could not lift it further. Aim for a gearmotor with about twice the maximum torque shown for the weight you want to lift.

Q: Who has the most powerful flipper: 'Ziggy', 'Toro', 'The Matador', or 'The Gap'? How much does The Gap weigh?

A: 'The Matador' from Inertia Labs claims the most powerful flipper title with 20,000 pounds of lifting force. Team Run Amok's The Gap is a heavyweight that last weighed in at 208 pounds with an empty CO2 tank.

Q: Is Matador's flipper more powerful than Toro's?

A: What did I just say? Matador's is the more powerful. Toro's superheavyweight CO2 flipper had 7000 pounds of lift. The Matador's 'next generation' flipper had 20,000 pounds of lift in a heavyweight.

Q: Aaron, do you know how many pounds of force Ziggy's lifting arm has? Do you know what psi Ziggy's pneumatic system runs on? Could you give me a link to Ziggy's website if it has one?

A: CM Robotics claims 14,000 pounds of lifter force for 'Ziggy', but the 4-bar flipper mechanism makes comparison with direct-acting flippers like 'Toro' difficult.

Ziggy's flipper runs on high-pressure air, someplace in the 3000 to 5000 psi range.

Some robot teams, such as Ziggy's CM Robotics, don't have websites. It isn't that they aren't capable, it's just that they have a problem with sharing.

Q: You said Ziggy's lifter has 14,000 pounds of force. How do you calculate that force?

A: Mark J. here: that number comes from CM Robotics -- I don't know how they are calculating. In a conventional single-pivot lifter it's fairly easy to apply a little geometry to figure the lifter force based on the force available at the pneumatic cylinder (surface area of piston times available pressure), the angle at which the force is applied, and the leverage between the force application point and the point of lift. Ziggy has a four-bar flipper mechanism, which makes the calculation more difficult. They may simply be giving the raw force available at the cylinder.

Actual flipper performance depends not only on the maximum force available, but also on the gas flow capacity of the pneumatic system. Valves and hoses must be large enough to flood the cylinder with an instantaneous burst of high-pressure gas or the 'flip' becomes a gentle 'lift'.

Q: What do you think of 'The Matador'? What is it's fight record?

A: The Matador, like all of Inertia Labs' robots, was beautifully made and well thought out. The amount of force in it's weapon was certainly overkill. A little less weight devoted to the weapon and a little more to a more controllable chassis might have worked out better.

It's record was 4 wins and 2 losses -- the last loss to the fearsome 'Warhead' at BattleBots 5.0. I don't remember the match, but there may not have been enough of 'The Matador' left to put back together.

Q: Are there any good gearmotors I can use in a simple beetleweight lifter? I want to be able to rotate the arm all the way around, something I can't do with a servo.

A: There are certainly some gearmotors that will work for your purpose, but it is also a fairly simple task to hack a servo for continuous rotation.

The power you'll need from a gearmotor (or hacked servo) will depend on the length of the lifter arm attached to it. For reasonable speed and reliability, select a gearmotor that will stall with no less than 1.67 times the weightclass on the end of the lifter. Here's the formula to calculate the desired torque:

Stall Torque (oz-in) = length of lifter arm (inches) × weight class (ounces) × 1.67

Example - a gearmotor for a 4" lifter arm on a beetleweight should have:

4 inches × 48 ounces × 1.67 = 321 oz-in stall torque

The 'B231 231:1 gearmotor' [no longer available] running at 12 volts would be a good choice. You could also use the BaneBots 64:1 24mm gearmotor [no longer available] running at 7.2 volts, but it's higher speed could be difficult to control on a lifter.

Some builders believe you can use a gearmotor with much less torque than we recommend. There is an extended discussion on gearmotors for lifting arms in this archive: search the page for 'RS-540'.

Q: What antweight weapon would you suggest for a beginner robot builder, beside a wedge and lifter?

A: That's a fairly complete list -- you might want to remove 'lifter'. Active weapons add several layers of additional complexity onto an already complex undertaking. I recommend that first-time builders keep their robots very simple. You'll have enough new things to worry about with battery maintenance, R/C system set-up, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems.

Q: How does 'Original Sin's hinged wedge work?

A: The front wedge on the current #1 ranked heavyweight is mounted on a heavy rod that pivots in very sturdy mounts attached to the chassis. This allows the front of the wedge to pivot down and scrape along the surface of the arena, or drop and lie flat if the 'bot is inverted. The pivot point is low -- near the centerline of the robot -- which helps prevent the wedge from folding back under on heavy impact.

Zero-clearance wedges have a drawback: they can get hung up on joints in the arena floor. In a smooth arena a low-pivot hinged wedge can be a plus.

Q: What do you think about 'The Butcher' from Inertia Labs?

A: Mark J. here: it was in interesting experiment: a pneumatic powered rotary weapon with a claimed 150 horsepower. Record: 1 win, 1 loss. The problem was that it isn't possible to effectively store enough energy in the form of compressed gas to power such a weapon for more than a very few seconds. Pound for pound, electric batteries store much more energy than compressed gas cylinders.

Q: Is 300-400 Joules decent for a beetleweight drum with the current competition?

A: That's way better than decent, assuming that you have a reasonable spin-up time.

Q: What size screws would be best for use as teeth on a 2.5" OD Beetleweight drum spinning at 18,000 RPM?

A: Mark J. here: unless that is a very narrow drum, it isn't reasonable to try to spin it at 18K RPM. Anything less than perfect dynamic balance will shake your 'bot around like jello in an earthquake. Even if it's perfectly balanced to start with, it won't be after the first hit. Worse, with two rows of teeth a tooth will pass by every 2 milliseconds -- you can't get any 'bite' into your opponent with teeth passing by that fast.

Slow down the drum by a factor of at least two. I'd suggest even slower. Speed and Joules are less important on a small drum than are bite and torque. For teeth try 3/16" hex cap screws. Anything smaller might shear off.

Q: How well did the flamethrower antweight 'Crack Torch' do? Wouldn't flame weapons be good for the insect classes because a lot of the robots are not fully covered in armor and protection?

A: 'Crack Torch' never won a fight. Although they are crowd favorites, flame weapons are ineffective in any weight class. Your opponent would have to sit still while you positioned your flame weapon and held it long enough to do some damage. If your opponent is sitting that still, he's disabled and you've already won the match.

You can research the fight history of any robot at

Q: What about Texas Heat? It is an effective lightweight flame thrower robot that is currently ranked 2nd. Wouldn't a similar design work for an antweight?

A: 'Texas Heat' is an effective wedge robot designed by the very experienced and well financed CM Robotics. It would be equally effective without the flame weapon. Flames are for show.

Q: How do you design a drum weapon with the motor inside the drum, like Inertia Labs' 'Hummer'?

A: Small drums may have the rotating can of a brushless outrunner motor press-fit into the inner diameter of a custom machined drum. The motor mount becomes the support for one end of the drum. This design is simple and rugged, but gives you no gear reduction control for maximum drum speed and spin-up time.

I've also seen larger drum weapons with an internal motor mounted on a hollow non-rotating 'dead shaft' with the drum supported on bearings. Power wires can be run to the motor thru the center of the dead shaft. Power was transferred to the inner surface of the drum by a friction drive. In general, an external motor with a belt drive is a simpler and better choice.

Q: Do you like Blendo? Do you know how much horsepower Blendo's Briggs and Stratton ICE had?

A: 'Blendo' was a great robot and a real benchmark in robot design. Their Briggs & Stratton internal combustion engine was rated at 5 HP -- not much by today's standards for heavyweight spinner power.

Q: How do I calculate the length of the bars in the four bar linkage to get a scooping motion similar to Ziggy's flipper weapon? [Leo]

A: There are several tips on 4-bar mechanism design in this archive. The T.i. Combat Robotics 4-Bar Simulator is a valuable design tool and a good place to start.

Q: Are pneumatics expensive? About how heavy are they? I'm looking to build a 60lb robot that can toss my opponents a few feet in the air, but I don't know how much weight and cash it would cost me.

A: Combat robots are generally expensive. Larger combat robots are more expensive. You'd better have an overall budget of at least a couple grand to cover a minimal lightweight flipper.

Standard pneumatic components are off-the-shelf products available thru industrial suppliers. If you can put your system together from these standard components you can keep the price reasonable. Pricing for a budget CO2 low-pressure (150 psi actuation pressure) lightweight pneumatic system might run something like this:

  • an actuator cylinder: less than $100
  • a useable 5-port valve: about $150
  • a small paintball regulator: about $100
  • a small CO2 paintball tank: less than $30
  • assorted connectors, hoses, valves, guages, electronics, and mounts: another $100.
That will get you a system that, if properly constructed, will toss an opponent over onto their back. Higher power systems to flip an opponent higher will require add-ins: buffer tank, quick-exhaust valve, higher flow solenoid valve, etc. A full blown high-pressure system will double or triple the cost.

How much will it weigh? That's like asking how much a rock weighs -- it depends on the rock. A basic lightweight system might weigh as little as 6 or 8 pounds, but adding on performance will quickly add to your weight as well as the bulk of the system. Pneumatics take up a lot of room, so your 'bot may need to be larger which makes for more weight in the armor and chassis as well.

There is also the matter of assembling a safe and reliable system. Pneumatics are not for beginners. They can generate a lot of force and move very quickly. That is a recipe for severe injury if you don't know what you're doing. A first-time builder has plenty to worry with just getting the radio, electrical, and mechanical systems of a simple combat robot working correctly. Don't add on the complexity of an active weapon 'til you get the basics down.

Q: How do you create a flywheel powered flipper?

A: Mark J. here: Although it is an interesting theoretical design, a flywheel powered flipper has a number of engineering challenges that render it inferior to pneumatic flippers. The idea is to store energy from an electric motor in a rotating disk, just like a conventional spinning weapon, but then to engage a clutch mechanism to transfer that stored energy via a linkage to a flipping arm or platform. The clutch must be able to engage quickly, transmit enormous force, and disengage quickly to prevent excess energy from ripping the weapon apart.

I've seen a few clever designs on paper, but no one has actually built a successful flywheel flipper. The complexity, weight, and expense would be much greater than a pneumatic system of similar capacity. Stick with pneumatics.

Q: I want to use some pneumatics in my next 'bot. Is HPA simply pressurized Oxygen? I have a compressor, could I use that to fill my tanks? Also, do I have to buy a special type of air tank for HPA?

A: Mark J. here: HPA is High Pressure Air (78% nitrogen, 20% oxygen) right out of the atmosphere. HPA systems in combat robots typically have their tanks pressurized at 2500 psi or above -- likely much greater than the capability of your compressor. The'RFL rules' require HPA pressure tanks to be rated for al least 120% of the maximum pressure they will be used at, and to have a current hydro test certifying that capacity. High pressure paintball tanks are commonly used in small 'bots, and SCUBA tanks in larger 'bots.

Before you go any farther, read the Team Da Vinci Understanding Pneumatics page. That will answer a lot of questions you didn't even know you should be asking.

Q: How is the spinning ring on a robot like 'Ringmaster' attached?

A: Mark J. here: there have been several attempts to build a successful invertible 'full body' spinner, with wheels sticking out both top and bottom. Ringmaster fought as a heavyweight at BattleBots 5.0 and had a good run, winning 4 fights before losing to Omega-13.

A rotating ring weapon is typically supported by a number of small wheels attached around the upper and lower perimeter of the chassis. The wheels run in grooves in the inner surface of the ring, supporting the ring and allowing it to rotate.

Q: I want to build a vert spinner that has a wedge in front so it can get at and chew up the soft underbellies of the current lightweights, destroying anything attached. How many joules should such a weapon pack?

A: Mark J. here: it sounds like you're planning a small diameter weapon with the intent to grind away at the bottom armor rather than a traditional impact spinner weapon. What you're building is more of a mobile power tool than a sledge hammer. Joules of stored kinetic energy don't apply -- you're not going to rely on stored energy to do damage, but on continuous application of power.

You'll need enough power to keep the weapon spinning while in contact with the other 'bot and their full weight bearing down on the spinning blade/bar/grinder. I don't have any simple formulas to calculate the power you'll need for that. I'd suggest keeping the RPMs relatively low and gearing for torque.

Q: I'm building a featherweight (12 kilo) robot for entry in to comptitions in the U.K. How should I power my four bar lifter: pneumatics, linear actuators or a high torque electric gearbox? And how much weight should it be able to lift?

A: Mark J. here: each of the power options you list have strengths and weaknesses:

  • Pneumatics components are widely available and can provide high thrust, speed, and design flexibility. They do require additional support equipment and add complexity to the confined space inside your robot.

  • Linear actuators are self-contained, compact, and have the same design flexibility found in pneumatic systems without the need for additional support equipment. They are, however, much slower and potentially less powerful than pneumatic systems.

  • An electric gearbox capable of supplying the torque needed for a 12 kilo robot would be bulky and heavy. The T.i. Combat Robotics 4-Bar Simulator can give you torque requirements for a gearbox powered 4-bar lifter. You might be surprized by the torque required.

I'd suggest linear actuators if you can locate suitable units. Calculating required thrust is a little tricky, but there are some notes in this archive that will help.

If you are using electric power for your lifter, I recommend designing to stall the lift motors at twice the weight of the weightclass (24 kilos in your case). That will load the motors down to about half speed when lifting the maximum weight you will likely face -- right at the horsepower peak of the motors. Many builders design for less lift in an effort to get more speed, but their lifters bog down under load.

A similar amount of lift should work well with a pneumatic system. You may be able to get much more, but you did say 'lifter' and not 'flipper'.

Q: I'm thinking of using an Axi 2808 press fit into my beetleweight's drum. How would I get the press fit?

A: A true 'press fit' involves machining the inside of the drum to a diameter a couple of thousandths of an inch smaller than the outside diameter of the motor and using a hydraulic press to shove the motor into place. If your machining isn't accurate, the drum won't run true.

Q: I was thinking of using two Magmotor E-150 motors at 36 volts to power my middleweight sminner weapon. They would give me a good amount of power at a reasonable price. How does that sound?

A: For weapon power, two E-150s would weigh nearly twice as much as an A28-150 and produce 1/3 the power. Weight is the greatest challenge in building a combat robot. You need to make the best use of every ounce. The weight you save in the motor can go into the spinner disk for more stored energy and damage potential, while the added power will improve your spin-up time and maximum RPM. Well worth the added expense.

Q: I've been thinking about mounting a spinning disk on an arm and moving it like a chop saw. Do you think this is a good idea?

A: If you're building a large weapon I would recommend putting all available weight into the battery, motor, and weapon mass rather than on structures that do not increase the impact and energy of the weapon. The Robot Wars housebot 'Dead Metal' used a circular saw weapon that swung forward and down, but a cutting saw does not produce anywhere near the kick-back force of a rotary impact weapon.

Every action produces an equal and opposite reaction, so your weapon arm would need to absorb the same amount of force that the weapon inflicts on your opponent. That would take a massively reinforced arm assembly! Keep it simple.

Q: How many RPM will I want for a 3 foot diameter spinning disk?

A: The energy stored in a spinning weapon is dependent on the speed of the disk, the mass of the disk, and how the mass is distributed on the disk. There is also the question of the time needed to spin the weapon up to an effective energy level. Search this archive for information on calculating weapon energy, appropriate energy levels, and spin-up time.

Q: How were the chains attached to the weapon sprockets on 'The Judge'?

A: Careful examiniation of the photo at right shows that the last chain link is bent inward toward the shaft and riveted thru the sprocket. An animation of this weapon is in this archive -- scroll down about 40 questions.

Q: Can I use an IFI Victor 883 ESC to control a lifter instead of a relay and solenoid?

A: Yep.

Q: Is there a guideline or a formula for figuring out battery requirements a spinner motor needs? My spinner runs at 36 volts and needs a separate pack, but don't know how much capacity it should have.

A: There are a lot of variables in calculating weapon battery requirements, but we gave a 'rule of thumb' and approximate formulas in an earlier post. Search the archive for 'battery capacity'.

Q: I took a look at your formulas to figure out weapon battery requirements and added them into your Spinning Weapon Excel Spreadsheet. It's formatted exactly like your work, and the formulas are all correct. Would you like to add it to your spreadsheet?

A: Nice idea! Michael Maples sent me his addition to the spreadsheet and it's way cool. I've replaced the downloadable file with the new version. Thanks, Michael.

Q: How do I decide how much 'burst current' is enough for my weapon battery?

A: Mark J. here: ideally you should be able to meet the full stall current demand of your weapon motor. Current equals torque, and if you can't supply the current the spin-up time of your weapon will suffer. From a practical standpoint, if you can supply 80% of your weapon motor stall current you'll be fine.

Note that LiPoly batteries can be damaged by current drain above their rated capacity. If your LiPoly pack is rated below stall current, you might want to use a 'servo slower' module to 'feather in' the throttle to your weapon ESC and avoid the big start-up amp rush.

Q: I'd like to be able to use my Thor spin controller with my featherweight's 6 pound disc weapon, but I can't find any brushed motors other than the Mini-mag (a bit overkill) for my use. Do you have any suggestions?

A: How about the classic DeWalt 18 volt drill motor? At 24 volts it kicks out 1.5 horsepower and it's less than a third the weight of the A28-150 Magmotor motor. The problem with it and other small brushed motors is high RPM. You'll have to figure out a weapon drive that will stay together at 20K RPM.

Check out the performance of your weapon/motor combination with the Team Run Amok Spinning Weapon Excel Spreadsheet.

Q: I'm running my featherweight's weapon Green Wave style: motor on top. As a result, I have a dead shaft. How should I put the bearing in my weapon bar?

A: Mark J. here: 'Green Wave' was a middleweight spinner with the weapon motor mounted above the robot body on a short, stationary (dead) shaft. The motor support shaft also functioned as the axle upon which the bar weapon spun. The design allowed the bar weapon to be mounted very low on the chassis.

'Green Wave' had serious problems with weapon bearing disintegration caused by heavy off-axis loads on impact. To prevent this problem with your 'bot, the weapon bar assembly should be supported by two bearings separated on the shaft to spread the load and prevent twisting. I'd suggest one bearing in the belt drive pulley and a second bearing in a flange mount on the bottom of the weapon bar.

Q: How are most eggbeaters constructed? Are they simply several steel bars welded into a rectangle shape?

A: Eggbeaters take enormous punishment. Most are cut from a single plate of metal for greatest strength.

Q: How thick should heavyweight eggbeater's shaft be?

A: Mark J. here: come on, guys -- you've got to give me more info than the general type of weapon and the weight class! It's like asking, "What should I feed my yellow pet?" Dog? Canary? Goldfish? See comment in the next question below on 'how thick'.

Q: How thick should the wall of my lightweight's drum weapon be?

A: Mark J. here: my standard answer to a 'how thick' question is, "as thick as you can make it and still make weight." In this case, a thicker drum wall will mean more energy storage and more durability. Like most design factors in a combat robot, there is a trade-off in adding weight to one system while pulling weight out of another.

A typical drum makes up around 20% of the total weight of the 'bot. About 3/16" aluminum might be a good starting point for a lightweight drum, but the size, speed, and design of the drum can modify this.

Q: Say I have a 6 inch diameter, 6 inch long drum. How far should the teeth extend?

A: Optimum drum tooth length depends on tooth spacing and the axial speed of the teeth, not the size of the drum. You want your target to get as far inside the arc of the rotating teeth as possible in order to get maximum 'bite'. Faster spinning drums require wider tooth spacing and run shorter teeth, while slower spinning drums can have closer and deeper teeth.

You need to calculate the amount of time between teeth when the weapon is up to speed, then calculate how far your 'bot is likely to move forward in that time period when attacking. That distance is the optimum tooth length.

Alternately, since your attack speed is so variable, you could just stick the teeth out about 3/8" and go for it!

Q: How are bolts used as weapon teeth (such as on 'Tillah') effective? Wouldn't the dull edges keep them from catching on an opponent?

A: Bolts are not ideal drum teeth -- but they are inexpensive, easy to obtain, and simple to replace. The simple blunt impact is fairly effective, plus a little time with a hand file or Dremel tool can put a nice, sharp edge on a bolt head.

Q: What would be the best way to mount a bar or other weapon to the TWM3R Gearbox?

A: Mark J. here: the Team Whyachi TWM3R gearbox has a nominal 1 inch titanium shaft with a 3/8-16 tapped hole in the end and a .25 inch keyway down the side. This makes it convenient to lock a keyed weapon hub onto the shaft. I'm sure that Team Whyachi would be willing to machine a suitable weapon hub for you.

Note that a keyed hub has no slip or give and will transmit all weapon shock loads back to the gearbox, potentially destroying it. I prefer a slipable weapon drive like a v-belt, but the Team Whyachi gearbox is built to take high loads. Your choice.

Q: In most cases, would a drum's teeth have to be hardened?

A: Mark J. here: If you are using hardenable metal it's a waste not to harden them, but I see a lot of non-hardened drum teeth. For longest life and greatest damage, harden the teeth.

Q: What is the maximum actuation pressure of a pneumatics system?

A: There is no maximum pressure specified in the RFL rules, but all components in the system must be rated for the pressure used. Above 250 psi, components must be over-rated to 120% of the pressure used. Systems operating above 2500 psi require pre-qualification by the event officials.

Consult section 7 of the 'RFL rules' for details.

Q: How did Son of Smashy's spring powered weapon work? Would it be possible to do something similar in a lightweight?

A: Son of Smashy used an electric winch to reset the spring after each strike. The difficult part in building a set-up like this is the release mechanism that allows the weapon to fire then re-engages the winch. In theory it could be done in any weight class but pneumatic systems are more powerful, quicker to reset, and can be built from off-the-shelf components. Spring powered weapons are rare for a reason.

Search the archive for 'winch' for other posts on this topic, and scroll down about eight questions for the post on 'Red Square'.

Q: Do builders make or buy axes?

A: Axes are neither popular nor effective weapons, but I've seen hardware store axes, picks, hammers, and chisels used. High energy weapons usually have custom made impact pieces.

Q: I want to build a super heavyweight with a three arm caged rotor (like 'Son of Whyachi') spun by 2 Etek motors. What is the best way to connect them to the weapon -- gears, chains, belts? What kind of battery power would I need?

A: The Etek motors have very large armature intertia which will cause the rotor to shift on the shaft if stopped abruptly. This effectively destroys the motor. To prevent this you should drive a rotary weapon with a slipable system like a v-belt. You may need several parallel belts to handle the power output of twin Eteks. Gears, chains, or timing belts will lead to trouble.

Interestingly, the battery capacity needed for a rotary weapon is more dependent on the weight, geometry, and speed of the weapon than on the motor powering it. An example, with formulas, of calculating spinning weapon battery capacity requirement is given in a previous answer in the Radio and Electrical archive.

Date marker: March 2008
Q: Can I use a stainless steel knife for a ramming spike?

A: You can use a slice of well-frozen pepperoni pizza if you like -- the result will be much the same. Neither the pizza nor the knife are designed to take large loads at the pointy end. Either will simply shatter on good impact. You need something that you could hammer into concrete. Repeatedly.

Q: How did KillerHurtz's weapon work? How was it actuated? What kind of linkage was used?

A: KillerHurtz is an unusual robot in nearly all design areas: all-plastic chassis, combined differential and pivot steering, computer controlled ESCs, and a pneumatic actuated overhead axe weapon with a chain drive.

The weapon is powered by a 100 mm bore pneumatic cylinder operating at 150 psi. The cylinder is attached to a lever directly connected to a large chain sprocket. A chain carries the force to a smaller sprocket attached to the axe. There is a good photo on an archive of their site: KillerHurtz Design page.

Q: Did KillerHurtz use a double-acting pneumatic cylinder?

A: Yes. The pneumatic plumbing appears to be incomplete in the photo -- see the KillerHurtz pneumatics page for details.

Q: How would I make a 'Frostbite' like thresher weapon?

A: Frostbite's thresher is simply eight short vertical bars mounted on hubs along a common central support shaft. Mount the shaft on bearings and spin it with a belt drive.

Mark J. here: the most efficient design for a spinner weapon is a drum -- it can store more energy for size and weight than other configurations. A disc weapon is less efficient, and a rotating bar even less efficient. Multiple short bars, like the thresher, are very poor at storing energy.

Q: How would I use your spinner spreadsheet with an eggbeater weapon?

A: Mark J. here: it takes just a little extra work.

  1. Input a bar the correct length and width to represent one side bar on your eggbeater. Double the thickness to set the mass correct for two bars.

  2. Manually calculate the mass of your two crossbars.

  3. Represent the mass of the crossbars with a cylinder the diameter of your weapon - tinker with the thickness to get the mass equal to the calculated crossbar mass.
That will get you very close to correct.
Q: How do you make a full-body drum?

A: There is plenty of information on drum weapons in this archive. You may also want to get a copy of Combat Robot Weapons by Chris Hannold -- it has specific information on many types of weapons in detail that I cannot afford to provide here.

Q: What's a leaf-spring flipper?

A: A leaf spring is a curved metal plate that has been tempered to 'spring back' to it's curved shape after being flattened. One end of the leaf spring is securely fastened to a chassis and the other end is winched down to flatten the spring, This stores a large amount of energy in a thin and flat profile. When another 'bot is on top of the spring the free end is released and the spring tosses the other 'bot skyward.

The principle is simple, but designing the winch and release mechanism is tricky. Team Whyachi's middleweight 'Red Square' was a successful leaf spring flipper -- ranked 15th, with a record of 19 wins and 12 losses.

Q: Would it be possible to make a spring powered flipper in a featherweight?

A: Possible, sure -- but there are reasons that spring flippers are not a popular design. Team Whyachi was successful because they are a very experienced team with great resources.

Q: How would I make a spring powered flipper in a hobbyweight?

A: No answer that I can give in a couple of paragraphs is going to help you much. Very few spring flippers have been built due to the difficulties in constructing a releaseable winch system to reset the spring. The mechanism from an automotive power seat would be a good start, but custom machine work would be needed for the release spool. I'd suggest building something else.

Q: Would it be better to make my vertical spinning weapon spin 'up' or 'down'? What are the advantages of spinning 'up' versus 'down'?

A: Mark J. here: Newtonian physics states, "For every action there is an equal and opposite reaction." When your spinning weapon strikes your opponent, equal forces are applied to both robots.

If the front edge of your weapon is spinning upward: the force applied to your opponent will be upward and your robot will be subject to an equal downward force. Since a solid surface is holding your robot up, nothing much will happen to it. Since nothing except gravity is holding your opponent down, they will fly end over end into the air -- greatly impressing the judges and spectators in your favor.

If the front edge of your weapon is spinning downward: the force applied to your opponent will be downward and your robot will be subject to an equal upward force. Since a solid surface is holding your opponent up, nothing much will happen to them. Since nothing except gravity is holding your robot down, you will fly end over end into the air -- greatly impressing the judges and spectators, but not in your favor.

Upward spinning, please.

Q: I'm considering adding a lifting arm to my lightweight. What would be the best way to implement this? If I choose to go pneumatic, does RoboGames have filling stations?

A: Mark J. here: The 'best way' depends on a balance of your budget, experience, skills, and expectations.

  • Pneumatic lifters are very quick and powerful but require complex, bulky components and a fair bit of weight allowance. Pneumatics are applicable to either a simple single-pivot lifter or a more efficient 4-bar design.

  • Electric actuator lifters are typically slow but are comparatively compact, safe, and simple to support and maintain. Again, they can be used with single pivot or 4-bar lifters.

  • Driving one arm of a 4-bar lifter with a gearmotor requires a heavy gearbox capable of handling enormous torque, and careful design work -- but the result can be quick and effective.

RoboGames is not a 'competitor friendly' event -- don't expect more than table space and an electrical outlet for support. Also, be aware that there are a number of 'improvements' to their procedures for 2008 that include very strict match schedules and new radio requirements that prohibit standard AM / FM / FM-PCM systems. You can email RoboGames organizer Dave Calkins with specific questions.
Q: In season five of Battlebots, 'El Diablo' and 'El Diablo Grande' both sported new thresher weapons. How would I go about building a weapon similar to the ones they used?

A: I can't find a picture of the weapon used by middleweight 'El Diablo' at BattleBots 5.0, but I did find some construction notes in a build report:

The weapon was made from four 1" thick polyethylene discs, each with a central bearing riding on a stationary steel shaft. Bridging across the four discs on opposite sides were two 1/4" thick 6160 aluminum plates with bolts threaded thru from the back side for 'teeth'. The entire assembly weighed 24 pounds and was reduction belt-driven by a Bosch GPA 750 motor to about 1300 RPM.

'El Diablo' went 0 for 1 at BB 5.0, losing their opening match to 'Ankle Biter'. Heavyweight 'El Diablo Grande' did better with a similar weapon, winning 3 matches before a loss to 'MechaVore'.

You can find pictures of El Diablo and El Diablo Grande as they fought at BB 5.0 at, under 'Event Reports'.

Thanks! I'd forgotten about their extensive collection of BattleBots photos.

It looks like the team changed the polyethylene and aluminum design before the competition. Those appear to be four thick aluminum discs with steel striker hooks bolted in. I don't think I could recommend a similar design -- a drum weapon of the same size, weight, and speed would store much greater energy than the 4-disc setup.

Q: I'm building an antweight but I'm having trouble with the drum weapon. If I make the drum out of a metal tube and weld or screw on teeth it becomes extremely unbalanced. Is there a trick to making one? Is there a way I could buy one?

A: Drum weapons are not projects for the average builder without access to and experience with precision machine tools. An error of a couple hundredths of an inch will leave you with a useless paperweight. A competent machine shop could certainly make a custom drum weapon for you, but the cost would be prohibitive (hundreds of dollars). Consider switching to an eggbeater weapon -- much easier to construct and balance. See also the previous post on balancing rotary weapons.

Q: How does Ziggy's flipping arm work? Why is it so powerful compared to other super-heavyweight flippers?

A: Clan MacCanIKill's 'Ziggy' has a 4-bar flipper mechanism with a pneumatic actuator acting on the front bar. Click the photo for a larger image. The pneumatics operate on high-pressure nitrogen at up to 3000 psi. Most pneumatic flippers use carbon dioxide gas which gives more 'flips' per tank but which also restricts their maximum pressure to about 850 psi.

In spite of the high-pressure nitrogen pneumatics, I don't believe that Ziggy is more powerful than other top-rank flippers. The 'laydown' position of Ziggy's actuator cannot match the mechanical efficiency of the twin upright actuators in Toro.

Q: What's the ideal material to use in a lightweight spinner killer's wedge?

A: 6AL-4V titanium, about 5/16" thick should do.

Q: Can I have a link to Team Entropy's site? 'Redrum' is my favori