Questions and Answers about Combat Robotics
from Team Run Amok

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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 robot weapon related posts prior to 2017. Such posts from 2017 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.

You can now take a tour of posts in the Ask Aaron Archives that have been referenced to answer new questions -- let's call them Less Frequently Asked Questions. Click the 'Mystery Post Tour' button above to get started.

Date marker: January 2017
Q: Why does RioBotz choose to have 13 radii and 18 sections [in the design of their integrated single-tooth 'snail drum' weapon]? [Dublin, Ohio]

A: [Mark J.] The selection of 18 initial sections was arbitrary -- it came from dividing the 360 degree polar coordinate plot into twenty-degree sections. That seemed to be a reasonable number of facets to machine into the drum in the final design. Two of the sections (40 degrees) were combined for the impact tooth at full radius and one section is the curved 'tooth notch' designed to reduce stress concentration that has no single radius. Then:

"After observing the nearly flat shape of the optimal solution in the regions between 220o and 320o... the algorithm [was] re-evaluated considering only 2 flat sides in such region. This new optimal solution is very similar to the previous one, but it is easier to machine due to the reduced number of facets." -- Drum Shape Design and Optimization Using Genetic Algorithms

The creation of just two facets ('k' and 'l') in the region from 220o and 320o reduced the number of radii to thirteen.

Q: Hi Aaron, could you tell me, for a spinning drum that stores 10,000J [30 Lbs class] what is the difference between a 10mm bite and a 20mm bite? Thanks. [Valle del Cauca, Colombia]

A: [Mark J.] Bite (what's bite?) is calculated as a maximum depth of opponent insertion into the arc of a spinning weapon at a given weapon RPM and forward velocity. You'll get that maximum bite rarely, just like 13 black only comes around rarely on a roulette wheel. Sometimes your luck will be very poor and you'll hit your opponent just as an impactor is facing them and get no bite at all! On average, you'll get half the max bite -- less as your attack speed drops.

  • Consider an opponent who was wise enough to avoid exposed sharp edges in their robot design. Given a nice sharp angle to attack you don't need a lot of bite, but if you're forced to attack a flat or gently curved surface you need all the bite you can get.
  • Greater bite also is a bonus when your attack velocity drops. A weapon with a lot of bite can still be effective in close quarters when you don't have a chance to back away and take a high-speed run. Watch some combat videos and pay attention to the speed at which most weapon hits are made.
What's the difference between 10mm and 20mm bite? A few more match wins. Bite is good -- more bite is better.

I've been writing quite a bit of JavaScript lately, so what's a little more? Take a look at the new 'Bite Calculator' in the Spinner Weapon FAQ.

Q: Why doesn't 'Witch Doctor' have gyroscopic forces acting on it? One side doesn't lift. [Reston, Virginia]

A: [Mark J.] See this post on gyroscopic forces a little farther down in this archive.

Q: Hey Mark,

How does the flipper on Lock jaw in Battlebots Season 2 harness the power of the springs? From what I could tell it was winched back but how was it able to fire then reset again? Additionally, could this method of flipping be utilized in all other weight classes as an alternative to pneumatics? Thanks in advance! [Straight Outta Facebook]

A: [Mark J.] Donald H. doesn't divulge much about his robot designs, and I can't see enough detail in the photos of 'Lock-Jaw' to understand the clutch mechanism. Fortunately there are builders who do share their spring-powered flipper designs:

  • Dale Hetherington has built pretty much every exotic robot design there is. Take a look at his very detailed build log for 'Dead Air'.
  • I'm personally very fond of the snail-cam spring loader for it's simplicity and adaptability to small weight classes.
That will get you a start.
Q: Hello Mark,

I've heard it said that Wrecks' vertical disk (30-35 lbs) has a much higher Moment of Inertia than Electric Boogaloo's vertical spinning weapon (~70 lbs). With the assumption that Wrecks is using a similar motor (big assumption), how is this possible?

-David R. [Livermore, CA]

A: [Mark J.] A little clarification:

The Moment of Inertia (MOI) is NOT a direct measure of how much energy a spinning weapon stores; the motors or speed of rotation have no bearing on the moment. MOI is a measure of the energy needed to change the rate at which the weapon is spinning. Its value depends on the mass of the weapon and (most importantly) on how that mass is distributed relative to the rotational axis.


  • A small chunk of matter is rotating around an axis at a given RPM at a distance of six inches. With each rotation the chunk of matter travels a distance of ( 2 × 6" × pi = ) approximately 37.7".
  • That same small chunk of matter is now rotating around an axis at the same RPM at a distance of 12 inches. With each rotation the chunk of matter travels a distance of ( 2 × 12" × pi = ) approximately 75.4".
  • The chunk of matter 12" from the axis must be travelling twice as fast as it did when only 6" from the axis in order to complete a revolution in the same length of time.
  • The kinetic energy of a moving object increases with the square of its velocity (K = ½mv2), the mass 12" away from the axis has four times as much kinetic energy as an equal mass 6" from the axis when rotating at the same RPM.
  • Doubling the distance of the mass from the rotational axis doubled the speed of the mass, which raised the energy stored by the spinner at any given speed by a factor of four (velocity2). This also increases the MOI of the spinner by a factor of four, even though the mass of the spinner has not increased.
Now, let's take a look at the spinner weapons on 'Electric Boogaloo' and 'Wrecks': 'Electric Boogaloo' has a bar spinner weapon - closer to 50 pounds than 70. A bar has a lot of its mass close to the rotation axis and relatively little mass far from the axis. The formula for the MOI of a rectangular bar spinner is:

(mass ÷ 12) × (length2 + width2)

'Wrecks' has a large diameter spinner with most of the mass concentrated in a ring at the outer edge -- far away from the axis of rotation. The formula for a the MOI of a thick ring (discounting the supporting spokes) is:

(mass ÷ 2) × (inner radius2 + outer radius2)

Let's use the new Run Amok JavaScript Spinner Weapon Calculator to compare the MOI of the two designs based on rough estimates of their sizes:

  • A steel bar similar to the spinner weapon used by 'Electric Boogaloo': 455 mm long by 160 mm wide by 40 mm thick weighs 22.7 kg and has an MOI of 0.440 kg·m2.
  • A steel ring similar to the spinner weapon used by 'Wrecks': 300 mm outer radius, 200 mm inner radius, and 12 mm thick weighs 14.7 kg and has an MOI of 0.956 kg·m2.
The ring weapon is less than 2/3 the mass of the bar weapon, yet has more than twice the Mass Moment of Inertia. That's a much more efficient use of weapon mass for energy storage.

Q: How do electric hammers not burn out [...just down the road from Ashburn, Virginia]

A: [Mark J.] If you aren't careful they do burn out.

The motor for an electrically powered hammer weapon needs to be powered off at either end of the weapon swing to avoid an extended 'stall' condition where the motor would consume damaging current levels. This can be done a couple of ways:

  • Most builders control the hammer with one of the spring-centered joysticks on the transmitter and release the stick at either end of the weapon swing. Just don't forget to release the stick!
  • Some R/C relay boards and a few ESCs (like the Talon SRX) have provisions for 'limit switches' to sense when the controlled device has reached the end of its travel and automatically shut off power in that direction. This allows an electric hammer to be actuated by a simple single-throw switch on the transmitter -- typically channel 5.

Q: What is the best motor to use for a horizontal spinner [Los Angeles, California]

A: [Mark J.] The Hamburger is Bad.

See Ask Aaron Spinner Weapon FAQ.

Q: Hello! I am a high school student that I building a new robot. I am building a drum spinner, and that spinner will be operated by a Brushless motor with the specs of:
  • Turns: 10T
  • Voltage: 12S Lipoly
  • RPM/V: 560kv
  • Motor Poles: 10
  • Internal resistance: 0.017 Ohm
  • Max Loading: 100A
  • Max Power: 4200W
I currently do not have the dimensions of weapon itself, but I am interested in knowing the "equation" in finding the speed of the weapon at full speed. I have the Wh of the battery, Volts, and most of the other specs. Also, where can I find the equations to calculate torque, and stall torque. [Dublin, Ohio?]

A: [Mark J.] What's wrong with this group of questions?

Given the syntax, grammar, and language structure, I have trouble believing that the author is from Ohio. Further, the motor and weapon design are unusual for a combat robot that might be constructed by an American high school student in the mid-west.

If the author is a high school student in Ohio and they are building a combat robot this size, they should have a local mentor to guide them in design, construction, and safety. The mentor should be providing the answers to questions like these.

Either way, I'm not comfortable answering your questions. The best I'm willing to do is to point you to this Wikipedia article on 'Motor Constants' and warn you that stall torque on brushless motors is much less than the calculated value due to the software in the motor controller limiting current at low motor speed.

Q: How to make 2 [pneumatic] cylinders work in sync? Use 2 buff tank and 2 valve or 1 buff tank with 1 valve [to] supply 2 cylinders? Thank you (just like Subzero) [Yunnan, China]

A: [Mark J.] I see that you've just asked the same question of Subzero's builders on their Facebook page. The better question might be, "Why use two pneumatic cylinders instead of just one of larger diameter?"

If I had to use two cylinders I'd want both to fill from a single gas supply thru a single valve to avoid any pressure imbalance. Why don't we wait a few hours and see what the builders say?

Subzero's builders did reply a few hours later:

Team Hammertime / Teamxd: That version was one supply tank with 2 rams, one valve and no buffer tank.

Q: What's the best & safest way for someone who's only done non-weaponed bots to do their first weaponed one? [New Jersey]

A: [Mark J.] I'm not sure how to respond to 'best' but I can offer some safety guidance. You didn't mention how large a robot you are interested in building or the type of active weapon you are considering, so I'll have to keep this general.

  1. Review your basic workshop safety practices:
    • Wear eye protection when there is danger of flying chips, abrasive dust, or irritants.
    • Remove rings and other jewelry before operating machinery.
    • Keep your workspace and the floor around it clean and un-cluttered.
    • Securely anchor materials being drilled, ground, or machined.
    • Cover and secure sharp edges and points in the work area when not in use. Use gloves where appropriate.
    • Do not wear loose clothing around power tools -- a work apron may be appropriate.
    • Monitor your lithium battery charging, and use a suitable charging container if needed.
    • If it's gonna be loud, wear hearing protection.
  2. Keep safety restraints on weapons that prevent them from operating unexpectedly until you are ready to test/use the weapon.
  3. Do not test or operate your weapon without containment suited to your weapon and robot.
  4. Be cautious of mechanism pinch-points: articulated levers, chains, hinged surfaces.
  5. Keep your design and expectations at a level appropriate for your building skills and resources.
  6. If you aren't sure you understand the safe operation of specialized equipment, fittings, or assemblies -- ask!

Q: Dear Aaron, which horizantal spinner has more effect on the other bot? And to you? Thx, [Google Fiber ISP]

A: [Mark J.] I'm not sure I understand your question. The base physics of a horizontal spinner are the same if it's a bar or disk -- or a top/mid/undercutter. The effectiveness depends on other factors such as energy storage, 'bite', and chassis stability. Suggest you read the Ask Aaron Spinner Weapon FAQ and then send in a more focused question.

Q: Is there a way to calculate a spinning weapon's gyro effect? I've seen bots that were similar to each other yet one had HUGE problems with gyro and one didn't. How can I make sure ours isn't like the first one? [Kansas City, MO]

A: [Mark J.] There are multiple posts about designing to minimize gyroscopic forces on your 'bot in the Ask Aaron Combat Robot Design Archive -- search there for 'gyroscopic'.

Many of those posts refer to the Total Insanity Gyroscopic Effect Calculator as a tool useful in adjusting robot design to better cope with the weapon gyro forces. The T.i. gyro calculator requires the 'Mass Moment of Inertia of Weapon' as an input, which can be calculated with the Run Amok Spinner Weapon Calculator

Q: Hi again, I've stumbled upon a problem, the snail cam spring reloader [needs to stop] after one full rotation, I cannot seem to find a suitable solution to do so with a gear motor. Do you think a stepper motor is better for this or is there a way to make motor start and stop after one rotation and a push of one button if you will. [Bristol, UK]

A: [Mark J.] When you turn off the windshield wipers on an automobile have you noticed how they continue for the rest of the wipe stroke and then stop in the park position? That's what you're looking for and here's how you do it:

  • The circuit shown at right allows the gearmotor to run until the flipper arm (not shown) reaches the fully loaded position and presses down to open the 'interrupter' micro switch -- stopping the gearmotor.
  • A momentary closure of the normally open R/C switch re-starts the gearmotor long enough to fire the flipper, which closes the interrupter switch.
  • The gearmotor then continues to run thru the rest of the reload cycle to an automatic stop.
The positioning of the micro switch is simplified if you use a lever-style switch that can be bent to fine-tune the switch point. The micro and R/C switches must have enough current capacity to handle the gearmotor, and the micro switch must be wired 'normally closed' (NC). For larger robots the micro switch can trigger a relay with the capacity to handle the motor load.

Q: I was actually thinking to have snail cam consist of 2 shapes, 1) the main cam and 2) a smaller cam with sudden increase in radius where reloading needs to stop that way there is no need for finest tuning. [Bristol, UK]

A: You have lots of options on the interrupter. A micro switch is simple, but you can certainly use other sensor types: infrared emitter/detector, inductive proximity, magnetic... whatever you're comfortable with.

Q: I was wondering if you have heard of or made any progress on getting the T.i. 4 Bar Simulator ported to a newer version of Windows. Thank you for any information and providing a valuable service. [Kansas]

A: [Mark J.] The author of the T.i Four Bar Simulator wrote the code back in 2007 when he was still in school. He tells me that he's not sure where the source code is, but that it may or may not be on an old desktop which he believes is stored in his parent's attic. The best he could offer was to take a look in the attic the next time he went home for a visit. In my mind I picture a dust-covered computer leaning up against the holy grail and partially covered by a lost da Vinci manuscript.

I keep an old Windows XP desktop next to my 'Super Nintendo' console in a dark corner of my basement just so I can run 'Four Bar' and play 'Donkey Kong Country'. A similar set-up may be your best option as well.

Q: Hi there,
I've got an idea for a spinner robot where the rotating ring spins inside a circular chassis, and once it reaches a sufficiently high speed, two 'teeth' on the ring are extended (to protrude beyond the chassis) by centrifugal force. On impact, the ring is slowed, meaning the teeth retract again (using a spring), leaving the ring to spin up again without external resistance. This would mean the ring could spin up and inflict damage on an opponent, even if the opponent was continuously in contact, since the teeth would be shielded by the chassis until the ring was at full speed.

I haven't been able to find any examples of a robot which uses this system, so my question is, are there any, and if not, why not? I feel there must be some fundamental reason why robots don't use such a technique - do you have any suggestions?

Many thanks, M [Bournville, England]

A: [Mark J.] I know of no examples of such a weapon system in use, and I think I know why there are none.

Spinner weapon teeth take huge abuse. The entire force of the weapon is transferred thru them to the opponent. Typically they are made of hardened tool steel and set into well braced recesses in the weapon body where they are secured by the best quality bolts obtainable. Still, an impactor tooth's life is short. A good impact can and will shear them away.

Your proposed design places the impactors on pivots which would themselves bear great impact forces, as would the stops required to restrict the tooth's outward motion. Moving parts subject to high loading are bad. Adding more parts subjected to high loading is worse. Simple is good.

Consider the situation just after your weapon impacts your opponent. If your opponent is still there to restrict your weapon from spinning back up, then your weapon isn't doing its job. Your opponent should be flying away from you at great speed, unable to prevent your weapon from spinning back up.

I don't believe that the benefits of a retracting-tooth weapon would offset the added complexity and fragility. The mass of the circular chassis shield would be better put to use in the mass of the spinner and the weapon motor. Keep it simple; simple robots win.

I just got more curious than normal and ran a calculation on the force needed for a spring to hold an impactor in against the outward centrifugal acceleration on a typical weapon. At 3000 RPM an object 250 mm from the axis of rotation sees 2500 gravities!

Q: Hey, M's weapon sounds like 'Greenspan' that used a flywheel with free spinning hammers. [Dublin, Ohio]

A: [Mark J.] I don't think that's what 'M' has in mind. There were a lot of 'flail' and 'pivot hammer' weapons in the early days, but those designs have been replaced by fixed impactors that are better at transmitting the full energy stored in a flywheel as a single big hit on the opponent.

The 'M' weapon shields the flywheel from direct contact with the opponent which allows it to spin-up even if the 'bot is in contact with the opponent or another obstruction. The impactors are held inside the protective bumper until they approach full speed, and then they either slide or pivot outward beyond the bumper in a fashion that locks them against lateral movement. See my sketch of a (poor) pivoting impactor design at right.

These types of retractable impact teeth would not swing out of the way on impact in the way Greenspan's hammer did -- they would deliver an unyielding blow. That's good, but my objection is that the sliding or pivoting mechanism would be a weak point subject to failure. Complex is bad -- simple is good.

Comment: To back your statement on M's weapon, I remember there was a Beyblade battling top kit called "Wing Attacker" which had that very setup. It... wasn't very good. [Arden, North Carolina]

Reply: [Mark J.] I'm not sure that Beyblade performance transfers to combat robots, but someone might appreciate that data point.
Q: After doing the calculation for my featherweight horizontal spinner, i got 1484 joules of energy. Is it enough? [Quebec, Canada]

A: [Mark J.] "How much energy should my spinning weapon store?" is the first question in the Ask Aaron Spinner Weapon FAQ. You can read your answer there. I suggest that you read the rest of the FAQ as well.

Q: I saw in my beta question you don't like electric hammers, what about pneumatic? in my first look at the math, it seems like pneumatic hammers are pretty tame too. Unless I did the work math wrong, you'd need a gargantuan cylinder like chomps to even break 1 KiloJoule which even some nastier 3 pound spinners beat. [Dublin, Ohio]

A: [Mark J.] Since you didn't include your calculations I can't check them, but a pneumatic system can provide much greater force than an electric system of comparable weight.

  • Are there any electric flipper robots out there? There are lifters, but there are no flippers -- an electric motor/actuator/solenoid cannot provide anywhere near the explosive release of power available from a comparable pneumatic system.
  • Also consider the great complexity, expense, and effort expended by Team Hurtz to construct their electric hammer. It is a beautiful piece of work, but a comparable pneumatic weapon could be built from off-the-shelf components at a fraction of the cost.
I'm not a fan of hammers in general, but if you're going to build one it makes sense to use pneumatics. Assuming perfect gas flow, a 3" diameter pneumatic actuator at 250 psi provides 3.14 × (1.5^2) × 250 = 1,766 pounds of accelerating force. Try to match that with an electric motor. The trick is in getting that 'perfect' gas flow...
Q: I have a third grader trying to answer a science fair question about the effectiveness of vertical vs horizontal spinners. He's built a bot out of a Thames and Kosmos building kit but he can't get enough power out of the motor to get spinners to do any damage to a piece of styrofoam. Spinner just stalls when it hits the foam. That's our problem to deal with, but:
  1. Are there equations we can run to determine the answer to this question?

  2. Does the angular velocity of a spinner change if it's in the vertical position, if all other things are equal?

  3. Does gravity assist or impede?

My engineer-son is asking questions his English-major mother can't answer. [Raleigh, North Carolina]

A: [Mark J.] You want to run some angular momentum equations for your third grader's science fair project?!?! My third grade teacher was still trying to get us to stop counting on our fingers. I guess things have changed.

It was my third-grade son who led his biology-major father into this mayhem. I'll be pleased to assist as best I can.

  1. Read thru the Ask Aaron Spinner Weapon FAQ. The entire FAQ will prove instructive, but you may find particular interest in the large, friendly blue text box near the top of that page describes the principle of spinning flywheel weaponry:

    General Principle

    Spinning weapons are flywheels. They rely on rotational inertia to collect energy from a continuous power source (electric motor, internal combustion engine...) over time and store it as rotational kinetic energy. On impact, the flywheel releases the stored energy in a blow that far exceeds the energy directly available from the continuous source.

    From your description of your son's spinner stalling, it is apparent that it does not have adequate rotational inertia to store sufficient kinetic energy from the small motor powering it. You could use a more powerful motor, but as a display of physics it would be much more interesting to increase the rotational mass of the weapon and note the change in the performance of the spinner.

    Here are your equations: How to Calculate Rotational Kinetic Energy, and I think this explanation of Kinetic Energy and Mass Moment of Inertia in Combat Robot Weapons might fill in some of the gaps.

  2. If you look thru the equations referenced above you'll discover that nowhere in the calculation of momentum or velocity is there a mention of horizontal vs. vertical orientation; the energy of the spinner system is not changed by its orientation.

    However, going from a horizontal to a vertical orientation does effect the performance of the weapon in another way. When a spinner weapon impacts the opponent there is both an action on your opponent and a reaction on your 'bot.

    • With a vertical spinner the action propels your opponent upward and the reaction simply presses your 'bot down. Since your 'bot is supported by the arena surface, it does not move and most of the impact energy is transferred to your opponent.
    • With a horizontal spinner the action propels your opponent left or right and the reaction throws your 'bot in the other direction. The force of the impact is split between moving the two 'bots in opposite directions. The desired transfer of damaging impact energy to your opponent is much less efficient.

  3. A balanced spinning mass is neither assisted nor impeded by gravity. In a vertical orientation the effect of gravity on the rising side of the mass is perfectly offset by the gravitational effect on the descending side. In a horizontal orientation nothing is rising or descending.
I threw a whole lot of information at you, but I think you can pick thru it to find answers that make some sense to you. New questions will arise -- write back as needed.
Q: I am in 150g weight competition, I was wondering if its possible to make a spring loaded spike/ram with ability to reload it, do you have any resources I could have a look at? [Bristol, England]

A: [Mark J.] Take a look at this archived post describing a spring-powered flipper reset by a rotating snail cam. With a little imagination it could be oriented to reload a spike, although a flipper is a more effective weapon.

Comment: Thank you, i was in fact going to do a flipper inspired but that very video, there is a lot of useful info in that other answer!

Q: "Melty Brain" robots do not count as having an active weapon under current Battle Bot rules. Have you ever seen a melty brain style robot that was paired with another weapon? I imagine that the robot's rotation could add even more energy to a hit from a spinner. [Westerville, Ohio]

A: [Mark J.] No, and it won't.

The concept of a 'melty brain' spinner is that the entire mass of the robot becomes a spinning weapon. Stealing mass and energy from this very efficient primary weapon and trying to add it back with a secondary weapon will do no better than break even on energy, and will add undesired complexity. Use all your weapon weight allowance on a single weapon. Simple robots win.

Q: Has there ever been a counter rotating vertical spinner in a robot? I'm thinking of two large disks parallel and close to each other on the same dead shaft. Only one would have teeth and they would have one or more small perpendicular drive wheels between them causing the counter spin. The reasoning behind building this would be to lessen gyroscopic effects. Thanks! [Minnesota]

A: [Mark J.] I know of two big 'bots that were designed to nullify the annoying gyroscopic forces associated with vertical spinners. Both used mechanically simpler solutions than your proposed co-axial counter-rotating disks:

  • Team Boilerbots built 'Counter Revolution' to compete at BattleBots. The twin counter-rotating vertical disks are not co-axial, but the counter rotation largely cancels the net gyro effects when the robot turns.
  • Richard Chandler campaigned superheavyweight 'Strike Terror' at BattleBots 4.0 and 5.0 with a vertical spinning weapon that was free to pivot in the longitudinal axis. This allowed the weapon to twist near-horizontal when turning without effecting the chassis, and then re-establish a vertical spin when turning ceased.
Neither 'bot was particularly successful. I'd recommend against adding the mechanical complexity your design requires. Simple 'bots win.

'Wedgemaster Wedge' writes in to remind us of 'CounterStryker' -- a 6-pound 'mantisweight' with counter rotating vertical disks built for Bot Bash:

Comment: Zac O built and documented this bot which is pretty close to what that dude wanted.

Thanks, Wedge.

Comment: Thanks Mark and Wedge that is exactly what I was thinking about. I found a video of CounterStryker fighting and it seemed to handle the turns well.

A: 'CounterStryker' has a good record: 2nd at Bot Bash '15 and 3rd at Bot Bash '16. I'm not a fan of friction drive for weapons -- Zac took care with the design and it works well in this insect class 'bot, but I wouldn't try it in a larger 'bot.

Q: How do horizontal spinners keep their weapons off of the frame? I know for example the most iconic horizontal spinner Last Rights/Tombstone has an adjustable height blade, meaning it isn't riding on the bottom frame. Is the friction of the bearing on the shaft enough to keep the blade from shifting during big collisions or is there something more to it that I am missing because that doesn't seem adequate? [Cleveland, Ohio]

A: [Mark J.] Typically the weapon hub and pulley/sprocket fill the entire space between the frame members. They ride against the inner bearing races or against 'thrust bearings' that take displacement loading during a 'hit'. The diagram shows a 'live shaft' arrangement -- in a 'dead shaft' design where the shaft does not rotate the bearings are incorporated into the weapon/pulley hub and the spacer is part of the hub assembly.

If the design allows additional space between the weapon bearings for blade height adjustment, tubular spacers (orange in the diagram at left) slide over the weapon shaft above and/or below the weapon hub to raise or lower its position.

Q: How does a horizontal full body spinner like 'Barber-ous' work? Is it a shell spinner like 'Ringmaster' on its side? [a server in California]

A: [Mark J.] First, a little terminology clarification:

  • Spiners are classified by the direction their impact, not by axis orientation. 'Barber-ous' is a vertical spinner and 'Ringmaster' is a horizontal spinner.
  • A 'shell spinner' has the entire exterior of the robot spinning. 'Ringmaster' is not a shell spinner -- it is an example of the rare and complex 'ring spinner' where only the outer edge of the body spins, leaving the wheels exposed at top and bottom to allow inverted operation.
Team Rotractor's original Barber-ous webpage (archived) has the worst build report I've ever seen, but I'm still grateful that it exists. The chassis photo at right came from another source. The shell and electricals have been removed and you can see that the layout is not nearly as complex as Ringmaster's:
  1. The weapon motor is mounted on the central chassis that also carries the batteries and electronics. The entire central chassis is concealed by the weapon shell when assembled.
  2. Non-rotating stub axles come off each end of the chassis. This version of 'Barber-ous' has worm-drive gearmotors bolted to the ends of the stub axles to power the drive wheels. The gearmotors are obscured in the photo by the wheels and hubs.
  3. Laying on the floor is one of the two large weapon hubs. With the gearmotor removed, the free-spinning hub slides onto the stub axle and a drive chain connects the hub sprocket to the weapon motor. The weapon hub on the far side is already in place. With the weapon hubs in place, the weapon cylinder slides on over the hubs and chassis and is bolted to the hubs. The drive-wheel assembly can then be re-mounted.
'Barber-ous' went thru many revisions and updates to the chassis and drive motors, but the weapon drive principle remained the same.

Q: What are the best motors currently for horizontal spinner type robots? Our weight limit is 85 pounds, and we're looking for something with relatively high torque and low spin-up time for the weapon bar. Also not sure whether to use brushed or brushless. [a server in Illinois...]

A: [Mark J.] The hamburger is bad. I cannot match a weapon motor to a weapon based only on the robot's weight class and weapon type. See the Ask Aaron Spinner Weapon FAQ to find out why and to learn what information is needed to calculate weapon performance.

If you don't know whether to use brushed or brushless, use brushed. Brushless adds a level of complexity and many pitfalls for builders unfamiliar with their quirks -- particularly in a heavier weight class such as this. Brushed is simple. Simple is good.

Now, it's possible that you just want me to take a blind guess and recommend an affordable and reliable old school motor that will make your entirely undescribed heavier-than-lightweight bar spin-up and look like a combat robot. If that's the case, Robot Marketplace has found a few of the classic EV Warrior motors. Run one at 24 volts thru a 2:1 pulley reduction to your bar weapon and you're probably in the ballpark.

What event runs an 85 pound weight limit? That's not a standard US weight class.

Q: Why can't 'Warrior Clan' launch bots into the ceiling like its previous form 'Warrior SKF' could? [Massachusetts]

A: [Mark J.] There are no significant changes to the 'bot, so the capacity is still there. I can only speculate that they have not had opponents that are particularly vulnerable to their flipper weapon. The SKF weapon is powerful but it does not have a great deal of vertical motion; their ideal opponent would have a low structural edge close to its center of mass.

Q: Sorry sir for posting a question but can you please clear my doubt " heavier the drum,Oppenent will find difficult to Topel you" is it true???? [Maharashtra, India]

A: [Mark J.] Stop asking about weapons and start building Indian arenas to contain the weapons you have.

Q: Hi There

First off, a huge thanks for keeping this site going, it's a hugely valuable source of information and by far one of the most comprehensive sites on combat robotics on the web. I'm currently designing a heavyweight (110Kg) robot after an extended break - I last built autonomous antweight/sumo bots in 2005.

My question is about spinners - namely, getting a large bar - al la Tombstone/Last Rites - up to speed in a respectable fashion. The current Robot Wars arena is 22m x 22m, and allows for around 2.5s of spin up time (on average) before first impact occurs.

Bar specs:
1300mm x 125mm x 30mm
Weight is roughly 38Kg

Motor Spec:
8500RPM, 42Nm Peak Torque (stall torque isn't stated, nor is the internal resistance, so I've used the peak torque figure in the Run Amok spinner calculator)

Results using a 4:1 gear ratio:
1339 RPM in 7.18 seconds, storing 53397 Joules.

Now, obviously this isn't [quick] enough - is this a case of me just not working out the stall torque correctly, or a case of choose a different motor? (If it is, which ones would you reccomend?)

Thanks [Fulwood, England]

A: [Mark J.] Welcome back to combat robotics, and thank you for the kind comment.

The Team Run Amok Spinner Spreadsheet - like any modeling software - is only as good as the data that goes into it. A motor with uncertain specs or a brushless motor with a non-linear torque curve results in questionable output, but in this case I don't think the motor specs are the problem.

If we conservatively assume that 'peak torque' is the same as 'stall torque' as you have done, the power numbers for the motor are still very impressive. Let's run a comparison by replacing your un-named motor in the spreadsheet with the weapon motor 'Tombstone' currently uses -- the mighty 'E-Tek-R' at 56 volts. We'll set the reduction to 2:1 to get comparable weapon RPM from the slow-spinning E-Tek:

E-Tek-R with 2:1 ratio: 1360 RPM in 6.04 seconds, storing 55,120 joules
Not a whole lot different from your results, so motor power isn't the problem. The real issue is that you're spinning up a big, heavy chunk of steel with a huge moment of inertia and the motor needs more help on low end torque to get a quicker spin-up. Let's try modeling your weapon with increased reduction ratios at meaningful time intervals:

Reductionjoules @ 2.5 secjoules @ 5.0 sec

Energy storage in the kilo-teens range is plenty to warmly welcome hard-charging opponents, and potential energy storage that takes more than 6 or 8 seconds to obtain is effectively useless for anything but 'showboat' hits on an already-beaten opponent. Dial in some additional reduction. It will put less stress on your motor and battery, and will give you a better balance of spin-up time to useable peak energy storage.

Q: Hello Mark! It's said that the flipper of Warrior Clan (Warrior SKF) is powered by its spinning ring, that's amazing! Can you show me how it works? [Guangdong, China]

A: [Mark J.] Team Whyachi's 'Warrior SKF' has been around for several years and we've had quite a few questions about its design and function. Search this archive for "Warrior SKF" and "dog clutch" to find those posts. You'll also discover what 'SKF' stands for.

The best explanation of how a flywheel flipper works is the well-illustrated build report for Dale Hetherington's 'Flip-O-Matic'. If flywheel flippers were a good idea you'd see lots of them; you don't see lots of them.

Q: I've been thinking about this one weapon design that I haven't really seen anywhere. Normally, pneumatic "poking" weapons tend to not be very good in terms of effectiveness, but I was thinking about taking this weapon design to a logical extreme by making the entire front the robot a heavy pneumatic battering ram/plow.

The idea would be to push the other robot at full speed towards an arena wall, much like the typical strategy with a simple wedge or rambox, but then use the pneumatic ram to shove the other robot into the wall with even greater force than simply slamming it into the wall under conventional drive power.

Coupled with a powerful drivetrain and a sturdy supporting structure, this could end up doing some damage (though not necessarily as much as a spinner) while also being a potential counter to spinners, essentially putting the paper back in rock paper scissors without violating the active weapon requirement. Your thoughts? [That one guy from Asheville, NC, who occasionally also posts from Chicago, IL, and has a battlekit drum spinner and a Fingertech Viper with a ridiculous and excessively long name]

A: [Mark J.] Newton's third law is working against this weapon:

For every action, there is an equal and opposite reaction.

You're pushing an opponent of equal mass across the arena. When you fire the weapon it will shove your opponent forward and shove you backward with equal force. The energy of the system of the two 'bots has a net gain of... (wait for it)... zero. The impact of the two-robot system impacting the wall is unchanged, and no additional damage is done to your opponent. Not much of a weapon. Better to spend the weight used on the weapon on extra drive power.

It is an active weapon, but it is not a 'damaging' weapon. It adds complexity and weight with no improvement in performance. I suspect that's why you've never seen such a weapon.

Q: I am looking for slipping clutch on a horizontal spinner. The speed is going to be about 8,000 RPM to 12,000 RPM and the torque is from 1.8 Nm to 4 Nm and I will be happy if I can use the clutch on a dead shaft kind of system. I am using RS 40 chain to spin up the spinner disk and it is for Lightweight Robot. So where can I buy the clutch suitable for this job and usually in combat robots what brand(s) are used and who supply them? [Kuala Lumpur, Malaysia]

A: [Mark J.] Torque limiting clutches like you describe are not off-the-shelf items. Commercially available clutches are bulky, heavy, expensive, and poorly matched to the speed and shock-loading found in robot combat applications. A very few builders construct their own slip clutches thru a trial and error approach -- a great deal of error.

The standard method to limit torque in robot spinner weapons is to use a v-belt drive and set the belt tension to slip at your required loading. Don't make it complicated if you don't have to.

Q: I have now 2 questions that I would like to ask:

1) Is there any tutorial on making slipping clutches, any guide or someone who can help me on this issue that you know of?

2) I checked couple of places for V belt design. The Rio tutorial is not very detailed on the V part section. So is there any reference for designing V belts systems and describing the types of the belts and things like that. I am not looking for a straight solution, I prefer to dig and read to understand what I am doing.

Also if I understood correctly, the torque limitation using the belt is going to cause a force on the shaft of the motor. Since I am planing to use an R/C Brushless (On this part I know what I am doing so please do not wipe it out of the question :) ) without a gearbox, doesn't the extra force slow down the motor significantly and reduce the lifespan of the motor? (I think it does and pretty sure about it, but I want to know your opinion too and how significant you think the difference is)

Thank you for helping many people including me.

A: Slip clutch design is a rare engineering specialty. I can point you to a NASA Tech Brief on Slip Clutches for an overview, but so few people are involved in the actual design of such equipment that there is no tutorial. As I said above, trial and error would be your instructor.

In contrast, there is a great deal of V-belt design help out on the web. Here's a good place to start: Machine Design: V-belt selection.

Chains, gears, and belt drive systems all create a side loading on the drive motor shaft. The side load places stress on the motor bearing but does not directly place a drag on motor output -- a lateral load is not 'work' and does not subtract from output power. A properly tensioned V-belt is quite efficient at energy transfer; it is better than 95% efficient in many cases. Given that the expected lifespan of a combat robot is measured in minutes, I believe there are more important design issues you should be tackling.

Design Philosophy

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

R/C brushless hobby motors have become the standard for combat robot spinner weapon power. While I consider their use in drivetrain systems to still be experimental, I certainly would not question your selection of a brushless motor for your weapon.

Q: Can you tell me which motor is best for cylindrical fly wheels like the Minotaur bot who compete in BattelBots Against Blacksmith [Azad Kashmir, Pakistan]

A: [Mark J.] I have several problems answering your question:

  • I don't think they build 250 pound robots in Pakistan, and you didn't tell me how heavy a 'bot you are building;
  • I have no idea what motors are available to you in Pakistan; and
  • I am reluctant to assist in weapon design for competitors in the sub-continent due to poor arena safety in the region.
I suggest you read the Ask Aaron Spinner Weapon FAQ for weapon design help and evaluate the motors available to you according to the guidelines given there.
Q: Hi, I have a hammer robot and I choose to directly attach the ram 3 inches below the fulcrum. The pivot point is also just a pin though an aluminum bar. The ram has a 6 inch throw so the hammer has a roughly 100 to 120 degree angle cocked. My question is, would it be worth it to add gearing to get a 180 degree swing and bearings for better efficiency or keep the rugged, less component design. Thank you for you help - Team Humphrey [West Virginia]

A: [Mark J.] The amount of 'work energy' available from your actuator is not increased by adding gearing to expand the range of motion, but a rack and pinion can increase efficiency in converting the linear motion of the actuator into the rotary motion of the hammer. However, careful attention to the hammer linkage geometry can keep the thrust vector favorable without the added weight and complexity of a gear system -- see example at right. I recommend keeping it simple.

Your pivot is a weak point in the structure of your weapon arm. You didn't share any details of your arm design, but you should be very cautious about enlarging the hole in the arm at this highly stressed point. Without knowing more about the design I can't make a recommendation on the benefits. If you've got a big, meaty chunk of aluminum around that pivot I'd recommend boring the hole just enough to press in an oilite bushing to avoid steel-on-aluminum purely for reliability. If well lubricated your simple pivot has minimal frictional loss, but the bushing will prevent spalling and wear that can lead to failure.

Q: Hi, again thank you for your suggestions on my hammer bot. For the new Battlebot show design I am trying to upgrade my old 120 pound bot design to the new 250 pound. So my question is 250 psi components instead of my old 150 psi components. I have found a 250 psi 3.25 inch bore 6 inch stroke cylinder. But a 24-48v 5-port solenoid and quick exhaust valves rated at 250 psi are very hard to find in McMaster Carr and Grainger. Do you have any suggestions? Thanks!

A: The old standard 5-port valves no longer deliver the performance expected from high-performance pneumatic weapons. The current standard uses individual high-flow solenoid assisted valves to pressurize and vent your actuator. The R/C controls are more complicated than the simple bang-bang switch control for the 5-port as you have to control each individual valve in the correct order, but it's worth the effort.

The preferred valve is the Burkert Type 5404. They're expensive, but if you're gonna play with the big boys you're gonna need big valves, big actuator ports, and a high-flow regulator.

Q: Hi, I used to compete in battlebot season 5, battle at the beach, RFL nationals etc. I had a middleweight robot Major Punishment it was a 150 psi pneumatic hammer sort of. I actually used 2 quick exhaust valves one was set up normal for exhausting co2 out but the other was arranged to exhaust my buffer tank into the firing side making a cheaper(smaller 5 port solenoid) lighter and high cv flow (3/4 inch quick exhaust). I may do the same at 250 unless you see a flaw? [West Virginia]

A: [Mark J.] Your use of the 'quick exhaust' valve to dump your buffer tank is functionally identical to the way Burkert valves (mentioned in your previous post) work. The retract on you hammer can use smaller valves or even a simple spring return. I still don't have a source for a workable 250 psi 5-port valve.

You mention CO2 -- BattleBots rules no longer allow CO2, so you'll have to go with High Pressure Air (HPA) or nitrogen.

Q: Also I used a Flail Medieval style spiked ball and chain which I think gives me advantages like hitting spinners and drums and more likely keeping my hammer head attached and not bent. It also separates me from some of the rebound force into my bot weapon and arm. My question is what do you think are the good and bad to my weapon? and my bot weapon design. Thankyou!

A: I'm familiar with 'Major Punishment'. Tough competitor with good maneuverability!

The small mass of the spiked ball makes for poor energy storage, and the chain separates the ball from the additional energy stored in the arm. I suspect those are reasons why chain-flails are no longer seen in robot combat.

Your points about durability and rebound are well taken, but you're losing a great deal of attack energy in trade. BattleBots is looking for competitors with weapons that can cause massive damage. The solid hammer weapon on 'Beta' claims an impact energy of 3000 joules from their 11 kg hammer, and you're going to have a great deal of difficulty matching that with a small flail weapon. Think bigger -- much bigger!

Aside: unless the ratings for ABC BattleBots second season pick up there may not be a third season. Consider a 220 pound main 'bot and a 30 pound 'assistant' so you'll have something in a standard weight class if the 250s evaporate.

Q: Hi again and thankyou for your good knowledge of many robot tech subjects. Do you know how and why Beta's hammer is so powerful, my previous thought was pneumatics was much more powerful for the weight? [West Virginia]

A: [Mark J.] 'Beta' does have a powerful electric hammer, but your belief that a pneumatic hammer can be more powerful for the weight is entirely correct. Go back and look at old video of 'The Judge', or run the power calculations and you can satisfy your beliefs. But 'Beta' is an effective competitor where no previous electric hammer has been. What has changed? Batteries have changed.

With a PMDC brushed motor: amperage equals torque and torque equals hammer power. In the past it simply wasn't realistic from a weight perspective to have a weapon battery pack that could deliver somewhere in the neighborhood of 1000 amps that the Briggs & Stratton E-Tek motor could turn into torque. Modern battery technology has made that entirely feasible, and electric hammer 'bots are now a competitive option -- particularly in a competition where pneumatic systems are limited to 250 psi.

Q: Dear Mark,in the new season of Battlebots lots of vertical spinning weaponed-bots are seemingly getting smaller to give weight allowances for better armour,like Poison Arrow and Witch Doctor.But from Witch Doctor's rather shocking loss to Red Devil in the round of 32 I think being small is not a really good idea,which makes them become perfect targets for clampbots to get a hold of,do you agree? [Chinese Forum]

A: [Mark J.] I've known Red Devil's builder Jerome Miles for many years. He is a fine young man, a great builder, and a talented driver. He also got very, very lucky in his fight against 'Witch Doctor'.

Improvements in battery and brushless motor technology have made it possible to shrink the mass and size of effective spinner weapons. Robots with these more advanced weapons are quick, maneuverable, and deadly efficient in deploying their weaponry. Any weapon system has weaknesses against specific counter attacks but the high-speed single tooth disks can certainly hold their own in a tournament. Don't form your opinion on the outcome of a single battle.

Q: How is T-minus's flipping device so effective, since the ram is nearly horizontal when actuated. Wouldn't this initially direct the majority of the force horizontally instead of vertically? I would have thought a flipper would become more powerful the closer to vertical the ram is oriented; how does the T-minus design allow such force upon actuation? I'm trying to see it in terms of the statics behind the design. Thanks! [Grand Rapids, Michigan]

A: [Mark J.] You're entirely correct; from a statics point of view the design of the 'T-Minus' flipper is terribly inefficient. To efficiently convert the linear action of the actuator to rotational motion of the single-pivot lifter, the actuator should pivot to remain perpendicular to the lifter arm motion. This was a primary design consideration for my heavyweight lifter 'The Gap'.

The complication is that all of the robot systems must work together, and concentrating on the efficiency of any single system leads to design compromises in other systems. Inertia Labs elected to concentrate on a well-armored compact and maneuverable low-profile design. That design required a 'lay-down' initial position for the actuator. The actuator never gets close to perpendicular alignment with the flipper arm motion and the force vectors are horribly inefficient.

So, back to your original question: how is T-Minus's flipper so effective? BRUTE FORCE! Inertia Labs made up for inefficiency with a big actuator, huge valves, large ports, and scary high gas pressure. If you have enough power you can get away with inefficiency, and the overall robot design is brilliant.

Q: I had an idea and I wanna know if it could work, i dont think I saw this design anywhere. Imagine a hammerbot a little like terrorhurtz or the one i send you videos a in aquestion below. But, the rack and pinion isnt connected directly to the shaft of the hammer. The pinion is on a dead shaft, bolted or weld on a sprocket. Above, we have a smaller sprocket, wich is bolted on the hammer. In theory, i could get more speed out of the same actuator, by gearing it with a ratio of 4:1, for example. Do you think it could work? It's not for any weight class in particular, just a design i had in my head that i thouh was worth sharing to you.

Thanks a lot for all you do for the combat robot community, you inspired me to build robot, you showed me it wasnt only reserved to pros :) [Quebec, Canada]

A: [Mark J.] I'm glad to see you're enjoying combat robotics and spending some time thinking about design improvements. In the gear train you describe, the 'pinion' on the dead shaft is called an idler gear. An idler gear has no effect on the gear ratio -- you would get the same gearing if your 'smaller sprocket' rode directly on the rack without the added complexity and weight.

About Gearing: Your pneumatic actuator can produce only a certain amount of power as defined by the cylinder bore, the gas pressure, and the rate at which the gas can flow thru the valves and ports into the cylinder. Power is a function of time and is described by the formula:

Power(t) = Force × Velocity

Gearing changes the ratio of force to velocity, but does not change power. You can 'gear down' to get greater force (torque in this case) and reduced speed, or you can 'gear up' to increase speed with reduced torque.

To be effective your hammer weapon must accelerate to as great a speed as possible in only half a revolution -- it is torque that creates that acceleration.

  • If you reduce the torque by gearing up too much the hammer will accelerate slowly and not achieve its best speed in the distance available.
  • If you reduce the speed by gearing down too much the hammer will accelerate quickly to a peak speed that is much lower than it might have if it were geared to use the full distance available.
What you're looking for is the gearing that provides the torque needed to accelerate the hammer to peak speed just as it impacts your opponent, maximizing the power from the actuator.

Q: ok, but if i use a chain instead of a gear? would it still nt affect the gearing of the hammer?

A: I don't see how the system you described could be implemented with a chain, but regardless... number of intermediate idler gears, idler wheels, or idler sprockets in a drivetrain will impact the overall gear reduction. The reduction ratio is calculated from only the sizes of the first and last elements in the sequence.

Q: Mark, why does 'Lucky' and 'Son of Ziggy' take a lot of time to make their weapons ready before they can use weapons again? That's a deadly drawback! [Jiangsu, China]

A: I haven't noticed any particular delay on weapon reset for 'Son of Ziggy' (video). I believe SOZ uses a spring powered return on the flipper and it takes just a moment for the high pressure gas vent from the pneumatic actuator so the spring can pull the weapon back down.

'Lucky' was rushed into battle before the flipper could be fully sorted and a lot of problems surfaced at BattleBots. Scroll down two posts to find a report.

Q: does some kind of rack attachment exist to put on a pneumatic ram? I would want something like the one terrorhurtz use. Thanks a lot :) [Quebec, Canada]

A: [Mark J.] The 'Terrorhurtz' weapon rack is all custom machine work. There are industrial pneumatic actuators that perform a similar function, but they're expensive and heavy [example]. Google: 'pneumatic rotary actuator'.

Q: how did john reid make the rack and pinion system? did he just put grooves on a longer shaft he then put in the pneumatic ram? it seems like it in the picture. or what about the robot in this video? or this video?

A: John Reid cut precision gear teeth into an extended length shaft on the pneumatic actuator. He has also provided a roller to support that shaft from the underside. I would think that the gear teeth would interfere with the front seal on the actuator, so there's more going on than is explained in the photos.

The robots in the other videos have their hammers driven by rack gears that have been attached to the pneumatic system. The attachment and support of the racks is critical, and the method used by the builders in the videos is not clearly shown.

British builders are famous for scrounging parts from scrapyards and repurposing them, so I suspect that those racks were scavenged from a discarded mechanical assembly. I would point out that neither of the bots in the videos are delivering what I would consider 'damaging' blows.

There are other designs for pneumatic axes: see this post for a discussion of the pneumatic weapons on 'SlamJob' and 'The Judge'.

Q: how does Ziggy's flipper seems so powerful compared to Lucky's? Aren't they built by the same guys? [Quebec, Canada]

A: [Mark J.] In my original answer to this question I attributed the reduced performance to BattleBots rules prohibiting the use of custom pneumatic components. It seems that I may be misreading the BattleBots Design Rules. I thought section 10 was quite clear on pneumatic components:

"There are no specific restrictions on the system design; however, the pneumatic system must use best practices and commercially available components that are rated for the operating pressures used."

I received a note from a reader in Massachusetts who was on-site at BattleBots 2016 and who offers a better explanation:

I have to disagree with you on the Lucky vs. Ziggy question. If Battlebots rules prohibited custom pneumatic components, how do you explain Bronco's black-box variable pressure system or Chomp's custom everything-except-the-tank? Also, having been at the event I know that there were custom components inside of Lucky, although not to the same degree as the other pneumatic bots.

You can feel free to ask Mark or Rob for the specific details, but the story is that the short time period they had for building prevented testing until the last minute. At that point, it was shown that the spring-retract and release valve system simply wouldn't do, and firing the system on full pressure (which was significantly greater than Ziggy's) would guarantee the arm being jammed or unable to cycle. Even operating at a greatly reduced pressure, the arm still got jammed repeatedly, and was only occasionally able to cycle back down.

There were other fundamental design issues in Lucky's flipper that kept it from being as potent as Ziggy's, but they're of a more mathematical nature and since I wasn't privy to the whole design process I can't really answer in good faith. Really, all of Lucky's issues came down to their status as a last-minute addition and the extremely short time that they actually had to build in.

Thanks, Massachusetts. I look forward to seeing what 'Lucky' can do with the bugs sorted.

Q: what kind of ICE engine people use to power spinner? i know that Icewave uses a fireman saw engine, but i dont seem to be able to find one anywhere (ebay, mcmaster carr). Is there other type can use? i know it might not be the most efficient way or the most simple way to power a spinner.. I just want to see if i can make one.. like i always love hammerbot, even if they are not really that efficient. [Quebec, Canada]

A: [Mark J.] Chainsaw engines are popular choices for ICE spinners -- light, powerful, and easy to obtain. Outputs around one horsepower for every 20 pounds of robot weight are about right. Check carefully with event organizers for rules specific to internal combustion engines at their event. Many events simply do not allow ICE. Current BattleBots rules (Rev. 2016.2):

Internal combustion engines are allowed, but with the following requirements:
  • The engine must use a self-starter that is activated by remote control.
  • Any electric fuel pumps must be able to be shut off by remote control.
  • If the engine uses a separate fuel tank, the tank and fuel line must be well protected.
  • The fuel tank must be vented (no pressurized tanks) with a vent system that will not continuously leak fuel if the bot is upside-down.

ICE weapons are temperamental, unreliable, and have a poor record in combat. They are most certainly not a sane choice for novice builders.

Q: I am building a hobbyweight with a small (~2 lb.) vertical spinning bar sticking out of the front wedge. I recently attended an event in which a couple of the other competitors were running Turnigy brushless motors for their belt-driven weapons (both of them did well). I am basically just trying to replace my heavy brushed motor with a lighter brushless motor while keeping the belt drive. What should I attach to a brushless motor (5mm shaft) to spin that 2 pound bar on my hobbyweight? [Albany, Oregon]

A: [Mark J.] I need more info:

  • Dimensions of your spinning bar (length, width, thickness);
  • Diameter of the pulley on the bar;
  • What brushed motor you are replacing, and at what voltage; and
  • Which Turnigy brushless motor you want to swap in.
I suspect you'll need a larger reduction ratio than you have with your brushed motor, and so will need a smaller motor pulley. Send me the info and I'll run the numbers.

Q: The steel bar is 5" x 3" x 1/2" (roughly, a couple of the corners are taken off a little bit). It is attached to a 3" pulley. I was running a Kawasaki 21.6V circular saw motor with a 7s lipo. I haven't yet selected which Turningy motor but was thinking something along the lines of the Turnigy XK3665-1200KV and running it with a 3s or 4s.

A: Hmmm... I have no clue about the power output of your circular saw motor, and I suspect you don't either. How did you decide on a 2" to 3" pulley ratio for the weapon?

The Turnigy XK3665-1200KV is an inrunner motor that would spin at close to 18,000 RPM on a 4-cell battery, but running it on 4 cells rather than its rated 7 cells reduces the output power by almost 70% [1 ÷ (7 ÷ 4)^2 = 33% of max power]. Pick a motor rated for the number of cells you want to use. For 4-cells something like the Turnigy Aerodrive SK3-3548-1050kv would be about right.

Your weapon bar is puny. At 8000 RPM (too fast) it stores less than 570 joules of energy. Consider adding thickness, increasing the diameter, or going to a full disk. Changing out the bar for a 6" diameter steel disk 1/2" thick bumps the 8000 RPM energy storage to nearly 1700 joules -- no longer puny.

Running a 1" diameter motor pulley to a 2" pulley on the weapon could work nicely for this set-up. Make sure the belt width is adequate to carry this amount of power.

Q: What are the benefits of an asymmetrical spinning blade versus a symmetrical one? [California]

A: [Mark J.] Briefly, you can spin your weapon twice as fast and store four times as much energy without losing critical weapon 'bite'.

From the Ask Aaron Spinner Weapon FAQ:

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read. It turns out that [weapon bite at a given weapon speed] depends on the spacing of the impactors and how fast your 'bot moves forward during an attack. You can effectively use greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to obtain if you have multiple impactors and a timid attack.

There are also multiple posts in this archive discussing single-tooth weapons. Search here for 'asymmetric'.

Q: hi do u have a way to calculate how much torque my lifter will produce? what gearing should i use for the gearboxe in a 30 lbs robot? [Quebec, Canada]

A: [Mark J.] You asked a question a couple months ago about chain driving a featherweight lifter. My reply to that question featured a link to the formulas needed to calculate torque for a simple lever arm lifter (not a 4-bar lifter). Suggest you re-read that post and follow the link.

You should gear the lifter motor so that maximum lifter load requires only about half of the motor's stall torque (torque overage factor = 2). That assures the fastest lifter speed when fully loaded. Here's an example:

  • Lifter arm length (pivot to tip): 1.0 feet
  • Maximum lifting weight: 30 pounds
  • Maximum torque at gearbox (ft-lb): 1.0 feet × 30 pounds = 30 lb-ft
  • Max Load at Gearbox (in-oz): 30 lb-ft × 192 = 5760 oz-in
  • Motor Stall Torque: 166 oz-in (BaneBots RS-775 18v @ 18v)
  • Torque Overage Factor: 2.0
  • Gear Ratio Required: (5760 ÷ 166) × 2.0 = 69.4:1

In the example given, the BaneBots 64:1 P60 Gearbox would do nicely. Run your own design numbers thru the same process to get your ratio.

Note: although a torque overage factor of 'two' provides the fastest lift at maximum load and keeps motor loading reasonable, some builders prefer a smaller torque overage factor to give a faster lift when the lifter is only raising one end of the opponent rather than the entire 'bot. This places a greater load on the lifter motor, but is an option. As long as the torque overage factor exceeds 'one' the lifter will function without stalling.

Q: hi srry if im annoying.. but do the lifter have to be a straight bar for the math to work? because my idea was to use something similar to the one nyx has [pictured at right]. I have done very basic physic at school, since i am only 17 so pardon my ignorance.

A: I'm always pleased to get follow-on questions. I have to make assumptions about most questions to keep the answers short, so don't be hesitant to ask for clarification. Curiosity and persistence are virtues.

The math works for any single-pivot lifting arm: measure the arm length as a straight line perpendicular to the pivot axle -- from center of axle to far tip of arm. Bends and angles along the way don't count.

Note the big gears 'Nyx' uses to handle the large torque loads on this long lift arm. A small BaneBots gearbox used as the pivot is unlikely to survive torque forces so great as seen in this design.

Q: so. can i use like a 16:1 ratio gearboxe then use gears to acheve the right ration, i assume thats how nyx work. Im i right?

A: Yes. That will reduce the torque load on the BaneBots gearbox. Chains require less precise positioning than gears and are more forgiving of minor misalignment -- easier for a novice builder.

Q: do u have a diagram on how an horizontal spinner like tombstone or mechavore is mounted? [Quebec, Canada]

A: [Mark J.] There are two common methods to mount spinner weapons; 'Tombstone' uses one, and 'Mechavore' the other:

  • 'Tombstone' uses a non-rotating 'dead shaft' solidly attached to the chassis as a structural member. The weapon blade and drive sprocket are attached to a tubular hub with bearings on each end that ride on the shaft. You can see spare weapon blade/sprocket assemblies for 'Last Rites' ready to slide onto the shaft in the picture at right.
  • 'Mechavore' has its spinner weapon and drive sprocket attached directly to a rotating 'live shaft'. The shaft itself rides in bearings attached to the chassis.
There is a diagram of live and dead shaft designs with a discussion in the Ask Aaron Weapon Archive: Live Shaft vs. Dead Shaft.

Q: hello im the guy that asked the question about the horizontal spinner mounting option. Do i need to put something between the bushing and the frame under the spnner? it seems like the friction at this point will be very high. Im sorry if my english was not really good ; )

A: Vous parlez mieux l'anglais que moi le français. Je parle le français comme une vache espagnole.

I assume we're talking about a 'dead shaft' design? Some bearings come with 'extended' inner races that will space the weapon hub up away from the chassis to prevent rubbing. If the selected bearings do not have an extended inner race, a small washer placed on the shaft will rest against the race and rise the hub up for clearance. I left the washers out of the linked diagram for the sake of simplicity, but then I left out the chassis as well.

Q: Hi Aaron,
First of all thanks for making your Excel Spinner Spreadsheet, it's amazing! There's one thing I'm not sure about it though.

I've got a Scorpion HKIII-4020-890KV brushless motor with a 6S LiPo battery to power my weapon. My horizontal bar is made out of 4340 stell (around 7.83 g/cc) and has Lenght = 0,42m Width = 0,04m and thickness = 15mm so my moment of inertia is 0,29269 KgM^2 and the mass is roughly 2Kg

So if I use a 3:1 gear ratio and consider that the no load speed would be 80% of the KV times the voltage (24V) I will get at my maximum energy (4724 J) and speed (5411rpm) in less than a second (around 0,85s) Would you say that's correct? I think I've mess up the no load speed.

Thanks in advance! [Brazil]

A: [Mark J.] Brushless motors in combat robots remain more of a black art than a science, but I can assure you that you're not going to spin up that bar in 0.85 second with that motor.

The Team Run Amok Excel Spinner Spreadsheet works quite well to estimate the performance of spinner weapons with brushed motors. As warned (perhaps inadequately) in the current version of the spreadsheet, brushless motor startup torque depends a great deal on the brushless motor controller. Differing controller hardware, firmware, and user settings will greatly impact the low-speed torque of a brushless motor. In particular, a sensorless motor like the Scorpion HKIII-4020-890KV will not come close to the 'best case' torque calculated by the simple brushless torque estimator included with the Spinner Spreadsheet because a sensorless controller must take an educated guess at the stator position relative to the magnet array. Also, the motor would need to pull over 1800 amps at start-up to produce that 'best case' torque, and your controller most certainly isn't going to allow that much current even if your battery pack could supply it.

I'd de-rate the stall torque of the Scorpion motor by a factor of at least six -- call it 3.3 Newton-meters. Aerodynamic drag will slow total spin-up time as well as reducing top RPM. Assuming 20% speed loss due to aerodynamic drag, I 'eyeball' weapon speed around 5200 RPM with 4200 joules in something like 6 seconds. That's still very nice performance for a featherweight.

Now the bad news: if that motor is bogged down to continuous operation at 80% of free running RPM it may pull in excess of its 65 amp continuous current rating. So much depends on who the motor controller behaves in situations like this. Brushless motors do poorly when bogged down -- you might consider increasing the reduction ratio.

Again, that stall torque rating could vary a good deal from the estimate and the performance figures would vary with it.

Q: I have a couple more questions about the size of my bar and the axis that supports it. This is all for a featherweight horizontal spinner, and if the model from above is correct I have 157 J per pound of my class which is pretty nice. I'm thinking about using the bar I've talked before with an axis of 10mm, both of them made out of 4340 Stell at 34HRC. Do you think that's enough thickness for my axis? How would you simulate that in a CAE program? I have experience with PTC Creo Simulate academic edition, I'm just not sure how to model this.

A: See Frequently Asked Questions #17. 'Ask Aaron' is not a free engineering service -- and if it was, no competent engineer would spec that weapon shaft without knowing a great deal more about the support the shaft would be given in your design.

Q: Hey Mark,
Are bearings absolutely necessary for a spinning disc/bar weapon? I was looking at Team Nightmare's build of Backlash (1st version) and nothing was mentioned about bearings. It's kinda hard to tell in the pics, but it appears that the disc is attached to the pulley and they both spin on a dead shaft. Are there some type of bearings in there that I'm not seeing or is it possible to spin a weapon on nothing but a greased up shaft? [A few miles south of here, Oregon]

A: [Mark J.] Take another look at the weapon pulley in the 'Backlash' build report -- that's a sealed bearing at the hub. The loading at the hub of a high-energy spinning weapon far exceeds the capacity of a 'greased up shaft'. On a good hit it would spall and seize. You can (and a fair number of builders do) use oilite bronze bushings in place of ball/roller bearings to support weapon shafts. They have a bit more friction but can survive enormous shock loadings. Keep them well oiled.

Q: So if I wanted to use something like a needle roller bearing, I would put it on the dead shaft and then attach the weapon/pulley to the outside of the bearing? If so, what is the best attachment method? I know set screws aren't great but could they work for a 12 pounder?

A: Not a great idea to apply significant point force (like a set screw) to the outside race of a bearing. A needle bearing would typically be installed in the weapon/pulley as an interference press-fit with thrust bearings and spacers taking up any lateral motion on the shaft.

Alternately, flanged ball bearings can be simply inserted into the sides of the weapon/pulley assembly to be held in place by the structure supporting the dead shaft. Washers or tubular spacers can take up any excess clearance.

Q: I've been noticing that some spinners that use outrunners to mount the motor pulley to the can of the motor and not the to shaft. Best example I can think of is the Australian featherweight 'Decimator'. Other than reducing height, does this offer any other advantages? Any significant disadvantages with this? [A few miles north of here, Oregon]

A: [Mark J.] A design like Aussie 'Decimator' (photo) mounts the weapon pulley on the can and tucks the entire weapon drive inside the chassis without requiring an internal bracket to support the weapon motor. It's clean and simple, and it does save a bit of height while still protecting the weapon belt. Simple is good.

The drawback is that the pulley is now located at the far end of the motor from the motor mount. This violates the general engineering rule of providing support as near as possible to the point where lateral force is applied to a structure. Decimator's design places great mechanical loading on the motor's internal bearing support compared to mounting the pulley in the conventional manner. The short height of Decimator's weapon motor keeps the load from becoming too great -- stresses on a taller motor would be worse.

Q: How can I install a pulley on a brushless outrunner motor? [Quebec, Canada]

A: [Mark J.] Well, you could the obvious and slide a nice FingerTech pulley onto the output shaft of the outrunner and tighten down the Allen screw. That works. I'd file a flat spot on the shaft to give the Allen screw a better bite, and I'd put a drop of threadlocker on the screw to keep it from loosening.

Q: how could i give a blue, energetic look to my flamethrower like th one used in complete control? [Quebec, Canada]

A: [Mark J.] We don't discuss flame weapons here at Ask Aaron. See Frequently Asked Questions #28.

Q: We're competing in a junior high battlebot competition. 50 pounds maximum on robot. We're wanting to do a spinning weapon like tombstone. Any thoughts on motor, maybe a cog belt. and pulleys? [Amarillo, Texas]

A: [Mark J.] Start by reading the Ask Aaron Spinner Weapon FAQ. The FAQ outlines the key elements in spinner weapon design and points you to tools to match a specific weapon design to a suitable motor and weapon drive.

There are also a great many posts in this archive about weapon motor selection.

As for drive components, browse the Mechanical and Drive Components selection at Robot Marketplace.

I'm pleased to provide the tools needed to design your weapon, and when you have your design roughed out I'll be pleased to look it over and offer suggestions. I won't design your weapon for you, but I'll give you a start:

An AmpFlow F30-150 motor @ 24 volts spinning a 24" x 3" x 1" steel bar via a 4:1 belt drive reduction. The bar weighs 20 pounds, spins up to more than 1600 RPM in 4 seconds, and stores better than 4200 joules of kinetic energy at that speed.

Q: Is this Park 370 Outrunner EFL-370-1360 Brushless Motor powerful enough to spin a 2.5 lb steel bar (overhead spinner curved down like that in one of the designs in the riobotz book) or a fbs shell? Live shaft most likely but dead shaft is possible. Using Timing belts and a gear ratio of about 3.5:1 with and rpm of about 4000. I would like to know if it is strong enough to spin the weapon up to speed in a reasonable amount of time under combat conditions (being hit while attempting to spin up). 11.1v lipo.

Or should I use this Feigo inrunner motor?

Thank you. I know my questions could probably be answered in the excel spinner spreadsheet but I am not very good at using that particular tool. Using the lighter motor would be amazingly helpful for weight but I fear it is too small to spin that much mass effectively under combat conditions and will get burned out. [Pennsylvania]

A: [Mark J.] I'd suggest you start by reading the Ask Aaron Spinner Weapon FAQ, wherein you will find out why a spinner weapon cannot be evaluated based only on the weight and material of the spinner, and why the dimensions of the spinning mass are critical in the calculations.

It's also not reasonable for me to comment on adequate performance of a spinner weapon without knowing in what weight class the robot will compete. Is this for a mantisweight, hobbyweight, BotsIQ, or maybe something else?

Neither of the motors you ask about are suitable for your purpose:

  • The Park 370 has a power output around 100 watts -- suitable for an antweight class spinner, but way too small for you.
  • The Feigo does not list any specifications except RPM per volt. All I can say is that it spins way too fast for your design.
If you aren't willing to do the calculations to pick out a suitable weapon motor, look at successful robots in your weight class with designs like yours to see what weapon motors they use. That will get you close.

Important note: you don't see any successful examples of that 'overhead spinner curved down' design in actual combat because that particular rotor shape in unstable! See this post elsewhere in this archive. Pick another design.

Q: Do you have any thought on the PERM PMG 132 motor vs a Motenergy ME0708 or ME1003? This will be for use in spinning blade or drum robots.

I understand the performance differences but wonder if there are any strengths or weakness in either of them that would be specifically important to combat robots. [Orange County, California]

A: [Mark J.] I'm going to defer to Ray Billings on this one. Ray and Team Hardcore have a whole lot of experience torturing big pancake motors in heavyweight weapons. They've gone thru a BIG stack of original Eteks, PERM PMG-132s, and Motenergys. None of them hold up for long, but if you need big weapon power you have to live with short lifespans. Ray estimates it costs him about $1000 a match for equipment replacement -- mostly batteries and weapon motors.

His current choice - for reliability and power - is the Motenergy ME0708, also known as the 'Etek-R'. If Ray likes it, I like it.

Q: Would a alien power system c8080 brushless motor work for a heavyweight vertical spinner? If not, what brushless motor would you recommend for a heavyweight vertical spinner? [State of Tennessee Department of Education]

A: [Mark J.] It's poor design practice to start with a weapon motor and build the rest of the robot around it. Design your robot to do what you need it to do, then build to meet that specification. All of the components of the weapon (and the robot) must work together.

That said, a 5000 watt brushless weapon motor like the C8080 is underpowered by current heavyweight standards. Design a suitable heavyweight-class weapon (think 'Tombstone') and plug in the performance specs for different motors until total weapon performance meets your goals. There are heavyweights that use Alien Power weapon motors, but not the cute little C8080.

Q: Hello,
Why are all full-body spinners' drive systems so slow? I know they need good stability especially with the gyro effect, but why can't they be fast? And would making the top part spin the opposite direction as the bottom tooth part eliminate gyro and maybe even help with the "throwback" con of those spinners?
Thanks, Missouri

A: [Mark J.] Let's talk a bit about the 'gyro effect':

  • The gyro effect -- more correctly 'gyroscopic precession' (video) -- is a force acting at a right angle to a force that's acting to change the direction the axis of a rotating body is pointing.
  • The axis of a vertical spinner weapon points left/right in relation to the robot. Turning DOES change the direction that axis is pointing. A vertical spinner (like a drum spinner) DOES exhibit a 'gyro effect' when turning that can lift one side of the robot off the arena floor.
  • The axis of a horizontal spinner weapon points straight up/down in relation to the robot. Turning DOES NOT change the direction that axis is pointing. A horizontal spinner (like a full-body spinner - FBS) DOES NOT exhibit a 'gyro effect' when turning.
Horizontal spinners don't suffer from the adverse 'gyro effect', but they can have trouble with simple torque reaction. Newton's third law says that for every action there is an equal and opposite reaction, so when you apply torque to spin-up a big horizontal spinner, an equal torque is applied that attempts to spin the robot in the other direction. This force greatly diminishes once the weapon is up to speed, but at weapon start-up it can be quite troublesome. Now that we have the terminology sorted out, let's get back to your questions.

Not all FBS are slow, but most builders skimp on the drive system in order to put additional mass and power into the weapon system. Let's face it, a FBS is all about the weapon. The attack strategy doesn't depend on speed, and any side of the robot is as dangerous as any other so maneuverability isn't an important factor either. All you need to do is move toward your opponent and wait for contact. Adding a powerful drive system steals power from the weapon and doesn't significantly add to the effectiveness of the robot.

Splitting the shell into two counter-rotating components adds significant complexity and fragility to the weapon system and results in less energy storage in the part that actually hits your opponent. It could eliminate the torque reaction on spin-up, but the 'throwback' on impact is a consequence of Newton's third law and the horizontal impact vector. You can't cheat Newton.

Q: So the rotational energy from the counter-rotating top wouldn't stop the bot from spinning away from impact?

A: That's right -- Newtonian counter-reaction trumps counter-rotation every time.

Q: What I'm trying to figure out is how to not get beat up from super-low wedge spinner bots. If they come in slow enough to move the gyro they can get under you. Unless you have a fast drive you can't get where they can be hit.

A: Like I said, not all FBS are slow. If you're willing to trade off some weapon power for robot speed there's nothing in the design to stop you.

Q: How do you defeat a bot that has a knife edge wedge and titanium spinner on top of that?

A: People beat wedges all the time. Suggest you watch some video of matches with this type of opponent to see how it's done in whatever weight class you're building. Modify your design and strategy accordingly.

Q: Greetings:
I was wondering how effective the following weapons would be on a robot. Note that I've ordered these roughly from "least likely to work" to "most likely to work", and am only seriously considering building the last three.
  1. A vertical spinning weapon on a pneumatic piston
  2. A horizontal spinning weapon on a pneumatic piston
  3. A drill/'Niterider'-style weapon on a pneumatic piston
  4. A pneumatic piston on a horizontal spinning weapon
  5. A lifter that deposits the opponent onto a spinning weapon on top of the robot (if you need a better description I can link you to a picture.
  6. A horizontal spinning drum weapon
  7. A "face spinner" (think 'Invertibrat' from Season 4)
  8. A front-hinge flipper a la 'Firestorm' or 'Cassius', except the lifting arm is replaced by a blade to potentially damage the opponent (obviously this would have to be a high-powered pneumatic flipper)
And yes, I know your "efficient, simple, easy to use" mantra, I just wanted your opinion on this. [Portland State University]

A: [Mark J.] I don't consider any of the weapons to be practical, so let's talk about the designs purely from a conceptual standpoint.

  1. A spinner weapon gets much better 'bite' if the closing speed on the opponent is greater. In close quarters, having the ability to quickly thrust the spinner forward is conceptually valuable.
    Conceptual score: A
  2. Same benefit as #1, but horizontal spinners throw both the attacker and target in opposite directions. Less desirable.
    Conceptual score: B
  3. 'Niterider' had a 'disemboweler' spike intended to penetrate another robot then twirl some stiff wires around to chop up internals. Would work really well if combat robots were armored with cardboard and filled with eggs. Adding pneumatics won't help.
    Conceptual score: F
  4. Whaaatttt?? How does that accomplish anything except sudden imbalance?
    Conceptual score: F
  5. I really hate dual-weapons. Decide whether you want to flip 'em or shread 'em, then put all your weight allowance into a single effective weapon.
    Conceptual score: D
  6. OK, maybe this one is practical. The drum stores more energy than a bar or disk, so it has some promise. Keep the drum fairly short and with a large diameter and you might have something.
    Too practical for a conceptual score
  7. 'Invertabrat' was an unsuccessful 'flipper' robot that violated the 'two-weapon rule' and added an ineffectual milling head to the back of the robot. Spinning at full speed the milling head might possibly leave a nasty scratch on aluminum or plastic. Enlarged, one edge goes up (good) and the other edge goes down (bad). Hit with the wrong edge and YOU fly upward.
    Conceptual score: F
  8. Blades can't cut modern robot armor, and a sharp edge makes it easy for your opponent to slip off to one side or the other. If you wanna flip 'em, flip 'em. If you want a pneumatic pickaxe, build that. Points for visual appeal.
    Conceptual score: C

Q: Hi Mark! I have a quick question on drumbots. How is a drum usually mounted and supported on a dead shaft, such that it can handle direct impacts and keep the shaft from bending? [Midvale, Utah]

A: [Mark J.] Usually? Sturdy endplates, great big bearings and a big hardened steel shaft anchored securely to the chassis as close to the bearings as possible.

Q: Hi Mark, I'm following up on my drum spinner mounting question. What if the drum was split in half with the pulley mounted in between? In my case this would be necessary given the compactness of the bot and motor mounting restrictions. So the shaft would either be 1.5" thick titanium or 1.1" thick steel anchored on the far left and right sides of the bot, and the drum itself is about 2" thick with a 6.3" OD not including teeth. It just has that vulnerability in the middle where the drum is split to be able to mount the pulley in between. Is this a viable option if built as robust as possible? If not, another option could be using the drum itself as a pulley? (Assuming the gearing could still be made correct)

A: I really dislike splitting the drum right at it's point of greatest structural weakness. Adding additional bearings near the center to support the split would load the shaft at its weakest point -- a poor engineering choice.

Your drum is amply thick to machine a v-belt groove (or two) into the drum. That will weaken the drum a bit, but not nearly so much as splitting the drum to add a central pulley.

Q: Hi Mark. That sounds good. I was leaning towards a built-in V-groove as well. Tolerances will have to be very high on the machining though, since the plan is to have a single tooth and a counterweight spanning across the entire drum. So the V-groove will have to go through those two AND the drum, misalignment between the three could cause troubles. It should work out fine though.

I could also just split the tooth and the counterweight in the middle to allow the pulley to go around just the drum. Is there a disadvantage to splitting a tooth into two pieces? When the drum hits it can potentially put the entire load on just one tooth.

A: You haven't mentioned the length of your drum. Assuming that it's fairly long I'd split the impactor tooth and the counterweight. At the energy levels a large drum weapon generates even a tool steel impactor will elastically deform enough on impact to localize the loading, so there is little benefit to a long-span tooth.

Q: Drum guy here again, thanks so much for your help so far! The total drum length is 11", but it actually won't be split directly down the middle, based on the latest design... more like into a 3.5" piece and a 7.5" piece. By the way, just wanted to mention that with the current materials and dimensions the drum will generate just under 20KJ at 90% RPM.

A: That's a lot of energy for a weapon that size. An aluminum drum with the dimensions you give plus steel tooth & counterweight weighs about 33 pounds and needs to spin close to 8000 RPM to store that much energy. Balance is going to be critical. Best luck.

Q: Drum guy here again. Thanks for the input! You are exactly right about the total weight and RPM. I'm curious though, what dimensions did you estimate the steel tooth would be, and how much it extends out of the drum? I've been trying to balance how far it extends out vs how far it is supported in the drum, and have yet to determine the sweet spot.

A: I didn't assign specific tooth dimensions. I just added a bit of mass to the outside diameter of a drum with the dimensions you gave to bump up the weight and get the RPM for 20K joules to come out a round number. There is a formula to calculate the maximum useable tooth height for a specific weapon at a specific closing rate:

Tooth Height (inches) = Closing Speed (inches per second) × 60
Weapon RPM × Number of Impactors

The formula requires some assumptions on closing rate, but does at least provide some guidance. How deep to set it into the drum? Best engineering judgement.

Q: Is there any difference between a pneumatic setup for co2 and nitrogen use? [Third Tube from the Right, Internet]

A: [Mark J.] Yes. CO2 converts to a liquid form under pressure and is stored at about 850 PSI. The storage tank must be mounted to assure that only the gaseous CO2 from the top of the tank enters the pressure regulator and actuator. As CO2 changes from a liquid to a gas it gets VERY cold -- cold enough to 'freeze up' a regulator designed for other gasses, so you need to make sure your regulator is made for use with carbon dioxide.

Nitrogen can be stored at pressures up to 5000 PSI and does not convert to a liquid form under pressure -- you'll need a larger tank to hold the same quantity of gas as the liquid CO2 system, but you don't need to worry about tank orientation or the problem with extreme cold. The higher pressure requires that the storage tank and regulator both be rated appropriately.

See the 'What a gas!' section of the Team Da Vinci: Understanding Pneumatics page for more details, and read the whole page while you're there.

Q: I can't think of one, and there may not be one just because I haven't seen many bots with it; but is there any pros to having a non circular kinetic weapon? Like Tillah from Team Juggerbots' square drum? [Spring, Texas]

A: [Mark J.] Can't think of one, you say?
  • Bar spinners like 'Tombstone'?
  • Asymmetric mini-discs like 'Witch Doctor'?
  • Cutaway discs like the 2015 version of 'Nightmare'?
  • Snail drums like 'Touro Maximus' (image right)?
  • Monotooth drum hybrids like 'HyperShock'?
Plenty of examples, some of them requiring a great deal of engineering and construction effort to produce. As covered in the Ask Aaron Spinner FAQ the best energy storage does not come from these designs, but builders don't generally waste effort so it's a safe bet that there is an advantage to be gained. The advantage varies with the style of weapon:
  • Bar spinners are simple to construct, robust, and durable;
  • Asymmetric mini-discs are single-pieced, durable, and can spin at high RPM while retaining 'bite';
  • Cutaway discs are... Well, they're pretty;
  • Snail drums are durable and show that you have serious design and machine skills; and
  • Monotooth drum hybrids have the advantages of asymmetric mini-disks but can store more energy.
Q: I think I worded my question wrong lol. I meant are there any advantages to a polygonal kinetic weapon like a square (or cube like I guess) drum. Then again, there are eggbeaters that are apparently good

A: Like it says in the 'Energy Storage' section of Ask Aaron Spinner FAQ an ideal design places as much of the mass as far from the rotational axis as possible. A circular cross-section achieves this goal for a given diameter, while something like the weapon on 'Tillah' (photo at left) has a good portion of the mass located at less than the max diameter and loses energy storage efficiency. As I recall, Ron Ender found that big square steel tube in a scrap yard and just really liked the look of it. It does look awesome, and that counts for something.

Eggbeaters can store a great amount of kinetic energy for their mass because they place a lot of the material a long way from the rotational axis where it does the most good. They are a viable design choice for insect class robots but they don't scale up well to heavier weight classes; they're too fragile at larger sizes.

Q: Is making a counter-rotating saw blade a good idea at all? I did a little test on some aluminum with my dad's saw (counter-rotating). It didn't have any throwback, and it made a clean cut just touching it. Using a single blade, it had a LOT of throwback, made not very deep but nasty looking cuts, but wasn't good at cutting from the flat side. Any thoughts? [Dearborn, Michigan?]

A: [Mark J.] I know that the new batch of 'BattleBots' are all complex examples of 'machine porn', but that's only because the audition process for the show demands that type of design. If you want to build a robot to win matches in open competition you need to keep the design simple and robust.

Design Philosophy

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

So, no counter-rotating saws. In fact, no saws at all. Effective use of a saw requires that you immobilize your opponent, and your opponent has other ideas about that. Keep it simple.

Q: How can u self right using a 4 bar lifter? i saw this video on youtube and even after watching it i just cant figure it out..

P.S. i really love ur site, its the best out there and u gave me the inspiration the build bot thank u :) [Quebec, Canada]

A: [Mark J.] Thanks for the props, Quebec.

The video of antweight 'Pad Thai Doodle Ninja' self-righting is taken from an awful angle to actually see what's happening. I think you'll get a much better idea of the process by watching this video of 'BioHazard' self-righting. Getting a 4-bar lifter to flop back upright requires extensive pre-planning and a fair amount of tinkering. You'll notice small extension 'claws' on the back of PTDN's lifter that I'm sure were added to get the self-righting to work.

Charles Guan's 'Equals Zero' website has an archive for PTDN that includes the design requirements for getting a 4-bar to self-right:

"[Self-righting] is kind of tricky with 4-bar lifters. You really have to take into account the center of gravity of the bot, and the length and extension of the arm, in order to facilitate this. Generally, 4-bar lifter bots flop onto their backs and come to rest on the arm whenever it is then deployed, as the CG is too far forward, and no self-righting is possible. [The classic video of former Battlebots heavyweight Biohazard shows how a 4-bar can self right.]

Notice how [BioHazard's] center of gravity is far enough back that the bot hinges on its rear edge and does not come to rest on the arm. The arm’s retraction then keeps the CG within the line drawn between the arm’s contact point and the bot’s rear edge, and it gathers enough momentum to push back over. Making it able to do this meant making the arm extend all the way back across the bot. Notice also how Biohazard had a ‘tang’ at the very back of the arm, a part that sticks up – this aids in the maneuver by making the contact point with the ground further forward, so the ‘line’ is longer.

This goal meant that I was continually watching the bot’s center of gravity in autodesk Inventor, and also continually modifying the linkage to suit. The arm had to have a certain amount of extension to make sure the CG was in the right place, and that extension had to jive with everything else’s placement."

The full text, with some pictures, is way down at the bottom of the Equals Zero archive for Pad Thai Doodle Noodle. When you scroll down to the part of the archive where Charles is doing a brake job on his van you're about half-way there. Keep going. WAY DOWN. Seriously.
Q: Hi Mark,

First of all, I know. You heavily advise against ring-spinners. However, there is one that catches my eye due to what seems to be a pretty solid design. This upcoming season of ABC's Battlebots will feature The Ringmaster. A Single-tooth ring spinner with the weapon system being driven via gears rather than friction wheel. You can see all how it all works on their facebook page.

I'm heavily considering using a similar drive design in a lightweight ring-spinner that i'd like to build (not a ripoff, I have standards.)

Now, you've almost completely crushed my dreams of building a ring-spinner. But if I would be wasting my time by building one with this type of drive system and ring support, I would indeed like to know.

For knowledge-sake, lets assume that I built a lightweight clone of The Ringmaster. Good RPM, Quick Spin up, "perfectly" Counter-balanced weapon, and so on. What deal-breaking issues or concerns do you think I'd most likely run into?

Thank you sir, David R. (Livermore, CA)

Quick addendum

I know that I am speculating that The Ringmaster works well even though we haven't even seen it in action, and we probably won't get a good idea of that until the new season airs. In fact, just about ALL of my question involved speculation. So I understand that nothing from the scenario I gave you is completely concrete. So upon your response, I'll be sure to have a pinch of salt ready. Thanks :)

A: [Mark J.] 'Crusher of Dreams' -- maybe I should put that on my résumé?

Where to start...

  • First: run a Google search for Ringmaster's team leader 'Hal Rucker'.
  • Second: examine your budget and technical skillset to see if they match Hal's.
  • Third: no-spoilers, but check in on the historical success of ring spinners.
  • Fourth: consider the wisdom of the Team Run Amok Design Philosophy:

Design Philosophy

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

Here's the problem with shell and ring spinners in general: the impact vector is lateral, parallel to the floor, and uncomfortably close to the center of mass of the robot. You hit your opponent and Newtonian physics throws you off in the opposite direction like a high-powered hockey puck. They are nearly as dangerous to themselves as to their opponents. A ring spinner has the advantage of being able to operate when inverted at the cost of much greater mechanical complexity. An extended center pole of the type used by lightweight champion shell spinner 'Ziggo' is a much simpler solution to this problem.

If you want to build a ring spinner as an example of 'machine porn' go ahead and do it for that reason -- and yes, gear drive it. Just don't expect it to dominate.

Quick addendum

1999: people watch BattleBots, see some frat boys build a robot out of a beer keg, say to themselves 'I can do that!', hundreds of teams do just that and combat robotics as a popular hobby takes off.

2016: people watch BattleBots, see robots that appear to be built by NASA contractors under DARPA funding, say to themselves 'I can't do that!' and combat robotics as a popular hobby dies.

Just a theory.

Q: Hello Aaron:
[pneumatic] cylinder has two holes: inlet and vent. As we know the speed of cylinder depend on air pressure and flow, So I got a idea: if we make cylinder have two "Inlet holes" the flow of the cylinder will be larger, so the cylinder speed will be faster? [Yunnan, China]

A: [Mark J.] A pneumatic system has multiple restriction points: the pressure tank valve, the pressure regulator, the solenoid control valves, and the actuator cylinder ports. Typically the speed of the system is limited by the performance of the regulator and valves. Gas flow at the actuator port is typically quite good; improving the flow there will not provide noticeably greater speed.

Q: do u have a diagram on how is actuated an axe like on shunt? can u explain how the linkage works? thx [Quebec, Canada]

A: [Mark J.] 'Shunt' uses a complex pneumatic powered multi-bar linkage to provide a full 180 degrees of axe motion. You can see Shunt's designer explaining and demonstrating that linkage in this video.

The diagram at top right is from Chris Hannold's book "Combat Robot Weapons". It shows a much simpler overhead axe/hammer weapon linkage.

Additional linkage options and a full explanation of pneumatic systems can be found at Team Da Vinci: Understanding Pneumatics.

Q: hi axe guy again in the picture u put, is the hammer able to do a 180 degree rotation?

A: No -- see the small diagram at right. When the axe retracts the axe bracket runs into the pneumatic actuator, and when the axe extends the actuator shaft runs into the axe pivot axle. It's only good for about 90 degrees.

You can play with the design and get a little bit more, but a simple single-pivot design like this is limited to an efficient swing of about 120 degrees. That's why Shunt's designer went to the complex linkage to get 180 degrees.

Q: axe guy again, do u have any diagram or image of the multi-bar linkage u were talking about?

A: The only images I have of Shunt's axe linkage are in the video link I provided above. I've sketched up an approximation of the linkage (right) taken from the video. A couple of the links are fused, so this is functionally a 4-bar linkage.

Note: I cannot recommend that a novice builder attempt to duplicate the 'Shunt' linkage or the other full-range linkage diagrammed on the Da Vinci Pneumatics page. Overhead axe weaponry is not effective in modern robot combat.

Q: is there a way for a featherweight lifter to be actuate by a motor with gear and chain, without any sprocket hanging out of the bot and without having it be a 4 bar lifter?
how could i make it to be like a rear hinged flipper? i dont want to have something like sewer snake... more like dantomkia.. but not gas operated
thx :) [Quebec, Canada]

A: [Mark J.] A long-armed rear-hinged featherweight lifter requires enormous torque to lift an opponent out at the far end of the arm. A short-armed featherweight lifter like 'Nyx' (pictured) can get away with a small sprocket on the lifter axle, but each time the length of the lifter arm doubles so also does the torque required to operate the lifter double. By the time the arm reaches all the way back to the rear of the robot the torque needed raises to gearbox-shattering levels unless a much larger sprocket is used on the lifter axle.

You can run the torque calculations to see for yourself how much torque is needed and shop around for a gearbox that can survive the load and provide the needed reduction ratio -- but you'll find that a suitable unit is heavy, bulky, and expensive. Four-bar mechanisms are popular for electric lifters because the torque requirement for a long lifter arm is greatly reduced. Perhaps you should reconsider your design.

Q: I just had a thought about how I could make the drum for my 60lb robot. My original plan was to machine the drum and drum tooth as one single piece of tool steel, but I've recently been considering forging a quarter-inch square bar of folded tamahagane (Japanese "Jewel" Steel, the material used in Japanese knives and swords) and welding it to a tool steel tube. Do you think this would be worth trying or should I just stick to my original plan? Or is the hamburger bad?

To note, the drum is going to be a simple counterbalanced single-tooth design either way (basically, two teeth but one is on the inside of the drum rather than the outside). [Arden, North Carolina]

A: [Mark J.] There are good reasons why most drum weapons have bolt-in steel impactors setting in pockets machined into a thick aluminum tube:

  • Impactor teeth take a beating. No matter what material is used the critical leading edge gets rounded off, they crack, and they deform. Throwing the whole drum away when this happens is expensive. Bolting in a new impactor makes better sense.
  • Correctly welding tool steel is a huge pain.
  • Welds break.
Tamahagane is a high carbon 'bloom' steel. Its performance in knives and swords comes from forging higher and lower carbon layers of tamahagane together to provide both resilience and strength. Blades made in this way were state of the art... 400 years ago. Even if you were able to obtain the different grades of tamahagane and correctly forge the multi-layered block via the intensive and laborious process, the performance of modern 'shock resistant' tool steel is much superior for your purpose.

I'd suggest building a conventional drum for your first attempt and saving the fancy stuff 'til after you've seen the challenges first-hand.

About balancing a single-tooth drum: I'm a little worried by your description of "two teeth but one is on the inside of the drum rather than the outside". That's like having two kids on a teeter-totter and moving one of them in toward the center -- they won't balance unless the kid moved toward the center is heavier than the kid that stays out on the end. I'm sure you had this figured out, but the next reader might be confused.

Q: Does a flamethrower count as an active weapon? [Quebec, Canada]

A: [Mark J.] In general, an active weapon is any weapon whose operation is controlled by a radio channel separate from those used to control the robot drivetrain. If your flamethrower meets that criteria it is 'active', however there may be additional constraints imposed by the event organizer.
  • Many events forbid the use of flame weapons entirely.
  • Some events require that an active weapon be 'effective' - a condition I have never seen a combat robot flame weapon meet.
Check with the organizer of the event you plan to enter for an official ruling, and please be careful with flame weapons.

Q: What do you think is the best way to get an impact slip on a drum spinner? So far we have two different pulleys that we can chose from, one with very shallow teeth and the other with no teeth. We are going to use a timing belt, and one thing to consider is that our pulley is mounted directly on the motor while having no support, so it can't take a whole lot of load. [North Kansas City, Missouri]

A: [Mark J.] Slippy belt drives vary a bit with weight class.

  • Big 'bots use v-belts that are easy to adjust for the degree of slip you want.
  • Insect classes tend to use timing belts or round (o-ring) belts. The round belts absorb shock and are easier to set-up for slip.
  • The in-between classes run all sorts of belts. I've seen flat belts, regular and inside-out timing belts, small v-belts, chains -- everything.
I don't know the weight class of your robot, the mass of the drum spinner, or the power/speed of your weapon motor. In general, I like the idea of a shallow-toothed pulley with a timing belt for a sub-light weapon. Set it up with enough slack that it can slip without too much load on the motor pulley. Specifics depend on the details of your weapon construction.
Q: Good Evening, Mr. Mark Joerger

Could you explain why Stinger's Mace is able to blow down whenever Scott inverted the movement?
Thank you! [New Jersey]

A: [Mark J.] I could, and I have. Stinger's weapon design is a 'torque-reaction hammer'. There is an explanation of torque-reaction hammers down the page in this archive.

Q:Hello! Mark
  Why isn't Stinger's mace considered as movable weapon, which was every competitor must have in RW series 7?
  If Scott used the "pizza cutter" did it still count?
Thank you!
Leo [Maryland]

A: [Mark J.] I'm willing to answer questions about robot design that use a Robot Wars competitor as an example, but Ask Aaron does not answer questions specific to UK Robot Wars events or competitors. The Ask Aaron Frequently Asked Questions #37 explains:

Q: Why are you no longer accepting questions about UK Robot Wars events or competitors?

A: Mark J. here: for many years Aaron and I did our best to answer any and all questions on the broad topic of robot combat. A few years ago we were flooded with a large volume of trivial UK Robot Wars questions that were well outside the mission and focus of this website. 'Ask Aaron' is not a 'fanboy' site and we are not interested in investing our time in this area. We regret having to cut off all UK Robot Wars questions, but the fanboys proved themselves to be both persistent and quite rude.

Q: I'm curious why the "rule of thumb" for spinners uses energy storage and not momentum storage. Because the acceleration/deceleration frame is so short momentum is conserved and not energy. As a side benefit, builders may see improvement by not optimizing for speed so much (get better bite) I've seen lots of matches of drum spinners ineffectually rolling against the opponent. It would also mean you could reduce the speed and get much better spin up time.

As a sidenote, I asked about gyroscopes earlier and want to thank you for the help. I used some of the info in a school project and it went really well: autonomous rickshaw walker (video) [Ontario, Canada]

A: [Mark J.] The rickshaw walker video is great -- thanks for sending the link!

I've deleted an extended exchange between myself and 'Ontario' wherein I inadequately answered his question, needlessly confused the issue, was in part simply incorrect, and generally took us down the rabbit hole. I'm taking a mulligan on this one and replacing the confounding original exchange with a brief note and an external link that succinctly answers the question. I'm embarrassed, but I'm willing to admit that I screwed this one up. My apologies to Ontario.

Here's a link that gives an example of the differences between momentum and kinetic energy plus a 'thought experiment' that clearly shows why robot spinner weapon design is based on energy storage: Kinetic Energy is NOT Momentum.

If the link doesn't adequately answer your question, you may wish to pursue the question on a physics forum. Ask Aaron provides tools and assistance in the design of combat robots, but a deep diversion into physics theory is outside our scope.

Q: Thanks for the help, despite mistakes being made on both sides, it was a really interesting discussion to have and I still learned quite a bit (damn moment of inertia and angular velocity messing with units). You do great work and discussions like this are why I love this sport. Keep it up.

A: Thank you, Ontario. Next time we'll take the discussion off-line until we get an answer that can be posted.

Q: In Featherweight robots, would a hydraulic system for a horizontal crusher (which works by pinning the opponent and then crushing their undersides) be feasible without compromising drivetrain or the wedges ? [Jawa Barat, Indonesia]

A: [Mark J.] In any weight class it's difficult to build a weapon and chassis structurally strong enough to survive the forces necessary to pierce or crush an opponent. It requires serious compromises of all the other systems of your robot in favor of the weapon. Most 'crusher' weapons turn out to be nothing more than slow and unreliable 'clampers'.

Search this archive for "hydraulic crusher" to find several previous posts on this topic.

Q: Hi. Is it at all a good idea to have a drum spinner with an 1/4" steel plate mounted at 45 degrees for a tooth? The diameter of the spinner is 2", width is 5", and the bot weight is 15lbs. I don't like it, but my team thinks it's a good design. Is there a scientific way you could convince them to change it, and to what?

Also now we are thinking about having a sort of drum with the center cut out, so basically there's two spinners. That a good idea either? I've sent sketches of the drums.

Thanks [Missouri]

A: The drawings help -- it's clear what you're thinking about. I had , and I got the wrong idea from your description. I've added arrows showing the presumed direction of rotation to your drawing.

There are good reasons why drum weapons look the way they do. Very clever builders have been tweaking the design for twenty years, but you can't cheat physics. Most of that is covered in the Ask Aaron Spinning Weapon FAQ so I won't repeat it all here. Let's concentrate on the unusual aspects of your designs.

The 45 degree Impactor: the drum certainly looks tempting with those up-swept impactors, but there are reasons to use more conventional 'straight' impactors:

  • To stick out a given distance from the drum an angled tooth has to be longer (heavier) than a straight tooth. That means that a straight tooth could be made thicker (stronger) and still weigh the same as a longer angled tooth.
  • Angling the tooth exposes the 'face' of the tooth to possible impact. If the face impacts the opponent it will hit at an angle that will push the opponent away rather than digging in and launching. This is a particular problem when the weapon spins very fast and forward speed is low. If you do angle the tooth, you'll need to grind away the face so that the leading edge of the tooth will always strike the opponent -- never the face. Unfortunately, that may weaken the impact area.
  • The angled 'pocket' needed to seat the tooth into the drum is a much nastier bit of machining than the simple pocket needed for a straight tooth. It's a lot of extra work for no real advantage.
  • A seldom considered factor, the aerodynamic drag of a tooth sticking out of a drum running at a few thousand RPM is significant. Adding to that drag by tipping the tooth into the airstream is only going to increase that drag, slowing the drum speed, and reducing the energy storage of the weapon.
The design has little if any advantage and several very real problems. I'd ditch it.

The 'Dumbbell' Drum: another 'looks cool' design, but there are reasons to avoid it:

  • Leaving out the center of the drum reduces the mass and rotational inertia of the weapon. Not good. You can use that weight savings to add to the diameter of the remaining sections of the drum and gain some additional rotational inertia, but if you leave the diameter the same you're losing weapon capacity.
  • Two small drums on a single axle will place a great load on the smaller-diameter axle when one drum strikes the opponent and slows while the momentum of the other drum tries to keep spinning. That makes for a weak point in the design that is prone to failure.
Same story as the angled tooth -- you aren't gaining anything and you're taking on new troubles.

Q: Hello! Would you please tell me how to intensify the instant power of a pneumatic flipper? Will extra gas bottle work? Thank you! [Maryland]

A: It's all about gas flow. When you trigger the flipper you need the gas to flow as quickly as possible from the pressure tank into the actuator, but there are lots of things in your pneumatic system that can slow down that flow.

  • Pressure regulator - selection of a 'high flow' pressure regulator is critical. Look for a regulator with a highest flow coefficient (Cv) you can find.
  • Valves - like regulators, actuator valves have flow coefficients that indicate how freely gas flows thru them. Higher is better.
  • Actuator ports - your pneumatic actuator has 'ports' -- openings for the gas to enter and leave the cylinder. Larger ports equal greater gas flow.
  • Hoses and fittings - these convey pressurized gas between your components. If the hoses are small and the fittings are restrictive the gas flow suffers.
It's important to note that the most restrictive element of your pneumatic system will be the limiting factor in system performance. It does no good to have a super high flow regulator if the actuator valve can't keep up. If everything isn't flowing well, the system won't 'pop'.

Adding an additional or larger gas bottle will give you a larger number of system actuations, but will not improve the 'pop'.

If you're determined to add an additional tank, read up on 'buffer tanks' at the Team Da Vinci Pneumatics page. A buffer tank can help overcome a system 'bottleneck' caused by a poorly performing pressure regulator, but it won't help if your other components are the problem.

Date marker: March 2016
Q: hi, i am having problem in entering details for spinner calculation entry.drum size (outer dia 9.5 cm, inner dia=5 cm , length=20cm). 2 tooth (2.5 cm, thicness =1.5) [Delhi, India]

A: [Mark J.] -- Click here

Q: Hello sir.please direct me to the archive where it tells me the calculations required to make single tooth to find the center of mass.i also want to know if turnigy rotomax 150cc is a good weapon motor for a drum in 40kg catogary [Chennai, Tamil Nadu, India]

A: [Mark J.] Click here.

One hour later...

Q: Is first CIM motor ok for 15kg robowars to run 4kg drum 120mm dia.its for a safe event called evvaa cup [Chennai, Tamil Nadu, India]

A: The purported safety of a single Indian event is not relevant. Read the "click here" again -- particularly the last sentence:

Until there is a significant and universal change in Indian arena safety and/or rules to control dangerous weapons, I can no longer in good conscience accept questions from the region.

One day later...

Q: Hello sir pls answer my previously asked question. I promise its for a safe event and im a school student studying 8th grade and my parents wont allow me to play on unsafe arenas [Chennai, Tamil Nadu, India]

A: I'm genuinely sorry, but until rules are in place to make all Indian arenas safe my conscience prohibits me from answering any combat robot questions from builders competing in India. People are being seriously injured at Indian combat events and I will not be part of that continuing problem.

You are welcome to search thru the Ask Aaron archives and the Spinner Weapon FAQ for assistance in the design of your robot, but I will accept no new questions from you or your fellow competitors.

Five days later...

Q: Sir please give an idea for mounting a pulley on rs 775 and if i ratio the speed of 19500 rpm will i get my torque incresed and how to drive the drum with it.sir please help.i beg you. [Chennai, Tamil Nadu, India]

A: I admire persistence, but I will not dishonor the purpose and spirit of Ask Aaron by continuing to answer questions from a region with such horrible safety control at so many robot combat events. Do not ask me again.

The questions you ask have all been previously asked and answered here at Ask Aaron. The answers and tools you need are in the links I provided in your previous request. If you'd been searching the archives instead of begging me for answers, you'd already have the knowledge you seek.

Q: Hey Aaron.
I am trying to build a single teeth bot and I have managed to balance the weight now the problem is should I use an amp flow motor or the starter motor which has considerably high torque than ampflow and I will be connecting 2 starter motors the one which are used in cars .. And it would be really helpful if u could sent me the formulas for energy storage calculations for single teeth wepon .. The design is similar to that of witch doctor from international robowars plz help me out [Quantil Inc - Pasadena, California]

A: [Mark J.] Why would you seek advice from someone you believe to be so stupid as to fall for this sham? The sentence structure, abbreviations, grammar, punctuation, and use of starter motors is not California - you're a conniving Indian builder from Bangalore using a SoCal proxy to sneak a question in thru the back door.

You know full well why I don't answer questions from builders competing in India and you've read how strongly I feel about this issue. In spite of this you disrespect me, 'Ask Aaron', your countrymen, and the sport of combat robotics by attempting to gain personal advantage over your competitors thru deceit.

Stop trying to build more powerful weaponry and start trying to build safer combat arenas, Julab!

Q: I'm the beetle spinner builder again [from Oregon]. My design is coming along alright, right now I just need some assistance on some of the most critical aspects of the design: the weapon shaft, attachment method, and reduction method.

I was originally going to use a Fingertech Blade hub to mount my weapon to the weapon shaft, but I had two major problems with that:

1: The hubs are notorious for loosing up and coming off
2: The maximum drill out size for the bore is 1/4"

The first problem I can work with, as Fingertech are redoing the design entirely, but the second part is a big problem. My original weapon of 6"x2"x.125" 4130 steel has become 10"x2"x.25" 4130 steel, running off of a 2:1 ratio of my motor(Maximum RPM of the motor is 21460rpm). While max speed of the weapon will be theoretically about 10100rpm(A whopping 2000 joules of energy storage in a 3lb robot), I really don't expect more than the mid end of 8000rpm(The less insane but still very high energy storage of 1200 joules) due to air resistance. With that much energy being thrown around, I don't expect a 1/4" shaft of kind to survive the sheer force. I know to make it as thick as I can while still being in weight, but I'm wondering if a .375" grade 8 bolt is enough handle the forces, or should I go up to .5" shaft for the weapon? I know bigger is better, but I'm on a very tight weight budget with so much of the bot dedicated to the weapon, and anywhere that I can save as much weight as possible, the better (Then again, the weapon shaft may be the last place I want to steal weight from).

The second issue is getting the weapon to stay on. Because my shaft needs to be bigger to handle the forces required, I was looking at Servocity hubs(The .770" bolt pattern ones to be specific)and bolt on the weapon, but I'm pretty sure if the screws didn't shear themselves first, the hub would snap like a cracker. I was wondering if I could take some washers(Like NORD-LOCK washers) and clamp tightly down on the blade with a pair of shaft collars like Hazard did. If I have to get the weapon mount custom machined for me, what would be the absolute best way for it to stay on the hub, and the hub stay on the shaft, no matter what? I feel like a weapon that under performs is still usable, but a weapon that flies off is worse than useless.

A: [Mark J.] I've never been able to figure out how 'Hazard' managed to get enough clamping force from the shaft collars to adequately hold the weapon blade in place. Tony B. got it to work, but I can't tell you how he did it. I also can't recommend a keyed shaft for a bar spinner -- too much localized stress around the key. I think the optimum solution is a keyless lock bushing that will position and clamp your weapon bar to the shaft. Shop around a bit to find one that meets your design needs.

I do not recommend bolts - grade 8 or otherwise - for weapon shafts. Bolts are designed with the metallurgy to survive high tension loadings, not the shear loading you would expose them to as a weapon shaft. Bolts should always be protected from shear loads, either thru the design of the elements they are holding or by insertion of hardened pins that will take the loading before it is transferred to the bolts. Use a shaft suited to the type of load it will encounter.

You didn't ask about the wisdom of spinning a bar weapon that accounts for 45% of the robot's weight at 8500 RPM, but I think I should warn you of a few things.

1) You didn't mention what specific weapon motor you plan to use. I'll assume it's brushless. Brushless motors put out enormous power for their weight, but they do not like to be bogged down spinning up a heavy weapon connected via a small reduction ratio. Also, pushing against aerodynamic drag that drops the free RPM of the motor by a very conservative 15% will likely place a continuous load and amp draw on the motor outside its design parameters. Combined result - you're gonna melt your weapon motor, and quickly.

2) There is a design issue about weapon 'bite' that I didn't put in the Ask Aaron Spinner Weapon FAQ because it's a trigonometry issue that's difficult to explain, but I'll give it a shot. Let's call it 'bite angle':

The larger the radius of the weapon, the shallower its impact angle will be for a given bite depth.

You can see in the diagram that for any given amount of bite depth a larger diameter weapon will impact at a much shallower angle than a smaller diameter weapon -- causing a more 'glancing' blow and decreasing energy transfer into the target. A larger diameter weapon typically more than makes up for this by spinning at a slower speed while carrying the same energy as the smaller weapon that spins faster -- giving a greater bite depth.

Bottom line: high RPM hurts the effectiveness of a large diameter weapon more than it would hurt a smaller diameter weapon. The large weapon is good, but slow it down and you'll get a much better hit.

Q: Last thing is getting the energy from the motor to the weapon. I was thinking of the Fingertech timing belts, but I feel like the forces the weapon would create would snap the belt(I've seen so many Last Rites clones on Youtube fail because the belt/chain failed). I also feel like O-rings would also snap on start up, so they're out. Should I move up to the wider 1/4" MXL belts, go with XL belts, or should I go for chain and sprocket, like the ones found on Servocity(Assuming the above problems aren't a problem for them, which I have a good feeling that they might be)? Obviously, this risks the weapon motor and its associated electronics, but I want to make sure that if weapon motor/ESC does die, its after delivering a huge hit that KO's my opponent.

A: You haven't told me about your motor torque. Drive belts are rated by torque and RPM and you've only given me the last half of that info. All I can suggest is that you look at the drives used by weapons in heavier weight classes that transmit a comparable level of torque at comparable RPM.

Q: Also, one last thing: How do you tell when a weapon bar is on its last legs and should be replaced before it snaps in half?

A: 'Taint easy. Generally a bar looks just fine right up to when it snaps. You may get lucky and detect a small change in the sound it makes when you strike it and let it 'ring', but that isn't a reliable test. The 'real' method to detect early signs of trouble is a magnetic dye penetration test. It takes some time, it isn't cheap, and it's not a perfect indicator. My recommendation is to carry a spare blade and hope for the best.

Q: Beetle spinner guy again, thanks for the help. Those keyless bushings you linked to seem like they would be perfect method to mount my weapon. They're a bit expensive, but for the weapon hub I think I should spend the extra bucks.

I completely forgot that motor torque is kinda of a big deal in transmission selection. The motor is question is Turnigy D3536/5 1450KV motor at 14.8 volts, a fair bit bigger than what most beetles run. I'm also thinking that I should bump up the gear ratio of the weapon from 2:1 to 4:1. Even if its not Tombstone ratios of energy storage, 500 or so joules of storage should more than sufficient for a beetle. Plus, if I want more energy, I can always change my weapons shape.

A: I think those are fine decisions.

The keyless bushing is an under-appreciated hub option. Shop around a bit -- there are different manufacturers and different designs that might click with your ideas, and might save you a few bucks a well.

Yea, that motor's a 'fair bit bigger' than the standard beetle weapon motor alright. The little bugger pumps out close to a full horsepower! Start-up torque is always a question mark with sensorless brushless motors -- a lot depends on the controller software. I think we can safely guess at around 300 oz-in of torque. Yes, I think 500 joules at around 5000 RPM will be VERY impressive in a beetle. You'll have a quick spin-up, good bite, and moderate motor loading.

In commercial usage, the XL timing belts are used for up to about 160 oz-in of torque and 4000 RPM. Combat robots overstress everything, so I wouldn't hesitate to use an XL belt in this application. I'd suggest the 3/8" width and the largest pulleys that work for your design and selected ratio -- larger pulleys reduce the effective torque transmitted by the belt. Be extra careful with pulley alignment; at the speed you're running alignment is critical.

It's going on ten years since I wrote the Team Run Amok Spinner Weapon Excel Spreadsheet. There have been several upgrades over the years (battery capacity guidance, english/metric conversion calculator, brushless motor torque estimator...) but it needed an update in the recommendations for energy capacity and spin-up time, as well as new default specs for the 'example' weapon.

Version 1.6 is now available for download: Team Run Amok Spinner Weapon Excel Spreadsheet

Q: Hi, Mark. Team WhoopAss's two flippers, HexaDecimator and Hexy jr, seemed quite different from some orthodox US flippers, such as Intertia Labs' flippers in many aspects. Do you think the two flippers should be classified as orthodox US flippers? What do you think is the edge of the two flippers? [Jiangsu, China]

A: [Mark J.] The only unusual element in the design of the Team WhoopAss flippers is the forward placement of the flipper hinge. A conventional flipper design has the hinge point as far to the rear of the chassis and as high as possible in order to maintain a constant mechanical advantage for the pneumatic actuator throughout its range of motion. Placing the hinge so far forward reduces the effective power of the weapon, but does have the advantage of allowing the actuator to be placed low and flat in the chassis -- lowering the height of the robot.

As is so often the case, the weapon is not what made the Team WhoopAss robots successful. Although not nearly as powerful as the flippers from Inertia Labs, the relatively modest weapons could be employed effectively because the rest of the robot was well designed and well driven.

Q: I am currently building my first combat robot. It's a featherweight bar spinner. I just have a few questions about motor and gear selection after using the spinning weapon spreadsheet.

My favorite option so far is using a geared AmpFlow E30-150. This geared motor has a no-load RPM of 670 and a stall-torque of 360 in-lbs. I plan on using a single stage pulley around 2.5:1 to bring the RPM up to the 1600-1700 range. My weapon is going to be approximately 16 inches long and weigh about a quarter of the robot's weight. Using the spreadsheet, the weapon will spin up in about 2 seconds with a tip speed of around 80 mph and store 880 joules of energy.

Is this a good design for what I hope to be a hard-hitting featherweight? What would be an ideal RPM for this weapon/weight class? Are there any other geared motors that you know of which would be better for this application? Any advice is appreciated! [Albany, Oregon]

A: [Mark J.] Several suggestions:

  • The AmpFlow E30-150 is a fine weapon motor, but consider how you plan to use it. You're attaching it to an expensive, heavy, and power-wasting gearbox that takes the RPM down too low, and then you're running it thru a belt drive to speed it back up. Spinner weapons typically use a single belt drive reduction to drop the motor speed and increase the torque to the weapon. Cheaper, lighter, more efficient, and less to go wrong.

  • The AmpFlow is a brushed motor: durable, easy to control, high in torque, and difficult to screw up. Brushless motors are currently very popular for spinner drives -- they have more power per pound than brushed motors, but they are also less durable, tricky to control, low in torque, and very easy to screw up. As this is your first robot, I think the brushed AmpFlow is a fine choice for a weapon motor.

  • Your energy storage is too low for a modern feather bar spinner. Trash the gearbox and run a 2:1 reduction belt drive to your bar. That gives you 2800 RPM and 1750 joules in 4 seconds -- 770 joules in the first 1.3 seconds. Much better.

  • Your remaining problem is stuffing that E30-150 down low enough to get an effective spinner height -- or is this a vertical spinner?
Have some fun with it, tear up a few opponents, and learn a whole bunch. Remember: a combat robot is more than just a weapon. Don't put all your effort into the weapon at the expense of the rest of the 'bot.

Q: Hey Mark, thanks a lot for the advice. I'm happy to get to trash the gear box. I guess my only concern was the weapon storing too much energy for the size of the robot (based on the 19 J/lb. mentioned in the spreadsheet).

I plan on matching the motor with a Talon SRX ESC and angling my bar so that it hits low while the motor can be mounted up higher (if this configuration raises any red flags, please let me know).

Other than that, my main concern is this: I have a half inch cold-drawn steel (C1018) shaft with compatible mounted bearings. My ever-changing CAD model has the shaft length around 5-6 inches. Would this shaft be sufficient for my weapon setup? Or should I spend a little more on going up to 3/4" or 1"?

A: The Weapon Spreadsheet was written about ten years ago -- back when weapons were much 'kinder and gentler' than the current standard. Around 60 joules per pound is entirely in line, particularly given your relatively low weapon speed.

Angled bar spinners were once fairly common -- I recall Chris Hannold's 'Six Million Dollar Mouse' at Robot Wars Extreme Warriors. The design has no serious flaws and solves a number of design problems. I haven't seen one built in quite some time, and it may come as a surprise to your opponents.

The Talon SRX has tested very well in similar applications. It should be fine -- just don't try a high-speed reverse of the weapon when it's spinning!

Your 1/2" shaft is likely too small for a weapon with this much power and that great a distance between supports. I don't have the details of your weapon construction, but I'd spend some of that money you saved by scrapping the gearbox on a 3/4" shaft and bearings.

Q: Hi Aaron. I want to know what Ziggy`s pneumatic pressure use, the Store pressure and the Regulator pressure? Ziggy builder's website can not open [Yunnan, China]

A: [Mark J.] There are descriptions of Ziggy's weapon system elsewhere in this archive.

Quick summary:

  • Gas of choice: high pressure air or nitrogen
  • Storage pressure: 3000 psi (~200 bar)
  • Regulated pressure: unregulated [correction -- see below]
Testing and operating such a weapon system is  EXTREMELY DANGEROUS  -- Do Not Attempt to construct a similar weapon without extensive experience in pneumatics and mechanical design.

Q: Hi Mark, I noticed it in the recent post that the Ziggy use a unregulated system which means the pressure in actuator is 3000psi, same as the pressure in the tank. However, I believe the Robogames rules limit the pressure at 250psi, Ziggy seems violated the rules. Is there some reason or just I made a wrong judgment about the pressure in the actuator is same as it in the tank? [Guangdong, China]

A: [Mark J.] Hmmm...

I had assumed that 'Ziggy' received an 'event organizer exception' for a high-pressure system, as allowed under the rules. The standard photos of 'Ziggy don't show a pressure regulator, but just to make sure I went thru my photo archives for a picture from a different angle. Guess what I found - a pressure regulator! According to my sources, that is a PR-59 GO Regulator.

So, I stand corrected. It appears that 'Ziggy' ran a 250 psi regulated pneumatic system -- at least some of the time. That amount of pressure does not mesh with the team's claim that their weapon put out 14,000 pounds of flipping force. You just can't get that much out of their actuator on 250 psi. My warning about the danger of a powerful pneumatic weapon like this still holds.

Q: Mark,
I came across this video of a spring powered flipper and it inspired me. I have been wanting to build a flipper for a long time but my budget wouldn't allow anything bigger than a beetle and ant/beetle weight pneumatics aren't quite up to combat specs. My question is how would you determine the torque needed to turn the cam in order to load the flipper? Thanks [Cincinnati, Ohio]

A: [Mark J.] Oh my! The concept is sound, but the components and layout of the mechanism in the video are horribly designed. The motor torque requirement is constantly varying as the oddly shaped lifting cam rotates. The motor visibly slows at one torque peak. In order to minimize the motor torque needed to load the flipper:

  • The spiral 'snail cam' driven by the motor must have a continuous and gradual lift.
  • The spring should be given a straight pull -- none of that 'bend in the middle' crud that requires calculus to figure out the non-linear change in spring tension.
  • Assuming a straight spring pull, the profile of the spiral cam must be a parabolic spiral to correctly adjust the pull rate to the increasing load imposed by the spring as it is extended (see Hooke's law).
I've given you just a start on how the calculation might be done, but it's almost midnight and I have to get some sleep. Let me think about this for a day to see if I can sort out the equations.

Q: Mark,
Spring powered flipper guy again. Would this be a better layout for the components?

A: Yes! Much better -- a proper snail cam and a nice straight spring extension. I'm not certain that the cam profile is entirely correct to provide an even loading on the motor, but the general layout is very nice.

Q: Also would using conservation of energy be a good approximation (i.e. rotational work converted into spring energy)?

A: Your question beat my update to the prior post. Calculating the rotational work needed to arm the spring and backing into the required torque is the path I decided would be best. Assuming that you get a straight pull on the spring and get the spiral cam correctly profiled to even out the torque requirement to a constant level, we can derive the required torque from the rotational work formula:

Work = Torque × Angle Thru Which the Axle Rotates [in radians]

In our case the work is the extension of the spring, which takes place over the course of one revolution of the axle. The spring extension force is a linear function (Hooke's law), so we can calculate the average force required to extend the spring as follows:

Average Spring Force = (Spring Force at Start + Spring Force at End) ÷ 2

That plugs into the basic linear work equation [Work = Force × Distance] like this:

Work = Average Spring Force × Distance Extended

Getting close now. One full rotation is 2𝜋 radians, so with a little algebra the rotational work formula transposes to our needed torque equation:

Torque = (Average Spring Force × Distance Extended) ÷ 2𝜋

Example: if you're extending a spring with a rate of 200 ounces per inch from its rest state (zero force) to one inch of extension with a parabolic snail cam over one full rotation, the torque required will be:

Torque = (((0 oz + 200 oz) ÷ 2) × 1 inch) ÷ 2𝜋 = 100 oz-in ÷ 6.283 = 15.9 oz-in

There are some losses due to angular inefficiencies and rubbing friction with the cam, and you will want the motor to deliver that level of torque at some reasonable motor speed to extend the spring quickly. I'd use a gearmotor with a stall torque about three times the formula result to avoid bogging and let the motor operate up near its horsepower peak.

Addendum: There is some discussion out in the forums about the amount of force required for an effective spring flipper. There are too many variables (force, stroke, geometry, angle, expectations, ect.) to give a good calculated result to that question. Further, the dynamics of a spring flipper are different than those for a pneumatic flipper and cannot be directly compared. I'd suggest mocking up the weapon geometry and trying differing spring rates on a proxy opponent to find a result you like.

Spring Powered Flipper Weapons

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

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

Ask Aaron: Four Spring Flipper Mechanisms

Q: Sewer Snake [Hebei, China]

A: [Mark J.] Well, art is art, isn't it? Still, on the other hand, water is water. And east is east and west is west, and if you take cranberries and stew them like applesauce, they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know. 1

Q: I think this compatriot from Hebei don't understand how the weapon of Sewer Snake works. He asked this question in Baidu Tieba and it was our administrator that recommend Ask Aaron. He is still confused with it because you explained it in English. Would you please draw a design diagram to help this guy, Mark?

By the way,there will be a robot combat competion held in China. That is why there are so many Chinese that ask questions. I am afraid that it will be a competition between institutions of higher in China. It is said that it will be broadcast on TV in the third quarter next year to schedule. [Jiangsu, China]

A: There are two nicely detailed photos plus a full description of Sewer Snake's weapon written by the builder himself in this post. I can't do much better than that!

I think the best thing I can do is repeat the advice I give to all first-time robot builders -- Keep it Simple! Adding a complex weapon system will multiply the number of things that can fail and take you out of the tournament. See Frequently Asked Questions #8.

I'd appreciate updates on the Chinese combat robot tournament -- could be interesting!

  1 Groucho Marx, 1930

Q: i am building a 15 pound robot for bots IQ and i am using a beater bar as my weapon. i would like to know what is the best way to make it a dead shaft [Manchester, Connecticut]

A: [Mark J.] If you want a non-rotating (dead) shaft for a rotary weapon the bearings must be mounted to the weapon itself rather than the chassis. A 'beater bar' weapon typically does not have a lot of room to mount a bearing without excessively weakening the structure of the bar.

A common solution is to use needle roller bearings that add very little to the diameter of the supported shaft. This is the design used in the beater for the beetleweight 'Weta' kits (photo at right). Additional detail on the Weta beater bar can be found here.

Q: Hey Mark, quick question:

When two horizontal spinners meet weapon to weapon, which one wins? [Woodburn, Oregon]

A: [Mark J.] Generally, the one with the greater tip speed has the advantage.

Tip Speed (MPH) = RPM × Pi × Weapon Diameter (inches) × 0.000947

Q: Hi, Aaron, I am from Chinese, I wonder the statistics of the motor of the Tombstone, and the speed of the weapon! [Heilongjiang, China]

A: [Mark J.] At ABC BattleBots season 1, Tombstone's weapon was powered by an Motenergy ME0708 Motor (also called 'ETEK-R') at 59 volts. The motor produces nearly 15,000 in-oz of torque at stall with a peak output just over 16 horsepower. Speed is around 4000 RPM. The bar weapon is driven by chain and sprocket without speed reduction.

Previous versions of heavyweight bar spinners from Hardcore Robotics have used the mighty 'Perm PMG-132' motor which produces close to 26 horsepower and 3000 RPM at 59 volts. Hardcore currently uses the ETEK-R because they have found it to be more durable in combat. Given the price of the PMG-132 it's entirely understandable that durability is a key consideration.

Q: Hi I'm participating in robo games though I'm from India can u pls tell me any energy calculation software or the calculations required to design the best drum in a 75kg category robot.what motors do u smudges [suggest?] the for drive and weapon [Tamil Nadu, India]

A: [Mark J.] I'm very puzzled by recent questions from India. Safety concerns prevent 'Ask Aaron' from accepting new questions from builders competing in Indian combat robot events (click here for details) -- but the questions being asked are ones that have been answered many, many times here.

If you're not willing to spend five minutes searching the Ask Aaron Archives and FAQs for answers to these very common questions, I don't think you have the right mind set to build a combat robot.

I'll make it really easy for you this time -- Start Here.

Q: What is the minimum energy for a drum for thrashing a 75kg robot?and what is the length of teeth I should use [Tamil Nadu, India]

A: [Mark J.] Two things:

  1. We request that you search the Ask Aaron Archives before submitting a question to see if it has already been answered. Weapon energy requirements and correct tooth length have been discussed here multiple times.

  2. As noted in the submittal box into which you typed your question, due to safety concerns Ask Aaron is not currently accepting questions from builders competing in Indian robot combat events -- click here.

Q: I've designed and built a featherweight combat robot with a drum weapon.

The drum is mild steel, 75mm diameter with a 12mm wall thickness and 160mm long. End plates are 10mm aluminum and the full-length live shaft is 22mm aluminum [alloy unknown]. The weapon motor is an Ampflow E30-400 with a gear ratio of 0.6:1. Drum energy storage is 1100 joules.

I'm concerned that my aluminum shaft will not be able to withstand the impact of the drum and that it will crack and break. Would you advise that I keep it the same or change it. I only have 300gms left to add in the robots weight.

Please reply soon as the event is in 3 days. [Eastern Hemisphere]

A: [Mark J.] Three days 'til your event? Since you don't know what alloy your weapon shaft is, we'll have to resort to testing.

  • Find a safe testing environment.
  • Spin up your weapon and charge at full speed into an immoveable object.
  • Spin back up and hit it again.
  • If the weapon survives you're good to go.
  • If the shaft bends/breaks you've got three days to repair and
I know builders who throw their new 'bots off the roof to see how well they survive a good impact. Better to break your bot now than in the tournament. I suspect that a 22 mm shaft is adequate for an 1100 joule weapon, but I don't know the details of how your shaft is supported. Test it and find out.

P.S. - You didn't ask, but your drum design needs work. Read thru this archive for pointers on drum design and discussions about 'bite' before you build your next robot.


A: [Mark J.] Click here.

Q: Why doesn't Manta's flipper work well? [Chevy Chase, Maryland]

A: [Mark J.] Are you talking about the 'Manta' with these achievements?

  • Fighting Robots European Championships 2013 Runner-Up
  • Fighting Robots UK Championships 2013 Runner-Up
  • Robot Wars Winter Tour 2013 Champion
  • 3rd in Fighting Robots UK Championships 2014
  • Robots Live! - Whitwick 2015 Winner
  • 4th in Robot Wars World Championships 2015
Seems like a fine robot to me. Nothing wrong with their flipper.
Q: Hi, Mark, I want to ensure that will it be safe to use a 1 MPa ram in a 7 MPa pneumatic system? If not, what kind of ram the competitors are using? I didn't find any ram is designed to hold such pressure. [Guangdong, China]

A: [Mark J.] 

Running 7 MPa pressure (70 Bar, 1015 PSI) with a 1 MPa rated ram turns it into a bomb -- metal shards embedded in the walls and in anyone so unwise to be present. Don't even consider such an action! Your 'competitors' are most certainly not using 1 MPa rams and valves at 7 MPa.

'Ask Aaron' does not offer advice on the construction of pneumatic systems exceeding 10 Bar pressure. 'Full pressure' pneumatic systems are extremely dangerous for inexperienced builders. Even with the correct components a small mistake can be fatal. There are components that can operate at 70 Bar, but for safety reasons I'm not going to provide info on where to find them. By the time you're experienced enough to know how to use them you won't have to ask me where to get them.

Read the Team Da Vinci pneumatics guide for information on combat robot pneumatic systems.

Q: Sir I want to know which motor is good for rotating an 2-3 kg weapon? [India, masked as a SoCal ISP]

A: [Mark J.] Click here.

Q: hello sir, i m looking forward towards a vertical spinner. I just want to know which is the better option to go for, smthing like "Electric bogaloo" or "the witch doctor". What are the +ve and -ve aspects of these bots??? please help me out. [India]

A: [Mark J.] Click here.

Q: In the archives drum weapons are mentioned numerous times but no where does it actually [show how to] attach the drum to the [live] weapon shaft. Could you explain that to me? [Greenville, Pennsylvania]

A: [Mark J.] Live shafts are not common for drum weapons. A non-rotating dead shaft can be made into a structural member of the chassis to strengthen the weapon mounting area. That's a major plus.

If you have a good reason to use a live shaft, the method of attaching the drum to the shaft is similar to attaching drive pulleys or wheel hubs to shafts:

  • Heavier 'bots with shafts large enough to be broached for a keyway can make good use of that technique to lock the drum and pulley to the shaft.
  • Smaller 'bots can attach the belt pulley to the drum with screws and use the pulley hub to clamp to the shaft. If using a set screw hub, be sure to file a flat on the shaft and use a threadlocking compound to keep the screw tight!
I think that in general you'll do better with a dead shaft.

Q: Second, how in an insect class could you make a weapon with a dead shaft? Every way I can think to configure a dead shaft requires an unusablely large weapon hub/reduction pulley to attach to.

A: What do you consider to be 'unusably large'? Take a look at the beater-bar weapon made for the 'Weta1' beetleweight kit. The weapon rides on small outer diameter needle bearings to reduce the size of the bearing support, and the belt pulley is attached directly to the weapon.

It's also possible to eliminate the belt drive and embed a small outrunner motor into the weapon drum with the entire drum/rotor assembly riding on a dead shaft. I'm not a big fan of direct-driving a spinner, but the new 'Saifu 2' antweight kit powers their compact spinner in this manner. Note that it does require some good machining skills to implement.

Q: How does a fbs [full body spinner] prevent itself from getting fillpped by a wedge in a small arena? [Pennsylvania]

A: [Mark J.] Often, it doesn't. If you look thru the 'Ask Aaron' archives you'll see that we repeatedly warn about big spinners in small arenas. The standard tactic for a wedge vs. spinner is for the wedge to 'box rush' the spinner and trap it against the arena wall before it can spin up to speed. It's quite effective. There are only two practical things a FBS can do to counter this:

  • design around massive weapon motor power to spin-up before your opponent can get to you; or
  • work up enough drive power and control to be able to dodge your opponent's charge while you spin-up.
Another worry: a good hit by a FBS can send it ricocheting off its opponent and into the arena walls, turning itself into a self-propelled runaway pinball. Under the best conditions a FBS is nearly as dangerous to itself as it is to its opponent, and this is greatly magnified in a small arena. Audiences and builders love full body spinners, but they're a real crapshoot to drive.
Q: To comment on the number of teeth of beetle fbs, would it not be better to have a lesser bite given the size of the arena and that you will most certainly be box rushed? Less teeth would make it easier to balance and when impacted the shell rpm would not drop as severely. If the impactors would be sharpened it would increase the bite into plastic or thin sheet metal. [Pennsylvania]

A: [Mark J.] I don't think you have a good understanding of 'bite'. As defined in section 6.3 of the RioBotz Combat Tutorial:

"The tooth bite is a distance that measures how much the tips/teeth of the spinner weapon will get into the opponent before hitting it."

There are numerous posts about 'bite' in this archive. Some highlights:

  • Fewer teeth give better 'bite'.
  • Lower rotational speed gives better 'bite'.
  • Higher closing speed on your opponent gives better 'bite'.
  • A single-tooth spinner has the best 'bite' but can be a challenge to balance.
  • The ability of sharpened impactors to 'dig in' to a soft or deformable surface is not the same thing as 'bite'.
  • If you have poor 'bite' at low RPM you'll have really awful 'bite' at high RPM and you never will get a good hit.
  • A weapon with good 'bite' will be effective at transferring destructive force to its opponent and will lose nearly all of its speed on impact. Less speed drop = less energy transfer = less damage to opponent = less desirable outcome.
  • If you're being 'box rushed' you're going to have great 'bite' due to the low rotation speed but little stored energy. You should gear and power the spinner to provide effective energy storage quickly while retaining good 'bite'. Evasive maneuvers while your spinner gets up to speed is a viable strategy.

Q: Hi there,

Regarding wedge design. I know that offensive wedge serve well as a spinner killer, but I would like to know if it will launch another wedge or pushers similarly? My wedge will be at an angle of 35 Deg from a initial angle of 15 Deg from the scooping tip , it will be moving at a speed of approx 2.5 m/s. Please note that I am building a sumo robot, not a combat robot. [Singapore]

A: [Mark J.] You're not going to get a 'launch' out of a shallow two-step wedge at that speed. Spinners provide most of the energy to launch themselves when they hit a wedge.

If you can get under your opponent's wedge, they may just be able to drive up and over your shallow wedge. Put a 'stop' barrier/lip at the top of the wedge to keep them from doing that!

Q: Hey Mark. I'm the beetle spinner guy, your answers have been extremely helpful, but I still have a few more questions:

1: There's been a lot of talk about wedges that can counter spinners, but what about the reverse? Whats a good weapon shape to counter wedges - specifically, a good weapon shape to counter the two big beetle brick kits, 'Trilobite' and 'D2'? If it wasn't important, then Ray wouldn't have three or four weapon bars to switch out for a given opponent.

2: On the same topic, whats a good blade shape for dealing with drums/egg-beaters('Grande Tambor' being the big one)? I know staying below or above the up-sweep zone is important, but other than that, whats a good shape for hitting drums where it hurts? [Molalla, Oregon]

A: [Mark J.] Ray Billings' mind works in mysterious ways, and I don't know what specific logic he uses to select a specific chunk of metal to beat up his opponents (photo of 'Last Rites' bars at right). He claims he has a method, but he could be using tarot cards for all I know.

You really don't have a lot of options for blade variation with your "huge horizontal" beetle bar weapon:

  • You can install a sharp-edged blade to cut into soft armor materials like UHMW or aluminum for better 'bite';

  • You can drop in a flat-edged blade against harder surfaces like steel or titanium to deliver maximum impact;

  • You can swap in a titanium blade to make pretty sparks against hard steel alloys just for show;

  • You can mount a lighter blade against a quick opponent in a small arena to get a faster spin-up time.

Bottom line: There isn't much you can do with swapping blades to go against specific weapon types. All you can do is to consider what surfaces your blade is actually going to impact on your opponent and go blunt or sharp, and then guess on how quickly you need the weapon to spin-up. Sparks are optional.

Q: is axe more effective or a hammer in robowar? [Bangalore, India]

A: [Mark J.] Neither is particularly effective. Of the two, I'd use a hammer -- axes can penetrate and get stuck in your opponent, which can cause all sorts of problems.

Q: Can you explain the gyroscopic effect on a bot??? [Pasadena, California]

A: Not if you put three question marks on it -- that makes it three times as hard. Let me try editing...

Q: Can you explain the gyroscopic effect on a bot?? [Pasadena, California]

A: Nope, still too hard...

Q: Can you explain the gyroscopic effect on a bot? [Pasadena, California]

A: Ahhh, that's got it! Much better.

[Mark J.] Now, are you asking for an explanation of:

  1. The gyroscopic effect that lifts one side of a vertical spinner robot when it turns; or

  2. The effect that gyroscopic precession robots like 'Gyrobot' and 'Wrecks' use to walk?

Technically it's the same effect, but the explanations and diagrams are different. I kinda hope it's the first choice because I had that diagram drawn and the answer largely complete before I considered the alternative question. I'll put that diagram in here just because it's kinda pretty and I'd hate to see it go to waste.

Write back and let me know which question you're asking.

['Pasadena' never wrote back -- anyone else curious about this?]

Q: i plan to do a 4 bar lifter and i want 2 know if i powered it by 2 linear actuator who can provide 150lb of maximal charge, does it mean i get 300lb of power out of them? [Quebec, Canada]

A: [Mark J.] Yes, but that doesn't mean that your lifter has a 300 pound capacity.

  • Two 150 pound force actuators give you a maximum 300 pounds of force input to the 4-bar system.

  • The actual output force of the 4-bar lifter will vary with the attachment point of the actuators and the geometry of the 4-bar mechanism. The actuators for the 4-bar lifter of heavyweight 'BioHazard' provide more than 2,800 pounds of force to lift a 220 pound opponent. You will benefit from reading the BioHazard Mechanical Design page.

  • The force analysis of a 4-bar system is complex.

  • Actuators slow down as the load on them increases. An actuator that has a speed of 4 inches per second and a 100 pound force rating will slow to half that speed when loaded with 50 pounds, a quarter of that speed at 75 pounds, and will come to a full stop at 100 pounds of load.

There are many posts about linear actuators and designing 4-bar lifters in this archive. I'd suggest reading them.

Side note: 'pounds' is not a measure of 'power'; 'pounds' and 'newtons' are measures of 'force'. If you're building combat robots you should learn the difference. Start here.

Q: Hey just wanna know i plan to do a heavyweight 4 bar lifter wich will looks very much like storm 2. My question is: the front ofthe robot will have a angle of 45 degree with the design i have in mind, i want to know if its too much to be able to push opponent? [Quebec, Canada]

A: [Mark J.] Well, 'Storm 2' had a 45 degree front wedge. It worked pretty well for them.

Q: i just wanna know if a 45 degree wedge will be able to ''deflect'' a spinner like defensive wedge do?

A: Take a look at successful 'spinner killer' wedges. They're all much shallower than 45 degrees.

Q: What is the best active weapon against a vertical sawblade? I know nonactive scoops and wedges do well (especially lifters) but for an active weapon would a faster spinning vertical spinner or a horizontal or full body spinner be more effective? I know that a great driver and a solid strategy with a nonactive will do extremely well but I want the match to be more exciting than a pushing match. [Panama City Beach, Florida]

A: [Mark J.] Several thoughts:

  • Team Run Amok offers advice to builders who want to win matches. It seems you already know how to do that.

  • There aren't many vertical saw spinners around. Why are you designing specifically to fight one?

  • Traditionally, vertical disk/saw spinners are vulnerable to horizontal spinners.

  • When two vertical spinners go weapon-to-weapon, the higher 'tip speed' wins. Tip speed is calculated as RPM times circumference.

  • A lifter is an active weapon.

In my book an ugly win is a whole lot better than an exciting loss. My advice is to keep it simple and 'build to win'.

Q: I am participating in IIT Guwahati robowar so for this, i am making a robot. In my design, for my weapon i am using a motor High Torque Robo war DC Motor 15000 RPM with model no RM0939 so is it wise to use this kind of motor. I will be using it for the rotating a drum with cutting wheels in it. [India]

A: [Mark J.] You've told me nothing about the weight of your robot or the size of your weapon, and the RoboMart website gives no specifications for the motor other than "15000 RPM". There is not enough information for me to answer your question -- the hamburger is bad.

Best I can tell, the motor is similar to the BaneBots RS-775, and it will require substantial gear reduction to be useable for either weapon or drive use. The statement on the RoboMart page that says the motors can be used with '69 mm or 87 mm wheels' is entirely misleading.

Guidance on spinning weapon design and motor selection is available here at Ask Aaron. Start with the Spinning Weapon FAQ.

Q: Is there a way to wire a brushless motor and your DC motors so that you can drive with one stick and control the rpm of the [weapon] motor with the other eliminating the need for rpm reduction? Is it wise to direct drive a weapon in this fashion? [Brooklyn, New York]

A: [Mark J.] Yes, that's a simple control option. No, it's not a good idea.

Speed reduction serves two purposes: it reduces speed and increases torque in equal proportions. Torque is what allows the weapon to run up to operating speed quickly. Without the torque multiplication offered by mechanical speed reduction weapon spin-up time greatly increases, as does current consumption, motor heating, and possibly a brushless motor issue known as 'cogging'.

You can compensate in small robot classes by using a much more powerful motor, but that costs weight and still leaves the high mechanical stresses on a motor directly connected to the weapon. My advice is to follow conventional design and belt drive your weapon with an appropriate speed reduction.

Q: I saw a post in the weapons archive about full body spinners and dead vs live shafts. After reading it I still am not sure what components are placed where for each. For a live shaft there needs to be a bearing on the bottom that the shaft goes into then a spacer to keep the pulley from moving up and down (I think, but am not sure how a spacer would do such a task), then the pulley (I am not sure how this connects to the shaft), then a top bearing and a weapon hub above that (again how does this connect to the shaft). [Duvall, Washington]

A: [Mark J.] The weapon and pulley hubs are ideally connected to the shaft by a keyed joint that locks rotational motion plus a pin or clamp that prevents the hub from sliding along the length of the shaft. A small robot might use just a clamping hub for this purpose. Do NOT attempt to use set-screw hubs for this application!

The spacer does not prevent just the pulley from moving up and down -- it prevents the entire pulley/shaft assembly from moving up and down because the pulley is locked to the shaft.

Q: For a dead shaft, why are 2 bearings used if the shaft does not spin? Could not the shaft just sit in the base, use a spacer, the pulley with a bearing in the center that just connects to the top (the pulley is the weapon hub). Sorry if this is confusing but well... I am confused. Thank you.

A: A single bearing is very good at absorbing a straight 'radial load', but an impact load applied at a point along the spinner shell not directly radial to the bearing creates a 'moment load' or 'torque' on the bearing that will destroy it. Using two bearings spaced well apart in an elongated hub redistributes the un-centered radial load into pure radial loads on the two bearings.

The dead shaft must also be firmly supported at two separated locations to help absorb the 'torque' placed on it by an impact -- once at the base and again as close to the hub as possible. The forces generated by a full-body spinner impact are as great on the support structure of your robot as they are on your opponent. Build strong!

Q: Do you know of any highly detailed (in pictures and text) build logs of full body spinners? For dead and live shafts? I read your response but still cannot picture the working solutions. Thank you again.

A: This is a very tough topic for the short answer format here at Ask Aaron. Some photos and drawings may help. There's a thread on the Robowars Australia forum that has several links to photo collections and a couple of build reports. I hope my short notes and the photos combine to get you that 'aha!' moment.

Q: hi aaron imagine a situation where two drum bots rotating at same rpm (say 5000rpm) but 1st bot having a drum dia 80mm,tooth height 50mm & drum length 300mm and other bot having 120mm dia, tooth height 35mm, and drum length is 200mm my question to is
(1) if the smaller drum bot is moving with 1m/s and bigger drum bot is sationary
(2) if the bigger drum bot is moving with 1m/s and smaller bot is stationary
(3) if both the bots are moving relative to each other

[Chhattisgarh, India]

A: [Mark J.] You defined two drum weapons and set up three scenarios, but you never actually got around to asking a question. Do you want to know which 'bot will have the advantage if those drumbots go 'head-to-head' in each of those situations?

  • Anytime two drumbots go 'head-to-head' the drum with the greater tooth tip-speed has the advantage. At 5000 RPM the smaller drum has a tip-speed of about 47 meters/sec and the larger drum has a tip speed of about 58 meters/sec. The larger drum has the advantage at any closing rate.

  • The calculation for weapon 'bite' depends on the closure rate of the two bots. It does not matter whether one or both of the robots are moving, it's simply the sum of their speeds relative to each other.

Q: Hi, lifter guy again. I downloaded 4 bar but I am not sure what values to input. You called it right, the picture is how I would like to design my lifter but I do not know how to input those values into a "4-bar" as pictured in the program. Thank you as always. [Bellevue, Washington]

A: [Mark J.] Bad timing... My home computers can't run the T.i. Combat Robot 4-Bar Calculator due to incompatible operating systems, and I won't be in my office to use the laptop I keep there that runs the calculator for 5 days. I can't immediately confirm what I'm about to suggest, but let me give it a shot:

  • The line segments in the diagram at right are labeled with letters corresponding to the inputs for the T.i. Calculator.
  • Segment 'J' has a zero length.
  • Length measurements should be in inches.
  • Weight should be in ounces -- 64 ounces if you plan on lifting the entire 4-pound weight of your opponent.
  • Calculations should be run as a 'Rear Bar' lifter.
The layout is somewhat unusual and the T.i. Calculator may balk because of the odd angles. If it won't run from the initial 'lowered' setting you can try inputting the 'A' angle for the elevated position and see what happens. Sorry I can't pre-check this for you.

Q: Hi mark...lifter guy again. Sorry to annoy you so much but I cannot get a single piece of useful information or any information infact out of 4-bar. Every time I attempt to "calculate" it says that I either need to change angle A or bars F and G are too short or too long. I tried for an hours adjusting the input values but with no success. Thanks for your help in these troubled times.

A: Troubled times indeed.

I had feared that the T.i. Four Bar Calculator might have trouble with this layout. A conventional 4-bar used in a lifter has the powered link pushing the 'G' bar forward to raise the mechanism. This design has the 'G' bar stationary and the powered link pushing the 'E' bar forward. That's backward logic to the calculator and it did balk on you.

Time for 'back of the envelope' calculations:

If you simply put a 3" long arm on the servo output shaft, how much torque would the servo require to provide 4 pounds of force at the end of that arm?

  • Four pounds = 16 ounces × 4 = 64 ounces
  • 64 ounces × 3 inches = 192 oz-in torque
The HS-5585 servo stalls at 236 oz-in torque, so it is capable of providing more than four pounds of force under these conditions. As long as you have a 4-bar design that does not require more torque than a simple bar lifter you'll be fine. How can we assure that?
  • Keep the 'F' bar shorter than the 'H' bar
  • Keep the angle between the 'H' and 'E' bars as close to 90 degrees as you can throughout the range of lifter motion.
If your layout looks more-or-less like the diagram you'll be fine. If your 'H' bar is 1.5" you'll want your 'F' bar to be at least 0.75" to get enough motion from the lifter. I can't provide you with a nice graph of torque requirement, but total lift speed when fully loaded will be somewhere in the 0.6 second range. Remember that you'll usually be lifting less than 4 pounds, because you're only lifting one end or side of your opponent -- but it's good to be prepared to lift the whole 4 pounds if the opportunity presents itself!

Q: Hi sir,what will be the best curve radius for the scoop to tackle out Indian drum bots my bots height is 100mm in 155lbs category [Mumbai, India]

A: [Mark J.] A 'spinner killer' scoop should ideally have a radius just a bit greater than the radius of the weapon it will face and should be mounted to match the curve of that weapon. It's better to have the radius too large than too small, so since you will face a variety of drum sizes you should design for the largest weapon you expect to encounter.

Q: Hi There, I am making a shell spinner. Ampflow A28-400 is used as weapon motor and A28-150 as driving motors. Contactor i am using is 586 White Rodgers solenoid. My question is how should i select RC Interface? [India]

A: [Mark J.] Selection of Electronic Speed Controllers (ESC) for your drive motors is covered in Frequently Asked Questions #21.

Selection of an R/C switch interface to control a weapon solenoid is covered in the Ask Aaron Solenoid FAQ.

Q: In your insect archive it is discussed that you need to "run numbers/do math" to calculate different aspects for lifters. How would one do this? Can this be done for 3 and 4 bar designs? This would be in a 4lb robot competition. Thank you. [Morris Plains, New Jersey]

A: [Mark J.] First, some nomenclature:

The internet is full of references to '3-bar' mechanisms, but this is a misnomer. What they're calling a '3-bar' is actually a '4-bar' in which they fail to count the base as the 4th bar. A true 3-bar mechanism forms a triangle and has zero degrees of motion freedom - it won't move at all. Lifters use a 4-bar mechanism. Don't spread the incorrect 3-bar usage.

The math is covered in multiple posts in the Ask Aaron archives:

  • Simple Lever: see this previous post for a discussion of calculating the torque requirements for a lifter motor attached to a simple lever. A more complete math analysis is available at the HyperPhysics website.

  • 4-Bar: there are more than a dozen posts on 4-bar lifters in this archive that include discussions of power options and design tools. Mentioned many times in those posts is the T.i. Combat Robotics 4-Bar Simulator -- a very useful tool to assist in designing 4-bar lifters. It will 'run the numbers' for you. You can find other tools with an internet search for '4-bar mechanism calculators'.

Q: Also how are typical [4-bar] servo lifters built? Thanks.

A: Plenty of links, diagrams, and photos for insect-class servo lifters in the Ants, Beetles, and Fairies archive.

Q: Do you have any good sources for building lifters so I don't constantly bombard you with questions? Can these sources please include common designs? Thanks. [Baden, Pennsylvania]

A: [Mark J.] For insect class lifters, a page search for 'lifter' in our Ants, Beetles, & Fairies archive returns more than 100 hits.

Larger robots? A page search for 'lifter' in this archive returns more than 200 hits! We've answered more than 5100 robot questions -- we are the good source.

Q: Hey Mark. I'm looking to mount a 3" Ampflow motor horizontally for use with a horizontal spinning weapon. I need a right-angle gearbox. Team Whyachi manufactures one (the TWM3R) but the obvious downside is the cost. Assuming I have access to a CNC mill, is this realistically something I can try to manufacture myself? I'm worried that controlling the exact positioning of my bevel gears will be difficult, and considering that the input rpm is in the realm of 10,000 rpm, I definitely don't want my gears to jam up when this thing gets spinning. Thanks! [Lansdale, Pennsylvania]

A: [Mark J.] I don't know the level of your machining skill so I can't comment on your likelihood of success. It is certainly within the capacity of a good machinist to produce a right-angle gearbox using bevel gears, but perhaps only after a few failed attempts. The box will take very large off-axis loading from weapon impacts, the gears must me positioned accurately, and material selection is critical. I'm going to predict that you will eventually regret your decision to not purchase the proven Whyachi gearbox.

Q: Hi Mark. My single tooth spinner spins at 3500 rpm. My drive motors run at 300 rpm with 20 cm dia tyres. So by calculating how much time it takes for one rotation of spinner and how much my bot moves forward in that time I get the tooth size. It's coming to more than 5cm!! Is it safe to keep that much tooth length? [Pune, Maharashtra, India]

A: [Mark J.] That is a very tall impactor! I'm assuming that this is a drum weapon. A taller tooth can put more strain on the junction between the tooth and drum, but there are a few things to consider...

  • The calculation provides the maximum tooth height that can be used under ideal conditions.

  • The proper speed to use in the calculation is the combined closing speed between your 'bot and your opponent. It they're charging toward you at the same speed you're charging at them, the closing speed is twice what you're calculating. You might be able to use a 10cm tooth!!!

  • Have you used the 'Acceleration Calculator' tab in the Tentacle Drivetrain Calculator to check the speed your 'bot can actually achieve in a charge half-way across the arena? That might be well less that the speed you think you have.

  • I've seen a lot of Indian robot combat videos and I don't recall ever seeing a high-speed charge attack with a drum weapon. What I generally see is two bots slowly closing to within about a foot of each other while their weapons spin up. Then one 'bot pops forward into the other. Closing speed is maybe 2 MPH. That will use MUCH less tooth height.
You can use a 5cm tall tooth if you like. I'd suggest making it quite broad at the base, and providing a strong anchorage into the drum. Personally, I'd make it about half that height. Even if you really do plan high-speed attacks and your drive motors can give you that acceleration, you'll still get good 'bite and have a stronger weapon.

Q: I ran some numbers on the [Run Amok Weapon] spreadsheet. My weapon spins up to 95% speed in 3.8 secs, making 4005 joules. Is this spin up and energy ok for small arenas like 20ft x 20ft 60 kg bot?

A: The energy level is adequate for a 60 Kg robot, but...

  • If you're going to be charging across the arena at full speed at the start trying to use that tall impact tooth, then it isn't a quick enough spin-up time.

  • If you're going to wait for the weapon to spin up while slowly closing on your opponent, then my argument for a shorter impact tooth makes more sense.

Q: Also, I am not getting S7 tool steel for teeth. Is EN24 with 54 hrc hardening ok? Or can you suggest the hardening. Will this material be tough and impact resistant for teeth. Thanks.

A: I've discussed EN24 steel for impact teeth previously -- see this post in the Ask Aaron Materials archive. If you decide to use EN24, I'd suggest making extra teeth and being prepared to replace them quickly when they blunt or break.

Q: Hi, Mark.

I've been playing around with your Spinner Spreadsheet, and I really would like to thank you and Team Run Amok for creating it. I've found it incredibly useful; I was doing all the calculations by hand before. I have a couple of questions: First, does the figure for the battery consumption include a safety factor to ensure the batteries won't drain before the match is over, or do I need to add a bit more to ensure I can make it through the full 3 minutes?

One more question, do you think you could create a similar spreadsheet for spinners powered by internal combustion engines? If that's too cumbersome of a project, could you direct me toward the correct calculations for determining the weapon kinetic energy vs. time data table, as well as the gas consumption in 3 minutes?

Thank you very much for your advice. [Austin, Texas]

A: [Mark J.] I'm always pleased to hear that builders find our modeling tools useful. I appreciate you taking time to comment on our Excel Spinner Spreadsheet.

The spinner battery calculations are 'conservative' in that they assume a worst-case energy consumption for each event. For example, they assume that a weapon 'hit' will bring the weapon to a full stop and it will need to spin back up to speed from that state. In an actual match it is unlikely that every hit would do that, so you can think of the difference as a 'safety factor'. That said, I like to add about 20% extra to the calculated battery consumption for both the weapon and the drive train for 'unexpected events'.

About ICE spinner calculations:

  • Permanent magnet direct current (PMDC) electric motors are a performance modeler's dream. They have a linear torque 'curve' and peak horsepower is always at 50% of unloaded RPM. That makes calculation of their power output versus a given load a relative snap.

  • The torque curve for an ICE is not at all linear and the power peak can move higher and lower in the RPM range depending on intake and exhaust cam timing variables and manifold design. The instantaneous torque available to spin the weapon is constantly varying in a non-linear fashion!

  • Two ICE each rated '10 peak horsepower' could provide VERY different spinner weapon performance due to differences in their torque curves. See this article on automotive ICE power curves.

  • An additional complication comes from the centrifugal clutch that ICE spinners use to engage power to the weapon. How quickly that clutch engages and at what RPM has a large impact on the performance of the weapon.

  • The above factors plus carburation settings and atmospheric conditions (temperature, humidity, barometric pressure) also have an impact on fuel consumption. The good news is that it's simpler to pour a little more gas into the tank than it is to cram a zillion more electrons into the battery.

In very general terms, compared to a PMDC motor of similar peak power an ICE will be slower in accelerating a weapon of given moment of inertia over the first half of the RPM range due to lower torque and clutch slippage. Acceleration from mid to top speed will be quicker due to greater torque in the upper half of the ICE power range. You can actually see this when watching an ICE spinner like 'Icewave' -- painfully slow initial acceleration with a big burst of speed as the engine reaches its high RPM power range.

As far as actually calculating ICE spinner performance, I'm afraid that you're on your own. Too many variables and non-linear calculations for me. I can't even give you a pointer to equations that might be of use. ICE spinners are much more of an art than a science.

Q: Hello mark . I have put my weapon values in the spinner spreadsheet for my 60 kg vertical spinner . it shows that it stores 4137 joules at 2.58 sec at 63% rpm (2250). total energy is 9406 joules, about 8 secs for complete spin rpm . will this be ok for a arena of 15ft x 15ft . or should I reduce the spin up time ? but then I will loose energy . the weight wih endcaps is 14.7 kg . Also I wanted to ask how broad the teeth should be .. not the projection out from the drum or the length along the drum , the breadth . thanks [Pune, Maharashtra, India]

A: [Mark J.] That's a VERY long spin-up time for so small an arena. It will put a VERY great load on your weapon motor. Increase the reduction ratio, lose some weapon mass, and/or increase motor power. You've got energy to spare.

Impactor breadth depends on the material from which it is made, the temper applied to that material, the support provided to the impactor, and the anticipated impact energy. Don't ask me to run those calculations for you -- I'm not running a free engineering service. Look at impactors on comparable spinners for guidance. Make it strong. If it breaks, make it stronger.

Q: What type of shaft is typically used for overhead spinners and fbs? How are they mounted? I know you had a diagram on here somewhere but I cannot find it again. [North Carolina]

A: [Mark J.] The diagram appears a couple times in this archive, but I'll reprint it for you here. A typical layout (as shown) is a live shaft supported by well separated bearings above and below the pulley/sprocket. If the pulley is pinned to the weapon shaft and tubular spacers are inserted to take up the extra space between the bearings, the pulley will locate the shaft.

Q: Is it possible to have a spinning weapon without the [top] bearing? For the design I had in mind the top and sides would spin with a stationary base. I thought it would be about a 5/8 live shaft with a bearing on the base, a spacer, the pulley, then hubs to attach the shaft to the top. Would this work?

A: Oh hell no!! That turns the weapon shaft into a lever that multiplies weapon impact reaction force by a factor of about ten to bend the shaft and/or destroy the bearing. Basic engineering practice is to place a support bearing as close as possible to the point of the applied force -- in this case that's the top bearing at the weapon hub. A second bearing is placed as far from the main bearing as possible to control shaft alignment and nullify off-axis forces with the advantage of a long lever arm. Your weapon will transmit HUGE loads back thru the shaft to your chassis and it must be VERY securely supported. Two bearings, as shown.

Q: Does the bearing have to be mounted to a stationary surface or can I attach it to the top with out building a box just to hold the bearing? Also could I put the top bearing under the pulley? Stationary or roof mounted? Thanks.

A: You need to build a chassis box to provide stationary support the top end of the shaft. Take a look at the photos of heavyweight spinner 'Megabyte' with its shell on and with the shell removed.. Its shaft runs all the way down thru the top of the chassis box and is supported firmly at both the base and top of the box.**

Yes, you can put both the pulley and weapon hub above the top bearing. 'Megabyte' does. It isn't quite as strong as putting it under the bearing, but many spinners do it for convenience.

** 'Megabyte' actually uses a non-rotating 'dead shaft' with a combined pulley/shell hub and well separated bearings that ride on the shaft -- a workable alternative to a live shaft -- but either way the shaft MUST be well supported by the chassis at two well separated points.

Q: Hi mark , my single tooth spinner is 20 cm in dia length 20cm and thickness of 10 mm . Now now how do I attach teeth to it ? As I have calculated, The max length of tooth I can keep I around is 5 cm (calculated from riobotz tutorial , of approach speed and weapon rpm).
I have only 1 cm of thickness ... so I have to make a slot and place it only?

also ... In arenas measuring 20ftx20ft ... how much spin up time should a spinner have (60 kg war) ? I read in the spinner sheet ... smaller the arena ... smaller the spin up time should be .... [Thane, Maharashtra, India]

A: [Mark J.] Impactor attachment is covered elsewhere in this archive [click here]. The slot does not have to be very deep, just enough to provide a little edge to support the impactor and keep it from shearing the attachment bolts.

A 5 cm tooth height?? That's a VERY tall tooth. Unless your robot is blindingly fast or your weapon spins very slowly, something's wrong. Re-do your calculations. If you get the same number, send me info on your robot speed and weapon RPM and I'll check it for you.

Spin-up time: estimate the time it will take your speediest opponent to cross the arena and hit you. Watch videos of prior tournaments if available. That's your spin-up time. Cut that time in half if your strategy is to charge at them at the start. You can gain a little time if your robot is nimble enough to dodge their first attack but you'd better be able to actually get out of their way, spin around, and position yourself to attack if you're counting on that strategy.

Q: Hello,

Question 1:
I sometimes see horizontal spinners and full body spinners experience sudden bouts of instability. I am not talking about wobbling after a big hit; I mean sudden and very rapid movement seemingly without a direct cause. 'Secto' does so a few times during this video as does 'Steel Shadow' in this match. Is this a question of high center of mass? poor spinner balance about the main vertical axis? some nuance of moment of inertia about a secondary axis? some resonant structural bending?

Question 2:
Do you know of any centered horizontal spinners, or full body spinners with an asymmetric single tooth design? If designed correctly is there any reason they would be more prone to instability? [California]

A: [Mark J.] You've opened up a can of upper-class level physics worms here. Take a deep breath.

  1. That sudden change in the axis of rotation of a spinning rigid body is a 'polhode motion'. The magnitude of the polhode and whether or not it is self-damping depends on the ratio of magnitudes of the moments of rotational inertia of the three principal axis of rotation for the rotating mass. Follow the polhode link, read the description, then watch this video and see if it starts to make sense. If it does, you have a future in classical mechanics.

    I suppose this does qualify as 'some nuance of moment of inertia about a secondary axis', and the problem can be compounded by 'some resonant structural bending' -- so you were on the right track. An FBS like 'Steel Shadow' can get into even more trouble when the edge of the shell makes contact with the floor and adds a new off-axis torque element to the mix.

    I'll leave it to you to track down the equations that predict the onset of troublesome instability but there is a certain 'look' to 'bots that have this problem. 'Steel Shadow' is tall and tapered, which places more mass low than high; common in unstable FBS. 'Secto' has a blade that is wider at the center than at the ends, which increases the rotational inertia of its unstable 'middle' axis; that spells trouble.

    In this video clip astronaut Don Pettit demonstrates stable and unstable modes for solid body rotation on the International Space Station. Using a hard cover textbook, he demonstrates that it will rotate stably about the longest and shortest axis, which represent the maximum and minimum movements of Inertia. Trying to rotate the book around an intermediate axis results in an unstable rotation in which the book appears to flip-flop while it rotates.

    That's about all the help I can give without writing a thesis on the topic. Try a web search for 'rotational stability' for additional tidbits.

  2. I don't know of any 'centered' single tooth spinners, but as long as you pay attention to the polhode issues discussed above there should be no instability problem. I've seen several 'offset' asymmetric spinners that are crosses between bars and disks that I'm pretty sure would be problematical if they were 'centered', but the offset design is more forgiving of instability issues.

Q: Can you explain the difference between a dead shaft and a live shaft in term of spinning weapons? Which one is easier to build and which one is more reliable? Thanks. [Kansas]

A: [Mark J.] First, the terminology:

  • A 'dead' shaft is fixed to the chassis and does not rotate. The weapon has bearings that ride on the shaft.

  • A 'live' shaft is fixed to the weapon and rotates. The shaft is supported by bearings on the chassis.

Which design is easier to construct and/or more reliable depends entirely on the type of spinner weapon you're building.

  • A 'thin' weapon like a disk or spinning bar can benefit from a live shaft that allows the support bearings to be spaced farther apart to better control off-axis loading. This also allows the pulley/sprocket to power the shaft from a more convenient location outside the bearing supports. See horizontal bar spinner 'Fiasco' as an example.

  • A 'thick' weapon like a drum can mount attached support bearings well apart within the weapon itself. This allows the dead shaft to become a fixed structural chassis member for greater strength and rigidity. The pulley/sprocket must be attached directly to the weapon as the shaft cannot be powered. Drum spinner 'Helios' is an example of good dead shaft design.

Q: For a design like Ice Cube (scoop robot) is it better to have the rear wheels extend over the rear of the robot or not? I ask this only because it seams that it makes his fully frontal attacks ineffective because he gets flipped over due to the large wheels hanging over the rear (Video). Without the wheels extending past the rear of his robot though he could get stuck on the back unable to move. Thank you for your time. [North Carolina]

A: [Mark J.] 'Ice Cube' is designed to allow contact between drive wheels and arena floor from nearly any orientation. To take it off its wheels you'd have to lift the tail and balance it up on the scoop. Designing to be 'always mobile' is certainly a valid tactic. As long as you have a drive wheel in contact you can influence the match. If you don't you're just a passenger.

Those instances in the video where driver 'Fuzzy' Mauldin flips the 'bot over with a 'wheelie' are entirely deliberate. Look closely and the only times he does it are when the 'bot is upside-down. Flipping up back upright with a quick power blip is a very quick method of restoring the correct forward/backward control orientation that is reversed when the 'bot is inverted. It's a strength to be able to do that -- not a weakness.

Q: I've seen some builders that are running direct [weapon] drive off a brushless out runner replace the shaft of the old motor to something thats longer and, or in some cases, larger in diameter that the old shaft was. How hard is it to modify a brushless outrunner to accept a longer and/or larger shaft? [Aumsville, Oregon]

A: [Mark J.] This video shows the process of replacing a typical outrunner shaft.

If you want a longer shaft, you'll need to make or find a shaft of the new length with the correct C-clip groove and flat. Difficulty level: easy if you have the proper equipment.

If you want a larger diameter shaft, you'll need the new shaft plus:

  • Shaft support bearings sized for the new shaft;
  • Machining the bearing seats for the larger bearings;
  • Precision boring the motor bell shaft hole for a proper press-fit; and
  • A new C-clip and washer to assemble the motor.

Difficulty level: senior machinist with a shop full of tools.

[Mark J.] I'm very pleased to have a guest commentary on the topic of brushless outrunner shaft replacement -- see the previous post in this archive. Mike Jeffries from Near Chaos Robotics has experience in replacing and modifying brushless outrunner shafts and offered to share his knowledge.

The spinner mini-drum weapon he describes for 'Algos' rides on a dead shaft and incorporates the 'ring of magnets' cut from an outrunner can. The stator bearings are removed and the bore is drilled out to slide over the dead shaft. This cross-section drawing of the weapon assembly may help you visualize this. I think Mike underestimates his builder chops!

For the most basic replacement option (same diameter shaft, better material) you can often do this with just a press of some sort. Care needs to be taken to avoid warping the motor can if you do this, but it's a very easy process if you can buy shafts that are the right diameter and temper. I've had decent luck doing garage heat treating and tempering, but for some of the tougher steels you need to temper the shafts at a temperature well above what most household ovens can reach so when possible buying pre-hard shafts is the better option. The added bonus there is you don't have to worry as much about the treating process warping the shafts as they should be in decent shape when you get them.

On the more complex side of things, upping the size of the shaft takes a bit more effort. On my 1lb bot Algos I replaced the 3mm live shaft with a 1/4" dead shaft. The process for doing this was as follows:

  • Remove bearings from stator
  • Drill out stator to accept new shaft diameter (I went for a snug slide fit)
  • Remove shaft from motor can via press or similar
  • Cut away most of the end of the motor can, taking care to leave the portion supporting the magnets
  • Press remaining portion of can into the weapon assembly
  • Reassemble motor with new dead shaft using small outer diameter shims to maintain intended weapon location

Assuming the weapon itself is made to fit the weapon motor, the process at least can be done without all that much equipment. I used a lathe to shave down the motor can, though a steady hand and a cutoff wheel could do the same. The portion of the assembly done with an arbor press could also be done with a vise in a pinch.

Thanks, Mike. I second Mike's suggestion that you find pre-hardened shafts if at all possible. I personally wouldn't try to use a vise instead of a press to insert the new shaft, and under no circumstance should you try to 'tap it in' with a hammer! I know how you guys think...

If you aren't familiar with the Near Chaos Robotics website, do yourself a favor and take a look.

Q: I've been trying to figure out the cross-section drawing of the spinner weapon assembly on 'Algos' you linked in your introduction to Mike Jeffries' guest post (just above). Can you walk me thru that drawing? [The Lower 48]

A: Yes, that's a very dense and 'busy' illustration that was pulled straight from a CAD cross-section view. I've taken a shot at stripping it down to the key elements: Cross-section of 'Algos' weapon hub motor

The outrunner motor stator is stripped of its bearings and drilled to accept a large steel shoulder bolt that supports the ball bearings upon which the weapon drum spins. The rotor magnet ring has been cut away and pressed into the drum -- effectively turning the drum itself into the rotor.
Combine the information ablove with the photos and description in Mike Jeffries' guest post and I think it will make sense. I've also redone the image link in the post to point to this new drawing.

Q: I'm having trouble relating the diagram of the 'Algos' drum to the outrunner motor before it was modified. Can you give me a sketch of the outrunner in the same format?

A: Sure. The only parts of the original outrunner that are used in the finished weapon are the motor stator and the rotor magnet ring. Cross-section of 'Algos' outrunner motor before conversion

The rotor and attached small diameter live shaft are removed and the magnet ring is cut free on a lathe and pressed into the recess in the weapon drum.

The small ball bearings are removed from the stator, and the tube that held them is drilled out to accept the larger shoulder bolt dead shaft that will support the larger ball bearings for the drum weapon.

Q: From recent photos I see Nightmare's spinning disc's shape changed a lot in order to compete in the all new Battlebots series. One thing I've been wondering is: does that change decrease the energy that blade would store greatly when spinning?

Thank you for taking your time to answer! [Chinese Forum]

A: Nightmare's old spinning disk was made from aluminum. The new 'butterfly' disc is 1/2" titanium -- denser than aluminum. By my rough calculations the new disc has at least as great a moment of inertia as the old disc, and it spins a bit faster. Jim Smentowski is a very experienced builder who knows exactly what he's doing.

Q: hi sir i had confusion with pulley motor pulley diameter 66 mm and my weapon pulley diameter is 44mm, .. whether its ratio is 0.68:1 or 1:1.45 which ratio is correct and how i have to calculate? [India]

A: [Mark J.] The reduction ratio is the relation between the diameter of the weapon pulley and the diameter of the motor pulley:

Ratio = driven pulley : driving pulley
44:66 = 2:3 = 0.67:1

I think you have the pulleys reversed for your purpose. If you're trying to increase the motor torque and get the weapon to spin more slowly than the motor, you want the larger pulley on the weapon and the smaller pulley on the motor. That would give you a 66:44 = 3:2 = 1.5:1 reduction ratio.

Q: hello sir..sorry to say but i read the whole solenoid section..i think only 1 DPDT solenoid can be used to control both forward and reverse direction of motor..
plzz sir suggest the connection diagram for DPDT solenoid control for both drive motors and for weapon.. thanxx alot.. [Chhattisgarh, India]

A: [Mark J.] Sorry to say, but your thinking is incomplete.

A single DPDT solenoid can provide forward/reverse directional control of a PMDC motor, but there is no 'center off' position with a DPDT solenoid. Your robot would be continuously 'on' in one direction or the other. To get the required forward/off/reverse control you need the circuits already diagramed in our Solenoid and Relay Guide.

Q: Is the drum weapon shouldn't be oversize? I think the larger drum will store more energy. In fact, the ICE powered big drum weapon bot 'REDRUM' always get disadvantage when fighting with horizontal spinners.(It only has few videos,mostly versus spinner) The drum didn't give out much damage and its gyroscopic forces makes 'REDRUM' difficult to drive.However, everyone can see 'Touro Maximus' with a small snail drum won the STEM and RG2015 champion.Is that because size of drum shouldn't be as large as possible or it is because of the successful snail drum design or it just because of 'REDRUM's problem itself? [Guangdong, China]

A: [Mark J.] A great many new builders share a mistaken belief that the success of a combat robot depends primarily on the design of the weapon. This is entirely false.

The weapon may be the least important system on a combat robot. If you're not winning matches it isn't because you have a poor weapon.
Any weapon is only as good as the robot that uses it, and placing too much design emphasis on the weapon will detract from the overall performance of the 'bot. See this post in the Design archive for a discussion, and this post on the design of the 'Touro Maximus' snail drum.

About drum weapon design:

  • A larger diameter drum weapon will store more energy than a smaller diameter drum of the same mass at the same speed -- see the Spinner Weapon FAQ for examples.

  • A larger diameter drum will also exert greater gyroscopic force on the robot that may cause control problems. Total Insanity Combat Robotics has a tutorial on weapon gyroscopic forces (archived) that fully explains the issue.

  • If your attack strategy requires high-speed rushing attacks and quick pivoting turns, a small diameter drum spinning at high speed as used by 'Touro Maximus' is the correct choice.

  • If your attack strategy allows a slow and cautious approach toward your opponent without high-rate turns (common in Indian robot combat) then a slower spinning large diameter drum weapon is ideal.

Different combat environments require different designs. Do not blindly emulate a weapon designed for a combat environment different from the one in which you will compete!


A: [Mark J.] Designing to maximize energy storage is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. Please read the FAQs before asking a question.


A: The relationship between weapon speed and bite is thoroughly covered in the Ask Aaron Spinning Weapon FAQ. PLEASE read the FAQs before asking a question.


A: Nobody has asked for an explanation, but yes I can provide.

63% of Max Generating that pretty chart in the Run Amok Excel Spinner Weapon Spreadsheet that shows weapon speed over time requires a fixed point of known speed-time. The formula to calculate that 'time constant' gives an answer for 63% of max RPM. The 63% is a logarithmic thing that would take too long to explain here; ask your physics professor. Since I have to generate that time constant anyway, I include it in an output field as a clue to math boffins that I know what I'm doing.

Once I have the time constant, a simple formula converts it to the spin-up time for other percentages of maximum speed:

Time to Speed = Time Constant × loge(1 - (speed ÷ max speed))

95% of Max Given mechanical and aerodynamic loss, the weapon will never reach 100% of the unloaded motor speed potential, so I cut off the calculation at an arbitrary 95%. In the real world, the weapon is unlikely to reach even that 95% figure, but it serves to point out that 100% will not be obtained.

Q: Theoretically, driving as fast as you can into the enemy increases the bite your spinning weapon will have. While I see drum-bots flying full speed across arenas, I am not aware of a shell spinner that drives even half so aggressively. Is there a particularly good reason for this? I ask because I want to size the thickness of my teeth, and my drive-train to be optimal for the speed that I will really operate the robot. [Palo Alto, California]

A: [Mark J.] First, I know I'm being picky but that's my prerogative:

  • Increasing bite with speed isn't theoretical, it's demonstrable fact.

  • The other 'bot isn't your enemy, it's your opponent. There's a difference.

So why don't Full Body Spinners (FBS) charge aggressively? Two things to consider:

  1. The large diameter of the spinning mass allows a FBS to store huge potential energy at a relatively slow spin speed. Slow spin speed will give good 'bite' even with a slow approach speed. This allows the designer to use a less powerful (slower) drivetrain and shift that mass into the weapon to increase the destructive potential even further.

    Small diameter drum weapons must spin at high RPM to match the energy storage of a FBS. The high weapon speed requires a high attack speed to match the 'bite' of a slower-spinning FBS.

  2. The result of an impact with a horizontal spinner is much different than the result of a vertical spinning drum impact:

    • The drum impact will impart an upward motion on the opponent, with an equal but opposite Newtonian reaction pressing the attacker downward. As this downward reaction is countered by support from the arena floor, the attacking drumbot is relatively unperturbed.

    • An impact from a horizontal FBS will impart a lateral (sideways) motion on the opponent, with an equal reaction throwing the attacking FBS laterally in the opposite direction. A high-speed attack that lands a glancing blow on the opponent can send a FBS caroming around the arena. Each time it strikes the wall the impact from the shell can add more speed and prolong the mayhem. A slower and more planned attack run can minimize the 'pinball' rebound issue and help keep the FBS under control.

General advice: be cautious with your FBS attacks or you can do more damage to yourself than to your opponent.

Q: i have seen most of the bot's teeth are not welded in the drum.....i know they are not strong enough to bear the impact instead of welding they uses some types of screw (may be Allen bolt) can you please explain which type of bolt they use and how they manage to fit in the drum....and are they strong enough to bear the impact [India]

A: [Mark J.] Impactors are typically set snuggly into a machined groove or pocket in the drum. The groove supports the base of the impactor and prevents the tooth from moving laterally. Bolts run radially thru the impactor and into tapped holes in the drum to hold the impactors in place. A very thin drum may use nuts or a nutstrip inside the drum to give enough thread depth for the bolts.

Do not attempt to simply bolt the teeth to the surface of the drum without providing a groove -- bolts are very strong in tension along their length but are not good at resisting 'shear forces' they would face if the tooth is not well supported.

The bolts themselves are usually coarse thread and as large a diameter as is practical for the impactor design. High strength 'grade 8' bolts are favored by some builders.

Strong enough? Depends on the materials and dimensions of your design. See Frequently Asked Questions #4.

Q: how to determine the tooth height of a drum drum is 160mm in dia and 20mm thick

A: Optimum tooth height does not depend on drum dimensions. Use the search box at the bottom of this page - immediately below the box where you entered your question - to search the Ask Aaron Archives for "tooth height" to find an explanation and an example calculation.

Q: sorry to say but sir i have seen most of the archives but i didn't get the topic of tooth height please help me [India]

A: You're building a combat robot but cannot search a website? Please allow me to spoon feed this to you:

Tooth Height Post in this Archive

There is also a complete discussion of tooth height in section 6.3 of the RioBotz Combat Tutorial.

Q: If thick walled drum weapons are not effective, why is the 'snail drum' on the heavyweight champion 'Touro Maximus' built to this design? [India]

A: [Mark J.] Team RioBotz know what they are doing. They fight in large arenas and their attack plan is to charge at high speed into their opponent -- much higher speed than Indian robots can use in their small arenas. If the attack speed is high, the drum can rotate at a higher speed and still retain good energy transfer ('bite') into the opponent. Double the attack speed allows double the weapon RPM -- and double the RPM gives four times the energy storage in the weapon!


  • A specific robot with an attack speed of 3 MPH finds that it must the limit weapon to 2000 RPM in order to maintain good weapon bite. At 2000 RPM its weapon can store 1500 joules of energy.

  • If the attack speed of the robot is increased to 6 MPH, the weapon RPM can be raised to 4000 RPM and still retain the same 'bite'. At 4000 RPM this same weapon can store 6000 joules of energy.

  • If the attack speed of the robot is increased to 12 MPH, the weapon RPM can be raised to 8000 RPM and still retain the same 'bite'. At 8000 RPM the weapon can store 24000 joules of energy.
So, at 4 times the attack speed, the weapon can store 16 times the energy and still have the same ability to bite into the opponent without 'skittering' off. When you have the capacity to store that much energy in the weapon, you can afford to decrease the storage capacity a bit to make the weapon more durable and better able to survive the massive hits it will deliver.

Recap: the snail drum on 'Touro Maximus' is a thick-walled small-diameter drum in order to better survive the ultra-high energy impacts it delivers because it spins at very high RPM. It is able to spin at such high RPM because 'Touro Maximus' has a very high attack speed, which gives the weapon good 'bite' even at such high RPM.

This works in arenas that are large enough to allow robots room to accelerate to high speed. In a small arena it's much better to concentrate on larger, thinner walled drums to store high energy at lower RPM due to their greater rotational inertia. Don't blindly copy the snail drum if you're going attack at low speed!

Q: i am having drum of 110dia (OD) and thickness is about 40mm and length is 20cms and using bearing caps at end i want to participate in a 60kg robowar dose it store enough energy at 3000 rpm [India]

A: [Mark J.] Have you been asleep in the back of the classroom?

There are at least four other very recent posts asking me to calculate the energy storage of drum weapons. My answer to all of them has been the same. Take a look at this post which makes the same error of data omission you have made.

Read the Ask Aaron Spinner FAQ. It will tell you how to calculate the energy your spinner weapon should store, and it gives you three options on how to calculate that figure.

Q: ...and also in your spreadsheet how can i give detail of tooth...thankyou

A: Ahhh! So you have been paying at least a little attention -- you know about the Run Amok Excel Spinner Weapon Spreadsheet. Good.

Impactor teeth and bars come in all shapes and sizes. Calculate the mass of the teeth (volume × density of tooth material) and increase the value for the drum radius until the mass is increased by the mass of the teeth. This will give you a good estimate of the contribution to energy storage the impact teeth will make.

Now, stop being lazy and run the numbers for your drum. You'll find that your drum is too small in diameter to store a reasonable amount of rotational energy. A larger radius and a thinner wall will greatly improve energy storage with the same mass.

Q: Thank you for i am making a new drum 15cm DIA and 25 cm length with thickness 35mm for 60kg weight limit ...but i am little bit confused about the diameter of dead shaft can you suggest me the dia of dead shaft

A: Is this a steel drum? Have you noticed that the drum will weigh nearly 25 kilos? That's a very heavy drum for a 60 kilo class robot, and it doesn't store a lot of energy for its weight. You may wish to work on the design a bit more. Here is your current design:

  • Diameter: 15cm
  • Wall Thickness: 35mm
  • Length: 25cm
  • Speed: 3000 RPM
  • Weight: 24.7 kilos
  • Energy Storage: 4420 joules

Reducing the wall thickness to 20mm and increasing the speed to 4000 RPM will significantly reduce weight and improve energy storage:

  • Diameter: 15cm
  • Wall Thickness: 20mm
  • Length: 25cm
  • Speed: 4000 RPM
  • Weight: 15.9 kilos
  • Energy Storage: 6074 joules

Better still, increase the diameter to 20cm, reduce the wall thickness to 11mm, and keep 4000 RPM:

  • Diameter: 20cm
  • Wall Thickness: 11mm
  • Length: 25cm
  • Speed: 4000 RPM
  • Weight: 12.7 kilos
  • Energy Storage: 10,063 joules

Thick walled, small diameter drums simply aren't effective at energy storage. Think bigger!

Back to your question -- the required diameter of the dead shaft depends on many factors:

  • shaft material
  • solid or hollow
  • energy storage capacity of the drum
  • distance from drum bearings to shaft support
  • strength of shaft mounting to chassis...

Assuming solid hardened steel alloy shaft and the drum design you've mentioned, 20mm to 25mm is a good starting point.

Q: in your spreadsheet it shows the weapon has 109 joules of energy is it sufficient for a drumbot of length 22cm, 11cm dia and 30mm thickness and it is hollowed so that a dead shaft is inserted and bearing at end plates....if the energy is not enough then what should i do so that it can stores more energy i am using ampflow e30-400 [India]

A: [Mark J.] The Ask Aaron Spinner Weapon FAQ answers all of these questions. Read it.

Q: hi sir
whether12V 5310 RPM "CIM" Brushed DC Motor
  • 2.5-inch CIM brushed DC motor
  • Stall torque: 2.42 N-m
  • No load RPM: 5,310
can be used for combat if it is shall i buy this to run my 4kg drum [Tamil Nadu, India]

A: [Mark J.] The CIM motor has been used for years in the FIRST robotics program, and is certainly capable of being used in combat. However, it weighs a lot (46 ounces) for the amount of power it produces (about 1/2 horsepower). Compare that to the RS-775 motor That weighs 12 ounces and produces 3/4 horsepower.

Should you use the CIM motor for your 4 kg drum weapon? You didn't tell ne enough about your weapon for me to answer that question (see the Spinner Weapon FAQ), but YOU can use the Team Run Amok Spinner Excel Spreadsheet to model the weapon performance with the CIM motor to see if it meets your needs. This is the same solution I offered to the two posts immediately below. I'm sensing a theme here. Maybe that spreadsheet is useful?


A: [Mark J.] See post immediately below.

If you are unable to use the Team Run Amok Spinner Excel Spreadsheet, you do have other options:

Q: my drumbot length is 20cms and OD is 100mm and ID is 75mm and dead shaft is inserted in drum and at the ends bearings is mounted how much is energy it is storing ar 3500 rpm [India]

A: [Mark J.] 'Ask Aaron' is not a free engineering service. We provide tools and information to assist combat robot builders in the design, construction, and operation of their machines. We don't do your design work for you.

I spent a good deal of time and effort to create the Team Run Amok Spinner Excel Spreadsheet to assist builders in modeling the performance of spinner weapon designs. The spreadsheet is mentioned prominently several times on this page, in the Frequently Asked Questions, and in this archive. A few questions farther down in this archive is a detailed example of the use of the spreadsheet in calculating the energy storage of a drumbot.

I will occasionally work thru the process of calculating some performance factors for a specific robot as an educational example. However, having already performed that exercise and having provided you with the tools required to calculate the energy storage capacity of your drum weapon, it does not serve the purpose of 'Ask Aaron' to perform that calculation for you.

I will point out that you failed to mention the material from which your drum is constructed. Without that information it is not possible to calculate the mass of the drum, and without the mass it is not possible to calculate the energy storage. I could not answer your question if I wanted to. Be so kind as to run the numbers yourself -- you might learn something.

Q: hi sir considering single teeth drum and snail drum two different drums which one will give best hit? [India]

A: [Mark J.] There is a previous post about the 'snail drum' -- search this archive for 'snail' to find that post.

The snail drum does not store more energy or deliver a better 'hit' than a single-tooth drum with comparable dimensions running at the same speed. The snail is tremendously difficult to design and construct, and the primary benefit appears to be the durability of the integrated impact tooth. The only real reason to build a snail drum is to impress your opponents with your machine shop skills. Avoid the temptation.

Q: I read through your page but could not understand clearly how to build a hollow drum.
Could you please tell me the processes involved and how to build the drum? [India]

A: [Mark J.] Most of the examples of drum construction in this archive come from the Team Cosmos site. Suggest you read the Team Cosmos build log for 'Solaris' for photos and a description of the process. There are build logs for other drumbots on their site as well.

Q: Hello,
I want to build a hollow drum of id- 25mm and od-70mm .
How am I supposed to build the drum ?
The length of the drum is 250mm.
What materials should I choose and how much energy will it store at 5700 rpm?
- Siddharth [India]

A: [Mark J.] You're approaching this backwards. You don't start with the weapon dimensions and speed -- how did you decide on those values? You start with the weapon requirements and work toward a combination of dimensions, speed, materials, and components that will meet those requirements. Here is the correct sequence of events in designing a spinning weapon:

  1. Determine the performance parameters needed to achieve an effective spinning weapon in your weight class. Consult the Ask Aaron Spinner Weapon FAQ for guidance.

  2. Download the Team Run Amok Spinner Excel Spreadsheet and model the performance of a trial weapon design to determine the weapon mass, energy storage, spin-up time, and battery power requirements of the weapon.

  3. Modify the design parameters of the weapon (dimensions, materials, motor specifications, drive reduction ratio...) and observe the effect of the modifications on the modeled performance of the weapon.

  4. When you have a design that provides a good combination of performance parameters, evaluate the design for construction practicality. There are many posts in this archive that discuss drum weapon construction. Read them.
I can tell you that your current design (if constructed from steel) would weigh 6.5 kilos and would store only 810 joules of energy at 5700 RPM. Minor changes to the weapon design could greatly improve your weapon performance. Start over.
Q: hi mark
making 2 teeth drum materials
(mild steel)
(teeth- en18)

od 150mm
id 20mm
shaft -15mm to 20mm

bearings- shall i go for needle or tapered one's??
and my drum rotates @5700rpm at 24v can you tell me the energy stored in it
whether high horse power motors plays the role in storing the goodenergy in a drum orelse a high RPM ed one's [India]

A: [Mark J.] You've designed your drum weapon without knowing how much energy it will store? That's a critical element in the design, not something you save to find out about after the design is fixed. The Team Run Amok Spinner Excel Spreadsheet can calculate the mass, moment of inertia, energy storage, and spinup times for rotating weapons given their dimensions, construction material, speed, and motor specifications. I strongly suggest that you learn to use this tool to assist in your weapon design.

Bearings: drum weapons typically use radial ball bearings. They are economical, tolerate a little misalignment, and are available in sizes and ratings adequate for the purpose.

Energy storage: you have failed to include the length of your drum, so I cannot tell you anything at all about your weapon -- even if I were in a mood to do your design work for you.

Horsepower vs. RPM: this has been discussed previously here at 'Ask Aaron'. Take a look at this post on drum power. You'll benefit from reading thru the Spinning Weapon FAQ as well -- you might learn something that you didn't even know you didn't know.

Q: length of the drum -150mm could u say me the energy storing capacity for it

A: [Mark J.] I'll do much better than that -- I'll show you how to use the Team Run Amok Spinner Excel Spreadsheet to calculate the mass, moment of inertia, energy storage, and spinup time for your weapon. You'll most certainly need to use the spreadsheet to evaluate new designs once you see the performance of the proposed weapon!

  • Download the spreadsheet on a computer with Microsoft Excel installed.

  • Select the 'Calculations' tab and fill in the specs for your motor. I've assumed that your 5700 RPM motor is an AmpFlow E30-150.

    • No-Load Speed: 5700 RPM
    • Stall Torque: 5 N-m
    • Gear Reduction: 1:1

  • Fill in the specs for your drum tube.

    • Material Density: 7800 kg/m2 [steel]
    • Length: 0.150 Meter
    • Outer Radius: 0.075 Meter [half of 150 mm]
    • Wall Thickness: 65.0 mm [(150 mm OD - 20 mm ID) ÷ 2]

  • You didn't provide dimensions for your impactor teeth, so we'll assume they're small in comparison to the drum and disregard their mass.

The output from the spreadsheet will tell you that the drum is quite heavy, and that it does store a lot of energy. It will also tell you that the spinup time is ridiculously long -- it takes a one horsepower motor a very long time to store that much energy in a spinning weapon. I suspect you'll want to re-design your weapon to weigh less, spin slower, and store a more reasonable amount of energy. Now you have a tool to help you do that, so go use it!

Q: Hi Sir...I am thinking of making a combat robot with two weapons: Wedge and Vertical Spinner. How can I use motors to operate these two weapons? Thank you in advance ! [India]

A: [Mark J.] Both horizontal and vertical spinner weapons are typically driven by a belt and pulley system. See the photo of horizontal spinner 'Fiasco' at right for an example of a belt driven spinner weapon. There are many posts about weapon motor selection and calculation of the correct belt drive reduction ratio in this archive.

A wedge is a 'passive' weapon that does not require its own motor. A wedge relies on a powerful drive train in the robot to ram and lift the opponent and break their traction, allowing you to push them with reduced resistance. There are many, many posts in the Motors and Controllers archive on selecting drive motors and calculating the correct gear reduction for your robot drivetrain.

Q: Hi Sir..I am completely new to a combat robot making.....I wanna have flipper as a weapon in it...Would you please tell me how can we transmit power from cylinder to function the weapon?And also,what can be the effective orientation of cylinder [with respect to] weapon? Thanks in advance !!

A: [Mark J.] There are DOZENS of posts about pneumatic flipper weapons in this archive. Search for 'pneumatic'. A few notes:

  • The Team Da Vinci Robotics Understanding Pneumatics page has several animated examples of pneumatic weapon designs, and a thorough discussion of pneumatic components.

  • Many Indian robot competitions do not allow pneumatic weapons. Check with the specific events you plan to enter before you build your flipper robot to make certain that you will be allowed to compete.

  • Pneumatic flipper weapons place great loads on the robot chassis and can be extremely dangerous if improper components or techniques are used. I can not recommend that anyone 'completely new' to combat robots attempt to construct and implement a pneumatic weapon. Pick something much simpler for your first combat robot.

Q: Hi Mark Sir......can you please tell me which motor is good to rotate a 13kg drum and 22kg single tooth spinner ,ampflo A28-400 or A28-150? [Mumbai, India]

A: [Mark J.] I can't recommend a motor based on just the mass of the weapon. I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in spinner weapon design and motor selection.

Q: Just wanted to know the Mechanism SKF Warrior by Team Whyachi !!!
what kind of mechanism is that ?
They say it stores energy when it spins, and then uses it to flip the opponent. How is this energy stored, spring ?
Or is it something else ? [Mumbai, India]

A: [Mark J.] Previously answered.

Search this archive for multiple posts on 'Warrior SKF' and its Spin Kinetic Force (SKF) weapon.

Q: thinking of building a bot with horizontal sppiner fr 25kg catg. compitations.. whic ampflow shud i use considering the cost factor also [India]

A: [Mark J.] It's generally not a great plan to start by picking a motor and then 'back into' a weapon design to suit that motor. You'll have greater success if you start with a weapon design (type, dimensions, material) that is capable of storing the energy you require at a reasonable speed -- then select a motor that can spin up the weapon to the desired energy level quickly enough to meet your needs.

I used the Run Amok Spinner Excel Spreadsheet to generate this simple example:

  • A steel bar 400 mm long, 90 mm wide, and 25mm thick weighs 7 kilos. That's a reasonable weight for a 25 Kg class spinner weapon.

  • Spinning like a lawnmower blade, the bar will store about 1000 joules of energy at 1400 RPM. That's an acceptable amount of energy for a 25 Kg class weapon and a reasonable speed.

Now that we have a practical weapon design, we can look at weapon performance when powered by a specific motor.

  • An economical AmpFlow E30-150 motor running on 24 volts with a 4:1 reduction belt drive to the bar weapon will spin the bar to about 1400 RPM in less than 2.5 seconds. That should be adequate performance in even a small arena.

That's only one possible weapon that may or may not suit your overall design. It does show that it's possible to design a 25 Kg class weapon powered by an Ampflow E30-150 motor. A more powerful motor could spin a weapon with greater destructive potential -- the choice is yours.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for additional guidance in weapon design and motor selection.

Q: hey mark, can you tell me that how much energy (J)/kg would be sufficient for a drum weapon to attack a bot of 15 kg?- sam [New Delhi, India]

A: [Mark J.] According to Stroker McGurk's Law:

"If some is good, more is better, and too much is just enough."

I suggest that you read the Ask Aaron Spinner Weapon FAQ for a more precise answer to your question, Sam. The first three questions there are:

  1. How much energy should my spinning weapon store?

  2. How do I calculate the kinetic energy storage capacity of a spinning weapon?

  3. How quickly should my weapon spin-up to speed?

Read the whole thing while you're there -- it might save you from asking a few more questions in the future.

Q: which type of dc motor suits to rotate 7kg horizontal drum (outer diameter 160mm, thick 40mm , made by MS) ?
it is advisable to use E-bike brushless dc motor having capacity of 200w , 12v , 20-18 A , 2000rmp (torque = p*30/3.14*2000)=0.9Nm) ?
or any other? [Gujarat, India]

A: [Mark J.] First, you've misapplied the formula used to calculate the torque constant (Kt) of a motor in an attempt to calculate the actual torque your motor supplies, and you've incorrectly entered the actual RPM of the motor into that formula instead of the motor's speed constant (Kv). The correct formula for estimating the stall torque of a brushless motor is given in the instructions page of the Run Amok Excel Spinner Spreadsheet. While you have the spreadsheet open, spend some time learning how to use it and you can do your own evaluation of motors for powering up a spinning weapon of any size, shape, and material.

You haven't given me enough info about your E-bike motor to model its performance in spinning up your weapon, but I can tell you that in general a 200 watt motor isn't going to be enough for the weapon you describe. The AmpFlow E30-150 motor provides 750 watts of output power, and would be a marginal motor for your purpose. If I correctly understand your description of the weapon, the E30-150 is capable of spinning it to about 1300 RPM and 1500 joules of energy in about 4.5 seconds (3:1 belt reduction). That would make an adequate weapon for a 40 kg robot.

Suggest you read the Ask Aaron Spinner Weapon FAQ for additional information in designing your weapon and selecting a weapon motor.

Q: sir which weapon is more effective from a Chucker or a lifter in 60kg bots ??and plz suggest me a best mechanism for it! [Low Earth Orbit over India]

A: [Mark J.] The answer depends on your competition rules and the design of the combat arena.

  • 'Flipper' weapons are quite effective in arenas where there is enough room between the arena boundary and the protective screening to toss your opponent out of the arena for a win. Flippers require an effective pneumatic system to provide the speed and power needed to toss an opponent end-over-end thru the air, and I know that pneumatics are not allowed in many Indian competitions. Direct use of electric power simply won't do.

  • 'Lifter' weapons are effective in breaking your opponent's traction and possibly toppling a top-heavy opponent. Few Indian robots can function when inverted or simply tipped on their side. Either pneumatic or electric power can be used for an effective lifter, although an electric lifter will require great gear reduction in order to provide the torque needed to lift a heavy opponent.

The 'best' design for a lifter or flipper will depend on the tournament rules, your ability as a builder, the materials and components available to you, and integration with the rest of your robot design. Suggest you search thru this archive for many previous posts about lifters and flippers to see what design might be best for you.

Q: can you please give the specifications of the dc motor we need to use to rotate a 3kg cicular disc at 1000 rpm ? [India]

A: [Mark J.] The power needed to spin a 3 KG disc to 1000 RPM depends on:

  • How quickly you want the disc to reach 1000 RPM; and
  • The diameter of the disc.
The power to sustain the disc at 1000 RPM is negligible, so it all comes down to how quickly you want your disc to reach the required speed. A larger diameter disc will take longer to spin up to speed than a smaller disc of the same mass, but it will store more energy for a greater impact.


  • An aluminum disc 300 mm in diameter and 15 mm thick weighs 3 kg. It will store about 170 joules of energy at 1000 RPM. A 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 0.8 second -- very quick, but very little stored energy.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. The same 'Small Johnson' motor with 0.56 N-m stall torque and a no-load 24,000 RPM speed geared down 24:1 will spin this disc up to 1000 RPM in about 3.5 seconds -- much slower, but much better energy storage.

  • An aluminum disc 600 mm in diameter and 4 mm thick weighs 3 kg. It will store about 730 joules of energy at 1000 RPM. An Ampflow E30-150 motor with 5.0 N-m stall torque and a no-load 5700 RPM speed geared down 5.7:1 will spin this disc up to 1000 RPM in about 1.7 seconds -- a good balance of speed and energy storage.
I will mention that most builders choose to spin their weapons faster than 1000 RPM. Doubling the speed to 2000 RPM will give four times the energy storage, but will require four times as long to spin up to full speed. You didn't mention the weight class of your robot, but a useful spinning weapon will require at least 40 joules of energy storage per kilo of robot weight. Plan accordingly.

Q: Hi Mark, what do you think of abrasive disks as cutting weapons? [India]

A: [Mark J.] NO CERAMIC CUTTING DISKS!!! It doesn't take much of an impact to shatter the disk, sending sharp shards at high speed in all directions. Given the average combat arena in India, that's a disaster. No sane event organizer would allow you to use such a weapon, plus they simply aren't effective at doing significant damage to your opponent.

If you are intent on using a cutting blade, an arrangement like that used by 'Gloomy' is your best option: an all-metal circular cutting saw attached to a movable arm that keeps the blade out of harm's way until the opponent is pinned and the blade can be put to good use. See: Gloomy vs. Hyperactive.

Q: ...and also about how much pressure would be needed to make a decent lifter in midweight class.

A: The power of a pneumatic ram is calculated by multiplying the gas pressure by the area of the piston. A lower pressure simply requires a larger diameter cylinder to compensate. I suggest that you read the Team Da Vinci Robotics Understanding Pneumatics page to get a good overview of combat pneumatic weapons and how to calculate the force of your lifter design.

A typical 'low pressure' pneumatic lifter will operate at 150 psi. Commercial pneumatic regulators, cylinders, and other components certified for this pressure are widely available and are relatively safe to work with.

Our own heavyweight lifter 'The Gap' uses a 150 psi pneumatic system to power a 3" diameter actuator. The actuator provides 1000 pounds of force, which translates to more than 400 pounds of lifting force out at the tip of the lifter arm.

Q: what is the best possible weapon against 'Tauro Maximus'?? [India]

A: [Mark J.] Let's look at the fight record. In US competition from 2009 thru 2014, heavyweight drum robot 'Tauro Maximus' lost 11 of its 32 fights:

  • One loss to 'DXTER' - a hinged wedge
  • One loss to 'Polar Vortex' - a lifting wedge
  • One loss to 'Last Rights' - a horizontal bar spinner
  • Two losses to 'Original Sin' - a pivoting wedge
  • Three losses to 'Great Pumpkin' - a simple wedge
  • Three losses to 'Sewer Snake' - a forked wedge/lifter
Ten of those eleven losses were to some form of wedge. A smooth low wedge or scoop surface gives no edge for a drum impactor to strike against, neutralizing the drum weapon.
Q: hi mark...i am having drum bot when i give a big hit to opponent my shaft bends inside. first i tried 20mm shaft in MS it bends. and then i used 25mm shaft at EN24 It also bends , what can i do at which mm and which material i have to use? [India]

A: [Mark J.] 'Ask Aaron' isn't an engineering service (see Frequently Asked Questions #17) but if I were spec'ing a weapon shaft I'd need a lot more information:

  • Drum weight?
  • Shaft length?
  • Weight class?
  • Peak drum speed?
  • Dimensions of the drum?
  • Live shaft or dead shaft?
  • Where exactly did the others bend?
  • What is the design of the shaft support?
I suspect that the problem isn't the diameter of the shaft or the material from which it is made -- a 25 mm EN24 shaft is difficult to bend, hardened or not. If I had to guess I'd say the problem is a flawed design in the support and bearing placement for the shaft, but I can't confirm that because you haven't told me anything about your design. The hamburger is bad.
Q: hi mark ... i am making a 10 kg war bot .. with a vertical disc ( very thick , like a fat drum ) of 4 kg . I was planning to use two small johnson motor ( the one on robotmarketplace ) , for the weapon geared down to 3000 rpm . will this much speed and torque be good enough? also is 4 kg god , or shall i increase the weight ? Each motor gives 0.36 HP . [India]

A: [Mark J.] Spinning weapons of a given mass can have very different performance characteristics depending on how that mass is distributed. Example - here are calculated energy storage values for three different hollow steel drum weapons with the same mass and length, but different diameters:

  • 15 cm long - 15 cm diameter - 14 mm wall thickness: mass is 7 kg and it stores 2900 joules of energy at 4000 RPM

  • 15 cm long - 20 cm diameter - 10 mm wall thickness: mass is 7 kg and it stores 5600 joules of energy at 4000 RPM

  • 15 cm long - 25 cm diameter -  8 mm wall thickness: mass is 7 kg and it stores 9200 joules of energy at 4000 RPM

For me to calculate the performance of your 'fat drum' weapon you'll need to give me more detail: material, length, diameter, and wall thickness if hollow. Tell me those things and I'll tell you if your weapon has enough energy and how long it will take to spin up.

Four kilos of weapon in a ten kilo 'bot is plenty -- if it's the right shape.

Q: I would like to add details ... the drum is 7.5cm dia , and 15 cm long solid mild steel . this is non- machined piece so not sure of weight. the pulley will be machined in the same piece and used with a dead shaft of 1.6 cm dia. I don't want to make it hollow , just enough required for the dead shaft to go in ... used with bearings . so will this setup be enough for 10 kg , and how much will be the energy stored?

A: A useful spinning weapon will require about 40 joules of energy storage per kilo of robot weight -- at a minimum. Using two 'Small Johnson' motors geared down to 3000 RPM, the Run Amok Excel Spinner Spreadsheet calculates:

  • A steel cylinder 7.5 cm in diameter and 15 cm in length with a 2 cm hole running thru the center will have a mass of 4.84 kilos (not counting impactor teeth). It will store 180 joules of energy at 3000 RPM and will spin up to speed in about 0.5 second. That's 18 joules per kilo of 'bot weight -- not nearly enough to be effective.
The problem isn't the mass of the drum. The problem is that the mass is concentrated very close to the rotational axis. Rotating mass stores more energy as it is placed farther away from the rotational axis. Let's run the numbers on some 4 kg weapons with different dimensions:
  • A steel disc 15 cm in diameter and 3 cm thick with a 2 cm hole in the center will have a mass of 4.06 kg. It will store 580 joules of energy at 3000 RPM and will spin up to speed in about 1.7 seconds. That's 58 joules per kilo of 'bot weight -- enough to be very effective.

  • A hollow steel tube 15 cm in diameter and 7.5 cm in length with a wall thickness of 16 mm will weigh 3.94 kg (not counting end plates -- just an example). It will store 890 joules of energy at 3000 RPM and will spin up to speed in about 2.7 seconds. That's 89 joules per kilo of 'bot weight -- a VERY dangerous weapon.
That's why drum weapons are usually hollow. Reconsider your 'solid drum' design, or at least increase the weapon diameter to make it a disc.

Q: Hey I am using a horizontal spinner I am confused which one is most important speed or bite .since I failed to a wedge bot even though I had enough speed and energy [India]

A: [Mark J.] Which is more important -- the front wheel on a bicycle or the rear wheel? You need both!

  • If you have no bite you have to rely on your opponent to make a mistake and leave a sharp edge exposed for your weapon to grab;
  • If you have no speed your stored energy suffers and even a good hit on your opponent does no damage.
The solution is to design a weapon that has high energy storage at a rotational speed slow enough to deliver great bite. Search this archive for more discussion on this topic.
Q: can a hollow pipe be used as a dead shaft for drum? [India]

A: [Mark J.] Can a wooden boat carry an elephant? Depends on the boat -- depends on the elephant. The hamburger is bad.

Q: I m making a drum bot having ID 120mm,so i will fit bearing housing to end caps as i cannot get 120mm OD bearing having smaller bore. my question is, will the bearing housing sustain impacts caused by drum? [India]

A: [Mark J.] Short answer - yes. The force of the impact will be borne by the impactor, the drum, the endplates, the bearings, the drum shaft, and the shaft supports. Each of these elements must be strong enough to withstand the full force of the weapon backlash. Don't skimp on the bearings!

Date marker: March 2015
Q: A builder on one of the forums claims that he spins his featherweight [30 pound class] drum weapon at 20,000 RPM and still gets bite. Is this possible? [Ohio]

A: [Mark J.] Usually, no way -- but I know the bot in question.

A conventional drumbot has the drum well out in front of the rest of the robot and rams the weapon directly into the opponent. In this particular robot the drum is set fairly well back and rather tall 'wedgelets' lead the opponent up and into the drum at a height close to the drum axis. This gives the drum a good chance to impact the sharp leading edge of the opponent and provides a favorable attack angle.

This isn't 'bite' in the conventional sense, but it does allow a very fast drum to connect and release a decent hit IF the driver can coax their opponent up the wedge.

Q: Hey mark.. I have seen Robo Bacon in antweight matches in robogames 13 and I have decided to do that robot weapon for 35 kg bot ... What kind of weapon is that? How it differs from double teethed drum? Which produces more impact on opponent? please prefer some material to do that weapon.... [India]

A: [Mark J.] How can you decide on a robot weapon before you even know if it has any advantages?

Team Uai!rrior's antweight 'Bacon' has a single-tooth spiral drum weapon, similar to hall-of-fame middleweight 'Professor Chaos'. Go read section 6.3 of the RioBotz Combat Tutorial for an explanation of the advantages of an asymmetric single-tooth spinner.

See this previous post for some notes and a diagram on spiral drum construction. The machining is difficult, and balancing the drum is a real challenge. A spiral drum is typically made of aluminum with steel inserts for balance and a hardened steel impactor.

Q: hey Mark., what are the merits of using chain drive and belt drive in LAST RITES ?? what would happen if we use belt drive instead of chain drive ., [India]

A: [Mark J.] The 'Last Rites' chain drive has been previously discussed. See this previous post.

Note: 'Last Rites' still has the chain drive, but now has a mechanical clutch mechanism to provide a little 'slip'.

Q: Hello, I am designing a 15lb horizontal spinner robot, with an uncanny resemblance to Hazard. I was trying to figure out a way to securely mount my 5.16lb, 24in bar across the top. I have visited Team Delta's website, and they seem to be using shaft collars both on the top and bottom. I am currently running a dual bearing support system, and a v-belt pulley driven by a beefy brushless motor. Any help would be appreciated! Thanks -- Erik [Winchester, Massachusetts]

A: [Mark J.] I wrote one answer to this question, briefly posted it, thought a little more, took the original post down, and started again. Let's see if this makes sense.

Hazard's 22 pound spinning blade is not directly fixed to the 1.5" diameter steel weapon shaft. It is squashed firmly between two shaft collars (see picture) that are tightened onto the shaft by machine screws that reduce the diameter of the collars for a clamping fit -- not held in place by set screws! Custom phenolic washers on either side of the blade form a mechanical clutch that allows the blade to slip on impact and limit the shock transmission back to the drivetrain (Hazard Build Report).

I'm not very keen on squishing a weapon blade between two collars and relying on friction to spin it up, but it worked very well for 'Hazard'. If you do decide to use shaft collars, I would strongly recommend clamping collars over set-screw collars, but I think I'd axially fix the blade by broaching the shaft and blade and fitting a key to lock rotation -- let your v-belt act as the slippy-clutch.

Q: if we want to give maximum impact of drum teeth to the oposition,then what should be the position of the skid,it should be closer to the drum or should be away. [India]

A: [Mark J.] Suggest you read section 6.6 of the RioBotz Combat Tutorial for a full treatment of weapon support and maximizing impact. Read the rest of the RioBotz tutorial while you're at it.

In general, your skid should be placed directly underneath the drum axle and be as stiff as possible.

Q: The power of robot rotary weapons is measured in joules, but joules doesn't mean much to me in real world terms. Can you give me an example of a 1000 joule impact with everyday objects? [West of the Pecos]

A: [Mark J.] Sure. A baseball bat hitting a major league home run is an impact of just about 1000 joules. Bear that in mind when you're designing and building your robot -- can you hit it as hard as you can with a baseball bat and have it bounce back for more?

Q: hi mark ,i was planning to build a 30 kg bot with vertical spinning bar of dimensions 20*15*5 cm (l*w*th) of mass around 7 kgs (mild steel) and i want to run it around 3000 rpm ,i was using an ampflow e30-400 to power my weapon and i want know the reduction ratio and energy storage of my weapon(i was unable to get through the spinner spread sheet)....and can i use ms for teeth ? [India]

A: [Mark J.] Lets start with the reduction ratio. From the AmpFlow website: the no-load speed of the E30-400 motor at 24 volts is 5700 RPM. Allowing for mechanical and aerodynamic drag on a spinning weapon will reduce that speed by about 15%, so let's call it 5000 RPM. You want a 3000 RPM weapon speed, so the formula is:

Motor Speed ÷ Weapon Speed = 5000 ÷ 3000 = 1.67 to 1 reduction.

A chunk of steel 20 cm by 15 cm by 5 cm isn't as much a bar as it is a square, and it will weigh closer to 12 kg than 7 kg. If you are removing material from that almost-a-square to reduce it's mass to 7 kg, I'd need details of the true shape of the rotating mass to calculate the energy storage.

IF the weapon is a simple 12 kg steel block of the dimensions you give, it will store about 3000 joules of energy at 3000 RPM. Spin-up time will be about 3 seconds.

Tooth material should be harder than the material you'll be hitting with the teeth. Mild steel isn't very hard. If your particular steel alloy can be hardened, I would consider at least surface hardening the teeth.

Q: hi mark ,iam the spinning bar guy again sorry i have given wrong info about it..... actually its dimensions are 20*12*4 cm (l*w*t) around 7.4 kg ,what will be the energy storage of my weapon ?and was that weapon is enough for my 30 kg bot ? OR i should chage my weapon dimensions ? thank you for your info about reduction ratio......

A: [Mark J.] That's a big difference. I'm not sure how you confused the dimensions that badly and sent me down the rabbit hole with the other numbers. The 'Ask Aaron' service is free, but that doesn't mean my time has no value. Please double check your numbers in the future.

A steel bar 20*12*4 cm spinning at 3000 RPM will store about 1700 joules of energy with a spinup time of about 2 seconds. That's better than 25 joules per pound of robot weight, which isn't bad for a bar spinner.

If your design allows, a longer and narrower bar of the same weight would store more energy. Examples: a 24*10*4 cm bar spinning at 3000 RPM will store about 2100 joules (2.5 second spinup), and a 28*8.5*4 cm bar will store more than 2600 joules (3 second spinup).

I'd strongly suggest that you learn to use the Run Amok Excel Spinner Spreadsheet so that you can examine many weapon designs and find the one that best fits your needs and your design parameters.

Q: Hello sir. I am new to this site. I would like to build a 30 kilo war robot with a drum weapon. Would a hollow drum or a solid drum be better? What kind of a material should I use? And how should I use this? [Maharashtra, India]

A: [Mark J.] Welcome to 'Ask Aaron'. I do ask that you do me the courtesy of reading thru the Frequently Asked Questions and the Recent Questions page, and then performing a keyword search thru the 'Ask Aaron Archives' to see if we've already answered your questions.

  • Your solid/hollow drum question is answered just a few questions below, and there is a great deal of additional information on drum construction elsewhere in this archive.

  • Your question on drum material is answered just below the answer to the hollow drum question, and again is discussed at other places in this archive.

  • Your question about how to build the weapon is -- you guessed it -- answered a bit further down this page and discussed in detail elsewhere in this archive.
Read thru these earlier questions and answers. If you have NEW questions, write back.
Q: Hello sir. Sorry to disturb you, I had go through your page for the last two days.

A: [Mark J.] No apology needed. I'm always pleased to receive robot questions, particularly from people who have taken time to read thru the website first!

Q: First: will you please tell me about energy storage capacity? What role does it play in the drumbot?

A: Rotational weapons in robot combat (drums, disks, blades) are all devices that accumulate and store the mechanical energy output from a motor over a period of time as kinetic energy. That stored kinetic energy will resist attempts to stop the rotation of the weapon. The greater the stored energy, the greater the potential impact the weapon will have on an object (like another robot) that comes into contact with an impact surface on the weapon. More energy = greater potential damage.

The energy storage of a spinning weapon depends on its rotational speed (RPM) and its 'moment of inertia' (MOI) about the center of rotation. The Run Amok Excel Spinner Spreadsheet will calculate the amount of kinetic energy stored by a spinning weapon based on RPM, material density, and the shape and dimensions of the spinning components.

See the Wikipedia article on Flywheel Physics for a taste of the math.

Q: Second: you talk about 'thickness' of the drum -- what do you mean by thickness?

A: Most drum weapons are hollow tubes with caps at each end to support the bearings. This type of design is used because hollow drums are MUCH more efficient (by mass) than solid cylinders of the same dimensions at storing kinetic energy. 'Thickness' refers to the thickness of the tube wall.

For example, a hollow drum may be 30 cm long and 20 cm in diameter with a wall thickness of 1 cm. A solid drum of the same length and diameter could be thought of as having a wall thickness of 10 cm -- all the way to the center!

Q: Third: in the Team Run Amok Spinner Spreadsheet, which calculations do I use for the drum bot?

A: The spreadsheet has sections for 'motor and drive', 'ring or tube', 'disk', and 'bar'. A drum weapon is a combination of a tube and two disk caps plus the motor and drive:

  • In the motor section, enter the no-load speed of the motor, the stall torque, and the drive reduction ratio between the motor and the weapon. The drive reduction ratio is typically the ratio of the motor and weapon pulley diameters. The 'no-load current' and 'voltage' are optional and only used in the battery capacity calculation.

  • In the tube section, enter the material density of the tube (from the handy chart at the top center of the sheet), the tube height ( = length in a horizontal drum), the tube radius (to the outer wall), and the tube wall thickness.

  • In the disk section, enter the material density and dimensions of one drum end cap -- but double the thickness to represent two caps.

  • Leave the bar section empty -- you have no bar element in a drum.
What about impact teeth? If the mass of the impact teeth is significant, you can manually calculate their mass and add a bit to the tube length dimension in the spreadsheet to increase it's mass by the mass of the teeth. That will give a good approximation.

Q: Which materials do you prefer for the drum and tooth in a drum with a length of 20cm and a diameter of 15cm?

A: Drums and end caps are typically made from aluminum tubing and plate -- 6061 alloy is commonly available and in suitable sizes and is easy to work with. Mild steel can also be used if it is more easily available.

Impact teeth are preferably made from tool steel. There are many posts about the suitability of various tool steel alloys in this archive.

Q: Where can I get those materials in Tamil Nadu, India?

A: I have no idea what materials are available in Tamil Nadu or elsewhere in India. I suggest that you join the 'Combat Robotics India' group on Facebook for support on local parts and materials.

Q: How do I make the end caps?

A: The end caps must be made with great precision or your drum will not be balanced. Typically they are turned on a metal lathe. Consult with a metal fabrication shop if you do not have the skills or equipment needed.

Q: mark lucky here tell me exactly which type of bearing i should use in the end rings of my drum [weapon]
drum diamensions :
od = 14 cm
length : 20 cm
weight : 14kg ( including tooth plate )
speed : 4500 to 5000 rpm
shaft dia : 3 cm
[Maharashtra, India]

A: [Mark J.] In general I'd use unmounted, shielded, pressed-steel cage, radial ball bearings with a 30 mm bore and about 16mm width -- something like these: shielded bearings.

Q: I want to use ampflow e-400 motor for my drum [weapon],but as it is not available [in India], can i use 2 e-150 motors, one on each side of d drum?rpm would be same but will d hp and torque increase? [Maharashtra, India]

A: [Mark J.] Yes, you can use a pair of AmpFlow E30-150 motors to power your drum. Performance comparison below -- and I'll add a third option:

  E30-400 (x1)
@ 24 volts
E30-150 (x2)
@ 24 volts
E30-150 (x1)
@ 36 volts
Price (US)$109$158$79
Weight5.9 lbs7.2 lbs3.6 lbs
Stall Torque1500 oz-in1420 oz-in1065 oz-in

As the table shows a pair of AmpFlow E30-150 motors will have performance quite similar to a single E30-400 motor, but at greater cost and weight. An attractive option is to overvolt a single E30-150 by 50% to 36 volts. I might not recommend overvolting the E30-150 this much for robot drive use because of the high and sustained torque loading imposed on drive motors, but for a weapon that spends much of its time at low loads it is an option that you might consider.

Q: Hi, this is Chaitanya again, from India [scroll down a few posts to see Chaitanya's previous post] thanks for your guidance. As you mentioned, you want ratings of pmdc motors to calculate battery , they are
Stall torque: 50 kg-cm
stall current : 54 amp
rpm: 260
diameter of wheel : 16 cm
as you told, we are thinking on drum of material EN41 metal, length 22 cm, wall thickness 20cm [you must mean millimeters], diameter [I hope you mean radius...] 6.5 cm (excluding teeth height of 1 cm). as you said, wall thickness is too much less for strength because we are inserting 1 cm of teeth height inside the drum. so please give some suggestions regarding drum design and teeth dimensions so that we can design in a better way? as you told, following are the metals available in India for teeth are SS304, SS316, HEHER D-2, HEHER D-3, HDS H-13, BN-24, BN-31, OHNS.
which one is best for teeth? Is there any need to harden teeth metal? if yes then up to what value it should be hardened in HRC ? Please guide us about drum teeth. We read all the riobotz combat pdf for height of teeth but calculations were nt match.
please help about required battery for drive motors for 5 min match.
Thanks. [India]

A: [Mark J.] Please re-check the pmdc motor ratings you provided. A 24 volt motor with a 54 amp stall current should produce much less than 50 kg-cm stall torque if ungeared, and should produce much more than 50 kg-cm stall torque if geared down to 260 RPM. You also failed to send me the gear reduction ratio of the gearbox. I need clarification on both of these. Might you send me a link to the motor and gearbox specs?

Of the options given, D2 tool steel (HEHER D-2) is probably the best for drum teeth. D2 tool steel can be hardened to HRC 58/60, but the drum design must support the teeth well as D2 has poor impact toughness and can shatter if left unsupported.

Tooth height depends on robot attack speed, drum RPM, and the arrangement of impactors on the drum:

  • With 260 RPM gearmotors and 16 cm wheels, your max attack speed is a bit under 5 MPH (200 cm/second).

  • Allowing for mechanical and aerodynamic loss, your max drum speed is about 5500 RPM (92 revs/second).

  • Assuming two full-width impactor bars on opposite sides of the drum, you have an impactor passing a fixed point on the radius every 5.4 milliseconds (1 ÷ (92 revs/second × 2 impactors) = 0.0054 second).

  • At top speed your robot can move forward 1.08 cm in 5.4 milliseconds (200 cm/sec × 0.0054 sec = 1.08 cm).
So, your optimum impactor height for a full-speed ramming attack is 1.08 cm. You got lucky in your guess of correct height.

Note: I've seen a lot of Indian robot matches and I have never seen a full-throttle, top-speed ramming attack by a drum robot. Indian combat arenas tend to be pretty small, and combat generally consists of the robots maneuvering into position and easing their drum weapon into the other robot with a short, darting attack. This is less than ideal and does not use the full depth of the impactor.

You have not mentioned the impactor design you have in mind. Something like the full-length impactor bars used by 'Solaris' (Team Cosmos - pictured) should do well for you. The Solaris drum has hardened steel bar impactors set into shallow grooves milled onto the drum that locate the bars and take the shear forces off the countersunk machine screws that hold it in place. There are a couple of additional impact teeth at the ends of the Solaris drum, but these were added to correct a specific weakness in the robot and may be ignored for your application.

Q: hey, this is chaitanya again, so sorry about those mistakes, the corrections are as follows:
voltage: 24 volt
stall current:- 9 amp
gear box ratio:- 4.5:1

so can you now tell me about battery for drive , pushing power of bot etc. for any other info, refer previous question. as you mentioned in the answer of First question, please tell me how many batteries of 12 volt 9 amp can we use (for both drive and weapon) so that bot can work properly for 5 min match?

New drum dimensions are length 22 cm, radius 6.5 cm, thickness 22 mm , are this dimensions suitable for our robot of weight 66 kg? if not then please guide us. which metal can we use for drum between EN31 or EN24? Is there any need of hardening drum metal? if yes then up to what value in HRC? Can you please tell me about tooth depth also. (length from surface of drum ). which metal can we use for dead shaft and tell us its radius also.
thnxs for helping us.

A: [Mark J.] I think you have some more work to do:

Drive Motors: the spec numbers you've given for your PMDC drive motors are quite odd, but if you're sure they're correct I'll continue.

The motors are not adequate for your drivetrain. I recommend a bare minimum of 4 watts of drivetrain output power per pound of robot, and your four-motor drivetrain produces less than 2 watts per pound of robot. A typical robot in your weight class might have 10 or 20 watts of drive power per pound.

With 16 cm diameter wheels, your four drive motors combined do not provide enough torque for reasonable pushing power in a 66 kg combat robot. Maximum pushing power will be well under 30 kg -- less than half what a robot in your weight class might be expected to produce. Acceleration will be very poor, and the drivetrain will stall under moderate load risking damage to the motors. I STRONGLY recommend that you obtain more powerful motors.

There is little point in calculating battery requirements for these motors, as I'm rather certain that the motors would fail well before the battery is depleted. For what it's worth, it comes out to just under 2 amp hours of battery capacity to operate the drivetrain for 5 minutes with these motors.

Drum: your drum dimensions and materials can store a reasonable amount of energy for a robot in your weight class. I can't comment on its overall suitability for your robot, as the weapon design must integrate with the rest of your robot design -- about which you have told me very little. The drum itself appears to be fine.

The material used for the drum is far less critical than the material used for the impactors. Given your dimensions, any grade of mild steel would work well. No need for hardening on the drum material.

The calculations for tooth depth were included in my answers to your last questions -- please find your answer there. None of the drum changes have altered those calculations, although they may change when you select new drive motors.

Dead Shaft: as discussed in Frequently Asked Questions #4 and #17, I do not provide a free engineering service. I'm pleased to discuss materials and dimensions in general, but I cannot recommend specific materials and dimensions for specific applications. There are too many unknowns in your design for me to have faith in an analysis at that level. I can recommend that you research designs similar to yours and examine the materials and dimensions other builders have successfully used.

Order of magnitude estimate: 3/4" chromoly steel, very well supported as close to the drum bearings as practical. When in doubt go bigger.

Q: Mark,
What do you think is a reasonable/appropriate [weapon] speed for a drumbot? Because I have heard claims of speeds upwards of 20,000 rpm and that seems ridiculously fast. At that speed isn't there no time for the robot to achieve bite? [New Richmond, Ohio]

A: [Mark J.] It's certainly tempting to spin a weapon up to stupid fast revs. Double the speed, get four times the energy storage -- awesome!

The problem, as you note, is that the faster the weapon spins, the harder it is to get the weapon to 'bite' into your opponent and get a powerful hit. A weapon without bite will just skitter across a smooth surface and do no harm at all. If you have no bite you must rely on your opponent to make the mistake of offering a sharp edge to give your weapon something to grab.

How fast is too fast? Depends on the spacing of the impactors and your attack speed. You can get away with greater RPM if you have a single counterweighted impactor and a high rate of closure on your opponent at impact. Decent bite can be very hard to come by if you have multiple impactors and a timid attack.

Section 6.3 in the RioBotz Combat Tutorial has a good explanation of weapon speed and bite, as well as the formulas for calculating bite depth. It's well worth a read.

There is one excuse for a hyper-speed drum weapon: when two drums go 'head-to-head' and their weapons meet, the faster drum wins. So, if you are expecting to fight a lot of other drumbots you might want to be able to reach for a few thousand extra RPM to see if you can launch them. The rest of the time you'd be much better off to throttle the weapon back and charge hard.

Q: In a pneumatic system for flipper which valve should i use ? Should i use 5 ports or 3 ports ? [West Bengal, India]

A: [Mark J.] I suggest you read the Team Da Vinci: Understanding Pneumatics page for a full description of 3, 4, and 5 port solenoid valves and their applications, as well as general design information and illustrations of robot flipper systems. I also suggest that you read thru the many posts about pneumatic systems in this archive for additional design considerations.

Q: Hi, this is Chaitany, from India, we are willing to design a robot same as tauro. We are using amflo f30-150 motor having noload current 2.5 amp and stall current is 375 amps for hollow drum having length 16 cm , diameter 14 cm and wall thickness 27 mm. We get the energy 4205 joules at 4347 Rpm at 3.02 sec. robot weight is 66 kg. Dimensions of bot is 60*70*14 cm. So the my question is how can we mount the drum on the shaft if dead shaft system is used ? Can i use bearings at edge of drum? any suggestions regarding drum and energy? We cannot increase the diameter.

We are using 4 pmdc motors for motion having rating 24 volt 9 amp. We are using 12 volt 9 amps battery, 2 for weapon motor and two for pmdc [drive] motors. So any suggestions about battery? If these batteris are use, then till what time robot work properly??? Is there any arrangement except dead shaft system?, if yes then guide us plz. Can we use carbide metal as a teeth? Plz help us. thnxs. [India]

A: [Mark J.] First, thank you for sending such complete information about your robot weapon. That makes it much easier for me to answer your questions.

One correction: the stall current on the AmpFlow F30-150 motor is misprinted as 375 amps on the Robot Marketplace site. Calculating the stall current from the spec numbers on the AmpFlow website gives either 296 amps (Volts/Resistance = 24/0.081 = 296 amps) or 294 amps (Stall Torque/Torque Constant Kt = 1370/4.66 = 294 amps) -- so let's split the difference and call it 295 amps.

Mounting the drum on a dead shaft: check a few posts down in this archive and you'll find an answer to this same question asked a few days ago -- with a photo. Hollow drum weapons typically have machined 'end caps' that slip into the ends of the drums and are held in place by machine screws inserted thru the drum face into threaded holes in the caps. The end caps have a hole sized for a bearing to support the drum on a dead shaft.

Drum suggestions: why so short a drum? A 24 cm long steel drum 14 cm in diameter with a 16 mm wall thickness will weigh the same (12 kg) as your short drum and will store more energy (4750 joules) at the same RPM (4347). For maximum energy storage, make the drum as long as practical for your design and reduce the wall thickness to keep the weight the same.

Battery suggestions - Weapon: if you look a bit further down the page of the Run Amok Excel Spinner Spreadsheet that you apparently used to calculate your drum energy, just below the 'Results' box is another box labeled 'Battery'. This box reports that your weapon system might be expected to use perhaps 1.45 amp-hours of current in a typical 3 minute match. With a 9 amp-hour battery you have many times the power needed for your weapon.

Battery suggestions - Drive: to calculate the current consumption of the drive motors I need much more information about the motors -- their stall torque, stall amperage, gear reduction, RPM, and wheel diameter. With that info I can estimate their current consumption as well as the robot speed, acceleration, and pushing power. That said, I very seriously doubt that the motors will come anyplace close to completely draining a 9 amp-hour battery pack in a typical match.

Commonly, combat robots have a single battery pack that operates both the weapon and the drive system -- consider using only two 12 volt 9 amp-hour batteries for the whole robot rather than four. You may likely be able to reduce the size of the batteries even further, but I need that drive motor info to be able to tell you by how much.

Alternative to dead shaft: sure -- you can use a live shaft firmly affixed to the drum endplates and supported by bearings in the robot chassis. A live shaft is rarely used because a rigidly fixed dead shaft can be a stiffening member joining the chassis supports together, but if a rotating live shaft suits your design you can certainly use it.

Carbide impactor teeth?: no! Tungsten carbide is very hard, but it is also very brittle and can SHATTER on impact and send sharp shards flying at high speed. US and European events typically forbid carbide impactors for safety reasons -- don't use them!

Q: Hi. For AmpFlow A28-150 motor, what is requirement of battery, and what will be the requirement of amp at 24 volts (for drum motor). [Maharashtra, India]

A: [Mark J.] Briefly, the amperage requirement for any electric motor depends on the torque load placed upon that motor, and you've given me inadequate information to calculate that load.

To calculate the torque load on your motor you need to first calculate the moment of inertia of your drum. This requires specific information on the dimensions of your drum and the material from which it is made. The Team Run Amok Excel Spinner Spreadsheet can perform those calculations for you and estimate the battery capacity requirement for a match of given length.

The AmpFlow A28-150 motor at 24 volts can pull 385 amps at full stall, and the closer you can come to providing that much amperage the faster your drum will spin-up. If your weapon should stall against an immoveable object, your motor may attempt to pull that full amperage from the battery for as long as the weapon is stalled and may damage a battery pack that is not rated for that great a drain. There is no point to having a powerful weapon motor if you do not supply it with the full current it needs to perform properly.

I suggest you search for the many previous posts in this archive covering the use of the Spinner Spreadsheet to calculate moment of inertia and the battery requirements for your weapon.

Q: Mark, I am a little confused about capacity of single tooth snail design drum of 'Touro'.
I am thinking to try that design in a local event.
I know it is very difficult in manufacturing but I wanna try.
Please tell me what I should do. [Tamil Nadu, India]

A: [Mark J.] RioBotz designed the "snail drum" for 'Touro Maximus' using the MatLab Genetic Algorithm Toolbox software. You can find the academic paper describing the drum and the design process here: Drum Shape Design and Optimization Using Genetic Algorithms. I suspect that the drum represents an investment of hundreds of hours and thousands of dollars. It's a whole lot of work for very little gain in drum performance -- largely an academic exercise.

A single-tooth drum is a fine concept, but it is MUCH easier for a competent builder to implement a design using a counterweight balancing insert on the opposite side of the drum from the uni-tooth. The guys at RioBotz were just showing off their technical muscle in designing the snail drum. You should NOT try to duplicate that effort.

Q: I'm making a drum with 14cm O.D. and 15mm wall thickness, which makes the I.D. 11cm. Drum weight will be around 7kg
What kind of bearings should I use for rotation of drum on rod? If I use 11cm OD bearings they will have a bore of 80-90mm which will increase the size and weight of the rod. Please help. [Maharashtra, India]

A: [Mark J.] There's no need for a huge bearing. Drum weapons typically have machined 'end caps' held in each end of the drum by screws. The end caps have holes to mount bearings of suitable size for the supporting rod.

Q: Am using 3HP AmpFlow motor for my drum bot. Weight of drum is 7-8 kg and the length of drum is 14-15 cm. Please suggest me the drum diameter and thickness of drum which stores maximum energy. [Maharashtra, India]

A: [Mark J.] I've spent considerable effort to put together the Team Run Amok Excel Spinner Spreadsheet that allows builders to model the performance of spinning weapons, but there still seems to be confusion about the basic physics of drum design and moment of inertia. I'd suggest that you study up on the topic before continuing your design efforts -- it's more than I can teach here.

The variables that determine the energy that a spinning drum holds at a given RPM are: mass, material density, diameter, and length. You've specified mass and length, and I'll assume that you're using steel. With mass, length, and speed held constant the energy storage will increase with increasing diameter. Examples - for a bare steel tube (no end caps or impactors):

  • 15 cm long - 15 cm diameter - 14 mm thickness: mass is 7 kg and it stores 2900 joules of energy at 4000 RPM

  • 15 cm long - 20 cm diameter - 10 mm thickness: mass is 7 kg and it stores 5600 joules of energy at 4000 RPM

  • 15 cm long - 25 cm diameter -  8 mm thickness: mass is 7 kg and it stores 9200 joules of energy at 4000 RPM
If your primary design consideration is greatest energy storage, make the drum as large in diameter as is practical for your overall design. There are other elements in drum design that should be examined before you get too excited about maximum energy storage -- see recent posts in this archive on drum design.
Q: Hi Mark. If I want 5000 joules of stored energy at 5000 RPM, what should be drum's outer dia and thickness if mass is limited upto 13 kg?
I am confused about it. I can't exceed outer dia 15 cm. Please help me. [Tamil Nadu, India]

A: [Mark J.] There seems to be a lot of confusion about stored energy in rotating weapons. Stored energy in a cylinder rotating around its radius center is a function of:

  • Rotational Speed (RPM)
  • Drum Diameter
  • Drum Length
  • Drum Wall Thickness
  • Material Density

You've given me a desired output and only two of the five variables (rotational speed and diameter). By selecting values for material density and drum length, I can give a design solution for any wall thickness to meet your criteria. For example:

  • A steel cylinder 15 cm in diameter, 100 mm in length, and solid to the center will weigh 11.5 kilos and will store 5000 joules of energy at 5000 RPM.

  • A steel cylinder 15 cm in diameter, 300 mm in length, with a 6.7 mm thick wall will weigh 7.1 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 300 mm in length, with a 31 mm thick wall will weigh 9.6 kilos and will store 5000 joules of energy at 5000 RPM.

  • An aluminum cylinder 15 cm in diameter, 2600 mm in length, with a 2 mm thick wall will weigh 6.7 kilos and will store 5000 joules of energy at 5000 RPM.

I suggest that you download the Team Run Amok Excel Spinner Spreadsheet to model the performance of various spinner dimensions and designs.

I also suggest that you browse thru this archive for several recent posts about drum weapon design.

It is quite odd to specify a weapon diameter, speed, and energy storage as starting parameters, and to then back into the other dimensions. In particular, specifying such a high rotational speed is detrimental to the overall performance of a spinner weapon. Were it my weapon, I would design it to maximize energy storage at the slowest possible speed -- something like:

  • A steel cylinder 15 cm in diameter, 380 mm in length, with a 10 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 3760 RPM.

A 5000 joule weapon does you no good at all if it's spinning too fast to have decent 'bite' and the ability to transfer that energy to your opponent in a single, huge impact. Slow it down a bit. A larger diameter drum would be able to store the same energy at an even slower speed -- example:

  • A steel cylinder 20 cm in diameter, 390 mm in length, with a 7 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2780 RPM.

Or even better:

  • A steel cylinder 25 cm in diameter, 360 mm in length, with a 6 mm thick wall will weigh 13 kilos and will store 5000 joules of energy at 2175 RPM.

Don't compromise weapon performance with dimensional restrictions that you can avoid, and design for the lowest weapon speed that will store enough energy to be effective.

Q: Hi Mark, is there an equation to calculate the force in a pneumatic ram?

A: [Mark J.] Sure - in pounds, inches, and psi:

Force = π × (0.5 × Ram Bore)2 × Pressure

Q: And is there an equation to calculate the force needed to flip a robot? Thank You. [San Francisco, California]

A: Unfortunately, no -- it takes both force and speed to flip an opponent. A ram with a great deal of force that extends slowly will lift but not flip a given weight into the air. A flipper must be able to flow pressurized gas through the regulator, valves, and ports at a very high rate to maintain pneumatic force as the ram quickly extends. The additional variables make this a difficult analysis. Most builders just go for as much speed and power as they can get and hope for the best.

Q: hi, i m planning to make a razer bot of 50kg, so for that hydraulic weapon , i have purchased a double acting hydraulic cylinder of 1 ton with 35-40mm bore and 150mm stroke, so for that can u give me what should be that psi of hydraulic pump(which pump is suitable)? and for up & down motion im using 12v solenoid valve, is it okay? and pls tell which motor i used to drive hydraulic pump? [Gujarat, India]

A: [Mark J.] Several thoughts:

  1. Your questions are very basic -- I'm guessing that you have no prior experience with pneumatics.

  2. Many very knowledgeable teams have tried to build 'Razer' style hydraulic weapon robots, but 'Razer' remains the ONLY successful hydraulic robot. A hydraulic crusher is a very difficult weapon to get right.

  3. If you knew enough to safely build a hydraulic weapon system, you wouldn't need to ask me how to do it.
Sorry, but I don't think you have enough knowledge or experience with pneumatics to take on building a complex and potentially dangerous crushing weapon. I certainly can't give you that knowledge in a few paragraphs here -- pick a different weapon system.
Q: hi mark lucky here how should i join my drum's impactor with my drum because i have made drum composite of mild steel and aluminium from inside as aluminium will cause major damage if i use the bolts while impacting ? [Tamil Nadu, India]

A: [Mark J.] You haven't told me nearly enough about your weapon drum for me to recommend an impactor attachment method. For example:

  • How thick is the drum?
  • How much of that is steel?
  • What aluminum alloy is the rest?
  • How much does the drum weigh?
  • How fast do you plan to spin it?
  • What is the drum diameter?
  • What do you plan to use for an impactor?
If I guessed wrong about any of these things I could give you a poor recommendation!

Q: lucky again steel hollow cylinder with 5mm thick and aluminum solid inserted in it i am thinkin to run it at 4000 rpm and material for impactor is mild steel and aluminum 6001

A: A solid core drum? Not efficient at storing energy -- the farther from the spin axis the mass is placed, the greater the energy storage. But you asked about impactor attachment...

You can certainly bolt thru the steel cylinder into a solid aluminum core, but bolts are not good at handling the shearing forces the impactor will be encountering. Ideally the impactor should be prevented from shifting on impact by insetting it into a groove machined in the drum and holding it in the groove with bolts. That takes the shear loading away from the retaining bolts, but a groove may not be practical given your composite design.

I'd be tempted to bolt down the impactor, then run a weld along the trailing edge to take some of the shear force off the bolts.

Q: Hi, still i have some doubt in Drum energy storage concept , already i read spread sheet calculation i didn't under stand the concept .. (if the weapon is 14kg and run at 5000 rpm with 5000 joules energy storage means ,what is the size of the drum length and diameter?) [Puducherry, India]

A: [Mark J.] There are a great many posts about spinner weapon design in this archive -- suggest you spend some time reading here.

The Team Run Amok Excel Spinner Spreadsheet can model spinner weapons and answer the type of question you are asking, although it's unusual to design a weapon by setting speed, mass, and energy targets and 'backing into' the drum size. There are several things to consider:

  • General Rule 1: for a given mass, increasing the diameter of the drum and thinning the wall thickness increases the energy storage.

  • General Rule 2: for a given mass, increasing the length of the drum and thinning the wall thickness has negligible impact on the energy storage.

  • 5000 RPM is too fast to spin a drum this massive. The problems with spinning a weapon too fast have been discussed repeatedly in the archives. Half that speed might be about right.

  • Why have you chosen 5000 joules of energy as your energy target? A good weapon design has other parameters that are of equal or greater importance as stored energy.

  • You'll need to consider the performance of the weapon motor you plan to use. Is it capable of spinning the drum to speed quickly enough to be useful?
Given your original parameters, the Spinner Spreadsheet quickly shows one possible solution: a steel drum 120 mm in diameter, 450 mm in length, with 10 mm wall thickness and 8 mm thick endplates will weigh close to 14 kilos (I left a little mass for impact teeth) and have about 5000 joules energy at 5000 RPM.

By using other materials and drum thicknesses, you can find a great many diameter and length combinations that will give the same energy storage at the same mass.

Q: As here you told 10mm thickness and 4mm thick end plate means the steel 10mm is i think its very small thickness hw can i fit 4mm end plate? and the center hole also big if the 120mm diameter means i fix big size of bearing?

A: I do not specifically recommend the given dimensions -- they were just the first solution that came from the spreadsheet after a few minutes of playing with it. If you don't like some of the dimensions, change them and use the spreadsheet to tinker with the other dimensions until the energy numbers come back into line.

The endplates may be welded into place, or machine screws may be run in to afix the end plates to the drum. The endplates may also be reduced in diameter a bit, and inserted into the ends of the drum tube before affixing. Look for a solution that suits your construction preferences.

The endplates may be drilled for any size bearing you like. Pick a bearing that will handle the high stress placed on the weapon and the speed of the drum.

Q: read spinner sheet there is one option for motor and drive which motor they asked driven motor (or) weapon motor?

A: This is a weapon spreadsheet. The 'motor' is the weapon motor and the 'drive' is the weapon drivetrain -- typically a belt and pulleys.

Q: if i can use Ampflow A28 400 means what is the gear ratio?

A: I really don't like the idea of running the drum at 5000 RPM. Search for 'bite' in this archive for discussion on problems with running a spinner weapon too fast. The AmpFlow A28-400 at 24 volts spins very close to 5000 RPM, so if you really insist on spinning the drum that fast, a 1:1 gear ratio would be correct.

Q: For disc column: what is material density Kg/m3? I'm not understand m3.

A: The spreadsheet uses units of kilograms per cubic meter (kg/m3) for material density. The spreadsheet includes a table of densities for common robot materials:

  • Steel is 7800 kg/m3
  • Titanium is 4500 kg/m3
  • Aluminum is 2700 kg/m3, and so on.

Q: actually 5k rpm with a 120mm dia weapon would be a decent tip speed. [Parts Unknown]

A: [Mark J.] My objection to a 5k RPM weapon speed has nothing to do with tip speed. Look up 'bite' in this archive, or read section 6.3.1 of the RioBotz Tutorial.

A weapon with two symmetrical impactors spinning at 5000 RPM has an impactor passing by any point in its arc every 6 milliseconds. How much of your opponent's 'bot are you going to be able to shove into your weapon arc in 6 milliseconds? Given that the question comes from India where the standard attack method is to slowly approach your opponent and ease your weapon into them, all a 5K weapon will do is skitter off a smooth surface. You might get lucky and find an exposed sharp edge for the weapon to 'bite', but I'd hate to rely on that luck to hold for a whole tournament.

The only real use for a 5K RPM weapon is if you're going head-to-head against another drumbot. In those fights, the faster tip speed wins.

Q: who say you have to spin your drum at full throttle?

You ALWAYS design a spinner to run at full throttle. Spinner energy increases with the square of speed -- if you cut your running speed by 50% your energy storage drops by 75%. Design the weapon to be effective at full speed or you're wasting motor power and weapon efficiency.

All these spinner design topics are well covered in the archives -- do your homework and read the earlier posts before you fire off another flip question.

Q: mark lucky again in case of drum's weapon [impactor] which material i should prefer i am thinking about cast iron and ms or i have also option of steel what u say? [India]

A: [Mark J.] NOT CAST IRON! You want a 'tough' material for your impactors -- a material that can take high impact. Cast iron is fairly strong, but it is brittle and can shatter on a high-energy impact sending metal shards in all directions. Various steel alloys cover a wide range of strength and toughness. I don't know what types of steel are available to you, but even mild steel is preferable to cast iron.

Q: Hello. We've all seen videos of antweight full body spinners (FBS) colliding with another bot and then flying off the walls of the arena (ultimately ending upside down or outside of the arena). This is why generally [Ask Aaron] has advised to avoid building FBS at this weight class (even though it is so fun to build them!).

What do you think of mounting the spinner on a suspension between the spinner and chassis. When the spinner hits another bot it will recoil into this suspension, attenuating the impact over a longer period of time so the bot as a whole will not bounce back as far. I imagine that it will not throw the other bot as far either. The shell of the bot at least is still very rigid. it just has a soft linkage to the chassis. not sure if there is a benefit to doing this, what do you think? [San Diego, California]

A: [Mark J.] Your analysis is correct. Placing suspension between the horizontal spinner shell and the chassis could reduce the recoil to your chassis, but it would also reduce the effective impact of the weapon on your opponent. Effectively, it would help your opponent as much as it would help you! You might just as well reduce the power of the weapon - it would be simpler. I've gotta vote 'no' on this idea.

For maximum weapon impact in a horizontal spinner you want the structural path from the point where the weapon strikes to the center of mass of the robot to be as stiff as possible. Vertical spinners have a strong advantage in this regard, as the recoil path (downward) is eventually supported by the arena floor (and the planet beneath). Section 6.6 of the RioBotz Combat Tutorial discusses the importance of mounting stiffness in maximizing weapon impact -- a good read.

Q: hey mark lucky here i am thinking to combine my design so that it should be combination of spearbot and drum bot whats your view [Maharashtra, India]

A: [Mark J.] See Frequently Asked Questions #26. My advice is to place your entire weapon weight allowance into a single weapon. Dual weapon robots have a serious drawback: two weak weapons that cannot work together are not as good as a single powerful weapon. Add the weight of that spear to the drum weapon to increase its power.

Q: Hello Mark. After seeing Biohazard being torn up into pieces in Combots Cup 2005 I was wondering a question. Could 4-Bar Lifting arms still be competitive in Heavyweight Class? [Beijing, China]

A: [Mark J.] The high-energy spinner weapons in the US heavyweight class are brutally destructive. With judging leaning so heavily on damage and no real possibility of boosting an opponent out of a US arena for a 'knockout', I don't think an electric lifter is a competitive design in the heavier weight classes.

Q: Dear Mark,I think Overkill and Toe-Crusher's design is quite different from other thwackbot. Is it has any advantages or disadvantages? [Yangzhou, China]

A: [Mark J.] 'Overkill' and 'Toe Crusher' are not technically 'thwackbots', they're torque-reaction hammers. There is a description of how they function and the drawbacks of the design in this archive -- search here for 'reaction hammer'. A torque-reaction hammer has far too little power to be a contender in current competition.

Q: Hey Mark,

I am building a 1lb ant weight vertical spinner bot. What is the best way to tell if a motor is adequate for a vertical spinner? I know that you can calculate what the kinetic energy in a spinning weapon. However, the motor I have (and I am trying to figure out if it is adequate) is a Rimfire 370 and is rated to 1000kV, weighs 1.4 oz and has a Burst Watts of 165. So basically, is there a way to look at the critical specs of a brushless outrunner (maybe there is a range of kV?) and know if it would be a good motor? I also know that the forces of a weapon are largely dependent on the radius and mass of the weapon itself, but I can assume that I can judge if said motor makes sense before designing the impactor. Does that make sense? If not, let me know.

Thank you very much,
New York

A: [Mark J.] You're on the right track, New York. The load on the motor when spinning up a weapon is dependent on the 'moment of inertia' (MoI) of the spinner, which does depend on the spinner shape, dimensions, and mass distribution. The challenge is in matching the MoI of the weapon to the power of the motor to assure that the motor can spin the weapon up to speed quickly enough to be useful.

If you're looking for a quick estimate of the suitability of the motor for a spinner, you'll want to look at the 'kV' and 'burst wattage' ratings:

  • The 'kV' of a motor tells you how fast the motor will spin for each volt of electricity it receives. Multiply the kV by your battery voltage for the no-load RPM of the motor. This is important because you don't want a motor that has to be 'geared down' a lot to provide a (not too fast) useful speed for your spinner. The Rimfire 370 has a kV of 1000, so with a 7.4 volt battery it will have a no-load speed of about 7400 RPM, which can be made into a reasonable spinner speed with a belt reduction of around 2:1. That's good!

  • The 'burst wattage' is an indication of the power the motor can produce to accelerate the spinning mass up to speed. Twice the wattage will spin a given weapon blade up to speed twice as quickly. The energy storage of the spinning weapon is measured in 'joules', and for an ant about 30 joules is a reasonable amount of stored energy. A joule is equivalent to one watt of power applied over one second of time, so your proposed 165 watt motor could (at peak output) store 30 Joules (watt-seconds) of energy in a spinner weapon in less than 1/5th of a second (30 watt-seconds ÷ 165 watts = 0.18 second). That's huge overkill!
Now, in actual operation the motor has to start from a standstill and generates only a fraction of it's rated output power until well up in the RPM range - but even estimating that the average output of the motor as it accelerates is half the peak output, this motor has WAY MORE than ample power for an ant spinner. The Rimfire 370 would be more than enough motor for a 3 pound 'beetleweight -- for an ant I'd go with a lighter motor and put the saved weight into a heavier spinner bar/disk/drum. Around 50 watts output per pound of 'bot is plenty!
Q: i want to make a drum of 8 kg with length 15cm and dia 13cm, what material should i use for drum and teeth, and what should be the thickness??[Maharashtra, India]

A: [Mark J.] That's a very small drum to weigh 8 kg. A solid aluminum drum that size only weighs 5.5 kilos, so I guess you'd best use a mild steel alloy.

A steel drum 15 cm long and 13 cm in diameter with a 15 mm thickness weighs 6.34 kg. Add two steel end plates 5mm thick and the total weight comes to about 7.4 kg. That leaves you 0.6 kg for your teeth. Energy storage works out to about 1800 joules at 3000 RPM.

Threading large, short bolts into the drum and leaving the heads exposed works reasonably well for teeth. They are easy to replace when damaged and give you flexibility in placement and size. If you have more conventional teeth in mind, a hardened tool-grade steel (like S7 alloy) is the preferred material.

I will mention that it is unusual to design a drum weapon starting with a specific size and mass. Changes to the dimensions can greatly impact the energy storage capacity of the drum for a given mass. Some time spent with the Team Run Amok Excel Spinner Spreadsheet may yield a large bonus in performance with small dimension changes. Example: increasing the drum diameter to 15 cm with a 12.5 mm thickness will increase the energy storage by 22% at the same weight.

Q: if i use same size pulley to rotate my 8kg drum from ampflow e30-400 how will be the loss and how much rpm will i get?

A: A V-belt has very high power transmission efficiency: up to 95%. There is very little power loss. Peak RPM will depend largely on the aerodynamic drag of the drum and teeth. As a guess, I'd say perhaps 5200 RPM @ 24 volts. Spin-up time will be about 4 seconds and energy storage close to 4000 joules.

Q: What's the name of the weapon used by Original Sin when fight with Last Rite? The yellow wedge can stop Last Rite's spinning bar immediatly. [Guangdong, China]

A: [Mark J.] You're greatly overstating the effectiveness of Original Sin's big yellow bar spinner trap. Team 'Late Night Racing' built the trap specifically to counter Last Rites' huge bar spinner, but as you can see in this video the defensive attributes of the device are not always effective.

The only real strategy when fighting 'Last Rites' is to hope they break down; sometimes you get lucky and they do!

Q: What mechanism do i use to lift the 4-bar lifter? I am using a 12V battery. The weight of my robot is around 40 kg. I am not allowed to use compressor. The dimension of my robot is 40*30. [Poona, Maharashtra, India]

A: [Mark J.] There are more than a dozen posts on 4-bar lifters in this archive that include power options and design tools. Start by reading thru those posts.

If you can't use pneumatics your only real power options are high reduction gearmotors directly driving the front or rear bar and linear actuators operating a bell crank attached to a bar. Both of these options are well discussed in the archive. The T.i. Combat Robotics 4-Bar Simulator is a very useful tool to assist in designing 4-bar lifters with electric power.

Q: Hey Mark! Last year I powered my robot's spinning weapon with a V-belt. Generally, I had no problems. I'm making some design changes this year including upping the weapon rpm, and I'm considering switching to a timing belt. I figured the wider contact area and teeth would boost my power trans efficiency. However, I still need the pulley to be able to 'slip' as I'm not using a clutch and don't want to wreck my motor during a hard stop when my weapon makes contact. Will using this style belt risk damage to my motor or will I have some flexibility as with the V-belt? Thanks for the help! [United States]

A: [Mark J.] The larger the robot, the more common it is to find a V-belt weapon drive.

  • A V-belt is simple to set up for slip -- loosen it a little and it slips more, tighten it a little and it slips less. A V-belt will also put up with a fair amount of misalignment, and the slippage is smooth.

  • A toothed timing belt is designed to not slip at all. It must be set up quite loose before it will slip at all, and you may need to set it up so loose that you risk the belt jumping off the pulley. Timing belts are often used in small robots simply because small V-belts are not available.
Both types of belt are very efficient at transmitting power. If you've had good results with the V-belt drive I'd recommend sticking with it.

There are many discussions of weapon belt drives in this archive that may give you additional help in your decision. Search here for 'timing belt'.

Q: Hi Mark i am from india making a bot of 60 kg. My weapon is a verical disk and it is bububling so any sugestion to make it stable? [India]

A: [Mark J.] I'm not sure what you mean by 'bububling', but if your disk spinner is unstable it's probably out of balance. I've previously posted the process for balancing a spinning bar or disk -- search this archive for 'How do I balance the weapon'.

The faster you spin a weapon the more critical weapon balance becomes. You haven't given me any info, but you may be attempting to spin your disk at an unrealistic speed. The more information you can provide, the better chance I have of giving you a useful answer.

Q: hello sir, can you suggest me some designs of counter robot against drum weapon robot??? bot weight class is 120lb... and please suggest me some designs of lifting and flipping mechanism also.... [India]

A: [Mark J.] It's disrespectful to ask me to invest my time to answer a question without first spending a bit of your time to see if the answer already exists here at Ask Aaron.

  • The 'counter drum' question has been answered multiple times -- most recently just a few posts below.

  • As to suggesting designs, I have stated (recently and often in the archives, and for a long time in the ) that we do not supply designs for robots or weapon systems. Ask Aaron is not a free engineering service. If you submit a design, I will offer a critique and perhaps provide some alternatives.
I suggest that you start by doing your homework and reading thru this archive for some design ideas.
Q: Hey, I am unable to [mount] my ampflow e 30-400 motor vertical on my base...those screw in the base of motor are [too] small to hold it...suggest some solution? [Tamil Nadu, India]

A: [Mark J.] The E30-400 motor (diagram) does not have the large mounting bolt holes found on some other AmpFlow motors. You haven't told me anything specific about your design, but I can offer some general suggestions.

  • You may remove the motor endplates, enlarge the mounting holes, and tap them for larger bolts.

  • A large motor like the E30-400 really must be mounted at or near both ends to spread the physical loading forces.

  • Clamp-style mounts do not rely on the endplate mounting holes and can be adapted to a wide range of support designs.

Q: Hi, I wanted to know how can we calculate power required for a flipper like Ziggy who throws away his opponents rather than mere inverting? For example how can we calculate how much energy the motor should provide or how much energy should a pneumatic actuator provide?

A: Mark J. here: there are a great many posts about flippers in this archive. I suggest you start by reading thru them.

A few significant points:

  • Forget about direct electric power for a flipper. Electric motors cannot realistically provide the explosive release of power needed to get the job done.

  • While it is possible to calculate the energy required to toss an opponent a given height, the formula assumes direct and instantaneous conversion of that energy at the center of mass of the opponent -- both highly unrealistic assumptions.

    Maximum Height Opponent Will Be Tossed = Weapon Joules ÷ (Opponent Mass (kilos) × 9.8)

  • 'Power' is a combination of force and speed. A pneumatic flipper must have very efficient gas flow thru large valves, ports, regulators and connectors to get that combination. It isn't cheap, and it can be very dangerous.
A successful flipper requires knowledge, experience, trial-and-error, and a fair scoop of luck. Study the design of good flippers until you can identify the factors they all share.
It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok

Q: Hi Mark, What according to you will be a better option as an impact weapon tooth - S1 or H13 tool steel?? [Las Vegas, Nevada]

A: [Mark J.] Without knowing the specifics of your design, I would in general pick the S1 tool steel.

The S-series tool steels are grouped together for their primary trait of shock resistance -- valuable in an impact tooth. S1 tool steel is fairly hard as well, which will give it an advantage in surface durability and the abilility to effectively transmit impact.

The H-series tool steels are known for their ability to maintain strength at high temperatures -- not a primary consideration for an impact tooth. H13 tool steel is very 'tough' but not particularly hard. It is also expensive due to its high chromium content.

Q: How would you calculate the oz-in [of torque] necessary for a beetleweight 4-bar lifter? I saw this in the Archives:

Stall Torque (oz-in) = length of lifter arm (inches) × weight class (ounces) × 1.67

...but how would this be transferred to a 4-bar lifter? [Fulton, Maryland]

A: [Mark J.] You didn't dig thru the archives quite far enough. The formula you found is for a simple single-pivot lifing arm, and it cannot be transferred to a 4-bar lifter due to the many design variables inherent in that design.

As noted several times in this archive, 'Total Insanity Combat Robotics' has a 4-Bar Simulator tool to assist in designing 4-bar lifters with electric power. Read thru their page and download the simulator. Play around with the lifter design until you get a good lift path and a reasonable torque requirement.

Q: When I tried to download the Setup for the simulator, an error message came up that said that the file requires .NET framework 1.1.4322, but when I went to the website it said that it is no longer offered that package. I have a Windows 8.1 computer and I'm not sure the other available packages will work?

A: .NET Framework 1.1 is still available for download direct from Microsoft, but it is not compatable with Windows 8.1. Microsoft offers some suggestions for Windows 8 users trying to run older .NET framework programs that might help.

If you simply can't get it to work on your computer, find a friend with an older version of Windows and install on their machine.

The Total Insanity simulator is very well suited for robot applications, but a quick seach for "4-bar simulator" reveals some other programs are out there. You may wish to investigate them as well.

Q: hi mark i'm using an ampflow E30-400 motor to drive a horizontal spinner of(40*20*2.5)cm(l*b*h) and its of 10 kgs. i just wanna know how much energy will it be storing and its effectiveness against drum bots? weight class is of 60 kgs. [Khamgaon, Maharashtra, India]

A: [Mark J.] I'm getting many questions for which 'Ask Aaron' already has tools that can provide the answer. In this case, the Run Amok Excel Spinner Spreadsheet is the tool. Let me walk you thru the steps.

Starting at the 'Motor and Drive' section (specs from the AmpFlow website):

  • No-Load Speed - The unloaded AmpFlow E30-400 motor spins 5700 RPM @ 24 volts.
  • Stall Torque - 1500 in-oz converts to 10.6 N-m.
  • You haven't told me what Reduction Ratio you'll use, so we'll leave it at the default 4:1 for now.

The default spreadsheet example is for a spinning disk and you're building a bar spinner, so in the 'For a disk' section change the Thickness from 6.4 mm to 0.0 mm to zero out the disk mass.

In the 'For a bar' section:

  • Material Density - 7800 (for steel)
  • Length - 0.4 meter
  • Width - 0.2 meter
  • Thickness - 25 mm

That's it. You can read the results right off the spreadsheet. The calculations say your bar weighs 9 kg, not 10. It also looks like the default 4:1 reduction ratio is too high -- the spin-up time is very short and the energy storage is small. A reduction around 2.5:1 looks about right. Want to know the energy storage? Run the spreadsheet for yourself.

You haven't told me enough about the weapon for me to even guess at its effectiveness. A weapon's effectiveness depends as much on the details and construction quality as on its type and dimensions. One critical factor will be the weapon height -- if you're building a drum-killer you'll want to set the weapon as low as possible in an 'undercutter' position. Hitting a drum weapon high is a major error.

I can say that horizontal bar spinners are at a general disadvantage. When your weapon hits it will throw your opponent in one direction and toss your 'bot in the other. This weakens the impact and can send your 'bot bouncing off arena walls like a pinball. Something to consider...

Q: Hi Aaron.. in a face to face combat between a drum and horizontal disk spinner, (both bots invertible ones) both having same mass and rpm, can you tell me which one will be more effective and which will have more impact?? [Orissa, India]

A: [Mark J.] Several considerations...

  • As detailed in the question immediately below, the power of a spinner weapon depends on more than just the mass and RPM.
  • A robot's overall effectiveness depends on more than just the weapon.
  • Different weapon types seem to do better in specific weight classes.
Being very general, the vertical attack angle and more robust structure of a typical drum weapon is usually a winner in a rock/paper/scissors comparison against a horizontal spinner. An exception may be in the hobbyweight class, where a lot of top competitors are horizontal spinners. Not sure why.

A horizontal spinner has the potential for a greater 'impact' than a typical drum weapon, but the horizontal impact tends to throw both robots in opposite directions. You may bounce around the arena and do as much damage to yourself as to your opponent.

Q: Hi Mark...same guy with drum and horizontal disk question.... Err! I made a mistake...instead of horizontal disk it's a vertical spinning disk. The weapons' maximum dia. (outer dia. for drum) 6 inches. Weight classes: 30kg as well as 60kg. Considering both bots are strongly built with 2 teeth each, which one will win or at least be more effective over the other??

P.S.: I am very sorry for Aaron. I came to know of his demise few days back. And thank you for continuing this website. May God bless you!

A: I can't tell you which will win. As I mentioned above, a robot's effectiveness depends on more than just the weapon. All of the components must work together reliably or the 'weak link' will fail and spell disaster. It's a serious mistake to concentrate on the weapon system to the detriment of the rest of the 'bot.

As to which is more effective, take note that there are no successful big vertical spinners in the current combat scene. They have several problems:

  • A big vertical disk has maneuvering problems -- a quick turn tries to tip the whole 'bot on its side due to gyroscopic forces. Awkward!

  • A large diameter vertical disk exerts a lot of impact force back into its mounting structure. The bigger it is the greater the difficulty in controlling the impact forces.

  • A large vertical disk will tend to strike the opponent as the impact tooth is traveling predominantly 'forward' rather than 'up'. This can result in a lot of 'kick back' that throws you and your opponent away from each other, rather than tossing your opponent upward while you remain braced by the floor.
Now, if you're just talking about some intermediate drum/disk hybrid design like the narrow drum on 'Professor Chaos' rather than the true vertical disk on 'Nightmare', I'd say that in general I would prefer a larger diameter drum to a smaller one. For any given mass and speed, a larger diameter drum will have greater stored energy for a more powerful impact.
Q: Sir, I am making a bot. I have used a A28-400 motor for weapon to drive a 16 kg hollow drum using a pulley belt mechanism. Voltage provided is 24V. I have to use the weapon for about 5 mins. total no. of spin ups will be around 10-15. How many Amps will it discharge?

I have a confusion of buying a Lipo and want to know which battery should i opt for- should i use a 6S 5000mAh 65C-130C battery or a 6S 5000mAh 45C-90C battery. Which one will be efficient considering the cost too? [Mumbai, India]

A: [Mark J.] You've given me too little information to answer your questions.

The amount of energy storage in a spinning weapon depends on:

  • The mass of the weapon;
  • The speed of the weapon (RPM); and
  • The shape and dimensions of the weapon.
Without a full description of the weapon dimensions and the speed at which it spins, the energy needed to spin the weapon up to speed cannot be calculated. Everything else being the same, a long but small diameter drum will store MUCH less energy than a short but large diameter drum. Example -- a hollow steel drum with a 1/2" thick wall spinning at 2400 RPM:
  • a drum 4" in diameter and 24" long will weigh 16 kilos and will store 920 joules of energy; but
  • a drum 9" in diameter and 10" long will still weigh 16 kilos but will store 4800 joules of energy -- more than five times as much!
The Run Amok Excel Spinner Spreadsheet can perform all of the calculations needed to determine the energy storage of your spinner weapon, and can estimate the battery capacity requirement. I strongly suggest that you make use of this powerful tool.

As to your battery selection: the 5000mAh capacity you propose is MUCH larger than just your weapon might use -- I assume you will use it to power the robot drivetrain as well. You have given me no information about your drivetrain, so I cannot comment on the overall suitability of the battery.

Q: Hey wassup, I have a question about horizontal spinner. I am planning to place a 7 kg horizontal spinner with a drive motor of ampflow E30-400, the robot weight category is 40 kg. so please tell will that motor be enough to take on the opponent? If yes, tell me which drive is more effective: belt drive or chain drive...

A: [Mark J.] The motor and total weapon weight are OK for a weapon on a 40 kilo robot, but the effectiveness of the weapon depends as much on the dimensions and shape of the rotating mass as it does on the mass of the weapon and motor power. Suggest you search thru this archive for prior posts on calculating weapon effectiveness.

Some quick calculations assuming a simple steel bar spinner (600 mm X 80 mm X 18 mm = ~7 kg) and an 8:1 weapon motor gear reduction show good energy storage and acceptable spin-up speed: about 3000 joules in 3 seconds. That would do nicely.

Robots in this size range typically use a V-belt drive for their weapon drive. A V-belt can be set to slip if the weapon stalls, which can keep the drive motor from stalling. A stalled weapon motor draws high current which can heat-damage the motor.

Q: Hey,thanks for the reply. I am planning to use a horizontal bar of steel weighing 7 kg(400mm,100mm,22mm) which has 1800 joules in 2 second. Weight class is 25kg, and i am going to use an ampflow e30-400 motor for the drive at 3:1 reduction. My question is, will my spinner stop if it hits the opponent?

A: I see you've been making good use of the Run Amok Spinner Weapon Excel Spreadsheet. Is it a 40 or 25 kilo weight class? You've mentioned both.

It's very difficult to model the precise results of a spinner hit on a complex shape such as a combat robot -- particularly for a horizontal spinner. I think it is safe to say that a hit by an 1800 joule horizontal spinner weapon on a 25 kilo opponent will send both you and your opponent skittering away in opposite directions, and if the weapon is stopped, the impact should push it clear and allow it to resume spinning.

An instantaneous stoppage like this is not a problem, but you want to avoid a prolonged stall that could damage the battery, speed controller, and motor. That type of stall can happen if the 'bot is trapped against the arena wall, or anytime your weapon is held immobile with power on.

Strange and unpredictable things happen in robot combat, and I advise use of a slip-capable belt drive for spinner weapons to keep potential stall damage to a minimum.

Q: Hi Aaron! I want to build a flipper robot for featherweight. not for some particular event, but for interschool and inter university events. Can i use an air retract for lifting a 6-7kg robot?

I found this air retract kit, let me know if its okay for my use.

I am new to pneumatics, read all your answered questions already, excellent responses!

A: [Mark J.] Model aircraft air retracts are designed to raise and lower a few ounces of landing gear in a slow and realistic manner. The valves, ports, and connectors are all low-flow rate to keep the actuation rate slow -- you don't want to wildly snap aircraft landing gear up and down.

The power of the actuators is calculated by multiplying the bore area of the cylinder by the operating pressure of the system; neither of those are given in the description. Guessing at 15mm bore and 80 psi pressure, the maximum thrust of the actuator would be about 20 pounds. Given the geometry of the retract mechanism that I can see in the photos, the thrust would be reduced to maybe 2 pounds at the flipper.

Two pounds of slowly applied thrust is not going to do you any good at all in a 7 kilo 'flipper'. Suggest you seach thru the this archive for previous posts on pneumatic flippers for component sources and suggestions.

Date marker: 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