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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.
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.
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:
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.
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:
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.
Consider:
'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:
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:
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:
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:
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: 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.
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]
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]
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]
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.
Q: Hey, M's weapon sounds like 'Greenspan' that used a flywheel with free spinning hammers. [Dublin, Ohio]
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]
What event runs an 85 pound weight limit? That's not a standard US weight class.
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. 1300mm x 125mm x 30mm Weight is roughly 38Kg
Motor Spec:
Results using a 4:1 gear ratio: 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:
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: 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:
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.
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:
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]
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]
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]
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:
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.
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...
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: 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:
I received a note from a reader in Massachusetts who was on-site at BattleBots 2016 and who offers a better explanation:
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):
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:
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:
There are also multiple posts in this archive discussing single-tooth weapons. Search here for 'asymmetric'.
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:
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.
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: 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.
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]
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]
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.
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: 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!
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:
The 'Dumbbell' Drum: another 'looks cool' design, but there are reasons to avoid it:
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]
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.
Q: Hello sir.please direct me to the archive where it tells me the calculations required to make single tooth drum.how 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:
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...
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 ..so plz help me out [Quantil Inc - Pasadena, California]
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 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)
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.
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]
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:
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:
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,
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:
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:
That plugs into the basic linear work equation [Work = Force × Distance] like this:
Getting close now. One full rotation is 2𝜋 radians, so with a little algebra the rotational work formula transposes to our needed torque equation:
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:
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.
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.
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.
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.
Q: Why doesn't Manta's flipper work well? [Chevy Chase, Maryland]
A: [Mark J.] Are you talking about the 'Manta' with these achievements?
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]
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: 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:
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:
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:
There are numerous posts about 'bite' in this archive. Some highlights:
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! 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]
You really don't have a lot of options for blade variation with your "huge horizontal" beetle bar weapon:
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:
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.
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:
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.
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?
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?
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 math is covered in multiple posts in the Ask Aaron archives:
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...
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...
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]
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:
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: 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:
Question 2:
A: [Mark J.] You've opened up a can of upper-class level physics worms here. Take a deep breath.
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.
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.
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]
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:
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!
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:
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. 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?
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:
Q: I'm having trouble relating the diagram of the 'Algos' drum to the outrunner motor before it was modified. Can you give me a sketch of the outrunner in the same format?
A: Sure. The only parts of the original outrunner that are used in the finished weapon are the motor stator and the rotor magnet ring.
The small ball bearings are removed from the stator, and the tube that held them is drilled out to accept the larger shoulder bolt dead shaft that will support the larger ball bearings for the drum weapon.
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.
A: [Mark J.] The reduction ratio is the relation between the diameter of the weapon pulley and the diameter of the motor pulley:
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: 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.
About drum weapon design:
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!
Q: HOW WE CAN BUILD A DRUMBOT WHICH ROTATES AT LOW RPM(2500-3500) AND CAN STORE MAX AMOUNT OF ENERGY [Bhilai, Chhattisgarh, India]
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. Q: WHAT SHOULD BE THE IDEAL SPEED FOR A TWO TEETH DRUM BOT SO THAT IT GET A BETTER BITE(15CM OD 20MM THOCK) 25CMS LONG GEAR RATOL 1.5(NOW) 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. Q: .......AND ALSO IN YOUR SPREADSHEET IT IS SHOWING 2 SPEED I.E (RPM [ 63% Max ]) AND (RPM [ 95% Max ]) CAN YOU EXPLAIN THAT
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:
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:
So why don't Full Body Spinners (FBS) charge aggressively? Two things to consider:
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.
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]
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 bot...my 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: 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]
Example:
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.
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 help...now 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:
Reducing the wall thickness to 20mm and increasing the speed to 4000 RPM will significantly reduce weight and improve energy storage:
Better still, increase the diameter to 20cm, reduce the wall thickness to 11mm, and keep 4000 RPM:
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:
Assuming solid hardened steel alloy shaft and the drum design you've mentioned, 20mm to 25mm is a good starting point.
Q: hi sir whether12V 5310 RPM "CIM" Brushed DC Motor
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? Q: HELLO SIR I AM HAVING A DRUM OF 20N CM LENGTH 110MM OD AND 90MM ID DEAD SHAFT DIA 5CMS AND USING DEAD SHAFT TO SUPPORT THE DRUM WITH BEARING I AM USING MILD STEEL AS DRUM AND THE TOOL IS EN24 SIR PLEASE I AM NOT ABLE TO CALCULATE THE ENERGY STORED IN MY BOT [India]
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]
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: 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:
Q: hi mark making 2 teeth drum materials (mild steel) (teeth- en18)
od 150mm
bearings- shall i go for needle or tapered one's??
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
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 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:
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:
Now that we have a practical weapon design, we can look at weapon performance when powered by a specific motor.
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:
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:
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.
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:
Examples:
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:
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:
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:
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:
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!
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! 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., 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: 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:
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.
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?
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?
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:
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 id=10cm 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:
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:
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:
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.
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: 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: 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):
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:
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:
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 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:
Or even better:
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:
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:
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:
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:
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:
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: 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,
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:
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: 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.
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.
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:
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:
...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):
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:
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...
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:
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:
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... Cheers.
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. 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:
Q: Hi Aaron, i have read in the archives that by using the timing belts instead of chain for a drum weapon..it 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:
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:
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:
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.
Robot haiku:
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]
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
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:
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?
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:
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:
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.
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.
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
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:
A: See Robot haiku:
Q: hey aaron, A: Many problems.
Robot haiku:
Q: Hi.. I plan to build a drum spinner weapon i did browse the 'spinner weapon starter package' at Robotmarketplace.com [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 solenoid..is that actually the better way to control it? 2ndly...my 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 (http://robotcombat.com/products/0-BATTLSW1.html) 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?
Thanks 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!! hu!!! 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
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:
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: 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!
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]
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.
A: Two things we won't discuss here at Ask Aaron:
Q: Dear Mark/Aaron
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 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: 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:
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.
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]
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
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 Q: how to build a simple flame thrower robot explain [Chandigarh, India] A: Persistant, aren't you?
As explained in 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:
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:
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:
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: 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.
Advantage to the horizontal spinner: the spinning mass of the weapon exerts gyroscopic forces on the robot if the rotational axis is deflected.
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: 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: 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: 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? 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:
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:
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:
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.
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:
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:
Q: Is there a way to beat an undercutter?
A: There must be -- people do it all the time. 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!
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 |
'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:
Q: What is the best motor to use for a horizontal spinner [Los Angeles, California]
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.] 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:
A: [Mark J.] The author of the 
A: [Mark J.] I know of no examples of such a weapon system in use, and I think I know why there are none.
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.
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.] No, and it won't.
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:
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.
A: [Mark J.] First, a little terminology clarification:
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.
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.
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.
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 '
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]
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: [Mark J.] There are two common methods to mount spinner weapons; 'Tombstone' uses one, and 'Mechavore' the other:
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]
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]
Q: how could i give a blue, energetic look to my flamethrower like th one used in complete control? [Quebec, Canada]
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.
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: [Mark J.] Can't think of one, you say?
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
"[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.]
A: [Mark J.] 'Crusher of Dreams' -- maybe I should put that on my résumé?
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 
Q: hi axe guy again in the picture u put, is the hammer able to do a 180 degree rotation?
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:
Q: Good Evening, Mr. Mark Joerger
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.
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.
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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':
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.
Q: Sewer Snake [Hebei, China]
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]
Two 150 pound force actuators give you a maximum 300 pounds of force input to the 4-bar system.
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: [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:
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.] I'm always pleased to hear that builders find our modeling tools useful. I appreciate you taking time to comment on our 
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: 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.
A: [Mark J.] First, the terminology:
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..
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.
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.
A: [Mark J.] I'll do much better than that -- I'll show you how to use the 
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.
Q: Hi Mark, what do you think of abrasive disks as cutting weapons? [India]
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]
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: 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.
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
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
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]
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]
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: 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'.
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.
A: You're right -- big spinning disks/saws are 'out' and smaller diameter drums are 'in'. Several reasons:
Q: Hey aaron
Q: what is the difference between inrunner and outrunner dc motor.. [Pune, India]
Q: How do you make an antweight beater weapon without machining it out of a single piece of material?
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:
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: Yes, but there are several drawbacks.
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:
Q: Dear Aaron, can you show me how Panzer MK3's and Panzer MK4's weapons work?
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 
