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4796 Questions and Answers about Combat Robotics
from Team Run Amok
Ten Years of Ask Aaron

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) requested a question and answer page to document our responses.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok

Welcome to my website Recent Questions


Not all the questions we receive at Ask Aaron are serious. Some are odd, some misdirected, and a few are incomprehensible. Aaron enjoyed dealing with these questions, and I've collected some of his best responses in a in a special page:
Aaron's Greatest Hits
Aaron's Greatest Hits

Q: On the subject of sumo robot. What kind of motor can you recommend to achieve great speeds and great impact? [Guayaquil, Equador]

A: [Mark J.] See previous answer six posts down this page.

Team Run Amok does not compete in sumo so I cannot personally recommend motors for the sumo classes. I do know that some successful competitors use the 12 volt Maxon RE40 motors in the 3kg class, but they are VERY expensive.

I suggest that you find builders who compete in sumo to discuss drivetrains.

Q: I've been working hard on the drum weapon design for my bot. Is there anything else I want to concentrate on other than the weapon? [India]

A: [Mark J.] Your weapon is pretty much the LAST thing you should worry about. The really important things are the basics.

  • If your speed controllers fail, you lose.
  • If your drive motors fail, you lose.
  • If your wheel hubs fail, you lose.
  • If your gearboxes fail, you lose.
  • If your battery fails, you lose.
  • If your radio fails, you lose.
But if your weapon fails, you still have a chance to win. Make sure you get the basics right before you decide to complicate things by adding an active weapon.

Q: hello sir,i am building a thirty kg drum bot i am deciding to use radio system for drive the four locomtion motor and a weapon motor.But still now i have no idea about the radio system ,as i am a eee student i have good knowledge on now tell me where exactly i have to start and learn and do it? [India]

A: [Mark J.] Read thru the first six or eight posts in the Ask Aaron Radio and Electrical archive. In those posts you will find links to:

  • our R/C Radio Reception guide;
  • an article on How R/C Radios Work;
  • our Combat Radio Function Guide;
  • an R/C robot circuit diagram in our FAQ;
  • our Radio Transmitter Programming Guide. and
  • our Combat Robot R/C Gyro Guide.
It's not like I'm trying to hide this stuff.

Update: I decided to put a single reference to all of the Run Amok radio guides in the FAQ. I believe it's FAQ #20.

Q: Hello will you please help me out by suggesting the torque and rpm required to move a robo car which is having a weight of 60kg and need to push a weight of about 40kg of a DC electric motor if possible mention in brief what should be specification i am planning for a four wheel drive. [India]

A: [Mark J.] The answer depends on how fast you want the 'bot to push that weight, how fast you want it to accelerate, and the diameter of the driven wheels. A motor with very little torque and speed can be geared down to push a 60kg 'bot and a 40kg weight slowly, but if you want more speed you'll need more power. Examples:

  • A 60kg 'bot with 3" diameter wheels powered by four gearmotors each delivering 3 kg-cm of torque and 150 unloaded RPM could push a sliding 40kg weight at about 0.5 MPH.

  • The same 'bot with 6" wheels and gearmotors delivering 130 kg-cm torque with 1000 RPM could push a sliding 40kg at about 12 MPH.

You can't calculate the power you need until you figure out your performance requirements. There are MANY posts discussing motor selection in the Motors & Controllers archive.

Q: The point (6,a) and (b,8) lie in the line y=3x-7, find the value of a and b [Glasgow, United Kingdom]

A: [Mark J.] Algebra homework? No robots? Sigh... just for fun:

If the two points are on the line described by the equation, they must both satisfy that equation. We'll substitute the given x and y and solve the equation to get a and b:

  • For the point (6,a): x=6, and we plug that into the equation to get

    y=3(6)-7 = 18-7 = 11, a = 11.

  • For the point (b,8): y=8, and we plug that into the equation to get


    ...add 7 to each side of the equation to get


    ...and divide each side of the equation by 3 to get

    5=x, b = 5.

You're welcome.

Q: hi mark ,i was planning to build a 30 kg bot with vertical spinning bar of dimensions 20*15*5 cm (l*w*th) of mass around 7 kgs (mild steel) and i want to run it around 3000 rpm ,i was using an ampflow e30-400 to power my weapon and i want know the reduction ratio and energy storage of my weapon(i was unable to get through the spinner spread sheet)....and can i use ms for teeth ? [India]

A: [Mark J.] Lets start with the reduction ratio. From the AmpFlow website: the no-load speed of the E30-400 motor at 24 volts is 5700 RPM. Allowing for mechanical and aerodynamic drag on a spinning weapon will reduce that speed by about 15%, so let's call it 5000 RPM. You want a 3000 RPM weapon speed, so the formula is:

Motor Speed / Weapon Speed = 5000 / 3000 = 1.67 to 1 reduction.

A chunk of steel 20 cm by 15 cm by 5 cm isn't as much a bar as it is a square, and it will weigh closer to 12 kg than 7 kg. If you are removing material from that almost-a-square to reduce it's mass to 7 kg, I'd need details of the true shape of the rotating mass to calculate the energy storage.

IF the weapon is a simple 12 kg steel block of the dimensions you give, it will store about 3000 joules of energy at 3000 RPM. Spin-up time will be about 3 seconds.

Tooth material should be harder than the material you'll be hitting with the teeth. Mild steel isn't very hard. If your particular steel alloy can be hardened, I would consider at least surface hardening the teeth.

Q: hi mark ,iam the spinning bar guy again sorry i have given wrong info about it..... actually its dimensions are 20*12*4 cm (l*w*t) around 7.4 kg ,what will be the energy storage of my weapon ?and was that weapon is enough for my 30 kg bot ? OR i should chage my weapon dimensions ? thank you for your info about reduction ratio......

A: [Mark J.] That's a big difference. I'm not sure how you confused the dimensions that badly and sent me down the rabbit hole with the other numbers. The 'Ask Aaron' service is free, but that doesn't mean my time has no value. Please double check your numbers in the future.

A steel bar 20*12*4 cm spinning at 3000 RPM will store about 1700 joules of energy with a spinup time of about 2 seconds. That's better than 25 joules per pound of robot weight, which isn't bad for a bar spinner.

If your design allows, a longer and narrower bar of the same weight would store more energy. Examples: a 24*10*4 cm bar spinning at 3000 RPM will store about 2100 joules (2.5 second spinup), and a 28*8.5*4 cm bar will store more than 2600 joules (3 second spinup).

I'd strongly suggest that you learn to use the Run Amok Excel Spinner preadsheet so that you can examine many weapon designs and find the one that best fits your needs and your design parameters.

Q: Greetings, my question: are two Banebots RS-550 motors with 16:1 P60 gearboxes enough for a 3kg sumo robot for metal dohyo? [Quito, Ecuador]

A: [Mark J.] We don't compete in sumo, and my initial reaction was that a pair of RS-550 motors can provide WAY more than enough power for a 3 kilo robot, even with magnetic downforce. Then I went out and found some video of recent 3 kilo sumo matches. YIKES! Take a look:

If you're competing at this level, I think you need to find someone who is a current competitor in 3 kg sumo to discuss drivetrains. The RS-550 motors may be adequate, but the BaneBots P60 gearboxes are probably too heavy for your purpose -- they are built to take heavy impacts that you aren't going to have in sumo. I suspect you'll want more than a 16:1 gear reduction to get that lightning acceleration in a very small arena -- maybe 26:1? Sorry I can't be of more help.

Q: Dear Mark,
Beetleweight eggbeater 'Wave of Mutilation' I am looking into building a beetleweight Eggbeater robot. I am trying to find a place to manufacture the actual eggbeater part of the robot. Its 2.5"x3.5"x.625" made out of steel (preferably tool steel). I looked at e-Machine shop to try to manufacture it and their price that they gave me was around $600! That seemed outrageous to me. That's over twice what I am spending on parts for the rest of the robot. I know that robot combat is an expensive hobby, I've built two already, and that the weapon needs to be the sturdiest part of the robot but my question is does this price seem reasonable to you? [location withheld]

A: [Mark J.] You didn't include a link to a drawing of your beater design, so I can't comment directly. It's possible that some element of your design may unintentionally call for very time-consuming work. If the design is a conventional rectangle with simple holes for the shaft, the price is not reasonable.

I'd suggest that you post your design to the Facebook 'Robotics Community' for comment and a referal to a shop to water cut your beater bar at a reasonable price.

Q: VestedMadScientist here again. I recently noticed that there is a robot in the lightweight class known as 'GLaDOS'. Do you know if that team affiliated with VALVe Software, or are they just using the name? [North Carolina]

A: [Mark J.] Lightweight 'GLaDOS' is the most recent robot from the very successful Team Adrenaline -- a group of students from Olathe Northwest High School in Olathe, Kansas. They have no connection to VALVe.

Q: Hello sir. I am new to this site. I would like to build a 30 kilo war robot with a drum weapon. Would a hollow drum or a solid drum be better? What kind of a material should I use? And how should I use this? [Maharashtra, India]

A: [Mark J.] Welcome to 'Ask Aaron'. I do ask that you do me the courtesy of reading thru the Frequently Asked Questions and the 'Recent Questions' page, and then performing a keyword search thru the 'Ask Aaron Archives' to see if we've already answered your questions.

  • Your solid/hollow drum question is answered just a few questions below on this page, and there is a great deal of additional information on drum construction in the Robot Weapons archive.

  • Your question on drum material is answered two questions below the answer to the hollow drum question, and again is discussed in the Robot Weapons archive.

  • Your question about how to build the weapon is -- you guessed it -- answered below on this page and discussed in detail in the Robot Weapons archive.
Read thru the earlier questions and answers. If you have new questions, write back.

Q: I bought the wrong BaneBots gearboxes! I have the 256:1 gearboxes which are far too slow for my purpose. Is there something I can do to change the gear ratio and speed up the gearbox? [India]

A: [Mark J.] BaneBots P60 gearboxes are modular. Your gearboxes have four 4:1 reduction stages housed inside a '4-stage' cylindrical ring gear. BanBots sells ring gears in 1, 2, 3, and 4-stage lengths. You can disassemble your gearboxes and reassemble them into one of the shorter housings while leaving out 1, 2, or 3 of the reduction stages. You'll end up with a 3-stage 64:1 gearbox, a 2-stage 16:1 gearbox, or a single-stage 4:1 gearbox. One of those ratios will likely be close to what you need.

Q: As I told you, I bought four BaneBots 256:1 gearmotors for locomotion in my 30 kilo robot. Since it is very slow I am planning to make an 18" wheel and make a dead wheel unit in that. If I do this will I increase the speed? Is it reliable? [India]

A: I'm not sure if I know what you're calling a 'dead wheel'. Are you talking about having the gearmotor drive a small wheel and use the small wheel to friction drive the outside of the large wheel? That will not increase the speed, and friction drives are not reliable in robot combat.

Do not compound the error you made in buying the wrong gearboxes by making an unreliable drivetrain. Chain and sprocket drive

Q: I actually meant this (see drawing) as a dead wheel unit. Now tell me, if I am doing like this in addition to increase of wheel diameter, will I increase the speed and is it reliable?

A: Every gear reduction stage you put between the motor and the wheel steals power from the motor. You've already got 4 gear stages in your gearmotor, and you want to add another one? The chain reduction will sap still more power, add extra weight, and create a new mechanical system that can fail.

A simpler, stronger, lighter, and more reliable solution is to remove one gear reduction stage from the gearmotor. If you are unable to obtain the correct shorter housing, you may reduce the length of the current housing on a lathe. That will give you a 64:1 reduction, which will work well with about 7' wheels in a small arena.

Q: As a newbie I have been trying desperately to find a template list of basic components for a hobbyweight robot but so far nothing. Its not that I am lazy or want a short-cut and in fact I have already read loads of articles on robot building and have browsed endless parts catalogues.

What I am after is a set of basic components (motor/hub/wheel/controller/battery) to use as a base reference (a sort of "minimum specs" with which the robot would at least function).

The thing is some of these motors are really expensive! I certainly don't want to buy a motor that is overkill but neither do I want my robot to fall apart due to plastic gears or weak shafts or some silly thing like that! So its not just a trivial decision. [Tarxien, Malta]

A: [Mark J.] I can certainly understand your desire for guidance on components, but combat robotics really isn't a 'template' type of endevour. A fighting robot is often greater than the sum of its parts, and a good portion of the challenge is to create a design that compliments the strengths and minimizes the weaknesses of available components.

  • All the components of a 'bot must function well together, so choice of 'motor/hub/wheel/controller/battery' will vary with the design you have in mind for the rest of the robot. For example, a wedge or rambot will call for more speed and power from the drivtrain than would be appropriate or needed for a drivetrain that will push around a big spinner weapon.

  • The hobbyweight class is currently lacking in off-the-shelf components that are well suited to the purpose. In particular, robust gearmotors with reasonable output and simple mounting options are just not available. The traditional source of hobbyweight motors has been hacked cordless drills, but they require some work to adapt to the purpose and are awkward to mount.

  • If you ask a dozen different sources for recommendations on hobbyweight 'motor/hub/wheel/controller/battery' you'll get no concensus on components. Everyone has had a good or bad experience with specific items.
Bottom line: the hobbyweight class is currently a poor entry point for new builders. If you have the option to build for a different weight class I'd recommend you give that some consideration.

Q: Hello sir. Sorry to disturb you, I had go through your page for the last two days.

A: [Mark J.] No apology needed. I'm always pleased to receive robot questions, particularly from people who have taken time to read thru the website first!

Q: First: will you please tell me about energy storage capacity? What role does it play in the drumbot?

A: Rotational weapons in robot combat (drums, disks, blades) are all devices that accumulate and store the mechanical energy output from a motor over a period of time as kinetic energy. That stored kinetic energy will resist attempts to stop the rotation of the weapon. The greater the stored energy, the greater the potential impact the weapon will have on an object (like another robot) that comes into contact with an impact surface on the weapon. More energy = greater potential damage.

The energy storage of a spinning weapon depends on its rotational speed (RPM) and its 'moment of inertia' (MOI) about the center of rotation. The Run Amok Excel Spinner Spreadsheet will calculate the amount of kinetic energy stored by a spinning weapon based on RPM, material density, and the shape and dimensions of the spinning components. Team Cosmos drum weapons - drums are hollow!

See the Wikipedia article on Flywheel Physics for a taste of the math.

Q: Second: you talk about 'thickness' of the drum -- what do you mean by thickness?

A: Most drum weapons are hollow tubes with caps at each end to support the bearings. This type of design is used because hollow drums are MUCH more efficient (by mass) than solid cylinders of the same dimensions at storing kinetic energy. 'Thickness' refers to the thickness of the tube wall.

For example, a hollow drum may be 30 cm long and 20 cm in diameter with a wall thickness of 1 cm. A solid drum of the same length and diameter could be thought of as having a wall thickness of 10 cm -- all the way to the center!

Q: Third: in the Team Run Amok Spinner Spreadsheet, which calculations do I use for the drum bot?

A: The spreadsheet has sections for 'motor and drive', 'ring or tube', 'disk', and 'bar'. A drum weapon is a combination of a tube and two disk caps plus the motor and drive:

  • In the motor section, enter the no-load speed of the motor, the stall torque, and the drive reduction ratio between the motor and the weapon. The drive reduction ratio is typically the ratio of the motor and weapon pulley diameters. The 'no-load current' and 'voltage' are optional and only used in the battery capacity calculation.

  • In the tube section, enter the material density of the tube (from the handy chart at the top center of the sheet), the tube height ( = length in a horizontal drum), the tube radius (to the outer wall), and the tube wall thickness.

  • In the disk section, enter the material density and dimensions of one drum end cap -- but double the thickness to represent two caps.

  • Leave the bar section empty -- you have no bar element in a drum.
What about impact teeth? If the mass of the impact teeth is significant, you can manually calculate their mass and add a bit to the tube length dimension in the spreadsheet to increase it's mass by the mass of the teeth. That will give a good approximation.

Q: Which materials do you prefer for the drum and tooth in a drum with a length of 20cm and a diameter of 15cm?

A: Drums and end caps are typically made from aluminum tubing and plate -- 6061 alloy is commonly available and in suitable sizes and is easy to work with. Mild steel can also be used if it is more easily available.

Impact teeth are preferably made from tool steel. There are many posts about the suitability of various tool steel alloys in the weapon archive, and one from India about seven posts further down this page.

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. Tube Measurements

Q: Still I can't get the concept of hollow drum and wall thickness. Can you please show a picture of a drum with the wall identified?

A: Sure - see at right.

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: Hi Mark, in our last competition (30kg category), we were beaten by an opponent with a slightly powerful pushing power (looking at their motors, 4 gearmotors combined providing total of 2HP). For our next tournament, I am replacing the old motors (Ampflow E30-150) with triple amount of horsepower provided by a pair of Ampflow A28-150. I have read in the archives that "a permanent magnet DC motor generates maximum horsepower at 50% of maximum RPM". Considering this fact, should i run the Ampflow motors with just 12v instead of the usual 24v? [Malaysia]

A: [Mark J.] Let's see if I have this straight: your 30 kilo robot with two E30-150 motors (2 horsepower total) got out-pushed by a 30 kilo robot with four gearmotors (2 horsepower total), and you've decided that the problem was that you didn't have enough horsepower? Think about that for a second.

Adding more power can give you greater speed and/or greater acceleration -- but once you have enough power to break traction and spin your wheels in a pushing battle, adding more power will just spin your wheels faster without generating more push.

Pushing ability is limited by the weight pushing down on your drive wheels and the 'grippiness' of your tires. If you want more pushing power you'll need to look to those factors to find it. Throwing money into more power isn't going to get you there.

Back to your original question: no! AT A GIVEN VOLTAGE a permanent magnet DC motor generates maximum horsepower WHEN THE LOAD PLACED ON THE MOTOR RESTRICTS ITS SPEED to 50% of maximum RPM. Artificial methods of restricting the RPM (reduced voltage, artificially induced load) simply reduce the total power available. See this post in the archives for a more complete explanation and a graph.

Q: mark lucky here tell me exactly which type of bearing i should use in the end rings of my drum
drum diamensions :
od = 14 cm
length : 20 cm
weight : 14kg ( including tooth plate )
speed : 4500 to 5000 rpm
shaft dia : 3 cm
[Maharashtra, India]

A: [Mark J.] In general I'd use unmounted, shielded, pressed-steel cage, radial ball bearings with a 30 mm bore and 16mm width -- something like these: shielded bearings.

Q: Dear Mark,a question I've been wondering for quite some time:Why do featherweight class in America was rather unpopular when compared to lightweight and 15lbs class,thus made Robogames cancelled its fight some years ago,and why are they extremely popular in UK?Because of Robochallenge's promotion over the years? [Chinese Forum]

A: [Mark J.] Popularity had very little to do with RoboGames removing the 30-pound Featherweight class from their tournament in 2012. RoboGames removed the very popular 12-pound Hobbyweight class from the competition in 2008 even though it had more competitors than any of the other 'big' robot classes. The organizer simply preferred to have the heavier weight classes featured in the large combat arena, and removed the 'sub-light' classes from the event. The Hobbyweights continue to thrive, and the Featherweight class lives on in two popular 'flavors' at the Northeast Robotics Club (NERC) events -- regular and 'sportsman'.

I count ten(!) active weight classes in current US robot combat competition, and only three in the UK: heavy, feather, and 150-gram ants. I haven't followed the UK events closely enough to say why the competitions shook out to those three classes, but I think that concentrating in just a few classes has helped to strengthen robot combat in the UK.

Q: Mark,since RG was discontinued in 2013 and now America's only "major" event seems like is STEM Tech Olympiad,would you add the result of this year's STEM event to your "tournament tree" page? [Chinese Forum]

A: [Mark J.] It was several years after the start of Robo Games before I decided they were a 'major' event and added their tournament trees to the Team Run Amok Team Run Amok 'Who Won' page. One event does not make a major series. I'll give it some consideration.

UPDATE: I had a little time, so went ahead and worked out the STEM Tech Olympiad tournament trees. You can access them from our Who Won? page, or jump directly to the STEM Tech 2014 trees.

Q: I want to use ampflow e-400 motor for my drum,but as it is not available [in India], can i use 2 e-150 motors, one on each side of d drum?rpm would be same but will d hp and torque increase? [Maharashtra, India]

A: [Mark J.] Yes, you can use a pair of AmpFlow E30-150 motors to power your drum. Performance comparison below -- and I'll add a third option:

 E30-400 (x1)
@ 24 volts
E30-150 (x2)
@ 24 volts
E30-150 (x1)
@ 36 volts
Price (US)$109$158$79
Weight5.9 lbs7.2 lbs3.6 lbs
Stall Torque1500 oz-in1420 oz-in1065 oz-in

As the table shows a pair of AmpFlow E30-150 motors will have performance quite similar to a single E30-400 motor, but at greater cost and weight. An attractive option is to overvolt a single E30-150 by 50% to 36 volts. I might not recommend overvolting the E30-150 this much for robot drive use because of the high and sustained torque loading imposed on drive motors, but for a weapon that spends much of its time at low loads it is an option that you might consider.

Q: Hi, this is Chaitanya again, from India [scroll down about 10 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?

D2 tool steel (HEHER D-2) has a good combination of the hardness and toughness needed for an impactor. I would suggest hardening and tempering to HRC 58/60.

Tooth height depends on robot attack speed, drum RPM, and the arrangement of impactors on the drum:

  • With 260 RPM gearmotors and 16 cm wheels, your max attack speed is a bit under 5 MPH (200 cm/second).

  • Allowing for mechanical and aerodynamic loss, your max drum speed is about 5500 RPM (92 revs/second).

    Weapon drum from Team Cosmos 'Solaris' robot

  • Assuming two full-width impactor bars on opposite sides of the drum, you have an impactor passing a fixed point on the radius every 5.4 milliseconds (1 / (92 revs/second * 2 impactors) = 0.0054 second).

  • At top speed your robot can move forward 1.08 cm in 5.4 millisconds (200 cm/sec * 0.0054 sec = 1.08 cm).
So, your optimum impactor height for a full-speed ramming attack is 1.08 cm. You got lucky in your guess of correct height.

Note: I've seen a lot of Indian robot matches and I have never seen a full-throttle, top-speed ramming attack by a drum robot. Indian combat arenas tend to be pretty small, and combat generally consists of the robots maneuvering into position and easing their drum weapon into the other robot with a short, darting attack. This is less than ideal and does not use the full depth of the impactor.

You have not mentioned the impactor design you have in mind. Something like the full-length impactor bars used by 'Solaris' (Team Cosmos - pictured) should do well for you. The Solaris drum has hardened steel bar impactors set into shallow grooves milled onto the drum that locate the bars and take the shear forces off the countersunk machine screws that hold it in place. There are a couple of additional impact teeth at the ends of the drum, but these were added to correct a specific weakness in Solaris and can be ignored for your application.

Q: hey, this is chaitanya again, so sorry about those mistakes, the corrections are as follows:
voltage: 24 volt
stall current:- 9 amp
gear box ratio:- 4.5:1

so can you now tell me about battery for drive , pushing power of bot etc. for any other info, refer previous question. as you mentioned in the answer of First question, please tell me how many batteries of 12 volt 9 amp can we use (for both drive and weapon) so that bot can work properly for 5 min match?

New drum dimensions are length 22 cm, radius 6.5 cm, thickness 22 mm , are this dimensions suitable for our robot of weight 66 kg? if not then please guide us. which metal can we use for drum between EN31 or EN24? Is there any need of hardening drum metal? if yes then up to what value in HRC? Can you please tell me about tooth depth also. (length from surface of drum ). which metal can we use for dead shaft and tell us its radius also.
thnxs for helping us.

A: [Mark J.] I think you have some more work to do:

Drive Motors: the spec numbers you've given for your PMDC drive motors are quite odd, but if you're sure they're correct I'll continue.

The motors are not adequate for your drivetrain. I recommend a bare minimum of 4 watts of drivetrain output power per pound of robot, and your four-motor drivetrain produces less than 2 watts per pound of robot. A typical robot in your weight class might have 10 or 20 watts of drive power per pound.

With 16 cm diameter wheels, your four drive motors combined do not provide enough torque for reasonable pushing power in a 66 kg combat robot. Maximum pushing power will be well under 30 kg -- less than half what a robot in your weight class might be expected to produce. Acceleration will be very poor, and the drivetrain will stall under moderate load risking damage to the motors. I STRONGLY recommend that you obtain more powerful motors.

There is little point in calculating battery requirements for these motors, as I'm rather certain that the motors would fail well before the battery is depleated. 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 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: Hey Mark, I've been planing a new beetle weight after my first competition (An 8"x8" plate of steel with chunky lite flites) and I was planing on using modded VEXtrollers with the kitbots 1000RPM motors. The problem is that VEXtrollers don't have BEC built in, and I really don't have much money to buy a dedicated BEC of reasonable quality, so I was wondering if I could an off the self Radio Shack voltage regulator. If I can, what should I look for, but if I can't, why? There must be a reason why, because if they worked easily, RC guys would be using them. [Woodburn, Oregon]

A: [Mark J.] If all you're powering at 5 volts is your receiver, you can certainly stop by Radio Shack and buy yourself a 7805 linear voltage regulator IC and wire it as shown in the diagram to power your receiver. The capacitors (35 volt rated) are optional, but will give cleaner power to your receiver.

So why don't R/C guys do this? Linear voltage regulators are inefficient. They waste a lot of power to do the job and lose capacity as the input voltage rises. That makes them a poor choice if you have multiple servos plus your receiver drawing power.

A 'dedicated BEC of reasonable quality' like you are trying to avoid buying is a switching power regulator. More complex but more efficient, a switching regulator also doesn't lose capacity at increased voltage inputs. Take your pick.

7805 voltage regulator circuit

Q: In a pneumatic system for flipper which valve should i use ? Should i use 5 ports or 3 ports ? [West Bengal, India]

A: [Mark J.] I suggest you read the Team Da Vinci: Understanding Pneumatics page for a full description of 3, 4, and 5 port solenoid valves and their applications, as well as general design information and illustrations of robot flipper systems. I also suggest that you read thru the many posts about pneumatic systems in the Robot Weapons archive for additional design considerations.

Basic Pnumatic System Diagram from Team DaVinci.

Note: questions sent in to 'Ask Aaron' thru our embedded webmail system do not use your email. If you write in with a question and ask for a confidential response, you must include your email address. Without your address I have no way to send you a confidential reply.

Q: A few months back I designed a beetle weight wedge with a polycarbonate shell over a 3D printed skeleton. The skeleton had nuts hot glued on the inside which the polycarb bolted to. The printer I had access to could print either ABS or PLA; I chose PLA because it prints better in that printer. By the end of the first event that I took it to, the printed substructure had broken in several places. Many of the fractures occurred at places that obviously should have been thicker and I also plan on reprinting with a higher fill percent.

Do you think the substructure would be more resilient if I printed with ABS instead of PLA? [New York]

A: [Mark J.] It's a little complicated. ABS is an overall stronger plastic than is PLA, so if you carved a chassis from a solid block of each material the ABS chassis would be considerably stronger.

Complication: PLA bonds to itself much better than ABS does when you're layering it up with a 3D printer, which means the ABS has a tendency to delaminate. This makes a structure subject to loading from all angles (like a combat robot chassis) much weaker.

The final verdict would depend on the intricacies of your chassis design, but in general PLA makes a stronger and more resilient 3D printed structure than does ABS.

Comment: Thank you! I hadn't thought about delamination. I think I'll stick with PLA and thicken the weak areas and print with a higher fill.

Q: should i buy colson wheels and live hub or directly ampflow wheels? m using local gearbox for ampflow motors [Maharashtra, India]

A: [Mark J.] The 4" thru 10" AmpFlow wheels are Colson wheels with a hub that mates with the AmpFlow gearboxes. If your 'local' gearboxes have output shafts identical to the AmpFlow gearboxes the AmpFlow wheel/hub combos will work very well for you. If your gearboxes have a different output shaft you'll need different hubs for Colson wheels.

Q: Hey mark, can u help me with designing flipper bot ? I am using 2 pneumatic rams. [West Bengal, India]

A: [Mark J.] Sure. My billing rate for engineering and design services is $60/hour. Send me an advance for the first 10 hours and I'll get right on it.

Alternately, see Frequently Asked Questions #4 -- and read the rest of the FAQ while you're there.

Q: is there anything such as cotton balls that would really get jammed into a pinch point of a drum and stop the weapon? can you attach things such as this to your bots armor or would it constitute an entanglement device aka against the rules?

A: [Mark J.] I don't know about cotton balls, but it's certainly possible to jam a rotary weapon with nets, ropes, and fabric. Take a look at 'Juggerbot 3.0' Vs 'Ultra-Violence' at Robotica. Entanglement devices were legal at Robotica, but the current Robot Fighting League rules prohibit "...nets, tapes, strings, and other entangling materials", so I think you're out of luck.

Q: doesnt the sticky stuff people paint on their foam wheels in the insect division violate the "no liquids" rule? Thanks again [San Diego, California]

A: The RFL rules prohibit liquid weapons, liquid that can spill out of a superficially damaged robot, and weapons that require significant cleanup. I don't think that tire traction goo falls into any of those categories, so its use becomes a call for the event organizer.

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 about the third post in the Weapons 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 diamter 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 ridgidly 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: Dear Mark,I think most US robot can't sustain the hit 'Touro Maximus' gave to 'Tanto',Maybe 'Tanto'just too bad luck.'Tanto''s Hardox fork wedge just bent a little bits,that's too much energy release in one little places.Maybe that's 'Tanto''s design problem,the design is bad at against a drum spinner.'Touro Maximus' also caused fire by that hit.Though 'Tanto''s battery dropped on it,I don't think battery outside can cause fire inside.The hit may also out of 'Touro Maximus''s design. [Guangdong, China]

A: [Mark J.] Many opinions, but no question.

Q: hi.. for amp flow -A150 motor what is requirement of battery,and what will be the requirement of amp. and max limit is 24 volts (i.e 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 imoveable 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 the Robot Weapons archive covering the use of the Spinner Spreadsheet to calculate moment of inertia and the battery requirements for your weapon.

Remembering Aaron...


Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

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