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Questions and Answers about Combat Robotics from Team Run Amok.


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6767 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email about designing and building combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for 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.

Q: Hi there, I'm currently trying to construct a circular arena for a Hobbyweight class competition in order to kickstart a robot combat scene here in Indonesia, do you have any specific pointers in order to make the arena safe? I want the arena to have big ring out zones like the kind of arenas you'd see in the UK in order to make flippers viable - and indirectly make ultra-high powered spinners worse because they run the risk of flinging themselves out of the arena. [Jakarta, Indonesia]

A: [Mark J.] The information I have on safe arena construction can be found in Frequently Asked Questions #38.

Q: I've been trying to figure out the cross-section drawing of the spinner weapon assembly on 'Algos' you linked in your introduction to Mike Jeffries' guest post in the Ask Aaron archives. Can you walk me thru that drawing?

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

The outrunner motor stator been stripped of its bearings and drilled to accept a shoulder bolt that supports the ball bearings upon which the weapon drum spins. The rotor magnet ring has been cut away and pressed into the drum -- effectively turning the drum itself into the rotor.
Combine the information ablove with the photos and description in Mike Jeffries' guest post in the Ask Aaron archives and I think it will make sense.
Q: Personally, what are your thoughts on the viability of Mecanum wheels in combat robots in 2020? The traditional thought was that they just don't have enough grip compared to regular wheels, but has that changed in the recent years? [Banten, Indonesia]

A: [Mark J.] Can you name a Mecanum-wheeled combat robot with a winning record? The closest I can come is superheavyweight 'Alcoholic Stepfather' with a 5 win / 5 loss record. They managed to get pretty good traction, but their custom wheels cost $1000 each and were regularly destroyed in combat.

The problems associated with Mecanum wheels go far beyond traction. Here's what the Stepfather team had to say about them in a 2015 Reddit post:

Mecanum wheels have a LOT of disadvantages: expensive custom parts (including custom shaped rollers); wheels are heavy; needs a complex control system (long time to debug); needs a suspension to ensure all wheels touch the uneven steel floor; you give up a BIG chunk of your traction; use a lot of channels of RC unit; need two different versions (only opposite corner wheels match each other) so twice as many costly spares to build.

Advantages: great orientation control (keeps our uberstrong front end pointing at the opponent). But the biggest advantage is they look really cool when you can get them to work well. That is really why we used them.

Nothing has changed since that Reddit post. If you want to look cool, by all means give Mecanum wheels a shot. If you're trying to win matches they're best left alone. There are multiple posts about Mecanum and Omniwheels in the Ask Aaron Robot Design and Construction archive.
Q: In that "Actual" pic of "LOLcano" [two posts below] it looks like the wheels don't even touch the ground. Am I seeing that right? [Wellesley, Massachusetts]

A: [Mark J.] No wheels are shown in either the 'Rendered' or 'Actual' pics of 'LOLcano V'. Large diameter wheels mount to the hexagonal section hubs you can see in the 'actual' pic, and square guards then mount to the shaft outside the wheeels. I've added a screen grab from the 'fight video' linked in the main post that shows the whole robot.

Q: I'm looking to make a Beetle weight hammer/pick ax bot and saw you mention a pneumatic hammer system is the way to go. Would it be possible to fit into the US Beetle class weight limit if not what is the best substitute? [Spartanburg, South Carolina]

A: [Mark J.] Did I really say that? A pneumatic system can deliver a great deal more speed and power to a hammer or lifter weapon than a direct-acting electric system -- but they are also a great deal more complex and dangerous. Whether they are 'the way to go' depends on your experience in building combat robots, expertise in metalworking, access to a machine shop, and expectations of performance.

  1. Take a look at the Team DaVinci Guide to Understanding Pneumatics for an overview of the combat robot pneumatic systems. Also check with the organizers of any events where you wish to compete for special tournament rules and restrictions on pneumatic systems.
  2. Search the Ask Aaron Ants, Beetles, and Fairies archive for 'pneumatic' to find dozens of earlier posts on insect class pneumatic systems. You'll find some alternatives with that same search.
  3. There have been many pneumatic beetles, ants, and even fairies, but there is no 'off-the shelf' set of components currently available that will work for insect-class combat robots. You'll need to carefully modify existing pieces or make your own -- not a simple task and not for the inexperienced.
  4. You can download the Team Run Amok Electric Hammer Spreadsheet and model the performance of an electric hammer sized for your 'bot -- but you'll find that the destructive energy available from such systems is disappointingly small compared to other weapon choices. A pneumatic hammer would be more powerful, but would still fall well short of other weapons.
No type of beetleweight hammer is likely to be effective at winning matches against spinners or powerful wedges, so 'the way to go' here might be to make it easy and just mount a hammer on a quick R/C servo. It won't do much damage, but neither will a more complex hammer system.
Q: A robot that has fascinated me and a design I can’t recall being repeated was that of 'LOLCano V' (as it’s called on BuildersDB). One thing that isn’t really explained, however, is how exactly they managed to get both the drivetrain and the weapon to move on the same axle. What are some ways to accomplish this, and what am I missing with these designs that I’m most likely overlooking?

I know this design is one that’s probably better suited to show off than to be competitive, but it’s one that I enjoy and would like to put my own spin on... whenever I get around to doing so. [Joliet, Illinois]

A: [Mark J.] 'LOLCano V' (fight video) is not a unique design. It's a eggbeater variant of a "full-body drum spinner" (FBDS) of which there are several well-known examples:

  • Derek Zahn competed successfully with ant and hobbyweight versions of his FBDS 'Daffodil' at NERC events in the early 2000's;
  • Team Anomoly campaigned an antweight FBDS 'Tumbleweed' at NERC events in 2005-6;
  • Team Rotractor's heavyweight FBDS 'Barber-ous' fought at Robot Wars series 5 thru 7; and
  • Team KurTrox's 2019 BattleBots heavyweight 'Axe Backwards' is a FBDS.
You're having trouble figuring out the 15-pound 'LOLCano V' because:
  1. The builders got carried away with their design software and made it overly complex; and
  2. The render left off key pieces (like wheels, hubs, and belts) and included extra stuff that didn't make it onto the finished 'bot.
Here's how it works:
  • There is a solid, structural 'dead shaft' running thru the robot and terminating on either end at a squared wheel guard plate that prevents shaft rotation.
  • The internal chassis is mounted to that shaft and does not rotate.
  • Chassis-mounted gearmotors belt-drive long hex-ended wheel hubs that rotate freely on the dead shaft.
  • The belt-driven beater-bar rides on bushings that bear on the round-section portion of the extended wheel hubs.
Summary There is a 'dead shaft' with wheel hubs rotating on it, and rotating independently against the outside of the wheel hubs are bushings supporting the beater bar. Very messy.
In a more common FBDS design layout as used by 'Daffodil' (archived build report):
  1. There is no continuous shaft thru the robot -- a structural chassis fits inside the drum and carries wheel and weapon forces.
  2. The drum rides on bearings supported by hollow 'stub' axles on either end of the chassis and is powered by a friction drive wheel running against the inside of the drum.
  3. The wheels are powered by chassis-mounted gearmotors with shafts extending thru the hollow stub axles to the wheel hubs.
  4. The stub axles also support one or more firmly mounted arms that press against the arena surface to prevent the chassis from spinning around inside the drum.
See this Full Body Drum Spinner post in the Ask Aaron Archives for a photo and description of the layout of 'Barber-ous'. The chassis shown there is unusual in that it has the drive motors mounted externally on the trailing arms with the wiring running to the chassis thru the stub axles. Later versions of 'Barber-ous' moved the drive motors to more common internal mounts on the chassis.

Q: In a recent post you mentioned that there are specific transmitter adjustments that could help with keeping a two-wheeled robot running straight instead of weaving or veering from the target. Are there things other than adding a gyro? Can you list some specific adjustments? [Way Up in the Rockies]

A: [Mark J.] I wasn't trying to be cryptic. I've covered specific transmitter tweeks that can help with straight tracking in assorted posts and radio articles and I assumed that I'd tied them all together in a single place. Apparently that is not the case, so I've created a new transmitter guide covering this topic:

Note that transmitter tweeks aren't going to solve mechanical issues with your robot. If your chassis isn't straight, a gearbox is binding, or a motor controller is faulty, you need to address those physical problems first.
Q: We're having no luck pairing our Turnigy T6A-V2 transmitter to an Endbots Lemon RX DESC. Do we have to use the Turinigy receiver/transmitter as a pair or is it possible to connect the new Endbots DESC to our transmitter? [The Aether]

A: [Mark J.] The Lemon receiver and your Turnigy transmitter operate in the same frequency range, but they speak different languages.

  • Your Endbots Lemon RX DESC requires a transmitter operating on the DSM2 radio protocol;
  • Your Turnigy transmitter sends signals in a 'flavor' of the AFHDS protocol that is compatible only with the HK-T6Av2 receiver.
Several manufacturers make transmitters capable of communicating via DSMX, but they'll cost a bit more than you paid for your entry-level T6A.
Q: I'm building a featherweight with RS-775 motors and banebots gearboxes. I'm looking for cheap brushed motor controllers rated for 130 amps and 36 volts. Can you help? [Social Media] Performance calculations for featherweight robot

A: [Mark J.] The bad news is that there are no cheap brushed motor controllers with those specs. The good news is that you've made some bad assumptions in designing your drivetrain and don't need those specs.

  • An RS-775 motor can draw as much as 130 amps at 18 volts when stalled -- but a properly geared featherweight drivetrain will never come close to stalling its drive motors. With a 20:1 gear reduction and 3" wheels, the Tentacle Torque Calculator shows a peak current draw per motor of less than 13 amps.
  • At higher voltages the stall current for the RS-775 motor rises. At 36 volts the stall current rises to 260 amps, but the RS-775 motors are not suitable for use at 36 volts. They also will not survive long at current draws much above 25 amps.
I would be comfortable running motor controllers in the true 30-40 amp continuous range, and at no more than 24 volts. You should have no difficulty finding controllers with those specs.

Q: I've read posts where you recommend twin stick transmitters over pistol grip transmitters for "very good reasons". What are the very good reasons? I always thought it was more personal preference than one was actually better than the other. I initially used twin sticks but found it frustrating as I couldn't go exactly straight forward or backwards without turning a little bit when driving a two wheeled bot. [Denver, Colorado]

A: [Mark J.] Builders inexperienced with the full range of control options available on an aircraft radio may well believe that any radio that they can slap a steering/throttle mix onto is going to be just fine for their 'bot. In the heat of a fight with some adrenaline pumping, they find their 'bot quite difficult to control: start speed motor differences, over-correcting on steering, trouble carving a smooth turn, and (yes) weaving along a straight line. They start to think they aren't good drivers, when the problem actually lays in their equipment and setup.

The requirements for precise control of a differential-steer robot are much closer to the requirements for control of a fixed-wing aircraft than those needed for a car or boat. Just having sticks instead of a knob/trigger won't make you a better driver, but the suite of settings that come with a good twin-stick aircraft transmitter certainly can -- if you're willing to learn what they do and how to apply them. You'll also have a much larger pool of users to help with any difficulties you encounter, and a greater selection of transmitters and receivers. You'll also pay less for equivalent quality equipment.

If you're having trouble getting a twin-stick transmitter set-up to your liking, I'll be pleased to help.

Transmitter Functions Quiz

There are features available on your transmitter that will make your robot respond better to the commands you give and make you more comfortable at the controls. Chances are that you're overlooking at least a couple tweeks that could make you a better driver. Let's see how much you know about transmitter functions.
  1. Which common low-speed robot control problem can be corrected by appropriate use of the 'Subtrim' transmitter function?
  2. If you use the 'Mode' function to switch your transmitter from the US standard 'Mode 2' to the European standard 'Mode 1' stick assignments, what will happen to your EleVon mix rotation (steering) stick assignment?
  3. Which specific transmitter features are needed to program a switch that will correct stick commands when your robot is flipped over? Can the FlySky FS-i6 do this?
  4. What is the effect of adding positive exponential response to the rotation (steering) channel on your transmitter?
  5. The servo powering your antweight lifter has a little less range of motion than you planned for when you designed the mechanism. Which transmitter functions can help with this problem?
Click here for answers

Q: I'm using a fingertech tinyMixer in my beetleweight for its invert function 'cause my flysky i6 can't be programmed for a transmitter invert. I have the tinymixer plugged into receiver ports 1 and 2 for right stick control, but I'm having trouble getting the mixing to work correctly. The troubleshooting plan you have in the 'transmitter-side' channel mixing doesn't seem to work with on-board mixing. Can you step me through setting up the tinyMixer? [Robot Forum]

A: [Mark J.] You're correct, the transmitter mix troubleshooting protocol likely won't sort out your tinyMixer without causing invert function problems. This alternate protocol will do the trick with your tinyMixer:

  • The tinyMixer 'LR' lead must be plugged into the steering channel port on the receiver (Ch1 for an Elevon mix).
  • The tinyMixer 'FB' lead must be plugged into the throttle channel port on the receiver (Ch2 for an Elevon mix).
If the mixer plugs are wrong the invert function won't work correctly, so we can't swap them to correct mixing problems.

Throttle Test Power up the transmitter and robot, then push the throttle stick forward a little.

  • If the robot moves forward: good! Skip down to the Turning Test.
  • If the robot moves backward: select REVERSE from the transmitter functions menu screen and reverse channel 2.
  • If the robot spins to the left (counterclockwise): physically reverse the polarity of the power leads to the left motor.
  • If the robot spins to the right (clockwise): physically reverse the polarity of the power leads to the left motor.
Turning Test Push the steering stick a little to the right -- a slow 'spin right' command.
  • If the robot spins right: you're done!
  • If the robot spins left: select REVERSE from the transmitter functions menu screen and reverse channel 1.
That should do it. There are additional tweaks available from the transmitter's Dual Rate , End Points, and Exponential Response adjustments that may make your robot easier to operate. You can learn about them here.

Q: I'm flying to my first combat tournament as a competitor in a couple weeks. How do I pack my beetle and tools to pass security and get onto the plane? Checked baggage? Carry on? Do I need some sort of permit? [Columbus, Ohio]

A: [Mark J.] It's not terribly difficult to fly in the US with a small 'bot. No special permit is needed. The items you'll travel with fall into three categories:

  1. Items that MUST be in checked baggage:
    Includes all sharp edged tools, any tool longer than seven inches, your 'bot if it looks dangerous, and liquids/gels/aerosols in volumes greater than 100 ml.
  2. Items that MUST be carried in the cabin:
    For our purposes this is primarily lithium batteries not in a personal electronic device. Your 'bot does not qualify as a PED so pull the lipo from your checked 'bot and pack it and spares in your carry-on bag. Tape over the connectors just in case the TSA is being picky that day. You may carry any number of batteries up to 100 watt hours each, and most airlines will allow two larger lithium batteries (up to 160 watt hours) in addition to the smaller ones. For reference, a 2000 mAh 4s lipo is less than 50 watt hours. FAA Lithium Battery Guidelines
  3. Items that may be either checked or carried:
    Small tools, battery chargers, radio gear, most spare parts, your harmless-looking robot. You've most likely got a checked bag with some items in it, so you might as well pack the rest. Most airlines allow a 'personal item' to be carried aboard in addition to your carry-on bag. Some builders like to carry their transmitter as their personal item.
Enjoy your flight, and good luck at your first tournament!
Q: Hi there I’m currently making fan made fan robot wars series. For YouTube do you know I find pictures of competitors robots what competed in robot wars 1997 [London, England]

A: [Mark J.] A London fanboy... You'll be after pics from the 1997 UK Robot Wars -- 'cause London fanboys refuse to acknowledge that there ever was a 1997 US Robot Wars (video).

The archived Tectonic Robot Wars site has a complete set of competitor photos from the 97 UK series. Click the show number links to get to the photos.

Please Note 'Ask Aaron' does not generally answer questions about UK Robot Wars (reason). I've made an exception in this case to encourage interest in the early history of robot combat.

New! Run Amok 4-Bar Lifter Spreadsheet
The T.i. Combat Robotics 4-Bar Simulator was an awesome design tool... until Windows abandoned its version of Visual Basic and it stopped working. There was no joy in Mudville.

I grew tired of this situation and started development of a replacement tool in a format that isn't likely to lose support: an Excel spreadsheet. Several months and a couple of beta versions later it's ready for a full release. The new Run Amok 4-Bar Lifter spreadsheet calculates and graphically displays the range of motion and torque requirements for front-bar and rear-bar torque-powered 4-bar lifter designs. The user interface will be familiar to users of the T.i. simulator, and the graphic outputs are directly comparable.

Run Amok 4-Bar Lifter Spreadsheet

Q: How to customise the starter motor of 3000 cc car? [Uttar Pradesh, India]

A: [Mark J.] I'd chrome it and add some pin stripes. मैं भारत के लड़ाकू रोबोटों के बारे में पूछे गए सवालों के जवाब नहीं देता.

Q: Hey Guys:

With tip speed limits being imposed on the higher classes and at times the lower ones I feel like super high speed robots that go fast for the purpose of going fast are very rare. What is the fastest weapon tip speed that has been hit in combat robotics, I assume that it would be an insect weight robot. Has anyone ever hit Mach 1? There would be no point in going this fast other than for the show and an impact with anything at this speed would certainly be robot suicide but damned if I dont want to hit a sonic boom.

Interested to hear your take. [Ontario, Canada]

A: [Mark J.] Tip speed limits are all based on 'theoretical' calculated values that do not account for aerodynamic drag. At low speeds these calculated estimates aren't too far off, but as speeds rise they become increasingly optimistic.

Unlike bearing and belt drag, aerodynamic losses are exponential. When you double the speed of a spinning weapon it must push twice the volume of air out of the way per unit of time, and it must move each unit of air twice as quickly. This means that aero drag increases with the cube of speed.
By the time you reach transonic speed you've got a mountain of pressure drag to overcome and the simple tip speed calculation is meaningless. I know of a few 'bots that have claimed calculated supersonic tip speed, but very few builders go to the effort of actually measuring true speed. Whos's actually fastest? No way to tell.

About Sonic Booms - A 'sonic boom' is not a noise made by an object when it 'breaks' the sound barrier. Any object travelling thru air faster than the speed of sound creates a pressure wave in front of itself that travels along with that object. As the object passes your loction that pressure wave and the low pressure zone immediately behind it strike you, creating the 'boom'. If you could achieve a weapon tip rotating at the speed of sound it would strike you with a small pressure wave on every rotation, resulting in a loud 'sonic siren' -- not a boom.

Q: I audibly went "ooooo" when I read the words "sonic siren", its too bad I can't seem to find a good video of a similar stationary sonic boom effect. I feel that losses at supersonic speed would be massive. I'm certain there is very little information on the kinda effects a 3ibs supersonic spinner would face however in your professional guess is there a ball park efficiency loss I should aim for? Does the AskAaron spinner calculator factor in for any efficiency losses already?

I've calculated that with a 60% efficiency loss I should be able to hit 767mph if I run a fat brush less motor at a 0.5:1 overdrive. The idea would be to use a disengagable brushed motor to get the spinner up to a speed before the brush less motor can take over.

The robot would be startlingly noncompetitive I'm sure but I gotta see what this sonic siren sounds like now! Hopefully it won't kill itself until after it hits the magic speed.

You provide a ridiculously large service to people in the combat community and I really do appreciate that. Cheers

A: I do admire enthusiasm, but as part of my 'service to the combat community' I am required to throw a few roadblocks in your path.

  1. The Ask Aaron Spinner Calculator does not factor in aerodynamic drag.
  2. Give a little thought to "aero drag increases with the cube of speed". To double tip speed requires [23 =] eight times as much power. If you have a weapon that requires 100 watts of motor power to overcome aero drag at 100 MPH tip speed, pushing to 200 MPH would take 800 watts of motor power...
    100 MPH = 100 watts

    200 MPH = 800 watts

    400 MPH = 6400 watts

    800 MPH = 51,200 watts

    We've gone from about 1/8th horsepower to overcome aero drag at 100 MPH to about 70 horsepower at 800 MPH. Bummer.
  3. That 767 MPH number you're aiming for is relative to the surrounding air. An airplane screaming along at 800 MPH relative to the ground is not breaking the 'sound barrier' if it has a 50 MPH tail wind. A spinning weapon is going to create a serious air vortex spinning along with it, so hitting an RPM mark that will give transonic tip speed relative to the robot will still not get you your 'sonic siren' because of the self-created tailwind from the weapon.

Spherical chicken Comment: Well I'm kind of sad to learn there wont be a sonic siren even if I can hit 767mph. I appreciate you bringing these up as they are important considerations. While it sounds like hitting a super sonic siren is out of the question the engineering challenge of designing a supersonic spinner is still very tantalizing. I'll look into using the most aerodynamically efficient blade I can and see if I can hook up 2 massive brushless motors plus a kick starting disengagable brushed to the weapon to overcome the massive cubed losses. Can't wait to find out what kind of physics gyro BS happens at massively high speeds. Time to talk to some overhead builders :P . Thanks for the advice guys!

Response: See if you can talk an event organizer into some special arena conditions. If you can pump the air pressure down to the equivalent of 60,000 feet and reduce the temperature to 216 Kelvin, air resistance is greatly reduced and Mach 1 drops to 573 knots -- but I suspect this only works with spherical chickens.

Q: are there any 60 lb lightweight competitions in the us now that robogames is gone [Lexington Park, Maryland]

A: [Mark J.] Two answers:

  • There are no currently scheduled tournaments that include a 60 pound weight class.
  • This isn't the first time that RoboGames skipped a year. Just 'cause there wasn't a 2019 RoboGames doesn't mean they're "gone".

Q: RE: Robogames. You say it skipped a year, but I was under the impression it was on indefinite hiatus due to retirement? [Sunbury-on-Thames, England]

A: Robogames is an ego-based project that comes, goes, and changes at the whim of its creator. In 2014 Robogames 'skipped a year' and in 2019 it skipped another year. I choose not to guess at when or if Robogames will return but - given its history - a skipped year or two doesn't mean that it is 'gone'.

Q: I've got a Banggood FlySky 2A mini receiver bound to my FS-i6X transmitter. The transmitter failsafes are all turned on but the receiver just ignores them. The failsafe setting instructions on the Banggood site don't make sense to me:
Failsafe settings:
The receiver can normally receive the transmitter signal, push the rocker to the the failsafe set, long press the receiver bind button, the blue light flashes quickly several times and then always bright, failsafe settings is successful.
I know that other builders use this receiver. What am I doing wrong? [Punta Gorda, Florida]

A: [Mark J.] The Banggood FS2A receiver doesn't communicate with the transmitter in the same way a real FlySky receiver does, so setting the failsafes has a different process. I can't read Chinese, but I've become pretty good at translating "Banggood English" to [Actual English]:

  • "The receiver can normally receive the transmitter signal"
     [Turn on the transmitter and receiver.]
  • "Push the rocker to the the failsafe set"
     [Move the transmitter sticks and switches to the desired failsafe positions and hold them there for the next step.]
  • "Long press the receiver bind button, the blue light flashes quickly several times and then always bright"
     [Hold down the receiver bind button 'til the LED flashes, then stays steady.]
  • "Failsafe settings is successful"
     [You're done.]
That's what the instructions are trying to tell you. Next time buy a receiver with instructions that you understand.

NOTE - The very similar Banggood Mini Frsky D8 receiver does not respond to the above failsafe setting process, or to any failsafe process. It is not suitable for combat robot use. Sorry.

Do not feed the troll Q: Who first discovered that wedglets/“Wolverine Claws” beat out a single hinged sheet in a wedge-war? [Mount Laurel, New Jersey]

A: [Mark J.] I'm pretty sure it was the same guy who discovered that anything beats out a flaming hammerbot.

New! Electric Lifter FAQ
There's been increasing interest in electric lifter weapons in recent months. I think the interest may stem from designs that have appeared at BattleBots and the various Chinese events. I had previously edited together posts from 'Ask Aaron' into topic area FAQs for LiPoly Batteries and Spinner Weapons, so I decided to try that same approach with electric lifters. Let me know what you think...

Electric Lifter FAQ

Q: So, I'm a bit befuddled... what exactly is Torque Overage Factor? I've read the Lifter FAQ and the archives, but there's not really a description as to what it is or what exactly it represents, unless I'm missing something. [Mattoon, Illinois]

A: [Mark J.] Take another look at the first two questions in the Electric Lifter FAQ:

Q: How do I calculate the torque needed from a gearmotor to operate a simple beetleweight lifter?


Q: Is the 1.67 multiplication to provide safety or is the number produced by the equation the actual minimum oz-in?

I've gone in and highlighted the reference there to the torque overage factor. Write back if it still doesn't make sense.
Q: Hey guys!

Beetleweight hammersaw weapon Since the last time I posted my beetleweight has gone from having a hammer to a hammersaw. I had a major issue with the hammer this entire season where the pinion gear thats pressfit onto the hammer motor keeps breaking free of the shaft, killing the hammer.

Version 1 My hammer system used a 370W 3.17mm shaft brushless motor that went directly into a 32mm planetary gearbox with and then to a gear driven 3:1. Total ratio 42:1. The issue happened when the hammer had no where else to go. If the hammer is back and we tried to push the hammer back further it just broke the pinion loose immediately. The pinion also broke free over general use, we struggled to get through a 3 min match with the hammer flailing 24/7. Our strategy of "just dont mess up controlling it" didn't work well. We used green loctite and superglue to try to mend this issue, it didn't work, it just popped loose.

Version 2 When rebuilding the hammersaw I downsized the hammer arm power massively and swapped in a toothed belt drive to curb this issue. Unfortunately, the motor shaft was even smaller at only 2mm diameter. We run a 2440 brushless into a 22mm gearbox. The arm """worked""" at one comp - was only swung maybe 6 times, and only took one full impact hit. During a post comp weapon test however, the arm started slipping again and I just know it's the same stupid pinion issue.

I found a setscrewed 3.17mm gear and dremeled a 'D' shape in the motor shaft for the V1 hammer, but the entire gearbox locks up so I haven't been able to durability test it yet. I think I need to clean and re-grease the 32mm gearbox.

Mini question: how important is grease in planetary gearboxes?

Mini answer: Grease is critical, but that setscrew pinion is probably interfering with rotation and causing the lock-up.

Unfortunately there is no way I can find a set screwed 2mm gear pinion. I've seen videos of people soldering pinions, does this work?

Other than soldering the gears and incorporating a slip clutch into the arm for V3 I have no idea what else to do to fix this. Do you have any tips? [Ontario, Canada]

A: [Mark J.] Your problem isn't the pinion gear or the press-fit attachment method -- the problem is poor quality motor shafts.

  • Hobby-grade brushless motors are designed to be used with clamp-style propeller hubs on model aircraft or with large diameter setscrew pinions on R/C cars. The shaft tolerances in those applications are not critical, so there is no need to add to manufacturing expense by using high-precision shafts.
  • A press-fit pinion gear requires precise tolerance on shaft diameter. Without a correct interference fit the pinion will not hold. Attempting to bodge the fit with Loctite or super glue is futile, and the gear fit is too tight to allow a proper solder joint -- there has to be a little room for the solder to wick in between the gear and shaft.
  • It is possible to obtain and install precision shafts on small brushless motors, but it's a difficult job with an inrunner and an unsensored brushless motor is a poor choice for the job of operating a hammer in the first place.
I'd replace the brushless motor with a brushed Nerf motor. These motors have precision shafts designed for press-fit gears, and their low-end torque is much superior to your 2440 brushless. Peak power output is lower, but the added stall torque will make up for it.
Press-Fit Pinion Tip Don't re-use a press-fit pinion. If you pull one off a shaft, replace it with a new pinion. Press-fitting a second time is not quite as secure as it was the first time.

Q: If your canoe gets stuck in a tree with the headlights still on, how many pancakes does it take to get to the moon? Very important, please respond as I need to know for my combat robot's weight class. [Kansas City, Missouri]

A: [Mark J.] None -- snakes don't have armpits.

Well, Art is Art, isn't it? Still, on the other hand, water is water. And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now go finish your beetleweight. [Groucho Marx, 1930]

Two photos of Aaron Joerger Remembering Aaron Joerger, 1991 - 2013

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

- Mark Joerger   
Killer Robot drawing by Garrett Shikuma

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.

Robot haiku:

That's obviously
A question from your homework.
Do your own research.

Aaron's Greatest Hits! More of Aaron's Poems Aaron's Minecraft High Dive Video Aaron's World of Warcraft Player Guide

It's a mystery!
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