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My motor (SunnySky X series V3 2814 1400 kv) has a max output of 1,180W, so using the Vector bar statistics in this post for a rough reference, I should be okay in terms of air resistance. By using these newfangled LiHV cells I found, I was able to push even more power than my planned 14.8V into the weapon for the same weight, giving me 15.2V. The pulley would be 3D printed nylon with a central hub containing a bearing running through the hole in the bar. With two Dartbox Viper motors for drive, I should have more than adequate speed for the weight class. Chassis setup is probably going to be 0.25" UHMW sides with 3mm carbon fiber top and bottom panels/weapon supports.
Recoil is a consideration, but it's a 3 lb bot, and beetleweights are known to hockey puck across the arena like crazy without much internal shock damage. Suffice to say, I won't see the catastrophic failures of modern Tombstone. Plus, shallow wedges won't reflect the full force of my weapon back into me, and anything that engages heavily enough to do so will end up obliterated.
There's also bite, but in my opinion, a shallow engagement isn't the end of the world on a horizontal spinner that doesn't need to grab and toss like a vert. Hell, the high engagement that allows verts to toss their opponent 15 feet into the air can even be bad, as it will contribute to sending me flying back in a horizontal that can't push energy into the ground on impact. Just look at Last Rites vs Whoops from Robogames 2017 where Ray was kicking Whoops' ass until he got a big engagement strike that sent him recoiling violently.
A: Mark J. When you're atop a determined horse that manages to work its bridle into a position where it can grab the bit with its teeth, you can pull that bridle in any direction you like but the horse will pay you no mind and keep to the path it chooses. I don't think my input will effect your design decisions, but perhaps other builders will benefit.
You may want to stop reading at this point and click this button:
From the numbers you gave I was able to back into some of the numbers you did not supply. It looks like you're planning to use a 1.5:1 pully reduction to spin your weapon bar up to an attempted 14,200 RPM with a 425 MPH tip speed and store 2500 joules. I believe you are minimizing multiple issues. Here are a couple:
Stored energy is only as useful as the ability you have to transfer that energy in an impact. At 14,000 RPM a weapon would be fortunate to transfer a tenth of its stored joules to your opponent. Most likely would be a series of little "bump backs" as the blade skitters across a surface. What that stored energy will be effective at doing is to provide a prolonged sequence of pinball rebounds once you start bouncing off the arena walls.
Consequences of Heat Saturation
The motor has a claimed max current of 80 amps for 30 seconds -- assuming that it is starting from a cold zero amp condition. During your prolonged calculated spin-up your motor is being bogged down far below its design operating RPM and pulling a lot of current. If your weapon spin-up is interrupted by a series of slowing impacts the motor will remain in that amperage danger zome for an extended period of time. Worse, your weapon motor will expend a continuous ~900 watts of power to maintain 14,000 RPM against weapon aero drag, which will place it (and likely the ESC) fairly close to heat saturation. Having to spin back up from this condition by drawing an average 160 amps over four seconds is a sure-fire way to blow the motor/ESC/battery.
Reply: "Shades of Hellachopper" here -- dropping to 2:1 reduction. Thanks for the reality check.
Comment: That's dangerously close to rational, Arlington.
Don't do it.
Before someone writes in to tell me that 'back in the day' some builders used two ESCs to "split the current load" on high-current Magmotors and Bosch GPAs, let me clarify. Those motors had a system of four brushes set 90 degrees from each other, and each of the ESCs were wired to a different pair of those brushes. This was a special application for specialised motors and the outputs of the ESCs were not "spliced together".
See also this previous post in the Ask Aaron Motors and Controllers archive.
A: Mark J. There is a similar situation that involves operating a lifter servo weapon at 2S when it is plugged into a receiver that can take no more than the voltage from a Battery Eliminator Circuit (BEC). The two situations have similar solutions.
With your Malenki, it is possible to use a battery with more than 2S to provide power to your brushless ESC by tapping into the balance plug of the same battery to obtain 2S power for the ESC/receiver and your drive motors. A diagram for a 3S battery is provided below.
There are a few drawbacks to this solution:
A: Mark J. This is a job for the Google A.I.: How does a vertical spinner weapon on a robot work?. The A.I. is much more patient in answering this type of broad question than I am. Each time you ask it generates a slightly different answer, but the answers I've seen for this particular question have all been reasonably complete and accurate.
If you want more detailed technical information, see the Ask Aaron Spinner Weapon FAQ.
If you have specific questions I'm glad to help.
A: Mark J. Someone had to count the laps. Someone had to keep the clock. Someone had to decide who left the platform first. Someone had to determine if a goal in 'The Maze' had been blocked. Someone had to award me a penalty for going out-of-bounds in 'The Speedway'.
For Season 1 that 'someone' was Terry Huntsberger, the NASA-JPL Roboticist listed in the end credits as "Robot Consultant". I mention Terry a few times as the Robotica 'tech judge' in my Robotica Journal.
Terry was not listed in the credits for seasons 2 and 3 and no one is credited as a robot consultant -- but someone had to make those calls...
I know this is a lot in one post, but I know you're the guy to ask. So let me know your thoughts on my setup. [Mount Prospect, Illinois]
A: Mark J. I see that you have made some changes to your design since your earlier post. A few observations:
A: It is not a matter of "Featherweights should be around a 20:1 drive system" -- different drive motors will require differing gear reductions for any given combination of robot weight, wheel diameter, and arena size. See: Optimizing Drivetrains.
You have the relationship between traction and gearing reversed. Adding magnetic downforce to a robot creates a need for greater torque from greater gear reduction -- not the other way around. The Team Tentacle Drivetrain Calculator includes a field to enter magnetic downforce and can calculate adjustments to the drivetrain requirements based on that added traction.
A: As noted in my answer to your earlier post, AndyMark Redlines are quite commonly run at 6S voltage in combat robots . If run with a reasonable gear reduction they can handle the heat. If run with insufficient gear reduction they will melt. Wrapping them in a TPU heatsink would be worse than doing nothing as TPU is a much better insulator than heat conductor.
A: There are essentially no differences in bearing protection. speed, or rotational inertia between a Live Shaft that spins with the weapon and a Dead Shaft that is fixed relative to the chassis. The primary difference is that a 'dead shaft' can be a structural member of the chassis that adds to both the strength and rigidity of the robot.
A: Angular contact bearings (expensive) are used where a single bearing must absorb forces from all directions. This is not the case for your weapon. A pair of tapered roller bearings (much less expensive - and stronger) will do very nicely.
Some builders prefer oilite bushings for their weapons. These bushings can absorb huge impact loads and are inexpensive -- at the cost of a bit greater friction.
A: See Frequently Asked Questions #17. Note also the difference between 'countersink' and 'counter bore'.
- sincerely, Iceywave
A: Mark J. Sincerely? I suspect you're just prompting me to post a pic of Team Run Amok's infamous antweight snapper/crusher/flipper 'Rat Amok'.
In truth a rat trap can store far more energy than is required for a properly designed antweight spring powered flipper -- a mouse trap spring should be adequate. I found an analysis on the energy stored in a typical mouse trap at the Physics Stack Exchange:
I then used τ = −kθ (torque applied to the spring arm by the weight is equal to torsional constant times the angle rotated) along with the values from one of the weights to calculate how far the spring is twisted by default [θ = (τweight / k) − θweight ]. This came out to about 73.63°.
I used this 'starting angle' to calculate how much potential energy (U = 1/2kθ2) the spring would have when totally open (an additional 180° from that last angle we found). The maximum energy that you could get out of this "standard" mousetrap is approximately 0.815 J.
A: Mark J. Depends on what you consider "heavier weight classes". Last I heard you were building a beetle and/or a hobbyweight.
Keyed hubs are the standard for combat robots above the insect classes for good reasons: compact, reliable, and widely supported by manufacturers. You need good reason to not use them.
I have used Trantorque keyless bushings (cross-section pictured) to lock pulleys and sprockets onto smooth shafts with good success. They come in a good range of sizes but are generally too bulky to use on motor shafts. They are also a bit pricey.
Down on the lower end of 'big bots' I see some builders using clamping hubs that surface-bolt onto pulleys and sprockets. A seach for 'clamping hubs' will turn up a variety of styles.
With all the talk of motors getting burned out due to aerodynamic resistance lately on the site, I don't want to waste money and time implementing a motor that melts when I turn on the blade. I'd then have to redesign the bar, chassis, and pulley to accommodate the bigger motor in weight and space.
A: Mark J. If you don't want to waste money and time you should pick a different hobby.
CAUTION: Entering the Deep End of the physics pool. If you Google "aerodynamic drag equation" you'll get this tidy little equation that calculates the drag on an object moving thru still air:
The Big Problem: Our rotating weapon is not moving uniformly thru still air. Velocity ranges from the greatest out at the tips down to zero at the hub center. How good is your calculus?
The Bigger Problem: The motion of the weapon creates a vortex of wind that changes the velocities of the weapon relative to the air. This is non-trivial and would keep a supercomputer busy for a week to calculate.
The Bottom Line: It is unrealistic to attempt to calculate the power needed to offset the drag on a defined spinner weapon at a given speed. But this is not to say that it is impossible to gain guidance on the power requirement for your weapon:
Watts Required =
A: Mark J. The 4mm wide S3M FingerTech belts are uncommon in beetleweight spinners -- particularly for a belt long enough to reach out to a horizontal weapon. The S3M tooth profile is OK but the magnitude of the power transfer calls for a wider belt:
A: Mark J. You have correctly entered the motor specs given by FingerTech (and other sources) for this motor -- but those specs are incorrect . Per the spec sheet, the 12 volt stall torque for the gearmotor is 27.8 oz-in @ 4.9 amps (Kt = 5.67) but actual test results from multiple builders yield an average stall torque almost three times greater.
For comparision: At 12 volts, the very much smaller FingerTech Silver Spark 22:1 has a similar no-load speed and stalls at 22.7 oz-in @ 2.1 amps for a Kt of 10.8 -- the spec for the 22mm stall torque is obviously too low. See: Converting Motor Specs. Based on the real world numbers I estimate the torque constant for these 22mm gearmotors at 16.3 oz-in per amp. Plug that into the drivetrain calculator and your output will make sense.
Also, the "Torque (per motor) to spin wheels" calculation in the drivetrain calculator does not refer to the torque required to simply drive the robot across the arena. It is the torque required to provide the full pushing force and "beak traction" to spin the wheels and prevent motor stall under heavy pushing. This value should ideally be attained at no more than half the stall amperage of the drive motor. See the Optimizing Drivetrains page for a full explanation.
PS: How much leftover battery life should I pack? The spinner weapon spreadsheet tells me that I'll be using 1.03 amp hours per match, so would a 1500 maH battery work? How do I know if the margin is too small, and if the battery will start to die before the end?
A: The Spinner Weapon Spreadsheet provides a gross estimate of the battery capacity required to spin up your weapon a specified number of times and maintain top speed for the given duration of the match. Take that number with a grain of salt. It does not factor in things like excessive aerodynamic drag from spinning a weapon at stupid-fast speed. It also does not include the battery capacity required by the robot drivetrain.
Generally, rounding up capacity in the 25% to 50% range is about right. You will find out if the margin is too small by testing. Most battery chargers will tell you the mAh required to restore the battery to full charge after a match so that you may determine if the battery has too much or too little capacity.
Three Minutes Later...
Just realized that the literal top post [on the Ask Aaron page] answered my initial question, oops. Do I need to worry about tensioning with one smooth side using a fingertech belt? [Ashburn, Virginia]
A: Mark J. If you run two toothed pulleys you can run a calculated fixed pulley spacing and be fine. If you have one smooth pulley you will need some method of tension adjustment -- one or two tensioning idlers as in your example photo or perhaps adjustable weapon motor mounts.
Beetleweights are still small enough to consider running two toothed pulleys with a timing belt -- you can do that and avoid the tensioning issue. Take a look at the STL files for the EndbotsVector Beetle kit. You might find it easy to modify the motor can 0.25" XL pulley from the Vector to fit your weapon motor. Consider battle hardening your weapon motor if you choose to do this.
Thanks! [Masked Server]
A: Mark J. Is this purely a theoretical inquiry or are you constructing a featherweight robot? I ask because someone building a featherweight with an active weapon would likely have gained sufficient experience in lighter weight classes to understand why chains are not used and timing belts are commonly used with at least one smooth pulley.
The abrupt deceleration of a spinning weapon striking its target places an enormous load on the weapon motor. The larger the motor the greater the risk of motor damage from this load. Current practice for 30-pound robots favors a narrow v-belt set loose enough to slip a bit under decelleration loading for protection of the weapon motor. Heavyweight robots may use chain drive with an industrial slip clutch built into the weapon or motor hub.
There are dozens of posts on the use of chains, belts, and slip clutches for weapons in the Ask Aaron Robot Weapons Archive.
A: Mark J. If your design does not support the shaft on the far end of the outrunner you should place the pulley as close to the motor base as possible to limit stress on the motor's internal bearing tube. Wrapping a pulley around the outrunner has the advantage of creating a very compact package and is common in beetleweight horizontal spinners -- see the Press Fit post farther down this page. Upscaling that design to a hobbyweight is fine, but featherweights and up should look to designs that do not stress the motor can.
I'm guessing aero forces will be the limiting factor here and overload the motor, but can you weigh in on how this weapon would perform? Would it be better to go with a lower kv? [An iCloud Server in New York]
A: Mark J. I recognize those weapon numbers. You either have or are duplicating a Vector beetleweight kit. The Vector had the 980 Kv SunnySky 2212 weapon motor that would draw a continuous 8 amps @ 14.8 volts to overcome the aerodynamic drag at 6500 RPM (370 joules). That's about 90 watts and the motor is rated for a continuous power output of 385 watts -- it could do that all day.
You are correct in worrying about aero forces at high RPM. Increasing weapon speed increases aerodynamic drag with the cube of speed:
A: Mark J. Typically, yes. Actually the pulley inner diameter is a few thousandths of an inch smaller than the can diameter for a tight press fit. See this post in the Ants, Beetles, and Fairies archive for a photo and a link.
Is there is software that will model the performance of a specific flipper design in the way your Spinner Weapon Spreadsheet models spinner weapon performance?
A: Mark J. Spinner weapon analysis is simple in comparison to the fluid dynamics flow calculations needed to model a pneumatic flipper. Most gas flipper builders will simply max out the components and hope for the best, but I can see how a reality check would be useful in a weightclass where such weapons are few and far between.
The builders of Robot Wars flipper 'Hassocks Hog' have a Guide to Designing a Pneumatic Flipper that includes a 'Flipper Calc' Excel spreadsheet. The interface is not very user friendly, but with determination it can get you some performance answers. From the webpage:
To help me improve my own woefully inadequate flipper, I wanted some way of simulating its performance before building it. I needed to check its performance first, before wasting money buying the wrong parts. As a benchmark, I wanted to know how high my flipper would throw another robot, but after trawling the Internet for some form of simulator, I gave up and set about creating a spreadsheet for myself.
"Flipper Calc" is the result of a few months work picking peoples brains and surfing pneumatic suppliers and gurus. I won't pretend that "Flipper Calc" is a 100% accurate simulation, but none the less it gives a close enough indication of flipper performance. In any case, the theory may be all well and good, but there are practical aspects such as curvatures of pipes, the type of connectors used, etc, that will affect the performance of your flipper too. Once the theory has identified the parts you need, the practical aspects will have to be addressed as well.
A: Mark J. Your event is on a Tuesday? Does that make this a school project?
It would help to know a bit about the robot you're building. Different designs, different builders, different shop tools, and differing experience levels all call for differing timetables -- but I can give some general advice.
A: Mark J. Yes, it is entirely true. The girl in the mouse suit had little to do with it, but the yam (it may have been a sweet potato) and the nails were critically important.
I prefer to not put the story in print as it is best told with a great many gestures, sound effects, and exagerated facial expressions. As I said in the original post, "...you'll need to buy me a beer to hear that story."
A: Mark J. You are entirely correct to worry about this, Zanesville. Tangential drive (YouTube video) and squishy foam tires do not mix. The transfer of power from the drive shaft to the tire is the product of the coefficient of friction and the force by which the shaft is pressed against the tire. Power in excess of this power limit results in slippage, and getting enough force against a foam tire will depress the shaft so deeply into the tire that drive force vectors would no longer be 'tangential' and power transfer efficiency plummets -- with or without a traction coating on the tire.
A: Mark J. I've never actually tried, but it shouldn't be very hard. You can only go where the rails go, so just give it a little throttle and toot the whistle once in a while.
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
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:
A nice image of HellaChopper - just to break up the blocks of text.
A change from a 1.5:1 reduction to a 2:1 reduction:
A: Mark J. No. ESCs operate with internal frequency generators to produce the current pulses used to control motor speed. The pulse generators for two ESCs -- even from identical sources and receiving identical receiver signals -- will not be 'synched'. At times the full current of the motor will be passed thru one unit, then the other. This negates their ability to "split the current load" and will very likely smoke both ESCs. Equally bad, while they survive you will get irregular pulsating speed variances rendering consistent control impossible.
First off with just the specs I would be curious any advice for the design just based off the electronics and CAD photo.
Q: Getting into my questions though, my first is about the drive. In another post you said that Featherweights should be around a 20:1 drive system. My first question is, if I have an arena that is utilizing a steel floor, and I add magnets for extra down force, would 10:1 be suitable? Or does the extra downforce not affect the torque?
Q: Another comment about the drive system, is when running the redlines and gearbox at 12v, they seem to get hot very fast. Besides adding a cooling fan, is there an easy way to dissipate the heat? Would 3D printing a TPU heatsink be beneficial? Or maybe it isn't a concern since the matches are only 3 mins max.
Q: Another question I have is about the weapon system. I am trying to decide whether it would be better to have the bearings spin on the inner or outer ring. The main difference in setup would be spinning the Weapon + Shaft, and attaching the bearings to a side plate. Or attaching the bearings inside the weapon and spinning it around the stationary shaft. I am not sure which would protect the bearing most from impact, as well as produce the most RPM/Inertia.
Q: Along with that, I've done a lot of research on bearings and have heard a lot of good things about angular contact bearings. But did not know the best type for high RPM yet high force.
Q: My third question [You and I count differently!] is about bolts/tapping plates connection. I currently have 1/16", 1/8", 1/4" and 1/2" aluminum to work with for plating. I also have M3, M4, M5, M6 bolts to work with. In my design I wanted to countersink my bolts so that I didn't leave an exposed bolt to be sheared off by the enemy weapon. But my main confusion is the best combination of these. Because say if we use a 1/2" plate, the bigger the bore a tap we make, the less wall material we leave to support the bolt.
Q: How do builders in the heavier weight classes go about attaching a pulley to their weapon motor shaft? I know set screws can't be the answer here. I've seen some builders reshaft their motors with a thicker shaft for a keyway but are there any other methods out there? [Sacramento-ish]
Fd – Drag force (N)
ρ – Air density (kg/m3)
A – Frontal area (m2)
C – Drag coefficient (unitless)
v – Relative velocity (m/s)
Q: These weapon performance numbers [at right] from the Run Amok Spinner Weapon Energy Calculator are looking a bit optimistic. I used ChatGPT to calculate the internal resistance of the SunnySky 2450 Kv 2212 motor since they only have the specs up to 1400 Kv version on the site.
Q: How do robots like Sauron and Vector design their pulleys to fit around the outrunner can? Do they just build it with equal diameter and press fit it on? [Ashburn, Virginia]
Q: I have been building featherweight spinners for a couple of years now and I want to look into making a pneumatic flipper. I have experience with paintball pneumatic systems and a good machine shop. I've read the Team Da Vinci pneumatics overview and your own Pneumatics Tricks and Tips page, but I'm uncertain about the sizes of components needed to produce a good flip in this weightclass.
I have been pondering the usefulness of tangential drive for antweight (1 lb) applications, but have concerns regarding its compatibility with foam wheels. In particular, the Repeat Robotics Repeat Tangent motors seem to be effective on the likes of robots like "Super Space Turtle", but I have only seen them used with tires made via custom-molded polyurethane/rubber. Could it be that foam wheels are too "squishy" to create any meaningful contact points with the shaft or simply have a poor friction coefficient (if the latter, could this be mitigated using liquid latex/some other traction coating)? This seemed like a "too good to be true" lightweight 4WD solution. Any feedback is greatly appreciated. [Zanesville, Ohio]
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