SERIOUS Issues With Keyangs, Bending/Twisting Drive Shafts

hey all,

Team 1757 is running the shoulder joint of our arm with a pair of Keyang motors and an ANSI 25 chain, geared down by a 1:6 ratio. We’re generating enough torque no problemo, but we just bent the axel that our drive sprocket is sitting on. We’d had problems with set screws before and managed to compe with those, but we’re stumped now. The bent shaft is definitely a result of the motors; our hypothesis is that the motors have a bias in one direction (they’re facing each other), and that they are twisting our shaft, thuereby weakening it and rendering it pretty vulnerable to bendage.

Has anyone observed bias in the Keyang motors? If so, has anyone quantified it so that we can compensate for it in our code? We have limited access to the machine shop at a neighboring school, and the lead time on these shafts is a few days. We would really love to stop beating them up.

Any help would be greatly appreciated, and repaid in full with a life debt. Well, maybe not a life debt, but lots of gratitude.


Be careful with pairing the keyangs. If one doesn’t move for some reason (i.e. circuit breaker, wrong code, wiring, etc…) it is not back drivable so bad things will happen when the other tries to move.

I don’t mean to be harsh or anything, but this sounds like mopre of an issue with the design of your mechanism than with the motors themselves, most likely for the reason stated above. To me, your “drive shaft” should be designed to withstand at least twice stall torque.

Try connecting both to 1 victor, so they will definitely stay in sync. Since the stall current of the Keyangs is 20 amps each, 2 will not blow out a victor.

If you want to be sure about the bias,mark the shaft, run them each way for a minute, and count revs.

in retrospect, our driveshaft was kind of wimpy - we’re working on a tougher replacement. thanks for the advice, too; we spun the keyangs for 60 rotations each and timed them, and found that there was a slight bias (very slight). it actually shouldn’t have been a problem, but we had been running them with only our driveshaft and not our arm mounted. the prolonged single-directional rotation was probably the issue - as they rotated, they put more and more torsion on the shaft (even though we should have realized it anyway, we had no way to tell, since the motors were bolted down). better to find out now than in boston!

thanks again for all the help.

I’m pretty sure that this is illegal…

I suggested it for testing, however in ‘FIRST 2007 GUIDELINES, TIPS, & GOOD
PRACTICES’ page 22 it does say:

“In some cases, more than one low current small motor (window/Mabuchi) or actuator may be optionally connected to a single speed controller.”

OK, I see, Thank you… I was just reading this backwards I guess:

<R91> Each motor, actuator, and compressor must be connected to one, and only one, speed controller or relay module.

Sorry for the hub-ub.

is there a way to change the name of this thread? i was thinking something along the lines of “really just minor problems with keyangs, not actually a whole lot of bending/twisting drive shafts anymore, sorry about the commotion; we were scared”.

Dad and Cory,
The two rules contradict each other but the robot rulebook R91 is the one I would believe. I will suggest that the Tips be corrected. To my knowledge, two motors have never been allowed to connect to one controller. However two controllers can be fed with one PWM via a “Y” cord. Still, no two motors are alike and without internal feedback from motor to control system no two motor will ever run in sync. Production variations will make this impossible.

<R91> Each motor, actuator, and compressor must be connected to one, and only one, speed controller or relay module.
The way I read this is how it’s written. If you have a motor, it can only be connected to one speed controller. What is not allowed is connecting a motor to multiple speed controller.

This implies nothing about connecting multiple motors to a single speed controller. I don’t see how this case breaks the rule. Each motor is connected to one and only one speed controller.

Item 31 on the 2007 Robot Inspection Checklist:

Motors can only be driven by one Victor (although a Victor can drive more than 1 motor).

Please stanby on this. I believe it may be an error.

This is a Q&A answered the other day by the GDC…
Re: Victor 884 Speed controller connected to two window motors.
Connecting multiple motors to a single speed controller is not recommended, but is permitted. Per Rule <R91>, you may connect more than one motor to a speed controller, but not more than one speed controller to a motor.

All of the documents do not yet agree but this is the answer. However, as I stated before in this application it won’t get the results hoped for. I cannot explain why it is being allowed this year. I would highly recommend against it.