Motor / Encoder "Modification" and Rule Intent

Hello all-

We would like to mount an encoder on the backside of a standard CIM style motor, which does not have an exposed backshaft. We were considering drilling a hole through the back cover, and tapping the back side of the shaft to port a small supplementary back-shaft which would turn a back-mounted encoder. This would be a benefit for teams using CIM motors, making available a standard, portable motor/encoder package.

This possibly violates “the mechanical system of the motor”, but does not seem to violate the intent of the rule: maintaining the overall power and weight of the motor. We feel this can be accomplished without spoiling the structural integrity of the motor.

Do you think this modification would be in violation of typical rule sets?

Or - are we missing some other obvious method? (FYI- We don’t typically wind up with gear boxes with spare rotating shafts, and didn’t want to use up the main shaft length if unnecessary or add extension couplers to it. Back mounting seemed the most compact and portable solution).

Thanks for the help and advice!!!

Take out the “possibly” part of the rule violation. I quote the rule, with emphasis:

The integral mechanical and electrical system of any motor may not be modified. Motors, servos, and electric solenoids used on the ROBOT shall not be modified in any way, except as follows:

A. The mounting brackets and/or output shaft/interface may be modified to facilitate the physical connection of the motor to the ROBOT and actuated part.
[Parts B through G do not apply to the question.]

The way I read what you want to do, and the rule, you are proposing modifying a part that is neither mounting bracket nor output shaft (part of the motor case, in fact), to facilitate the connection of an external device to the motor’ s output shaft, which is not covered under any part of the exception list, and which is therefore illegal. You might be able to argue the intent with one inspector, but you probably won’t be using that motor once you hit the LRI, and almost certainly won’t be if they happen to call Big Al. Unless, of course, you can get Q&A to specifically allow that sort of use, but that’s kind of a long shot (my prediction of their answer ranges from a straight “No” to “Rule R33 does not allow that because…”).

The intent part–the way I read that blue box (which, BTW, is NOT a rule, just a clarifying statement) is that the exceptions are allowed for “ease of use”–mounting to the robot or whatever the motor is moving–rather than “we think we can do this without violating the maximum power rules”.

Now, that said: Why don’t you, instead of drilling and tapping the back of the shaft, drill and tap the other end? Most gearboxes I’ve seen don’t block that from being accessed. Add your encoder to that end–you may need a plug or something to adapt it–and you should be OK.

Standard disclaimer - this is based on the 2013 rules…

In my opinion, such a motor modification would be illegal. I do agree, it does not affect the power output of the motor. However, the challenge of incorporating an encoder into your design without modifying the motor should just be considered part of the challenge each year.

Allowing a modification of this sort would then open up a whole slippery slope of other teams wanting to do other things to motors to simplify their design process. Which will then lead to inexperienced teams doing things they should not do, like drilling through the important parts. And it just gets too hard to inspect and have to make judgement calls at the event as to if a certain modification is legal or not. It’s hard enough telling teams that their bumpers or frame are not in compliance with the rules, without having to tell someone that their custom motor modification is illegal and quite possibly render their entire robot or competition strategy useless. Allowing modifications of this sort would probably be enough to cause me to stop being an LRI.

One of the things we do in the manufacturing side of engineering is constantly implore the folks on the design side to learn to work with stock parts instead of custom “altered items” which always seem to experience lead time issues.