pic: Poly-carbonate Gearbox Plate



This is a slightly adjusted version of the Vex dog shiftier gearbox. It is similar to the original but has three changes. First it is made of 1/4’’ poly-carbonate not aluminum, the reason being our team has a massive sheet of it, this leads me to my second and third reason. I wanted to incorporate mounting for the AMT103-V Kit encoder. This lead me to a design decision that I am not sure about. Due to the placement of the mounting hole on the encoder I removed the 3/8" bearing hole since even the smallest bearings I could buy would not fit. Instead I decided to use the poly-carbonate plate as a bushing for the shaft. So this lead me to the question,

“Would I be able to run a shaft with out a bearing in a poly-carbonate plate?”

I have done several gearboxes with polycarb side plates in the past few years.

I have not ever done so except in extremely low speed/low load applications, but I would definitely not recommend ever using polycarb as a bearing surface. It’s actually comparatively rough on machined or cut surfaces, and doesn’t do as well in wear as plastics like delrin. Just make some kind of adaptor plate for the encoder, and put your mount holes somewhere else.

I would also not suggest blindly copying the lightening pattern of the aluminum plate to the polycarb. It is much more flexable than aluminum, and the thin ribs will cause the bearing holes to twist quite easily. I always do my polycarb gearboxes with solid side plates (the shift from aluminum to lexan is plenty of weight savings for us), and consider a more aggressive standoff pattern to stiffen it up as well.

Can you fit an Igus type plastic bushing in there? or a bronze bushing?

And yeah, all those irregular holes look like a waste of machining time, but then I’m a naturally lazy person.

You can’t afford to use the kind of aggressive lightening patterns in polycarb that you can in aluminum, particularly for gearboxes. Polycarbonate just isn’t rigid enough. I would not pocket the gearbox at all, and maybe add more counterbores standoffs to give you rigidity. All of the rules for pocketing aluminum gearboxes just don’t apply here.

To answer your question, could you run a shaft in polycarb without a bushing… Maybe, but probably not at 1200 RPM. You could easily heat the polycarbonate up to its glass transition point wih aggressive driving and things would get ugly fast. Even if it works though, your efficiency will tank beyond the point where it’s worth it. An encoder is a terrible reason to throw out ball bearings…

I definitely can/will/did remove the lightening holes. I can fit a bushing if I have to, I just wanted to check before if got them.

I would tend to agree with Chris, that I wouldn’t even go as far as to swap from a ball bearing in an application as crucial as a drivetrain. Just use a mount plate like this, you’ll be much better off.

I thought as much. My belief has been to design dangerously but build safely. I will see if a bronze bushing will work. We have two extra gear box’s that would use this plate, I think that I will make and install one with a bushing and run the gearbox for a good two hours on full checking in at ten minute intervals (don’t worry someone will be in the room) and take picture for records. while it may never go on a robot it would be a great learning experience for designing with plastic.

For that to be a realistic test, the gearbox must be put under loads similar to what it will see on an frc robot. Testing it as you described, with no load, will not expose any potential problems fully