03-03-2015 23:26

Joe G.

Re: pic: Poly-carbonate Gearbox 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.

04-03-2015 00:00

MrForbes

Re: pic: Poly-carbonate Gearbox Plate

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.

04-03-2015 00:46

Chris is me

Re: pic: Poly-carbonate Gearbox Plate

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...

04-03-2015 00:47

Munchskull

Re: pic: Poly-carbonate Gearbox Plate

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.

04-03-2015 00:55

Joe G.

Re: pic: Poly-carbonate Gearbox Plate

Quote:

Originally Posted by Munchskull 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.

04-03-2015 01:06

Munchskull

Re: pic: Poly-carbonate Gearbox Plate

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.

04-03-2015 11:34

Dunngeon

Re: pic: Poly-carbonate Gearbox Plate

Quote:

Originally Posted by Munchskull 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.