[Joe G.]

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.