[EricH]

Originally Posted by apples000

Okay, I have a few more questions.
I think we're going to be using some gearbox with a single chain reduction.
We are going to drive the arm pivot because we want the arm to be able to rotate 180 degrees.

This transmission can backdrive, so I'm looking for ways to prevent it.

I'll point you to one of the robots from 2005, namely 330's. Large sprocket driving a single pivot, dead-axle (sprocket directly attached to the base of the arm). To get to the sprocket from the FP motors (x2), the power first went through 1 FP gearbox/motor to a common shaft, to a small sprocket, to a medium sprocket on an intermediate shaft via chain, to a small sprocket on the same intermediate shaft, to the big sprocket. No backdriving without a bit of force (>8 lb@5'), and it could lift us under power). Also no lock...

There are a number of methods for reducing force on the chain; try running 2 chain runs in parallel, which will cut the amount of force each chain has to take in half. Also note the counterweight/counterforce suggestions. The counterweights will help stop backdriving.

For purposes of locking an arm, a small pneumatic cylinder fired into holes will do the trick. Or a small servo into a slot. Something like that.