Gearing a Rotating Arm

[Chris is me]

Arms can be tricky!

I would avoid driving a long lever arm with a live 1/2" hex axle, and I would avoid driving one directly off a VP, especially 100:1. The 10:1 VP stages are the least strong, and arms exert a lot of shock loading that can really do nasty things to the thinner gears of the VP.

How many positions does the arm need to be in? 2, 3, 4? If at all possible, using pneumatics to rotate the arm between a few positions will simplify control a great deal and is generally more durable.

If you must motorize a long lever arm, it's probably best to do the final reduction for the arm using chain, with the sprocket bolted to the arm to transmit torque. 973 RAMP has some good videos on how to design a simple durable arm power transmission system. The chain can deal with some of the shock loading the arm will face a bit better than a final gear stage could, as there is some flex in the chain and the sprockets are engaged at multiple teeth versus 1 or 2 teeth for a geared system.

Your gearbox will almost definitely backdrive, though you may be able to feed-forward the motor at a small voltage to prevent this. Only do this at low voltages or your motor will heat up quite fast.