[Adrian Clark]

Given the reduction of your gearbox, and how much torque you have at the shaft, I would extremely worried about using a set-screw to transmit power to the pulley. I wouldn't expect it to take more than a few matches for that set screw to start slipping (because of your high reduction I'm assuming this gearbox is being used in a high load application).

The best way to transmit power to and from a shaft is to use hex shaft and hex broached components. I can't see into your gearbox too well, but given that those are WCP gears I'm assuming that your output shaft is hex that's been lathed down to 3/8. I would suggest that you redesign your plate to accommodate a 1.125 OD bearing for your output shaft and use an AndyMark 3/8hex oversized bearing. Or if you don't want to redo your plate and still use that 7/8 OD bearing then I would suggest you mill a keyway into your shaft and broach a keyway into your sprocket, it wouldn't be a good as using hex shaft but it would be much stronger than it currently is.

Edit: I just noticed that you were using WCP's new 24DP delrin gear, where'd you get your 24DP pinion? The only place iv'e found to get a 24DP pinion that isn't brass in to buy SDP-SI carbon steel pinion wire, is that what yours is?