Even though I read your description and title, I'm still having a hard time trying to figure out exactly what this is... I looked up "capstan" and from that I'm guessing this contraption has something to do with flowing water through pipes? That's my best guess as to what it does. Also most people don't know this but a sprocket refers to a single profiled wheel with teeth... you have several cogs (correct term) sandwiched together to form a cogset, cluster or better known in the cycling world as a cassette in terms of newest development in bicycle performance. What I don't see is how the derailleur is suppose to alternate from one cog to the next, that task is usually left to a tensioned cable connected from a shifter. I would guess something as big is this machine wouldn't need manual support but perhaps an automatic mechanical shifter in conjunction with the revolutions of the capstan? I didn't mean to be overly observant or critical about your design, just stating my opinions is all. One last this, and this probably wont matter to you because the design is kept simple but, gear ratio's do matter (gear tooth count) and if set up wrong can give anyone a world of frustration.

Anyway, I like where this is going and I wish you the best of luck with it! <-- Is a road racing cyclist, if you haven't figured out already. XD

Yeah, I guessed it was a bit of an odd design. I don't have a bicycle, and diagrams on the internet were rather hard to follow. These diagrams are more to give the writers a general picture of what their characters are looking at in the story. It's an oversized jumble of cogs and wheels that make the water pumps go. XD

easy to understand ==> nuff said. lol