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I think my go-to invention would be some kind of bicycle or something that uses a similar mechanism
If you accept a lack of gears, a belt drive instead of a chain probably simplifies the most difficult part of actually making it. Best pick somewhere with good roads though, since you’re definitely not making pneumatic tyres or a pump to fill them with
I thought about something that stays in place, maybe a small millstone or a blower for a smith’s forge.
Shaft-drive bicycles are a thing too. That might be easier for an ancient blacksmith to tackle?
Edit: or to at least get a proof of concept you could bypass the whole torque-transmission issue by building a penny-farthing