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how do I make a joiner between a smoothbore (non-grooved) motor shaft & a bike gear sprocket? Answered

I have a small piece of brass stock and I want to join the motor & the sprocket with it, I have limited tools & its going to be part of a future instructable. I need some help on how to design it update: due to some answers I've gotten, I'm not using the brass stock, I'm using a larger block of alluminum, it an inch thick so it would be hard to heat.



Best Answer 9 years ago

Great question! Surely the shaft is keyed on the motor - Get a keyed adapter plate that fits on the motor shaft, (shouldn't be too hard to procure) then drill and tap holes to mount the sprocket. If the shaft isn't keyed, then drill a hole in some stock the same size as your shaft, and drill a hole across that hole to fit a pin through. Mount the assembled adapter plate on the shaft, and drill into the shaft (partway should do). Tap the hole, and use a machine screw to hold the adapter onto the motor.

I love the idea, it was not something I had thought of, yet based on the dialogue between me and lemonie, a set screw might not work very well to handle the torque. Perhaps you might have some understanding of thermodynamics, maybe enough to help solve this?

if it's only the torque of an electric motor/to bicycle sprocket, it should hold up. A set screw with a drilled support hole can take HUGE torque if you use the right pin. (hardened steel bolt, not light duty stuff). Using multiple ones would only make it easier :D If you get a tension fit on the shaft, the pilot holes would make great supports to drill into the round spindle.

What is the motor shaft designed to connect to? I wonder whether a belt drive might fit into this?


i have no clue what it was origionally designed for, but its going to be connected to a bike sprocket, its going on my electric bike.(different from any other design on instructables as of yet)

It's that big 24V thing? I'll be interested to see this when it's done. Looking at the smooth unkeyed shaft in the image, I'd be thinking like frollard but with a red-hot heat-shrinking to fit it rather than a set-screw. (I think there's far too much torque for a set-screw).


Id put 2 or more set screws in, and I would put some dimples in the shaft for the set screws. What do you mean by red-hot heat-shrinking? are we talking about electrical heatshrink, or heating the future adapter, machining the hole, and putting it on the motor really quick and letting the part cool & contract?

It's the "heating the future adapter, machining the hole, and putting it on the motor really quick and letting the part cool & contract" thing I was thinking of. Being round and relatively hard it's not that easy to drill dimples in the shaft. L

heating a part that size evenly (its an inch thick block of aluminum) and then drilling it out would be a bit of a challenge for me, might drilling a slightly larger hole in the block to accomidate a peice of pvc pipe on the shaft and melting the pvc to fill out the gap work? could pvc handle the torque?

You drill it first then put it in a fire / under a blow-lamp (and pail it on with a hammer). The 24V motor looks powerful, I'm thinking the PVC wouldn't do it on what looks like a smooth shaft. L

im guessing, but if you drilled first then heated, wouldn't all sides want to expand (and thus the hole would tend to shrink)?

I haven't found a good link to this sort of thing, but the principle is applied in various places. A star pattern device would get bigger more easily I guess L


I'm not an engineer yet (starting college in the fall) so all the thermodynamics I know is what I've learned on the web, so understandably I don't know how it will actually work. Personally, my gut feeling is telling me that it would grow away from a space physically occupied by the metal and displace as much air as possible(eg. pic 1). which of these pics might best describe the process?

thermal eexpansion.jpgthermal eexpansion2.jpg

we did the 'ring and ball' trick in something like grade 7 or 8. It involved a metal ball, and ring, that barely fit one through the other. When you heated the ball, it wouldnt fit. When you heated the ring, it would fit - so a ring shaped device's hole got bigger. Although I think that might only apply to smaller objects whose expansion forces all outwards... peculiar.


9 years ago

How about JB welding it or really welding it.

jb weld doesnt cope well with much torque, also i dont know how i could weld two dissimilar metals (AL and steel)