Idler Sprockets

[DampRobot]

My team is putting an AndyMark 16 tooth hubbed sprocket on a steel keyed 3/8" shaft as an idler sprocket. Originally, we planned on pressing a brass bushing into a .500 bore sprocket so that it could spin freely around the shaft, but unfortunately we have run out of bushings.

We are trying to finish our practice bot tomorrow, and to test it, we need some kind of idler sprocket. It needs to rotate freely around the shaft (the shaft drives something else), and more importantly it needs to be made quickly with stuff we have in the shop. My current thinking is simply take a .375 bore sprocket and put it on without a key.

What are your suggestions?

[Lil' Lavery]

What's the actual function of the piece, just to route the chain around the axle? If it doesn't absolutely have to be a sprocket, there are any number of alternatives you can use. A simple piece of PVC tubing over the shaft can be used (on a temporary/replaceable basis), for instance. Just make sure the PVC is large enough to free spin over the shaft and you'll be fine.

[DampRobot]

Originally Posted by Lil' Lavery

What's the actual function of the piece, just to route the chain around the axle? If it doesn't absolutely have to be a sprocket, there are any number of alternatives you can use. A simple piece of PVC tubing over the shaft can be used (on a temporary/replaceable basis), for instance. Just make sure the PVC is large enough to free spin over the shaft and you'll be fine.

It's function is to improve chain wrap on another sprocket. I don't know if there are any common PVC pipe sizes with an ID of .375. Do you think turning a Delrin piece to go around the shaft would work?

[EricH]

Originally Posted by DampRobot

It's function is to improve chain wrap on another sprocket. I don't know if there are any common PVC pipe sizes with an ID of .375. Do you think turning a Delrin piece to go around the shaft would work?

Make the Delrin a cam shape while you're at it, if you can, or make it slightly oversized. Gives you a tensioner built into the system, which can be helpful if you have problems with slack chain or tight chain.

[Lil' Lavery]

You could use 1/2" PVC, since it has to free spin anyway. The OD would be substantially less than the sprocket (~2"), but that may or may not matter depending on your design.

Delrin could also work. But with both PVC and delrin, you have to worry about the chain wearing through the material over time. Even "grooves" in the material start to add additional friction to your device. So the better it spins around the shaft and the easier it is to replace as parts get worn down, the better.

[DampRobot]

Originally Posted by EricH

Make the Delrin a cam shape while you're at it, if you can, or make it slightly oversized. Gives you a tensioner built into the system, which can be helpful if you have problems with slack chain or tight chain.

EricH,

Keep in mind that the shaft under the tensioner has to rotate unconstrained to the idler. I think I will try to have the bore made oversize though. Having tried to get some .374 ID spacers on that shaft earlier today, I don't think I'll make the same mistake twice...

[aheg1220]

Just make a temporary bushing out of aluminum until you can get your hands on some bronze bushings.