Timing Belt Wrap + Load

Our team is looking to use timing belts for a 3:1 reduction of 775pros, but we were worried about the contact of our belts and pulleys.

Here’s what it looks like now:

1 and 3 are 12T, 4 is 36T, and 4 is an idle tensioner. The system uses 9mm wide GT2 timing belts.

From what I’ve seen, the general consensus is that wrap angle should be >120. However, even though we have a wrap angle of ~155 on our small pulleys, there are only 5 teeth in contact.

Should I look to increase the contact angle even more? And will 9mm belt be able to handle the load? (we plan to use this for a flywheel shooter)

I think you meant 2 is an idle tensioner

If you have the hardware to create a test setup with these 775pro motors, I would suggest to test in the real world.
Im not a belt pro so im not sure but I think you might be better off if you move 1 and 3 downwards so it would more contact area

From what I know the 120 degree rule of thumb is more related to the Capstan Equation than the tooth interface, which has your supported load increase by the power of contact angle. So I personally wouldn’t worry only having 5 teeth if you have 155 degrees of contact. I’d be more worried about the ~90 degrees that you have on 2.

If you’re worried about the contact of the belt to 2, could you swap the locations of 2 and 4, or bring 1 and 3 closer together? Both look like they would increase the amount of contact.

Personally I don’t know much about the strength of belts, we mostly have moved to gears for stuff like this (plus they make metal sounds)

From a quick visual inspection the belt run looks fine. If the small pulleys are attached directly to 775pros they shouldn’t have that much torque going through them. You generally won’t see more than 180 degrees of wrap on small pulleys, which would be 6 teeth on a 12t pulley. Having 5 teeth is pretty close to that. Obviously the more wrap you have the better, so if your design can handle moving 1/3 down or 2 up that would be ideal. But if not, I would guess that it would still work fine.

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