pic: Belt Drive Chassis Take 2

[Jared Russell]

With a toothed belt (or chain), I see problems arising when there are multiple, non-continuous contact patches with the pulley (like with the center wheel in the configuration shown).

For the sake of argument, say the top part of the belt is fully engaged with the teeth at the top of the center pulley. Clearly, you also want the other contact patch to be fully engaged with the teeth of the center pulley. Let's say your pully has 40 teeth, and 10 of them are engaged with the top of the belt. Then there are 10 unengaged teeth (the gap between where the belts contact the pulley), then 10 teeth engaged with the bottom of the belt. In this configuration, there must be an integer number of teeth in the belt between where it leaves the top of the center pulley and returns to the bottom (since there is an integer number of teeth on the pulley between the same two points*).

How do you ensure this? You either design for the exact right belt length, or you add adjustment to your system. You already have a tensioner, but, depending on belt pitch, you may need more than one dimension of adjustment in order to find both a belt length and belt tension that work. Failing that, I could see (best case) uneven loading of the belt and (worst case) belts skipping out of their pulleys.

I hope that all made sense.

* There could of course be, say, 9.5 teeth between contact patches, in which case the requirement generalizes to X.5 teeth in the belt, etc.

[Sh1ine]

Originally Posted by Jared341

With a toothed belt (or chain), I see problems arising when there are multiple, non-continuous contact patches with the pulley (like with the center wheel in the configuration shown).

For the sake of argument, say the top part of the belt is fully engaged with the teeth at the top of the center pulley. Clearly, you also want the other contact patch to be fully engaged with the teeth of the center pulley. Let's say your pully has 40 teeth, and 10 of them are engaged with the top of the belt. Then there are 10 unengaged teeth (the gap between where the belts contact the pulley), then 10 teeth engaged with the bottom of the belt. In this configuration, there must be an integer number of teeth in the belt between where it leaves the top of the center pulley and returns to the bottom (since there is an integer number of teeth on the pulley between the same two points*).

How do you ensure this? You either design for the exact right belt length, or you add adjustment to your system. You already have a tensioner, but, depending on belt pitch, you may need more than one dimension of adjustment in order to find both a belt length and belt tension that work. Failing that, I could see (best case) uneven loading of the belt and (worst case) belts skipping out of their pulleys.

I hope that all made sense.

* There could of course be, say, 9.5 teeth between contact patches, in which case the requirement generalizes to X.5 teeth in the belt, etc.

The other mentor is concerned about the same thing. I am not sure that I completely follow you however. I am under the impression that if a belt has a 5mm pitch it is only a matter of turning a pulley (also with a 5mm pitch) to the correct orientation. And if that pulley has a 5mm pitch then the belt should match at all points, regardless of the path in between. But who knows, you answer sounds a lot better then mine. I have never been much of a math wiz.

Have you experienced a issue like this or are you just making a well educated guess (theory)?

[artdutra04]

Originally Posted by Sh1ine

The other mentor is concerned about the same thing. I am not sure that I completely follow you however. I am under the impression that if a belt has a 5mm pitch it is only a matter of turning a pulley (also with a 5mm pitch) to the correct orientation. And if that pulley has a 5mm pitch then the belt should match at all points, regardless of the path in between. But who knows, you answer sounds a lot better then mine. I have never been much of a math wiz.

Have you experienced a issue like this or are you just making a well educated guess (theory)?

This is a real issue with any toothed belt or chain if you attempt to engage different locations of the same sprocket.

[Sh1ine]

Originally Posted by artdutra04

This is a real issue with any toothed belt or chain if you attempt to engage different locations of the same sprocket.

Ok. That is a good image. Thank you. I have a idea to work around that. Thanks.