Undersize timing pulley 1

[kiwibrick]

So I have a question.
Two 3HTD pulleys, 40T and 20T, 20T pulley is 0.2mm too small in diameter, so it appears the 3HTD profile on the smaller pulley is not really 3HTD (its something like 2.968 pitch)

In theory could this give a different ratio than the tooth count would indicate?, ie. say 2.02:1. I'm of the mind that it could but just wanted some discussion.

Thanks

 

[3DDave]

Toothed items are proportional to tooth count. If the diameter is too wrong it will tear up the belt or allow skipping. That said, no pulley will be exactly perfect and 0.2mm seem like a small deviation.

 

[Compositepro]

Also, the "true" functional pulley diameter is not the O.D. of the pulley but the diameter to the neutral bending axis of the belt, which is at the center of the reinforcing cords in the belt. Changes in belt tension due to load changes will also cause temporary speed changes. But, as 3DDave said, longer term, toothed belts must stay exactly in sync. With non-toothed belts, load changes do result in speed changes due to belt stretch and slip.

Another interesting case is that a high stretch belt (i.e., a rubber band) can loop around a 4" pulley and a 6" that are both turning at the same rpm without any problem. There will simply be almost no tension on one side of the loop and 50% belt stretch on the other.