Toothed Belts and Regen Braking

Many people try to use a tooted belt as a reduction gear from a motor to a drive wheel. This is fine – as long as you do not expect to get regenerative braking through the belt!

The motor shaft will always rotates a lot faster than the drive wheel, so there is a fairly large reduction ratio via the belt. This involves a pinion on the motor with a small diameter and few teeth, with a larger pinion on the driven wheel. Regen braking requires feeding power in reverse through the belt with the large wheel driving the small one.

It is fairly well known in engineering circles that, if you try and feed power via a toothed belt from a large wheel to a small one, you must expect belt slippage. This is not acceptable with a toothed belt.

Consider the top diagram, left. A is the motor (drive) pinion, B the driven gear. B is under tension and is transferring power. Side C is slack and where C touches the large wheel there is no tension on the gear teeth. Belt tension (pressure on the gear teeth) increases as the belt travels to side D.

Now consider B driving and A driven. Side D will be slack, side C in tension and the teeth at side D will have no tension on them. The problem is that only very few teeth are in contact with the belt. If an A has 8 teeth, then it is pretty clear that only about 3 teeth will be able to do anything.

In the second diagram a third wheel is present as a pressure roller to try and maintain contact. Clearly the situation is better but, with our 8 tooth wheel we still only have probably 5 teeth doing anything. With carefully maintained alignment you might get away with this – but it certainly cannot be recommended.

So with a toothed belt, expect to have to forgo the benefits of regenerative braking.