Re: Re: What do we want?

I tried to think this one out when I was designing my machine and I came to
the conclusion that, if the toothed belt is wrapped around the pulleys to
any significant degree - that is with more than 3 or 4 teeth in contact at
any one time, then slight pitch irregularities will have no effect. What
would be more significant would be the closeness of fit of the belt teeth
into the pulley recesses. Unless you use (or make) pulleys with close to
100% match between tooth and recess size, there is the possibility of slip
between belt and pulley which would give inaccuracy. You actually need to do
quite close selection as there appear to be large variations in different
products. When I was looking for an economical system to use I decided on a
5mm pitch belt and checked several possible belts and pulleys. I got a
couple of industrial (very expensive) belts from engineering suppliers and a
couple of different lawn mower drive belts, I als had two or three old belts
out of photocopiers etc - about 7 in all. Only two of these were the same!
All the others had different width teeth/spaces and their respective pulleys
were designed for different amounts of freedom on the recesses. Predictably,
the lawnmowers were the sloppiest fit on their pulleys although the belts
seem to be made to quite close tolerances on pitch but I was surprised to
find the amount of variation on the industrial and commercial belts. Prices
for the small belts I bought ranged from 2.36UKP (lawnmower) to 10.87UKP
(industrial). The cheaper belts also had reinforcing cords of some type of
synthetic material whils the industrial ones have stainless steel wire
reinforcement which is wound as a spiral and sticks out of the edges of the
belt as sharp spikes! The worst case of pitch error would occur if one of
the teeth on the belt was wider than the slots in the pulley when the belt
would ride up and effectively follow a larger radius for the partial
revolution whilst that tooth is in contact with the pulley.

Ian
--
Ian W. Wright
Sheffield UK

> From: PTENGIN@...
>
>Cog belt drives do not
> have this error to any significant degree and are used almost exclusively

for

> this type of drive. Think of a flat belt with smooth round pulleys and

then

> cut notches for projections cast onto the belt