Why flat belts need crowned pulleys.

**metric_taper** · Jan 26, 2021, 07:56 PM

Originally Posted by tonyfoale

I am not surprised that you had problems with the convex crown on the work wheel. Terrible idea.
You did not describe your machine but I am guessing that you have 3+ pulleys. 1 on the motor, at least one idler and the work pulley. I would put the crown on the motor pulley.

The only reason that I know that would require a concave pulley is if one or more pulley axes is/are misaligned causing a tendency to throw the belt one way, compensated by the concave pulley trying to throw it the other way, hence compensating. On wide pulleys such as on a belt grinder it only takes a tiny axes misalignment to force the belt one way. In fact some machines do not have crowned wheels, instead there is a misalignment adjustment with which you can use to make the belt track true. The misalignment works like one half of a crowned pulley and obeys the same rules.

Glad that you have found a workable solution.

PS. You mentioned the problem got worse when warmed up. Expansion somewhere in the system may be causing any slight misalignment to change. 13" wide belts only need a tiny change to act differently.

What gets hot is the sanding belt, and elongates just enough to run away.
However, by having the idler with a concave crown, the belt can only get so long, and it is captured by this trough.
This is a two roller system;