Why flat belts need crowned pulleys.

[tonyfoale]

I have a 13" wide belt sander that will not keep track with a convexed crown, and even though theory says a concave crown fails, it did not......
The "original" China copy had the idler pulley a smooth cylinder, no crowning, and the motor driven pulley that contacts the wood work piece a convex crown. Well that didn't work, as it made the board have a dish concave scallop along it's length, and depending on where you fed the board, it would remove so much material, it would be burned by the belt.
So I pulled the whole thing apart and machined the pulley in contact with the wood board (sanding belt in between) to be a perfect cylinder.
I used duct tape on the idler pulley to make the crown convexed originally. But whenever the belt got warm, it would take off from center position, and the sanding belt would grind the sides of the machine internals. I muttled with it this way for years. Then last year I stripped off all the duct tape, and tried it with a convex crown. That has worked reliably.

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.