More often than not, if the belt is riding in the bottom of the sheave,
that means one of two things. Sides of the belts are worn out. Or, the
usual culprit is that the sheaves ( pulleys) are worn out. I have taken
them off, and threw them in my glass bead cabinet and roughed up the sides
of the sheaves. Replaced the belts, and belt squeal went away until the
sheaves polished back up. No substitute for replacing sheaves with unworn
ones, though.
Jim Hupy
Salem, Oregon