Worn sheaves will reduce belt life. If the grooves are worn, the belt will shift. Belts will slip and burn out, and if the sidewalls of the sheave groove are dished, as indicated below image, the bottom shoulder of the sheave will wear the bottom corner of the belt.

Gates 13998 Sheave Gauge Helps to identify worn sheaves and damaged belts with fractional and metric measurements. Gates Product No. 7401-0015.