Full definition
Progressive internal degradation of belt material caused by repeated bending cycles as the belt wraps around each pulley in the drive system. Every revolution subjects the belt to one complete flex cycle per pulley; at 1,750 RPM on a 100 mm pulley, the belt may endure 100+ million cycles per year. Flex fatigue manifests as transverse cracks on the inner (compression) surface, eventually propagating to the tension cords. The primary countermeasure is respecting minimum pulley diameters specified per belt profile: e.g., SPA min 90 mm, SPB min 140 mm, SPC min 224 mm per ISO 4184. Cogged (notched) belts tolerate 20-30% smaller diameters. Fatigue life also decreases with higher belt speed, ambient temperature, and under-tensioning (which increases flex amplitude from flutter).