Full definition
A pulley in a belt drive system that is not connected to the motor (driver) or load (driven) — it serves only to guide the belt path, increase wrap angle on the drive pulley, or act as a tensioning element. Always mounted on the slack (low-tension) side of the drive to minimize bearing loads and belt bending stress. Flat-faced idlers for flat and timing belts; grooved idlers for V-belt drives (though flat back-side idlers are common). Diameter should be at least equal to the minimum recommended for the belt profile. Bearings: sealed ball bearings sized for belt tension and speed. Per ISO 1081 guidelines. An idler that increases wrap angle on the drive pulley from 150° to 200° can boost drive capacity by 15-20%. Brands: Fenner, TB Woods, Gates.