Full definition
An auxiliary pulley in a belt drive system whose sole purpose is to maintain correct belt tension without transmitting power to or from any shaft. Can be smooth-faced (for back-side contact on V-belts or flat belts), grooved (for inside contact on V-belts), or toothed (for inside contact on timing belts). Always positioned on the slack (low-tension) side of the drive to minimize additional bending stress on the already highly loaded tight side. Mounting: fixed position with adjustment slot (manual tensioning) or spring-loaded arm (automatic tensioning). Diameter must be at least equal to the minimum recommended for the belt profile to avoid accelerating flex fatigue. Bearing rated for belt tension (radial load) and speed. Adding an idler tensioner near the smaller pulley also increases wrap angle, improving traction capacity. Per ISO 1081 guidelines. Spring-loaded versions (automotive-style automatic tensioners) require no periodic adjustment.