A belt idler pulley maintains proper tension on a fan belt, preventing excessive movement and belt pulling. It can be smooth or multi-ribbed and is commonly equipped with a zero-maintenance sealed bearing. A squeak or grinding sound may indicate a broken or weak spring or damaged bearing.
A belt idler is a pulley that applies subtle pressure to a fan belt to maintain proper tension on the belt system. Typically positioned in the middle of a long belt run, the belt tensioner pulley also prevents excessive belt movement and the possibility of belt pulling. In the case of a single serpentine belt system, the pulley often consists of a plastic pulley attached to a steel or aluminum arm that is spring-loaded. The pulley, while not actually attached to any hardware, serves two purposes in this setup: first, to maintain tension on the rubber belt, and second, the pulley arm is loosened with a wrench to allow the belt is removed from the system pulley.
When a rubber belt is called upon to span an open area without pulley support, it tends to flex, flap, and whip when moving at typical engine speeds. By placing a belt tensioner pulley in the middle of the span, the belt will typically run straight and smooth with no erratic movement at any speed. This type of pulley is so effective at eliminating unwanted belt movement that two idler pulley assemblies are commonly used in applications that require a fan belt that is too long. The spring loaded pulley bracket or arm eliminates the need for adjustments on belt driven components such as the alternator and power steering pump.
There are two different types of pulley used in a belt idler pulley assembly: a smooth flat faced pulley and a multi-rib pulley. The style of pulley used depends on the position on the belt that the pulley contacts. A flat face pulley is designed to mount against the reverse side of the belt, while the grooved face of the pulley is designed to mount to the front or side of the fan belt. Both styles are commonly equipped with a zero-maintenance sealed bearing that is designed to require no special maintenance for the life of the bearing.
A squeak coming from the fan belt on initial engine start-up is a common sign that the spring is broken or weak and requires replacement. A grinding sound or visible wiggle from the pulley or belt is a telltale sign that the bearing is damaged in the pulley. In most cases, the belt tensioner pulley and mounting arm or bracket are sold individually at most auto parts stores or dealer parts counters.
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