What Is the Function of an Idler Pulley?

by Andrea Stein

Pulleys are devices used to direct applied force in a rotational or linear system of motion. An idler pulley plays a key role in the operation of a vehicle’s belt system.


The idler pulley is one pulley in a set that drives the belt system of a vehicle. The idler pulley regulates the belts that connect to the crankshaft and are used to produce movement in numerous engine accessories, such as the alternator, steering pump and air-conditioner compressor.


The idler pulley consists of a pulley component called a sheave, or a wheel containing a groove that runs along its end. The idler pulley is braced between supports and holds a belt which sits in the groove and runs over the roller. Power generated by the crankshaft rotates the wheel, which turns the belt and enables torque, speed and mechanical power to be transmitted.


An idler pulley's components experience wear over time. Idler pulleys may often begin to slip, for example, as a result of damaged or worn bearing devices. If the idler pulley fails, the vehicle will not operate. Precautions must be taken when changing out an idler pulley, such as ensuring the vehicle is properly supported. A driver should always seek out a certified mechanic to replace the pulley.

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