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What is an Idler Wheel?

Lori Kilchermann
Lori Kilchermann

An idler wheel is used to keep slack out of a belt on an engine or machine. When routing a belt, it is necessary to have an idler wheel to pull the belt away from any obstructions and to put pressure on the belt to keep it taught. The same principle is applied to chain applications, where the idler wheel typically has a sprocket-like appearance. Commonly manufactured from plastic or aluminum molded around a bearing, the idler wheel runs smoothly and quietly, rarely requiring specialized servicing. A squeaky or grinding sound indicates the idler wheel needs to be replaced.

Most idler wheels are mounted to a spring-loaded bracket that allows the idler to be pulled away from the belt in order to remove the belt. Once the new belt is placed around all of the pulleys, the idler wheel can be released to ride against the belt. The spring-loaded bracket maintains pressure against the belt and keeps the belt from jumping off of the pulleys. This is especially important on a serpentine belt system.


A serpentine belt is a single rubber belt that drives all of an automobile engine's accessories. The serpentine belt is typically ribbed and rides on pulleys that have several grooves in them. The belt is wound around the pulleys in such a manner that the idler wheel typically rides against the smooth backside of the belt. This arrangement ensures that the idler does not damage the grooves on the belt.

The only adjustable component in the typical serpentine belt system is the idler bracket, and this single component is responsible for maintaining the integrity of the entire system. Occasionally, this spring-loaded mounting bracket will freeze into position or the spring will break. The result of a broken spring is often a squeaky belt. The belt will squeak as it slips on the pulleys due to the lack of tension. Automobile belts should be checked for wear every 50,000 miles (80467.2 km), and the idler components should also be checked at that time.

The bearing in an idler wheel is typically not able to be greased. When it begins to become noisy, it needs to be replaced. Most idler wheels are replaceable by simply removing the attaching nut and slipping the old pulley off of the shaft. The new pulley can be slid back on the shaft and tightened. Once the serpentine belt is placed back on the pulleys, the system is ready for use.

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