What Is an Idler Pulley?

Most modern cars have replaced the old arrangement of belts hooked to the central motor shaft with a single serpentine belt. This belt runs the alternator, water pump and air conditioning compressor. A close inspection will reveal one or two pulleys that don't appear to drive anything. These are the idlers.

Definition

An idler is a pulley that doesn't transfer power directly to a mechanical device. While it drives no system function directly, it performs many other necessary functions in a drive system.

Tensioning

Idler pulleys can be used to maintain constant pressure on a belt to prevent slippage.

Belt Guides

Idler pulleys are used to guide flat belts in a multi-pulley arrangement.

Power Transmission

Idler pulleys can be used to reverse the drive belt's direction of travel in applications requiring a single belt to power multiple operations.

Stepped Idlers

Stepped idler pulleys are pulleys with two, differently sized working surfaces. They are used to change torque and speed delivered to a working pulley.

References

• National Programme on Technology Enhanced Learning: Flat Belt Drives
• Iowa State University: Changing Torque