idler

An idler has two principal meanings. As a noun referring to a person, an idler is someone who spends time in inactivity or avoids work; the term can carry pejorative connotations and historically described someone not employed. In engineering, an idler is a non-driving component that guides, tensions, or supports moving parts in mechanical systems.

In belt and chain drives, idlers are pulleys or rollers that do not transmit power but maintain belt or chain tension, alignment, and wrap around drivers. An idler pulley redirects the belt path, while idler rollers support belt movement in conveyors and other machinery. Some idlers are designed to absorb impact or vibration, such as impact idlers used in bulk material handling.

In gear trains, an idler gear sits between a driver and a driven gear to change direction or spacing without altering the overall gear ratio. In bicycles and motorcycles, chain tensioners or guide pulleys function as idlers to keep the chain taut and aligned.

Etymology: the word idler derives from idle, reflecting inactivity in the human sense; the mechanical sense uses the term metaphorically for a component that does not drive motion.