An idler sprocket, also known as a tensioner sprocket or tensioner pulley, is a type of sprocket used in various mechanical systems. It is primarily employed to maintain proper tension in a chain or belt drive system by guiding and tensioning the chain or belt.

The idler sprocket is typically a smaller sprocket compared to the driving or driven sprockets in the system. It is mounted on a pivot or bearing and does not transmit power to other components directly. Instead, its purpose is to change the direction of the chain or belt and provide tension to ensure proper engagement and prevent slack or excessive movement.