An idler-wheel is a wheel which serves only to transmit rotation from one shaft to another, in applications where it is undesirable to connect them directly. For example, connecting a motor to the platter of a phonograph, or the crankshaft-to-camshaft gear train of an automobile.

Because it does no work itself, it is called an "idler".

An idler-wheel may be used as part of a friction drive mechanism. For example, to connect a metal motor shaft to a metal platter without gear noise, early phonographs used a rubber idler wheel.

Likewise, the pinch roller in a magnetic tape transport is a type of idler wheel, which presses against the driven capstan to increase friction.

In a belt drive system, idlers are often used to alter the path of the belt, where a direct path would be impractical.

Idler pulleys are also often used to press against the back of a pulley in order to increase the wrap angle (and thus contact area) of a belt against the working pulleys, increasing the force-transfer capacity.

Belt drive systems commonly incorporate one movable pulley which is spring- or gravity-loaded to act as a belt tensioner, to accommodate stretching of the belt due to temperature or wear. An idler wheel is usually used for this purpose, in order to avoid having to move the power-transfer shafts.

An idler gear is a gear inserted between two or more gears (which include at a minimum a drive gear and a driven gear) to either change the direction of rotation of the output shaft, reduce the size of either or both gears while maintaining the spacing of the shafts, or both.