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".

Friction drive

An idler-wheel may be used as part of a friction drive

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