<P> An idler - wheel may be used as part of a friction drive mechanism, as in a phonograph, or as a belt tensioner in a belt drive system . </P> <P> An idler gear is a gear wheel that is inserted between two or more other gear wheels . The purpose of an idler gear can be two-fold . Firstly, the idler gear will change the direction of rotation of the output shaft . Secondly, an idler gear can assist to reduce the size of the input / output gears whilst maintaining the spacing of the shafts . </P> <P> An idler gear does not affect the gear ratio between the input and output shafts . Note that in a sequence of gears chained together, the ratio depends only on the number of teeth on the first and last gear . The intermediate gears, regardless of their size, do not alter the overall gear ratio of the chain . But, of course, the addition of each intermediate gear reverses the direction of rotation of the final gear . </P> <P> Likewise, the size of an idler wheel in a non-geared friction drive system does not affect the gear ratio between the input and output shafts . The surface speed of the input shaft is transferred directly to the surface speed of the idler wheel, and then from the idler wheel to the output shaft . A larger or smaller idler wheel maintains the same surface speed (which equals the surface speed of the input shaft), therefore the output shaft is driven at a constant speed regardless of the size of the idler wheel (unless of course there is slippage, which should nor occur in most friction drive systems when operating correctly; however, there are instances where an idler wheel can double as a clutch, or if there is a sudden or unusually heavy load on the system . These situations can cause the ratio of rotations between the wheels to vary, unlike a gear system, which will always rotate at a certain rate unless something is very wrong and the gears starts skipping teeth, or teeth are broken off). </P>

Where can you find a simple gear train with idler