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For the Fiona Apple album, see The Idler Wheel....

An idler-wheel drive is a system used to transmit the rotation of the main shaft of a motor to another rotating device, for example the platter of a record-reproducing turntable or the crankshaft-to-camshaft gear train of an automobile.

Friction drive[edit]

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.

Idler gear[edit]

Part of a machine with three gears connected to one another.
Gears in a machine. The middle, medium-sized gear is an idler gear.

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.

Gear ratio[edit]

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.

In non-geared friction drive system, the size of all three wheels do matter in calculating final speed or distance. This is due to all systems being in constant contact due to a gain ratio. Geared systems have a direct ratio of teeth per size, but all three wheels in constant friction has influence on one another. You can calculate the distance the final output gear will travel by using this formula: the diameter of the drive wheel or input shaft x the size of the output shaft / the size of the idler wheel.[1] For instance, if you have a capstan drive shaft that is 8mm in diameter, and it is turning an idler wheel that is 40 mm in diameter that is then turning a wheel that is 250mm:

8mm input shaft x 250mm output shaft / 40mm idler wheel = 50 mm.

This means that one rotation of the input shaft would make the output shaft travel 50mm in rotational distance. Rotational distance can also be calculated in speed given the capstan is a constant speed. However, the size of all three friction wheels will effect final output speed (RPM) or distance. For the size of of the idler wheel: the bigger the idler wheel, the slower final output speed will be and the smaller the idler wheel, the faster final output speed or distance will be.



An intermediate gear which does not drive a shaft to perform any work is called an idler gear. Sometimes, a single idler gear is used to reverse the direction, in which case it may be referred to as a reverse idler. For instance, the typical automobile manual transmission engages reverse gear by means of inserting a reverse idler between two gears.

Transmission over distance

Idler gears can also transmit rotation among distant shafts in situations where it would be impractical to simply make the distant gears larger to bring them together. Not only do larger gears occupy more space, but the mass and rotational inertia (moment of inertia) of a gear is quadratic in proportion to its radius. Instead of idler gears, of course, a toothed belt or a roller chain can be used to transmit torque over distance.

Battle tanks

All the gears and wheels that turn inside the tracks of a battle tank are idler gears that transfer power from the input gear to the output gear to move the track and move the tank forward. The power take off mechanism includes a gear train with an input idler gear, a first intermediate idler gear, a second intermediate idler gear and an output gear. The input idler gear receives a rotary input and the first intermediate idler gear meshes with the input gear and the second intermediate idler gear. The output gears transmit rotary power to one of the first and second axles.


  1. ^ "Sheldon Brown's Bicycle Glossary G".