# Idler-wheel

(Redirected from Idler pulley)
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

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

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

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. The distance the final output gear will travel can be calculated 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, givan a capstan drive shaft that is 8mm in diameter, and it is turning an idler wheel that is 40mm in diameter that is then turning a wheel that is 250mm:

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

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 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.

### Applications

Reversing

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. Since a driven gear (gear "A") rotating clockwise will drive a second gear ("B") counterclockwise, adding a third gear to the string means that gear "C" will be spinning the same direction as "A". A typical transmission is designed with "A" and "B" gears, so when the engine spins, the outputs shaft spins the opposite direction, which drives the vehicle forward. A reverse idler gear setup is actually typically an "A" and a "C" gear, which are not in contact with each other until a "B" gear is moved between them. Since the transmission is designed to move the car forwards when the output is spinning in the opposite direction from the input shaft, when added to the "B" idler gear, it forces the "C" gear to spin in the same direction as the "A" gear, and thus the input and output shafts are spinning in the same direction, which drives the car in reverse.

Another scenario is a series of rollers, such as used for pressing paper. Each roller has to be powered, but adding a motor to each one is wasteful. One could simply add a gear onto the end of the shaft of each roller, but that means that each roller would be spinning the opposite direction of the one before. By simply adding a small idler gear between each larger gear, the result is a series of rollers, all being powered in the same direction.

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. For short distances, a train of idlers may be used; whether an odd or even number is used determines whether the final output gear rotates the same direction as the input gear or not. For longer distances, a roller chain or belt is quieter and creates less friction, although gears are typically stronger, depending and the strength of the roller chain. A case where numerous idler gears might be used is as described above, where there are a number of output gears that need to be driven simultaneously.

Caterpillar track idler wheels