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.

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.

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