Idler pulley

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.

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.

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.

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.

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.

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