Gearbox

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In mechanics, a transmission (also called a "standard" or "manual" transmission) or gearbox, is the system of gears and/or the hydraulic system (called variously "hydrodynamic", "fluid" or "automatic" transmission) that transmits mechanical power from a prime mover— such as an engine or electric motor—to some form of useful output device, normally rotary in form, and generally at a reduced rate of angular speed but at a higher motive torque.

Generally, transmissions will provide a significant speed-power conversion known as gear reduction (in speed) to a higher torque (rotational force or power). In motor vehicle transportation, a vehicle transmission may provide many different speed-power ratios known colloquially as 'gears' or 'speeds', and possibly several variant speeds in reverse direction as well— Tractors and large trucks especially may have a dozen or more forward 'gears' which vary from a crawling speed at high torque to high speed at low torque where the only torque needed with a load coasting along at a given speed are that small additional energy (force) needed to overcome ongoing friction and other road losses such as climbing a grade. When the torque needed to surmount a grade is insufficient at a higher rotational speed, the gearbox is shifted into a lower gear to provide more power, as was needed when initially accelerating said vehicle to the desired road speed. Gearing has much in common with the mechanics and mechanical factors present in pulley systems. One trades distance (numbers o rotations) for increased force.

Explanation

Early transmissions (gearboxes) included the right-angle drives and other gearing in windmills, horse-powered devices, and steam engines, mainly in support of pumping, milling, and hoisting. Most modern gearboxes will either reduce an unsuitable high speed and low torque of the prime mover output shaft to a more useable lower speed with higher torque, or do the opposite and provide a mechanical advantage (i.e increase in torque) to allow higher forces to be generated. However, some of the simplest gearboxes merely change the physical direction in which power is transmitted.

Many systems, such as typical automobile transmissions, include the ability to select one of several different gear ratios. In this case, most of the gear ratios (simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gear(s) may be an "overdrive" type that increases the output speed.

Uses

Gearboxes have found use in a wide variety of different—often stationary—applications.

Read more at Wikipedia.org