Planetary Gearboxes
A gearbox provides speed and torque conversions from a rotating power source to another device using gear ratios. The most common use is in motor vehicles, where the gearbox adapts the output of the internal combustion engines to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The gearbox reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Gearbox is also used on pedal bicycles, fixed machines, and anywhere else rotational speed and torque needs to be adapted. Often, a gearbox will have multiple gear ratios (or simply "gears"), with the ability to switch between them as speed varies. This switching may be done manually (by the operator), or automatically. Directional (forward and reverse) control may also be provided. Single-ratio gearbox also exist, which simply change the speed and torque (and sometimes direction) of motor output.
In motor vehicle applications, the gearbox will generally be connected to the crank shaft of the engine. The output of the gearbox is transmitted via drive shaft to one or more differentials, which in turn drive the wheels. While a differential may also provide gear reduction, its primary purpose is to change the direction of rotation. Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation (e.g. diesel electric transmission, hydraulic drive system, etc.). Hybrid configurations also exist.
Early gearbox included the right-angle drives and other gearing in windmills, horse powered devices, and steam engines, in support of pumping, milling, and hoisting. Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at slower rate than the input shaft, and this reduction in speed will produce mechanical advantages, causing an increase in torque. A gearbox can be setup to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in which power is transmitted. Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be overdrive types that increase the output speed.
In conclusion we can say that gearboxes have found use in a wide variety of different—often stationary—applications, such as wind turbines, agriculture, industrial, construction, mining and automotive equipment.