An epicyclic gear, also referred to as a planetary gear, is a gearing system in which gears known as planetary gears revolve around one central gear referred to as the sun gear. This system is reminiscent of the planets' orbits around the sun, thus giving the gear system its name. Most people encounter an epicyclic gear system every day when they drive their cars. The system is used by automatic transmission vehicles to switch between gears automatically.
Three major components make up an epicyclic gear. The first is the center gear, known as the sun gear. This gear turns all the gears around it and is at the very center of the entire gear system. Next are the planetary gears. Each one is positioned so the sun gear engages and turns all the planetary gears at once. Finally, the outer or ring gear contains both planetary and sun gears, and the planetary gears turn the ring gear in time with the rotating sun gear.
In the ring gear, the grooves or teeth are located on the inside instead of the outside. This allows the spinning planetary gears to engage and turn the ring gear from the inside so everything spins at the same time, and yet the center gear and ring gear can still be connected by a single axis through the center of both.
Gear ratios play the starring role in making an epicyclic gear system work. In a system such as this with different sized gears, the ratio between them determines the speed and power the gear system produces. How much power the car gets all depends on the input ratio, the amount of power coming in, and the output ratio, how much power is put out. This in turn depends on the design of the epicyclic gear system and can be manipulated up to a point to achieve different results.
In simplest terms, a gear ratio is an expression of the smaller sun gear in comparison to the larger ring gear. The planetary gears are simply there to connect the smaller and larger gears, and do not play a direct role in the basic ratio calculation. If, for example, a machine needed a ratio of 6 to 1 for two gears spinning around the same axis, the engineer could place a sun gear that is 1/6th the size of the ring gear inside that ring gear and thus the ratio for that gear system becomes 6:1.