Epicyclic gearing
From Academic Kids

Epicyclic_gear_ratios.png
Epicyclic gearing or planetary gearing is a gear system that consists of one or more outer gears, or planet gears, rotating about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Epicyclic gearing systems may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.
Gear ratio
Epicyclic_carrier_locked.png
The gear ratio in an epicyclic gearing system is somewhat nonintuitive, particularly because there are several ways in which an input rotation can be converted into an output rotation. The three basic components of the epicyclic gear are:
 Sun: The central gear
 Planet carrier: Holds one or more peripheral planet gears, of the same size, meshed with the sun gear
 Annulus: An outer ring with inwardfacing teeth that mesh with the planet gear or gears
In any epicyclic gearing system, one of these three basic components is held stationary; one of the two remaining components is an input, providing power to the system, while the last component is an output, receiving power from the system. The ratio of input rotation to output rotation is dependent upon the number of teeth in each gear, and upon which component is held stationary.
One situation is when the planetary carrier is held stationary, and the sun gear is used as input. In this case, the planetary gears simply rotate about their own axes at a rate determined by the number of teeth in each gear. If the sun gear has S teeth, and each planet gear has P teeth, then the ratio is equal to S/P. For instance, if the sun gear has 24 teeth, and each planet has 16 teeth, then the ratio is 24/16, or 3/2; this means that one clockwise turn of the sun gear produces 1.5 counterclockwise turns of the planet gears.
This rotation of the planet gears can in turn drive the annulus, in a corresponding ratio. If the annulus has A teeth, then the annulus will rotate by P/A turns for each turn of the planet gears. For instance, if the annulus has 64 teeth, and the planets 16, one clockwise turn of a planet gear results in 16/64, or 1/4 clockwise turns of the annulus. Extending this case from the one above:
 One turn of the sun gear results in S/P turns of the planets
 One turn of a planet gear results in P/A turns of the annulus
So, with the planetary carrier locked, one turn of the sun gear results in S/A turns of the annulus.
The annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to 1+A/S.
If the annulus is held stationary and the sun gear is used as the input, the planet carrier will be the output. The gear ratio in this case will be 1/(1+A/S). This is the lowest gear ratio attainable with an epicyclic gear train. This type of gearing is sometimes used in tractors and construction equipment to provide high torque to the drive wheels.
More planet and sun gear units can be placed in series in the same ring gear housing (where the output shaft of the first stage becomes the input shaft of the next stage) providing a larger (or smaller) gear ratio. This is the way some automatic transmissions work.
During World War II, a special variation of epicyclic gearing was developed for portable RADAR gear, where a very high reduction ratio in a small package was needed. This had two outer annular gears, each half the thickness of the other gears. One of these two annular gears was held fixed and had one fewer teeth than did the other. Therefore, several turns of the "sun" gear made the "planet" gears complete a single revolution, which in turn made the rotating annular gear rotate by a single tooth.