Epicyclic gearing

An epicyclic gear train (also known as a planetary gearset) is a gear reduction assembly consisting of two gears mounted so that the center of one gear (the "planet") revolves around the center of the other (the "sun"). A carrier connects the centers of the two gears and rotates, to carry the planet gear(s) around the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. If the sun gear is held fixed, then a point on the pitch circle of the planet gear traces an epicycloid curve.

An epicyclic gear train can be assembled so the planet gear rolls on the inside of the pitch circle of an outer gear ring, or ring gear, sometimes called an annulus gear. Such an assembly of a planet engaging both a sun gear and a ring gear is called a planetary gear train. By choosing to hold one component or another—the planetary carrier, the ring gear, or the sun gear—stationary, three different gear ratios can be realized.

Combining two rotations is also aided by a planetary gearset. When the sun gear and the carrier run in opposite directions, you can get the combined output through the ring gear, or vice versa. For example, when the carrier and the ring gear run in opposite directions, the sun gear experiences a complex speed variation due to the interaction of the inputs.

Technically, you can not get output by providing single rotational input. Because, if you cancel one of the inputs, the other rotational input will be wasted. But it can be solved by avoiding reverse rotations with ratchets. This approach contributes to the development of wave energy technology. It is possible to integrate irregular wave movements in a step-by-step manner and generate a continuous single-rotational output.