Epicyclic gearing

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An epicyclic gear train consists of two gears mounted so that the centre of one gear revolves around the centre of the other. A carrier connects the two gears' centres and rotates to carry one gear, called the planet gear or planet pinion, around the other, called the sun gear or sun wheel. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed, and the planetary gear roll around the sun gear. An epicyclic gear train can be assembled, so the planet gear rolls on the inside of the pitch circle of a fixed, outer gear ring or ring gear, sometimes called an annular gear. In this case, the curve traced by a point on the planet's pitch circle is a hypocycloid. The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is called a planetary gear train.