An
epicyclic gear train consists of two
gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates to carry one gear, called the
planet gear, around the other, called the
sun gear. 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(s) roll around the sun gear.