An epicyclic gear teach (also referred to as planetary gear) contains two gears mounted to ensure that the centre of 1 equipment revolves around the centre of the additional. A carrier connects the centres of both gears and rotates to transport one equipment, called the earth gear or planet pinion, around the additional, called the sun gear or sun wheel. The earth and sunlight gears mesh so that their pitch circles roll without slide. A spot on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, sunlight equipment is set and the planetary gear(s) roll around sunlight gear.

An epicyclic gear train can be assembled so the planet equipment rolls within the pitch circle of a set, outer gear band, or ring equipment, sometimes called an annular equipment. In cases like this, the curve traced by a spot on the pitch circle of the planet is a hypocycloid.

The combination of epicycle gear trains with a planet engaging both a sun gear and a ring gear is named a planetary gear train.[1][2] In cases like this, the ring gear is usually fixed and the sun gear is driven.

Epicyclic gears get their name from their earliest program, which was the modelling of the motions of the planets in the heavens. Believing the planets, as everything in the heavens, to be perfect, they could only travel in ideal circles, but their motions as seen from Earth could not be reconciled with circular movement. At around 500 BC, the Greeks developed the idea of epicycles, of circles traveling on the circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD was able to predict planetary orbital paths. The Antikythera Mechanism, circa 80 BC, had gearing which was able to approximate the moon’s elliptical path through the heavens, and even to improve for the nine-calendar year Drive Chain precession of that route.[3] (The Greeks could have seen it much less elliptical, but instead as epicyclic motion.)