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

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

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

Epicyclic gears obtain name from their earliest app, which was the modelling of the motions of the planets in the heavens. Believing the planets, as everything in the heavens, to end up being perfect, they could only travel in perfect circles, but their motions as seen from Earth could not end up being Conveyor Chain reconciled with circular motion. At around 500 BC, the Greeks created the idea of epicycles, of circles venturing on the circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD could predict planetary orbital paths. The Antikythera System, circa 80 BC, had gearing which was in a position to approximate the moon’s elliptical path through the heavens, and also to correct for the nine-12 months precession of that path.[3] (The Greeks would have seen it much less elliptical, but rather as epicyclic motion.)