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

An epicyclic gear teach can be assembled therefore the planet Leaf Chain equipment rolls within the pitch circle of a set, outer gear band, or ring equipment, sometimes named an annular equipment. In this case, 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 named a planetary gear train.[1][2] In this case, the ring equipment is generally fixed and the sun gear is driven.

Epicyclic gears get their name from their earliest program, that was the modelling of the movements of the planets in the heavens. Believing the planets, as everything in the heavens, to become perfect, they could only travel in ideal circles, but their motions as seen from Earth cannot become reconciled with circular movement. At around 500 BC, the Greeks developed the thought of epicycles, of circles venturing on the circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD was able to predict planetary orbital paths. The Antikythera System, circa 80 BC, got gearing which was in a position to approximate the moon’s elliptical route through the heavens, and also to improve for the nine-year precession of that route.[3] (The Greeks would have seen it not as elliptical, but instead as epicyclic motion.)