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

An epicyclic gear teach can be assembled so the planet equipment rolls within the pitch circle of a fixed, outer gear ring, or ring equipment, sometimes named an annular equipment. In this instance, 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 this instance, the ring equipment is generally fixed and sunlight gear is driven.

Epicyclic gears get their name from their earliest program, that was the modelling of the actions of the planets in the heavens. Believing the planets, as everything in the heavens, to be perfect, they could only travel in perfect circles, but their motions as seen from Earth could not be reconciled with circular motion. 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 Mechanism, circa 80 BC, experienced gearing which was in a position to approximate the moon’s elliptical route through the heavens, and even to correct for the nine-calendar year precession of that path.[3] (The Greeks would have seen it much less elliptical, but rather as epicyclic motion.)