An epicyclic gear train (also referred to as planetary gear) contains two gears mounted to Transmission Chain ensure that the centre of one equipment revolves around the center of the additional. A carrier links the centres of the two gears and rotates to carry one equipment, called the planet gear or planet pinion, around the other, called the sun gear or sunlight wheel. The planet and sunlight 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, the sun gear is fixed and the planetary gear(s) roll around sunlight gear.
An epicyclic gear teach can be assembled therefore the planet gear rolls on the inside of the pitch circle of a set, outer gear band, or ring equipment, sometimes called an annular gear. 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 gear is usually fixed and the sun gear is driven.
Epicyclic gears get their name from their earliest app, that was the modelling of 
the actions of the planets in the heavens. Believing the planets, as everything in the heavens, to become perfect, they could just 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, acquired gearing which was able to approximate the moon's elliptical path through the heavens, and even to correct for the nine-yr precession of that path.[3] (The Greeks would have seen it much less elliptical, but instead as epicyclic motion.)