An epicyclic gear train (also known as planetary gear) includes two gears mounted so that the agricultural Chain centre of one gear revolves around the center of the additional. A carrier links the centres of both gears and rotates to transport one gear, called the earth gear or world pinion, around the additional, called sunlight gear or sunlight wheel. The planet and sun gears mesh so 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 the sun gear.

An epicyclic gear teach can be assembled so the planet gear rolls on the inside of the pitch circle of a set, outer gear band, or ring gear, sometimes called an annular equipment. In this case, 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 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 cannot end up being reconciled with circular movement. At around 500 BC, the Greeks created the thought 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 System, circa 80 BC, had gearing which was able to approximate the moon’s elliptical path through the heavens, and actually to improve for the nine-season precession of that route.[3] (The Greeks would have seen it not as elliptical, but rather as epicyclic motion.)