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An epicyclic gear teach (also called planetary gear) contains two gears mounted to ensure that the centre of one equipment revolves around the centre of the various other. A carrier links the centres of the two gears and rotates to carry one equipment, called the Drive Chain planet gear or world pinion, around the other, called sunlight gear or sun wheel. The planet and sun gears mesh to ensure that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, sunlight equipment is set and the planetary equipment(s) roll around sunlight gear.

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

The combination 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 sunlight gear is driven.

Epicyclic gears obtain name from their earliest application, which was the modelling of the actions 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 become reconciled with circular movement. At around 500 BC, the Greeks invented the thought of epicycles, of circles traveling on the circular orbits. With this theory Claudius Ptolemy in the Almagest in 148 AD could predict planetary orbital paths. The Antikythera Mechanism, circa 80 BC, got gearing which was in a position to approximate the moon’s elliptical route through the heavens, and even to improve for the nine-12 months precession of that route.[3] (The Greeks would have seen it not as elliptical, but rather as epicyclic motion.)