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

An epicyclic gear teach can be assembled so the planet gear rolls within the pitch circle of a set, outer gear band, or ring gear, sometimes called an annular gear. In this case, the curve traced by a point 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 called a planetary gear train.[1][2] In this instance, the ring gear is usually fixed and the sun gear is driven.

Epicyclic gears get their name from their earliest software, which was the modelling of the motions of the planets in the heavens. Believing the planets, as everything in the heavens, to be perfect, they could just travel in ideal circles, but their motions as viewed from Earth could not become reconciled with circular motion. At around 500 BC, the Greeks invented 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, had gearing which was able to approximate the moon’s elliptical route through the heavens, and also to correct for the nine-yr precession of that route.[3] (The Greeks could have seen it much less elliptical, but rather as epicyclic motion.)