An epicyclic gear train (also referred to as planetary gear) consists of two gears mounted to ensure that the centre of one equipment revolves around the centre of the additional. A carrier connects the centres of the two gears and rotates to carry one gear, Transmission Chain called the earth gear or world pinion, around the various other, called sunlight gear or sunlight wheel. The planet and sunlight gears mesh to ensure that their pitch circles roll without slip. A spot on the pitch circle of the planet equipment traces an epicycloid curve. In this simplified case, sunlight equipment is set and the planetary gear(s) roll around sunlight gear.
An epicyclic gear train can be assembled so the planet equipment rolls within the pitch circle of a fixed, outer gear band, or ring equipment, sometimes called an annular gear. In cases like this, 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. In this case, the ring gear is normally fixed and the sun gear is driven.
Epicyclic gears obtain name from their earliest software, that was the modelling of the movements of the planets in the heavens. Believing the planets, as everything in the heavens, to end up being perfect, they could just travel in ideal circles, but their motions as viewed from Earth cannot be reconciled with circular movement. At around 500 BC, the Greeks invented the idea of epicycles, of circles travelling 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 path. (The Greeks would have seen it much less elliptical, but rather as epicyclic motion.)