The epicyclic gear train, also called the planetary gear train, is derived from appropriate combination of two or more gears and at least one gear axis is rotated about another gear axis. As shown in the figure, this epicyclic gear train comprises of three gears, where the central gear is called the sun gear, the outmost gear is called the ring gear, the gear between sun gear and ring gear is the planet gear, and the sun gear and planet gear are connected by the element called the arm. The axis of the sun gear and the ring gear are grounded. The planet gear can revolve on its own axis and have a revolution around the sun gear. So this mechanism has two degrees of freedom.