Epicyclic gear train, also known as planetary gear train, is an assembly containing more than one pair of gears, in which at least one gear is centered on the axis about which one another gear rotates. In an epicyclic gear train, the extreme gear with external teeth is called the sun gear; the extreme gear with internal gear teeth is called the ring gear; the gear that rotates on an axle whose own axis is constrained to rotate about another axis is called the planet gear; and the link on which gear axles are arranged is called the planet carrier. A distinct characteristic of epicyclic gear trains is that, based on the assembly of multiple gears, it can produce higher transmission ratio than those of other simple gear trains. Besides, in certain situations, each gear in an epicyclic gear train is rotatable. By fixing different gears, the gear train can obtain different transmission ratios. Hence the epicyclic gear trains are broadly applied in the gearbox of automobiles.