Planetary gear

A gear system that consists of one or more outer gears (or planet gears), rotating about a central gear (or sun gear). Planet gears are often mounted on a movable arm or carrier which also may rotate relative to the sun gear. Additionally, epicyclic gearing systems may incorporate the use of an outer ring gear which meshes with the planet gears. Planetary gear systems are used in automatic transmissions. Also known as epicyclic gearing.