Planetary gears are also known as Epicyclic gears are a gears system consisting of planet gears or outer gears, revolving about a central or sun gears. The planetary gears are mounted on a carrier or movable arm which rotates relative to the sun gear. Epicyclic gears use the outer ring gear or annulus that meshes with the planetary gears.

The axes of all gears are generally parallel, but for some cases like pencil sharpeners they placed at an angle, introducing elements of bevel gears. The planet carrier is usually concentric. The design of planetary gears consist of wheel set, from the inside outward of that Sun wheel carried planetary gears and internal gears with internal teeth exists.

In planetary gears the gear ratio is non-intuitive. The three basic component of epicyclic gear are: Sun, Planetary carrier and Annulus. Sun is the central gear of the system. The planetary carrier holds planetary gears of same size, meshed with sun gear. And lastly the Annulus is the outer ring with inward –facing teeth that mesh with the planetary gears.

In epicyclic gearing, one component is held stationary; one of the two remaining component is an input, while other acts as a output which receive power from the system. The ratio of input rotation to output rotation is depend on the number of teeth and the component which is being kept stationary.

The important application of planetary gears lies in the Automatic transmission system. Planetary gears provide large reduction in a small volume, high power density, multiple kinematic combinations, coaxial shafting and pure torsional reactions.