Planetary gearing is a gear system consisting of planet gears, revolving about a central sun gear. Typically, the planet gears are mounted on a movable arm or carrier, which itself may rotate relative to the sun gear. Planetary gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears. Ring gear, Sun gear, Planets and Planet Carrier comprises of one planetary stage which gives a fixed ratio. Several planetary stages can be combined to get higher ratios.
