A planetary gearbox refers to a gear system where one or more ‘planet’ gears rotate around a central ‘sun’ gear. Typically, a planet carrier is powered by the input torque which rotates the planets around a set outer ring (sometimes referred to as the annulus). This in turn drives sunlight gear which then provides the output torque. However there are types of the additional combinations, such as the sun gear being set and the band providing output torque.
The main advantage of planetary gearboxes are their high power transmission efficiencies. Typicially this is often around just 3% per stage. Further to the, the high effiency can be delivered regardless of the compact style giving a sizable torque to size/weight ratio. A few of this is because of the load sharing that occurs over the multiple planet gears.