A planetary gearbox identifies a gear system in which one or more ‘planet’ gears rotate around a central ‘sun’ equipment. Typically, a planet carrier is driven by the input torque which rotates the planets around a fixed outer ring (sometimes referred to as the annulus). This in turn drives sunlight gear which then supplies the output torque. However there are types of the various other combinations, such as the sun gear being set and the ring providing output torque.

The main benefit of planetary gearboxes are their high power transmission efficiencies. Typicially this can be around simply 3% per stage. Further to this, the high effiency is delivered regardless of the compact design giving a huge torque to size/weight ratio. Some of this is because of the load sharing occurring over the multiple planet gears.