A planetary gearbox identifies a gear system in which one or more ‘world’ gears rotate around a central ‘sun’ gear. Typically, a planet carrier is driven by the insight torque which rotates the planets around a fixed outer ring (sometimes known as the annulus). Therefore drives the sun gear which then provides the output torque. Nevertheless there are examples of the other combinations, like the sun gear being set and the ring providing output torque.

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