A planetary gearbox refers to a gear system where 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 known as the annulus). Therefore drives the sun gear which then supplies the output torque. Nevertheless there are examples of the various other combinations, such as 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 this, the high effiency is usually delivered regardless of the compact design giving a huge torque to size/weight ratio. A few of this is because of the load sharing occurring across the multiple planet gears.