A planetary gearbox refers to a gear system in which one or more ‘planet’ gears rotate around a central ‘sun’ gear. 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 the sun gear which then provides the output torque. However there are examples of the other combinations, such as the sun gear being fixed 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. Further to this, the high effiency is delivered despite the compact design giving a large torque to size/weight ratio. Some of this is thanks to the load sharing that occurs across the multiple planet gears.