A planetary gearbox is a mechanical device with an aligned input shaft and output shaft, and is used to transfer the torque density. Also known as an epicyclic gear train, the planetary gear comprises of two interlocking gears
that are mounted in a way that the centre of one gear rotates around the centre of the other. A carrier connects the centres of these two gears and rotates carrying one gear known as the planet pinion or planet gear around the other gear known as sun wheel or sun gear. The two gears mesh making their pitch circles roll around each other without slipping up. Simplified, the sun gear is fixed while the planetary gear rolls around the sun gear tracing an epicycloid curve. Sometimes a planetary gearbox can be assembled in a manner that the planet gears roll on the outside of an outer ring often called an annular gear. In this case, it traces a hypocloid. The epicyclic gear train gets its name from their earliest application which was modelled like the planetary system.