In an epicyclic gearbox (planetary gearbox), several spur gears are evenly distributed between an internally toothed gear and an externally toothed gear. The spur gears move on a concentric circular path.

Planetary gearboxes are of great importance in many areas of mechanical engineering and can be used for a wide range of applications. They transmit high power and achieve high ratios and speeds with a favorable mass inertia ratio. As the number of planetary gears increases, the load distribution and therefore the transmittable torque increases, which means that planetary gearboxes can transmit very high torques with high efficiency in a compact design.