Planetary Gear

The four main components of this speed reducer are the sun gear, the planet gears, the ring gear, and the carriage. It is characterized by the ring gear orbiting around the sun gear. It is compact and can handle high ratios, thus transmitting a lot of torque, and is effective even at low speeds. 
 

This efficiency ensures that most of the energy input to the reducer is multiplied and transferred to torque, rather than wasted on mechanical losses inside the gearbox. Another advantage is load distribution. The transmitted load is shared among multiple gears and planets, resulting in significantly increased torque capacity. They can transmit torque that is three times higher than that of a spur gearbox of similar size. The main benefits of this kind of gear reducer, apart from the aforementioned efficiency, is its compact size, low clearance, and high torque to weight ratio.

(The article quoted from: https://www.imts-exhibition.com/components-and-accessories/gear-reducer.html)