how do planetary gears work


How do Planetary Gears Work?

Planetary gears, also called epicyclic gearing, are widely used as power transmission systems in cars and various other machines. Planetary gearsets are composed of several gear-like components arranged to allow for the transmission of rotational motion. Learning how these systems work will help you understand the powerful efficiency behind this technology.

Overview & Components

A typical planetary gearset is composed of three main components:

  • Central Sun Gear: The central sun gear is the primary gear in the system. It offers an output shaft and rotates around its own axis.
  • Planetary Carrier Assembly: This component holds several gears, or “planet gears”, which are held in place by pins and roll around the teeth on the sun gear.
  • Ring or Annulus Gear: This is the second output shaft and is connected to the planet gears.


A planetary gearset works by using the motion of the sun gear and Ring Gear to adjust and transfer speed and torque. Depending on the arrangement of the gear teeth, the gearset can increase, decrease, or change the speed or torque of the system.

  • Speed Increase: To increase the speed of rotation within a planetary system, the sun gear is held stationery and the ring gear is rotated. This set up applies torque to the planet gears making them rotate around the sun gear. As this happens, a mechanical advantage is created turning the output shaft at a higher velocity than the ring gear.
  • Speed Decrease:In a planetary gearset arrangement, in order to decrease speed, the sun gear is rotated and the ring gear is held stationary. Applying torque to the sun gear causes it to rotate, while its planets move with it. The motion of the planets around the stationary ring gear creates a reduction of output speed from the system.
  • Torque Increase: To increase torque in a planetary system setup, the ring gear is held stationary and the sun gear is rotated. As the sun gear rotates its planets, they start rotating around the stationary ring gear, which creates a higher output torque from the system.
  • Torque Decrease: To reduce the for output torque, both the sun gear and the ring gear are rotated. As the planet gears are moved between the sun gear and the ring gear, they start spinning around their own axis, which in turn reduces the output torque.

This concludes our overview on how planetary gears work. As you can seen, this dizzying arrangement can become complex very quickly. If you find yourself stuck, it’s always best to consult a professional.