How Does A Planetary Gear System work?


The Planetary Gear System is a gear system capable of delivering multiple gear reductions by a set of planetary gears. The planetary gears set is another essential part of the overdrive. It is also used in automatic transmission.

A planetary gear system consists of 3 types of gears:

1. An outer ring gear having inside teeth. It is sometimes called internal gear.
2. Three planet pinions held on pinion shafts in a cage or carrier.
3. A sun gear at the center of the three planet pinions.


The planet pinions mesh with the ring gear internally and with the sun gear externally. The planet gear system gets its name from the fact that the pinion revolves around the sun gear and rotates at the same time, just as the planets in the solar system rotate and revolve around the sun.
The planetary system, when used in the overdrive, works as follows. The ring gear is mounted on the Output Shaft.

The three planet pinions are assembled into a cage that is splined to the transmission main shaft, The sun gear has an arrangement whereby it may be permitted to turn, or it may be locked in a stationary position. When it is locked, the ring gear and hence the output shaft is forced to turn faster than the transmission main shaft. That is, the output shaft overdrives the transmission main shaft.

planetary gear system
planetary gear system

Uses Of Planetary Gear System

A planetary gear system can be used for various functions by holding one of the three members (ring gear, sun gear, and planet pinion cage) stationary and turning another member. The various combinations are as follows:

Speed increase

  • Hold the sun gear stationary and turn the planet pinion cage, the ring gear will rotate faster than the planet pinion cage. The ratio between the planet pinion cage and the ring gear depends upon the sizes of the different gears.
  •  Another combination to get increased speed is to hold the ring gear stationary and turn the planet pinion cage. The sun gear will rotate faster than the cage.

Speed reduction

  • Hold the sun gear stationary and turn the ring gear. The planet gear case will turn slower than the ring gear.
  • Hold the ring gear stationary and tun the sun gear. The planet pinion cage will rotate at a speed less than the sun gear speed.


  •  Hold the planet pinion cage stationery and tum the ring gear. In this case, the planet pinion act as idlers and they cause the sun gear to turn in the reverse direction to the ring gear rotation. Thus the system functions as a reverse rotation system, with the sun gear turning faster than the ring gear.
  •  Hold the planet pinion cage stationery and tum the sun gear. The ring gear will turn in a reverse direction, but slower than the sun gear.

Direct drive

 If any two of the three members are locked together, then the entire planetary gear system is locked out; and the input shaft and output shaft must turn at the same speeds. On the other hand, if no member is held stationary and no two members are locked together, then the system will n transmit power at all. The input shaft may turn, but the output shaft does not.

planetary gear system
planetary gear system

Watch the planetary gear animation video for a better understanding of this concept.

Also, Read

  1. What is Overdrive Gearbox? How does it work?
  2. What is Freewheel Unit? Construction and Operation of the Freewheel Unit.


Creative Blogger And Automobile Engineer.

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