What is the Function and the Working of the Planetary Gearbox?

The planetary gearbox is the component used in the Overdrive system. The planetary gearbox increases the speed by arranging the main shaft below the cut-in-speed. This is the component used in an automatic transmission system. In this article, I will describe the function of the planetary gearbox system the working principle, and the components used in the system.


Planetary Gearbox

The planetary gear set is another essential part of the overdrive. It is also used in the automatic transmission. A planetary gearbox system consists of three types of gears:

  1. An outer ring gear has inside teeth. It is sometimes called internal gear.
  2. Three planet pinions are 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.

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The planetary system, when used in the overdrive, works as follows.  The ring gear is attached to 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.

Various Combinations or the Function of Planetary Gearbox


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

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Another combination to get increased speed is to hold the ring gear stationary and turn the planet pinion 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 more slowly than the ring gear.

Hold the ring gear stationary and turn the sun gear. The plant pinion cage will rotate at a speed less than the sun gear speed.

Reverse Gear

Hold the planet pinion cage stationary and turn the ring gear. In this case, the planet pinions 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 stationary and turn 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 not transmit power at all. The input shaft may turn, but the output shaft does not.

Components of the Planetary Gear Box


A planetary gear system, or planetary gearbox, consists of several essential components that work together to provide gear reduction and torque distribution. The primary components of a planetary gearbox are:

Sun Gear

The sun gear is the central gear in the system. It typically remains stationary and is driven by an input power source. Its teeth mesh with the planet’s gears, which orbit around it.

Planet Gears

Planet gears are smaller gears that revolve around the sun gear. They are mounted on a carrier, and their purpose is to transfer power from the sun gear to the ring gear.

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The carrier holds the planet gears in place and allows them to rotate around the sun gear. The carrier can also serve as an output connection point, depending on the design of the gearbox.

Ring Gear

The ring gear is the outermost gear in the system, and it encircles the planet’s gears. It can be fixed (stationary) or allowed to rotate, depending on the specific application and design. The ring gear meshes with the planet gears and helps transmit power to the output.

Input Shaft

The input shaft is the component that connects the sun gear to the power source, such as an electric motor or an engine. It’s responsible for driving the sun gear and initiating the motion of the planetary gears.

Output Shaft

The output shaft is where the output power is extracted from the planetary gearbox. Depending on the design, it can be connected to the carrier, the ring gear, or both, depending on the intended use and desired output characteristics.

Housing or Casing

The housing or casing encloses all the other components, protecting them from external contaminants and providing structural support for the gearbox.

These components work together to create various gear ratios and torque outputs, making planetary gearboxes versatile and suitable for a wide range of applications, including in vehicles, industrial machinery, and robotics. The specific arrangement and design of these components can vary depending on the application and desired performance characteristics.


The unique characteristic of a planetary gear system is that the motion of these components creates multiple paths for power transmission and various gear reduction possibilities. By controlling the input and output connections to these different components, you can achieve a range of gear ratios and torque outputs.

Frequently Asked Questions(FAQ’S)

What is a planetary gearbox?

A planetary gearbox, also known as an epicyclic gearbox, is a type of gear system that uses a central sun gear, surrounding planet gears, and an outer ring gear to achieve various gear ratios and torque outputs.

How does a planetary gearbox work?

A planetary gearbox operates by having the planet gears rotate around the sun gear while engaging with the ring gear. Depending on the configuration, this can lead to speed reduction, torque multiplication, or a combination of both.

Are planetary gearboxes reversible?

Yes, most planetary gearboxes are reversible, allowing for bidirectional rotation. The direction of rotation depends on the input and output connections.

How do I select the right planetary gearbox for my application?

Choosing the right planetary gearbox involves considering factors like required gear ratio, input and output speed and torque, space constraints, and environmental conditions. Consulting with a gearbox manufacturer or engineer is often helpful.

Can planetary gearboxes be used in high-speed applications?

Yes, planetary gearboxes can be designed for high-speed applications, but it’s essential to choose a gearbox with appropriate specifications and materials to handle the high rotational speeds.

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