What is Planetary Gearbox? | How does a Planetary Gear work?

An epicyclic gear is a most common type of gear. The epicyclic gear train is also called a planetary gearbox which plays a most important role in the motion controlling process. The planetary gearboxes are widely used as an important part of automotive mechanical motion transmission applications. In the case of the planetary gearboxes, the input gear and the output gear have identical turning center. In simple words, the input gear revolves around the output gear center. This output shaft is aligned with the input shaft. In this article, we will further discuss planetary gear, working, and types.

What is Planetary Gearbox?

The planetary gearbox has two mounted gears (i.e., input and output gears), one of which rotates around the center of the other gear. A carrier uses to connect the centers of the input gear and the output gear. This carrier rotates to rotate the one gear known as a planetary gear or planet pinion and around the other gear known as the sun wheel or sun gear.

The planet gear (also known as planetary gear) mesh with the sun gear in such a way that their pitch circles can roll without slippage. A sign on the planet gear’s pitch circle represents the planetary curve. In this planetary gearbox, the sun gear is stationary, and the planet gear is moveable. This planet gear rotates around the sun gear.

planetary gearbox
Planetary Gearbox diagram (Reference: motioncontroltips.com)

Planetary gears can install in such a way that these gears rotate within a fixed-pitch circle of outer ring gear or ring gear (in some cases, known as an annular gear).

In this case, the curve drawn through the point or sign on the planet gear’s pitch circle is a hypocycloid. Bicycle throttle hubs are a good example of a planetary gearbox.

These gearboxes may also use in combination with hydraulic motors, electric motors, or IC engines to increase efficiency, durability, and the ability to control the large load.

Working of Planetary Gearbox

The combination of the epicycle gear train and the planets gear, which mesh with the sun and ring gears at the same time, is known as a planetary gear train.

The planetary gearbox has an external ring gear that is known as the ring gear. The planetary gearbox has the following three different gears to transfer load:

  1. Ring gear
  2. Sun gear
  3. Planetary gear
planetary gearbox working
Planetary Gearbox working (Reference: lancereal.com)

The sun gear installs in the middle of the gear set. An electrical motor connects with the sun gear. This motor runs the sun gear. There are many planetary gears meshed with the ring and sun gears which are fixed in the gearbox casing.

When the motor’s electrical power drives the sun gear, it further drives the planet gears about their respective axes and the sun gear. The place of the planetary gear fixes via a carrier. This carrier further connects with the output shaft.

With the above-given gear’s arrangement, the torque is distributed between the teeth of different gears. This arrangement provides the planetary structure with a high degree of rigidity and helps to reduce backlash (i.e., one to two arc minutes in some designs). High rigidity is also another vital factor for applications that need fast start-stop cycles or quick variations in the rotational direction.

The planetary gearbox has a compact design that delivers a high-speed reduction ratio in a small overall bundle. These designs also have less inertia which is particularly advantageous in servo functions. This is because the inertia of the gearbox directly increases the load inertia that the motor has to balance.

These gearboxes are designed like any other gearbox. The oil or grease may use to lubricate planetary gearboxes, but maximum gearboxes are lubricated by the supplier company. Therefore, you don’t need to re-lubricate your gearboxes to extend their life.

Working Principle of Planetary Gearbox

The planetary gear is the heart of the new engineering technologies. The gearboxes use the gears to drive everything from simple machinery to the most modern electrical and mechanical systems.

Thousands of years ago, a simple arrangement of central drives and rotating gears were used to create the motion of the planets. Nowadays, manufacturers use planetary gears, which need high durability, functional efficiency, and torque density.

In planetary gearboxes, maximum teethes of the gears rotate at the same time. This motion delivers the high-speed deceleration, which obtains via relatively small gears and the low inertia reflected by the system.

The teethes of the gears distribute the load and enable planetary components to deliver high torque. The grouping of high torque transmission, compact size, and speed reduction makes the planetary gearbox a best choice for applications where space is limited.

These gearboxes are most commonly used in airplanes, motorcycles, cars, and many other vehicles.

Read Also: Working and Types of Steering System

What is Planetary Gearbox Used for?

A planetary gearbox is most commonly used in the following applications:  

  • It uses in the robots to increase the torque.
  • These gearboxes are also used for the printing presses to reduce the roller’s speed.
  • They also use for particular positioning
  • They use in the packaging machines
  • The planetary gearbox also uses to drive the wheels.
  • These gears also use to drive the cutter head.
  • They also use to drive the drilling machines.
  • They use in coil tubing injectors.
  • These gearboxes are used in pumps and compressors.
  • They use to drive hoist.
  • The epicyclic gearbox is most widely used in the mixing machines.

Types of Planetary Gearbox

The planetary gearboxes have the following major types:

  1. Wheel Drive
  2. Spindle Output
  3. Shaft Output

1) Wheel Drive Planetary Gearbox

In this type of planetary gearbox, the sun gear operates the surrounding planetary components connected with a carrier in a planetary gearbox.

Wheel Drive gearbox
Wheel Drive gearbox (Refer: lancereal.com)

As the sun gear starts rotating, the planetary gear rotates around the exterior ring gear. The wheels may be mounted on the system casing. You can optimize the size of your system by linking the wheels directly with the gearbox. The wheel drive gearboxes can use for torques up to 332,000 Nm.

2) Shaft Output

In the case of the shaft output gearbox, the sun gear drives the coupled planetary components that are connected in a rotating carrier of the gearbox.

Shaft Output planetary gearbox
Shaft Output Gearbox (Source: lancereal.com)

The ring part fixes via a rotating driver that transfers the drive force to the shaft. The system casing connects directly to the machine, and the output end is a rotating shaft. These types of gearboxes can provide outputs of up to 113,000 Nm.

3) Spindle Output

The working of this gearbox is the same as the shaft output gearbox. However, the spindle output gearbox provides output via a flange. The spindle output gearbox can supply an output of up to 113,000 Nm.

Spindle Output planetary gearbox

How Does a Planetary Gear Relate to Portal Axles?

Portal axles are employed to increase the clearance of the ground and enable gear drives to reduce the stress on the differentials. These axles are used in many automobiles like the Mercedes-Benz Unimog and the Mercedes-AMG G6 36×6 truck. These vehicles also have a planetary hub gearbox that permits the half shaft to rotate quicker than the wheel.

This decreases the torque required to produce the same power. In automobiles like the G63 and the Unimog, the portal axle function is somewhat different because the tires and large wheels already deliver the maximum part of the required ground clearance.


  1. The absolute ratios are between 3: 1 and 10: 1 per stage.
  2. Closed system.
  3. These gearboxes have low backlash.
  4. They have high efficiency.
  5. They have a compact design and low mass inertia.

How Do We Know What Planetary Gearbox We Need?

When choosing a planetary gearbox, you need to use the outcome to determine its size and gear ratio. This is a most important balance between performance, cost, size, and efficiency. Well-known manufacturers have advisory methods for modeling. They start each project with deep learning about specific speeds, torques, situations, and machine performance.

You may also use your experience and knowledge to identify and choose the cost-effective, correct, and reliable planetary gearbox. An expert provides you a gearbox that can work without any trouble for many years.

How to Choose a Planetary Gearbox?

The choice of a good planetary gearbox is very hard. Follow the below-given tips to choose the best planetary gearbox.

  1. Features required: First of all, you need to consider the ratio, backlash, torque, etc., to select your gearbox.
  2. Space: Multiple sizes of planetary gearboxes are available in the market. If you have low space, select a gearbox that has a small size so that you can install it easily.
  3. Environment: In some environments, moisture, dirt, or dust; may cause corrosion of the gearbox. Therefore, you must consider this factor for the selection of the gearbox and choose a gearbox that has a corrosion-resistant body.

FAQ Section

Do planetary gears need lubrication?

Yes. Like other gears and mechanical parts in an automobile, the planetary gear also needs lubrication to stop damage and wear. The lubrication is also very important for the smooth operation of the planetary gear.  

The transmission systems of your vehicle require proper maintenance and repair. This can contain minor tasks like changing the oil and proper lubrication of the system parts. Timely proper maintenance and lubrication are very important to correct the problem as soon as possible because the gears in the transmission wear out over time and cause skidding and noise.

What are the Drawbacks of Using a Planetary Gearbox?

Planetary gearboxes have many advantages and disadvantages. These gearboxes have complex modeling and manufacturing, making them more expansive than other types of gearboxes.  

The planetary gearbox requires very precise construction. If one planet gear is closer to the sun gear than others, it can lead to the imbalance of the planet gear, which can lead to increased wear and failure. Furthermore, the compact size of the planetary gear ensures a high level of heat dissipation, which may require cooling in applications that operate very quickly or have continuous power.

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