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 What Is A Planetary Gear?

What Is A Planetary Gear?

Definition Of Planetary Gear

A planetary gear, also known as an epicyclic gear or sun-and-planet gear. That is a type of gear mechanism that consists of three main components: a sun gear, planet gears, and a ring gear. These gears arranged in a way that enables torque transmission. speed reduction or amplification, and various output options.

The sun gear positioned at the center and a cylindrical gear with inward-facing teeth. Surrounding the sun gear are multiple planet gears. which are smaller gears that mesh with both the sun gear and the outer ring gear. The planet gears are often mounted on a carrier, which connects them together.

planetary gear

The outer ring gear, also called the annulus or ring gear, has teeth on the inside that mesh with the planet gears. The planet gears rotate around the sun gear while also rotating on their own axes. This configuration allows for different gear ratios and torque transmission options. depending on how the gears fixed or held stationary.

Planetary gears known for their compactness and versatility. They can achieve high gear reduction ratios in a smaller size compared to other gear mechanisms. They are commonly used in various applications, including automotive transmissions. industrial machinery, robotics, wind turbines, and more. where efficient power transmission, torque multiplication, and compact design required.

Table of Contents

Type of Planetary Gear

There are different types of planetary gears based on their design and configuration. Here are a few common types:

Spur Planetary Gear:

In a spur planetary gear system. all the gears have straight-cut teeth, similar to traditional spur gears. The planet gears mesh with both the sun gear and the ring gear. and the axes of the planet gears are parallel to the axis of the sun gear.

Helical Planetary Gear:

Helical planetary gears have helical-cut teeth, which angled rather than straight-cut. The helical teeth allow for smoother engagement and quieter operation compared to spur gears. The helix angle can be present on the sun gear, planet gears, or both.

Herringbone Planetary Gear:

Herringbone planetary gears, also known as double helical gears. have two sets of helical teeth with opposite helix angles. This design cancels out axial thrust forces and provides balanced torque transmission. Herringbone gears are often used in high-torque applications where precise load distribution required.

Bevel Planetary Gear:

Bevel planetary gears have conical-shaped gears instead of cylindrical gears. The sun gear, planet gears, and ring gear are all bevel gears. allowing for torque transmission and speed reduction at a right angle. Bevel planetary gears are commonly used in automotive differentials.

Cycloidal Planetary Gear:

Cycloidal planetary gears, also known as cycloidal drives. have a unique design that uses noncircular gears. The gears have lobes or pins that engage with each other. resulting in smooth and vibration-free motion. Cycloidal gears are often used in precision applications and robotics.

Harmonic Drive:

Although not strictly a planetary gear, the harmonic drive is a type of gear mechanism. that utilizes a flexible spline and a wave generator to achieve high gear reduction ratios with precise control. It offers high torque capacity, zero backlash, and compact size. making it suitable for robotics, aerospace, and other applications.

These are some of the common types of planetary gears. Each type has its own advantages and applications. and the choice of gear type depends on the specific requirements of the system or machinery in which it used.

Why Use Planetary Gearing?

Planetary gearing offers several advantages that make it a preferred choice in many applications. Here are some reasons why planetary gearing used:

Compact Size:

Planetary gears provide high gear reduction ratios in a compact form factor. The arrangement of the gears, with the planet gears rotating around the sun gear and meshing with the ring gear. allows for a significant reduction in size compared to other gear mechanisms. This compactness is especially valuable in applications . where space limited or weight needs to minimized.

what is a planetary gear

High Torque Capacity:

Planetary gears can handle high torque loads due to the multiple gear engagement points. The load distributed across multiple planet gears. allowing for greater torque transmission capacity compared to other types of gears. This makes planetary gears suitable for applications that require high torque. such as automotive transmissions, heavy machinery, and robotics.


Planetary gears offer high efficiency in power transmission. The meshing of multiple gears reduces backlash. minimizing energy losses and ensuring smooth torque transfer. The ability to distribute the load across multiple gears. also helps in achieving efficient power transmission.

Multiple Speed Ratios:

Planetary gears provide the flexibility to have multiple gear ratios within a single gear system. By fixing or holding different components of the gear system. such as the sun gear, planet carrier, or ring gear, various output speed options achieved. This versatility makes planetary gears well-suited for applications. that require different speed ranges or variable speed control.

Torque Amplification and Reduction:

Planetary gears allow for torque amplification or reduction, depending on the gear configuration. By fixing the input and output components and allowing the planet gears to rotate. torque multiplied. Conversely, by fixing the planet gears and allowing the input and output components to rotate, torque reduced. This versatility in torque management is beneficial in applications. where varying torque requirements exist.

Load Sharing:

The distribution of load among multiple planet gears in a planetary gear system. helps in sharing the load and reducing wear on individual gear teeth. This load sharing characteristic improves the durability and lifespan of the gear system.

Noise and Vibration Reduction:

Planetary gears, particularly those with helical or herringbone gear teeth. offer smoother and quieter operation compared to other gear types. The helical or herringbone teeth minimize gear meshing noise and vibration. resulting in quieter and more comfortable operation in various applications.

These advantages make planetary gearing a preferred choice in automotive transmissions, robotics. industrial machinery, aerospace, renewable energy, and many other industries. where compactness, high torque capacity, efficiency, and flexibility are essential requirements.

How Can You Use a Planetary Gear?

To use a planetary gear, you need to understand its components and their functions. Here are the steps involved in using a planetary gear:

Identify the Components:

A planetary gear consists of three main components. the sun gear, planet gears, and the ring gear (also known as the annulus). Familiarize yourself with these components and their respective positions.

Determine Input and Output:

Determine which component will serve as the input. and which one will be the output based on your specific application. The input component receives the initial power or rotation. while the output component delivers the final power or rotation.

Fix the Input Component:

Decide whether the input component will fixed or held stationary. By fixing or holding the input component. you determine how power transferred and how the gear system functions. There are typically three options for fixing the input component:

a. Fixing the Sun Gear:

If you fix the sun gear and provide rotation to it. the planet gears will rotate around the sun gear while also rotating on their own axes. The output will taken from the planet carrier.

b. Fixing the Planet Carrier:

If you fix the planet carrier and provide rotation to it. the sun gear and the planet gears will rotate around the stationary planet carrier. The output taken from the ring gear.

c. Fixing the Ring Gear:

If you fix the ring gear and hold it stationary. the sun gear and the planet gears will rotate around the fixed ring gear. The output taken from the planet carrier.

Determine Gear Ratio:

The gear ratio determined by the number of teeth on the gears and their arrangement. By choosing appropriate gear sizes and tooth counts for the sun gear, planet gears, and ring gear. you can achieve the desired gear ratio for your application. The gear ratio determines the relationship between the input and output speeds and torques.

Assemble and Align:

Once you have determined the input, output, and gear ratio. assemble the planetary gear system accordingly. Ensure proper alignment of the gears to enable smooth meshing and operation. Lubricate the gears if necessary.

Apply Power:

Provide power or rotation to the input component based on the specific requirements of your application. This achieved through a motor, engine, or any suitable power source.

Observe Output:

Monitor the output from the designated output component. The output used to drive machinery, transmit power. or perform any intended function based on the application.

By following these steps, you can effectively use a planetary gear in various applications. such as automotive transmissions, robotics, industrial machinery, and more. Understanding the gear arrangement, fixing the appropriate components. and determining the gear ratio are key aspects of using a planetary gear effectively.

How are Planetary Gears used?

Planetary gears used in various applications. that require speed reduction, torque multiplication, or both. Here are some common uses of planetary gears:

Automotive Transmissions:

Planetary gears are widely used in automatic transmissions. They allow for multiple gear ratios, enabling smooth and efficient shifting between gears. The planetary gear system helps in torque multiplication. and speed reduction, providing the appropriate power and speed to the wheels.


Planetary gears are commonly used in robotic systems. particularly in robotic arms and manipulators. They enable precise control of movement. and allow for high torque transmission in a compact form factor. Planetary gears used to drive joints. providing the necessary motion and strength for robotic tasks.

Wind Turbines:

In wind turbines, planetary gears employed to increase the rotational speed of the blades. and convert it into electrical energy. The wind turbine’s low-speed shaft connected to the sun gear. while the high-speed shaft connected to the ring gear. This arrangement allows for efficient energy conversion and optimal generator speed.

Industrial Machinery:

Planetary gears find applications in various industrial machinery. such as conveyor systems, machine tools, and heavy equipment. They provide speed reduction, torque multiplication, and precise control over rotational motion. Industrial mixers, extruders, and presses are some examples where planetary gears used.


Planetary gears utilized in aerospace applications. including aircraft landing gear systems and auxiliary power units (APUs). They provide the necessary torque and speed reduction for landing gear extension and retraction. In APUs, planetary gears help transfer power from the turbine. to the generator or other accessory drives.

Watches and Clocks:

In horology, planetary gears used in mechanical watches. and clocks to drive various complications and display functions. They allow for the smooth and precise movement of hour. and minute hands, date indicators, and other watch functions.

These are just a few examples of how planetary gears used. Their versatility, compactness, and ability to handle high torque make them suitable for a wide range of applications. that require speed reduction, torque multiplication, or precise control of rotational motion.

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