Planetary Gear

Planetary gears have the ability to rotate around their own axis of rotation (B-B) like a fixed-axis gear. Additionally, their axes of rotation can rotate with the blue bracket, known as the planet carrier, about the axis of the other gears (A-A). The rotation around its own axis is referred to as “rotation”, while the rotation around the other gear axis is known as “revolution”. The name “planetary” is derived from the resemblance to the motion of planets in the solar system.

Planetary Gear

The feature

The planetary gear transmission boasts small size, large carrying capacity, and stable operation as its main features. However, the structure of high-power, high-speed planetary gear transmission is complex and demands high manufacturing precision.

Certain types of planetary gears are highly efficient, but they have a limited gear ratio. Meanwhile, other types have a larger transmission ratio, but they are less efficient. When used as reducers, their efficiency decreases as the gear ratio increases. Conversely, when utilized as speed increasers, there is a possibility of generating self-locking.


  1. Reduced transmission with large transmission ratio
Reduced transmission with large transmission ratio

In a planetary gear system where the number of teeth on each wheel are z1 = 100, z2 = 101, z2′ = 100, and z3 = 99, the transmission ratio from the input member H to the output member 1 is 100.

This demonstrates that a high gear ratio can be achieved according to the requirements of the planetary gear system.

  1. Realize compact power transmission

A planetary gear system can utilize multiple evenly spaced planet gears to effectively transfer motion and power.

Realize compact power transmission

The centrifugal inertial force generated by the revolution of the planetary gears and the radial component force of the reaction force between the tooth profiles can be balanced, allowing for a small force on the main shaft and high transmission power.

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Furthermore, by utilizing internal gears, the transmission space is optimized, and the input and output shafts are aligned, resulting in a smaller system size compared to traditional fixed-axis gear systems under similar conditions.

This type of system is particularly fitting for use in aircraft.

  1. Achieve the synthesis of motion

Motion synthesis combines two input motions to generate a single output motion.

Achieve the synthesis of motion

The differential gear system has two degrees of freedom, which allows the motion of one member to be determined based on the determined motion of any two members.

This feature of the differential gear system is utilized to synthesize motion.

The rotational speed of the carrier, H, is a combination of the rotational speed of wheel 1 and wheel 3.

Therefore, this type of system can be used as an addition mechanism.

When the carrier H, sun gear 1 or 3 is the original, the wheel train can be used as a subtraction mechanism.

This characteristic of differential gear trains, which enables motion synthesis, is widely used in machine tools, computing mechanisms, and compensation adjustment devices.

  1. Achieve the decomposition of motion

The differential gear train is also capable of decomposing the rotation of one of the primary moving members into different rotations of the other two driven basic members.

Achieve the decomposition of motion

The image depicts a schematic diagram of the rear axle differential in a car. In the diagram, members 5 and 4 create a fixed axle train, where wheel 4 is attached to the carrier H, and the planet wheels 2 and 2′ are mounted on H.

Gears 1, 2, 2′, 3 and the planet carrier H together form a differential gear train that decomposes the motion of the engine to the gear 5 into different movements of the sun gears 1 and 3.

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