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What Is Planetary Gear?
Sep 03, 2018

 Concept:

        Planetary gears refer to a gear system that can rotate around its own axis like a fixed-axis gear, and their rotation axis also rotates along the axis of the planetary frame around other gears. The rotation around one's own axis is called "rotation" and the rotation around other gear axes is called "revolution", just like the planets in the solar system, so it is named.

     Compared with general gear transmission, planetary gear transmission has many unique advantages. The most remarkable feature is that power can be diverted when power is transmitted, and the input shaft and output shaft are on the same horizontal line. Therefore, planetary gear transmission has been widely used in various mechanical transmission system reducers, accelerators and transmission devices. Especially because of its "high load, large transmission ratio" characteristics, it has been widely used in aircraft and vehicles (especially heavy vehicles). The [1] planetary gear also plays a great role in torque transmission of the engine. Because the speed and torque of the engine are quite different from the road driving demand, the above characteristics of planetary gears can be used to convert the power of the engine to the driving wheels properly. Automatic transmission in automobiles also uses these characteristics of planetary gears to obtain different transmission ratios by changing the relative motion of each component through clutches and brakes.

      However, due to the complex structure and working state of planetary gears, the vibration and noise problems are also prominent. Fatigue pitting of gear teeth, root cracks and even fracture of gear teeth or shaft are easy to occur, which affects the operation accuracy, transmission efficiency and service life of the equipment.


Structure composition:

      The main transmission structure of planetary gear reducer is planetary gear, sun wheel and outer gear ring.

      Planetary gear mechanism is divided into single planetary row and double planetary row according to the number of planetary gears installed on the planetary frame.

    Simple (single row) planetary gear mechanism is the foundation of the transmission mechanism. Usually the transmission mechanism of automatic transmission is composed of two or more rows of planetary gear mechanisms. The simple planetary gear mechanism consists of a sun wheel, several planetary gears and a gear ring, in which the planetary gear is supported by a fixed shaft of the planetary frame, allowing the planetary gear to rotate on the supporting shaft. Planetary gears and adjacent sun gears and gear rings are always in constant meshing state, usually using helical gears to improve the stability of work.


Application area:

      Planetary gear train has been widely used in land, navigation, aviation and other transportation equipment, mainly in the following aspects:

       (1) Deceleration transmission with large transmission ratio.

In the planetary gear train shown below, if the number of teeth of each wheel is Z1 = 100, Z2 = 101, z2'= 100, Z3 = 99, the transmission ratio of input member H to output member 1 is equal to 100. It can be seen that the planetary gear train can achieve a large transmission ratio as required.

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        (2) High power transmission with compact structure.

Planetary gear trains can use several evenly distributed planetary wheels to transmit motion and power at the same time. The centrifugal inertia force produced by these planetary wheels and the radial component of the reaction force between the tooth profile can be balanced with each other, so the spindle has small force and large transmission power. In addition, because it uses the internal meshing gear, makes full use of the transmission space, and the input and output shaft in a straight line, so the space size of the whole gear system is much smaller than the ordinary fixed shaft gear system under the same conditions. This type of gear system is especially suitable for aircraft.



        (3) Realizing the synthesis of motion.

The synthesis of motion is the combination of two input motions into one output motion. The degree of freedom of the differential gear train is equal to 2. The motion of the other component can be determined only after the definite motion of any two components is given. The synthesis of motion can be realized by using this characteristic of differential gear trains.

The simplest motion synthesis gear train is shown in the following figure. The speed of planet carrier H is the synthesis of wheel 1 and wheel 3 speed. Therefore, the gear train can be used as an additional mechanism. When the planet carrier H, sun wheel 1 or 3 are the original moving parts, the gear train can also be used as a deceleration mechanism.

Differential gear trains are widely used in machine tools, computer mechanisms and compensating and adjusting devices.

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        (4) The decomposition of motion.

Differential gear train can also decompose the rotation of one original member into the different rotation of the other two follower basic members. The following figure shows the automobile rear axle differential diagram, in which the components 5 and 4 form a fixed axle gear train, the wheel 4 is fixed to the planetary frame H, and the H is equipped with planetary wheels 2 and 2. Gear 1, 2, 2', 3, and planetary frame H form a differential gear train, which divides the motion of the engine to gear 5 into different motions of sun gear 1 and 3.

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