# Working principle and structure analysis of planetary gear set

update:2020-10-14
abstract:Working principle and structure analysis of planetary gear set Planetary gear transmission principle...

Working principle and structure analysis of planetary gear set

Planetary gear transmission principle
Most of the gears we know are gears with a fixed axis of rotation. For example, mechanical clocks, ordinary mechanical gearboxes, and reducers. Although all the gears above are rotating, their rotation center (which coincides with the center of the circle) is often mounted on the casing through a bearing. Therefore, their rotation axis They are fixed relative to the casing, so they are also called "fixed shaft gear transmission."
There must be movement. Correspondingly, there is a less well-known type of gears called "planetary gears" whose axis of rotation is not fixed, but is mounted on a rotatable bracket (blue) (Figure The black part is the housing, and the yellow represents the bearing). Planetary gears (green) can rotate around their own axis of rotation (BB) like fixed shaft gears, their axis of rotation also follows the blue carrier (called planet carrier) around the axis of other gears (AA) Rotate. The rotation around its own axis is called "rotation", and the rotation around the axis of other gears is called "revolution", just like the planets in the solar system, hence the name "planetary gear".
Just like the solar system, the gears with fixed axes that become the center of planetary gear revolution are called "sun gears", as shown in the red gears. There are usually two meshing points on one planetary gear or on two mutually fixed planetary gears, which are in relationship with the two sun gears respectively. As shown in the picture on the right, the gray internal gear axis coincides with the red external gear axis, which is also the sun gear.

The principle of gear transmission with a fixed axis is very simple. In a pair of gears that mesh with each other, one gear is used as the driving wheel, and power is transmitted from it, and the other gear is used as a driven wheel, and the power is output from it. Some gears are only used as transfer stations, one side meshes with the driving wheel and the other side meshes with the driven wheel, and power passes through it.
Planetary gear transmission characteristics
Compared with ordinary gear transmission, planetary gear transmission has a series of outstanding advantages when their parts materials, mechanical properties, manufacturing accuracy, working conditions, etc. are the same, so it is often used as a planetary reducer/planet Gear reducer, speed increaser, differential and reversing mechanism, and other special purposes. The main features of planetary gear transmission are as follows:
(1) The planetary reducer is small in size, light in weight, compact in structure, large in transmission power, and high in load capacity; this feature is determined by the internal factors such as the structure of the planetary gear transmission, as shown in the figure below.
A. The planetary reducer has the principle of power splitting. Several identical planetary gears are evenly distributed around the center wheel to share the load, so that the load on each gear is smaller, and the corresponding gear modulus can be smaller. In the case of uniform load, as the planetary gear increases, its outer dimensions decrease.
B. Planetary reducer makes reasonable use of internal gearing. Make full use of the high internal meshing carrying capacity and the space volume of the internal gear (or internal gear ring), thereby reducing the radial and axial dimensions, making the structure compact and the carrying capacity high.
C. Planetary reducer is a coaxial transmission device. Each center wheel forms a coaxial transmission, and the input shaft and the output shaft are coaxial, so that the size of the transmission device in the length direction is greatly reduced.
(2) The planetary reducer has a large transmission ratio; as long as the type of planetary transmission and the gear allocation plan are appropriately selected, a few gears can be used to obtain a large transmission ratio. In a planetary mechanism that is not used as a power transmission but mainly used to transmit motion, its transmission ratio can reach several thousand. In addition, because the planetary gear transmission can rotate its three basic components, it can realize the synthesis and decomposition of motion, as well as complex motion such as stepped and continuously variable transmission.

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