# Structure and working principle of planetary gear mechanism

#### 1.Basic structure of planetary gear mechanism

There are many types of planetary gear mechanism, the simplest of which is composed of a sun wheel, a ring gear, a planet carrier and several planet gears supported on the planet carrier, called a planet row.

The sun gear, ring gear and planet carrier in the planetary gear mechanism have a common fixed axis, and the planet gear is supported on the planet gear shaft fixed on the planet carrier, and meshed with the sun gear and ring gear at the same time. When the planetary gear mechanism is running, several planetary gears on the planet gear shaft of the empty sleeve on the planet carrier can rotate around their own axis on the one hand, and on the other hand, they can rotate around the sun with the planet carrier together, just like the motion of the planets in the sky, and have two kinds of motion states: rotation and Revolution (hence the name of the star gear). In the planetary row, there are The sun gear, ring gear and planet carrier with fixed axis are called the three basic elements of planet row.

#### 2.Types of planetary gear units

Planetary gear units can be classified in different ways

(1) Classification according to gear engagement

According to the different meshing ways of gears, planetary gear mechanism can be divided into two types: external meshing and internal meshing. Because of its large volume and low transmission efficiency, the external meshing planetary gear mechanism has been eliminated in automobiles; because of its compact structure and high transmission efficiency, the internal meshing planetary gear mechanism is widely used in automatic transmissions.

(2) Classification according to the number of rows of gears

According to the number of rows of gears, planetary gear mechanism can be divided into single row and multi row. The multi row planetary gear mechanism is composed of several single row planetary gear mechanisms. In the automatic transmission of automobile, the number of planetary row varies according to the number of gears. Generally, there are two planetary rows in the third gear and three planetary rows in the fourth gear (with overdrive). Usually, the multi row planetary gear mechanism composed of two or two single row planetary gear mechanisms is used.

(3) According to the number of planetary gear sets between the sun gear and the ring gear

According to the number of planetary gear sets between the sun gear and the ring gear, the planetary gear mechanism can be divided into single planetary gear type and double planetary gear type.

There are two sets of meshing planetary gears between the sun gear and the ring gear in the double planetary gear mechanism. The outer set of planetary gears and the ring gear mesh, and the inner set of planetary gears and the sun gear mesh. Compared with the single planetary gear mechanism under the same conditions, the ring gear can get reverse transmission.

The planetary gear mechanism is used as the speed change mechanism, because there are several planetary gears transmitting power at the same time, and the internal meshing type is often used, making full use of the space in the middle of the ring gear, so compared with the common gear speed change mechanism, under the condition of transmitting the same power, the size and weight of the speed change mechanism can be greatly reduced, and the same direction and coaxial speed reduction drive can be realized; in addition, due to the adoption of the Constant mesh transmission, continuous power, good acceleration and reliable operation.

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