Planetary gear set speed reducer

Abstract

The utility model relates to the technical field of gear set speed reducers, in particular to a planetary gear set speed reducer. The planetary gear train comprises a first planetary gear set and a second planetary gear set, and the first planetary gear set comprises a first inner gear ring, a first sun gear, a first planet carrier and a plurality of first planet gears; the first inner gear ring is fixedly installed on a support, and a main shaft is rotationally arranged on the support. The first sun gear is installed on the main shaft, and the first planet carrier is installed on the main shaft. The second planetary gear set comprises a second inner gear ring, a second sun gear, a second planet carrier and a plurality of second planet gears; the second inner gear ring is rotationally mounted on the bracket; the second sun gear is fixedly installed on the main shaft, and the second planet carrier is rotationally installed on the main shaft and fixedly connected with the first planet carrier. The speed reducer has the characteristics of compact structure and large reduction ratio.

Description

Technical Field

[0001] The utility model relates to the technical field of gear set reducers, in particular to a planetary gear set reducer. Background Art

[0002] There are many rotating joints in small robot devices such as humanoid robots and mechanical dogs. Reducers are often required in these rotating joints to convert the input high-speed rotation into low-speed rotation, thereby achieving a more precise rotation range.

[0003] However, the existing planetary gear reducers for small robot equipment are usually large in size and complicated in structure when meeting the required transmission ratio. Using them for small robot equipment such as humanoid robots and mechanical dogs will seriously hinder the lightweight and small-volume design of these small robot equipment. In addition, it is difficult to achieve precise control of the rotation output end, which is not conducive to the precise control of important output parameters such as the rotation angle, speed, and acceleration of the rotation joints of small robots.

[0004] To this end, it is urgent to propose a reducer with a new structure. Under the premise of ensuring the reduction ratio required by small robot equipment, the size of the reducer can be reduced to ensure the lightweight and miniaturization of the robot. In addition, it can also achieve precise control of important output parameters such as the rotation angle, speed, acceleration, etc. of the rotating joints of small robots. Utility Model Content

[0005] The purpose of the utility model is to provide a planetary gear set reducer that can at least partially overcome the above-mentioned technical problems, can meet the reduction ratio required by the robot's rotating joints, and has a smaller volume. In addition, it can also achieve precise control of important output parameters such as the rotation angle, speed, acceleration, etc. of the rotating joints of small robots.

[0006] The utility model provides a planetary gear set reducer, comprising a first planetary gear set and a second planetary gear set, wherein the first planetary gear set comprises a first inner ring gear, a first sun gear, a first planet carrier and a plurality of first planet gears; the first inner ring gear is fixedly mounted on a bracket, and a main shaft coaxial with the first inner ring gear is also rotatably arranged on the bracket; the first sun gear is fixedly mounted on the main shaft, and the first planet carrier is rotatably mounted on the main shaft; for any of the first planet gears, the first planet gear is rotatably mounted on the first planet carrier and meshes with the first sun gear and the first inner ring gear; the second planetary gear set comprises a second inner ring gear, a second sun gear, a second planet carrier and a plurality of second planet gears; the second inner ring gear is rotatably mounted on the bracket; the second sun gear is fixedly mounted on the main shaft, and the second planet carrier is rotatably mounted on the main shaft and fixedly connected to the first planet carrier; for any of the second planet gears, the second planet gear is rotatably mounted on the second planet carrier and meshes with the second sun gear and the second inner ring gear.

[0007] Furthermore, the main shaft is a hollow shaft, a shaft is inserted into the main shaft, one end of the shaft is fixedly connected to the second inner gear ring, and the other end extends to a side of the first inner gear ring away from the second inner gear ring.

[0008] Furthermore, the second inner gear ring and the shaft rod are both used to connect to a load end.

[0009] Furthermore, a monitoring reference is provided on one end of the shaft away from the second inner gear ring, and a sensor is provided on the bracket, wherein the sensor is used to monitor the rotation parameters of the shaft, and the rotation parameters include rotation angle, rotation speed, and rotation angular acceleration.

[0010] Furthermore, the monitoring reference object is a magnet, and the sensor is a magnetically encoded angle sensor.

[0011] Furthermore, the second planetary gear set also includes a third sun gear and several third planetary gears; the third sun gear is fixedly mounted on the main shaft and is located on the side of the second sun gear away from the first sun gear, the number of teeth and the module of the third sun gear and the second sun gear are equal, and the projections of the third sun gear and the second sun gear along the axis of the main shaft do not overlap; for any third planetary gear, the third planetary gear is rotatably mounted on the second planet carrier and is meshed with the third sun gear and the second inner ring gear.

[0012] Further, the number of the second planetary gears and the number of the third planetary gears are equal, and the second planetary gears and the third planetary gears are arranged alternately.

[0013] Furthermore, the number of the first planetary gears is 3 to 6, and the number of the second planetary gears is 3 to 6.

[0014] Furthermore, the number of the first planetary wheels is 3, and the circumferences are evenly distributed on the first planet carrier; the number of the second planetary wheels is 3, and the circumferences are evenly distributed on the second planet carrier; the first planetary wheels and the second planetary wheels are arranged alternately.

[0015] Compared with the prior art, the utility model has the following advantages and beneficial effects:

[0016] 1. A planetary gear reducer provided by an embodiment of the present disclosure achieves a large reduction ratio on the basis of being small and compact by cleverly combining two planetary gear sets. By slightly adjusting the gear parameters of the planetary gear reducer, the reduction ratio can be greatly adjusted, and the rotation direction of the output end can be changed. Since multiple planetary gears are more evenly distributed on the circumference, it is conducive to achieving dynamic balance of the rotating shaft, thereby reducing vibration and noise caused by imbalance, and ensuring the smooth operation of the planetary gear reducer. Less vibration and smoother operation are conducive to reducing wear and extending the service life of the planetary gear reducer.

[0017] 2. A planetary gear reducer provided in an embodiment of the present disclosure can realize dual-axis output. When it is used for the rotating joint of a small robot device, the output of the second inner gear ring side and the output of the shaft are located on both sides of the planetary gear reducer. The target component is driven to rotate through the communication between the second inner gear ring and the shaft, which can make the component more balanced in installation, avoid unnecessary bending moment, and thus have better stress conditions;

[0018] 3. A planetary gear reducer provided by an embodiment of the present disclosure can realize real-time monitoring of the rotation parameters of the output end; by inputting the rotation parameters monitored by the magnetic encoding angle sensor into the control circuit for controlling the rotation parameters of the power source, the control circuit adjusts the rotation parameters of the power source according to the rotation parameters monitored by the magnetic encoding angle sensor, and then can timely adjust the rotation parameters of the output end, thereby realizing real-time monitoring and control of the rotation parameters of the output end of the planetary gear reducer;

[0019] 4. A planetary gear reducer provided in an embodiment of the present disclosure can make the actual meshing tooth clearance of the second planetary gear set smaller and the transmission between teeth smoother; and the actual meshing tooth clearance of the second planetary gear set is smaller, which can make the output accuracy of the planetary gear reducer higher, and is conducive to the precise control of the rotation angle of the output end of the planetary gear reducer; in addition, in the second planetary gear set, a third planetary gear equal in number to the second planetary gear is added, which can make the operation of the second planetary gear set smoother, can withstand a larger torque, and adapt to the characteristics of large torque output at the output end. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings described herein are used to provide a further understanding of the embodiments of the present utility model, constitute a part of this application, and do not constitute a limitation of the embodiments of the present utility model. In the drawings:

[0021] Figure 1 The exploded view of the planetary gear reducer according to the first embodiment of the present invention;

[0022] Figure 2 An exploded view of the planetary gear reducer according to Embodiment 2 of the present utility model;

[0023] Figure 3 It is a cross-sectional view of the planetary gear reducer according to Embodiment 3 of the present utility model;

[0024] Figure 4 An exploded view of the planetary gear reducer according to Embodiment 4 of the present utility model;

[0025] Figure 5 It is a cross-sectional view of the planetary gear set reducer according to embodiment 4 of the present utility model.

[0026] Marks and corresponding parts names in the attached drawings:

[0027] 11-first inner gear ring; 12-first sun gear; 13-first planet carrier; 131-shaft tube; 14-first planet gear; 21-second inner gear ring; 22-second sun gear; 23-second planet carrier; 24-second planet gear; 25-third sun gear; 26-third planet gear; 3-bracket; 31-sensor; 4-main shaft; 5-shaft rod; 51-monitoring reference object. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the utility model more clear, the utility model is further described in detail below in combination with the embodiments and drawings. The schematic implementation mode and description of the utility model are only used to explain the utility model and are not intended to limit the utility model. It should be noted that the utility model is already in the actual development and use stage.

[0029] There are many rotating joints in small robot devices such as humanoid robots and mechanical dogs. In these rotating joints, reducers are often required to convert the input high-speed rotation into low-speed rotation, so as to achieve a more accurate rotation range. However, the existing reducers are usually large in size and complicated in structure under the condition of meeting the reduction ratio required for the precise rotation of the robot's rotating joints. Using them in small robot devices such as humanoid robots and mechanical dogs will seriously hinder the lightweight and small-volume design of these small robot devices. In addition, these existing reducers are also difficult to achieve continuous monitoring and precise control of the rotating output end, which is not conducive to the precise control of important output parameters such as the rotation angle, speed, acceleration, etc. of the rotating joints of small robots. The utility model proposes a planetary gear reducer, which can reduce the size of the reducer and ensure the lightweight and miniaturization of the robot under the premise of ensuring the reduction ratio required for the rotating joints of the robot. In addition, it can also achieve precise control of important output parameters such as the rotation angle, speed, acceleration, etc. of the rotating joints of small robots.

[0030] Embodiment 1:

[0031] like Figure 1 As shown, a planetary gear set reducer provided in this embodiment includes a first planetary gear set and a second planetary gear set.

[0032] The first planetary gear set includes a first inner ring gear 11, a first sun gear 12, a first planet carrier 13 and a plurality of first planetary gears 14; the first inner ring gear 11 is fixedly mounted on a bracket 3, and a main shaft 4 coaxial with the first inner ring gear 11 is rotatably arranged on the bracket 3; the first sun gear 12 is fixedly mounted on the main shaft 4, and the first planetary carrier 13 is rotatably mounted on the main shaft 4; for any first planetary gear 14, the first planetary gear 14 is rotatably mounted on the first planet carrier 13 and meshes with both the first sun gear 12 and the first inner ring gear 11;

[0033] The second planetary gear set includes a second inner ring gear 21, a second sun gear 22, a second planet carrier 23 and a plurality of second planetary gears 24; the second inner ring gear 21 is rotatably mounted on the bracket 3; the second sun gear 22 is fixedly mounted on the main shaft 4, and the second planet carrier 23 is rotatably mounted on the main shaft 4 and fixedly connected to the first planet carrier 13; for any second planetary gear 24, the second planetary gear 24 is rotatably mounted on the second planet carrier 23 and meshes with the second sun gear 22 and the second inner ring gear 21.

[0034] Therefore, in a planetary gear reducer provided in this embodiment, the power provided by the power source can be input into the first planet carrier 13 (for example, a shaft tube portion 131 is formed on the first planet carrier 13, and the shaft tube portion 131 is sleeved on the main shaft 4, and the shaft tube portion 131 extends to the outer side of the end surface of the first inner gear ring 11, and the power is input into the shaft tube portion 131 to facilitate the power input. For example, the shaft tube portion 131 can be powered by a DC brushless hollow motor) so that the first planet carrier 13 rotates around the main shaft 4, and then the first planet carrier 13 will cause the first planetary gear 14 to rotate on the first inner gear ring 11 rotates around the main shaft 4, and the first planetary gear 14 drives the first sun gear 12 to rotate around the axis of the main shaft 4 during the process of meshing with the first internal gear ring 11, thereby driving the main shaft 4 to rotate; so far, for the second planetary gear set, since the second sun gear 22 is also fixedly mounted on the main shaft 4, the second sun gear 22 also rotates around the axis of the main shaft 4, and since the first planetary carrier 13 and the second planetary carrier 23 are fixedly connected, the second planetary carrier 23 also rotates around the main shaft 4, and the two power inputs acting on the second planetary gear set both cause the second internal gear ring 21 to rotate around its own axis. Alternatively, the power provided by the power source is input to the main shaft 4 (that is, it is equivalent to inputting the power to the first sun gear 12 and the second sun gear 22). In the first planetary gear set, the first sun gear 12 rotates around the axis of the main shaft 4. During the process of the first sun gear 12 meshing with the first planetary gear 14, the first planetary gear 14 drives the first planetary gear 14 to rotate around the main shaft 4 (that is, the first planetary carrier 13 rotates around the main shaft 4); in the second planetary gear set, under the action of the power source, the second sun gear 22 also rotates around the axis of the main shaft 4; since the first planetary carrier 13 and the second planetary carrier 23 are fixedly connected, the second planetary carrier 23 also rotates around the main shaft 4, and the two power inputs acting on the second planetary gear set both cause the second inner ring 21 to rotate around its own axis.

[0035] In order to specifically present the deceleration effect of a planetary gear set reducer provided by the present embodiment, the following lists the parameter configurations of the first internal gear ring 11, the first sun gear 12, the first planetary gear 14, the second internal gear ring 21, the second sun gear 22, and the second planetary gear 24 of the planetary gear set reducer and the corresponding reduction ratios in some specific practices of the present embodiment, as shown in the following table:

[0036] Table 1: Relationship between reducer gear parameters and reduction ratio

[0037]

[0038] It can be seen from the above that the planetary gear reducer provided in this embodiment achieves a larger reduction ratio (as can be seen from the above table, reaching 108.5:1 to 298.14:1) on the basis of meeting the requirements of small size and compactness by cleverly combining two planetary gear sets. According to the above table, it can be seen that by slightly adjusting the gear parameters of the planetary gear reducer (referring to only slightly changing the number of teeth, which has little effect on the size of the planetary gear reducer), a large adjustment of the reduction ratio can be achieved, and a change in the rotation direction of the output end can also be achieved (referring to different output rotation directions under the same input rotation direction).

[0039] Preferably, the number of the first planetary gears 14 is 3 to 6, and the number of the second planetary gears 24 is 3 to 6. More preferably, the number of the first planetary gears 14 is 3, and they are evenly distributed on the first planet carrier 13; the number of the second planetary gears 24 is 3, and they are evenly distributed on the second planet carrier 23; each of the first planetary gears 14 and each of the second planetary gears 24 is staggered (the staggered arrangement is as follows Figure 1 As shown, the projection of the first planetary gear 14 along the axis of the main shaft 4 is located between the projections of the second planetary gears 24 along the axis of the main shaft 4). Based on this, the embodiment provides a planetary gear reducer, because the multiple planetary gears are more evenly distributed on the circumference, it is conducive to achieving dynamic balance of the rotating shaft, thereby reducing vibration and noise caused by imbalance, and ensuring the smooth operation of the planetary gear reducer. Less vibration and smoother operation are conducive to reducing wear and extending the service life of the planetary gear reducer.

[0040] Embodiment 2:

[0041] like Figure 2 As shown, this embodiment is based on embodiment 1, and the difference is that, in a planetary gear set reducer provided in this embodiment, the main shaft 4 is a hollow shaft, and a shaft 5 is inserted in the main shaft 4, one end of the shaft 5 is fixedly connected to the second inner gear ring 21, and the other end extends to the side of the first inner gear ring 11 away from the second inner gear ring 21. The second inner gear ring 21 and the shaft 5 are both used to connect the load end.

[0042] Based on this, a planetary gear set reducer provided in this embodiment can achieve dual-axis output. When it is used for the rotating joints of small robotic equipment, the output on the second inner ring 21 side and the output of the shaft 5 are located on both sides of the planetary gear reducer. The target component is driven to rotate through the communication between the second inner ring 21 and the shaft 5, which can make the installation of the component more balanced and avoid unnecessary bending moment (for single-sided output, usually the connection ends of the two joint arms are located on one side respectively, which will produce lateral bending moment, while for double-sided output, usually one joint arm connection end clamps the other joint arm connection end in the middle), thereby achieving better force conditions.

[0043] Embodiment 3:

[0044] like Figure 3 As shown, this embodiment is based on embodiment 2, and the difference is that in the planetary gear reducer provided in this embodiment, the shaft 5 is not used to connect the load, but to monitor the rotation parameters of the output end. Specifically:

[0045] A monitoring reference object 51 is provided on one end of the shaft 5 away from the second inner gear ring 21, and a sensor 31 is provided on the bracket 3. The sensor 31 is used to monitor the rotation parameters of the shaft 5, and the rotation parameters include the rotation angle, the rotation speed (referring to the rotation angular velocity), and the rotation angular acceleration. Preferably, the monitoring reference object 51 is a magnet, and the sensor 31 is a magnetic encoding angle sensor 31.

[0046] Based on this, the embodiment of the utility model provides a planetary gear reducer, since the shaft 5 and the second inner gear ring 21 are fixedly connected, the rotation angle of the shaft 5 is equal to the rotation angle of the second inner gear ring 21, the rotation speed of the shaft 5 is equal to the rotation speed of the second inner gear ring 21, and the rotation angular acceleration of the shaft 5 is equal to the angular acceleration of the second inner gear ring 21. Based on this, real-time monitoring of the rotation parameters of the output end (i.e., the second inner gear ring 21) can be achieved. More specifically, by setting a magnet as a monitoring reference 51 on the end of the shaft 5 away from the second inner gear ring 21, a magnetic encoding angle sensor 31 is set on the bracket 3 to monitor the rotation parameters of the shaft 5, and the rotation angular velocity and rotation angular acceleration can be obtained through the monitored rotation angle. Preferably, the magnetic encoding angle sensor 31 is directly opposite to the end face of the shaft 5 where the magnet is installed. Therefore, by inputting the rotation parameters monitored by the magnetic encoding angle sensor 31 into the control circuit that controls the rotation parameters of the power source, the control circuit adjusts the rotation parameters of the power source according to the rotation parameters monitored by the magnetic encoding angle sensor 31, and then can timely adjust the rotation parameters of the output end, thereby realizing real-time monitoring and control of the rotation parameters of the output end of the planetary gear set reducer.

[0047] Embodiment 4:

[0048] like Figure 4 , Figure 5 As shown, this embodiment is based on embodiment 2, and the difference is that in a planetary gear set reducer provided in this embodiment, the second planetary gear set further includes a third sun gear 25 and a plurality of third planetary gears 26;

[0049] The third sun gear 25 is fixedly mounted on the main shaft 4 and is located on a side of the second sun gear 22 away from the first sun gear 12. The number of teeth and the module of the third sun gear 25 and the second sun gear 22 are equal, and the projections of the third sun gear 25 and the second sun gear 22 along the axis of the main shaft 4 do not overlap.

[0050] For any of the third planetary gears 26 , the third planetary gear 26 is rotatably mounted on the second planet carrier 23 and meshes with both the third sun gear 25 and the second inner gear ring 21 .

[0051] More preferably, the number of the second planetary gears 24 and the number of the third planetary gears 26 are equal, and the second planetary gears 24 and the third planetary gears 26 are arranged alternately.

[0052] It should be understood that the non-coincidence of the projections of the third sun gear 25 and the second sun gear 22 along the axis of the main shaft 4 is specifically manifested in that when the second sun gear 22 and the third sun gear 25 are installed, the gear teeth of the two are aligned correspondingly (that is, the projections of the third sun gear 25 and the second sun gear 22 along the axis of the main shaft 4 are overlapped), and then the second sun gear 22 or the third sun gear 25 is rotated by a certain angle so that the gear teeth of the two are no longer aligned. Preferably, the gear teeth of the second sun gear 22 are made to correspond to the tooth roots of the third sun gear 25 (that is, the gear teeth of the second sun gear 22 or the third sun gear 25 are rotated by half).

[0053] Based on this, in the second planetary gear set, when the gear teeth of the second planetary gear 24 just mesh with the tooth gap of the second inner gear ring 21, the gear teeth of the third planetary gear 26 partially mesh with the tooth gap of the second inner gear ring 21, and when the gear teeth of the second planetary gear 24 mesh out of the tooth gap of the second inner gear ring 21, the gear teeth of the third planetary gear 26 just mesh with the tooth gap of the second inner gear ring 21. As a result, the actual meshing tooth gap of the second planetary gear set can be made smaller, and the transmission between teeth is smoother. In addition, the actual meshing tooth gap of the second planetary gear set is smaller, which can make the output accuracy of the planetary gear reducer higher (due to the existence of the meshing gap, the output end will have a certain free rotation angle, that is, it will rotate freely between the two positions of the gear teeth on both sides of the tooth gap. In the case of a large tooth gap, the output end may have a large free rotation angle, and the reduction of the tooth gap directly reduces this free rotation angle), which is conducive to the precise control of the rotation angle of the output end of the planetary gear reducer. In addition, in the second planetary gear set, the third planetary gears 26 are added, which are equal in number to the second planetary gears 24, so that the second planetary gear set can run more smoothly, withstand a larger torque, and adapt to the characteristics of large torque output at the output end.

[0054] It should be understood that, in the planetary gear reducer provided by the present invention, appropriate bearings can be provided at each rotationally mating joint to provide axial support to ensure smooth rotation.

[0055] The specific implementation methods described above further illustrate the purpose, technical solutions and beneficial effects of the utility model in detail. It should be understood that the above description is only the specific implementation method of the utility model and is not used to limit the protection scope of the utility model. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the utility model should be included in the protection scope of the utility model.

Claims

1. A planetary gear set reducer, comprising a first planetary gear set and a second planetary gear set, characterized in that: The first planetary gear set comprises a first inner gear ring (11), a first sun gear (12), a first planet carrier (13) and a plurality of first planetary gears (14); the first inner gear ring (11) is fixedly mounted on a bracket (3), and a main shaft (4) coaxial with the first inner gear ring (11) is rotatably arranged on the bracket (3); the first sun gear (12) is fixedly mounted on the main shaft (4), and the first planetary carrier (13) is rotatably mounted on the main shaft (4); for any of the first planetary gears (14), the first planetary gear (14) is rotatably mounted on the first planet carrier (13) and meshes with both the first sun gear (12) and the first inner gear ring (11); The second planetary gear set comprises a second inner ring gear (21), a second sun gear (22), a second planet carrier (23) and a plurality of second planetary gears (24); the second inner ring gear (21) is rotatably mounted on the bracket (3); the second sun gear (22) is fixedly mounted on the main shaft (4); the second planet carrier (23) is rotatably mounted on the main shaft (4) and is fixedly connected to the first planet carrier (13); for any second planetary gear (24), the second planetary gear (24) is rotatably mounted on the second planet carrier (23) and is meshed with the second sun gear (22) and the second inner ring gear (21).

2. A planetary gear reducer according to claim 1, characterized in that: The main shaft (4) is a hollow shaft, and a shaft (5) is inserted into the main shaft (4); one end of the shaft (5) is fixedly connected to the second inner gear ring (21), and the other end extends to a side of the first inner gear ring (11) away from the second inner gear ring (21).

3. A planetary gear reducer according to claim 2, characterized in that: The second inner gear ring (21) and the shaft rod (5) are both used for connecting to a load end.

4. A planetary gear reducer according to claim 2, characterized in that: A monitoring reference object (51) is provided on one end of the shaft (5) away from the second inner gear ring (21), and a sensor (31) is provided on the bracket (3). The sensor (31) is used to monitor the rotation parameters of the shaft (5), wherein the rotation parameters include the rotation angle, the rotation speed, and the rotation angular acceleration.

5. A planetary gear reducer according to claim 4, characterized in that: The monitoring reference object (51) is a magnet, and the sensor (31) is a magnetic encoding angle sensor (31).

6. A planetary gear reducer according to claim 1, characterized in that: The second planetary gear set further includes a third sun gear (25) and a plurality of third planetary gears (26); The third sun gear (25) is fixedly mounted on the main shaft (4) and is located on a side of the second sun gear (22) away from the first sun gear (12); the number of teeth and the module of the third sun gear (25) and the second sun gear (22) are equal, and the projections of the third sun gear (25) and the second sun gear (22) along the axis of the main shaft (4) do not overlap; For any of the third planetary gears (26), the third planetary gear (26) is rotatably mounted on the second planet carrier (23) and meshes with both the third sun gear (25) and the second inner gear ring (21).

7. A planetary gear reducer according to claim 6, characterized in that: The number of the second planetary gears (24) and the number of the third planetary gears (26) are equal, and the second planetary gears (24) and the third planetary gears (26) are arranged in a staggered manner.

8. The planetary gear reducer according to claim 1, characterized in that: The number of the first planetary gears (14) is 3 to 6, and the number of the second planetary gears (24) is 3 to 6.

9. A planetary gear reducer according to claim 8, characterized in that: The number of the first planetary gears (14) is three, and the circumferences thereof are evenly distributed on the first planet carrier (13); the number of the second planetary gears (24) is three, and the circumferences thereof are evenly distributed on the second planet carrier (23); The first planetary gears (14) and the second planetary gears (24) are arranged alternately.

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