US20190389047A1

US20190389047A1 - Transmission system and electric tool provided with same

Info

Publication number: US20190389047A1
Application number: US16/114,250
Authority: US
Inventors: Lei Weng; Zhi Ouyang
Original assignee: ZHEJIANG MINGLEI TOOLS INDUSTRY Co Ltd
Current assignee: ZHEJIANG MINGLEI TOOLS INDUSTRY Co Ltd
Priority date: 2018-06-21
Filing date: 2018-08-28
Publication date: 2019-12-26
Also published as: US10513024B1; CN108626326B; CN108626326A

Abstract

Provided herein is a transmission system including: a housing; a pair of double-layered gears for transmitting a power from a motor to a screw driver shaft defining a transmission axis, the pair of gears including a first bigger gear and a first smaller gear; a torque cup for covering a leading end portion of the housing through which the shaft is extended from the housing; a first set of planetary gears for driving the shaft, regularly surrounding and meshing the first smaller gear; and a first ring gear for sliding axially against the housing and meshing the first bigger gear and/or the first set of planetary gears; wherein the torque cup can be turned to force the first ring gear to slide axially.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of Chinese Patent Application No. 201810646354.1, filed on Jun. 21, 2018 in the State Intellectual Property Office of China, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to an electric tool, in particular to a transmission system and an electric tool provided with same.

BACKGROUND ART

Transmission system is one of the main core components of an electric tool, and its main function is to provide the power needed for the electric tool to rotate. Existing electric tools all have multiple functions for the convenience of use, that is, they can be used for drilling and screwing at the same time.
In order to realize the above-mentioned operation functions, enterprises also make some structural improvements to the transmission system during transmission system production, i.e. the transmission system is equipped with a speed change mechanism and a torque cup at the same time, so as to switch between various functions.
For example, the Chinese patent application number 201120271287.3 discloses an improved structure for high-speed and low-speed switch of an electric tool, and a shift lever of the improved structure is driven by a shift frame which is directly exposed outside a transmission system housing for operation by workers. In this way, workers are likely to come into contact with the shift frame unintentionally during operation, thus increasing the probability of misoperation. When the electric tool works under the condition of large torque, if the rotating speed increases instantaneously, it is easy to cause damage to a transmission shaft.

SUMMARY OF THE INVENTION

Therefore, the invention provides A transmission system, comprising:

- a housing;
- a pair of double-layered gears for transmitting a power from a motor to a mounting shaft for clasping a screw driver bit or a drill bit and defining a transmission axis, the pair of double-layer gear comprising a first outer or bigger gear and a first inner or smaller gear;
- a torque cup mounted to the housing to cover an leading end portion of the housing through which the mounting shaft is extended partially outside the housing ;
- a first set of planetary gears for driving the mounting shaft, circumferentially regularly surrounding and meshing the first inner gear,
- a first ring gear fixed circumferentially and relatively to the housing so that it can slide axially to mesh both of the first outer gear and the first set of planetary gears or only the former;
- wherein the torque cup is configured so that it can be turned to force the first ring gear to slide axially.

Preferably, the first ring gear is surrounded with a gear sleeve, and the gear sleeve and the first ring gear are axially locked; and the torque cup can be turned to force the gear sleeve to slide axially.
Preferably, the transmission system further comprises:

- a sun gear ratatable around the transmission axis and having an axial end fixed with a plurality of axial stubs each of which co-axially supports one of the first set of planetary gears for rotation around itself; and
- a second set of planetary gears for driving the mounting shaft, meshing inwards with the sun gear and outwards with a second ring gear, which is mounted to the housing for a rotation restrained with respect to the housing.

Most preferably, the second ring gear is provided with an axial and endless profile or cam which is urged against the housing.
The invention relates also to an electric tool comprising such a transmission system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of a transmission system according to embodiment 1;

FIG. 2 is a first explosive view of the transmission system according to embodiment 1;

FIG. 3 is a second explosive view of the transmission system according to embodiment 1;

FIG. 4 is a third explosive view of the transmission system according to embodiment 1;

FIG. 5 is a partial schematic structural diagram of the transmission system according to embodiment 1;

FIG. 6 is a fourth explosive view of the transmission system according to embodiment 1;

FIG. 7 is a fifth explosive view of the transmission system according to embodiment 1;

FIG. 8 is a sixth explosive view of the transmission system according to embodiment 1;

FIG. 9 is a seventh explosive view of the transmission system according to embodiment 1;

FIG. 10 is an enlarged view of a part A of FIG. 9;

FIG. 11 is a structural view of a torque cup and a lifting pin of embodiment 2;

FIG. 12 is a structural view of a torque cup and a lifting pin of embodiment 3;

FIG. 13 is a partial schematic structural diagram of a transmission system according to embodiment 4;

FIG. 14 is a sectional view of the transmission system according to embodiment 4.

Reference Numerals: 1. housing; 1 a. front housing body; 1 a 1. guide hole; 1 a 2. second limiting block; 1 b. rear housing body; 1 b 1. first blind hole; 1 c. bottom cover; 1 c 1. through hole; 1 d. front cover; 1 d 1. limiting hole; 2. second planetary gear set; 3. second inner gear; 4. double-layer gear; 4 a. first outer ring gear; 4 b. first inner ring gear; 5. first planetary gear set; 6. first inner gear; 7. gear sleeve; 7 a. fixing block; 7 b. lifting pin; 7 b 1: spherical surface; 7 b 2: tapered; 7 c. biasing spring; 8 a. lug; 8 b. sliding groove; 9. torque cup; 9 a. inner edge; 9 b. protrusion; 9 b 1. concave cambered surface; 9 b 2. tapered groove; 9 c. steel ball; 9 d. second extension spring; 9 e. second blind hole; 10. sun gear; 10 a. disc; 10 b. second inner ring gear; 11. third planetary gear set; 12. third inner gear; 12 a. projection; 13. transmission ring; 13 a. first limiting block; 14. mounting shaft; 15. shaft sleeve; 15 a. clamping block; 16. thread bushing; 16 a. first spring; 17. torque washer ; 17 a. teeth; 17 b. lifting rod; 17 c. ball.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in further detail below with reference to the accompanying drawings.

Embodiment 1

A transmission system, as shown in FIG. 1 and FIG. 2, comprises a housing 1 which is divided into a front housing body 1 a and a rear housing body 1 b, the rear portion of the rear housing body 1 b is provided with a bottom cover 1 c, and the center of the bottom cover 1 c is provided with a through hole 1 c 1 through which an output shaft of a motor can pass. The rear housing body 1 b is internally provided with a second planetary gear set 2 which is attached to the bottom cover 1 c, and planetary gears of the second planetary gear set 2 are uniformly distributed around the through hole 1 c 1 and can mesh with the output shaft of the motor. The number of the planetary gears in the second planetary gear set 2 can be selected according to the actual situation, and here it is three. In addition, the second planetary gear set 2 is further sleeved with a second inner gear 3, and the second inner gear 3 is fixed to the rear housing body 1 b.
Furthermore, as shown in FIG. 3, the rear housing body 1 b is further provided with a double-layer gear 4 inside. Shafts of the planetary gears of the second planetary gear set 2 are fixed to a first outer ring gear 4 a of the double-layer gear 4 and are also uniformly distributed around the center of the double-layer gear 4. While a first inner ring gear 4 b of the double-layer gear 4 is located on the side, away from the second planetary gear set 2, of the first outer ring gear 4 a. Here, the center of the double-layer gear 4 is coaxial with the through hole 1 c 1.
Besides, as shown in FIG. 4, a first planetary gear set 5 is evenly distributed around the first inner ring gear 14 b, and the first planetary gear set 5 can indirectly drive a mounting shaft 14 for mounting a drill bit or a screwdriver. Also, each planet gear of the first planetary gear set 5 meshes with the first inner ring gear 4 b. At the same time, the first planetary gear set 5 and the first outer ring gear 4 a are sleeved with a same first inner gear 6, and the first inner gear 6 can slide between the first planetary gear set 5 and the first outer ring gear 4 a.
In addition, as shown in FIG. 4 and FIG. 5, there is a locking structure between the outer peripheral surface of the first inner gear 6 and the inner side of the front housing body 1 a, and the locking structure mainly comprises a plurality of lugs 8 a evenly distributed on the peripheral surface of the first inner gear 6 and sliding groove 8 b located on the inner side of the front housing body 1 a and matched with the lugs 8 a. When the first inner gear 6 is completely sleeved outside the first planetary gear set 5, the lugs 8 a can be inserted into the sliding groove 8 b, thus realizing the locking between the first inner gear 6 and the housing 1. When the first inner gear 6 is sleeved on the first planetary gear set 5 and the first outer ring gear 4 a at the same time, the lugs 8 a disengage from the sliding groove 8 b, thus realizing the unlocking between the first inner gear 6 and the front housing body 1 a. In addition, the first inner gear 6 is also sleeved with a gear sleeve 7. Here, the first inner gear 6 and the gear sleeve 7 are fixed in the axial direction, while the first inner gear 6 can rotate in the circumferential direction. When the first inner gear 6 is completely sleeved on the first planetary gear set 5, the first planetary gear set 5 rotates on its axis and revolves around the first inner ring gear 4 b, at this point, the revolution angular velocity of the first planetary gear set 5 is smaller than the rotation angular velocity of the double-layer gear 4. When the first inner gear 6 is sleeved outside the first planetary gear set 5 and the first outer ring gear 4 a at the same time, the first inner gear 6, the double-layer gear 4 and the first planetary gear set 5 are locked, at this point, the first planetary gear set 5 only revolves. At this point, the revolution angular velocity of the first planetary gear set 5 is equal to the rotation angular velocity of the double-layer gear 4. In this way, speed change of the transmission system can be realized by the sliding switch of the first inner gear 6 between the first planetary gear set 5 and the first outer ring gear 4 a.
Meanwhile, as shown in FIG. 6, the outer peripheral surface of the gear sleeve 7 is provided with fixing blocks 7 a, and the number of the fixing blocks 7 a can be determined according to the actual situation. Here, the number of the fixing blocks 7 a is two, and the fixing blocks 7 a are evenly distributed around the center of the gear sleeve 7. Each fixing block 7 a is provided with a lifting pin 7 b, and the lifting pin 7 b, the fixing block 7 a and the gear sleeve 7 are integrally formed. In addition, the lifting pins 7 b are arranged parallel to the axial direction of the gear sleeve 7. The end, facing the bottom cover 1 c, of each lifting pin 7 b is further sleeved with a biasing spring 7 c, and one end of each biasing spring 7 c abuts against the corresponding fixing block 7 a. In addition, the outer peripheral surface of the rear housing body 1 b is provided with first blind holes 1 b 1, and the other end of each biasing spring 7 c abuts against the bottom of the corresponding first blind hole 1 b 1. In this way, on the one hand, the biasing springs 7 c can support the lifting pins 7 b, which is conducive to keeping the stress on the lifting pins 7 b in balance, on the other hand, the first blind holes 1 b 1 can also limit the biasing springs 7 c, thus reducing the damage caused by the torsion of the biasing springs 7 c during stretching. Moreover, the outer peripheral surface of the front housing body 1 a is also provided with guide holes 1 a 1, and the lifting pins 7 b are inserted into the guide holes 1 a 1 and can slide in the guide holes 1 a 1. Thus, the guide holes 1 a 1 can protect the lifting pins 7 b and guide the lifting pins 7 b to slide in an accurate direction.
Furthermore, the side, away from the bottom cover 1 c, of the front housing body 1 a is also provided with a torque cup 9, and the inner peripheral surface of the torque cup 9 is provided with an inner edge 9 a. The inner edge 9 a is provided with protrusions 9 b corresponding to the lifting pins 7 b, and the transition between the protrusions 9 b and the inner edge 9 a is continuous and natural. When the torque cup 9 is rotated till the protrusions 9 b come into contact with the ends of the lifting pins 7 b, the protrusions 9 b start to push the lifting pins 7 b by continuing to rotate the torque cup 9. Accordingly, the lifting pins 7 b slide toward the side of the bottom cover 1 c and drive the first inner gear 6 to slide to the position where the first inner gear 6 is simultaneously sleeved on the first outer ring gear 4 a and the first planetary gear set 5. This also realizes the process of controlling the speed increase of the transmission system through the torque cup 9. Furthermore, when the torque cup 9 is rotated reversely, the acting force of the protrusions 9 b on the lifting pins 7 b will be removed, so that the lifting pins 7 b can reset under the action of the biasing springs 7 c, thus realizing the speed decrease of the transmission system.
In addition, as shown in FIG. 7, the front housing body 1 a is internally provided with a sun gear 10 which is composed of a disk 10 a and a second inner ring gear 10 b coaxial with the disk 10 a. Here, shafts of the planetary gears of the first planetary gear set 5 are all arranged on the side, away from the second inner ring gear 10 b, of the disc 10 a in a inserting mode, and the planetary gears are evenly distributed around the center of the sun gear 10. A third planetary gear set 11 is evenly distributed around the second inner ring gear 10 b, and each planetary gear of the third planetary gear set 11 meshes with the second inner ring gear 10 b. In addition, the third planetary gear set 11 is sleeved with a third inner gear 12 which meshes with each planetary gear of the third planetary gear set 11.
As shown in FIG. 8, shafts of the planetary gears of the third planetary gear set 11 are uniformly distributed around the center of a transmission ring 13 in the circumferential direction and are arranged on the transmission ring 13 in a penetrating mode, and the transmission ring 13 is coaxial with the through hole 1 c 1. First limiting blocks 13 a are evenly distributed on the side, away from the third planetary gear set 11, of the transmission ring 13. The end of the mounting shaft 14 is provided with a shaft sleeve 15, the mounting shaft 14 and the shaft sleeve 15 are locked in the circumferential direction, that is, the end of the mounting shaft is hexagonal, the inner ring of the shaft sleeve is also hexagonal, and the two are matched with each other. The circumferential surface of the shaft sleeve 15 is provided with clamping blocks 15 a matched with the first limiting blocks 13 a, and the clamping blocks 15 a can be embedded between two adjacent first limiting blocks 13 a, so that the transmission ring 13, the sleeve 15 and the mounting shaft 14 are relatively stationary.
As shown in FIG. 9, the mounting shaft 14 extends into the front housing body 1 a and is fixed in the front housing body 1 a through two bearings. In addition, the front housing body 1 a is sleeved with a thread bushing 16 on the outer peripheral surface where the mounting shaft 14 is located. Here, the outer side of the thread bushing 16 is matched with the inner side of the torque cup 9 through threads. The front housing body 1 a is further sleeved with a torque washer 17 at the side end, facing the bottom cover 1 c, of the thread bushing 16. Here, the inner ring of the torque washer 17 is provided with teeth 17 a, and the front housing body 1 a is provided with second limiting blocks 1 a 2 on the inner side of the thread bushing 16. The second limiting blocks 1 a 2 are evenly distributed along the axial center of the front housing body 1 a, and the second limiting blocks 1 a 2 are arranged along the axial direction of the mounting shaft 14. The second limiting blocks 1 a 2 can just pass through the space between two adjacent teeth 17 a. At the same time, lifting rods 17 b are evenly distributed in the circumferential direction on the side, facing the bottom cover 1 c, of the torque washer 17, one end of each lifting rod 17 b is fixed to the torque washer 17, and the other end of each lifting rod 17 b abuts against the surface of the third inner gear 12. In addition, the lifting rods 17 b extends into the front housing body 1 a.
In addition, as shown in FIG. 9 and FIG. 10, first extension springs 16 a are arranged between the inner side of the thread bushing 16 and the outer side of the front housing body 1 a. One end of each first extension spring 16 a abuts against the bottom of the thread bushing 16, and the other end abuts against the torque washer 17. The side, away from the thread bushing 16, of the bottom of the torque cup 9 is provided with a front cover 1 d which is fixed to the front housing body 1 a through a bolt. In addition, a plurality of limiting holes 1 d 1 are distributed around the center of the front cover 1 d, and the number of the limiting holes 1 d 1 can be set according to the actual situation. The side, facing the front cover 1 d, of the torque cup 9 is provided with steel balls 9 c matched with the limiting holes 1 d 1. The number of the steel balls 9 c can be determined according to the actual situation, and the steel balls 9 c are symmetrical about the axial center of the torque cup 9. In addition, the steel balls 9 c abut against the bottom of the torque cup 9 through second extension springs 9 d, and the bottom of the torque cup 9 is provided with second blind holes 9 e. Each second extension spring 9 d abuts against the bottom of the corresponding second blind hole 9 e. Through the interference fit between the steel balls 9 c and the different limiting holes 1 d 1, workers can select different gear positions conveniently.
Working principle of the transmission system: based on low-speed operation, the output shaft of the motor first drives the second planetary gear set 2 through gears to rotate on its axis and revolve at the same time. Since the shafts of the planetary gears of the second planetary gear set 2 are fixed to a first outer ring gear 4 a of the double-layer gear 4, the second planetary gear set 2 will drive the double-layer gear 4 to rotate on its axis. When the double-layer gear 4 rotates on its axis, the first inner ring gear 4 b will drive the first planetary gear set 5 to rotate on its axis and revolve. Since the first planetary gear set 5 is arranged on the sun gear 10 in a penetrating mode, the first planetary gear set 15 will drive the sun gear 10 to rotate on its axis. The second inner ring gear 10 b of the sun gear 10 will drive the third planetary gear set 11 to rotate on its axis and revolve. Since the third planetary gear set 11 is arranged on the transmission ring 13 in a penetrating mode, the transmission ring 13 will be driven to rotate on its axis, so as to drive the mounting shaft 14 to rotate on its axis through the shaft sleeve 15. Due to the low-speed state, a screwdriver bit can be installed at the end of the mounting shaft 14 to screw bolts or screws.
Torque adjustment: in order to increase the torque of screwing, workers can screw the torque cup 9, and the thread bushing 16 will move toward the side of the bottom cover 1 c under the action of the screw of the torque cup 9, so that the thread bushing 16 will compress the first extension springs 16 a. Then the first extension springs 16 a exert pressure on the torque washer 17, and the torque washer 17 exerts pressure on third inner gear 12 through the lifting rods 17 b. When the first extension springs 16 a are pressed more tightly, the third inner gear 12 will be more fixed firmly, and thus the torque will be greater.
Speed increasing process of the transmission system: when the torque cup 9 is reversely screwed, the pressure of the thread bushing 16 putting on the first extension springs 16 a will decrease, and the protrusions 9 b on the inner edge 9 a will gradually make contact with the ends of the lifting pins 7 b; when the protrusions 9 b push the lifting pins 7 b, the torque of the transmission system is the smallest, at this point, the lifting pins 7 b will push the first inner gear 6 to move till the first inner gear 6 is directly sleeved on the first planetary gear set and the first outer ring gear 4 a, so that the first planetary gear set, the first outer ring gear 4 a and the first inner gear 6 are locked. At this point, the rotation angular velocity of the double-layer gear 4 is equal to the rotation angular velocity of the sun gear 10, thus realizing the function of increasing speed. In this way, in the case of small torque and high rotating speed, drilling can be conducted as long as a drill bit is installed at the end of the mounting shaft 14.
The matching process of the steel balls 9 c and the different limiting holes 1 d 1 allows positioning workers to adjust to the required positions, and can also avoid the out-of-gear event.

Embodiment 2

A transmission system, as shown in FIG. 11, is based on embodiment 1, the side, facing the corresponding lifting pin 7 b, of each protrusion 9 b is provided with a concave cambered surface 9 b 1, and the end of each lifting pin 7 b is a spherical surface 7 b 1. Here, each concave cambered surface 9 b 1 is matched with the end of the corresponding lifting pin 7 b, so that when the protrusions 9 b push the lifting pins 7 b, the concave cambered surfaces 9 b 1 restrain the lifting pins 7 b, thus effectively ensuring the restriction of the concave cambered surfaces 9 b 1 to the lifting pins 7 b and avoiding the out-of-gear event during high-speed output of the transmission system.

Embodiment 3

A transmission system, as shown in FIG. 12, differs from embodiment 2 only in that the side, facing the corresponding lifting pin 7 b, of each protrusion 9 b is provided with a tapered groove 9 b 2, and the end of each lifting pin 7 b is tapered 7 b 2. In this way, each tapered groove 9 b 2 can be matched with the end of the corresponding lifting pin 7 b, so that on the one hand, the lifting pins 7 b can smoothly slide into the tapered grooves 9 b 2, and on the other hand, the tapered grooves 9 b 2 can limit the lifting pins 7 b, thus ensuring that the lifting pins 7 b and the protrusions 9 b can remain stable during high-speed output of the transmission system.

Embodiment 4

A transmission system, as shown in FIG. 13 and FIG. 14, is based on embodiment 1, projection 12 a are evenly distributed on the side, facing the lifting rods 17 b, of the third inner gear 12 along the axial center of the third inner gear, the end of each lifting rod 17 b contacts with a ball 17 c, and the balls 17 c are limited in the front housing body 1 a. When the resistance on the mounting shaft 14 is greater than the torque of the transmission system, the transmission ring 13 will be locked, that is, it will not rotate on its axis. However, the third planetary gear set 11 located on the transmission ring 13 will still rotate on its axis under the action of the second inner ring gear 10 b of the sun gear 10 and drive the third inner gear 12 sleeved outside to rotate. When the projection 12 a pass through the balls 17 c, the lifting rods 17 b push up the torque washer 17 so that the third inner gear 12 can rotate, thus realizing the function of shaft disengagement.
In addition, here, the number of the limiting holes 1 d 1 of the front cover 1 d is twelve, the limiting holes are equally divided into two groups, the two groups of limiting holes 1 d 1 are symmetrical about the center of the front cover 1 d, the number of the steel balls 9 c is two, and the steel balls are symmetrical about the center of the torque cup 9. The two groups of limiting holes 1 d 1 are equally divided into an equal number of low-speed gears and an equal number of high-speed gears, and there are four low-speed gears and two high-speed gears. Meanwhile, the limiting holes 1 d 1 of the low-speed gears are located on the front cover 1 d corresponding to the middle position of two adjacent teeth 17 a, while the limiting holes 1 d 1 of the high-speed gears are not located on the front cover 1 d corresponding to the middle position of two adjacent teeth 17 a.
When the transmission system is in a low gear, that is, when the steel balls 9 c are engaged with the limiting holes 1 d 1 of the low-speed gears, then the teeth 17 a is corresponding to the gap between the limiting blocks 1 a 2. If the mounting shaft 14 is locked due to excessive resistance, the teeth 17 a of the torque washer 17 can smoothly slide between two second limiting blocks 1 a 2 while the projection 12 a push up the lifting rods 17 b, thus normally realizing the process of shaft disengagement for avoiding danger. When the transmission system is in a high gear, the teeth 17 a are blocked by the second limiting blocks 1 a 2, which prevents the torque washer 17 from being pushed up by the projection 12 a, however, the resistance is not very large during this process, so that shaft disengagement does not occur, and thus the drilling process can proceed normally.

Embodiment 5

An electric tool is disclosed. A transmission system for the electric tool is a transmission system recorded in any one of the embodiments 1 to 4 of the invention.
The specific embodiments are only an explanation of the invention, but not a limitation to the invention. After reading this specification, a person skilled in the art can make modifications to the embodiments without creative contribution as needed, however, as long as they are within the scope of the claims of the invention, they are protected by the patent law.

Claims

1. A transmission system, comprising:

a housing (1);

a double-layered gears (4) for transmitting a power from a motor to a mounting shaft (14) for clasping a screw driver bit or a drill bit and defining a transmission axis, the double-layered gears (4) comprising a first outer or bigger gear (4 a) and a first inner or smaller gear (4 b);

a torque cup (9) mounted to the housing to cover an leading end portion of the housing through which the mounting shaft is extended partially outside the housing ;

a first set of planetary gears (5) for driving the mounting shaft, circumferentially regularly surrounding and meshing the first inner gear (4 b),

a first ring gear (6) fixed circumferentially and relatively to the housing so that it can slide axially to mesh both of the first outer gear (4 a) and the first set of planetary gears (5) or only the former;

wherein the torque cup (9) is configured so that it can be turned to force the first ring gear (6) to slide axially.

2. The transmission system according to claim 1, wherein the first ring gear (6) is surrounded with a gear sleeve (7), and the gear sleeve (7) and the first ring gear (6) are axially locked; and the torque cup can be turned to force the gear sleeve (7) to slide axially.

3. The transmission system according to claim 1, further comprising:

a sun gear (10) rotatable around the transmission axis and having an axial end fixed with a plurality of axial stubs each of which co-axially supports one of the first set of planetary gears (5) for rotation around itself; and

a second set of planetary gears (11) for driving the mounting shaft (14), meshing inwards with the sun gear (10) and outwards with a second ring gear (12), which is mounted to the housing for a rotation restrained with respect to the housing.

4. The transmission system according to claim 2, further comprising:

5. The transmission system according to claim 4, wherein the second ring gear is provided with an axial and endless profile or cam which is urged against the housing.

6. An electric tool comprising a transmission system according to claim 1.