A damper structure for a transmission
Patent Information
- Application Number
- CN202522631002.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-11
- Publication Date
- 2026-10-23
- Estimated Expiration
- 2035-12-11
Smart Images
Figure CN224882173U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of bicycle transmission technology, and in particular to a damping structure for a transmission. Background Technology
[0002] A Chinese patent with publication number TWI690452B discloses a bicycle rear derailleur, including a base member, a movable member, a chain guide assembly, a biasing device, and a damping device. The base member is mounted on a bicycle frame, the movable member is movably coupled to the base member, the movable member has an inner annular surface, the chain guide assembly is rotatably connected to the movable member, and the biasing device is configured to bias the chain guide assembly relative to the movable member in a first rotational direction. A damping device is disposed between the chain guide assembly and the movable member. The damping device is operable to apply a damping force to the chain guide assembly when the chain guide assembly rotates relative to the movable member in a second rotation direction opposite to the first rotation direction. The damping device includes a friction member having an outer annular surface that frictionally engages with the inner annular surface of the movable member. The friction member has a first end that is non-rotatably attached to the chain guide assembly and a second end that is a circumferentially free end.
[0003] However, the aforementioned bicycle rear derailleur has the following drawbacks: the damping force provided by the damping device of the bicycle rear derailleur is fixed, and the rider cannot change the magnitude of the damping force according to the road conditions. During the shifting process, the derailleur needs to overcome the damping force of the damping device, which will cause unnecessary energy loss and reduce the riding experience. It urgently needs to be improved. Utility Model Content
[0004] The purpose of this invention is to provide a damping structure for a transmission that can adapt to and adjust the damping force of the transmission according to road conditions and extend the service life of the transmission.
[0005] The above-mentioned technical objective of this utility model is achieved through the following technical solution: a damping structure for a transmission, disposed on a joint seat of a rear derailleur, wherein a chain guide assembly is connected to the joint seat via a main shaft, and an annular sleeve portion is coaxially connected to the end of the main shaft; a braking friction component is connected to the joint seat, the braking friction component including a fixing portion fixedly connected to the joint seat and a damping portion connected to the fixing portion; the damping portion is rotatably disposed within the annular sleeve portion, and the outer wall of the damping portion contacts and engages with the inner wall of the annular sleeve portion; A gear shifting sleeve is rotatably provided on the joint seat. The damping part is located between the gear shifting sleeve and the annular sleeve. The damping part is provided with a compression structure. Rotating the gear shifting sleeve drives the compression structure to push the damping part and the annular sleeve to compress and engage, thereby adjusting the frictional damping force between the damping part and the annular sleeve.
[0006] By adopting the above technical solution, when riding on flat roads, the shift knob sleeve remains in a position where it does not contact the damping part. At this time, there is only slight contact between the damping part on the brake friction component and the inner wall of the annular sleeve. The damping force between the two is small. The slight undulations generated during riding will cause the chain guide assembly to have a slight tendency to rotate in the direction of slack chain. By utilizing the small damping force generated between the damping part and the annular sleeve, the chain guide assembly can be prevented from driving the main shaft to rotate rapidly in the direction of slack chain through the one-way bearing, thereby preventing the chain from falling off due to excessive vibration. When riding on bumpy roads, by rotating the shift knob sleeve to a position where the compression structure is pressed against the damping part, the shift knob sleeve will exert a compression force on the damping part towards the annular sleeve, thereby damping... The outer wall of the brake damping part presses against the inner wall of the annular sleeve part, thereby increasing the frictional damping force between the brake damping part and the annular sleeve part. When the bicycle body experiences severe undulations during riding on bumpy roads, the chain guide assembly rotates in the direction of the slack chain, which drives the spindle to rotate. At this time, a large damping force is generated between the inner wall of the annular sleeve part at the end of the spindle and the outer wall of the damping part of the brake friction component. This is to prevent the chain guide assembly from rotating too much per unit time, which could cause the chain to fall off. The rider can adjust the damping force according to the road conditions, thereby avoiding the rider's energy being wasted during gear shifting when the damping structure provides a large damping force when it is not necessary. It can also cause wear on the damping surface. It has the effect of adapting the damping force of the gearbox to the road conditions and extending the service life of the gearbox.
[0007] A further feature of this invention is that the extrusion mechanism includes a plurality of protruding ribs on the outer wall of the gear shift sleeve, and the side wall of the damping part has a pressing surface corresponding to the protruding ribs. When the gear shift sleeve rotates to the point where the protruding ribs abut against the pressing surface, the protruding ribs push the damping part against the inner wall of the annular sleeve part through the pressing surface, thereby increasing the frictional damping force between the outer wall of the damping part and the inner wall of the annular sleeve part.
[0008] By adopting the above technical solution, rotating the gear screw sleeve causes the corresponding convex rib to press against the pressing surface of the damping part, pushing the damping part to fit tightly against the side wall of the annular sleeve part, thereby increasing the frictional damping force between the damping part and the annular sleeve part.
[0009] A further feature of this invention is that the rib extends along the axial direction of the gear shift sleeve, and a plurality of the ribs are disposed opposite to each other on the outer wall of the gear shift sleeve.
[0010] By adopting the above technical solution, the ribs that extend axially along the gear shift sleeve can increase the contact area between the ribs and the inner wall of the damping part, thereby making the ribs exert a more uniform extrusion force on the damping part, and thus improving the frictional damping effect between the outer wall of the damping part and the inner wall of the annular sleeve part.
[0011] A further feature of this invention is that the outer wall of the rib has an arc structure, and the center of the arc of each rib is located on the outer arc edge of the gear shift sleeve, and the arc radius of adjacent ribs gradually increases or decreases along the same rotation direction of the gear shift sleeve.
[0012] By adopting the above technical solution, rotating the gear shift knob sleeve causes the ribs of corresponding radius on the side wall of the gear shift knob sleeve to abut against the mating surface of the damping part, thereby pushing the damping part to move a corresponding distance away from the gear shift knob sleeve, thus adjusting the damping force of the damping part on the annular sleeve part. The radius of the ribs is set to gradually change along the rotation direction of the gear shift knob sleeve, making it convenient for the rider to adjust the damping force step by step.
[0013] A further feature of this invention is that the damping portions are symmetrically arranged on both sides of the fixing portion, and a gap is formed at the end of the damping portions on both sides away from the fixing portion.
[0014] By adopting the above technical solution, a gap is formed at the end of the damping part on both sides that is far from the fixed part. This is beneficial for the damping parts on both sides to be pushed and relatively opened by the convex ribs of the stop screw sleeve. At the same time, the symmetrically distributed damping parts are also beneficial for the uniformity of the extrusion pressure of the damping parts on the inner wall of the annular sleeve.
[0015] A further feature of this invention is that a drive handle is fixedly connected to the gear shifting sleeve by fasteners, an arrow mark is provided on the drive handle along its length, and a plurality of gear marks are provided on the joint seat. When the drive handle drives the gear shifting sleeve to rotate to a predetermined gear, the arrow mark aligns with the corresponding gear mark.
[0016] By adopting the above technical solution, the gear shift knob can be easily rotated using the drive handle to conveniently adjust the corresponding damping level. At the same time, when the arrow mark is aligned with the gear mark, the rider can easily and accurately understand the damping level of the gear shift knob.
[0017] A further feature of this invention is that: the gear shift sleeve is provided with a rotating cover by an anti-rotation structure, the joint seat is provided with a rotating hole corresponding to the rotating cover, the rotating cover is rotatably mounted on the joint seat through the rotating hole, and the rotating cover is provided with an annular boss, the annular boss being embedded and engaged with the rotating hole.
[0018] By adopting the above technical solution, the rotation of the rotating cover and the rotating hole can be matched to improve the rotational smoothness of the gear shifting sleeve, while the annular boss can improve the sealing performance of the rotating cover when it rotates relative to the rotating hole.
[0019] A further feature of this invention is that the anti-rotation structure includes an anti-rotation connecting part and an anti-rotation hole. The anti-rotation connecting part is disposed on the gear shift sleeve, and the anti-rotation hole is opened on the rotating cover. The anti-rotation connecting part and the anti-rotation hole cooperate to prevent rotation.
[0020] A further feature of this invention is that a rotating cover is connected to the gear shift knob to prevent rotation, and a positioning structure is provided between the joint seat and the rotating cover. When the rotating cover rotates relative to the joint seat to the corresponding gear position, the rotating cover is positioned and engaged with the joint seat through the positioning structure.
[0021] By adopting the above technical solution, when the gear shift sleeve is adjusted to the corresponding damping gear, the positioning structure can prevent the gear shift sleeve from rotating unexpectedly.
[0022] A further feature of this invention is that the positioning structure includes a positioning post and a positioning groove. The positioning groove is formed on the side wall of the rotating cover. The positioning post is slidably disposed on the joint seat. A spring is provided between the positioning post and the joint seat. The spring has a tendency to push the positioning post to lock and position itself in the corresponding positioning groove.
[0023] By adopting the above technical solution, the positioning effect of the positioning pin and the positioning groove can be used to improve the feel of the damping gear adjustment of the gear shift sleeve, while effectively preventing the gear shift sleeve from rotating accidentally.
[0024] In summary, this utility model has the following beneficial effects: A damping structure is installed on the joint seat of the rear derailleur. A chain guide assembly is connected to the joint seat via a main shaft. An annular sleeve is coaxially connected to the end of the shaft. A braking friction element, including a damping part rotatably housed within the annular sleeve, is connected to the joint seat. A shift knob is rotatably mounted on the joint seat, with several ribs on its outer wall. The side wall of the damping part has a contact surface. When riding on a flat road, the rider rotates the shift knob to a position where the ribs and contact surface abut against each other. At this position, there is only slight contact between the damping part of the braking friction element and the inner wall of the annular sleeve, resulting in a small damping force. Slight undulations during riding cause the chain guide assembly to slightly rotate in the direction of a slack chain. This small damping force between the damping part and the annular sleeve prevents the chain guide assembly from rapidly rotating the main shaft in the direction of a slack chain via a one-way bearing, thus preventing the chain from falling off due to excessive vibration. When riding on bumpy roads, turning the gear shift knob to the position where the rib aligns with the abutment surface creates an outward pressure on the damping part of the brake friction component. This forces the outer wall of the damping part against the inner wall of the annular sleeve, increasing the frictional damping force between the brake damping part and the annular sleeve. During bumpy rides, the bike experiences significant undulations. When the chain guide assembly rotates towards a looser chain, it drives the spindle to rotate. At this point, a large damping force is generated between the inner wall of the annular sleeve at the spindle end and the outer wall of the damping part of the brake friction component. This prevents the chain guide assembly from rotating too much within a unit of time, which could cause the chain to slip off. Riders can adjust the damping force according to road conditions, avoiding unnecessary energy loss during shifting due to excessive damping force provided by the damping structure when it's not needed, and preventing wear on the damping surface. This design allows for adaptive adjustment of the derailleur's damping force based on road conditions, extending the derailleur's lifespan. Attached Figure Description
[0025] Figure 1 This is an overall structural diagram of the present invention.
[0026] Figure 2 This is a longitudinal sectional view of the present invention located at the damping structure position.
[0027] Figure 3 This is an exploded view of this utility model.
[0028] Figure 4 This is a structural diagram of the braking friction component of this utility model.
[0029] Figure 5 This is a structural diagram of the gear shift knob of this utility model.
[0030] In the diagram: 1. Rear derailleur; 11. Chain guide assembly; 12. One-way bearing; 13. Torsion spring; 2. Joint seat; 201. Top cover; 202. Base; 21. Gear position indicator; 22. Rotating hole; 23. Positioning pin; 231. Spring; 3. Main shaft; 31. Annular sleeve; 4. Braking friction component; 41. Fixing part; 42. Damping part; 421. Abutting surface; 5. Gear position screw sleeve; 51. Anti-rotation connection part; 52. Rib; 6. Drive handle; 61. Arrow indicator; 7. Rotating cover; 71. Annular boss; 72. Anti-rotation hole; 73. Positioning groove. Detailed Implementation
[0031] The present invention will be further described below with reference to the accompanying drawings.
[0032] A damping structure for a transmission, such as Figures 1-5 As shown, the damping structure is located on the joint seat 2 of the rear derailleur 1. The joint seat 2 includes an upper cover 201 and a base 202. A chain guide assembly 11 is connected to the joint seat 2 via a main shaft 3. An annular sleeve portion 31 is coaxially connected to the end of the main shaft 3. The main shaft 3 and the chain guide assembly 11 are connected by a one-way bearing 12. The one-way bearing 12 allows the chain guide assembly 11 to rotate freely relative to the main shaft 3 when rotating in the chain tension direction, and to rotate freely in the chain slack direction. During rotation, the main shaft 3 can be driven to rotate synchronously through the one-way bearing 12, and the damping purpose is achieved through the damping structure; the joint seat 2 is connected to a braking friction element 4, which includes a fixed part 41 fixedly connected to the joint seat 2 and a damping part 42 connected to the fixed part 41. The damping part 42 is rotatably disposed inside the annular sleeve part 31, and the outer wall of the damping part 42 contacts and cooperates with the inner wall of the annular sleeve part 31; a gear shifting sleeve 5 is rotatably provided on the joint seat 2, and the outer wall of the gear shifting sleeve 5 is provided with several The side wall of the damping part 42 is provided with a pressing surface 421 corresponding to the rib 52. The pressing surface 421 is a plane that protrudes inward, and the two sides of the plane smoothly transition to the arc edge of the inner wall of the annular sleeve part 31. When the gear screw sleeve 5 is rotated to make the rib 52 abut against the pressing surface 421, the rib 52 pushes the damping part 42 against the inner wall of the annular sleeve part 31 through the pressing surface 421, so as to increase the frictional damping force between the outer wall of the damping part 42 and the inner wall of the annular sleeve part 31. A drive handle 6 is fixedly connected to the gear shift knob 5 by fasteners. The drive handle 6 allows for easy rotation of the gear shift knob 5 to conveniently adjust the corresponding damping level. An arrow mark 61 is provided along the length of the drive handle 6, and several gear mark 21 are provided on the joint seat 2. When the drive handle 6 drives the gear shift knob 5 to rotate to the predetermined gear, the arrow mark 61 aligns with the corresponding gear mark 21, making it easy for the rider to accurately understand the damping level of the gear shift knob 5.
[0033] like Figures 2-5As shown, the ribs 52 extend axially along the gear shift sleeve 5, and several ribs 52 are arranged opposite each other on the outer wall of the gear shift sleeve 5. The ribs 52 extending axially along the gear shift sleeve 5 can increase the contact area between the ribs 52 and the inner wall of the damping part 42, thereby making the ribs 52 exert a more uniform squeezing force on the damping part 42, and thus improving the friction damping effect between the outer wall of the damping part 42 and the inner wall of the annular sleeve part 31. The outer wall of the ribs 52 has an arc structure, and the center of the arc of each rib 52 is located on the outer arc edge of the gear shift sleeve 5. The arc radius of adjacent ribs 52 gradually increases or decreases along the same rotation direction of the gear shift sleeve 5. Rotating the gear shift sleeve 5 causes the ribs 52 with corresponding radii on the side wall of the gear shift sleeve 5 to... The rib 52 abuts against the mating surface 421 of the damping part 42, thereby pushing the damping part 42 to move a corresponding distance away from the gear shift sleeve 5, thereby adjusting the damping force of the damping part 42 on the annular sleeve part 31. The radius of the rib 52, which gradually changes along the rotation direction of the gear shift sleeve 5, is set to facilitate the rider to adjust the damping force step by step. The damping parts 42 are symmetrically arranged on both sides of the fixing part 41, and a gap is formed at the end of the damping parts 42 on both sides away from the fixing part 41. This gap at the end of the damping parts 42 on both sides away from the fixing part 41 is beneficial to the damping parts 42 being pushed and relatively opened by the rib 52 of the gear shift sleeve 5. At the same time, the symmetrical distribution of the damping parts 42 is also beneficial to the uniformity of the pressure of the damping part 42 on the inner wall of the annular sleeve part 31.
[0034] like Figures 1-5As shown, the gear shift sleeve 5 has a rotating cover 7 provided with an anti-rotation structure. The joint seat 2 has a rotating hole 22 corresponding to the rotating cover 7. The rotating cover 7 is rotatably mounted on the joint seat 2 through the rotating hole 22. The rotating cover 7 has an annular boss 71, which is embedded in the rotating hole 22. The engagement of the rotating cover 7 with the rotating hole 22 can improve the rotational stability of the gear shift sleeve 5. At the same time, the annular boss 71 can improve the sealing performance of the rotating cover 7 when it rotates relative to the rotating hole 22. The anti-rotation structure includes an anti-rotation connecting part 51 and an anti-rotation hole 72. The anti-rotation connecting part 51 is provided on the gear shift sleeve 5, and the anti-rotation hole 72 is provided on the rotating cover 7. The anti-rotation connecting part 51 and the anti-rotation hole 72 are engaged to prevent rotation. A positioning structure is provided between the joint seat 2 and the rotating cover 7. When the gear shift sleeve 5 is rotated, the anti-rotation hole 72 is provided to prevent rotation. When the rotating cover 7 rotates relative to the joint seat 2 to the corresponding position, the rotating cover 7 is positioned and engaged with the joint seat 2 through the positioning structure. When the gear shifting sleeve 5 is adjusted to the corresponding damping position, the positioning structure can prevent the gear shifting sleeve 5 from rotating accidentally. The positioning structure includes a positioning post 23 and a positioning groove 73. The positioning groove 73 is opened on the side wall of the rotating cover 7. The positioning post 23 is slidably disposed on the joint seat 2, and a spring 231 is provided between the positioning post 23 and the joint seat 2. The spring 231 has a tendency to push the positioning post 23 to lock and position the corresponding positioning groove 73. By utilizing the positioning effect of the positioning post 23 and the positioning groove 73, the feel of the gear shifting sleeve 5 in adjusting the damping position can be improved, and the gear shifting sleeve 5 can be effectively prevented from rotating accidentally.
[0035] The basic working principle of this utility model is as follows: A damping structure is set on the joint seat 2 of the rear derailleur 1. At the same time, the chain guide assembly 11 is connected to the joint seat 2 through the main shaft 3. The end of the shaft is coaxially connected to an annular sleeve 31. The joint seat 2 is connected to a braking friction element 4, which includes a damping part 42. The damping part 42 is rotatably disposed in the annular sleeve 31. A gear shifting sleeve 5 is rotatably disposed on the joint seat 2. The outer wall of the gear shifting sleeve 5 is provided with several protruding ribs 52. The side wall of the damping part 42 is provided with a pressing surface 421. When the rider rides on a flat road, the gear shifting sleeve... 5. Rotate to the position where the rib 52 and the abutment surface 421 avoid each other. At this time, there is only slight contact between the damping part 42 on the brake friction component 4 and the inner wall of the annular sleeve part 31. The damping force between them is small. The slight undulations generated during the ride will cause the chain guide assembly 11 to have a slight tendency to rotate in the direction of slack chain. By utilizing the small damping force generated between the damping part 42 and the annular sleeve part 31, the chain guide assembly 11 can be prevented from driving the main shaft 3 to rotate rapidly in the direction of slack chain through the one-way bearing 12, thereby preventing the chain from vibrating. Excessive force can cause the chain to detach. When riding on bumpy roads, the rider rotates the gear shift knob 5 to a position where the rib 52 abuts against the mating surface 421. At this time, the rib 52 exerts an outward squeezing force on the damping part 42 on the brake friction component 4, thereby pressing the outer wall of the damping part 42 against the inner wall of the annular sleeve part 31, thus increasing the frictional damping force between the brake damping part 42 and the annular sleeve part 31. The vehicle body will experience severe undulations during riding on bumpy roads. When the chain guide assembly 11 rotates in the direction of loosening the chain, it will drive the main shaft 3 to rotate. A large damping force is generated between the inner wall of the annular sleeve portion 31 at the end of shaft 3 and the outer wall of the damping portion 42 of the brake friction component 4. This is to prevent the chain guide assembly 11 from rotating too much per unit time, which could cause the chain to fall off. The rider can adjust the damping force according to the road conditions, thereby avoiding the rider's energy being wasted during gear shifting when the damping structure provides a large damping force when it is not necessary. This would also cause wear on the damping surface. It has the effect of adapting the damping force of the gearbox to the road conditions and extending the service life of the gearbox.
[0036] The above description is only a preferred embodiment of the present utility model. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the claims of the present utility model patent application are included in the scope of the present utility model patent application.
Claims
1. A damping structure for a transmission, disposed on a joint seat (2) of a rear derailleur (1), wherein a chain guide assembly (11) is connected to the joint seat (2) via a main shaft (3), characterized in that: The end of the main shaft (3) is coaxially connected to an annular sleeve (31), and the joint seat (2) is connected to a braking friction element (4). The braking friction element (4) includes a fixing part (41) fixedly connected to the joint seat (2) and a damping part (42) connected to the fixing part (41). The damping part (42) is rotatably disposed inside the annular sleeve (31), and the outer wall of the damping part (42) contacts and engages with the inner wall of the annular sleeve (31). A gear shifting sleeve (5) is rotatably provided on the joint seat (2). The damping part (42) is located between the gear shifting sleeve (5) and the annular sleeve part (31). The damping part (42) is provided with a pressing structure. Rotating the gear shifting sleeve (5) drives the pressing structure to push the damping part (42) and the annular sleeve part (31) to press and cooperate, so as to adjust the frictional damping force between the damping part (42) and the annular sleeve part (31).
2. The damping structure of a transmission according to claim 1, characterized in that: The extrusion structure includes a plurality of protruding ribs (52) on the outer wall of the gear shift sleeve (5). The side wall of the damping part (42) is provided with a pressing surface (421) corresponding to the protruding ribs (52). When the gear shift sleeve (5) is rotated to the point where the protruding ribs (52) abut against the pressing surface (421), the protruding ribs (52) push the damping part (42) against the inner wall of the annular sleeve part (31) through the pressing surface (421) to increase the frictional damping force between the outer wall of the damping part (42) and the inner wall of the annular sleeve part (31).
3. The damping structure of a transmission according to claim 2, characterized in that: The ribs (52) extend along the axial direction of the gear shift sleeve (5), and a plurality of the ribs (52) are disposed opposite to each other on the outer wall of the gear shift sleeve (5).
4. The damping structure of a transmission according to claim 2, characterized in that: The outer wall of the rib (52) is an arc structure, and the center of the arc of each rib (52) is located on the outer arc edge of the gear shift sleeve (5). The arc radius of adjacent ribs (52) gradually increases or decreases along the same rotation direction of the gear shift sleeve (5).
5. The damping structure of a transmission according to claim 1, characterized in that: The damping parts (42) are symmetrically arranged on both sides of the fixing part (41), and a gap is formed at the end of the damping parts (42) on both sides away from the fixing part (41).
6. The damping structure of a transmission according to claim 1, characterized in that: A drive handle (6) is fixedly connected to the gear shifting sleeve (5) by fasteners. An arrow mark (61) is provided on the drive handle (6) along the length direction. Several gear marks (21) are provided on the joint seat (2). When the drive handle (6) drives the gear shifting sleeve (5) to rotate to the predetermined gear, the arrow mark (61) is aligned with the corresponding gear mark (21).
7. The damping structure of a transmission according to claim 1, characterized in that: The gear shift sleeve (5) is provided with a rotating cover (7) by means of an anti-rotation structure. The joint seat (2) is provided with a rotating hole (22) corresponding to the rotating cover (7). The rotating cover (7) is rotatably mounted on the joint seat (2) through the rotating hole (22). The rotating cover (7) is provided with an annular boss (71). The annular boss (71) is embedded and engaged with the rotating hole (22).
8. The damping structure of a transmission according to claim 7, characterized in that: The anti-rotation structure includes an anti-rotation connecting part (51) and an anti-rotation hole (72). The anti-rotation connecting part (51) is provided on the gear shift sleeve (5), and the anti-rotation hole (72) is opened on the rotating cover (7). The anti-rotation connecting part (51) and the anti-rotation hole (72) are anti-rotation engaged.
9. The damping structure of a transmission according to claim 1, characterized in that: The gear shift knob (5) is connected to a rotating cover (7) for preventing rotation. A positioning structure is provided between the joint seat (2) and the rotating cover (7). When the rotating cover (7) rotates relative to the joint seat (2) to the corresponding gear position, the rotating cover (7) is positioned and engaged with the joint seat (2) through the positioning structure.
10. The damping structure of a transmission according to claim 9, characterized in that: The positioning structure includes a positioning post (23) and a positioning groove (73). The positioning groove (73) is opened on the side wall of the rotating cover (7). The positioning post (23) is slidably disposed on the joint seat (2). A spring (231) is provided between the positioning post (23) and the joint seat (2). The spring (231) has a tendency to push the positioning post (23) to lock and position with the corresponding positioning groove (73).