Constant-force elastic structure capable of linearly moving along front arm

Abstract

The utility model discloses a constant-force elastic structure linearly moving along a front arm, the front end and the rear end of the front arm are respectively mounted at the upper ends of a swivel seat and an adjusting seat in a manner of rotating around a first horizontal rotating shaft and a second horizontal rotating shaft, and the front end and the rear end of a support plate are respectively mounted at the lower ends of the swivel seat and the adjusting seat in a manner of rotating around a third horizontal rotating shaft and a fourth horizontal rotating shaft; the adjusting base, the front arm, the swivel base and the supporting plate form a parallel four-connecting-rod structure, the front end of a connecting piece of the elastic assembly can be installed on the swivel base in the mode of rotating around a third horizontal rotating shaft, and the rear end of the connecting piece is hinged to the front end of an elastic element of the elastic assembly through a fifth horizontal rotating shaft. The fifth horizontal rotating shaft can be installed on the front arm in a sliding mode in the elastic stretching direction of the elastic element along a straight line, an arc line or an oblique line forming an angle with the elastic stretching direction of the elastic element, and the rear end of the elastic element is installed on the front arm, the adjusting base or the second rotating shaft in a locking mode in the stretching direction of the elastic element. According to the utility model, the display can stably hover at any height, and the whole structure is small and beautiful.

Description

Technical Field

[0001] This utility model relates to a monitor bracket, and more particularly to a constant force elastic structure that extends the linear movement of the forearm. Background Technology

[0002] Monitors are used in many places, such as homes, offices, and public places. They are often mounted on monitor stands. Currently, the height and horizontal angle adjustment of monitors are achieved by clamping plastic pads between the forearms and relying on friction. The plastic pads are easily worn and damaged during long-term use, resulting in poor stability after the monitor's height and horizontal angle adjustment. It is difficult to maintain stable support and state for a long time. Moreover, the adjustment often requires tightening and loosening screws, which is cumbersome. It is also affected by the friction value, has a small load-bearing range for the monitor, and is complicated to assemble. Utility Model Content

[0003] To overcome the above-mentioned defects, this utility model provides a constant force elastic structure that extends the linear motion of the forearm. This constant force elastic structure that extends the linear motion of the forearm can automatically hover after the display height is adjusted. It has a simple structure and an aesthetically pleasing appearance.

[0004] The technical solution adopted by this utility model to solve its technical problem is as follows: a constant force elastic structure for linear movement of a forearm, including an adjusting seat, a rotating head seat, a forearm, a support plate, and an elastic component. The front and rear ends of the forearm, which can rotate around a first horizontal axis and a second horizontal axis respectively, are installed on the upper ends of the rotating head seat and the adjusting seat. The front and rear ends of the support plate, which can rotate around a third horizontal axis and a fourth horizontal axis respectively, are installed on the lower ends of the rotating head seat and the adjusting seat. The adjusting seat, the forearm, the rotating head seat, and the support plate together form a parallel four-bar linkage structure. The elastic component includes a connecting piece and an elastic element that can elastically extend and retract. The front end of the connecting piece, which can rotate around the third horizontal axis, is installed on the rotating head seat. The rear end of the connecting piece is hinged to the front end of the elastic element through a fifth horizontal axis. The fifth horizontal axis is installed on the forearm that can slide along a straight line, an arc, or an oblique line at an angle to the elastic extension and retraction direction of the elastic element. The rear end of the elastic element is stopped along its extension and retraction direction and is installed on the forearm, the adjusting seat, or the second axis.

[0005] As a further improvement of this utility model, a cavity structure is formed on the inner side of the forearm, and the support plate and elastic component are both accommodated in the cavity structure of the forearm. The front end and rear end of the forearm are respectively formed with clearance notches, and the rotating head seat and the adjusting seat can be rotatably inserted into the cavity structure of the front and rear walls through the clearance notches.

[0006] As a further improvement of this utility model, the left and right side walls of the forearm are provided with elongated guide grooves extending along the stretching direction of the elastic element, and the two ends of the fifth horizontal rotating shaft are respectively slidably inserted into the elongated guide grooves on the left and right side walls of the forearm.

[0007] As a further improvement of this utility model, bearings are respectively installed at both ends of the fifth horizontal rotating shaft.

[0008] As a further improvement of this utility model, the elastic element includes a spring, an adjusting bolt, and a connecting block. The connecting block is fixedly installed on the rear end of the spring. The stud of the adjusting bolt is movably screwed to the connecting block. The stud of the adjusting bolt can be accommodated in the inner ring of the spring. The rear end of the forearm has an clearance adjustment hole that is directly opposite the head of the adjusting bolt. Hardware tools can be inserted into the forearm through the clearance adjustment hole and rotate the adjusting bolt. Rotating the adjusting bolt can change the initial elastic force value of the elastic element.

[0009] As a further improvement of this utility model, the front end of the spring is formed with a hinged hook ring, which is sleeved on the outside of the fifth horizontal rotating shaft. The second horizontal rotating shaft is provided with a bolt hole extending radially therefrom. The adjusting bolt passes through the bolt hole, and the head of the adjusting bolt stops on the rear side of the outer circumference of the second horizontal rotating shaft. The upper end of the adjusting seat forms a U-shaped structure, and the adjusting bolt is accommodated in the opening of the U-shaped structure at the upper end of the adjusting seat.

[0010] As a further improvement of this utility model, the U-shaped structure of the adjustment seat is provided with first horizontal hinge holes on both sides of the side walls. The other end of the forearm is sleeved on the outside of the U-shaped structure at the upper end of the adjustment seat. The other end of the forearm is provided with second horizontal hinge holes on both sides of the side walls. The second horizontal rotating shaft includes an adjusting bolt limiting shaft and connecting screws. The bolt through hole is located in the middle part of the adjusting bolt limiting shaft. The two ends of the adjusting bolt limiting shaft are respectively inserted into the first horizontal hinge hole and the second horizontal hinge hole. The two connecting screws pass through the second horizontal hinge hole and are screwed to the two ends of the adjusting bolt limiting shaft. The heads of the two connecting screws are respectively pressed against the left and right sides of the rear end of the forearm.

[0011] As a further improvement of this utility model, the bottom surface of the U-shaped structure on the adjusting seat is a pointed bottom structure with a middle height higher than the front and rear sides, and the screw part and head of the adjusting bolt are respectively stopped on the front and rear sides of the bottom surface of the U-shaped structure on the adjusting seat.

[0012] As a further improvement of this utility model, the side wall of the rotating head seat is provided with an open groove structure, the front end of the connecting piece is inserted into the open groove structure, the rear end of the connecting piece forms a bifurcated structure, the hinge hook of the front end of the spring is accommodated in the bifurcated structure of the rear end of the connecting piece, the front end side wall of the connecting piece and the bifurcated structure side wall of the rear end of the connecting piece are respectively provided with connecting piece through holes, the connecting piece through holes at the front and rear ends of the connecting piece are respectively sleeved on the third horizontal rotating shaft and the fifth horizontal rotating shaft, the left and right side walls of the rotating head seat and the adjusting seat are also respectively provided with stepped surfaces with a recessed structure, and there are two support plates, the front and rear ends of the two support plates respectively cover the stepped surfaces of the left and right side walls of the rotating head seat and the adjusting seat.

[0013] As a further improvement of this utility model, the forearm includes a left forearm cover with a U-shaped cross-section and a right forearm cover with a U-shaped cross-section. The side walls of the left and right forearm covers are joined to form a sleeve-shaped structure. The rear end of at least one of the left and right forearm covers is located on a base plate. The base plate closes the rear end opening of the sleeve-shaped structure formed by the left and right forearm covers. The base plate is provided with an adjustment hole for hardware tools to be inserted into the forearm to rotate the adjustment bolt.

[0014] The beneficial effects of this utility model are as follows: This utility model forms a parallelogram structure by sequentially hinged together the forearm, rotating head seat, support plate, and adjustment seat to ensure that the rotating head seat only moves up and down during adjustment. The elastic component is designed as a hinged structure consisting of a connecting piece and an elastic element. When the arm moves up and down, the other end of the connecting piece moves back and forth along the straight line of the forearm, while one end of the connecting piece rotates around a third horizontal axis. The connecting piece, forearm, and rotating head seat of the elastic component together form a triangular structure, achieving stable support for the rotating head seat. When adjusting the height of the rotating head seat, the elastic element... The component automatically adjusts the position of the rear end of the connecting piece and the elastic support force on the connecting piece through elastic extension and retraction, so that the display can maintain stable support at any height, forming a height suspension function. This utility model uses the fifth horizontal rotating shaft to slide linearly in the long strip guide groove on the side wall of the forearm, so that the elastic element can only extend and retract linearly within the forearm and cannot produce movement in other directions. This can ensure that the output elastic force value provides stable support for the rear end of the connecting piece. The elastic component and the support plate of this application are hidden inside the forearm, avoiding the support plate being exposed, making the overall structure compact and beautiful. Attached Figure Description

[0015] Figure 1 This is an exploded perspective view of the present invention;

[0016] Figure 2 for Figure 1 Enlarged view of section A in the middle;

[0017] Figure 3 for Figure 1 Enlarged view of section B in the middle;

[0018] Figure 4 This is an exploded perspective view of the elastic component of this utility model;

[0019] Figure 5 This is a front view of the forearm of this utility model in an upward-facing position;

[0020] Figure 6 This is a top view of the forearm of this invention in an upward-facing position;

[0021] Figure 7 for Figure 6 C-axis sectional view;

[0022] Figure 8 This is a front view of the forearm of this utility model in a downward tilting position;

[0023] Figure 9 This is a schematic diagram of the internal structure of the forearm of this utility model in a downward tilting state;

[0024] Figure 10 This is a front view of the forearm of this utility model in a horizontal position. Detailed Implementation

[0025] Example: A constant-force elastic structure for linear motion of a forearm includes an adjusting seat 1, a rotating head seat 2, a forearm 3, a support plate 4, and an elastic component 5. The forearm 3, with its front and rear ends rotatable around a first horizontal axis 6 and a second horizontal axis respectively, is mounted on the upper ends of the rotating head seat 2 and the adjusting seat 1. The support plate 4, with its front and rear ends rotatable around a third horizontal axis 8 and a fourth horizontal axis 9 respectively, is mounted on the lower ends of the rotating head seat 2 and the adjusting seat 1. The adjusting seat 1, forearm 3, rotating head seat 2, and support plate 4 together form a parallel four-bar linkage structure. The elastic component... Component 5 includes a connecting piece 10 and an elastic element capable of elastic extension and retraction. The front end of the connecting piece 10 is rotatable around the third horizontal pivot 8 and is mounted on the rotating head seat 2. The rear end of the connecting piece 10 is hinged to the front end of the elastic element via a fifth horizontal pivot 11. The fifth horizontal pivot 11 is slidable along a straight line, an arc, or an oblique line at an angle to the elastic extension and retraction direction of the elastic element and is mounted on the forearm 3. The rear end of the elastic element is stopped along its extension and retraction direction and is mounted on the forearm 3, the adjusting seat 1, or the second pivot.

[0026] When adjusting the monitor height, due to the parallel four-bar linkage structure formed by the front arm 3, adjustment base 1, support plate 4, and rotating head, only the front arm 3 needs to be rotated to cause the rotating head base 2 to move up and down without rotating. Thus, the monitor only needs to be adjusted in height without affecting the monitor's tilt angle. At the same time, the front arm 3, rotating head base 2, and connecting piece 10 form a triangular structure to achieve stable support for the rotating head base 2. During the movement of the front arm 3, the fifth horizontal rotating shaft 11 slides linearly on the front arm 3. On the one hand, it adapts to the change in the angle between the connecting piece 10, the rotating head base 2, and the front arm 3. On the other hand, it automatically changes the elastic force value output by the elastic element, so that the rear end of the connecting piece 10 remains stable in the adjusted position. Thus, the monitor height remains stable in the adjusted position. Therefore, after the rotating head base 2 suspends the monitor, it can maintain vertical movement and hover under the constant force within the force range of the front arm 3.

[0027] The inner side of the forearm 3 forms a cavity structure, and the support plate 4 and elastic component 5 are both accommodated within the cavity structure of the forearm 3. The front and rear ends of the forearm 3 each have clearance notches, allowing the rotating head seat 2 and adjusting seat 1 to be rotatably inserted into the cavities of the front and rear walls via these clearance notches. This structure conceals all the hinge joints of the parallelogram within the cavity structure of the forearm 3, improving overall aesthetics and protecting the hinge joints.

[0028] The left and right sidewalls of the forearm 3 are provided with elongated guide grooves 12 extending along the extension and retraction direction of the elastic element. The two ends of the fifth horizontal rotating shaft 11 are respectively slidably inserted into the elongated guide grooves 12 on the left and right sidewalls of the forearm 3. The elongated guide grooves 12 guide the fifth horizontal rotating shaft 11 and limit its sliding distance, thereby ensuring that the elastic element always extends and retracts linearly within the forearm 3 without making other movements. At the same time, the sliding distance limitation restricts the pitch angle of the forearm 3.

[0029] Bearings 13 are installed at both ends of the fifth horizontal rotating shaft 11. When the fifth horizontal rotating shaft 11 slides in the elongated guide groove 12, the bearings 13 form rolling friction with the inner sidewall of the elongated guide groove 12, reducing friction.

[0030] The elastic element includes a spring 14, an adjusting bolt 15, and a connecting block 16. The connecting block 16 is fixedly mounted on the rear end of the spring 14. The stud of the adjusting bolt 15 is movably screwed to the connecting block 16, and the stud of the adjusting bolt 15 can be accommodated in the inner ring of the spring 14. The rear end of the forearm 3 has a clearance adjustment hole that faces the head of the adjusting bolt 15. Hardware tools can be inserted into the forearm 3 through the clearance adjustment hole and rotate the adjusting bolt 15. Rotating the adjusting bolt 15 can change the initial elastic force value of the elastic element. Rotating the adjusting bolt 15 can change the compression of the spring 14 in the initial state, thereby adjusting the elastic force value output by the elastic element in the initial state to adapt to the stable support of displays of different weights.

[0031] The spring 14 has a hinged hook 17 at its front end, which is sleeved on the outside of the fifth horizontal rotating shaft 11. The second horizontal rotating shaft has a bolt hole 18 extending radially therefrom. The adjusting bolt 15 passes through the bolt hole 18, and the head of the adjusting bolt 15 stops at the rear side of the outer circumference of the second horizontal rotating shaft. The upper end of the adjusting seat 1 forms a U-shaped structure, and the adjusting bolt 15 is accommodated in the opening of the U-shaped structure at the upper end of the adjusting seat 1. The second horizontal rotating shaft is sleeved on the adjusting bolt 15 through the bolt hole 18. When the adjusting bolt 15 rotates, it will not interfere with the second horizontal rotating shaft, and the extension direction of the elastic element is always consistent with the forearm 3.

[0032] The U-shaped structure of the adjusting seat 1 has first horizontal hinge holes 19 on both sides. The other end of the forearm 3 is sleeved on the outside of the U-shaped structure at the upper end of the adjusting seat 1. The other end of the forearm 3 has second horizontal hinge holes 20 on both sides. The second horizontal rotating shaft includes an adjusting bolt limiting shaft 7 and connecting screws 21. The bolt through hole 18 is located in the middle of the adjusting bolt limiting shaft 7. The two ends of the adjusting bolt limiting shaft 7 are respectively inserted into the first horizontal hinge holes 19 and the second horizontal hinge holes 20. The two connecting screws 21 pass through the second horizontal hinge holes 20 and are screwed to the two ends of the adjusting bolt limiting shaft 7. The heads of the two connecting screws 21 are respectively pressed against the left and right sides of the rear end of the forearm 3. The adjusting bolt limiting shaft 7 serves two purposes: firstly, it limits the position of the adjusting bolt 15; secondly, it is screwed into the connecting screw 21 to form an integral structure, which can act as a second horizontal pivot. To improve the smooth contact between the head of the adjusting bolt 15 and the rear side of the adjusting bolt limiting shaft 7, it is best to form a contact plane on the rear side of the adjusting bolt limiting shaft 7. The rear end of the bolt through hole 18 is located on this contact plane, and the head of the adjusting bolt 15 is stopped on this contact plane. It is also best to set a friction pad between the front arm 3 and the adjusting seat 1. By tightening the connecting screw 21, a large friction force is formed between the front arm 3 and the adjusting seat 1, which helps to position the angle of the front arm 3 and prevents the front arm 3 from swinging rapidly when the adjusting bolt 15 is rotated.

[0033] The bottom surface of the U-shaped structure on the adjusting seat 1 is a pointed structure with a middle height higher than the front and rear sides. The screw part and head of the adjusting bolt 15 are respectively stopped on the front and rear sides of the bottom surface of the U-shaped structure on the adjusting seat 1. By setting a front and rear inclined surface with the second horizontal axis as the dividing point on the bottom surface of the U-shaped structure of the adjusting seat 1, the adjustment bolt 15 can avoid the swing of the front arm 3 and the swing angle range can be limited.

[0034] The rotating head seat 2 has an open slot structure 22 on its side wall. The front end of the connecting piece 10 is inserted into the open slot structure 22. The rear end of the connecting piece 10 forms a bifurcated structure. The hinge hook 17 at the front end of the spring 14 is accommodated in the bifurcated structure at the rear end of the connecting piece 10. The front side wall of the connecting piece 10 and the side wall of the bifurcated structure at the rear end of the connecting piece 10 are respectively provided with connecting piece through holes 23. The connecting piece through holes 23 at the front and rear ends of the connecting piece 10 are respectively sleeved on the third horizontal rotating shaft 8 and the fifth horizontal rotating shaft 11. The left and right side walls of the rotating head seat 2 and the adjusting seat 1 are also respectively provided with recessed stepped surfaces 24. There are two support plates 4. The front and rear ends of the two support plates 4 respectively cover the stepped surfaces 24 on the left and right side walls of the rotating head seat 2 and the adjusting seat 1.

[0035] The rear end of the connecting piece 10 forms a bifurcated structure, which can be hinged to the front end of the elastic element through the fifth horizontal rotating shaft 11. It can also limit the hinge hook 17 of the front end of the elastic element in the left and right directions. Together with the U-shaped structure at the upper end of the adjusting seat 1, it limits the adjusting bolt 15 in the left and right directions, so that the elastic component 5 is always parallel to the middle of the left and right side walls of the front arm 3 and will not tilt, ensuring stable support for the rotating head seat 2. The left and right side walls of the rotating head seat 2 and the adjusting seat 1 form a stepped surface 24 with a recessed structure for installing the support plate 4 and ensuring that the support plate 4 does not interfere with the front arm 3 after installation.

[0036] The forearm 3 includes a left forearm cover 25 with a U-shaped cross-section and a right forearm cover 26 with a U-shaped cross-section. The sidewalls of the left and right forearm covers 25 and 26 are joined to form a sleeve-shaped structure. The rear end of at least one of the left and right forearm covers 25 and 26 is located on a base plate, which closes the rear opening of the sleeve-shaped structure formed by the left and right forearm covers 25 and 26. The base plate is provided with an adjustment hole for hardware tools to be inserted into the forearm 3 to rotate the adjustment bolt 15. The forearm 3 is designed with an interlocking structure to facilitate the assembly of the elastic component 5 and the support plate 4.

Claims

1. A constant-force elastic structure for linear motion of a forearm, comprising an adjustment seat (1), a rotating head seat (2), a forearm (3), a support plate (4), and an elastic component (5), wherein the forearm is mounted on the upper ends of the rotating head seat and the adjustment seat, with its front and rear ends respectively rotatable around a first horizontal axis (6) and a second horizontal axis; and the support plate is mounted on the lower ends of the rotating head seat and the adjustment seat, with its front and rear ends respectively rotatable around a third horizontal axis (8) and a fourth horizontal axis (9); wherein the adjustment seat, the forearm, the rotating head seat, and the support plate together form a parallel four-bar linkage structure, characterized in that: The elastic component includes a connecting piece (10) and an elastic element capable of elastic extension and retraction. The front end of the connecting piece is rotatable around a third horizontal axis and is mounted on a rotating head seat. The rear end of the connecting piece is hinged to the front end of the elastic element via a fifth horizontal axis (11). The fifth horizontal axis is slidable along a straight line, an arc, or an oblique line at an angle to the elastic extension and retraction direction of the elastic element and is mounted on the forearm. The rear end of the elastic element is stopped along its extension and retraction direction and is mounted on the forearm, the adjusting seat, or the second axis.

2. The constant-force resilient structure that delays the linear motion of the arm according to claim 1, characterized in that: The inner side of the forearm forms a cavity structure, and the support plate and elastic component are both accommodated in the cavity structure of the forearm. The front end and rear end of the forearm are respectively formed with clearance notches, and the rotating head seat and the adjusting seat can be rotatably inserted into the cavity structure of the front and rear walls through the clearance notches.

3. The constant-force resilient structure that delays the linear motion of the arm according to claim 2, characterized in that: The left and right sidewalls of the forearm are provided with elongated guide grooves (12) extending along the stretching direction of the elastic element, and the two ends of the fifth horizontal rotating shaft are respectively slidably inserted into the elongated guide grooves on the left and right sidewalls of the forearm.

4. The constant-force resilient structure that delays the linear motion of the arm according to claim 3, characterized in that: Bearings (13) are installed at both ends of the fifth horizontal rotating shaft.

5. The constant-force resilient structure that delays the linear motion of the arm according to claim 2, characterized by: The elastic element includes a spring (14), an adjusting bolt (15), and a connecting block (16). The connecting block is fixedly installed on the rear end of the spring. The stud of the adjusting bolt is movably screwed to the connecting block. The stud of the adjusting bolt can be accommodated in the inner ring of the spring. The rear end of the forearm has an clearance adjustment hole that is directly opposite the head of the adjusting bolt. Hardware tools can be inserted into the forearm through the clearance adjustment hole and rotate the adjusting bolt. Rotating the adjusting bolt can change the initial elastic force value of the elastic element.

6. The constant-force resilient structure that delays the linear motion of the arm according to claim 5, characterized in that: The spring has a hinged hook (17) at its front end. The hinged hook is sleeved on the outside of the fifth horizontal rotating shaft. The second horizontal rotating shaft has a bolt hole (18) extending radially therein. The adjusting bolt passes through the bolt hole, and the head of the adjusting bolt stops on the rear side of the outer circumference of the second horizontal rotating shaft. The upper end of the adjusting seat forms a U-shaped structure, and the adjusting bolt is accommodated in the opening of the U-shaped structure at the upper end of the adjusting seat.

7. The constant-force resilient structure that delays the linear motion of the arm according to claim 6, characterized in that: The U-shaped structure of the adjustment seat has a first horizontal hinge hole (19) on each of its two side walls. The other end of the forearm is sleeved on the outside of the U-shaped structure at the upper end of the adjustment seat. The other end of the forearm has a second horizontal hinge hole (20) on each of its two side walls. The second horizontal rotating shaft includes an adjustment bolt limiting shaft (7) and a connecting screw (21). The bolt through hole is located in the middle of the adjustment bolt limiting shaft. The two ends of the adjustment bolt limiting shaft are respectively inserted into the first horizontal hinge hole and the second horizontal hinge hole. The two connecting screws pass through the second horizontal hinge hole and are screwed to the two ends of the adjustment bolt limiting shaft. The heads of the two connecting screws are respectively pressed against the left and right side surfaces of the rear end of the forearm.

8. The constant-force resilient structure that delays the linear motion of the arm according to claim 7, characterized in that: The bottom surface of the U-shaped structure on the adjusting seat is a pointed structure with a middle height higher than the front and rear sides. The screw part and head of the adjusting bolt are respectively stopped on the front and rear sides of the bottom surface of the U-shaped structure on the adjusting seat.

9. The constant-force resilient structure that delays linear motion of an arm according to claim 6, characterized by: The rotating head seat has an open slot structure (22) on its side wall. The front end of the connecting piece is inserted into the open slot structure. The rear end of the connecting piece forms a bifurcated structure. The hinge hook at the front end of the spring is accommodated in the bifurcated structure at the rear end of the connecting piece. The front side wall of the connecting piece and the side wall of the bifurcated structure at the rear end of the connecting piece are respectively provided with connecting piece through holes (23). The connecting piece through holes at the front and rear ends of the connecting piece are respectively sleeved on the third horizontal rotating shaft and the fifth horizontal rotating shaft. The left and right side walls of the rotating head seat and the adjusting seat are also respectively provided with stepped surfaces (24) with a concave structure. There are two support plates. The front and rear ends of the two support plates respectively cover the stepped surfaces of the left and right side walls of the rotating head seat and the adjusting seat.

10. The constant-force resilient structure that delays the linear motion of the arm according to claim 2, characterized by: The forearm includes a left forearm cover (25) with a U-shaped cross-section and a right forearm cover (26) with a U-shaped cross-section. The side walls of the left and right forearm covers are joined to form a sleeve-shaped structure. The rear end of at least one of the left and right forearm covers is located on a base plate. The base plate closes the rear end opening of the sleeve-shaped structure formed by the left and right forearm covers. The base plate is provided with an adjustment hole for hardware tools to be inserted into the forearm to rotate the adjustment bolt.

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