Server sequential opening and closing device with locking function

CN123035635APending Publication Date: 2026-11-03GUANG DONG XING HUI CHUANG XIN JI SHU YOU XIAN GONG SI
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Patent Information

Application Number
CN202611503225.8
Authority / Receiving Office
CN · China
Patent Type
Applications(China)
Current Assignee / Owner
Filing Date
2026-09-24
Publication Date
2026-11-03

AI Technical Summary

Technical Problem

现有服务器开合装置在锁止安全性和运维便利性方面存在不足,难以实现稳定、可靠的锁止功能和便捷的复位操作。

Method used

A server sequential opening and closing device was designed, which includes a stop mechanism and a sequence component. Through the structural cooperation of a stop plate, a stop lever, a pull rod, and an elastic element, the device achieves the locking function of the inner rail in the fully open state, and achieves automatic reset through a reset elastic element. Combined with a sequence locking plate and a beaded nest limiting component, the device ensures the opening and closing sequence and stability.

Benefits of technology

It achieves the locking function of the inner rail in the fully open state, ensuring a safe and reliable locking effect, and improves the convenience of operation and maintenance and the stability of operation through automatic reset and sequential opening and closing.

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Abstract

The application provides a server sequential opening and closing device with locking function, which comprises an outer rail, a middle rail and an inner rail, the middle rail is provided with an outer nest group and an inner nest group, the inner rail is provided with a stop mechanism, the stop mechanism comprises a stop baffle, a stop lever and a pull rod, the stop baffle is installed at the front end of the middle rail, the positioning end of the stop lever is rotationally connected to the inner rail, the swing end of the stop lever is provided with a stop port, the inner rail is provided with a stop elastic element, the two elastic ends of the stop elastic element are connected to the stop lever and the inner rail respectively, the swing end of the stop lever is provided with a control groove, the pull rod is movably connected to the inner rail, and the front end of the pull rod is provided with a pull cone that can slide in the control groove and simultaneously push the stop lever outward to separate the stop port from the stop baffle. The device has simple structure, locking function, high running stability, reliability and practicality, and effectively solves the locking safety problem, and is convenient to reset and easy to operate.
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Description

Technical Field

[0001] This invention relates to the technical field of opening and closing devices, specifically a server sequential opening and closing device with locking function. Background Technology

[0002] With the rapid iteration and large-scale implementation of digital economy, artificial intelligence, cloud computing and big data technologies, the demand for data storage, computing power scheduling and network transmission across society is growing exponentially. As the core basic hardware equipment of data centers and computing power platforms, servers are the core carriers for massive data processing, resource scheduling and business operations, and generally adopt a rack-stacking installation structure.

[0003] Among them, the server opening and closing device is a core supporting component that enables the server to extend, push, pull, install, fix, and perform daily maintenance and repair within the rack. Its structural stability, ease of installation, smooth movement, and locking security directly determine the assembly efficiency, maintenance convenience, and long-term operational reliability of the entire server. It is a key basic structural component that ensures the stable operation of data center equipment.

[0004] Therefore, in order to solve problems such as maintenance and locking security, this patent designs a solution that is easy to maintain and disassemble while ensuring locking security. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a server sequential opening and closing device with locking function. It has a simple structure, a locking function, effectively solves the locking security problem, and is easy to reset, easy to operate, has high operational stability, and is reliable and practical.

[0006] The objective of this invention is achieved as follows: A server sequential opening and closing device with a locking function includes an outer rail, a middle rail, and an inner rail. The middle rail is provided with an outer bead socket assembly and an inner bead socket assembly for sliding connection with the outer rail and the inner rail, respectively. The inner rail is provided with a stop mechanism for locking in the fully open state. The stop mechanism includes a stop plate, a stop lever, and a pull rod. The stop plate is installed at the front end of the middle rail. The positioning end of the stop lever is rotatably connected to the inner rail. The swing end of the stop lever has a stop opening. The inner rail has an elastic element acting on the stop lever. The stop is fastened to a stop elastic element for use with a stop plate. The two elastic ends of the stop elastic element are respectively connected to a stop lever and an inner rail. The swing end of the stop lever is provided with a control groove. The pull rod is connected to the inner rail for moving back and forth. The front end of the pull rod is equipped with a pull platform that can slide in the control groove as it moves, thereby moving the stop lever upward and separating the stop from the stop plate. The inner rail is provided with a reset elastic element that can push the pull rod back to reset so that the stop lever moves downward. The two elastic ends of the reset elastic element are respectively connected to the end of the pull rod and the inner rail.

[0007] Based on the above optimization, the swing end of the stop lever is integrally formed with a front limiting protrusion and a rear limiting protrusion, and a stop opening is formed between the front limiting protrusion and the rear limiting protrusion.

[0008] Based on the above optimization, the control groove is configured as a groove with an opening towards the stop plate, and the front and rear sides of the groove are respectively provided with inclined surfaces that gradually slope outward from top to bottom.

[0009] Based on the above optimization, the inner rail is provided with a limiting boss for limiting the swing position of the swing end of the stop lever, and the swing end of the stop lever is slidably connected to the limiting boss.

[0010] Based on the above optimization, the inner rail is provided with a front pressure plate and a rear pressure plate to prevent the pull rod from deviating and sliding. The front pressure plate and the rear pressure plate are fixed on the inner rail and form a guide groove between them and the side wall of the inner rail for the pull rod to slide back and forth.

[0011] Based on the above optimization, the rear end of the pull rod is provided with a control pull ring for easy pulling and moving backward. The control pull ring is fastened to the rear end of the pull rod, and the control pull ring and the front pressure plate are respectively provided with a snap-fit ​​position for fastening with the elastic end of the reset elastic element.

[0012] Based on the above optimization, the middle rail is provided with a sequence component for the inner rail and the middle rail to open and close sequentially on the outer rail. The sequence component includes a sequence locking plate, an inner connecting locking plate, and an outer connecting locking plate. The inner connecting locking plate is installed at the rear end of the inner rail, and the outer connecting locking plate is installed at the front end of the middle rail. The sequence locking plate is rotatably connected to the rear end of the middle rail via a connecting shaft, and the swinging front end of the sequence locking plate is provided with a sequence buckle. The rear end of the sequence locking plate is connected to a sequence elastic element. The sequence elastic element elastically acts on the sequence locking plate, causing the sequence locking plate to engage with the inner connecting locking plate as the inner rail moves, or the sequence locking plate to separate from the inner connecting locking plate as the inner rail moves. The outer connecting locking plate is installed at the front end of the outer rail and rubs against the outer rail as the middle rail moves.

[0013] Based on the above optimization, the middle rail is provided with an inner bead nest limiting group for restricting the movement position of the inner bead nest group. The inner bead nest limiting group includes an inner elastic hook and an inner protrusion. The inner elastic hook is installed at the front end of the middle rail. The inner bead nest group is engaged or disengaged with the inner elastic hook as the inner rail moves. The inner protrusion is integrally formed at the rear end of the middle rail and abuts or disengages with the inner bead nest group as it moves.

[0014] Based on the above optimization, the outer rail is provided with an outer bead nest restriction group for restricting the movement position of the outer bead nest group. The outer bead nest restriction group includes a front stop block and a rear stop block. The front stop block is installed at the front end of the outer rail, and the rear stop block is installed at the rear end of the middle rail.

[0015] Based on the above optimization, the outer rail is equipped with an outer mounting plate and an inner mounting plate for easy installation in the cabinet. The inner mounting plate is fastened to the outer rail, and the outer mounting plate is adapted to be fastened to the inner mounting plate. The outer mounting plate is provided with a drawer anti-collision component, which includes a brake pad and a connecting boss. The connecting boss is located on the outer front end of the middle rail. The brake pad is installed on the outer mounting plate. The outer rail has a clearance opening for frictional connection between the brake pad and the connecting boss as the middle rail moves.

[0016] The advantages of this invention are: 1) The stop mechanism using this structure, through the structural cooperation of the stop baffle, stop lever, lever, and stop elastic element, ensures that the inner rail stops on the middle rail when the opening and closing device is fully opened, thus realizing the locking function of the mechanism. The machine box must be unlocked to be pulled out or closed. The structure is simple, the operation is stable, safe and reliable, ensuring the quality of product use.

[0017] 2) Under the action of the reset elastic element, the stop mechanism can be automatically reset after unlocking, which is convenient to operate, easy to maintain, reliable and practical.

[0018] 3) The use of sequential components allows the opening and closing device to open and close in sequence, improving operational stability and smoothness, and ensuring the reliability of the product's long-term operation. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a preferred embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the structure in the fully open state of a preferred embodiment of the present invention.

[0021] Figure 3 This is a schematic diagram of the unlocked state of a preferred embodiment of the present invention.

[0022] Figure 4 This is a schematic diagram of the inner rail structure in a preferred embodiment of the present invention.

[0023] Figure 5 This is an exploded view of a preferred embodiment of the present invention.

[0024] Figure 6 This is an exploded view of the stop mechanism according to a preferred embodiment of the present invention. Detailed Implementation

[0025] The present invention will now be further described with reference to the accompanying drawings.

[0026] According to the appendix Figures 1 to 6As shown, the server sequential opening and closing device with locking function of the present invention includes an outer rail 1, a middle rail 2, and an inner rail 3. The middle rail 2 is provided with an outer bead socket group 4 and an inner bead socket group 5, which are slidably connected to the outer rail 1 and the inner rail 3, respectively. The inner rail 3 is provided with a stop mechanism that has a locking function when fully open. The stop mechanism includes a stop plate 6, a stop lever 7, and a pull rod 8. The stop plate 6 is installed at the front end of the middle rail 2, the positioning end of the stop lever 7 is rotatably connected to the inner rail 3, and the swing end of the stop lever 7 is provided with a stop opening 9. The inner rail 3 is provided with a stop elastic element 10 that acts on the stop lever 7 to fasten the stop port 9 to the stop plate 6. The two elastic ends of the stop elastic element 10 are respectively connected to the stop lever 7 and the inner rail 3. The swing end of the stop lever 7 is provided with a control groove 11. The pull rod 8 is connected to the inner rail 3 for moving back and forth. The front end of the pull rod 8 is provided with a pull platform 12 that can slide in the control groove 11 as it moves and push the stop lever 7 upward to separate the stop port 9 from the stop plate 6.

[0027] The stop mechanism using this structure, with the coordinated structure of the stop baffle 6, stop lever 7, lever, and stop elastic element 10, ensures that the inner rail 3 stops on the middle rail 2 when the opening and closing device is fully opened, thus achieving a locking function. The chassis must be unlocked to be pulled out or closed. The structure is simple, the operation is stable, safe and reliable, ensuring the quality of product use.

[0028] Furthermore, the inner rail 3 is provided with a reset elastic element 13 that can push the pull rod 8 to reset so that the stop plate 7 moves downward to reset. The two elastic ends of the reset elastic element 13 are respectively connected to the end of the pull rod 8 and the inner rail 3.

[0029] In this way, under the action of the reset elastic element 13, the stop mechanism can be automatically reset after unlocking, which is convenient to operate, easy to maintain, reliable and practical.

[0030] Reference Figures 1 to 6 As shown in the figure, further detailed, the swing end of the stop lever 7 is integrally formed with a front limiting protrusion 71 and a rear limiting protrusion 72, and a stop opening 9 is formed between the front limiting protrusion 71 and the rear limiting protrusion 72.

[0031] The control groove 11 is configured as a groove with an opening in the direction of the stop plate 6, and the front and rear sides of the groove are respectively provided with inclined surfaces 111 that gradually slope outward from top to bottom.

[0032] Furthermore, the inner rail 3 is provided with a limiting boss 73 for limiting the swing position of the swing end of the stop lever 7, and the swing end of the stop lever 7 is slidably connected to the limiting boss 73.

[0033] In practical applications, the inner rail 3 is provided with a front pressure plate 14 and a rear pressure plate 15 to prevent the pull rod 8 from deviating and sliding. The front pressure plate 14 and the rear pressure plate 15 are fixed on the inner rail 3 and form a guide groove between them and the side wall of the inner rail 3 for the pull rod 8 to slide back and forth.

[0034] That is, the pull rod 8 is mounted on the inner surface of the inner rail 3, and the stop lever 7 is mounted on the surface of the inner rail 3 and placed on the pull rod 8, so that the pulling frustum 12 is tangent to the inclined surface 111 of the control groove 11. The stop elastic element 10 of this structure can also be fixedly installed on the inner surface of the inner rail 3 by a pressure block.

[0035] Furthermore, the rear end of the pull rod 8 is provided with a control pull ring 16 for easy pulling and moving backward. The control pull ring 16 is fastened to the rear end of the pull rod 8, and the control pull ring 16 and the front pressure plate 14 are respectively provided with a snap-fit ​​position for fastening with the elastic end of the reset elastic member 13.

[0036] Thus, when the drawer is opened, the inner rail 3 moves accordingly, and the pull rod 8, under the elastic action of the stop elastic element 10, always has a locking force towards the stop plate 6. When the inner rail 3 is fully open, the front limit protrusion 71, under the action of compression, passes over the stop plate 6, causing the stop plate 6 to stop between the front limit protrusion 71 and the rear limit protrusion 72, preventing the inner rail 3 from being pulled outwards or closed inwards, thus achieving the locking function of the inner rail 3 in the open state. When the control ring 16 is pulled in the opening direction of the inner rail 3, the control ring 16 moves a certain distance along the inner surface of the inner rail 3, thereby driving the pull rod 8 to move together, pulling the truncated cone 12 to move along the inclined surface 111 of the control groove 11, thereby causing the swing end of the stop lever to slide upwards along the limiting protrusion 73, causing the stop port 9 to move away from the stop plate 6, thereby unlocking, at which point the inner rail 3 can be freely closed or opened. When the control pull ring 16 is released, the stop mechanism is reset under the elastic action of the reset elastic element 13.

[0037] Reference Figures 1 to 6 As shown in the diagram, further detailed, the middle rail 2 is equipped with a sequence component for sequentially opening and closing the inner rail 3 and the middle rail 2 on the outer rail 1. The sequence component includes a sequence locking plate 17, an inner connecting locking plate 18, and an outer connecting locking plate 19. The inner connecting locking plate 18 is installed at the rear end of the inner rail 3, and the outer connecting locking plate 19 is installed at the front end of the middle rail 2. The sequence locking plate 17 is rotatably connected to the rear end of the middle rail 2 via a connecting shaft, and a sequence buckle is provided at the swinging front end of the sequence locking plate 17. A sequence elastic element 20 is connected to the rear end of the sequence locking plate 17. The sequence elastic element 20 elastically acts on the sequence locking plate 17, causing the sequence buckle to engage with the inner connecting locking plate 18 as the inner rail 3 moves, or causing the sequence buckle to separate from the inner connecting locking plate 18 as the sequence locking plate 17 moves along the inner rail 3. The outer connecting locking plate 19 is installed at the front end of the outer rail 1 and rubs against the outer rail 1 as the middle rail 2 moves.

[0038] Reference Figures 1 to 6 As shown in the diagram, further detailed, the middle rail 2 is provided with an inner bead nest limiting group for restricting the movement of the inner bead nest assembly 5. The inner nest limiting group includes an inner elastic hook 21 and an inner protrusion 22. The inner elastic hook 21 is installed at the front end of the middle rail 2, and the inner bead nest assembly 5 moves with the inner rail 3 and engages or disengages with the inner elastic hook 21. The inner protrusion 22 is integrally formed at the rear end of the middle rail 2 and abuts or disengages with the inner bead nest assembly 5 as it moves.

[0039] In addition, the outer rail 1 is provided with an outer bead nest limiting group for restricting the movement position of the outer bead nest group 4. The outer bead nest limiting group includes a front stop block 23 and a rear stop block 24. The front stop block 23 is installed at the front end of the outer rail 1, and the rear stop block 24 is installed at the rear end of the middle rail 2.

[0040] That is, in the closed state, the sequential elastic element 20 elastically acts on the sequential locking piece 17, ensuring that the sequential locking piece always has a tendency to engage with the inner connecting locking piece 18. When the inner rail 3 is opened, the middle rail 2 moves together with the inner rail 3 under the cooperative action of the sequential locking piece 17 and the inner connecting locking piece 18. When opened to a specific position, under sufficient tension, the inner connecting locking piece 18 squeezes the sequential locking piece 17, and the sequential elastic element 20 deforms, causing the inner connecting locking piece 18 to separate from the sequential locking piece 17. Subsequently, the outer connecting locking piece 19 can stop at the front end of the outer rail 1 under friction, causing the middle rail 2 to stop at the outer rail 1, and the inner rail 3 continues to open outward. When the inner rail 3 is fully open, the inner bead socket 5 moves to the designated position. The inner elastic hook 21, under its own elasticity, engages with the front end of the inner bead socket 5, and the stop mechanism activates, locking the inner rail 3 at the front end of the middle rail 2. Simultaneously, the outer bead socket 4 is stopped at the front end of the outer rail 1 by the front stop block 23. When the inner rail 3 is pushed to close, the control ring 16 of the stop mechanism is pulled, unlocking the inner rail 3 from the middle rail 2. Continuing to push the inner rail 3 causes the inner elastic hook 21 to deform and separate from the inner bead socket 5, allowing the inner bead socket 5 to move with the inner rail 3. Simultaneously, with sufficient pushing force, the inner protrusion 22 abuts against the inner rail 3, the sequential locking piece 17 engages with the inner connecting locking piece 18, and under the action of the rear stop block 24, the middle rail 2 and the inner rail 3 move together inward along the outer rail 1. During this process, because the front end of the middle rail 2 of this structure is equipped with a front stop block and the rear end of the outer rail 1 has a stop protrusion to stop the middle rail 2, the middle rail 2 and the inner rail 3 are completely closed on the outer rail 1 and do not detach from the outer rail 1.

[0041] By using sequential components, the opening and closing device can be opened and closed in sequence, improving operational stability and smoothness, and ensuring the reliability of the product's long-term operation.

[0042] Reference Figures 1 to 6As shown in the diagram, further details are provided. The outer rail 1 is equipped with an outer mounting plate 25 and an inner mounting plate 26 for easy installation in the cabinet. The inner mounting plate 26 is fastened to the outer rail 1, and the outer mounting plate 25 is adapted to be fastened to the inner mounting plate 26. The outer mounting plate 25 is provided with a drawer anti-collision component, which includes a brake pad 27 and a connecting boss 28. The connecting boss 28 is located on the outer front end of the middle rail 2. The brake pad 27 is installed on the outer mounting plate 25. The outer rail 1 has a clearance opening 29 for frictional connection between the brake pad 27 and the connecting boss 28 as the middle rail 2 moves.

[0043] The outer mounting plate 25 and the inner mounting plate 26 facilitate the assembly of the opening and closing mechanism, improving assembly efficiency. Simultaneously, the structural cooperation between the connecting boss 28 and the brake pad 27 prevents damage to other components when the chassis is closed, ensuring reliability and practicality.

[0044] The above specific embodiments are merely specific implementations of the present invention with better effects. Any structure that is the same as or equivalent to the server sequential opening and closing device with locking function of the present invention is within the protection scope of the present invention.

Claims

1. A server sequential opening and closing device with locking function, comprising an outer rail (1), a middle rail (2), and an inner rail (3), wherein the middle rail (2) is provided with an outer bead nest assembly (4) and an inner bead nest assembly (5) for sliding connection with the outer rail (1) and the inner rail (3), respectively, characterized in that: The inner rail (3) is provided with a stop mechanism that has a locking function when fully open. The stop mechanism includes a stop plate (6), a stop lever (7), and a pull rod (8). The stop plate (6) is installed at the front end of the middle rail (2). The positioning end of the stop lever (7) is rotatably connected to the inner rail (3). The swing end of the stop lever (7) is provided with a stop opening (9). The inner rail (3) is provided with a stop elastic member (10) that acts elastically on the stop lever (7) so that the stop opening (9) is fastened to the stop plate (6). The two elastic ends of the stop elastic member (10) are respectively connected to the stop lever (7). The inner rail (3) has a control groove (11) at the swing end of the stop lever (7). The pull rod (8) moves back and forth and is connected to the inner rail (3). The front end of the pull rod (8) is equipped with a pull platform (12) that can slide in the control groove (11) as it moves and push the stop lever (7) upward to separate the stop port (9) from the stop plate (6). The inner rail (3) is provided with a reset elastic element (13) that can push the pull rod (8) to reset so that the stop lever (7) moves downward to reset. The two elastic ends of the reset elastic element (13) are connected to the end of the pull rod (8) and the inner rail (3) respectively.

2. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The sway end of the stop lever (7) is integrally formed with a front limiting protrusion (71) and a rear limiting protrusion (72), and a stop opening (9) is formed between the front limiting protrusion (71) and the rear limiting protrusion (72).

3. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The control groove (11) is configured as a groove with an opening in the direction of the stop plate (6), and the front and rear sides of the groove are respectively provided with inclined surfaces (111) that gradually slope outward from top to bottom.

4. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The inner rail (3) is provided with a limiting boss (73) for limiting the swing position of the swing end of the stop lever (7), and the swing end of the stop lever (7) is slidably connected to the limiting boss (73) up and down.

5. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The inner rail (3) is provided with a front pressure plate (14) and a rear pressure plate (15) to prevent the pull rod (8) from deviating and sliding. The front pressure plate (14) and the rear pressure plate (15) are fixed on the inner rail (3) and form a guide groove between them and the side wall of the inner rail (3) for the pull rod (8) to slide back and forth.

6. The server sequential opening and closing device with locking function according to claim 5, characterized in that: The rear end of the pull rod (8) is provided with a control pull ring (16) for easy pulling and moving backward. The control pull ring (16) is fastened to the rear end of the pull rod (8), and the control pull ring (16) and the front pressure plate (14) are respectively provided with a snap-fit ​​position for fastening with the elastic end of the reset elastic member (13).

7. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The middle rail (2) is provided with a sequence component for the inner rail (3) and the middle rail (2) to open and close sequentially on the outer rail (1). The sequence component includes a sequence locking plate (17), an inner connecting locking plate (18), and an outer connecting locking plate (19). The inner connecting locking plate (18) is installed at the rear end of the inner rail (3), and the outer connecting locking plate (19) is installed at the front end of the middle rail (2). The sequence locking plate (17) is rotatably connected to the rear end of the middle rail (2) via a connecting shaft, and the sequence locking plate (17) is... The swing front end is provided with a sequence buckle, and the rear end of the sequence locking piece (17) is connected to a sequence elastic element (20). The sequence elastic element (20) acts elastically on the sequence locking piece (17), causing the sequence locking piece (17) to move with the inner rail (3) and engage with the inner connecting locking piece (18), or the sequence locking piece (17) moves along the inner rail (3) and separates from the inner connecting locking piece (18). The outer connecting locking piece (19) is installed at the front end of the outer rail (1) and moves with the middle rail (2) and rubs against the outer rail (1).

8. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The middle rail (2) is provided with an inner bead nest restriction group for restricting the movement of the inner bead nest group (5). The inner bead nest restriction group includes an inner elastic hook (21) and an inner protrusion (22). The inner elastic hook (21) is installed at the front end of the middle rail (2). The inner bead nest group (5) is engaged or disengaged from the inner elastic hook (21) as the inner rail (3) moves. The inner protrusion (22) is integrally formed at the rear end of the middle rail (2) and abuts or disengages from the inner bead nest group (5) as the inner bead nest group (5) moves.

9. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The outer rail (1) is provided with an outer bead nest restriction group for restricting the movement of the outer bead nest group (4). The outer bead nest restriction group includes a front stop (23) and a rear stop (24). The front stop (23) is installed at the front end of the outer rail (1), and the rear stop (24) is installed at the rear end of the middle rail (2).

10. The server sequential opening and closing device with locking function according to claim 1, characterized in that: The outer rail (1) is equipped with an outer mounting plate (25) and an inner mounting plate (26) for easy installation in the cabinet. The inner mounting plate (26) is fastened to the outer rail (1), and the outer mounting plate (25) is adapted to be fastened to the inner mounting plate (26). The outer mounting plate (25) is provided with a drawer anti-collision component, which includes a brake pad (27) and a connecting boss (28). The connecting boss (28) is located on the outer front end of the middle rail (2). The brake pad (27) is installed on the outer mounting plate (25). The outer rail (1) has a clearance opening (29) for frictional connection between the brake pad (27) and the connecting boss (28) as the middle rail (2) moves.