A sofa bed with a dual rocker assembly

Abstract

This utility model relates to a sofa bed with a dual rocker arm assembly. Existing sofa beds have complex backrest assembly structures, affecting the user experience. This utility model includes a base and a backrest located at the rear edge of the base. The bottom of the backrest and the rear of the base are connected by a dual rocker arm assembly. The dual rocker arm assembly includes a first rocker arm and a second rocker arm spanning between the backrest and the base. Guided by the cooperation of the first and second rocker arms, the backrest switches between a vertical sitting position and a horizontal reclining position along a preset path. By placing the first and second rocker arms between the backrest and the base, the posture and position of the backrest can be adjusted and controlled, allowing it to switch between sitting and reclining positions. This effectively simplifies the structure, facilitates assembly and maintenance, reduces production costs, and also facilitates switching between positions, improving the user experience.

Description

Technical Field

[0001] This utility model relates to the field of furniture, specifically to a sofa bed. Background Technology

[0002] Existing sofa beds include a base, armrests, and a backrest. The armrests are located on both sides of the base, and the backrest is located at the rear edge of the base. A movable platform is provided within the base, which cooperates with the backrest to allow the sofa bed to switch between a sitting and reclining position. When the sofa bed is in the sitting position, the backrest is upright, and the movable platform and backrest work together to provide support for the user. When the sofa bed is in the reclining position, the backrest is tilted and level with the top surface of the movable platform, so that the backrest and movable platform combine to form a support surface for the user to lie on, providing a comfortable reclining experience. The existing backrest assembly structure needs to balance positioning reliability and ease of movement, resulting in a complex structure that increases assembly and maintenance difficulty, hinders switching, increases production costs, and negatively impacts the user experience. Utility Model Content

[0003] To address the shortcomings of existing technologies, this utility model provides a sofa bed with a dual rocker assembly. A first rocker and a second rocker are arranged between the backrest and the base to adjust and control the posture and position of the backrest and switch it between a sitting and reclining position. This not only simplifies the structure, facilitates assembly and maintenance, and reduces production costs, but also makes it easier to switch between positions and improves the user experience.

[0004] This utility model achieves its purpose through the following method: a sofa bed with a dual rocker arm assembly, comprising a base and a backrest disposed at the rear edge of the base. The bottom of the backrest and the rear of the base are connected by a dual rocker arm assembly, which includes a first rocker arm and a second rocker arm spanning between the backrest and the base. Guided by the cooperation of the first and second rocker arms, the backrest can switch between a vertical sitting position and a horizontal reclining position along a preset path. The dual rocker arm assembly, consisting of a first rocker arm and a second rocker arm respectively connected to different areas of the backrest, adjusts and controls the posture and position of the backrest, allowing it to switch between sitting and reclining positions. This effectively simplifies the structure, facilitates assembly and maintenance, reduces production costs, and also facilitates switching between positions, reducing the failure rate and improving the user experience.

[0005] Preferably, the dual rocker assembly includes a fixed plate disposed at the rear edge of the base and a movable plate disposed at the bottom of the backrest sidewall. The ends of the first and second rockers are rotatably connected to the corresponding fixed and movable plates to guide the movable plate's movement. The two ends of the first and second rockers are respectively connected to the fixed and movable plates. The fixed plate limits and supports the first and second rockers, ensuring that both rockers swing at their bottom ends connected to the fixed plate. The movable plate is fixed to the backrest and rotatably connected to the top ends of the first and second rockers. Because the top ends of the first and second rockers have different movement paths, the corresponding area of ​​the movable plate moves synchronously with the top end of the corresponding first or second rocker, thereby enabling the movable plate to adjust its posture and move its position. This allows the backrest to precisely switch between a sitting and reclining position under the guidance of the movable plate. The movable plate can be an independent component mounted on the backrest sidewall or formed from the backrest sidewall, both serving to drive the backrest's movement.

[0006] Preferably, the fixed plate is triangular with a horizontal bottom edge and a rearward-sloping apex, while the movable plate is square. The first and second rockers are staggered and straddle the fixed and movable plates, allowing them to move independently. The fixed plate provides both limiting and support for the first and second rockers. Its triangular shape satisfies the positional requirements between the bottom ends of the first and second rockers while effectively reducing its size and facilitating disassembly and maintenance. The staggered placement of the bottom ends of the first and second rockers on the fixed plate allows for greater differences in their movement paths at their top ends, thus satisfying the requirements for adjusting the attitude and position of the movable plate.

[0007] Preferably, when the backrest is in a seated position, the bottom end of the first rocker arm is rotatably connected to the top corner of the fixed plate, and the top end is rotatably connected to the front edge of the top edge of the movable plate. The bottom end of the second rocker arm is rotatably connected to the front edge of the bottom edge of the fixed plate, and the top end is rotatably connected to the front edge of the bottom edge of the movable plate. Both the first and second rockers swing around their bottom ends, causing their top ends to swing along preset paths and drive the backrest to switch to a reclining position. The two ends of the movable plate move independently along the swing paths of the top ends of the first and second rockers, respectively, allowing the movable plate to simultaneously adjust its posture and position, thus ensuring that the backrest can smoothly switch between a reclining and seated position. The preset path of the top end of the first rocker arm refers to an arc path centered on the bottom end of the first rocker arm, and the preset path of the top end of the second rocker arm refers to an arc path centered on the bottom end of the second rocker arm.

[0008] Preferably, the dual rocker arm assemblies are in two sets and symmetrically arranged on both sides of the backrest, so that both sides of the backrest are subjected to force and move in unison. The symmetrical dual rocker arm assemblies on both sides of the backrest ensure that both sides of the backrest move synchronously under the guidance of the corresponding movable plates, preventing the backrest from getting stuck due to inconsistent movements on both sides.

[0009] Preferably, the base includes a retractable platform that can extend and retract forward and backward. Below the platform is a retractable support section that can also extend and retract forward and backward. The support section can switch between a support state flush with the top surface of the platform and a concealed state hidden beneath the platform. The platform moves forward relative to the base, providing space for the backrest to be switched to a reclining position. When the backrest is in a sitting position, the support section can be switched to a concealed state and stored under the platform, allowing the sofa bed to function as a sofa. Alternatively, when the backrest is in a reclining position, it can move forward and raise to the support state, ensuring that the top surfaces of the backrest, platform, and support section are at the same height and fit together to form a flat support surface.

[0010] Preferably, the fixed plate has a limiting pin at the middle of its inclined side for the second rocker arm to abut. When the backrest is switched to a sitting position, the second rocker arm swings backward and is positioned with the limiting pin to limit the backrest's backward displacement. Limiting pins on both fixed plates limit the swinging of the second rocker arms on both sides of the backrest, thereby limiting the backrest's position, effectively improving leaning stability, preventing the backrest from shifting backward due to leaning force, and enhancing leaning support comfort.

[0011] Preferably, the top surface of the base and the area adjacent to the fixing plate form a limiting surface. When the backrest is switched to the reclining position, the backrest rests on the limiting surface to limit downward displacement. When the backrest is switched to the reclining position, its two side edges rest on the limiting surface from top to bottom, providing support and ensuring that the backrest can withstand the user's weight, preventing it from collapsing due to force and improving reclining comfort.

[0012] Preferably, the bottom of the backrest moves from back to front, and the top of the backrest moves from top to bottom, so that the backrest can switch from a vertical sitting position to a horizontal reclining position. The backrest can adjust its orientation through reasonable posture adjustment. When the backrest is in a vertical sitting position, it faces forward and forms a support surface for the user's back to lean against. When the backrest is in a horizontal reclining position, it faces upward, and its top surface is flush with the top surface of the movable seat, which can be used to support the user's neck and head.

[0013] Preferably, the first rocker arm has a first anchor point at its center, and the movable plate has a second anchor point. The dual rocker arm assembly includes a support spring bridging the first and second anchor points. The support spring, through contraction and reset, drives the backrest to switch and position to a sitting or reclining position. The support spring utilizes the preload generated by its contraction and reset to drive the movable plate, thereby driving the backrest to switch to a sitting or reclining position. The support spring reaches its maximum length and obtains maximum preload at a specific position during the backrest switching process, allowing it to use this preload to drive the backrest to switch between sitting and reclining positions in both directions and provide a positioning force, preventing the backrest from detaching from the sitting or reclining position. When the backrest needs to switch positions, it must first receive external force and counteract the preload from the support spring before switching positions, effectively improving the reliability of the backrest's positioning.

[0014] Preferably, when the backrest is in a seated position, the second anchor point is located at the center of the front edge of the movable plate. When the backrest switches between a seated and a reclining position, the assist spring is stretched to obtain a preload force that drives the backrest to switch and position. When the backrest switches between a seated and a reclining position, the top of the first rocker arm spans between the two sides of the assist spring. When the top of the first rocker arm coincides with the assist spring, the assist spring is stretched to its longest length, at which point the assist spring has the maximum preload force. Regardless of which side the top of the first rocker arm tilts to, the preload force can be reduced by shortening the length of the assist spring, thereby driving the backrest to switch to the corresponding seated or reclining position, ensuring that the assist spring can provide bidirectional assistance for the backrest position switching.

[0015] Preferably, when the first anchor point, the top of the first rocker arm, and the second anchor point are aligned on the same straight line, the length of the assist spring is A, and the original length of the assist spring in the unloaded state is D, where A > D. The assist spring is always in a stretched state during use, ensuring that it consistently applies a preload force generated by contraction to the first and second anchor points, thus providing reliable positioning for the backrest.

[0016] Preferably, when the backrest is switched to a sitting position, the top of the first rocker arm is located on the side of the assist spring facing away from the second rocker arm, and the length of the assist spring is B, where A > B > D. Alternatively, when the backrest is switched to a reclining position, the top of the first rocker arm is located between the assist spring and the second rocker arm, and the length of the assist spring is C, where A > C > D. During use, the assist spring is stretched and can generate preload by contracting and returning to its original position. The magnitude of the preload generated by the assist spring is proportional to its stretched length. When the first anchor point, the top of the first rocker arm, and the second anchor point are on the same straight line, the length of the assist spring is A, and it has the maximum preload. Under the action of the preload, the assist spring can shorten its own length and drive the top of the first rocker arm to shift to both sides of the assist spring, thereby driving the backrest to switch to a sitting or reclining position, thus assisting the backrest in position switching and positioning.

[0017] The beneficial effects of this utility model are as follows: A dual rocker assembly is set between the backrest and the base. The dual rocker assembly includes a first rocker and a second rocker connected to different areas of the backrest, which adjusts and controls the posture and position of the backrest, allowing the backrest to switch between a sitting position and a reclining position. This not only simplifies the structure, facilitates assembly and maintenance, and reduces production costs, but also makes it easy to switch between uses, reduces the failure rate, and improves the user experience. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the assembly structure when the backrest is in a seated position.

[0019] Figure 2 This is a schematic diagram of the assembly structure when the backrest is in a reclining position.

[0020] Figure 3 This is a structural diagram of the sofa bed with the backrest in a seated position.

[0021] Figure 4 This is a structural diagram of the sofa bed with the backrest in a reclining position.

[0022] In the diagram: 1. Base, 2. Backrest, 3. First rocker arm, 4. Second rocker arm, 5. Fixed plate, 6. Movable plate, 7. Support part, 8. Assist spring, 9. First anchor point, 10. Second anchor point, 11. Movable platform, 12. Limiting pin, 13. Limiting surface. Detailed Implementation

[0023] The essential features of this utility model will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1A sofa bed with a dual rocker arm assembly is shown, comprising a base 1 and a backrest 2 disposed at the rear edge of the base 1. The bottom of the backrest 2 and the rear of the base 1 are connected by a dual rocker arm assembly, which includes a first rocker arm 3 and a second rocker arm 4 spanning between the backrest 2 and the base 1. Guided by the cooperation of the first rocker arm 3 and the second rocker arm 4, the backrest 2 can switch between a vertical sitting position and a horizontal reclining position along a preset path. The dual rocker arm assembly, consisting of a first rocker arm 3 and a second rocker arm 4 respectively connected to different areas of the backrest 2, adjusts and controls the posture and position of the backrest 2, allowing it to switch between sitting and reclining positions. This effectively simplifies the structure, facilitates assembly and maintenance, reduces production costs, and also facilitates switching between positions, reducing the failure rate and improving the user experience.

[0025] In actual operation, the dual rocker assembly includes a fixed plate 5 disposed at the rear edge of the base 1 and a movable plate 6 disposed at the bottom of the side wall of the backrest 2. The ends of the first rocker 3 and the second rocker 4 are rotatably connected to the corresponding fixed plate 5 and movable plate 6 to guide the movable plate 6 to move. The fixed plate 5 is a triangle with a horizontal bottom edge and a rearward tilted top angle, and the movable plate 6 is square. The first rocker 3 and the second rocker 4 are staggered across the fixed plate 5 and the movable plate 6 to allow the first rocker 3 and the second rocker 4 to move independently. Specifically, the first rocker 3 and the second rocker 4 both swing around their bottom ends, so that the top ends of the first rocker 3 and the second rocker 4 have corresponding swing paths. The ends of the movable plate 6 are driven and guided by the top ends of the first rocker 3 and the second rocker 4 respectively, realizing the posture adjustment and position adjustment of the backrest 2, thereby driving the backrest 2 to switch to a reclining position.

[0026] In actual operation, when the backrest 2 is in a sitting position, the bottom end of the first rocker arm 3 is rotatably connected to the top corner of the fixed plate 5, and the top end is rotatably connected to the front edge of the top edge of the movable plate 6. The bottom end of the second rocker arm 4 is rotatably connected to the front edge of the bottom edge of the fixed plate 5, and the top end is rotatably connected to the front edge of the bottom edge of the movable plate 6. When the backrest 2 switches from a sitting position to a reclining position, the bottom of the backrest 2 moves from back to front, and the top of the backrest 2 moves from top to bottom, so that the backrest 2 switches from a vertical sitting position to a horizontal reclining position. Specifically, the backrest 2 moves with the movable plate 6, the top edge of the movable plate 6 swings backward and descends, and the bottom end of the movable plate 6 swings forward and rises along an arc path, so that the backrest 2 rotates and adjusts from a sitting position with the front side wall facing forward to a reclining position with the front side wall facing upward (e.g., Figure 2 (As shown).

[0027] In actual operation, the first rocker arm 3 has a first anchor point 9 in the middle, and the movable plate 6 has a second anchor point 10. The dual rocker arm assembly includes a booster spring 8 bridging the first anchor point 9 and the second anchor point 10. The booster spring 8 drives the backrest 2 to switch and position to a sitting or lying position by retracting and resetting. The second anchor point 10 and the top of the first rocker arm 3 are offset from each other on the movable plate 6, so that the relative position between the second anchor point 10 and the top of the first rocker arm 3 changes as the movable plate 6 rotates, which in turn causes the extension length of the booster spring 8 to change accordingly. The booster spring 8 obtains a differentiated preload to drive and limit the movable plate 6. Specifically, when the backrest 2 is in a sitting position, the second anchor point 10 is located in the middle of the front edge of the movable plate 6. When the backrest 2 switches between a sitting and lying position, the booster spring 8 obtains a preload by stretching to drive the backrest 2 to switch and position.

[0028] In actual operation, when the backrest 2 is in a sitting position, the second anchor point 10 is located in front of the first rocker arm 3 and between the first rocker arm 3 and the second rocker arm 4. The length of the assist spring 8 at this time is set to B. When the backrest 2 is in a reclining position, the second anchor point 10 is located above the rotational connection point between the first rocker arm 3 and the movable plate 6. The top part of the first rocker arm 3 is located on the side of the assist spring 8 facing the second rocker arm 4. The length of the assist spring 8 at this time is set to C. When the backrest 2 switches between the sitting and reclining positions, the top part of the first rocker arm 3 will pass back and forth on both sides of the assist spring 8, so that the top part of the first rocker arm 3 will coincide with the assist spring 8 at a certain moment. At this time, the first anchor point 9, the top part of the first rocker arm 3 and the second anchor point 10 are on the same straight line and are equal to the sum of the distance between the first anchor point 9 and the top part of the first rocker arm 3 and the distance between the top part of the first rocker arm 3 and the second anchor point 10, so that the length of the assist spring 8 reaches its maximum value. The length of the assist spring 8 at this time is set to A, and it has the maximum preload for contraction. Let the original length of the assist spring 8 in the unloaded state be D. By setting A > B > D and A > C > D, it can be seen that the assist spring 8 is always in a stretched state. When the backrest 2 switches from a sitting position to a lying position, the length of the assist spring 8 will first extend from B to A, and then shorten from A to C when the top of the first rocker arm 3 crosses the assist spring 8. During the process of extending from B to A, an external force is required to stretch the assist spring 8, so that the assist spring 8 can use its own preload to restrict the backrest 2 from leaving the sitting position. During the process of shortening from A to C, since the assist spring 8 can release the preload by shortening the distance, the preload can drive the backrest 2 to switch to the lying position and ensure that the backrest 2 is positioned in the lying position. Conversely, when the backrest 2 switches from a reclining position to a sitting position, the length of the assist spring 8 will first extend from C to A, and then shorten from A to B when the top of the first rocker arm 3 crosses the assist spring 8. During the process of extending from C to A, an external force is required to stretch the assist spring 8, so that the assist spring 8 can use its own preload to prevent the backrest 2 from leaving the reclining position. During the process of shortening from A to B, since the assist spring 8 can release the preload by shortening the distance, the preload can drive the backrest 2 to switch to the sitting position and ensure that the backrest 2 is positioned in the sitting position.

[0029] In actual operation, the dual rocker arm assemblies consist of two sets, symmetrically arranged on both sides of the backrest 2, so that both sides of the backrest 2 are subjected to force and move in unison. The two sets of dual rocker arm assemblies ensure that both sides of the backrest 2 can rotate and displace synchronously, thereby preventing the backrest 2 from jamming due to differential movements on both sides.

[0030] In actual operation, the base 1 is equipped with a retractable platform 11, and a retractable support 7 is located below the platform 11. The support 7 can switch between a support state flush with the top surface of the platform 11 and a hidden state concealed beneath the platform 11. The support 7 can switch between these two states, thus enabling the use of a sofa bed (e.g.,...). Figure 3 and 4 (As shown). When the sofa bed needs to be switched to a sitting position, the backrest 2 is switched to the sitting position, using the front side wall of the backrest 2 to form a backrest surface. The support part 7 is switched to a hidden state and hidden under the movable platform 11, so that the top surface of the movable platform 11 forms a shorter support surface for the user. The backrest surface and the support surface are used to improve sitting comfort. When the sofa bed needs to be switched to a reclining position, the backrest 2 is switched to the reclining position, and the support part 7 moves forward out of the movable platform 11 and switches to the support state, so that the top surfaces of the backrest 2, the movable platform 11, and the support part 7 are flush and connected to form a support surface for the user to lie down, improving reclining comfort.

[0031] In actual operation, the middle of the inclined side of the fixed plate 5 is provided with a limiting pin 12 for the second rocker arm 4 to abut. When the backrest 2 is switched to the sitting position, the second rocker arm 4 swings backward and is positioned with the limiting pin 12 to limit the backrest 2 from shifting backward. The limiting pins 12 on both fixed plates 5 limit the swing of the second rocker arm 4 on both sides of the backrest 2, thereby limiting the backrest 2 and effectively improving the stability of leaning back, preventing the backrest 2 from shifting backward due to the leaning force, and improving the comfort of leaning support.

[0032] In actual operation, the top surface of the base 1 and the adjacent area of ​​the fixing plate 5 form a limiting surface 13. When the backrest 2 is switched to the reclining position, the backrest 2 rests on the limiting surface 13 to limit the downward displacement of the backrest 2. When the backrest 2 is switched to the reclining position, its two side edges rest on the limiting surface 13 from top to bottom, providing support for the backrest 2 and ensuring that the backrest 2 can withstand the weight from the user, preventing the backrest 2 from collapsing downward due to force, and improving reclining comfort.

[0033] In practical operation, the sofa bed can be equipped with armrests as needed. The armrests and the base 1 are independent components that can be connected by fasteners. The armrests are fixed to both sides of the base 1, and the fixing plate 5 is installed on the base 1. Furthermore, the base 1 can also be a component of the armrests, meaning that the function of the base 1 is achieved by an equivalent structure located at a corresponding position on the armrests; all of these should be considered specific embodiments of this utility model. The fixing plate 5 and the base 1 are preferably metal parts, which effectively improves structural strength, extends service life, and is less prone to damage.

Claims

1. A sofa bed with a dual rocker arm assembly, comprising a base (1) and a backrest (2) disposed at the rear edge of the base (1), characterized in that, The bottom of the backrest (2) and the rear of the base (1) are connected by a dual rocker assembly. The dual rocker assembly includes a first rocker (3) and a second rocker (4) that span between the backrest (2) and the base (1). The backrest (2) switches between a vertical sitting position and a horizontal lying position along a preset path under the guidance of the first rocker (3) and the second rocker (4).

2. The sofa bed with dual rocker assembly of claim 1, wherein, The dual rocker assembly includes a fixed plate (5) disposed at the rear edge of the base (1) and a movable plate (6) disposed at the bottom of the side wall of the backrest (2). The ends of the first rocker (3) and the second rocker (4) are rotatably connected to the corresponding fixed plate (5) and movable plate (6) to guide the movable plate (6) to move.

3. A sofa bed with a dual rocker assembly as defined in claim 2, wherein, The fixed plate (5) is a triangle with a horizontal bottom edge and a backward tilted top angle. The movable plate (6) is square. The first rocker (3) and the second rocker (4) are staggered across the fixed plate (5) and the movable plate (6) so that the first rocker (3) and the second rocker (4) can move independently.

4. The sofa bed with dual rocker assembly of claim 3, wherein, When the backrest (2) is in a sitting position, the bottom end of the first rocker (3) is rotatably connected to the top corner of the fixed plate (5), and the top end is rotatably connected to the front of the top edge of the movable plate (6). The bottom end of the second rocker (4) is rotatably connected to the front of the bottom edge of the fixed plate (5), and the top end is rotatably connected to the front of the bottom edge of the movable plate (6). The first rocker (3) and the second rocker (4) both swing around the bottom end as the center, so that the top ends of the first rocker (3) and the second rocker (4) swing along a preset path and drive the backrest (2) to switch to a lying position.

5. The sofa bed with dual rocker assembly of claim 2, wherein, The dual rocker arm assembly consists of two sets, symmetrically arranged on both sides of the backrest (2), so that both sides of the backrest (2) are simultaneously subjected to force and linked together; or, the base (1) is provided with a movable platform (11) that can extend and retract forward and backward, and a support part (7) that can extend and retract forward and backward is provided below the movable platform (11), and the support part (7) can switch between a support state flush with the top surface of the movable platform (11) and a hidden state hidden under the movable platform (11); or, the fixing plate (5 The backrest (2) is provided with a limiting pin (12) in the middle of the inclined side for the second rocker arm (4) to abut. When the backrest (2) is switched to the sitting position, the second rocker arm (4) swings backward and is positioned with the limiting pin (12) to limit the backrest (2) from shifting backward; or, the top surface of the base (1) and the adjacent area of ​​the fixing plate (5) form a limiting surface (13). When the backrest (2) is switched to the reclining position, the backrest (2) rests on the limiting surface (13) to limit the backrest (2) from shifting downward.

6. A sofa bed with a dual rocker assembly according to any one of claims 2-5, characterized in that, The bottom of the backrest (2) moves from back to front, and the top of the backrest (2) moves from top to bottom, so that the backrest (2) switches from a vertical sitting position to a horizontal lying position.

7. A sofa bed with a dual rocker assembly as defined in claim 6, wherein, The first rocker arm (3) has a first anchor point (9) in the middle, and the movable plate (6) has a second anchor point (10). The dual rocker arm assembly includes a booster spring (8) that spans between the first anchor point (9) and the second anchor point (10). The booster spring (8) drives the backrest (2) to switch to a sitting or lying position and position itself by retracting and resetting.

8. The sofa bed with dual rocker assembly of claim 7, wherein, When the backrest (2) is in a sitting position, the second anchor point (10) is located in the middle of the front edge of the movable plate (6). When the backrest (2) switches between a sitting position and a lying position, the assist spring (8) obtains a preload force to drive the backrest (2) to switch and position by stretching.

9. The sofa bed with dual rocker assembly of claim 8, wherein, When the first anchor point (9), the top of the first rocker arm (3) and the second anchor point (10) are on the same straight line, the length of the assist spring (8) is A, and the original length of the assist spring (8) in the non-forced state is D, where A > D.

10. The sofa bed with dual rocker assembly of claim 9, wherein, When the backrest (2) is switched to a sitting position, the top of the first rocker (3) is located on the side of the assist spring (8) facing away from the second rocker (4), and the length of the assist spring (8) is B, A > B > D; or, when the backrest (2) is switched to a reclining position, the top of the first rocker (3) is located on the side of the assist spring (8) facing the second rocker (4), and the length of the assist spring (8) is C, A > C > D.

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