Battery module pressure maintaining tray

Abstract

According to the battery module pressure maintaining tray, when the battery module pressure maintaining tray is applied to production of short battery modules, a double-module simultaneous pressure maintaining mode with the middle as a fixed reference and the movable pressing assemblies on the two sides is adopted, pressure maintaining is conducted on the two short battery modules at the same time, the resource utilization rate is increased, and the overall production efficiency and economic benefits are improved; when the battery module pressure maintaining tray is applied to production of a long battery module, a single-module pressure maintaining mode that a single-side fixed reference is adopted, and the other side of the battery module pressure maintaining tray is provided with a movable pressing assembly is adopted, so that the tray space can be effectively utilized and the stroke capacity of corresponding equipment can be fully applied under the condition of trays with the same size. By adjusting the positions of the first pressing movable end and the second pressing movable end (through the first adjusting structure and the second adjusting structure) and the detachable reference fixing assembly, the tray can be compatible with a short battery module and a long battery module, and the size compatibility of equipment is improved.

Description

Technical Field

[0001] The present application relates to the field of power battery technology, and in particular to a battery module pressure-maintaining tray. Background Art

[0002] In the current battery module production process, battery cells are primarily assembled by bonding with foam or uncured structural adhesive followed by extrusion. However, the resilience of these materials can cause the actual module length to exceed the design standard, thus affecting the quality of subsequent processing. Therefore, before and after CCS welding, a pressure-maintaining tray is required to apply constant pressure to the module to ensure stable module length, protect the precise CCS welding position, and maintain the consistency of the module's overall dimensions to support side panel welding and smooth installation into the pack box. As a key tool, the performance of the pressure-maintaining tray is directly related to the quality of module production.

[0003] Traditional pressure-holding pallet designs rely on a fixed datum on one side, with the movable end utilizing a trapezoidal lead screw coupled with an external servo motor for precise extrusion, and the self-locking nature of the lead screw to maintain the pressure-holding state. However, with the rapid advancement of product technology and evolving market demands, the integration of PACK systems has significantly increased, with module sizes increasing from small to large, the number of cells doubling, and the module length doubling accordingly.

[0004] Faced with this transformation, traditional pressure-maintaining pallets face challenges in being compatible with the sizes of new and old modules: to accommodate both long and short modules, the size of the pallets and supporting equipment needs to be doubled, resulting in extremely low resource utilization when processing shorter modules, affecting overall production efficiency and economic benefits. Utility Model Content

[0005] The purpose of the embodiments of the present application is to provide a pallet device for a battery assembly line to solve the problems of existing pressure-maintaining pallets having poor dimensional compatibility, extremely low resource utilization, and affecting overall production efficiency and economic benefits.

[0006] An embodiment of the present application provides a battery module pressure-maintaining tray, comprising: a chassis, and a reference fixing assembly mounted on the chassis, a first pressing movable end, a second pressing movable end, a first adjustment structure, and a second adjustment structure;

[0007] The first pressing movable end and the second pressing movable end are respectively arranged on both sides of the length direction of the chassis;

[0008] The first adjusting structure is provided near the first pressing movable end, and the first adjusting structure is used to drive the first pressing movable end to approach or move away from the second pressing movable end along the length direction;

[0009] The second adjusting structure is provided near the second pressing movable end, and the second adjusting structure is used to drive the second pressing movable end to approach or move away from the first pressing movable end along the length direction;

[0010] The reference fixing assembly is detachably mounted in the middle of the chassis.

[0011] In the above technical solution, when the battery module pressure-maintaining pallet is used in the production of short battery modules, a dual-module simultaneous pressure-maintaining method with the middle as a fixed reference and movable clamping components on both sides is adopted. The simultaneous pressure-maintaining of two short battery modules improves resource utilization, and improves overall production efficiency and economic benefits. When the battery module pressure-maintaining pallet is used in the production of long battery modules, a single-module pressure-maintaining method with a fixed reference on one side and a movable clamping component on the other side is adopted. Under the same size pallet, the pallet space can be effectively utilized and the travel capacity of the corresponding equipment can be fully utilized. By adjusting the position of the first clamping movable end and the second clamping movable end (through the first adjustment structure and the second adjustment structure), as well as the detachable reference fixing component, the pallet can be compatible with short battery modules and long battery modules, improving the size compatibility of the equipment.

[0012] In some optional embodiments, a slide rail is further included;

[0013] The slide rail is mounted on the chassis, and the slide rail is arranged along the length direction;

[0014] The first pressing movable end and the second pressing movable end are arranged on the slide rail, and the first pressing movable end and the second pressing movable end move on the slide rail;

[0015] When the reference fixing assembly is installed on the chassis, the slide rail passes through the reference fixing assembly.

[0016] In some optional embodiments, the first pressing movable end includes a first slide and a first clamping plate;

[0017] The first slide is fixedly connected to the first clamping plate, and the first slide is arranged on the slide rail and moves on the slide rail;

[0018] The second pressing movable end includes a second slide and a second clamping plate;

[0019] The second slide is fixedly connected to the second clamping plate, the second slide is arranged on the slide rail and the first slide moves on the slide rail;

[0020] A battery module of a first length is clamped between the first clamping plate and the second clamping plate, a battery module of a second length is clamped between the first clamping plate and the reference fixing assembly, and a battery module of a second length is clamped between the second clamping plate and the reference fixing assembly; wherein the first length is greater than the second length.

[0021] In some optional embodiments, the first adjustment structure includes a first drive screw;

[0022] The first slide is sleeved on the first transmission screw, and the first pressing movable end is driven to move along the length direction by rotating the first transmission screw;

[0023] The second adjustment structure includes a second transmission screw;

[0024] The second slide is sleeved on the second transmission screw, and the second pressing movable end is driven to move along the length direction by rotating the second transmission screw.

[0025] In some optional embodiments, the first transmission screw and the second transmission screw are ball screws.

[0026] In the above technical solution, the use of a ball screw enables rapid compression and decompression of the battery module, improving the cycle efficiency of the production line equipment. The ball screw achieves transmission through the rolling of balls between the screw and the nut. This rolling friction method has higher transmission efficiency than sliding friction. The ball screw can achieve gapless transmission and has high positioning accuracy and rigidity. Because the ball screw uses rolling friction, it has less wear and a low friction coefficient, thus having a long service life. The design structure of the ball screw enables it to withstand large radial and axial loads and has high rigidity. This enables the ball screw to maintain a stable working state under high-speed movement and heavy load conditions.

[0027] In some optional embodiments, the first adjustment structure further includes a first connector;

[0028] The first connecting head is fixedly mounted on one end of the first transmission screw;

[0029] The second adjustment structure also includes a second connector;

[0030] The second connecting head is fixedly mounted on one end of the second transmission screw.

[0031] In some optional embodiments, the first connector and the second connector are respectively located on both sides of the chassis in the length direction.

[0032] In some optional embodiments, the first adjustment structure further includes a first drive motor;

[0033] The output end of the first driving motor is equipped with a first driving connector, and the first driving connector is adapted to the first connector;

[0034] The second adjustment structure also includes a second drive motor;

[0035] The output end of the second drive motor is equipped with a second drive connector, which is adapted to the second connector.

[0036] In some optional embodiments, the first adjustment structure further includes a first ratchet self-locking assembly;

[0037] The first ratchet self-locking assembly is used to lock the first transmission screw when the first transmission screw is rotated to a pressing position, thereby maintaining the pressure of the corresponding battery module;

[0038] The second adjustment structure also includes a second ratchet self-locking assembly;

[0039] The second ratchet self-locking assembly is used to lock the second transmission screw when the second transmission screw is rotated to reach the pressing position, thereby maintaining the pressure of the corresponding battery module.

[0040] In some optional embodiments, the first ratchet self-locking assembly includes: a first ratchet, a first pawl, a first spring, and a first unlocking mechanism;

[0041] The first ratchet is fixedly mounted on one end of the first transmission screw, and the first pawl is connected to the first spring; when the first ratchet rotates counterclockwise, the first ratchet moves the first pawl and compresses the first spring until the first clamping movable end reaches the clamping position, the first spring resets, and the first pawl presses against the gap in the first ratchet to achieve self-locking; the first unlocking mechanism is connected to the first pawl;

[0042] The second ratchet self-locking assembly includes: a second ratchet, a second pawl, a second spring and a second unlocking mechanism;

[0043] The second ratchet is fixedly mounted on one end of the second transmission screw, and the second pawl is connected to the second spring; when the second ratchet rotates counterclockwise, the second ratchet moves the second pawl and compresses the second spring until the second clamping movable end reaches the clamping position, the second spring resets, and the second pawl presses against the gap in the second ratchet to achieve self-locking; the second unlocking mechanism is connected to the second pawl.

[0044] In the above technical solution, the ratchet method is used for self-locking, which greatly improves the dimensional consistency and accuracy of the battery module pressure maintenance and improves the production yield of the battery module. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0046] Figure 1 A top view of a battery module pressure-maintaining tray provided in an embodiment of the present application;

[0047] Figure 2 A three-dimensional diagram of a battery module pressure-maintaining tray provided in an embodiment of the present application;

[0048] Figure 3This is a structural diagram of the second ratchet self-locking assembly provided in an embodiment of the present application.

[0049] Icons: 1-reference fixing assembly, 2-slide rail, 3-first transmission screw, 4-first clamping movable end, 5-second transmission screw, 6-second clamping movable end, 7-second ratchet self-locking assembly, 71-screw support seat, 72-second ratchet, 73-second connecting head, 74-second unlocking mechanism, 75-second pawl, 76-second spring, 8-chassis. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for which protection is claimed, but merely represents selected embodiments of the present application. Based on the embodiments in this application, all other embodiments obtained by users of ordinary skill in the art without making creative work are within the scope of protection of this application.

[0051] Please refer to Figure 1 and Figure 2 , Figure 1 A top view of a battery module pressure-maintaining tray provided in an embodiment of the present application. Figure 2 A three-dimensional diagram of a battery module pressure-maintaining tray provided in an embodiment of the present application. The battery module pressure-maintaining tray of this embodiment includes: a chassis 8, and a reference fixing component 1, a first clamping movable end 4, a second clamping movable end 6, a first adjustment structure, and a second adjustment structure installed on the chassis 8; the first clamping movable end 4 and the second clamping movable end 6 are respectively arranged on both sides of the length direction of the chassis 8; the first adjustment structure is arranged near the first clamping movable end 4, and the first adjustment structure is used to drive the first clamping movable end 4 to approach or move away from the second clamping movable end 6 along the length direction; the second adjustment structure is arranged near the second clamping movable end 6, and the second adjustment structure is used to drive the second clamping movable end 6 to approach or move away from the first clamping movable end 4 along the length direction; the reference fixing component 1 is detachably installed in the middle of the chassis 8.

[0052] In the embodiment of the present application, when the battery module pressure-maintaining pallet is used in the production of short battery modules, a dual-module simultaneous pressure-maintaining method with the middle as a fixed reference and movable clamping components on both sides is adopted. The simultaneous pressure-maintaining of the two short battery modules improves resource utilization, and improves overall production efficiency and economic benefits. When the battery module pressure-maintaining pallet is used in the production of long battery modules, a single-module pressure-maintaining method with a fixed reference on one side and a movable clamping component on the other side is adopted. Under the same size pallet, the pallet space can be effectively utilized and the travel capacity of the corresponding equipment can be fully utilized. By adjusting the position of the first clamping movable end 4 and the second clamping movable end 6 (through the first adjustment structure and the second adjustment structure), as well as the detachable reference fixing component 1, the pallet can be compatible with short battery modules and long battery modules, thereby improving the size compatibility of the equipment.

[0053] In some optional embodiments, a slide rail 2 is further included; the slide rail 2 is installed on the chassis 8, and the slide rail 2 is arranged along the length direction; the first clamping movable end 4 and the second clamping movable end 6 are arranged on the slide rail 2, and the first clamping movable end 4 and the second clamping movable end 6 move on the slide rail 2; when the reference fixing component 1 is installed on the chassis 8, the slide rail 2 passes through the reference fixing component 1.

[0054] In some optional embodiments, the first clamping movable end 4 includes a first slide and a first clamping plate; the first slide and the first clamping plate are fixedly connected, the first slide is disposed on the slide rail 2, and the first slide moves on the slide rail 2; the second clamping movable end 6 includes a second slide and a second clamping plate; the second slide and the second clamping plate are fixedly connected, the second slide is disposed on the slide rail 2, and the first slide moves on the slide rail 2;

[0055] A battery module of a first length is clamped between the first clamping plate and the second clamping plate, a battery module of a second length is clamped between the first clamping plate and the reference fixing assembly 1, and a battery module of a second length is clamped between the second clamping plate and the reference fixing assembly 1; wherein the first length is greater than the second length.

[0056] In some optional embodiments, the first adjustment structure includes a first transmission screw 3; the first slide is sleeved on the first transmission screw 3, and the first pressing movable end 4 is driven to move along the length direction by rotating the first transmission screw 3;

[0057] The second adjustment structure includes a second transmission screw 5; the second slide is sleeved on the second transmission screw 5, and the second pressing movable end 6 is driven to move along the length direction by rotating the second transmission screw 5.

[0058] In some optional embodiments, the first transmission screw 3 and the second transmission screw 5 are ball screws.

[0059] In the embodiment of the present application, by adopting a ball screw, the battery module can be quickly compressed and quickly decompressed, thereby improving the beat efficiency of the production line equipment. The ball screw realizes transmission by the rolling of the balls between the screw and the nut. This rolling friction method has higher transmission efficiency than sliding friction. The ball screw can achieve gapless transmission and has high positioning accuracy and rigidity. Because the ball screw adopts rolling friction, it has less wear and a low friction coefficient, so it has a longer service life. The design structure of the ball screw enables it to withstand large radial loads and axial loads, and has high rigidity. This enables the ball screw to maintain a stable working state under high-speed movement and heavy-load conditions.

[0060] In some optional embodiments, the first adjustment structure also includes a first connector; the first connector is fixedly mounted on one end of the first transmission screw 3; the second adjustment structure also includes a second connector 73; the second connector 73 is fixedly mounted on one end of the second transmission screw 5.

[0061] In some optional embodiments, the first connector and the second connector 73 are respectively located on both sides of the chassis 8 in the length direction.

[0062] In some optional embodiments, the first adjustment structure also includes a first drive motor; the output end of the first drive motor is equipped with a first drive connector, and the first drive connector and the first connector are adapted to each other; the second adjustment structure also includes a second drive motor; the output end of the second drive motor is equipped with a second drive connector, and the second drive connector and the second connector 73 are adapted to each other.

[0063] In some optional embodiments, the first adjustment structure further includes a first ratchet self-locking assembly; the first ratchet self-locking assembly is used to lock the first transmission screw 3 when the first transmission screw 3 is rotated to reach the pressing position, thereby maintaining the pressure of the corresponding battery module;

[0064] The second adjustment structure also includes a second ratchet self-locking assembly 7; the second ratchet self-locking assembly 7 is used to lock the second transmission screw 5 when the second transmission screw 5 is rotated to reach the pressing position, thereby maintaining the pressure of the corresponding battery module.

[0065] For details, please refer to Figure 3 , Figure 3 This is a structural diagram of the second ratchet self-locking assembly 7 provided in an embodiment of the present application. The first ratchet self-locking assembly and the second ratchet self-locking assembly of this embodiment have the same structure, and the first ratchet self-locking assembly is not shown in the figure.

[0066] In some optional embodiments, the first ratchet self-locking assembly includes: a first ratchet, a first pawl, a first spring, and a first unlocking mechanism; the first ratchet is fixedly mounted on one end of the first transmission screw 3, and the first pawl is connected to the first spring; when the first ratchet rotates counterclockwise, the first ratchet shifts the first pawl and compresses the first spring until the first clamping movable end 4 reaches the clamping position, the first spring resets, and the first pawl presses against the gap in the first ratchet to achieve self-locking; the first unlocking mechanism is connected to the first pawl;

[0067] The second ratchet self-locking assembly 7 includes: a second ratchet 72, a second pawl 75, a second spring 76 and a second unlocking mechanism 74; the second ratchet 72 is fixedly mounted on one end of the second transmission screw 5, the second ratchet 72 is arranged close to the screw support seat 71, and the second pawl 75 is connected to the second spring 76; during the counterclockwise rotation of the second ratchet 72, the second ratchet 72 moves the second pawl 75 and compresses the second spring 76 until the second clamping movable end 6 reaches the clamping position, the second spring 76 is reset, and the second pawl 75 presses against the gap of the second ratchet 72 to achieve self-locking; the second unlocking mechanism 74 is connected to the second pawl 75.

[0068] In the embodiment of the present application, a ratchet method is used for self-locking, which greatly improves the dimensional consistency and accuracy of the battery module pressure maintenance and improves the production yield of the battery module.

[0069] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0070] In the description of the present application, it should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, or are the orientations or positional relationships in which the products of the present solution are usually placed when in use. These are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limitations on the present application. In addition, the terms "first", "second", "third" and the like are only used to distinguish descriptions, and cannot be understood as indicating or implying relative importance. In addition, terms such as "horizontal" and "vertical" do not mean that the components are required to be absolutely horizontal or suspended, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted. It should also be noted that, in the description of this application, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Users of ordinary skill in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0071] In the embodiments provided in this application, it should be understood that the disclosed devices can be implemented in other ways. The device embodiments described above are merely schematic. For example, the division of the units is merely a logical function division. There may be other division methods in actual implementation. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed may be through some communication interface, the indirect coupling or communication connection of the device or unit may be electrical, mechanical or other forms.

[0072] In addition, the units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed across multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0073] Furthermore, the functional modules in each embodiment of the present application can be integrated together to form an independent part, or each module can exist independently, or two or more modules can be integrated to form an independent part.

[0074] In this document, relational terms such as first and second, etc. are used merely to distinguish one entity or operation from another entity or operation, but do not necessarily require or imply any actual relationship or order between these entities or operations.

[0075] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection of the present application. For those skilled in the art, various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A battery module pressure-maintaining tray, characterized in that: include: A chassis, and a reference fixing assembly, a first pressing movable end, a second pressing movable end, a first adjustment structure, and a second adjustment structure mounted on the chassis; The first pressing movable end and the second pressing movable end are respectively arranged on both sides of the length direction of the chassis; The first adjusting structure is disposed near the first pressing movable end, and the first adjusting structure is used to drive the first pressing movable end to approach or move away from the second pressing movable end along the length direction; The second adjusting structure is provided near the second pressing movable end, and the second adjusting structure is used to drive the second pressing movable end to approach or move away from the first pressing movable end along the length direction; The reference fixing assembly is detachably mounted on the middle portion of the chassis.

2. The battery module pressure-maintaining tray according to claim 1, wherein: Also includes slide rails; The slide rail is mounted on the chassis, and the slide rail is arranged along the length direction; The first pressing movable end and the second pressing movable end are arranged on the slide rail, and the first pressing movable end and the second pressing movable end move on the slide rail; When the reference fixing assembly is mounted on the chassis, the slide rail passes through the reference fixing assembly.

3. The battery module pressure-maintaining tray according to claim 2, wherein: The first pressing movable end includes a first slide and a first clamping plate; The first slide is fixedly connected to the first clamping plate, the first slide is arranged on the slide rail and moves on the slide rail; The second pressing movable end includes a second slide and a second clamping plate; The second slide is fixedly connected to the second clamping plate, the second slide is arranged on the slide rail and the first slide moves on the slide rail; A battery module of a first length is clamped between the first clamping plate and the second clamping plate, a battery module of a second length is clamped between the first clamping plate and the reference fixing assembly, and a battery module of a second length is clamped between the second clamping plate and the reference fixing assembly; wherein the first length is greater than the second length.

4. The battery module pressure-maintaining tray according to claim 3, wherein: The first adjustment structure includes a first transmission screw; The first slide is sleeved on the first transmission screw, and the first pressing movable end is driven to move along the length direction by rotating the first transmission screw; The second adjustment structure includes a second transmission screw; The second slide is sleeved on the second transmission screw, and the second pressing movable end is driven to move along the length direction by rotating the second transmission screw.

5. The battery module pressure-maintaining tray according to claim 4, wherein: The first transmission screw and the second transmission screw are ball screws.

6. The battery module pressure-maintaining tray according to claim 5, wherein: The first adjustment structure further includes a first connector; The first connecting head is fixedly mounted on one end of the first transmission screw; The second adjustment structure further includes a second connector; The second connecting head is fixedly mounted on one end of the second transmission screw.

7. The battery module pressure-maintaining tray according to claim 6, wherein: The first connector and the second connector are respectively located on both sides of the chassis in the length direction.

8. The battery module pressure-maintaining tray according to claim 7, wherein: The first regulating structure further includes a first driving motor; The output end of the first driving motor is equipped with a first driving connector, and the first driving connector is adapted to the first connector; The second adjustment structure further includes a second drive motor; The output end of the second driving motor is equipped with a second driving connector, and the second driving connector is adapted to the second connector.

9. The battery module pressure-maintaining tray according to claim 8, wherein: The first adjustment structure further includes a first ratchet self-locking assembly; The first ratchet self-locking assembly is used to lock the first transmission screw when the first transmission screw is rotated to a pressing position, thereby maintaining the pressure of the corresponding battery module; The second adjustment structure further includes a second ratchet self-locking assembly; The second ratchet self-locking assembly is used to lock the second transmission screw when the second transmission screw is rotated to reach the pressing position, thereby maintaining the pressure of the corresponding battery module.

10. The battery module pressure-maintaining tray according to claim 9, wherein: The first ratchet self-locking assembly includes: a first ratchet, a first pawl, a first spring and a first unlocking mechanism; The first ratchet is fixedly mounted on one end of the first transmission screw, and the first pawl is connected to the first spring. When the first ratchet rotates counterclockwise, the first ratchet moves the first pawl and compresses the first spring until the first pressing movable end reaches the pressing position, the first spring resets, and the first pawl presses against the gap in the first ratchet to achieve self-locking. The first unlocking mechanism is connected to the first pawl. The second ratchet self-locking assembly includes: a second ratchet, a second pawl, a second spring and a second unlocking mechanism; The second ratchet is fixedly mounted on one end of the second transmission screw, and the second pawl is connected to the second spring; when the second ratchet rotates counterclockwise, the second ratchet moves the second pawl and compresses the second spring until the second clamping movable end reaches the clamping position, the second spring resets, and the second pawl presses against the gap of the second ratchet to achieve self-locking; the second unlocking mechanism is connected to the second pawl.

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