Hydraulic truck spring with anti-load biasing structure

Abstract

The utility model discloses a hydraulic carrier spring with anti -eccentric load structure, including reset spring, the adaptive alignment washer subassembly is set up in the bottom of reset spring, and its bottom is fixed on the support, sleeve subassembly is coaxially sleeved in the outside of reset spring, and its bottom is fixedly connected with adaptive alignment washer subassembly, a hydraulic cylinder of inverted installation, and the tail of cylinder body can be swingly connected on the support, and the end of piston rod downward stretches out and abuts on the top of sleeve subassembly, and the end of piston rod is fixedly connected with the lifting frame. The utility model belongs to spring technical field, and specifically refers to a hydraulic carrier spring with anti -eccentric load structure.

Description

Technical Field

[0001] This utility model belongs to the field of spring technology, specifically referring to a hydraulic transport vehicle spring with an anti-eccentric load structure. Background Technology

[0002] Manual hydraulic pallet trucks (commonly known as "pallet jacks") are indispensable short-distance handling equipment in logistics warehousing and production workshops. They use a hydraulic system driven by human power to lift and move goods. Their general design (rigid / hinged connection between hydraulic cylinder and lifting frame, with independent return spring) is basically reliable under normal working conditions.

[0003] The current mainstream design involves using an inverted hydraulic cylinder to lift the piston rod, which is rigidly or hingedly connected to the lifting frame to transmit lifting force. Simultaneously, a tension or compression spring is independently installed between the vehicle frame and the lifting frame to automatically reset the lifting frame when the hydraulic system depressurizes. This general solution can operate reliably under ideal conditions where the center of gravity is centered and the load is balanced.

[0004] However, in specific industrial sectors such as heavy machinery manufacturing, metal casting, building material handling, and large mold turnover, material handling conditions exhibit the following extreme characteristics: 1. Extremely heavy loads and unstable center of gravity: The handled objects are often heavy castings, large molds, irregular stone materials, or steel structural components, whose center of gravity is difficult to center and may dynamically shift; 2. Handling irregularly shaped goods with shifted centers of gravity or inaccurate placement is common and frequently used. This results in long-term uneven stress on both sides of the lifting frame, inevitably generating huge and continuous off-center load moments during operation, far exceeding the design capacity of general-purpose handling vehicles. Under these widespread non-ideal working conditions, the aforementioned general solutions gradually reveal the following structural limitations: 1. The problem of lateral force on hydraulic cylinders becomes prominent: Because the piston rod is directly connected to the lifting frame, the off-center load torque is converted into a lateral force acting on the piston rod. The internal piston and cylinder, and piston rod and seals of the hydraulic cylinder have a precise clearance fit, making them very sensitive to lateral forces. Under long-term or severe off-center load conditions, this lateral force will cause additional friction and compression between the piston rod and the seals and the inner wall of the cylinder, accelerating seal wear, increasing the risk of hydraulic oil leakage and lifting force reduction, and affecting the service life and reliability of the hydraulic cylinder.

[0005] 2. Reduced effectiveness of independent return springs: When the lifting frame tilts due to uneven load, the force axis of the independently installed return spring will form an angle with the actual movement direction of the lifting frame. This will reduce the effective return force of the spring, which may cause slow or stuck return action, or in extreme cases, failure to return to the correct position, affecting the smoothness of equipment operation and the certainty of the return action. Utility Model Content

[0006] To address the problems of reduced reliability of hydraulic cylinders due to lateral forces and insufficient efficiency of independent return springs in existing general solutions under off-center load conditions, this utility model provides a hydraulic pallet truck spring with an anti-off-center load structure.

[0007] To achieve the above functions, the technical solution adopted by this utility model is as follows: a hydraulic pallet truck spring with an anti-eccentric load structure is installed between the support and the lifting frame of the hydraulic pallet truck, including a return spring; An adaptive self-aligning washer assembly is disposed at the bottom of the return spring, and its bottom is fixed to the bracket; A sleeve assembly is coaxially sleeved on the outside of the return spring, and its bottom is fixedly connected to the self-aligning washer assembly; An inverted hydraulic cylinder, the tail of which is pivotally connected to the bracket, has its piston rod end extending downward and abutting against the top of the sleeve assembly; The end of the piston rod is fixedly connected to the lifting frame.

[0008] Furthermore, the self-aligning washer assembly includes a ball joint support fixed to the bracket, a convex ball movably accommodated within the ball joint support, and a ball joint top cover that covers the ball joint support to form a convex ball groove. The convex sphere and the convex spherical groove are in a spherical sliding fit. The ball joint top cover is fixed to the ball joint support seat by circumferentially distributed fasteners, which constrains the convex ball in the convex ball groove and restricts its axial dislodgement, while allowing the convex ball to rotate around its center within a predetermined angle. A cylinder is fixed on the convex ball, extending upward through the ball hinge top cover, and the top end of the cylinder is fixedly connected to the bottom of the sleeve assembly.

[0009] Furthermore, the sleeve assembly includes a bottom cylinder fixedly connected to the top of the cylinder and a top cylinder whose bottom is axially slidably inserted into the top of the bottom cylinder; The reset spring is housed within a cylindrical cavity formed by the bottom cylinder and the top cylinder.

[0010] Furthermore, a circular groove is formed at the top of the bottom cylinder, and the bottom of the top cylinder is slidably installed in the circular groove; The outer wall of the bottom cylinder is provided with a limiting groove communicating with the circular groove along the axial direction, and the outer wall of the top cylinder is fixedly provided with a limiting block, which is slidably disposed in the limiting groove.

[0011] Furthermore, an insertion hole is provided at the center of the top of the top cylinder; The end of the piston rod passes downward through the insertion hole and extends to the central axis position of the return spring; The lower end of the return spring is supported on the inner bottom surface of the bottom cylinder, while the upper end abuts against the inner top surface of the top cylinder. The piston rod is fixed with a radially protruding round shoulder; the axial distance between the lower surface of the round shoulder and the upper surface of the top wall of the top cylinder is set to be less than the effective support stroke of the hydraulic cylinder piston rod.

[0012] Furthermore, the top cylinder is provided with auxiliary grooves extending downward from the top surface symmetrically along the insertion hole; An auxiliary insert is fixedly provided at the end of the piston rod corresponding to the auxiliary groove, and the auxiliary insert slides in conjunction with the auxiliary groove.

[0013] Furthermore, the tail end of the hydraulic cylinder is mounted on the bracket via a spherical support.

[0014] Furthermore, a connecting plate is fixedly connected to the round shoulder, and the connecting plate is fixedly connected to the lifting frame.

[0015] Furthermore, the contact surface between the convex ball and the convex ball groove is provided with a wear-resistant and friction-reducing coating.

[0016] Compared with the prior art, the present invention achieves the following beneficial effects by adopting the above structure: 1. Effectively isolates lateral forces, protecting core components of the hydraulic cylinder. When the lifting frame tilts due to eccentric loading, the self-aligning washer assembly can automatically release constraints through its ball joint structure, dissipating potentially harmful lateral bending moments on the piston rod into the constraint force within the ball joint and the elastic deformation energy of the return spring. This mechanism fundamentally avoids abnormal friction and compression between the piston rod and the hydraulic cylinder seals under eccentric loading conditions, significantly reducing the risk of seal wear and hydraulic leakage, thereby improving the reliability and service life of the hydraulic cylinder under complex loads.

[0017] 2. Ensures immediate, reliable, and unaffected reset action, free from off-center load. The sleeve assembly provides precise linear guidance for the reset spring. By setting the gap between the round shoulder and the top cylinder to be less than the effective stroke of the piston rod, the reset spring is ensured to be pre-compressed throughout the lifting process, completely eliminating reset idle stroke and action delay. Combined with the spherical support at the tail of the hydraulic cylinder, the entire system allows the lifting frame to reset immediately and smoothly after depressurization. Its reset efficiency is no longer diminished by the tilt of the lifting frame, effectively solving the reset delay and jamming problems that may occur with traditional independent reset springs under off-center load, significantly improving operational safety and equipment reliability. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of a hydraulic pallet truck spring with an anti-eccentric load structure proposed in this utility model; Figure 2for Figure 1 A sectional view of; Figure 3 This is a schematic diagram of the overall structure of the adaptive self-aligning washer assembly and sleeve assembly proposed in this utility model; Figure 4 for Figure 3 Cross-section Figure 1 ; Figure 5 for Figure 3 Cross-section Figure 2 ; Figure 6 This is a schematic diagram of the overall structure of the hydraulic cylinder, piston rod, round shoulder, connecting plate, and spherical support seat proposed in this utility model.

[0019] Among them, 1. return spring, 2. self-aligning washer assembly, 21. ball joint support, 22. convex ball, 23. ball joint top cover, 24. cylinder, 3. sleeve assembly, 31. bottom cylinder, 311. circular groove, 312. limiting groove, 32. top cylinder, 321. limiting block, 322. insertion hole, 323. auxiliary groove, 4. hydraulic cylinder, 41. piston rod, 411. round shoulder, 412. connecting plate, 413. auxiliary insertion block, 42. spherical support. Detailed Implementation

[0020] The technical solution of this utility model will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this utility model. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this utility model.

[0021] In the description of this utility model, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0022] Unless otherwise expressly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances. The present invention will be further described in detail below with reference to the accompanying drawings.

[0023] like Figure 1-6 As shown, the present invention provides a hydraulic pallet truck spring with an anti-eccentric load structure, which is installed between the support and the lifting frame of the hydraulic pallet truck. It includes a return spring 1, which provides elastic force to reset the lifting frame; an adaptive self-aligning washer assembly 2, which is disposed at the bottom of the return spring 1 and whose bottom is fixed to the support; a sleeve assembly 3, which is coaxially sleeved on the outside of the return spring 1 and whose bottom is fixedly connected to the adaptive self-aligning washer assembly 2; and an inverted hydraulic cylinder 4, whose cylinder tail is swayably connected to the support, and whose piston rod 41 extends downward and abuts against the top of the sleeve assembly 3. The cylinder tail of the hydraulic cylinder 4 is mounted on the support through a spherical support seat 42, which allows the overall axis of the hydraulic cylinder 4 to swing slightly in multiple directions around its center. The bracket is a vertical support structure set on the main frame of the hydraulic pallet truck. It is usually a channel steel with an L-shaped or C-shaped cross section, and one side of it provides a mounting plane for the spring system. The self-aligning washer assembly 2 and the hydraulic cylinder 4 are both installed and fixed on different mounting positions on the upper and lower parts of the bracket, that is, they share the same mounting foundation. The end of the piston rod 41 is fixedly connected to the lifting frame. When the lifting frame tilts due to off-center load, the tilted lifting frame drives the top of the sleeve assembly 3 to tilt synchronously through the piston rod 41 fixed to it. This tilting motion forces the adaptive self-aligning washer assembly 2 to move, allowing the sleeve assembly 3 and the return spring 1 to swing around a self-aligning center as a whole, thereby releasing the constraint bending moment caused by the frame tilt. During this process, the return spring 1 generates uneven compression in the sleeve, forming an elastic return torque opposite to the direction of the off-center load. This torque reacts to the lifting frame through the self-aligning structure, helping it resist the off-center load, maintain stability, and drive it to return smoothly after pressure relief.

[0024] like Figure 1-5 As shown, the self-aligning washer assembly 2 includes a ball joint support 21 fixed on a bracket, a convex ball 22 movably accommodated within the ball joint support 21, and a ball joint top cover 23 covering the ball joint support 21 to form a convex ball groove; the convex ball 22 and the convex ball groove are in a spherical sliding fit; the contact surface between the convex ball 22 and the convex ball groove is provided with a wear-resistant and friction-reducing coating, and the wear-resistant coating (such as molybdenum disulfide or Teflon) can reduce the surface friction coefficient and reduce wear; The ball joint top cover 23 is fixed to the ball joint support 21 by circumferentially distributed fasteners, which constrains the convex ball 22 in the convex ball groove and restricts its axial dislodgement, while allowing the convex ball 22 to rotate around its center within a predetermined angle. A cylinder 24 is fixed on the convex ball 22, extending upward through the ball joint top cover 23. The top of the cylinder 24 is fixedly connected to the bottom of the sleeve assembly 3. This ball joint structure provides a multi-directional swinging bottom fulcrum for the entire spring and sleeve system. When the system is tilted under force, the convex ball 22 rotates in the ball groove, which can adaptively change the axial direction of the cylinder 24 and the sleeve, thereby transforming the bending stress that originally acted on the rigid connection into the constraint internal force at the spherical contact and the elastic deformation energy of the spring, realizing the flexible release of torque.

[0025] like Figure 1-6 As shown, the sleeve assembly 3 includes a bottom cylinder 31 fixedly connected to the top of the cylinder 24 and a top cylinder 32 whose bottom is axially slidably inserted into the top of the bottom cylinder 31; the return spring 1 is housed in the cylindrical cavity formed by the bottom cylinder 31 and the top cylinder 32; the sleeve assembly 3 forms a guiding and housing structure, the bottom cylinder 31 is fixedly connected to the self-aligning assembly as a fixed reference; the top cylinder 32 is a moving part used to transmit the action of the piston rod 41 and compress the spring; the cylindrical cavity provides radial constraint and protection for the return spring 1, ensuring that the spring always maintains a stable linear motion trajectory during compression and reset, and preventing the spring from becoming unstable or bending laterally; The bottom cylinder 31 has a downward-facing circular groove 311 at its top, and the bottom of the top cylinder 32 is slidably installed in the circular groove 311. The outer wall of the bottom cylinder 31 has a limiting groove 312 that communicates with the circular groove 311 along its axial direction. The outer wall of the top cylinder 32 is fixedly provided with a limiting block 321, which is slidably installed in the limiting groove 312 to constrain the top cylinder 32 to only move axially relative to the bottom cylinder 31. The circular groove 311 provides radial positioning and the main sliding support surface for the bottom of the top cylinder 32, bearing the main lateral force. The cooperation between the limiting groove 312 and the limiting block 321 strictly limits the circumferential rotational freedom of the top cylinder 32, while allowing it to slide freely axially. This design ensures that the top cylinder 32 can still make accurate linear movements when it is subjected to lateral force that may be caused by eccentric load, and will not jam or loosen from the bottom cylinder 31. This is the basis for achieving stable guidance and anti-eccentric load movement.

[0026] like Figure 1-6As shown, a socket 322 is provided at the center of the top of the top cylinder 32; the end of the piston rod 41 passes downward through the socket 322 and extends to the central axis of the return spring 1; the lower end of the return spring 1 is supported on the inner bottom surface of the bottom cylinder 31, and the upper end abuts against the inner top surface of the top cylinder 32; a radially protruding round shoulder 411 is fixed on the piston rod 41; the axial distance between the lower surface of the round shoulder 411 and the upper surface of the top wall of the top cylinder 32 is set to be less than the effective support stroke of the piston rod 41 of the hydraulic cylinder 4; thus, when the piston rod 41 of the hydraulic cylinder 4 extends and supports the lifting frame, the lower surface of the round shoulder 411 will press against the top wall of the top cylinder 32 and push the top cylinder 32 to move downward relative to the bottom cylinder 31, so that the return spring 1 is maintained throughout the entire lifting stroke of the lifting frame. The system is held in a compressed state, i.e., a pre-compressed state. The spring force generated in the pre-compressed state is configured to be greater than the reset resistance generated by the weight of the lifting frame and system friction, so as to ensure that when the hydraulic system is depressurized, the lifting frame can start immediately and smoothly and complete the reset action under the action of the reset spring 1. By accurately setting the axial assembly clearance between the round shoulder 411 and the top wall of the top cylinder 32, it is ensured that once the hydraulic cylinder 4 starts working and bears the load, the small extension of the piston rod 41 can eliminate the clearance, so that the round shoulder 411 and the top cylinder 32 form a rigid contact. After that, the continued extension of the piston rod 41 will directly be converted into the downward pressure on the top cylinder 32 and the compression of the reset spring 1. This design eliminates the delay and idle stroke of the reset action, making the reset response instantaneous and direct. The top cylinder 32 has symmetrically arranged auxiliary grooves 323 extending downward from the top surface along the insertion hole 322; the end of the piston rod 41 is fixedly provided with an auxiliary insertion block 413 corresponding to the auxiliary groove 323, and the auxiliary insertion block 413 slides with the auxiliary groove 323 to prevent relative rotation between the piston rod 41 and the top cylinder 32; the auxiliary insertion block 413 and the auxiliary groove 323 constitute a simple and reliable anti-rotation key; under off-center load conditions, there may be a torque between the piston rod 41 and the top cylinder 32 that causes relative rotation between the two; this anti-rotation structure can effectively resist such torsional torque, ensure that the force applied by the piston rod 41 to the top cylinder 32 is always kept in the expected direction, prevent loosening of the connection, abnormal wear or reduction of force transmission efficiency caused by relative rotation, and ensure the rigidity and stability of the entire force transmission chain.

[0027] like Figure 1 , 2 As shown in Figure 6, a connecting plate 412 is fixedly connected to the round shoulder 411. The connecting plate 412 is fixedly connected to the lifting frame. As a standardized mechanical interface, the connecting plate 412 realizes reliable force transmission and structural connection between the piston rod 41 and the lifting frame. It transforms the complex force of the lifting frame into concentrated force and bending moment acting on the axis of the piston rod 41. It is an important bridge connecting the actuator (hydraulic cylinder 4 and spring system) and the working part (lifting frame).

[0028] In practical use 1. Installation and positioning: First, the ball joint support 21 of the self-aligning washer assembly 2 is firmly fixed to the predetermined installation position of the bracket on one side of the bottom of the transport vehicle with multiple sets of bolts; similarly, the hydraulic cylinder 4 is installed on the corresponding position above the bracket on the other side through the ball support 42 at its tail.

[0029] 2. Connect the lifting frame: Connect the connecting plates 412 on the round shoulders 411 at the ends of the piston rods 41 of the hydraulic cylinders 4 on both sides to the corresponding sides of the lifting frame, adjust and ensure that the connection is firm, so that the lifting frame and the piston rods 41 on both sides become a linkage unit.

[0030] 3. Pre-compression state confirmation: Operate the hydraulic system to extend the piston rod 41 downwards to lift the lifting frame; during the lifting process, the round shoulder 411 of the piston rod 41 will press against the top cylinder 32 of the sleeve assembly 3, thereby causing the internal return spring 1 to enter and remain in the pre-compression state; this state is the key to ensuring that the automatic reset function is effective, and it must be confirmed that the return spring 1 is in the compressed state at any lifting height of the lifting frame.

[0031] 4. Normal use and anti-eccentric load process: The hydraulic pump is operated to lift the goods; when the goods with the center of gravity shifted are being transported, the eccentric load torque will cause the lifting frame to tilt; at this time, the tilt will be transmitted through the piston rod 41, forcing the top of the sleeve to tilt accordingly; this movement triggers the action of the bottom ball joint self-aligning washer, allowing the entire spring and sleeve unit to swing, thereby absorbing and offsetting the eccentric load torque and preventing the hydraulic cylinder 4 from bearing lateral force; at the same time, the unevenly compressed return spring 1 will generate a reverse stabilizing torque.

[0032] 5. Reset: After unloading or transferring, operate the pressure relief valve; under the pre-compression force of the reset spring 1, the lifting frame will be smoothly and automatically pushed back to the initial low position, completing one work cycle.

[0033] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the inventive spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A spring for a hydraulic pallet truck with an anti-eccentric load structure, installed between the support and the lifting frame of the hydraulic pallet truck, characterized in that: Includes a return spring (1); An adaptive self-aligning washer assembly (2) is disposed at the bottom of the reset spring (1), and its bottom is fixed to the bracket; The sleeve assembly (3) is coaxially sleeved on the outside of the return spring (1), and its bottom is fixedly connected to the self-aligning washer assembly (2); An inverted hydraulic cylinder (4) has its cylinder body tail swayably connected to the bracket, and the end of its piston rod (41) extends downward and abuts against the top of the sleeve assembly (3); The end of the piston rod (41) is fixedly connected to the lifting frame.

2. The hydraulic pallet truck spring with anti-eccentric load structure according to claim 1, characterized in that: The self-aligning washer assembly (2) includes a ball joint support (21) fixed to the bracket, a convex ball (22) movably accommodated in the ball joint support (21), and a ball joint top cover (23) covering the ball joint support (21) to form a convex ball groove. The convex ball (22) and the convex ball groove are in a spherical sliding fit; The ball joint top cover (23) is fixed to the ball joint support seat (21) by circumferentially distributed fasteners, which constrains the convex ball (22) in the convex ball groove and restricts its axial dislodgement, while allowing the convex ball (22) to rotate around its center within a predetermined angle. A cylinder (24) is fixed on the convex ball (22) and extends upward through the ball hinge top cover (23). The top end of the cylinder (24) is fixedly connected to the bottom of the sleeve assembly (3).

3. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 2, characterized in that: The sleeve assembly (3) includes a bottom cylinder (31) fixedly connected to the top of the cylinder (24) and a top cylinder (32) whose bottom is axially slidably inserted into the top of the bottom cylinder (31). The reset spring (1) is housed in a cylindrical cavity formed by the bottom cylinder (31) and the top cylinder (32).

4. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 3, characterized in that: The bottom cylinder (31) has a downward-facing circular groove (311) at its top, and the bottom of the top cylinder (32) is slidably installed in the circular groove (311). The outer wall of the bottom cylinder (31) is provided with a limiting groove (312) that communicates with the circular groove (311) along the axial direction. The outer wall of the top cylinder (32) is fixedly provided with a limiting block (321), and the limiting block (321) is slidably disposed in the limiting groove (312).

5. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 3, characterized in that: The top of the top cylinder (32) is provided with an insertion hole (322) at the center of the top. The end of the piston rod (41) passes downward through the insertion hole (322) and extends to the central axis position of the return spring (1); The lower end of the return spring (1) is supported on the inner bottom surface of the bottom cylinder (31), while the upper end abuts against the inner top surface of the top cylinder (32). The piston rod (41) is fixed with a radially protruding round shoulder (411).

6. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 5, characterized in that: The axial distance between the lower surface of the round shoulder (411) and the upper surface of the top wall of the top cylinder (32) is set to be less than the effective support stroke of the piston rod (41) of the hydraulic cylinder (4).

7. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 5, characterized in that: The top cylinder (32) has symmetrically provided auxiliary grooves (323) extending downward from the top surface along the insertion hole (322). An auxiliary plug (413) is fixedly provided at the end of the piston rod (41) corresponding to the auxiliary groove (323), and the auxiliary plug (413) slides in cooperation with the auxiliary groove (323).

8. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 1, characterized in that: The tail of the hydraulic cylinder (4) is mounted on the bracket via a spherical support (42).

9. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 5, characterized in that: A connecting plate (412) is fixedly connected to the round shoulder (411), and the connecting plate (412) is fixedly connected to the lifting frame.

10. A hydraulic pallet truck spring with an anti-eccentric load structure according to claim 2, characterized in that: The contact surface between the convex ball (22) and the convex ball groove is provided with a wear-resistant and friction-reducing coating.

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