Ball screw pair for a steering gear of a motor vehicle

Abstract

The application relates to a ball screw pair of an automobile steering gear, and belongs to the technical field of automobile steering gear accessories. The ball screw pair of the automobile steering gear comprises a screw rod, a nut, a reverser and balls; the screw rod is externally provided with the nut; the screw rod and the nut screw groove are matched with each other; the inside of the nut is an internal thread raceway; the outside of the screw rod is an external thread raceway; the balls are transmission bodies between the nut and the ball screw; a plurality of waist round holes are arranged in the circumferential direction of the nut and are arranged in a circumferential manner; the waist round holes cross the screw groove and connect two adjacent nut raceways; the reverser is arranged in the waist round hole. The ball screw pair of the automobile steering gear and the design method of the reverser are especially suitable for the production and use of the ball screw pair reverser of the automobile steering gear.

Description

Technical Field

[0001] This invention relates to a design method for a ball screw pair reverser in an automotive steering system, belonging to the technical field of automotive steering system components. Background Technology

[0002] Due to their high transmission efficiency, low wear, and long service life, ball screw pairs are playing an increasingly important role in mechanical transmissions that require high speed, high precision, and high efficiency.

[0003] Ball screw pairs are classified into internal circulation ball screw pairs and external circulation ball screw pairs according to the ball circulation method. They differ depending on the usage conditions. Currently, research on ball screw pairs mainly focuses on the smoothness of the ball movement process. The design of the reversing device in the internal circulation ball screw pair determines the service life of the ball screw pair. The seventh parabolic ball return curve disclosed in CN 119900801 A is applied to the design of the reversing device of the ball screw pair, which includes four parts: screw, nut, ball, and reversing device. The screw has an external thread, and the inner surface of the nut has an internal thread. The external thread and the internal thread match and correspond. Together with the reversing device, they form a closed channel for the ball to roll.

[0004] However, existing technologies and products for internal circulation ball screw assemblies, including the structure disclosed in CN 119900801 A, have technical defects. Current ball screw assemblies on the market focus on smooth operation and high-speed ball performance, paying less attention to the reversing mechanism itself. The reversing mechanism is prone to damage during use, resulting in short product lifespan and high maintenance and replacement costs. The content disclosed in CN 119900801 A (seventh-order parabola) and CN 119042278 A (five-order parabola) primarily aims to reduce ball energy loss in the ball screw assembly. The design parameters for the reversing mechanism only consider the individual ball return curve design, without analyzing the overall structural parameters. This leads to products designed using existing methods exhibiting problems of easy breakage and short lifespan. Therefore, it is necessary to develop new design methods to solve the above-mentioned problems of existing methods. Summary of the Invention

[0005] The purpose of this invention is to provide a design method for a ball screw pair reverser for automotive steering systems, in order to solve the problems of easy breakage and short lifespan of existing ball screw pair reversers.

[0006] The technical solution of this invention is:

[0007] A ball screw assembly for an automotive steering system includes a screw, a nut, a reverser, and balls; the nut is disposed on the outside of the screw.

[0008] The lead screw and nut have matching raceways between their spiral grooves. The nut has an internal thread raceway, and the lead screw has an external thread raceway. A ball bearing acts as a transmission element between the nut and the lead screw. The nut has multiple oblong holes machined circumferentially, arranged in a circular pattern. These oblong holes span the spiral groove, connecting adjacent nut raceways. A reversing device is installed within each oblong hole. The characteristic feature is that:

[0009] The reverser is flat and round, and its surface curve is a ball return curve. The ball return curve satisfies the equation of a fifth-order parabolic curve. The ball return curve is connected to the lead screw raceway, and a ball stress relief area is set on the ball return curve. The balls always roll along the nut raceway and the lead screw raceway, climbing over the reverser and the raceway to achieve a closed loop.

[0010] The design method for the return bead curve is as follows:

[0011] Establish a three-dimensional coordinate system with the lead screw axis as the y-axis and the z-axis as the axis of symmetry to form a fifth-order parabolic cylindrical surface. Combine this with the characteristics of the ball screw to form a ball return curve.

[0012] The return curve satisfies the equation of a quintic parabolic cylindrical curve shown in equation (1-1):

[0013] (1-1);

[0014] The equation of a quintic parabolic cylindrical curve with x as the parameter variable:

[0015] (1-2);

[0016] The coefficients A, B, and C in the above formula satisfy the following equation:

[0017] (1-3);

[0018] The values ​​of ω, δ, and λ are shown in the following equation:

[0019] (1-4);

[0020] Where x, y, and z are parameters; θ is the parameter angle in radians; ρ is the distance from the ball return curve to the screw axis in mm; xF is the abscissa of point F on the ball return curve in mm; Ph is the lead of the ball screw pair in mm; d0 is the diameter of the center circle of the ball screw pair in mm; λ is the thread helix angle of the nut raceway in radians; and ω and δ are calculation coefficients introduced in the derivation process.

[0021] Based on the ball screw pair return curve structure and the driving performance requirements of automotive steering systems, the return curve is optimized in the z-axis direction. The ball stress relief curve of the return curve satisfies the following equation:

[0022] (1-5);

[0023] The aforementioned ball return curve is corrected in the ball stress relief region of the reverser. From the transition curve segment to the ball climbing over the top of the leadscrew and back to the transition curve segment, optimization is performed based on the aforementioned control points. The optimized control points are then fitted to derive the corrected and optimized curve equation:

[0024] (1-6);

[0025] Where x is a parameter variable, in mm; Mz, Nz, Pz, Qz, and Tz are the coefficients of the coordinate point z on the return curve, in mm.

[0026] The specific values ​​of the above coefficients depend on the specific parameter structure of the ball screw pair. The ball return curve of other reversers can be calculated using the same method.

[0027] The advantages of this invention are:

[0028] The design method for the ball screw pair and its reverser in this automotive steering system increases the service life of the reverser by adding a ball stress relief region correction curve to the ball return curve. This solves the problem that traditional design methods, which mostly rely on theoretical calculations and analyses based on curves, fail to consider the impact of ball loads on the return ball raceway in practical applications. The impact of ball loads on the return ball raceway directly affects the service life of the reverser, making this method particularly suitable for the production and use needs of ball screw pair reversers in automotive steering systems. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the structure of the present invention;

[0030] Figure 2 This is a schematic cross-sectional view of the nut of the present invention;

[0031] Figure 3 This is a schematic diagram of the inverter of the present invention;

[0032] Figure 4 A diagram illustrating the establishment of a three-dimensional coordinate system in the design method of this invention;

[0033] Figure 5-14 Photographs of experimental products used in the comparative experiments for this invention;

[0034] Figure 15 This is a graph obtained from an embodiment of the present invention.

[0035] In the diagram: 1. Lead screw; 2. Nut; 3. Reversing device; 4. Ball bearing. Detailed Implementation

[0036] The ball screw assembly of the automotive steering system includes a screw 1, a nut 2, a reverser 3, and balls 4; the nut 2 is provided on the outside of the screw 1.

[0037] The lead screw 1 and the nut 2 have matching raceways between their spiral grooves. The nut 2 has an internal thread raceway inside, and the lead screw 1 has an external thread raceway outside. The ball 4 is located between the nut 2 and the lead screw 1 as a transmission element. The nut 2 has multiple oval holes machined in the circumferential direction, arranged in a circular pattern. The oval holes cross the spiral groove and connect two adjacent nut raceways. A reverser 3 is installed inside the oval holes.

[0038] The reverser 3 is flat and round, and the surface curve of the reverser 3 is a ball return curve. The ball return curve satisfies the equation of a fifth-order parabola curve. The ball return curve is connected to the raceway of the lead screw 1. A ball stress relief area is set on the ball return curve. The ball 4 always rolls along the nut raceway and the raceway of the lead screw 1, climbing over the reverser 3 and the raceway to achieve a closed loop.

[0039] The design method for the return bead curve is as follows:

[0040] Establish a three-dimensional coordinate system (see the instruction manual appendix). Figure 4 Using the lead screw axis as the y-axis and the z-axis as the axis of symmetry, a fifth-order parabolic cylindrical surface is formed, which, combined with the characteristics of the ball screw, forms a ball return curve.

[0041] The return curve satisfies the equation of a quintic parabolic cylindrical curve shown in equation (1-1):

[0042] (1-1);

[0043] The equation of a quintic parabolic cylindrical curve with x as the parameter variable:

[0044] (1-2);

[0045] The coefficients A, B, and C in the above formula satisfy the following equation:

[0046] (1-3);

[0047] The values ​​of ω, δ, and λ are shown in the following equation:

[0048] (1-4);

[0049] Where x, y, and z are parameters; θ is the parameter angle in radians; ρ is the distance from the ball return curve to the screw axis in mm; xF is the abscissa of point F on the ball return curve in mm; Ph is the lead of the ball screw pair in mm; d0 is the diameter of the center circle of the ball screw pair in mm; λ is the thread helix angle of the nut raceway in radians; and ω and δ are calculation coefficients introduced in the derivation process.

[0050] Based on the ball screw pair return curve structure and the driving performance requirements of automotive steering systems, the return curve is optimized in the z-axis direction. The ball stress relief curve of the return curve satisfies the following equation:

[0051] (1-5);

[0052] Where x is a parameter variable, in mm; Mz, Nz, Pz, Qz, and Tz are the coefficients of the coordinate point z on the return curve, in mm.

[0053] The aforementioned ball return curve is corrected in the ball stress release region of the reverser. From the transition curve segment to the ball climbing over the top of the screw and back to the transition curve segment, optimization is performed based on the aforementioned control points. The optimized control points are then fitted to obtain the corrected and optimized curve equation:

[0054] (1-6);

[0055] The specific values ​​of the above coefficients depend on the specific parameter structure of the ball screw pair. The ball return curve of other reversers can be calculated using the same method.

[0056] Example: The applicant designed the return curve of a ball screw pair with a thread helix angle λ of 4.4°, a ball diameter of 5mm, and a center circle diameter d0 of 33.1mm using the method described in this application. The specific design steps are as follows:

[0057] The return curve satisfies the equation of a quintic parabolic cylindrical curve shown in equation (1-1):

[0058] (1-1);

[0059] The equation of a quintic parabolic cylindrical curve with x as the parameter variable:

[0060] (1-2);

[0061] The coefficients A, B, and C in the above formula satisfy the following equation:

[0062] (1-3);

[0063] The values ​​of ω, δ, and λ are shown in the following equation:

[0064] (1-4);

[0065] Where x, y, and z are parameters; θ is the parameter angle in radians; ρ is the distance from the ball return curve to the screw axis in mm; xF is the abscissa of point F on the ball return curve in mm; Ph is the lead of the ball screw pair in mm; d0 is the diameter of the center circle of the ball screw pair in mm; λ is the thread helix angle of the nut raceway in radians; and ω and δ are calculation coefficients introduced in the derivation process.

[0066] Based on the ball screw pair return curve structure and the driving performance requirements of automotive steering systems, the return curve is optimized in the z-axis direction. The ball stress relief curve of the return curve satisfies the following equation:

[0067] (1-5);

[0068] Where x is a parameter variable, in mm; Mz, Nz, Pz, Qz, and Tz are the coefficients of the coordinate point z on the return curve, in mm.

[0069] The aforementioned ball return curve is corrected in the ball stress release region of the reverser. From the transition curve segment to the ball climbing over the top of the screw and back to the transition curve segment, optimization is performed based on the aforementioned control points. The optimized control points are then fitted to obtain the corrected and optimized curve equation:

[0070] (1-6);

[0071] The graph is attached to the instruction manual. Figure 15 .

[0072] To verify the superiority of this application, the applicant conducted the following comparative experiment:

[0073] Experimental group: The experimental group is a ball screw assembly with reverser return curve parameters designed according to the method of this application;

[0074] Comparative group: The comparative group is a ball screw pair with reverser return curve parameters designed according to the method disclosed in patent CN 119042278 A;

[0075] Experimental methods:

[0076] Currently, national and industry standards do not include a separate life test for the reverser; instead, the life test is performed on the entire ball screw assembly. The test procedure is as follows: The ball screw assembly is installed on the test device, and the test bench is checked to ensure that all transmission chains are correct, unobstructed, and without abnormalities. The test load is applied along the axial direction of the screw, with loads of 5kN, 8kN, 10kN, 12kN, and 16kN respectively, moving at a constant speed. The load on the test bench is kept within ±5% variation. The loading device is started, and the load is controlled to reach the specified value. The test is then conducted with the load kept constant, with a life test limit of 100,000 cycles.

[0077] The parameters and results of the experimental and control groups are shown in the table below: ;

[0078] The experimental results are as follows:

[0079] In the control group, the first group of reversers did not fail, while the other control groups all suffered varying degrees of damage: side support fracture (e.g., control group 2), reverser groove damage (e.g., control group 3), and simultaneous side support fracture and reverser groove damage (e.g., control groups 4 and 5). None of the experimental groups corresponding to each load failed in the 100,000-cycle test. The above experimental results show that the correction curve with the ball stress release area is significantly better than that of the control group. Therefore, the technical solution of this application solves the problems of easy damage to the reverser and short life of the ball screw pair in the existing solution.

[0080] This automotive steering gear ball screw pair incorporates a ball stress relief region correction curve in the return ball curve of the reverser, thereby increasing the service life of the reverser. This addresses the problem that traditional design methods, which often rely on theoretical curve calculations and analyses without considering the impact of ball loads on the return ball raceway in practical applications—a problem where the impact of ball loads on the return ball raceway directly affects the reverser's service life—making this design particularly suitable for the production and use needs of automotive steering gear ball screw pair reversers.

Claims

1. A ball screw assembly for an automotive steering gear, comprising a screw (1), a nut (2), a reverser (3), and balls (4); the screw (1) is provided with a nut (2) on its outside; the screw (1) and the nut (2) have matching raceways between their helical grooves, the nut (2) having an internal thread raceway inside and the screw (1) having an external thread raceway outside; the balls (4) are positioned between the nut (2) and the screw (1) as a transmission element; the nut (2) has multiple oblong holes machined in the circumferential direction, arranged in a circular pattern, the oblong holes spanning the helical grooves and connecting adjacent nut raceways; the oblong holes are fitted with a reverser (3); characterized in that: The reverser (3) is flat and round. The surface curve of the reverser (3) is the ball return curve. The ball return curve satisfies the equation of a fifth-order parabolic curve. The ball return curve is connected to the raceway of the lead screw (1). A ball stress release area is set on the ball return curve. The ball (4) always rolls along the raceway of the nut and the raceway of the lead screw (1), and climbs over the reverser (3) and the raceway to achieve a closed loop.

2. The ball screw assembly of an automotive steering system according to claim 1, characterized in that: The design method of the ball return curve includes: establishing a three-dimensional coordinate system, taking the axis of the lead screw (1) as the y-axis and the z-axis as the axis of symmetry to form a fifth-order parabolic cylindrical surface, and combining the characteristics of the ball screw to form a ball return curve.

3. The ball screw assembly of an automotive steering system according to claim 2, characterized in that: The design method for the beaded return curve includes: the beaded return curve satisfies the equation of a quintic parabolic cylindrical curve as shown in equation (1-1): （1-1）； The equation of a quintic parabolic cylindrical curve with x as the parameter variable: （1-2）； The coefficients A, B, and C in the above formula satisfy the following equation: （1-3）； The values ​​of ω, δ, and λ are shown in the following equation: （1-4）； Where x, y, and z are parameters; θ is the parameter angle in radians; ρ is the distance from the ball return curve to the screw axis in mm; xF is the abscissa of point F on the ball return curve in mm; Ph is the lead of the ball screw pair in mm; d0 is the diameter of the center circle of the ball screw pair in mm; λ is the thread helix angle of the nut raceway in radians; and ω and δ are calculation coefficients introduced in the derivation process.

4. The ball screw assembly of an automotive steering system according to claim 3, characterized in that: The design method for the return bead curve includes: Based on the ball screw pair return curve structure and the driving performance requirements of automotive steering systems, the return curve is optimized in the z-axis direction. The ball stress relief curve of the return curve satisfies the following equation: （1-5）； Where x is a parameter variable, in mm; Mz, Nz, Pz, Qz, and Tz are the coefficients of the coordinate point z on the return curve, in mm. The aforementioned ball return curve is corrected in the ball stress release region of the reverser. From the transition curve segment to the ball climbing over the top of the screw and back to the transition curve segment, optimization is performed based on the aforementioned control points. The optimized control points are then fitted to obtain the corrected and optimized curve equation: （1-6）。

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