Ball Screw Drive and Motor Vehicle Steering System Equipped with Same

Abstract

A ball screw drive includes a lead screw for converting an auxiliary torque produced by an electric motor into an auxiliary translational force acting on a steering rack in a steering system of a motor vehicle. One end of the lead screw and one end of the steering rack are each connected to a track rod via an axial link, and the ball screw drive has a ball nut mounted in a housing with a fixed bearing. In order to withstand strong lateral loads or lateral forces coming from the track rod or rods and acting in the radial direction there is provided between the end of the lead screw connected to the track rod and the ball screw drive a plain bearing element which supports the lead screw radially against the housing. The support is provided by an adjustable radial clearance, and the plain bearing element is a sleeve.

Claims

1. A ball screw drive comprising: a threaded spindle configured to convert an assistance torque generated by an electric motor into a translational assistance force which acts on a toothed rack in a steering system of a motor vehicle, one end of the threaded spindle and one end of the toothed rack connected in each case by an axial joint to a track rod; a ball nut mounted in a housing by a fixed bearing; and a plain bearing element arranged between the one end of the threaded spindle connected to the track rod and the toothed rack, the plain bearing element configured to support the threaded spindle radially with respect to the housing the plain bearing element including a sleeve surrounding the threaded spindle the sleeve having a conically shaped section which is of conical shape in an axial direction at an outer circumference of the conically shaped section, the conically shaped section bearing against a conically shaped inner wall in the housing, the sleeve having a smooth inner wall against which an outer circumference of the threaded spindle bears with a settable radial play, wherein the settable radial play is settable by a position, of the conically shaped section relative to the conically shaped inner wall, and wherein the position of the conically shaped section is set by an external thread of the sleeve.

2. The ball screw drive as claimed in claim 1, wherein: the sleeve has a ring-shaped section including the external thread, and the external thread engages an internal thread formed in the housing.

3. The ball screw drive as claimed in claim 1 or 2, wherein the conically shaped section has multiple slots which run in the axial direction and which divide the conically shaped section into multiple segments.

4. The ball screw drive as claimed in claim 1, wherein the radial play is set such that the sleeve supports the threaded spindle radially with respect to the housing only in the event of an exceedance of a defined transverse load.

5. The ball screw drive as claimed in claim 1, 2, or 4, wherein the conically shaped section has a gradient of between 1 and 1.5.

6. The ball screw drive as claimed in claim 1, 2, or 4, wherein the sleeve is manufactured from metal.

7. A steering system with electrical power steering assistance for a motor vehicle comprising: a track rod; a toothed rack; an electric motor; and a ball screw drive including a threaded spindle configured to convert an assistance torque generated by the electric motor into a translational assistance force which acts on the toothed rack, one end of the threaded spindle and one end of the toothed rack connected in each case by an axial joint to the track rod, a ball nut mounted in a housing by a fixed bearing, and a plain bearing element arranged between the one end of the threaded spindle connected to the track rod and the toothed rack, the plain bearing element configured to support the threaded spindle radially with respect to the housing, the plain bearing element including a sleeve surrounding the threaded spindle, the sleeve having a conically shaped section which is of conical shape in an axial direction at an outer circumference of the conically shaped section, the conically shaped section bearing against a conically shaped inner wall in the housing, the sleeve having a smooth inner wall against which an outer circumference of the threaded spindle bears with a settable radial play, wherein the settable radial play is settable by a position of the conically shaped section relative to the conically shaped inner wall, and wherein the position of the conically shaped section is set by an external thread of the sleeve.

8. A plain bearing element for a ball screw drive including a threaded spindle, the plain bearing element comprising: a housing; and a sleeve surrounding the threaded spindle, the sleeve having a conically shaped section which is of conical shape in an axial direction at an outer circumference of the conically shaped section, the conically shaped section bearing against a conically shaped inner wall in the housing, the sleeve having a smooth inner wall against which an outer circumference of the threaded spindle bears with a settable radial play, wherein the settable radial play is settable by a position of the conically shaped section relative to the conically shaped inner wall, and wherein the position of the conically shaped section is set by an external thread of the sleeve.

9. (canceled)

10. (canceled)

11. (canceled)

12. The ball screw drive as claimed in claim 1, wherein the radial play is set such that the sleeve supports the threaded spindle radially with respect to the housing only in the event of an exceedance of a transverse load that arises when a transverse force component amounts to more than 10% of a track rod force.

13. The ball screw drive as claimed in claim 1, wherein the sleeve is manufactured from a plastic.

14. The ball screw drive as claimed in claim 13, wherein the plastic is a polyamide.

Description

[0020] The invention will be described in detail below on the basis of exemplary embodiments which relate to a steering system for a motor vehicle and with reference to the appended drawings FIGS. 3, 4 and 5a/b, which show the following schematic illustrations:

[0021] FIG. 3 shows, in cross section, the construction of a steering system which is equipped with a ball screw drive according to the invention;

[0022] FIG. 4 shows, in greater detail, the installation of a plain bearing element in the form of a sleeve within the steering system; and

[0023] FIGS. 5a/b show further detail views illustrating the installation and construction of the plain bearing element formed as a sleeve.

[0024] The ball screw drives illustrated in FIGS. 3 and 4 also have components which are already present and denoted by reference designations in FIGS. 1 and 2. Therefore, the reference designations for the following components are maintained: steering system housing GB with steering gear LG and ball screw drive 10 in the housing (part) 1; ball nut 5; inner region or ball circuit with balls 6; threaded spindle 2b with its axis of rotation D; fixed bearing 4; toothed rack 2b; joint G for the coupling of the respective track rod 3 or 3; belt R, and motor M.

[0025] Below, for the description of the first exemplary embodiment, reference will be made to FIGS. 3-5a/b:

[0026] FIG. 3 shows, in cross section, the construction of a steering system which is equipped with a ball screw drive according to the invention. The ball screw drive 10 comprises the threaded spindle 2a, which is formed so as to be structurally integral with the toothed rack 2b of the steering system. The collective unit will hereinafter also be referred to in simplified form as toothed rack. To the two free ends of the collective unit there is articulatedly connected a respective track rod 3 and 3. To prevent high transverse loads, that is to say high transverse forces in the radial direction Y, overloading the ball circuit of the ball screw drive 10, a plain bearing element 20 is arranged between the ball screw drive 10 and the free end of the threaded spindle 2a, that is to say the end connected to the track rod 3, which plain bearing element supports the threaded spindle 2a radially with respect to the housing GH.

[0027] As shown in greater detail in FIGS. 4 and 5a/b, the plain bearing element is formed as a sleeve or bushing 20 which surrounds the threaded spindle 2a. The sleeve 20 has a conically shaped section 22 which is of conical shape in the axial direction x at its outer circumference and which bears against a conically shaped inner wall 23 in the housing GH. In the illustrated example, the cone of the section 22 narrows in the direction X, that is to say toward the ball nut. The sleeve 20 also has a ring-shaped section 21 which is equipped with an external thread 25 which engages with an internal thread formed in the housing GH. The sleeve 20 or the section 21 may be understood as a setting screw which, by being screwed into the internal thread of the housing GH, presses the conical section 22 with greater or lesser intensity against the conical inner wall of the housing GH. In this way, the position of the cone, or the pressing action thereof, is defined, whereby, in turn, it is determined whether, and what degree of, radial play RS exists between the smooth inner wall of the sleeve 20 and the threaded spindle 2a.

[0028] As shown in particular in FIG. 5a, the conically shaped section 22 is equipped with multiple slots 24 which run in the axial direction x, whereby said slots divide the section 22 into multiple segments. In this way, a desired elasticity of the sleeve (setting screw) 20 is determined. The material selection, for example PA6 plastic, and the dimensioning of the conical section 22 also co-determine the elasticity of the sleeve.

[0029] The radial play (RS), which is illustrated in particular on the basis of FIG. 5b, is settable during the course of the installation of the sleeve (20) by virtue of the position of the conically shaped section 22 relative to the conically shaped inner wall 23 being set to the desired dimension by means of the external thread 25. The greater the extent to which the cone is pressed against the conical inner wall of the housing GH, the smaller the radial play becomes. It can thus be achieved that a radial play always exists when no transverse load or only a small transverse load acts. Thus, the function of the ball screw drive 10 initially remains unaffected; in effect, the threaded spindle 2a freewheels within the sleeve 20.

[0030] The sleeve or setting screw is now set such that the threaded spindle 2a is supported radially with respect to the housing GH only in the event of an exceedance of a defined transverse load, that is to say it is only then that the threaded spindle bends to such an extent as to bear against the smooth inner wall 26 of the sleeve. For example, the position of the sleeve or the degree of the conical pressing action may be set such that, in the case of a transverse force component (see c in FIG. 2), amounts to at least 10% of the track rod force (see a). In this way, it is the intention for the radial play RS to be set such that the radial supporting action of the plain bearing or of the sleeve 20 takes effect only in the event of an exceedance of said value. The limit may for example also be set to at least 20%, such that the supporting action takes effect only in the presence of particularly high transverse loads, such as arise in extreme situations (exceptional events, misuse).

[0031] The setting can also be performed through the dimensioning of the gradient of the cone. It has been found that a gradient of between 1 and 1.5 is particularly advantageous here. The total length of the sleeve 20 may amount to for example approximately 15 mm. As regards the length 11 of the threaded section 21 and the length 12 of the conical section 22 (see FIG. 5b), 12 is considerably longer than 11; for example, 12 is at least 2 times as long as 11.

[0032] The installation location of the sleeve 20 is preferably situated close to the free end with the connection joint for the track rod 3. In this way, the transverse force components acting on the track rod can be better supported/accommodated. In the present example, the sleeve itself is composed of a polyamide of type PA-6, though may also be manufactured from other suitable plastics, and alternatively from metal, such as for example steel or aluminum.

[0033] The invention is particularly suitable for being installed in the steering system of a motor vehicle, in particular in a steering system with electrical power steering assistance. By means of the invention, lighter-weight design and construction variants can be realized, because smaller threaded spindles or toothed rack diameters are possible. A shorter construction of the ball nut is also possible, because fewer thread turns are required. The performance and operating characteristics are considerably improved by means of a non-braced ball chain.

[0034] In summary, the invention relates to a ball screw drive which is used in a steering system. The ball screw drive 10 comprises a threaded spindle 2a for converting an assistance torque generated by an electric motor N into a translational assistance force which acts on a toothed rack 2b in the steering system of a motor vehicle, wherein one end of the threaded spindle 2a and one end of the toothed rack 2b are connected in each case by means of an axial joint G to a track rod 3, wherein the ball screw drive 10 has a ball nut 5 which is mounted in a housing GH by means of a fixed bearing 4. In order to be able to withstand high transverse loads or transverse forces originating from the track rod(s) 3, 3 and acting in the radial direction y, a plain bearing element 20 is arranged between that end of the threaded spindle 2a which is connected to the track rod 3 and the ball screw drive 10, which plain bearing element supports the threaded spindle 2a radially with respect to the housing GH. The support is realized by means of a settable radial play, wherein the plain bearing element is preferably formed as a sleeve 20 which surrounds the threaded spindle 2a, wherein the sleeve 20 has a conically shaped section 22 which is of conical shape in an axial direction x at its outer circumference and which bears against a conically shaped inner wall 23 in the housing GH.

LIST OF REFERENCE DESIGNATIONS

[0035] 10 Ball screw drive [0036] 1 Housing of the ball screw drive [0037] 2a Threaded spindle [0038] 2b Toothed rack [0039] 3, 3 Track rod [0040] 4 Fixed bearing [0041] 4A Outer ring [0042] 5 Ball nut [0043] 6 Balls or ball circuit [0044] GH Housing of the steering system [0045] L Steering gear [0046] D Axis of rotation [0047] G Axial joint for the articulated connection of the track rod to the threaded spindle or toothed rack [0048] a Track rod force (can be broken down into the components b and c) [0049] b Axial force component [0050] c Transverse force component [0051] K Tilting moment [0052] x, y, z Spatial axes or direction coordinates [0053] R Belt [0054] M Electric motor [0055] 20 Plain bearing in the form of a setting screw as radial support bearing for the threaded spindle [0056] 21 Ring-shaped section of the sleeve with external thread [0057] 22 Conical sections/segments [0058] 23 Conical inner wall of the housing GH [0059] 24 Slots [0060] 25 External thread [0061] 26 Inner wall of the sleeve [0062] RS Radial play (settable)