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 which acts on a steering rack in the 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 lateral loads or lateral forces coming from the track rod or rods and acting in the radial direction, there is provided between an 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.

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 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.

2. The ball screw drive as claimed in claim 1, wherein: the plain bearing element includes a plain bearing bushing surrounding the threaded spindle, and the plain bearing bushing is mounted cardanically in the housing so as to be pivotable at least about a transverse axis which is oriented perpendicular to a direction in which a transverse force component of a track rod force arises.

3. The ball screw drive as claimed in claim 2, wherein the plain bearing bushing has a convexly curved ring-shaped outer circumference.

4. The ball screw drive as claimed in claim 3, wherein: at least a first subregion of the outer circumference of the plain bearing bushing bears against a concavely curved inner wall, of the housing.

5. The ball screw drive as claimed in claim 2, wherein: at least a second subregion of the outer circumference of the plain bearing bushing bears against a concavely curved inner wall of a ring, and the ring is insertable into the housing.

6. The ball screw drive as claimed in claim 5, wherein the ring is fixed in a desired position by a fixing element.

7. The ball screw drive as claimed in claim 2, wherein: a first round-wire ring secures a first end of the plain bearing bushing against displacement in an axial direction, and a second round-wire ring secures a second end of the plain bearing bushing against displacement in the axial direction.

8. The ball screw drive as claimed in claim 7, wherein an outer circumference of the plain bearing bushing has two grooves in which the first and second round-wire rings are received.

9. The ball screw drive as claimed in claim 7, wherein: the first end of the plain bearing bushing has a first bevel, and the second end of the plain bearing bushing has a second bevel.

10. 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 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.

11. A plain bearing element for a ball screw drive including a threaded spindle, the plain bearing element comprising: a housing; and a plain bearing bushing surrounding the threaded spindle and mounted in the housing, the plain bearing bushing configured to support the threaded spindle radially with respect to the housing.

Description

[0019] 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, 4a/b and 5a/b, which show the following schematic illustrations:

[0020] FIG. 3 shows, in cross section, the construction of a steering system which is equipped with a ball screw drive according to the invention;

[0021] FIGS. 4a/b show, in detail, the installation of a plain bearing element according to a first exemplary embodiment in the form of a cardanically mounted plain bearing bushing; and

[0022] FIGS. 5a/b show, in detail, the installation of a plain bearing element according to a second exemplary embodiment in the form of a plain bearing bushing secured by means of round-wire rings.

[0023] 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 GH 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.

[0024] Below, for the description of the first exemplary embodiment, reference will be made to FIGS. 3-5a/b:

[0025] FIG. 3 shows, in cross section, the construction of a steering system which is equipped with a ball screw drive 10 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 30 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.

[0026] As shown in greater detail in FIGS. 4a/b, the plain bearing element is formed as a plain bearing bushing 30 which surrounds the threaded spindle 2a. The bushing 30 is composed substantially of a ring-shaped body with a smooth inner wall 32 and with a convexly curved outer circumference 31. In the example shown here, said part of the bushing has the form of a spherical cap, approximately half (right-hand half in FIG. 4b) of which is received in a concavely shaped section of the housing GH. The other half of the spherical cap 31 (left-hand half in FIG. 4b) is held by a ring 33 which is insertable into the housing or by the concavely shaped inner wall of said ring, such that the spherical cap and thus the entire plain bearing bushing 30 can be positioned at the desired location so as to be pivotable about the z axis. A fixing element in the form of a ring 35 serves for fixing the arrangement.

[0027] The exemplary embodiment illustrated in FIGS. 4a/b thus shows a cardanically mounted bushing 30 which is pivotable about the z axis, which radially supports the threaded spindle 2a, and which can accommodate transverse forces (in the y direction). In this way, the ball circuit, in particular the first and final ball raceways, are protected against overloading. The contacting or the play between the plain bearing bushing 30 and the housing GH can be optimally set by means of the fixing element 35. The bushing itself or the ring-shaped body 30 may have incisions (as shown in FIG. 4b) for the purposes of saving material and weight. If desired, the incisions or narrowings of the material may also be of relatively thin dimensions in order to achieve a certain elasticity.

[0028] A second embodiment of the invention will be described below on the basis of FIGS. 5a/b: a likewise ring-shaped body is used as plain bearing element 40, which body has the form of a plain bearing bushing and comprises the threaded spindle 2a. The inner wall of the plain bearing element or of the plain bearing bushing 40 is likewise of flat and smooth form in the inner region. The two ends of the bushing 40 may be beveled, that is to say may have bevels 42. The bevels facilitate the assembly process and are advantageous with regard to the bending line and the rotation of the thread.

[0029] On the outer circumference of the bushing 40 there are provided grooves which extend in each case in a circumferential direction and which receive in each case one round-wire ring 43 and 44 respectively. The round-wire ring 43 and 44 make contact with the inner wall of the housing GH, such that the plain bearing bushing is (cardanically) mounted so as to be pivotable to a small extent. Furthermore, in the center of the bushing 40 (see FIG. 5b), the outer circumference 41 has a slight convex curvature, and is formed for example in the manner of a spherical cap. The spherical cap is axially fixed by means of one of the round-wire rings, in this case by means of the right-hand ring 44. After a deflection of the arrangement (under the action of an occurring transverse load), a central position is automatically set by means of the O-ring. In this way, automatic centering is realized in the solution illustrated in FIGS. 5a/b. This function is not implemented in the example from FIGS. 4a/b.

[0030] In most applications, the installation location of the plain bearing bushing 30 or 40 should preferably be situated close to the free end of the threaded spindle 2a, that is to say as close as possible to the connection joint for the track rod 3 (see also FIG. 3). In this way, transverse force components acting on the track rod can be better supported/accommodated. In the present example, the bushing 30 or 40 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.

[0031] 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.

[0032] In summary, the invention relates to a ball screw drive 10 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 M 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, 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 30 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 plain bearing element is preferably formed as a plain bearing bushing 30 which is cardanically mounted in the housing GH so as to be pivotable at least about a transverse axis z which is oriented perpendicular to a direction y in which a transverse force component c of a track rod force a arises. The plain bearing bushing 30 preferably has a ring-shaped outer circumference 31 which is convexly curved, in particular is curved in the manner of a spherical cap. At least a first subregion of the convex outer circumference 31 bears against a concave subregion of the inner wall of the housing GH. The BSD bearing, using the bearing arrangement proposed here, accommodates only axial forces, and may thus be designed as a pure axial ball bearing.

[0033] The use of round steel rings or O-rings (as shown in FIGS. 5a/b) gives rise to automatic centering of the slide bushing: if the toothed rack is subjected to radial load, the bending line profile of the toothed rack results in relation to the bearing arrangement. The cardanic plain bearing arrangement in this case supports the radial force. Since the bushing, contrary to the theoretical calculation approach, does not have the ideal structural length of 0 mm, it follows, along its length, the profile of the bending line of the toothed rack. If the transverse force is now eliminated, the toothed rack assumes the neutral position again within its bearing arrangement, and carries the cardanic plain bearing along, but not completely. Owing to bearing play between toothed rack and plain bearing, a small residual angle remains, which has an adverse effect with regard to contact pattern and in particular friction. If an O-ring is now installed between housing and plain bearing, said O-ring likewise deforms elastically under the action of transverse force during pivoting of the plain bearing. If the transverse force is now eliminated, the O-ring seeks to assume its original shape again, and returns the plain bearing into the central position (the O-ring generates a restoring moment).

LIST OF REFERENCE DESIGNATIONS

[0034] 10 Ball screw drive

[0035] 1 Housing of the ball screw drive

[0036] 2a Threaded spindle and 2b Toothed rack

[0037] 3, 3 Track rod

[0038] 4 Fixed bearing with 4A Outer ring

[0039] 5 Ball nut

[0040] 6 Balls or ball circuit

[0041] GH Housing of the steering system

[0042] L Steering gear

[0043] D Axis of rotation

[0044] G Axial joint for the articulated connection of the track rod to the threaded spindle or toothed rack

[0045] a Track rod force (can be broken down into the components b and c)

[0046] b Axial force component

[0047] c Transverse force component

[0048] K Tilting moment

[0049] x, y, z Spatial axes or direction coordinates

[0050] R Belt

[0051] M Electric motor

[0052] 30 Plain bearing (first example) in the form of a cardanically mounted plain bearing bushing without centering

[0053] 31 Ring-shaped outer circumference, convexly curved, for example in the manner of a spherical cap

[0054] 32 Inner wall of the plain bearing bushing

[0055] 33 Wedge-shaped/concavely shaped ring

[0056] 35 Fixing element/fixing ring

[0057] 40 Plain bearing (second example) in the form of a cardanically mounted plain bearing bushing with centering

[0058] 41 Outer circumference, slightly convexly curved and equipped with grooves

[0059] 42 Beveled ends

[0060] 43, 44 Round-wire rings in the grooves