WO2009049700A1

WO2009049700A1 - Electrical power steering system with a novel axial bearing

Info

Publication number: WO2009049700A1
Application number: PCT/EP2008/006082
Authority: WO
Inventors: Frank Birkwald; Alexander Kiforiuk; Michael Stange
Original assignee: Thyssenkrupp Presta Ag
Priority date: 2007-10-16
Filing date: 2008-07-24
Publication date: 2009-04-23
Also published as: DE102007049748A1

Abstract

The invention relates to a bearing for a ball screw drive nut in vehicle steering systems. Conventional bearings for ball screw drive nuts are generally characterized by the use of grooved ball bearings. The substantial strain in the steering systems is due to translational forces, for which the grooved ball bearings are not very suitable. Other solutions, for example, with four-point bearings or expensive individual solutions, restrict flexibility of design and lead to an unjustified additional expense. According to the invention, the bearing for ball drive screw nuts is designed in a manner conforming to requirements, using line-contacting rolling bodies with a high bearing capacity, preferably by means of roller bearings and needle-roller assemblies. Necessary installation is conducted by simple handling, which may be performed by trained personnel. This results in steering systems with increased rigidity and structural flexibility of design.

Description

ThyssenKrupp Presta AG FL-9492 Eschen Principality of Liechtenstein

Electric power steering with new thrust bearing

The present invention relates to a motor vehicle power steering system with the features of the preamble of claim 1.

Power steering systems draw the steering power exclusively from an existing energy source in the vehicle, while power steering systems supplement the muscle power of the driver with another source of energy. In motor vehicles with foreign power steering systems or

Power steering systems can cause large static and dynamic forces. Increasing customer demands for comfort, performance, safety and space have allowed the vehicle masses to increase steadily and significantly increases the forces acting on the steering elements.

Preferred embodiment for initiating the steering forces is the immediate positioning of the actuator on the push rod. Hydraulically assisted steering systems are widely used as power steering systems. Increasing competition gets the hydraulic power steering system on the market by the energetically effective, so-called electric steering. The steering forces, whether as a foreign power steering system or power steering system, an electric motor ready. The system technology of this steering type is advantageous for steer-by-wire steering systems. A favorable design of the electric steering is the APA version, whose characteristic feature is the axis-parallel arrangement of electric motor and push rod. Such compact packaging solutions, especially in car front-axle steering systems, allow for an advantageous assembly.

Screw drives, of the type of Wälzschraubtriebe, convert a rotational movement into a linear movement. Between the flanks of the nuts and spindles roll off balls and erqeben in analog

Filling of the thread Lx iebniuLLex ini l Wälzkϋi.pfcü.11 high SLeiliykeiLen. In lead screws for steering systems, the driven nut of the screw drive has a degree of freedom about the axis of rotation and the spindle, designed as a push rod of the steering, is axially displaced.

State of the art

There are various technologies and marketable products for steering systems, such as the following documents.

A generic device is known from DE 100 32 120 Al.

Document DE 100 32 120 A1 discloses an electric motor-driven worm drive of the push rod. Worm drives, Figure 1 position 12, are inherently with a moderate

Efficiency affected. The bearing of the worm-driven screw nut takes place in Figure 1 position 6, with an unspecified rolling bearing, which must absorb the entire translational force of the push rod. Figure 4 of the same document shows an APA version with an adjustable friction drive and Figure 5 a toothed drive.

The document EP 0 963 900 B1, published a power steering system with a rod located on the pushrod, which is operated directly by the muscular force of the driver by means of a pinion, position 30. Spaced apart from the rod toothing, externally threaded screw threads are present on the push rod, into which a threaded nut indirectly engages. The threaded nut, position 140, is enveloped by the rotor, position 132, of a driving electric motor. Two unspecified bearings, positions 146 and 170, store the rotor, the so-called rotor shaft and the screw nut.

The document DE 103 53 161 A1, discloses an electric motor driven power steering system in APA design. A characteristic feature of the application, according to claim 7, is the storage of the output gear and the screw nut by a single double-row roller bearing. Drive and driven gear can form a Hüllgetriebe or be designed as meshing teeth. To avoid an undesirable overturning moment, the normal of the rolling bearing axis is located directly on the normal of the driven wheel axis.

Deep groove ball bearings are limited in principle due to their nature

Absorption capacity for the number of round rolling elements. Due to the point contact between the rolling track and the rolling element, the axial load capacity is low and the suitability for the high stress in a motor vehicle steering low. Big advantage of the deep groove ball bearing is the high product variety and the favorable price.

Deep groove ball bearings with split inner ring, also called four-point bearing, can be filled with rolling elements up to the geometric limits of the inner and outer rings as well as the cage. Various bearing manufacturers offer special, very expensive bearings for screw drives. However, these bearings are expensive and the variety offered is small. A designed for such a bearing systems construction for motor vehicle steering would depend on singular supplier parts. task

Object of the present invention is to improve the vehicle steering in terms of rigidity and structural freedom.

According to the invention the object is achieved by the features in the characterizing part of claim 1 in conjunction with the preamble.

The object is achieved by the ball screw nut with at least one roller bearing with line-contacting rolling parallel to the axis of rotation and at least two bearings with rolling elements whose axes of rotation are largely perpendicular to the axis of rotation of the ball screw nut is stored.

Due to the principle of line-contacting rolling elements for higher static and dynamic loads, usually explicitly for radial or axial loads. Point contact rolling elements are comparatively less resilient, as for example the deep groove ball bearings. One

Comparison of ball screws 1 to axial needle roller rings with comparable dimensions clarifies the differences (FAG roller bearings, January 2006).

D = maximum outside diameter d = bore diameter

C stat. = maximum load capacity for stationary storage at ideal conditions

Conditions, radial load capacity of the deep groove ball bearing, axial

Load capacity of the axial needle ring An advantage for a rigid radial bearing of the screw nut is that the rolling parallel to the rotational axis of the ball screw nut rolling bearing is designed as a roller bearing. An axial securing of the roller bearing during assembly is unnecessary, by the axial limitation in the steering housing and on the

Ball screw nut pushed pulley as a floating bearing. In particular, this bearing can be rolling elements with a large axial extent, which withstand the tilting moment, caused by the tension forces of the belt drive spaced from the roller bearing, with an equilibrium-forming moment.

An advantage of the present invention is further that the largely perpendicular to the rotational axis of the ball screw nut rolling bearings are formed as needle rims. Characteristic feature of axial needle roller covers is the small axial extent and the usually high number of rolling elements. As a result, these bearings are very stiff and resist shocks mostly undamaged. Axial needle rings presuppose hardened and ground raceways, which is why, as a rule, housing disks between the rolling needles and the adjoining Ma.schinent.fii become intact. Housing disks, also called axial bearing washers, are usually offered by the manufacturers of rolling bearings as inexpensive mass parts in hardened and ground design.

The inventive mounting of the ball screw nut allows a compact design with simple components and a mounting largely consists of the juxtaposition of components. Another advantage is that even with a total failure of the axial bearing, the vehicle steering has good emergency running properties.

The embodiments listed below suggest according to the invention a variant with one-sided housing disks. In Variant two are available for an axial needle ring with two housing washers.

Embodiment The invention will be explained below with reference to the drawing. Show it :

Fig. 1 is a sectional view of the steering system according to the invention in a first variant

Fig. 2 is a sectional view of a variant with two housing disks per rolling bearing

Figure 1 shows a ball screw nut 1 and surrounding elements. Between several Wälzrlanken 2 and not shown

Push rod roll off the rolling elements, not shown, of a recirculating ball. The drive of the ball screw nut 1 is effected by a belt 3 on a pulley 4. As a driven wheel, the pulley 4 is driven by a motor, not shown. The belt 3 may, for example, be a positive fit

Act belt element (toothed belt) or a non-positive belt. In an alternative embodiment, a toothing or a friction drive could be used.

An advantageous embodiment of the invention for receiving the radial forces is a wide roller bearing 5 between the ball screw nut 1 and a steering housing 6. The roller bearing 5 is axially secured without additional measures from the steering housing 6 and the pulley 4. A further advantageous absorption of radial forces is effected by the rolling elements, not shown, on the Wälzflanken 2 between the ball screw nut 1 and the not shown push rod. A bearing base 7 is pressed by a nut 8 to the shoulder of the ball screw nut 1. The with the ball screw nut

I rotating bearing base 7 passes the translational forces of the push rod via the indirectly connected ball screw nut 1 advantageously to the axial bearing present in a small axial extension. The bearing base 7 is merely frictionally connected to the ball screw nut 1.

The horizontal rolling tracks of the bearing base 7 preferably have surface hardening by case hardening or nitriding, for example

Vickers hardness of about 600. Both hardening processes produce an advantageous material structure with hard surfaces and elastic cores. The hardening process is followed by a reworking by grinding, which eliminates the distortion and ensures the parallelism of the Wälzbahnen and the bore size of the bearing base 7.

Locknuts are usually provided with at least four radial recesses and formed with fine threads which perform low axial movements per revolution on a shaft thread. The circlip 9 is a cost-effective stamped part of a plastically good deformable steel with an inner flap, which is rotatably connected by engagement in an axial groove in the shaft, with the same. One of the approximately 20 outer tabs of the locking ring 9 can be bent in axial alignment with a radial recess of the nut 8 in the recess contour and secure the lock nut 8 permanently positive manner.

In two-sided axial extension of the plane-parallel hardened surfaces of the bearing base 7 are immediately identical needle rings 10 and

II arranged. The needle-shaped rolling elements are fixed in position on all sides by plastic or metal cages and the rotation axis of the needles rotate during operation with a pasty lubricant around the axis of rotation of the ball screw nut 1. The storage base 7 abge- turned sides of the needle rings 10 and 11 roll on non-rotating identical housing discs 12. The housing discs 12, often called thrust washers, are inexpensive mass parts which usually belong to the product range of the bearing manufacturer. The plane-parallel rings of sheet metal of small axial extent are ground after hardening, in some cases also polished.

On a bearing cap 13 with an internal thread preferably small pitch is one of the housing discs 12 at. In the case of the final machining of the internal work and the thread, the workpiece is machined without re-tensioning the workpiece so that the normal of the axis of rotation of the thread is parallel to the contact surface of the housing disk 12. A hood 14 is designed with an external thread corresponding to the internal thread of the bearing cap 13. The inner contour of the hood 14 with the contact surface of the housing plate 12 and the external thread are produced in one clamping After assembly and adjustment of the bearing clearance, a steering box cover 15 is connected to a releasable fastening means (not shown) on the steering box 6 and at the same time the bearing cap 13 is clamped to the steering box 6.

A particularly advantageous embodiment of the invention is shown in FIG 2. For this design, the bearing base 7 may be formed uncured. The needle rings are each separated by a hardened housing disc 12 of soft components and have rolling surfaces and rolling elements with the same required hardness. Since after a surface hardening of the bearing base 7 according to the design according to FIG. 1 with a grinding process the distortion would have to be eliminated, the invention according to FIG. 2 can be designed by the use of inexpensive serial parts. For mounting the roller bearing 5 is already sufficient lubricant in its bearing seat in the steering housing 6. The roller bearing now takes the ball screw nut 1 in the uncoupled state including the pulley 4 and the push rod, not shown.

It follows the bearing cap 6 with the pulley 4 side facing away from the thread. The housing plate 12 is placed without consideration of a rolling surface, since both sides are identical. This is followed by the Nadelkranz 10, possibly with addition of the lubricant. The bearing base 7 is pushed with little radial play on the ball screw nut 1 and the second needle ring 11 with sufficient lubricant, which is identical to the needle ring 10.

After laying the second housing disc on the T.agfirhasi s 7, the threaded eyes of the cleaned hood 14 echo a liquid screw lock. The hood 14 is temporarily screwed into the bearing cap 13. After sliding the locking ring 9, the screwing of the nut 8 is performed with a predetermined torque, the Anschraubmoment is countered by a radial form-fitting design with a suitable tool on the opposite side of the screw nut 1. Depending on the relative position of the radial recesses of the nut 8 to the outer lobe of the circlip 9, the mother has to be turned back less than 18 ° with assumed twenty lobes. In axial alignment of the radial recess and lobe this is bent into the contour of the recess inside.

In order to adjust the bearing clearance when the steering housing cover 15 has not yet been mounted, the dimension "x" in the figure is measured and the difference formed in the maximum axial positions of the ball nut 1 achieved by manual force The measuring points are positioned, for example, on the hood 14 which is stationary during operation and the rotating one Slot nut 8. Conveniently the game is adjusted by rotation of the hood 14 according to an angle at a known thread pitch would, for example, a Difference of 0.25 mm require a rotation of the hood 14 of 36 ° to obtain at 2 mm thread pitch 0.05 mm axial play.

The assembly process of the device according to the invention is characterized by simple juxtaposition of components, which can be performed by semi-skilled workers. Assembly of bearings known from the art, for example with the heating or cooling of bearing rings or complex fuses, is almost completely eliminated in the case of the steering system according to the invention.

LIST OF REFERENCE NUMBERS

1 ball screw nut

2 rolling edges

3 straps

4 pulley

5 roller bearings

6 steering housing

7 storage base

8 Nutmuttcr

9 circlip

10 needle ring 1

11 Needle ring 2

12 housing disc

13 bearing cap

14 hood

15 steering box cover

Claims

claims

1. vehicle steering with electric motor drive, with a steering housing (6) and arranged therein

Ball screw, wherein the ball screw in the steering housing (6) about a rotational axis rotatably mounted ball screw nut (1), a

Number of rolling elements and a linear vprlagprharp in the direction of the axis Srhnh.st ange with a ball screw, characterized in that the ball screw nut (1) with respect to the axis of rotation in at least one rolling bearing (5) is supported in the radial direction and in at least two other bearings { 10, 11) is supported in the axial direction, wherein the rolling elements of the further rolling bearing (10,

11) with their axes of rotation radially to the axis of rotation of

Ball screw nut (1) are aligned.

2. Vehicle steering system according to claim 1, characterized in that the further rolling bearings (10, 11) are Nadelkranzlagcr.

3. Vehicle steering system according to one of the preceding claims, characterized in that the needle bearing (10, 11) abut respectively an annular, circumferential collar of a bearing base (1).

4. Vehicle steering system according to one of the preceding claims, characterized in that the collar has a hardened surface.

5. Vehicle steering system according to one of the preceding claims, characterized in that the collar carries housing discs (12) made of a hardened material.

6. Vehicle steering system according to one of the preceding claims, characterized in that the needle roller bearings abut against the bearing base facing away from sides of the housing discs (12).

7. Vehicle steering system according to one of the preceding claims, characterized in that the needle bearing (10, 11) by means of threaded means (14) are free of play adjustable.