Steering system for a motor vehicle and method for mounting a steering system for a motor vehicle

Description

[0001] The invention relates to a steering system for a motor vehicle and a method for assembling a steering system of a motor vehicle according to the preambles of the independent claims.

[0002] Steering systems in motor vehicles are often designed in such a way that the operation of a steering wheel by the driver or—particularly in the case of so-called autonomous driving—by a corresponding actuator, causes a push rod to move. This push rod is typically movable along its longitudinal direction and causes the steering movements of the wheels. For this purpose, the push rod is usually connected at its ends to steering knuckles via tie rods. The push rod is also referred to as an intermediate rod or rack, depending on the precise design of the steering system.

[0003] Such steering systems with push rods are known from the prior art, for example, DE 10 2017 103 975 A1 or DE 203 10 872 U1. A problem with the steering systems shown therein, as well as with a multitude of other possible implementations of the type in question, is that when the forces required to move the push rod are introduced into the push rod, torques are also introduced into the push rod, which would cause the push rod to twist if this twisting is not counteracted by a suitable design measure.

[0004] For example, it is known from DE 10 2017 103 975 A1 that the steering system can have a guide device for guiding the push rod, wherein the guide device is designed to prevent rotation of the push rod about its longitudinal axis. For this purpose, the push rod has a flattened portion. A roller rests against the flattened portion under preload. The roller is rotatably mounted on a roller axle.

[0005] In practice, such systems face the challenge of ensuring play-free guidance of the push rod while keeping the frictional forces generated by the guide mechanism, against which the push rod must be moved, within acceptable limits. To ensure this in individual cases, it is desirable to be able to adjust the resistance.

[0006] In DE 10 2017 103 975 A1, this is solved by the conically shaped rollers being fastened in rings that surround the push rod. By twisting the rings, the rollers can be clamped against the push rod in a kind of torsional movement. However, this requires the use of several Allen screws to clamp the clamping rings. Setting a defined resistance is therefore comparatively complex. Furthermore, it is not possible to use a push rod that has the flattened portion against which the rollers rest only over a section of the push rod remote from the ends of the push rod. Such push rods have larger cross-sections at both ends than in the area of ​​the flattened portion. It is not possible, or at least extremely difficult, to guide such push rods through a guide device such as that shown in DE 10 2017 103 975 A1 when the steering system is being assembled.

[0007] In the push rod shown in DE 203 10 872 U1, a spring-loaded pressure piece engages an area of ​​the tie rod provided with trapezoidal flats. Assembly can be accomplished by first inserting the tie rod into the guide device and then inserting the pressure piece into it. The resistance that such a guide device exerts against displacement of the push rod in the guide device determines the force of a spring loading the pressure piece. Precise adjustment is therefore not possible. To reduce frictional forces, rolling elements are provided between the pressure piece and the push rod. However, these only have small-area contact with the push rod, resulting in high local surface pressures. Accordingly, the material of the push rod must be hardened. This is one of the reasons why the production of such a guide device is comparatively expensive.

[0008] US 2005 / 0061575 A1 describes a steering system in which a steering shaft is coupled to a push rod via a toothed system. The push rod is subjected to compressive forces via a spring-loaded roller assembly arranged on flattened V-shaped surfaces. This generates a contact force in the toothed system between the steering shaft and the push rod.

[0009] DE 10 2019 208 451 A1 discloses a steering gear for a motor vehicle in which a push rod is prevented from twisting by a roller rolling on a running surface of the push rod. The push rod and roller are preloaded against each other by elastic forces.

[0010] JP 2018 111 426 A shows a steer-by-wire steering system in which rollers are pressed against a flattened surface of a push rod by means of spring forces.

[0011] JP H06 239 247 A shows a steering system in which rollers rest on a rounded surface of the push rod of the steering system and preload the push rod against the steering column.

[0012] JP 2007 062 412 A shows a steering system in which rollers rest on opposite flattened surfaces of the push rod of the steering system and press the push rods against each other.

[0013] The invention is based on the object of providing a steering system for a motor vehicle and a method for assembling such a steering system, which enables cost-effective production, simple assembly and reliable adjustment of the resistance against the movement of the push rod in the guide device.

[0014] The object is achieved according to the invention with the features of the independent claims. Further practical embodiments and advantages of the invention are described in conjunction with the dependent claims.

[0015] According to the invention, this object is achieved in that, in the steering system according to the invention, for adjusting the resistance against displacement of the push rod in the guide device, the roller axis can be moved towards the push rod to generate the preload and can be fixed in its position. If the roller axis, which is preferably oriented at right angles to the longitudinal direction of the push rod, is moved towards the push rod, the preload with which the roller rests on the flattened area increases. By increasing this preload, the friction forces that arise when the push rod moves along its longitudinal direction in the guide device. By being able to fix the roller axis in a defined position, a defined resistance against displacement of the push rod in the guide device can be permanently set. A line contact resulting between the roller and the flattened area is particularly advantageous.Due to its comparatively large contact surface, compared to ball bearings, for example, this reduces the requirements for the hardness of the tie rod in the area of ​​the flattening; this can in particular be made of an unhardened material.

[0016] The roller can be formed, in particular, by the outer ring of a rolling bearing. Rolling bearings with outer rings exist as standardized components and can represent a cost-effective and reliable way to implement the roller mounted on the roller axle, one that also offers comparatively low frictional resistance due to the rolling friction. The outer ring of such a roller bearing can then roll directly on the flattened surface of the push rod. This allows for a cost-effective roller design.

[0017] The rolling bearing can, in particular, be a needle bearing. Needle bearings have the advantage of being smaller than ball bearings.

[0018] The roller spindle is mounted in the guide device so that it can rotate about an eccentric axis of rotation that is parallel and eccentric to the roller axis. This makes it possible for the roller spindle to move towards the push rod by rotating the roller spindle about the axis of rotation. Such an arrangement is relatively easy to construct. All that is required at the ends of the roller spindle are elements that are arranged eccentrically to the roller axis and that must be mounted so that they can rotate accordingly in the guide device. One of the elements can in particular have a thread. In this way, the roller spindle can be screwed into the guide device. The screwing-in movement simultaneously generates a rotating movement of the roller spindle, during which it describes a circular path.In this way, the roller axle can be mounted in the guide device by first screwing it into the guide device, with a final section of this movement serving to move the roller axle towards the push rod in order to create the preload with which the roller rests against the push rod. It makes sense to first insert the push rod into the guide device after screwing in the roller axle with the roller mounted on it up to a certain point in the screwing-in movement. Continuing the screwing-in process by up to half a turn then causes the roller to move towards the push rod and builds up the preload from the moment the roller comes into contact with the push rod.

[0019] The guide device can have a counterbearing to support the push rod on the side facing away from the roller. Such a counterbearing has the advantage that it exerts a force on the push rod that opposes the force exerted on the push rod by the roller. The counterbearing is therefore arranged in particular in the immediate vicinity of the roller in the guide device. In this way, the counterforce is introduced in spatial proximity to the introduction of the force resulting from the preload with which the roller rests on the push rod. Arranging the roller and counterbearing in close spatial proximity therefore ensures that the introduced forces are not transmitted over long distances in the longitudinal direction of the push rod, which would result in bending moments in the push rod.

[0020] The counterbearing can be a guide bush. Guide bushes of the type in question are essentially circular-cylindrical elements, designed particularly as plain bearings for the push rod. The guide bush can be made of polyoxymethylene. Polyoxymethylene (POM for short) can be a homopolymer and / or a copolymer. It is characterized by high rigidity combined with low friction values. Such guide bushes have been shown to represent a cost-effective and reliable solution as a counterbearing.

[0021] The steering system can, in particular, be a steer-by-wire steering system. Steer-by-wire steering systems are steering systems in which the steering wheel is not connected to the push rod via a mechanical connection, such as a steering column, which transmits the movement of the steering wheel to the push rod. Instead, only an actuator is required that causes the push rod to move along its longitudinal direction. The steer-by-wire steering system can also feature a force feedback actuator. This is an actuator that acts on the steering wheel, thereby giving the driver of the motor vehicle the impression of opposing forces acting on the steering wheel, which result from the steering.

[0022] Since such steer-by-wire steering systems do not have a mechanical coupling between the push rod and the steering column, there is no way to prevent unintentional twisting of the push rod about its longitudinal axis within the framework of this mechanical coupling. A steering system according to the invention with a guide device of the type described is therefore particularly advantageous in conjunction with a steer-by-wire steering system.

[0023] The steering system may include an electric motor to drive and / or assist the movement of the push rod. The electric motor may be arranged with the rotational axis of its shaft parallel to the push axis. Such arrangements of an electric motor with the shaft parallel to the push rod are also referred to as the APA concept.

[0024] The torque generated by the electric motor can be transmitted to the push rod by means of a traction mechanism. The traction mechanism can, in particular, be a toothed belt. In particular, the torque can first be transmitted to a ball screw drive, which converts the torque into a force for moving the push rod. Such a ball screw drive has the advantage that it can generate a backlash-free and low-friction transmission for transmitting the torque as a force to the push rod. The transmission of the torque generated by the electric motor to the push rod typically also creates torques that are introduced into the push rod. These torques can be counteracted by the present invention.

[0025] The method according to the present invention relates to the assembly of a steering system as described above. During assembly of the steering system, the push rod is first inserted into the guide device. The guide device is already pre-assembled at least to the extent that the roller is already located in the guide device. In particular, the roller axis can already be screwed into the guide device. During the insertion of the push rod into the guide device, the roller axis is moved into a position that enables the insertion of the push rod into the guide device. This is in particular a position in which the roller axis is at a greater distance from the intended end position of the push rod in the guide device than is later the case during operation of the steering system.In particular, if the cross-section of the push rod at both ends is larger than the cross-section of the push rod in the area of ​​the flattened portion, one end of the push rod can initially pass the roller, as the roller is advantageously initially at a distance from the push rod that allows this to happen. In a further method step, the push rod is moved in the guide device and the mechanical resistance of the guide device to this movement is measured. This can be done in particular in a test bench set up for this purpose. During this time, the roller axis is moved towards the push rod until it reaches a position at which the resistance of the guide device to the movement of the push rod has reached a desired value. The roller axis is then fixed in this position.This can be done, for example, by caulking, in particular one of the elements arranged eccentrically to the roller axis.

[0026] In this way, the resistance to movement of the push rod in the steering system can be precisely and permanently adjusted in a simple and comparatively cost-effective operation. The advantage of this procedure, which is made possible in particular by a steering system according to the invention, is that the force required to move the push rod in the steering system can be adjusted directly. The adjustment is therefore ultimately reliable and precise because tolerances in the components involved can be compensated for by directly measuring the resistance of the guide device to movement of the push rod, i.e. in particular by measuring the force required to move the push rod in the guide device.

[0027] In this context, a particularly advantageous design for the steering system is one in which the roller axis is mounted in the guide device so that it can rotate about an axis of rotation that is parallel and eccentric to the roller axis. This allows for rapid adjustment by a rotational movement of up to 180°, which can be easily accomplished, for example, using a simple tool. The movement characteristics are also particularly advantageous when the roller axis is moved toward the push rod by rotating about the axis of rotation that is eccentric to the roller axis. During this movement, the distance between the roller axis and the push rod is sinusoidally related to the angle of rotation about the axis of rotation.In this way, a rapid adjustment of the distance between the roller and the push rod is possible, whereby a precise adjustment can be made on the last section of the approach movement, since with appropriate dimensioning of the distances there only a minimal approach of the axis of rotation to the push rod occurs due to the rotational movement.

[0028] Further practical embodiments of the invention are described below in conjunction with the drawings. They show: Fig. 1 an exemplary steering system, Fig. 2 the exemplary steering system from Figure 1 omitting the housing, Fig. 3 a sectional view through the steering system according to Figure 1 in the area of ​​the guide device, Fig. 4 a selection of exemplary push rods.

[0029] The steering system 10 shown as an example has a push rod 12. Furthermore, the steering system 10 has a guide device 14 for guiding the movement of the push rod in its longitudinal direction X. The guide device 14 is designed to prevent rotation of the push rod 12 about its longitudinal axis. The push rod 12 has a flattened portion 16. A roller 18 rests against the flattened portion 16 under preload. The roller 18 is rotatably mounted on a roller axle 20.

[0030] To adjust the resistance against displacement of the push rod 12 in the guide device 14, the roller axis 20 can be moved towards the push rod 12 to generate the preload and fixed in its position.

[0031] As in the example shown, the roller 18 can be formed by the outer ring of a rolling bearing 22. The rolling bearing 22 can be a needle bearing, as shown.

[0032] As shown by way of example in the figures, the roller axle 20 is accommodated in the guide device 14 so as to be rotatable about an axis of rotation that is parallel and eccentric to the roller axis 20. In the example shown, this is made possible by the fact that eccentric elements 24 and 26 are provided at the ends of the roller axle 20 relative to the roller 18 and / or to the roller axis 20. One of the eccentric elements 24, 26, for example the eccentric element 24, can have a thread. With this thread, the unit consisting of roller 18 and roller axle 20 can be screwed into the housing 28 of the guide device 14. For this purpose, the eccentric element 24 can have a suitable tool interface 30, for example a polygon, as shown. In particular, such a tool interface 30 can cause the roller axle 20 to rotate about the axis of rotation and thus move the roller axle 20 onto the push rod 12.

[0033] The guide device 14 can further comprise a counterbearing 32 for supporting the push rod 12 on its side facing away from the roller 18. The counterbearing 32 can be a guide bushing, as shown. The counterbearing 32 can, in particular, be arranged in the immediate vicinity of the roller 18, as shown.

[0034] As shown by way of example in the figures, the steering system 10 can be a steer-by-wire steering system. As shown, the steering system 10 can have an electric motor 34. As shown by way of example, the electric motor 34 can be arranged with its rotational axis or the rotational axis of its shaft parallel to the push rod 12.

[0035] The electric motor 34 can convert the generated torque, as shown by way of example, in particular via a ball screw drive into a force for moving the push rod 12.

[0036] The end region 38 of the push rod 12 can be designed in different ways. Figure 4 Different push rods 12 with flats 16 are shown as examples. These each have different end areas 38. In the case of the push rod 12 in Figure 4a ) the cross section of the end price 38 corresponds to the cross section in the area of ​​the flattening 16. Such a push rod can be particularly easily inserted past the roller 18 into an already pre-assembled guide device 14 during assembly of the steering system 10.

[0037] In the case of Fig. 4b ), the end region 38 has a larger cross-section than the area of ​​the flattened portion 16. However, in the area of ​​the end region 38, there is also a reduction in the cross-section in the form of a flattened portion 16. This simplifies the passage of the roller 18 during assembly of the push rod.

[0038] In the case of Figure 4c ) the end region 38 of the push rod 12 has a circular cross-section. The cross-section is here in comparison with the Figure 4a) and 4b ) shown push rods 12 is the largest, which has a beneficial effect on the introduction of force, for example, into a tie rod (not shown) connected to the push rod 12. Assembly is comparatively the most difficult with a push rod 12 designed in this way, but it has been shown that the steering system described here and in particular also the assembly of the steering system, in which the roller 18 is already mounted in the guide device 14 when the push rod 12 is inserted therein, can also be realized with push rods 12 designed in this way. List of reference symbols

[0039] 10Steering system 12Push rod 14Guide device 16Flat 18Roller 20Roller axle 22Rolling bearing 24Eccentric element 26Eccentric element 28Housing 30Tool interface 32Counter bearing 34Electric motor 36Ball screw 38End section XLongitudinal direction

Claims

1. Steering system (10) for a motor vehicle, having a push rod (12), the steering system (10) comprising a guide device (14) for guiding the movement of the push rod (12) in its longitudinal direction, the guide device (14) being designed to prevent rotation of the push rod (12) about its longitudinal axis, the push rod (12) comprising a flattened portion (16), a roller (18) abutting the flattened portion (16) under pretension, the roller (18) being rotatably mounted on a roller axle (20), the roller axle (20), in order to adjust the resistance to displacement of the push rod (12) in the guide device (14), being movable towards the push rod (12) to generate the pretension and can be fixed in its position, characterized in that the roller axle (20) is rotatably received in the guide device (14) about an eccentric axis of rotation parallel to the roller axle (20), so that the movement of the roller axle (20) towards the push rod (12) can be effected by rotating the roller axle (20) about the axis of rotation.

2. Steering system (10) according to claim 1, characterized in that the roller is formed by the outer ring of a rolling bearing (22).

3. Steering system (10) according to claim 2, characterized in that the rolling bearing (22) is a needle bearing.

4. Steering system (10) according to any of the preceding claims, characterized in that the guide device (14) comprises a counter bearing (32) for supporting the push rod (12) on the side thereof that faces away from the roller (18).

5. Steering system (10) according to claim 4, characterized in that the counter bearing (32) is a guide bush made of polyoxymethylene.

6. Steering system (10) according to any of the preceding claims, characterized in that the steering system (10) is a steer-by-wire steering system.

7. Steering system (10) according to any of the preceding claims, characterized in that the steering system (10), for effecting and / or assisting the movement of the push rod (12), comprises an electric motor (34) which is arranged with the rotational axis of its shaft parallel to the push rod (12).

8. Steering system (10) according to claim 7, characterized in that the torque generated by the electric motor (34) is transmitted by means of a traction means to a ball screw drive and is converted by the latter into a force for moving the push rod (12).

9. Method for assembling a steering system (10) according to any of the preceding claims, characterized in that during assembly of the steering system (10), the push rod (12) is first inserted into the guide device (14), the roller axle (20) being moved into a position which enables the push rod (12) to be inserted into the guide device (14), and in that in a further method step, the push rod (12) is moved in the guide device (14) and the mechanical resistance of the guide device (14) to this movement is measured, the roller axle (20) being moved towards the push rod (12) until it reaches a position in which the resistance of the guide device (14) to the movement of the push rod (12) has reached a desired value, and the roller axle (20) then being fixed in the position reached.