Electromechanical power steering with pivot bearing arrangement

Abstract

The invention relates to an electromechanical power steering system with an electric servomotor having a motor shaft which drives a shaft (2) meshing with a helical gear (4), wherein the shaft (2) is arranged in a gearbox housing (8) and is rotatably mounted in a bearing arrangement (7) having a rolling bearing (9) about an axis of rotation (100), wherein a preloading device (18) is included which spring-elastically preloads a movable bearing element of the bearing arrangement (7) against the gearbox housing (8) for adjusting the clearance of the engagement between the helical gear (4) and the shaft (2), and wherein the gearbox housing (8) has a housing section (16) with a first contact surface (33) and a second contact surface (32) whose normals do not intersect, and wherein the bearing arrangement (7) has a bearing support (13) for the rolling bearing (9).which forms the movable bearing element and which has a lever arm (19) extending from a base body (24) of the bearing support (13) with a free end (21) which rests on the gearbox housing (8) at the second contact surface (32) in a second contact area (34), wherein the bearing support (13) has an eccentric cam (14) on its base body (24) which rests on the first contact surface (33) on the housing (8) in a first contact area (35).

Description

[0001] The present invention relates to an electromechanical power steering system having the features of the preamble of claim 1.

[0002] In electromechanical power steering systems, a torque is generated by an electric motor, which is transferred to a gearbox and superimposed there on the steering torque provided by the driver.

[0003] A generic electromechanical power steering system has an electric servo motor which drives a worm shaft which meshes with a worm gear arranged on a steering shaft, wherein the worm gear is operatively connected to an input shaft of a steering gear and wherein the worm shaft and the steering shaft are rotatably mounted in a common gear housing.

[0004] The bearings of a worm gear should be as free of play as possible to avoid acoustic problems such as deflection knocking or rattling. At the same time, however, some play must be guaranteed in order to compensate for tolerances and environmental influences such as changes in humidity, temperature, etc. Conversely, however, the preload applied to the gearing must not be too great, as otherwise the slippage torque and the associated wear will become too great. The angles in the gearing of the worm shaft and worm wheel, as well as the bearings of the worm, result in asymmetric loads on the worm shaft bearings. Over time, this causes wear to the components, which increasingly increases the play in the gearing. It has been shown that setting the play to zero during assembly is not sufficient. The increase in the play in the gearing leads to acoustic problems.

[0005] The published patent application DE 10 2014 110 306 A1 discloses a worm shaft which, at its end remote from the motor, is rotatably mounted in a bearing which is accommodated in a holder in which a bearing ring, which forms an outer part of the bearing, is non-rotatably mounted. The holder is displaceably mounted in the worm gear such that the bearing ring can be moved toward the toothing of the worm wheel, whereby the worm shaft can be preloaded into backlash-free meshing engagement with the worm wheel. A spring, for example, can be provided as a preloading element, which engages the holder and elastically presses the worm shaft against the toothing of the worm wheel without backlash. Furthermore, the holder is pivotally mounted in the gear housing via a pin forming a pivot axis.

[0006] It is an object of the present invention to provide an electromechanical power steering system with a helical gear, in particular a worm gear, in which noise and play can be reduced effectively and in the long term.

[0007] This object is achieved by an electromechanical power steering system having the features of claim 1. Further advantageous embodiments can be found in the subclaims.

[0008] Accordingly, an electromechanical power steering system is provided with an electric servomotor having a motor shaft which drives a shaft meshing with a helical gear, wherein the shaft is arranged in a gear housing and is mounted in a bearing arrangement having a rolling bearing so as to be rotatable about a rotational axis, wherein a pretensioning device is included which spring-elastically pretensions a movable bearing element of the bearing arrangement against the gear housing to adjust the play of the engagement between the helical gear and the shaft, wherein the gear housing has a housing section with a first contact surface and a second contact surface, wherein the bearing arrangement has a bearing carrier for the rolling bearing which forms the movable bearing element and which has a lever arm extending from a base body of the bearing carrier and having a free end which bears against the gear housing on the second contact surface in a second contact region.The bearing carrier further comprises an eccentric cam on its base body, which bears against the first contact surface on the housing in a first contact area. The normals of the first contact surface and the second contact surface can have a change of sign in one or more spatial directions. It is also conceivable and possible for the two contact surfaces to be designed as contact points. The bearing carrier is thus preferably in contact with the housing exclusively via the contact surfaces. The bearing arrangement can be arranged at the end of the shaft closest to the motor and preferably at the end of the shaft far from the motor.

[0009] Due to the asymmetric two-point support of the bearing carrier on the gearbox housing, the bearing assembly itself is play-free under all load conditions, thus effectively reducing noise. The bearing carrier is preferably formed as a single piece from a single workpiece, for example, a plastic injection-molded part.

[0010] Preferably, the part of the gear housing forming the first and second contact surfaces is wedge-shaped with flat surfaces. This part can be designed to be movable to prevent locking of the shaft.

[0011] It is advantageous if the first and second contact areas lie in a common plane perpendicular to the axis of rotation.

[0012] The lever arm preferably has at its free end a projection extending in the longitudinal direction, which is resiliently supported on the gear housing and which bears against the second contact surface of the housing part.

[0013] In an advantageous embodiment, a free space is formed between the projection and the base body of the bearing carrier, in which space the part of the gear housing forming the first and second contact surface is received.

[0014] The rolling bearing is preferably concentrically surrounded by the base body of the bearing carrier and the bearing carrier has a seat for the rolling bearing in a central opening of the base body, which seat is formed by a shoulder in the opening.

[0015] It is advantageously provided that a spring element held in the housing presses against the free end of the lever arm to elastically preload the shaft in the housing. Preferably, a rigid stop, for example, a contact surface of the housing or a pressed-in pin, is provided, which interacts with the preloaded spring element. It is also conceivable and possible for the elastic preload of the shaft in the housing to be realized by means of a spring-preloaded pin. The projection of the lever arm is preferably supported on the gear housing by means of the pin.

[0016] To ensure good adjustment options, the eccentric cam preferably extends over a circular sector in a range between 10° and 100° to the rotation axis of the shaft.

[0017] The eccentric cam preferably has a bulbous protrusion that extends asymmetrically in the circumferential direction and is either uniformly or, particularly advantageously, convex, domed, or curved in the longitudinal direction to compensate for tolerances. The radius of curvature in the cross-section is smaller or larger than the radius of the bearing seat. The radius of curvature is selected to be smaller if the position of the contact point is to be controlled, and larger to minimize contact pressure. The longitudinal direction is defined by the direction of the shaft's rotational axis.

[0018] The eccentric cam is preferably shaped such that when the bearing outer ring of the rolling bearing moves about the axis of rotation, the bearing outer ring moves away from or towards the housing section (the first contact surface) to adjust the play of the engagement between the helical gear and the shaft.

[0019] In order to ensure a long service life of the bearing arrangement, the first and / or second contact surface of the gearbox housing preferably has a higher rigidity than the bearing carrier.

[0020] In order to be able to absorb the asymmetric forces, the angular distance between the first contact area and the second contact area in the circumferential direction to the axis of rotation is preferably between 20° and 50°, and particularly preferably between 30° and 40°.

[0021] In one embodiment, the helical gear is a worm gear and the shaft is a worm shaft.

[0022] An exemplary embodiment of the present invention is described below with reference to the drawings. Identical components or components with identical functions bear identical reference numerals. They show: Fig. 1: a spatial representation of a gear of an electromechanical power steering system; Fig. 2: another spatial representation of the Fig. 1 shown gearbox; Fig. 3: a spatial representation of the engagement between the worm wheel and the worm shaft of the gearbox; Fig. 4: a cross-section through the worm shaft at the level of a bearing, Fig. 5: a spatial view of a component of the bearing, as well as Fig. 6: a schematic representation of the operation of the bearing.

[0023] In the Fig. 1 to 3 illustrate a transmission 1 of an electromechanical power steering system of a motor vehicle. The electromechanical power steering system is designed to assist the driver during a steering movement. In addition to the transmission, the electromechanical power steering system includes an electric motor or servomotor 150. The servo unit can be arranged as a superimposed steering system on a steering column or as an auxiliary power assistance device on a pinion or rack.

[0024] The servomotor drives a worm shaft 2, which, via its worm 3, engages a worm gear 4 that is rotationally fixedly connected to the lower steering shaft 5 or the pinion. When the electric motor 150 is operated, the worm shaft 2 is driven, and the worm 3 and the worm gear 4 rotate accordingly to provide rotational support for the lower steering shaft 5 or to directly introduce an assist torque into the pinion.

[0025] The worm shaft 2 is mounted at its free end 6 in a bearing arrangement 7 in the gear housing 8 surrounding the gear.

[0026] The bearing arrangement 7 is shown in detail in Fig. 4. It has a rolling bearing 9, which is designed as a ball bearing and allows slight axial movements and changes in the axis of rotation of the worm shaft 2 relative to the gear housing 8. The rolling bearing 9 has an inner ring 10, rolling elements 11 and an outer ring 12. The rolling elements 11 run in grooves between the inner ring 10 and the outer ring 12. The outer ring 12 is received in a bearing seat of a bearing carrier 13. The bearing carrier 13 surrounds the outer carrier on the circumference. The bearing carrier 13 has an eccentric cam 14. The eccentric cam 14 is formed on the circumferential surface of the bearing carrier 13 and has a bulbous elevation 15, which extends asymmetrically in the circumferential direction and is constant in the longitudinal direction and whose radius of curvature in cross-section is smaller or larger than the radius of the bearing seat.The eccentric cam 14 extends over a circular sector in a range between 20° and 100° to the rotational axis 100 of the worm shaft 2. The eccentric cam 14 is supported with its circumferential surface on a housing section 16. This housing section 16 is preferably formed in the shape of a wedge and has a flat plane 17 on which the eccentric cam 14 can roll up and down or slide along, the latter particularly when the eccentric cam forms a sliding support point. When the bearing outer ring 12 moves about the rotational axis 100, schematically represented by the arrows, the bearing outer ring 12 moves away from or toward the housing section 16, thus increasing or decreasing the engagement play between the worm gear 4 and the worm shaft 2 depending on the direction of rotation.However, it is particularly advantageous if there is no movement of the bearing outer ring 12 or only a small tolerable movement, which increases the distance between the two contact surfaces 32, 33. The housing section 16 preferably has a higher rigidity than that of the eccentric cam 14, so that the housing 8 is not damaged by the rolling of the eccentric cam 14 on the plane 17.

[0027] The bearing bracket 13 further comprises a preloading device 18 having a lever arm 19. The lever arm 19 is designed as an arm 20 with a free end 21. It extends from the circumferential surface of the bearing bracket 13 in the radial direction to the rotation axis 100. The bearing bracket 13 is also supported on the gear housing 8 at the free end 21 of the lever arm 19. For this purpose, a spring-loaded pin 22 held in the housing 8 presses on the free end 21. The position of the worm shaft 2 relative to the worm wheel 4 can be adjusted by means of the preloading device 18. The eccentric cam 14 slides up or down on the inner wall of the gear housing 8 depending on the force exerted by the spring 23 of the preloading device 18 on the pin 22.

[0028] Due to the two-point support of the bearing carrier 13 on the gearbox housing 8, the bearing arrangement 7 is free of play even in all load situations, so that noise can be effectively reduced.

[0029] The Fig. 5 and Fig. 6 shows the details of the bearing bracket 13. The bearing bracket 13 has a substantially cylindrical base body 24, through which passes a circular opening 25, in which a circumferential shoulder 26 is provided on the inside, which forms an annular contact surface 27 in the longitudinal direction as a seat for the rolling bearing. The eccentric cam 14 is arranged on the outside of the base body 24 and extends longitudinally over the entire height of the bearing bracket 13. The lever arm 19, also on the outside of the base body 24, is offset in the circumferential direction from the eccentric cam 14. The lever arm 19 extends longitudinally only over part of the height of the base body 24. A distance is provided between an upper side 28 of the lever arm 19 and an end face 29 of the base body 24 lying parallel thereto. The lever arm 19 has a projection 30 at its free end 21.The projection 30 extends longitudinally from the top side of the free end 21, so that a free space 31 is formed between the peripheral side of the base body 24 and an inner side of the projection 30 facing the base body side. In this free space, the wedge-shaped housing part is arranged, as shown in . Fig. 4. The spring element 23 rests against the projection 30 of the lever arm 19. The movement of the projection 30 of the lever arm 19 is guided on the wedge-shaped housing part. The projection 30 rests against a first side 32 of the wedge-shaped housing part. The eccentric cam 14 rests against a second side 33 of the wedge-shaped housing part, opposite the first side 32. The wedge-shaped housing part thus forms two bearing surfaces 32, 33, each with a contact point 34, 35 or a contact surface for the bearing carrier 13. The bearing carrier 13 has a flat surface on its side facing away from the worm shaft.

[0030] Fig. Figure 6 schematically shows the arrangement of the contact surfaces 32, 33 after assembly of the gearbox 1, without external forces acting on the worm shaft. The worm shaft (not shown) is located in the y-direction above the schematically shown worm gear 4. The x-axis extends perpendicular to the y-axis and perpendicular to the rotation axis 100 of the worm shaft. The origin of the coordinate system lies at the center of the opening 25 of the bearing bracket 13 or at the center of the base body 24 of the bearing bracket 13 and thus on the rotation axis 100. The contact surfaces 32, 33 lie in a common plane in the z-direction.

[0031] The contact point 35 or area of ​​the eccentric cam 14 on the gear housing has an angular distance α from the center point to the x-axis. The contact surface 33 of the gear housing at this first contact point 35 (tangent) extends at an angle δ to the y-axis and has a distance d1 from the y-axis. The contact point 34 or area of ​​the lever arm 19 on the gear housing has an angular distance β from the center point to the x-axis. The contact surface 32 of the gear housing at this second contact point 34 (tangent) extends at an angle γ to the perpendicular line connecting the line between the center point and the second contact point 34 in the second contact point 34. The distance between the center point and the second contact point 34 is defined as d2. The spring 23 has a spring force F1. The pretensioning device has a play d3.

[0032] The geometry of the contact surfaces (angles and distances) and the friction coefficients of the contact points and contact surface can be selected so that the worm cannot lift off (adjuster) (γ<0), or that lift-off is only possible with very high forces, or that lift-off is possible even with small forces. The configuration can operate radially into the gearing with small contact forces (in passive operation / slip torque).

[0033] The two contact surfaces 32, 33, which theoretically can also be point-shaped, are arranged offset from one another and enable compensation of the asymmetrical toothing forces and thus also a reduction in noise.

[0034] The angle δ can be used to adjust the force balance between the two directions of rotation of the worm shaft.

[0035] In an advantageous embodiment, the angles α and δ are 0°. The angle β is approximately 35° and the angle γ is 9°. The angle γ must be chosen to be relatively large in order to overcome frictional forces. Without friction, the angle γ is in a range from 0.5° to 2.5°, in particular between 1° and 2°. With friction in the second contact area 34, the angle γ is in a range between 7° and 13°. In the advantageous embodiment, the coefficient of friction is approximately 0.1. The distance d1 is approximately 16 mm and the distance d2 approximately 22 mm. The spring force is preferably 5 N. The play d3 in the pretensioning device is set to 0.2 mm. The remaining contacts have no play. The radii of the contact points 34, 35, starting from the center point, are at least 7 mm.

[0036] A further adjustment option can be provided by a movable housing part. The part of the housing located between the two contact points, which is particularly wedge-shaped, can prevent the worm shaft from locking by deflecting in the γ-direction.

[0037] It is possible to arrange the contact points in such a way that high forces are required for the bearing carrier to move relative to the worm shaft. To keep the slip torque as low as possible, a spring can be provided at the second contact point 34 between the projection of the lever arm and the housing.

[0038] The bearing carrier described above has a particularly simple design and is formed with the preloading device in a single component, preferably in one piece. Due to its adjuster-like properties, the bearing carrier's function can be fulfilled over the service life of the transmission. Wear in the transmission can also be compensated for.□

Claims

[1] Electromechanical power steering system with an electric servo motor (150) with a motor shaft, which drives a shaft (2) meshing with a helical gear (4), wherein the shaft (2) is arranged in a gear housing (8) and is rotatably mounted about a rotation axis (100) in a bearing arrangement (7) having a rolling bearing (9), wherein a pretensioning device (18) is included, which pretensions a movable bearing element of the bearing arrangement (7) resiliently against the gear housing (8) to adjust the play of the engagement between the helical gear (4) and the shaft (2), characterized byin that the gear housing (8) has a housing section (16) with a first contact surface (33) and a second contact surface (32), wherein the bearing arrangement (7) has a bearing carrier (13) for the rolling bearing (9), which forms the movable bearing element and which has a lever arm (19) extending from a base body (24) of the bearing carrier (13) with a free end (21) which bears against the gear housing (8) on the second contact surface (32) in a second contact region (34), wherein the bearing carrier (13) has an eccentric cam (14) on its base body (24) which bears against the first contact surface (33) on the housing (8) in a first contact region (35). [2] Electromechanical power steering according to claim 1, characterized by that the housing section (16) of the gear housing (16) forming the first and second contact surfaces (32, 33) is wedge-shaped with flat sides (17). [3] Electromechanical power steering according to claim 1 or 2, characterized by that the first and second contact areas (34,35) lie in a common plane perpendicular to the axis of rotation (100). [4] Electromechanical power steering according to one of the preceding claims, characterized by that the lever arm (19) has at its free end (21) a projection (30) extending in the longitudinal direction, which is resiliently supported on the gear housing (8) and which bears against the second contact surface (32). [5] Electromechanical power steering according to claim 4, characterized by that a free space (31) is formed between the projection (30) and the base body (24) of the bearing carrier (13), in which space the housing section (16) of the gear housing (8) forming the first and second contact surfaces (32, 33) is received. [6] Electromechanical power steering according to one of the preceding claims, characterized bythat the rolling bearing (9) is concentrically surrounded by the base body (24) of the bearing support (13) and that the bearing support (13) has a seat for the rolling bearing (9) in a central opening (25) of the base body (24), which seat is formed by a shoulder (26) in the opening (25). [7] Electromechanical power steering according to one of the preceding claims, characterized by that a spring element (23) held in the gear housing (8) presses onto the free end (21) of the lever arm (19) for spring-elastic preloading of the worm (2) in the gear housing (8). [8] Electromechanical power steering according to one of the preceding claims, characterized by that the eccentric cam (14) extends over a circular sector in a range between 10° and 100° to the axis of rotation (100) of the worm shaft (2). [9] Electromechanical power steering according to one of the preceding claims, characterized bythat the eccentric cam (14) has a bulbous elevation (15) which extends asymmetrically in the circumferential direction and is constant in the longitudinal direction and whose radius of curvature in cross-section is smaller than the radius of the bearing seat. [10] Electromechanical power steering according to one of the preceding claims, characterized by that the eccentric cam (14) is shaped in such a way that when a bearing outer ring (12) of the rolling bearing (9) moves about the rotation axis (100), the bearing outer ring (12) moves away from or towards the housing section (16) in order to adjust the play of the engagement between the helical gear (4) and the shaft (2). [11] Electromechanical power steering according to one of the preceding claims, characterized by that the first and / or second contact surface (32,33) of the gear housing (8) has a higher rigidity than the bearing carrier (13). [12] Electromechanical power steering according to one of the preceding claims, characterized by that the angular distance between the first contact region (35) and the second contact region (34) in the circumferential direction to the axis of rotation (100) is between 20° and 50°, in particular between 30° and 40°. [13] Electromechanical power steering according to one of the preceding claims, characterized by that the helical gear is a worm gear (4) and the shaft is a worm shaft (2).

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