Planetary roller gear, steering actuator and rear axle steering

Abstract

Planetary roller gear (1), comprising a threaded spindle (2), a number of planets (6) contacting the threaded spindle (2), and a rotatable drive element cooperating with the planets (6), which is mounted in a housing (30) by means of a radial bearing (13, 14), characterized in that a central axis (A 14 ) the radial bearing (13, 14) - with reference to the mechanically unloaded state - is spaced parallel from a central axis (A2) of the threaded spindle (2).

Description

[0001] The invention relates to a planetary roller gear designed according to the preamble of claim 1. Furthermore, the invention relates to a steering actuator intended for use in a motor vehicle and to a rear axle steering system.

[0002] A steering actuator of this type is known, for example, from DE 10 2021 104 649 A1. The known steering actuator, intended for use in a rear axle steering system, comprises a multi-part housing and a rotary-linear transmission arranged in the housing, which is designed as a planetary roller gear. Several housing components of the steering actuator according to DE 10 2021 104 649 A1 are connected to one another by a press fit.

[0003] Further design possibilities for planetary gear drives for steering systems of motor vehicles can be found, for example, in documents WO 2020 / 164654 A1 and WO 2022 / 117142 A1.

[0004] US patent 6,343,671 B1 relates to an actuator for correcting a steering angle input via the steering wheel of a vehicle to the wheels of a steered vehicle axle. This actuator operates with a pitch-preserving planetary roller gear, to which a sensor for determining the position of a spindle rod is assigned.

[0005] The invention is based on the objective of further developing a planetary roller gear, which is particularly suitable for use in a steering actuator of a rear axle steering system, compared to the aforementioned prior art, whereby a particularly favorable ratio between manufacturability-friendly design and long-term, consistently stable operating characteristics, in particular with a low noise and vibration level, is sought.

[0006] This problem is solved according to the invention by a planetary roller gear with the features of claim 1. The planetary roller gear is suitable, among other things, for use in a steering actuator according to claim 8, in particular as part of a rear axle steering system according to claim 10.

[0007] In its basic design, the planetary gear unit comprises a threaded spindle with a single or multi-start thread, a number of planets contacting the threaded spindle, and a rotatable drive element cooperating with the planets, which is mounted in a housing, i.e., a gear housing, in particular an actuator housing, by means of a radial bearing.

[0008] According to claim 1, the central axis of said radial bearing, with reference to the mechanically unloaded state of this bearing, is spaced parallel to the central axis of the threaded spindle, whereby the position of the central axis of the threaded spindle in the mechanically unloaded state is also to be considered.

[0009] The invention is based on the consideration that various operational forces and moments act on components of a planetary gear unit, which is part of an actuator that converts the rotation of a driving element into a linear adjustment of an output-side element. This applies particularly to cases in which an input-side element of the planetary gear unit is subjected to radial forces, for example, originating from a rotary-rotary gearbox upstream of the planetary gear unit. To prevent axial misalignment and / or elastic deformation of actuator components under real operating conditions, for example, in the form of shaft bending or housing deformation, conventional approaches involve making the corresponding components more robust.

[0010] The solution proposed in the application deliberately departs from such approaches by suggesting that different axes are offset relative to each other: on the one hand, the geometric axis of the input-side, rotatable element of the planetary gear set, and on the other hand, the central axis of the output-side element, i.e., the threaded spindle of the planetary gear set. During normal operation, this offset is at least approximately eliminated by the operational forces, resulting in an approximation of kinematically ideal operation with minimal vibration, noise, and wear. Particular attention is paid to the fact that the bearings of various components of the planetary gear set may exhibit significant elasticity or play.

[0011] The radial bearing used to support the drive element of the planetary gear unit can be designed, for example, as a rolling bearing, in particular as a roller or needle bearing. Ball bearings are also suitable for radial bearing applications. In this context, the term "radial bearing" also includes bearing types that can absorb forces in the axial direction in addition to radial forces. This applies, for example, to double-row roller or ball bearings in an X or O arrangement.

[0012] The position of the threaded spindle's central axis can also be determined by various bearing types. A plain bearing is particularly suitable. Alternatively, a linear rolling bearing, for example, can be used to support the threaded spindle. In any case, the threaded spindle is guided within a housing to prevent rotation.

[0013] Regardless of the bearing arrangements of the input and output elements of the planetary gear drive—that is, the rotating drive element on the one hand and the threaded spindle on the other—the offset between the central axis of the threaded spindle and the central axis of the radial bearing of the drive element in its mechanically unloaded state can be, for example, at least 100 ppm and at most 2.5% of the mean radius of the radial bearing. In particular, this offset can range from 500 ppm to 1.5% of the mean radius of the radial bearing. In the case of a radial bearing designed as a rolling element bearing, the mean radius of this bearing is understood to be the radius of the pitch circle, that is, the circle passing through the centers of all rolling elements.In the case of a sliding bearing present at the relevant location, the mean radius is defined by the circle which is placed centrally in the gap between the mutually supported elements, i.e., on the one hand the rotatable drive element and on the other hand a housing-fixed element of the radial sliding bearing.

[0014] The offset between the central axis of the threaded spindle and the central axis of the radial bearing of the drive element can also be used, if necessary, to selectively reduce the efficiency of the planetary gear unit. In particular, this allows for a self-locking design of the planetary gear unit.

[0015] One possible further development involves surrounding the radial bearing of the drive element with a damping element. In the case of a double-row radial bearing, for example, two ring-shaped damping elements made of a non-metallic material, in particular an elastomer or a plastic, can be provided for damping, each of which incorporates a radial bearing, particularly in the form of a roller or needle bearing.

[0016] Several design options are possible for the drive element of the planetary gear unit. For example, the drive element could be the nut of the planetary gear unit. Alternatively, a cage guiding the planets of the planetary gear unit, i.e., a planet carrier, can serve as the drive element. In this case, the planetary gear unit is designed as a pitch-preserving screw drive (SPWG). This means that there is always a clear correlation between the angular position of the driving element and the position of the movable output element.

[0017] Regardless of which element of the planetary gear set is used as its rotatably mounted drive element, various methods exist for introducing torque into the drive element. For example, a direct electric, i.e., gearless, drive is possible. In this case, the drive element can be connected to the rotor of an electric motor, optionally via a compensating coupling, or it can be an integral part of such a rotor.

[0018] Alternatively, the planetary gear unit can be part of a gear arrangement that also includes a rotary-rotary gear unit. For example, a recirculating gear unit, designed as a reduction gear, particularly in the form of a chain or belt drive, is connected upstream of the planetary gear unit, wherein the output element of the recirculating gear unit is rotationally fixed to the input element of the planetary gear unit.

[0019] The planetary roller gear is suitable not only for use in a rear axle steering actuator, but also, for example, in a steer-by-wire system for steering the front wheels of a vehicle. Furthermore, applications in brake, clutch, or transmission actuators are also possible.

[0020] Two exemplary embodiments of the invention are explained in more detail below with reference to a drawing. This drawing shows: Fig. 1. Sectional view of a steering actuator of a rear axle steering system including a planetary roller gear, Fig. 2. In schematic, exaggerated representation, geometric relations within the planetary gear system according to Fig. 1, Fig. 3 a modified embodiment of a rear axle steering system, which includes a planetary roller gear as a rotary-linear gear.

[0021] Unless otherwise stated, the following explanations refer to both embodiments. Corresponding or essentially equivalent parts are marked with the same reference numerals in all figures.

[0022] A planetary gear set, designated by reference numeral 1, is used as a rotary-linear gear set in a steering actuator 20 within the rear axle steering system 10 of a motor vehicle. Regarding the basic design and function of the rear axle steering system 10, reference is made to the prior art cited above.

[0023] The planetary gear set 1 comprises a threaded spindle 2, the thread of which is designated 3. Profiles 4 of several planets 6 mesh with the thread 3. The profiles 4 are formed in a central section 7 of each planet 6. The central section 7 transitions into two relatively thin side sections 8, which are provided with a profile 5. All profiles 4, 5 of the planet 6 are smooth (without a pitch). Following the two side sections 8, each planet 6 terminates in a smooth cylindrical end section 9. The end sections 9 are mounted in disk elements 23 of a planet carrier 16.

[0024] During operation of the planetary roller gear 1, the planets 6 contact the threaded spindle 2 exclusively with their central sections 7. The profiles 5 of the side sections 8, on the other hand, mesh with profiles 11 of a nut 12 that are also without a pitch. The nut 12 can be a single piece, as in the Fig. 1 and Fig. 3 outlined, or may be designed in multiple parts, whereby in the case of a multi-part design the nut parts may be pre-tensioned against each other.

[0025] The nut 12 is mounted opposite the planet carrier 16 by means of internal axial bearings 24, namely axial roller bearings, such that axial forces can be transmitted between the nut 12 and the disc elements 23 of the planet carrier 16. However, the transmission of torque via the nut 12 is not provided in this case. Instead, an outer sleeve 15, which is rigidly connected to the planet carrier 16, exists for this purpose. The outer sleeve 15 has a toothed section in its central region, viewed in the axial direction, which can be used to drive the planet carrier 16 by means of a drive element, namely a belt 28. The belt 28 is part of a rotary-rotary drive designed as a reduction gear, namely a belt drive 26.An output-side element of the belt drive 26, in the form of a pulley 27, is rigidly connected to or integrally formed with the outer sleeve 15 and thus functions as the input-side element of the planetary gear set 1. Alternatively, for example, a design of the rotary-rotary gear set as a chain drive is also possible. Thanks to the driven planet carrier 16, i.e., used as a drive element, the planetary gear set 1 is designed as a pitch-preserving screw drive (SPWG).

[0026] On the input side of the belt drive 26, there is a pulley 35 which is rigidly connected to a shaft 34 of an electric motor 29. The rotor of the electric motor 29 is designated 33.

[0027] The threaded spindle 2, which is longitudinally displaceable by means of the gear arrangement comprising the belt drive 26 and the planetary roller gear 1 downstream thereof, is guided against rotation in a housing 30 of the steering actuator 20. At both ends, the threaded spindle 2 is connected to connecting elements 32, which are provided for articulated coupling with chassis components by means of which the steering angle of the rear wheels of the vehicle is adjustable. Between the connecting elements 32 and the housing 30, as shown in Fig. 3, each a bellows 36. The entire steering actuator 20 can be connected to the chassis of the vehicle by means of fastening contours 31 formed by the housing 30.

[0028] A sliding bearing 17 is provided for supporting the threaded spindle 2 in the housing 30, wherein two sleeve-shaped sliding bearing elements of the sliding bearing 17 are located asymmetrically next to the arrangement consisting of the planet carrier 16, i.e., the drive element of the planetary roller gear 1, the nut 12, and the planets 6. The central axis of the threaded spindle 2, which is identical to the central axis of the sliding bearing 17 in the mechanically unloaded state, is designated A2.

[0029] The planet carrier 16 is axially supported in the housing 30 by means of two external thrust bearings 25, namely thrust roller bearings. A radial bearing arrangement 13, 14 is provided for the radial support of the planet carrier 16 in the housing 30, comprising a first rolling bearing 13 and a second rolling bearing 14. The central axis of the radial bearing arrangement 13, 14, again with reference to the mechanically unloaded state, is defined by A 14Each rolling bearing 13, 14 is designed as a needle bearing and comprises two bearing rings 19, 22, namely an outer ring 19 and an inner ring 22, as well as rolling elements 21, i.e. needles, rolling between the bearing rings 19, 22.

[0030] The pitch circle of the rolling bearing 13, 14 is determined by the position of the centers of the rolling elements 21. The radius of the pitch circle is given by R. 14 described. As from Fig. As can be seen in the exaggerated representation of 2, the central axes A2, A 14 not together. Rather, there is a gap between the central axes A2, A 14 , as long as no radial forces act on the various bearings 13, 14, 17, an offset VA is given.

[0031] During driving, 10 forces F act on the rear axle steering, which are in Fig. 3 are indicated by way of example. Among other things, forces F result in tensile stresses that prevail within the belt drive 26. Bending loads within the threaded spindle 2 also play a role. Overall, the forces F contribute to reducing the offset VA, that is, the center axes A2, A 14 at least approximately coincide. In contrast to unloaded configurations, where the bearings 13, 14, 17 of the planet carrier 16 and the threaded spindle 2 are coaxial in the mechanically unloaded state, the actual operation of the planetary roller drive 1 thus represents an approximation of the kinematically ideal operation. This results in particularly favorable noise characteristics and low wear of the entire steering actuator 20.

[0032] In the exemplary embodiment according to Fig. 1. The outer rings 19 of the rolling bearings 13, 14 are each inserted into an annular damping element 18. The offset VA between the central axes A2, A is determined by... 14 Properties of the damping element 18 are also taken into account. In contrast to the embodiment according to Fig. 1 are in the case of Fig. 3. The outer rings 19 are inserted directly into recesses of the housing 30, in particular pressed in. The geometric relationships according to Fig. 2 also apply to the embodiment according to Fig. 3. Reference symbol list 1 planetary roller gear 2 threaded spindles 3 threads 4 Profiling the central section of a planet 5 Profiling the side section of a planet 6 Planet 7 Middle section Section 8 9 Final Section 10 Rear axle steering 11 Profiling the mother 12 Mother 13 Radial bearing, rolling bearing 14 Radial bearing, rolling bearing 15 Outer sleeve 16 planetary carriers 17 Plain bearing 18 damping element 19 Bearing ring, outer ring 20 Steering actuator 21 rolling elements, needle 22 Bearing ring, inner ring 23 disc element 24 inner axial bearing 25 outer axial bearing 26 Rotary-rotary gearbox, belt drive 27 Pulley on the side of the PWG 28 belts 29 Electric motor 30 cases 31 Mounting contour 32 Connection element 33 Rotor 34 Shaft of the electric motor 35 Pulley, electric motor side 36 bellows A2 Central axis of the threaded spindle A 14 central axis of the radial bearing F force R 14 mean radius of the radial bearing VA offset between the center axes A2 and A 14

Claims

[1] Planetary gear unit (1), comprising a threaded spindle (2), a number of planets (6) contacting the threaded spindle (2), and a rotatable drive element cooperating with the planets (6), which is mounted in a housing (30) by means of a radial bearing (13, 14), characterized by , that a central axis (A 14 ) the radial bearing (13, 14) - with reference to the mechanically unloaded state - is spaced parallel from a central axis (A2) of the threaded spindle (2). [2] Planetary roller gear (1) according to claim 1, characterized by , that the radial bearing (13, 14) is designed as a rolling bearing. [3] Planetary roller gear (1) according to claim 1 or 2, characterized by , that the position of the central axis (A2) of the threaded spindle (2) is determined by a sliding bearing (17). [4] Planetary roller gear (1) according to any one of claims 1 to 3, characterized by, that an offset (VA) exists between the central axis (A2) of the threaded spindle (2) and the central axis (A) as it exists in the mechanically unloaded state 14 ) of the radial bearing (13, 14) at least 100 ppm and at most 2.5% of a mean radius (R 14 ) of the radial bearing (13, 14). [5] Planetary roller gear (1) according to any one of claims 1 to 4, characterized by a damping element (18) surrounding the radial bearing (13, 14) of the drive element. [6] Planetary roller gear (1) according to any one of claims 1 to 5, characterized by a spindle nut as its drive element. [7] Planetary roller gear (1) according to any one of claims 1 to 5, characterized by a planet carrier (16) as its drive element. [8] Steering actuator (20) comprising a planetary roller gear (1) according to claim 1. [9] Steering actuator (20) according to claim 8, characterized by, that a belt drive (26) designed as a reduction gear is connected upstream of the planetary roller gear (1), wherein the output element of the belt drive (26) is connected to the drive element of the planetary roller gear (1) in a rotationally fixed manner. [10] Rear axle steering (10) comprising a steering actuator (20) according to claim 9.

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