Planetary roller gear, steering actuator, and rear-axle steering system

Abstract

The invention relates to a planetary roller gear (1), in particular in a steering actuator (20) of a rear-axle steering unit (10), comprising a threaded spindle (2), a number of planets (6) which contact the threaded spindle (2), and a rotatable drive element (16) which interacts with the planets (6) and which is mounted in a housing (30) by means of a radial bearing (13, 14), in particular a roller bearing. In a mechanically unloaded state, the central axis (A14) of the radial bearing (13, 14) and the central axis (A2) of the threaded spindle (2) are spaced apart parallel to each other.

Description

[0001] Planetary roller gears, steering actuator and rear axle steering

[0002] 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 a rear-axle steering system.

[0003] 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 gear 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 an interference fit.

[0004] Further design options for planetary roller gears for steering systems of motor vehicles can be found, for example, in the documents WO 2020 / 164654 A1 and WO 2022 / 117142 A1.

[0005] US 6,343,671 B1 concerns an actuator for correcting a steering angle input to the wheels of a steered vehicle axle via the steering wheel. This actuator operates with a pitch-accurate planetary roller gear, which is associated with a sensor for determining the position of a spindle rod.

[0006] The invention is based on the object 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 cited prior art, while striving for a particularly favorable balance between a production-friendly design and long-term, as consistent as possible operating characteristics, in particular with a low noise and vibration level. This object is achieved according to the invention by a planetary roller gear having 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 a basic concept known per se, the planetary roller gear comprises a threaded spindle with a single or multiple thread, a number of planets contacting the threaded spindle, and a rotatable drive element interacting with the planets, which is mounted by means of a radial bearing in a housing, i.e. gear housing, in particular actuator housing.

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

[0009] The invention is based on the consideration that various operational forces and moments act on components of a planetary roller gear, 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 in particular to cases in which an input-side element of the planetary roller gear is exposed to radial forces, which originate, for example, from a rotary-rotary gear connected upstream of the planetary roller gear. 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, the corresponding components can be designed to be more solid according to common approaches.

[0010] The solution according to the application deliberately departs from such approaches by proposing the offsetting of various axes, namely, on the one hand, the geometric axis of the input-side, rotatable element of the planetary roller gear, and, on the other hand, the center axis of the output-side element, i.e., the threaded spindle of the planetary roller gear. During normal operation, this offset is at least approximately canceled out by the operational forces, thus approximating kinematically ideal, low-vibration, low-noise, and low-wear operation. In particular, this approach takes into account the fact that the bearings of various components of the planetary roller gear may exhibit significant elasticity or be subject to play.

[0011] The radial bearing supporting the drive element of the planetary roller gear can be designed, for example, as a rolling bearing, particularly a roller or needle bearing. Ball bearings can also be considered as radial bearings.

[0012] In this case, the term "radial bearing" also includes bearing types that can accommodate axial forces in addition to radial forces. This applies, for example, to double-row roller or ball bearings in an X- or O-arrangement.

[0013] The position of the center axis of the threaded spindle can also be determined by a variety of bearing types. A plain bearing is particularly suitable for this purpose. Alternatively, a linear roller bearing, for example, can be used to support the threaded spindle. In either case, the threaded spindle is guided in a housing that is secured against rotation.

[0014] Regardless of the bearings of the input and output-side elements of the planetary roller gear, i.e., the rotatable drive element on the one hand and the threaded spindle on the other, the offset between the center axis of the threaded spindle and the center axis of the radial bearing of the drive element in the mechanically unloaded state can, for example, be at least 100 ppm and a maximum of 2.5% of the mean radius of the radial bearing. In particular, the said offset can be in the 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 bearing, the mean radius of this bearing is understood to be the radius of the pitch circle, i.e., the circle passing through the centers of all rolling elements.In the case of a plain bearing present at the corresponding location, the mean radius is defined by the circle which is placed centrally in the gap between the elements mounted against each other, i.e. on the one hand the rotatable drive element and on the other hand a housing-fixed element of the radial plain bearing.

[0015] The offset between the center axis of the threaded spindle and the center axis of the radial bearing of the drive element can also be used, if necessary, to specifically reduce the efficiency of the planetary roller gear. In particular, this allows for a self-locking design of the planetary roller gear.

[0016] A possible further development provides for the radial bearing of the drive element to be surrounded by a damping element. In the case of a double-row radial bearing, for example, two annular damping elements made of a non-metallic material, in particular an elastomer or plastic, can be provided for damping, each of which houses a radial bearing, in particular in the form of a roller or needle bearing.

[0017] Regarding the drive element of the planetary roller gear, various design variants are possible. For example, the drive element is the nut of the planetary roller gear. Alternatively, a cage guiding the planets of the planetary roller gear, i.e., a planet carrier, can act as the drive element of the planetary roller gear. In this case, the planetary roller gear is designed as a pitch-stable 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.

[0018] Regardless of which element of the planetary roller gear is used as its rotatably mounted drive element, various options are available 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 can be an integral part of such a rotor.

[0019] Alternatively, the planetary roller gear can be part of a gear arrangement that also includes a rotary-rotary gear. For example, a belt-and-chain transmission designed as a reduction gear, particularly in the form of a chain or belt drive, is connected upstream of the planetary roller gear, with the output element of the belt-and-chain transmission being rotationally connected to the input element of the planetary roller gear.

[0020] 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 steering system for steering the front wheels of a vehicle. Applications in brake, clutch, or transmission actuators are also possible.

[0021] Two exemplary embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings:

[0022] Fig. 1 shows a detail of a steering actuator of a rear axle steering system including a planetary roller gear,

[0023] Fig. 2 in a schematic, exaggerated representation of geometric relations within the planetary roller gear according to Fig. 1 ,

[0024] Fig. 3 shows a modified embodiment of a rear axle steering system, which comprises a planetary roller gear as a rotary-linear gear.

[0025] Unless otherwise stated, the following explanations refer to both exemplary embodiments. Corresponding or essentially equivalent parts are identified by the same reference numerals in all figures. A planetary roller gear, designated overall by reference numeral 1, is used as a rotary-linear gear of a steering actuator 20 in a rear-axle steering system 10 of a motor vehicle. Regarding the basic structure and function of the rear-axle steering system 10, reference is made to the prior art cited at the beginning.

[0026] The planetary roller gear 1 comprises a threaded spindle 2, the thread of which is designated 3. Profilings 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 comparatively thin side sections 8, which are provided with a profile 5. All profiles 4, 5 of the planet 6 are designed without a pitch. Adjacent to the two side sections 8, each planet 6 terminates in the form of a smooth, cylindrical end section 9. The end sections 9 are mounted in disk elements 23 of a planet carrier 16.

[0027] 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, which are also pitchless. The nut 12 can be designed in one piece, as sketched in Figures 1 and 3, or in several parts, whereby in the case of a multi-part design the nut parts can be preloaded against each other.

[0028] The nut 12 is mounted relative to 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 disk elements 23 of the planet carrier 10. However, a transmission of torque via the nut 12 is not provided in the present case. Rather, an outer sleeve 15, which is firmly connected to the planet carrier 10, exists for this purpose. The outer sleeve 15 has a toothing in its central region, viewed in the axial direction, which can be used to drive the planet carrier 16 by means of a belt 28. The belt 28 is part of a rotary-rotary gear 26, namely a belt drive, designed as a reduction gear.An output-side element of the belt drive 26, in the form of a pulley 27, is rigidly connected to the outer sleeve 15 or formed integrally, and thus functions as the input-side element of the planetary roller gear 1. Alternatively, a design of the rotary-rotary gear 26 as a chain drive is also possible. Thanks to the driven planet carrier 16, i.e., used as the drive element, the planetary roller gear 1 is designed as a pitch-stable screw drive (SPWG).

[0029] 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.

[0030] The threaded spindle 2, which is displaceable in its longitudinal direction with the aid of the gear arrangement comprising the belt transmission 26 and the planetary roller gear 1 connected downstream thereof, is guided in a rotationally secured manner 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 to chassis components, via which the steering angle of the rear wheels of the motor vehicle can be adjusted. As can be seen from Fig. 3, a bellows 36 is located between each connecting element 32 and the housing 30. 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.

[0031] A plain bearing 17 is provided for supporting the threaded spindle 2 in the housing 30, wherein two sleeve-shaped plain bearing elements of the plain bearing 17 are located asymmetrically next to the arrangement comprising the planet carrier 16, i.e. the drive element of the planetary roller gear 1, the nut 12 and the planets 6 in the present case. The central axis of the threaded spindle 2, which is identical to the central axis of the plain bearing 17 in the mechanically unloaded state, is designated A2. The planet carrier 16 is mounted in the axial direction in the housing 30 by means of two outer axial bearings 25, namely axial roller bearings. For radial mounting of the planet carrier 16 in the housing 30, a radial bearing 13, 14 is provided which comprises a first roller bearing 13 and a second roller bearing 14. The central axis of the radial bearing 13, 14, again related to the mechanically unloaded state, is designated Au.Each 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.

[0032] 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 designated Ru. As can be seen in Fig. 2 in an exaggerated representation, the axes A2, Au do not coincide. Rather, an offset VA exists between the axes A2, Au, as long as no radial forces act on the various bearings 13, 14, 17.

[0033] During driving operation, forces F act on the rear-axle steering 10, which are indicated by way of example in Fig. 3. Among other things, forces F result from tensile stresses existing 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, i.e., the axes A2, Au at least approximately coincide. In contrast to non-stressed designs, in which the bearings 13, 14, 17 of the planet carrier 16, on the one hand, and the threaded spindle 2, on the other hand, are coaxial in the mechanically unloaded state, the actual operation of the planetary roller gear 1 thus represents an approximation of kinematically ideal operation. This results in particularly favorable noise behavior and low wear of the entire steering actuator 20.

[0034] In the embodiment shown in Fig. 1, the outer rings 19 of the rolling bearings 13, 14 are each inserted into an annular damping element 18. The design of the offset VA between the axes A2, Au also takes into account the properties of the damping element 18. In contrast to the embodiment shown in Fig. 1, in the case shown in Fig. 3, the outer rings 19 are inserted, in particular pressed, directly into recesses in the housing 30. The geometric relationships shown in Fig. 2 also apply to the embodiment shown in Fig. 3.

[0035] List of reference symbols

[0036] Planetary roller gear

[0037] threaded spindle

[0038] thread

[0039] Profiling the midsection of a planet

[0040] Profiling the lateral section of a planet

[0041] planet

[0042] Middle section

[0043] Page section

[0044] final section

[0045] Rear-axle steering

[0046] Profiling the mother

[0047] Mother

[0048] Radial bearings, rolling bearings

[0049] Radial bearings, rolling bearings

[0050] Outer sleeve

[0051] planet carrier

[0052] Plain bearing

[0053] Damping element

[0054] Bearing ring, outer ring

[0055] Steering actuator

[0056] Rolling element, needle

[0057] Bearing ring, inner ring

[0058] Disc element inner thrust bearing outer thrust bearing

[0059] Rotary-rotary gear, belt drive

[0060] Pulley on the PWG side

[0061] belt

[0062] electric motor

[0063] Housing

[0064] Mounting contour 32 connecting element

[0065] 33 Rotor

[0066] 34 Electric motor shaft

[0067] 35 Pulley, electric motor side

[0068] 36 Bellows

[0069] A2 Axis of the threaded spindle

[0070] Au axis of the radial bearing

[0071] F Force

[0072] R14 mean radius of the radial bearing

[0073] VA offset between axes A2 and A14

Claims

Patent claims 1 . Planetary roller gear (1), with a threaded spindle (2), a number of planets (6) contacting the threaded spindle (2), and a rotatable drive element (16) which interacts with the planets (6) and is mounted in a housing (30) by means of a radial bearing (13, 14), characterized in that the central axis (A14) of the radial bearing (13, 14) - based on the mechanically unloaded state - is spaced parallel from the central axis (A2) of the threaded spindle (2).

2. Planetary roller gear (1) according to claim 1, characterized in that the radial bearing (13, 14) is designed as a rolling bearing.

3. Planetary roller gear (1) according to claim 1 or 2, characterized in that the position of the central axis (A2) of the threaded spindle (2) is fixed by a plain bearing (17).

4. Planetary roller gear (1) according to one of claims 1 to 3, characterized in that the offset (VA) between the center axis (A2) of the threaded spindle (2) and the center axis (A14) of the radial bearing (13, 14) given in the mechanically unloaded state is at least 100 ppm and at most 2.5% of the mean radius (R14) of the radial bearing (13, 14).

5. Planetary roller gear (1) according to one of claims 1 to 4, characterized by a damping element (18) surrounding the radial bearing (13, 14) of the drive element (16).

6. Planetary roller gear (1) according to one of claims 1 to 5, characterized by a spindle nut as its drive element (16).

7. Planetary roller gear (1) according to one of claims 1 to 5, characterized by a planet carrier as its drive element (16).

8. Steering actuator (20) comprising a planetary roller gear (1) according to claim 1.

9. Steering actuator (20) according to claim 8, characterized in that a belt transmission (26) designed as a reduction gear is connected upstream of the planetary roller gear (1), wherein the output element (27) of the belt transmission (26) is connected in a rotationally fixed manner to the drive element (16) of the planetary roller gear (1).

10. Rear axle steering (10) comprising a steering actuator (20) according to claim 9.

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