Actuator and rear-axle steering system

Abstract

An actuator (1), in particular a rear-axle steering system (10), operates with a planetary roller screw drive (2) as rotary-linear transmission. Multiple planets (5) of pitchless profile mesh with a multi-start thread (4) of a threaded spindle (3), which also forms part of the planetary roller screw drive (2). At least at one end of the threaded spindle (3), a guide bolt (23) is screwed on the multi-start thread (4).

Description

[0001] Actuator and rear axle steering

[0002] The invention relates to an actuator, in particular in the form of a steering actuator, with a rotary-linear transmission designed as a planetary roller screw drive. Furthermore, the invention relates to a rear-axle steering system with such an actuator, which can in particular be an electromechanical actuator.

[0003] A steering actuator with a planetary roller gear, i.e., a planetary roller screw drive, is known, for example, from DE 10 2021 104646 A1. Components of the known steering actuator are connected to each other by interference fit.

[0004] Another actuator intended for use in a rear-axle steering system of a motor vehicle is described in DE 10 2018 213 776 A1. This actuator comprises a housing and a spindle that is movable relative to the housing and has at least one fastening end. Furthermore, the actuator according to DE 10 2018 213 776 A1 comprises a bearing journal with a fastening opening in which the fastening end is axially fixed. The fastening end is to be supported and clamped relative to the bearing journal via a ring, wherein the ring has a conical inner surface and an end face perpendicular to the longitudinal axis of the bearing journal.

[0005] DE 10 2014 206 934 A1 relates to a servomotor intended for use as a central actuator for a rear-wheel steering system. The servomotor comprises a movable spindle with a moving thread. A bearing sleeve is attached to one end of the spindle. For this purpose, the bearing sleeve has a mounting opening in which the mounting end is received and axially fixed. According to DE 10 2014 206 934 A1, the diameter of the mounting end is equal to or smaller than the core diameter of the moving thread. The connection between the mounting end and the bearing sleeve is to be realized using a conical sleeve.

[0006] The invention is based on the object of further developing actuators, in particular with an electric drive, which are suitable, among other things, for adjusting the steering angle of the rear wheels of a vehicle, compared to the cited prior art, in particular with regard to manufacturing technology.

[0007] This object is achieved according to the invention by an actuator having the features of claim 1. It is in the form of a steering actuator suitable for use in a rear axle steering system according to claim 10.

[0008] In general, the actuator according to the application can be used not only in vehicles, but also in stationary machines or other systems. Where reference is made to "steering actuator" in the following, the corresponding statements also apply, if transferable, to applications outside of automotive technology.

[0009] The actuator, in particular a steering actuator, operates with a rotary-linear gear designed as a planetary roller screw drive, which comprises a plurality of pitchless profiled planets. The planets mesh with a multi-start thread of a threaded spindle, also part of the planetary roller screw drive, which is displaced in its axial direction when the planetary roller screw drive is actuated. A guide pin is screwed onto the multi-start thread at at least one end of the threaded spindle.

[0010] The multi-start thread thus has multiple functions: Firstly, it transmits forces within the planetary roller screw drive, performing a function in converting rotation into linear movement and interacting directly with the planets, which are arranged in an annular space surrounding the threaded spindle. Secondly, a guide pin is screwed onto the same multi-start thread at least at one end of the threaded spindle. In particular, in this way, a guide pin, which has a multi-start internal thread corresponding to the design of the thread of the spindle, can be screwed to the threaded spindle at each end of the threaded spindle. The threads of the spindle and of the at least one guide pin can be designed as double-start or triple-start threads. A higher number of threads is also possible.

[0011] According to various possible embodiments, the guide pin has a base against which the threaded spindle abuts on the end face. The base is not necessarily closed. For example, the base can be degenerated into a rim which extends from a cylindrical section of the guide pin and is formed on one end face of the guide pin. Likewise, a base, a radially inward-facing rim, or another structure which provides an axial stop for the threaded spindle can be arranged between the two end faces of the guide pin. Segmentation of a rim or a structure with the same function is also possible. For example, a spline is formed by such a structure of the guide pin and a mating contour interacting with it on the threaded spindle side.

[0012] In all of these cases, in which there is an axial stop between the threaded spindle and the guide pin, there may be an axial distance between the base or the functionally comparable contour of the guide pin and a screw connection area in which the multi-start thread of the threaded spindle is screwed into the equally multi-start internal thread of the guide pin. This axial distance creates a preload area that is distinct from the screw connection area and allows at least minimal, but technically relevant, elastic deformation of the threaded spindle and / or the guide pin. The axial distance is, for example, at least half and at most 1.5 times the pitch diameter of the multi-start thread of the threaded spindle.

[0013] A first design variant, in which an axial distance is provided between the base of the guide sleeve and the screw connection area, provides that in a section of the at least approximately cylindrical inner circumferential surface of the guide pin close to the base, an enlarged inner diameter of the guide pin is provided compared to the multi-start internal thread of the guide pin formed in the screw connection area and to be screwed onto the thread of the threaded spindle. Thus, in this variant, an undercut is provided in the guide pin, which represents a clearance through which the preload area is created. The extent to which the multi-start thread of the spindle extends within the preload area is irrelevant for the function of the preload area. In particular, the thread of the threaded spindle can extend to its front face. Alternatively, the thread of the spindle can be missing in the preload area.

[0014] A second design variant also features an unthreaded end section of the threaded spindle that extends into the guide pin and also provides an axial distance between the base of the guide sleeve and the screw connection area. In this case, the outer diameter of the end section is reduced compared to the remaining section of the threaded spindle, which is provided with the multi-start external thread, so that a clearance is also created, which is essential for the function of the preload area. Due to the tapered, unthreaded end section, it is irrelevant in this case whether the multi-start internal thread of the guide sleeve, which is matched to the dimensions of the external thread of the spindle, extends into the preload area or merely covers the screw connection area.

[0015] Regardless of the type of connection between the guide pin and the lead screw, either the lead screw nut or a cage guiding the planets can act as the driving element of the planetary roller screw drive. In the latter case, the planetary roller screw drive is designed as a pitch-stable planetary roller gear (SPWG). Regarding the design features of pitch-stable planetary roller gears, reference is made to documents DE 10 2021 100 275 A1 and DE 10 2019 103 384 A1 for background information. An SPWG, in principle in the same way as a simple linear motion screw that operates without rolling elements, converts a defined rotational movement of the drive element into a defined linear movement of the output element, i.e., the lead screw, without slippage. This requires a less extreme transmission ratio than a planetary roller gear with a driven nut.

[0016] In the case of the actuator according to the application, in particular a steering actuator, the spindle nut, which has no input or output function when the cage is driven, can be mounted in the spindle nut by means of a bearing, which can be a plain bearing or a rolling bearing. This bearing transmits primarily or exclusively axial forces and can, for example, be in the form of two axial roller bearings or axial needle bearings. This bearing can also be designed as a plain bearing, which absorbs forces in both axial directions, relative to the longitudinal axis of the spindle nut, the cage, and the entire steering actuator.

[0017] As for the bearings of the cage guiding the planets in the actuator housing, a rolling bearing or a plain bearing can also be considered. In the case of a rolling bearing, this bearing can be designed, for example, as an angular contact roller bearing or an angular contact ball bearing. In the case of a plain bearing, conical plain bearing surfaces can be used.

[0018] In any embodiment, the steering actuator is particularly suitable for use in a rear-axle steering system of a motor vehicle. Furthermore, the steering actuator can be used as a component of a steer-by-wire steering system for steering the front wheels of a vehicle. Regarding the manufacture of the actuator's threaded spindle, regardless of whether it is a steering actuator or another, particularly electrically operated, actuator, it is rational to start with a longer threaded rod, which is cut to length to obtain sections that are used as the threaded spindle within the fully assembled actuator in normal operation. In any case, the threaded spindle, which is a possibly reworked section of the threaded rod, has an additional function, namely the retention of at least one guide pin.

[0019] Especially in variants where the multi-start thread of the lead screw extends to its end faces, rework, for example, in the form of machining, on sections of the threaded rod is reduced to a minimum. However, if the end sections of the lead screw are designed as cylindrical sections with a reduced diameter, this requires additional machining steps on the spindle, which, however, are offset by a simplified geometry of the inner surface of the guide pins.

[0020] Several embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings:

[0021] Fig. 1 shows a detail of a first embodiment of a steering actuator of a rear axle steering system,

[0022] Fig. 2 shows a detail of the steering actuator according to Figure 1,

[0023] Fig. 3 a guide pin of a steering actuator, Fig. 4 a threaded spindle for a planetary roller screw drive of a steering actuator,

[0024] Fig. 5 shows a threaded spindle modified compared to the design according to Figure 4,

[0025] Fig. 6 shows a detail of a planetary roller screw drive including the threaded spindle according to Figure 5 in a representation analogous to Figure 2.

[0026] Unless otherwise stated, the following explanations refer to all exemplary embodiments. Corresponding or essentially equivalent parts are identified by the same reference numerals in all figures.

[0027] A steering actuator, designated overall by reference numeral 1 and only partially illustrated in the figures, is intended for use in a rear-axle steering system 10 of a motor vehicle. The actuator 1, in this case a steering actuator, comprises an electric motor, which enables the axial adjustment of a threaded spindle 3 via a belt drive or other rotary-rotary gear, for example a planetary gear, and a planetary roller screw drive 2, i.e., a rotary-linear gear. Regarding the basic function of the rear-axle steering system 10, reference is made to the prior art cited above.

[0028] The thread of the threaded spindle 3 is multi-start, in this case two-start, and designated 4. In contrast to the threaded spindle 3, the planets of the planetary roller screw drive 2, designated 5, have a pitchless profile. Each planet 5 has a central section 6 and two adjoining side sections 7, 8 with a comparatively small diameter. Only the central section 6 meshes with the thread 4 of the threaded spindle 3. The two side sections 7, 8, on the other hand, are lifted off the thread 4 in every operating state of the planetary roller screw drive 2 and only mesh with the profiles of a nut 9, i.e. the spindle nut, which also have a pitchless profile. The nut 9 is freely rotatable in the planetary roller screw drive 2 and functions neither as a drive nor as an output element.

[0029] Rather, a cage 11, which guides the planets 5, is provided as the drive element of the planetary roller screw drive 2. The cage 11 comprises two cage discs 12, 13 and a cage sleeve 14. The cage sleeve 14 is rotationally fixedly connected to the cage discs 12, 13, between which the planets 5 are guided. The cage sleeve 14 represents the output element of the aforementioned belt drive, whose belt is designated 15. A pulley, provided as the drive element of the belt drive, is rotationally fixedly connected to the motor shaft of an electric motor (not shown). The threaded spindle 3 is thus linearly displaceable via a combination of a rotary-rotary gear, namely a belt drive, and a rotary-linear gear, which is present in the form of the planetary roller screw drive 2.

[0030] For the transmission of axial forces between the freely rotatable nut 9 and the cage 11, a first bearing 16 is provided, which in the present case is designed as a rolling bearing and comprises two axial roller bearings 17, 18, which are arranged between the two end faces of the nut 9 and the inner sides of the cage discs 12, 13.

[0031] The entire cage 11 is mounted by means of a second bearing 19 in the housing of the steering actuator 1, designated overall by 20. In the present case, the second bearing 19 is also designed as a rolling bearing. The second bearing 19 comprises two tapered roller bearings 21, 22, which are mirror images of one another and are arranged between conical regions of the outer surfaces of the cage discs 12, 13 and likewise conical inner surfaces of the housing 20. In a modified manner, not shown, a first bearing 16 and / or the second bearing 19 can be designed as a plain bearing. In all exemplary embodiments, guide bolts 23 are screwed onto the two ends of the threaded spindle 3, using the multi-start thread 4 for this purpose, which also interacts with the planets 5 in the manner described. The guide bolts 23 are guided in plain bearing elements 24, which are inserted into the housing 20.In a manner not shown, the guide pins 23 are articulated to chassis elements that enable the steering angle of the rear wheels to be varied. Figure 3 shows a guide pin 23 modified compared to the exemplary embodiment according to Figures 1 and 2, which performs the same function within the steering actuator 1 as in the case of Figures 1 and 2. The threaded spindle 3 according to Figure 4 is installed in the steering actuator 1 according to Figures 1 and 2, but can also be combined with the guide pin 23 according to Figure 3. The threaded spindle 3 is manufactured in a manner not shown by cutting a longer threaded rod to length.

[0032] The guide pin 23, which is concentric with the threaded spindle 3, has a receptacle designed as a blind hole 25 for the end of the threaded spindle 3. A base of the guide pin 23 is designated 26. In all embodiments, the end face of the threaded spindle 3, designated 27, abuts the base 26 under preload. Deviating from the illustrated embodiment, variants of the base 26 that are not fully covered are also possible. For example, the base 26 can have a central opening, so that only a rim, which assumes the function of the base 26, is present. Likewise, a segmentation of the base 26 or a structure that has the same function, namely a stop function, is possible.

[0033] Within the blind hole 25, a screw connection area 28, in which the thread 4 of the spindle 3 is screwed to the guide pin 23, must be distinguished from a preload area 29, which borders the base 26. The uniform pitch diameter of the thread 4, even in the screw connection area 28, is designated d2. The length of the preload area 29, designated L29 and measured in the longitudinal direction of the threaded spindle 3, corresponds in the present case to at least half the pitch diameter d2 of the multi-start thread 4. The length L29 of the preload area 29 represents the axial distance between the base 26 and the screw connection area 28.

[0034] With regard to the contours of the threaded spindle 3 and the guide pin 23 in the preload area 29, various design options exist, as is particularly evident from a comparison of Figures 2 and 6, wherein Figure 5 shows an isolated illustration of the threaded spindle 3 of the steering actuator 1 according to Figure 6. In the variant according to Figures 1 and 2, the thread 4 of the spindle 3 extends approximately to its end face 27. In this case, a clearance in the preload area 29 is achieved by having an enlarged inner diameter Di in the corresponding area 29. In the example according to Figures 1 and 2, the diameter Di in the preload area 29 is at least 5% and a maximum of 15% larger than the pitch diameter d2 of the threaded spindle 3.

[0035] In the case of Figure 6, the release in the preload area 29 is achieved by the threaded spindle 3 having a cylindrical surface with an outer diameter D in the entire preload area 29 aThe outer diameter D a In the present case, it is in the range of 85% to 95% of the pitch diameter d2 of the multi-start thread 4. Deviating from the design shown in Figure 6, in which the internal thread of the guide pin 23 extends approximately to the bottom 26, variants can also be realized in which the guide pin 23 has a smooth cylindrical inner surface in the largest part of the preload area 29.

[0036] In each embodiment, in the fully assembled steering actuator 1, at least minimal elastic deformation of the threaded spindle 2 and the guide pin 23 is present within the preload area 29. List of reference symbols

[0037] Actuator, steering actuator planetary roller screw drive threaded spindle

[0038] multi-start thread

[0039] planet

[0040] Middle section

[0041] Page section

[0042] Page section

[0043] Mother

[0044] Rear-axle steering

[0045] cage

[0046] Cage disc

[0047] Cage disc

[0048] Cage sleeve

[0049] Belt first storage

[0050] Axial roller bearings

[0051] Axial roller bearing second bearing housing

[0052] angular contact roller bearings

[0053] angular contact roller bearings

[0054] Guide bolt

[0055] Plain bearing element

[0056] Blind hole, bottom mounting

[0057] frontal surface

[0058] Screwing area

[0059] Preload range d2 pitch diameter

[0060] Since outer diameter

[0061] Di inner diameter l_29 length of the preload area

Claims

Patent claims 1. Actuator (1) with a rotary-linear gear designed as a planetary roller screw drive (2), which comprises a plurality of pitchless profiled planets (5) which mesh with a multi-start thread (4) of a threaded spindle (3) which is also part of the planetary roller screw drive (2), wherein at least one end of the threaded spindle (3) has a guide pin (23) screwed onto the multi-start thread (4).

2. Actuator (1) according to claim 1, characterized in that the thread (4) of the threaded spindle (3) is designed as a double-start thread.

3. Actuator (1) according to claim 1 or 2, characterized in that the guide pin (23) has a base (26) against which the threaded spindle (3) abuts on the end face.

4. Actuator (1) according to claim 3, characterized in that an axial distance (L29) is provided between the base (26) and a screwing area (28) between the threaded spindle (3) and the guide pin (23).

5. Actuator (1) according to claim 3, characterized in that said axial distance (L29) corresponds to at least half and at most 1.5 times the flank diameter (d2) of the multi-start thread (4) of the threaded spindle (3).

6. Actuator (1) according to claim 4 or 5, characterized in that the axial distance (L29) is given by an enlarged inner diameter (Di) of the guide pin (23) in a section near the bottom, that is to say prestressing region (29), of the guide pin (23) compared to the multi-start internal thread of the guide pin (23) screwed onto the thread (4) of the threaded spindle (3).

7. Actuator (1) according to claim 4 or 5, characterized in that the axial distance (L29) is given by a threadless end section of the threaded spindle (3).

8. Actuator (1) according to one of claims 1 to 7, characterized in that the planetary roller screw drive (2) has a driven cage (11) guiding the planets (5).

9. Actuator (1) according to claim 8, characterized in that a first bearing (16) is provided between the cage (11) and a spindle nut (9) of the planetary roller screw drive (2) and a further bearing (19) is provided between the cage (11) and a housing (20) of the actuator (1).

10. Rear axle steering (10), comprising an actuator (1) designed as a steering actuator according to claim 1.

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