Planetary roller gear and steering actuator

Abstract

Planetary rolling gear (1), comprising a threaded spindle (2), several planets (4) contacting the threaded spindle (2), and a spindle nut (9) in which the planets (4) roll in contact with a lubricant, characterized in that a lubricant reservoir (14) with a non-circular cross-sectional contour (Q14) describing an ellipse or a uniform thickness is formed between the planets (4) and an inner circumferential surface of the spindle nut (9).

Description

[0001] The invention relates to a planetary gear unit suitable, for example, for use in an electromechanical actuator according to the preamble of claim 1. The invention further relates to a steering actuator operating with a planetary gear unit.

[0002] A planetary gear system of this type is known, for example, from WO 2013 / 004469 A1. The known planetary gear system is designed for use in a submarine or diving boat and can be actuated, in particular, by means of a brushless electric motor. A lubrication supply system for the device according to WO 2013 / 004469 A1 ensures a constant supply of lubricant to one of the planetary gears' recirculating chambers.

[0003] An electromechanical steering actuator, which operates with a planetary roller gear as part of a linear drive, is disclosed, for example, in DE 10 2019 127 328 A1. The mechanical efficiency of the linear drive is between 40% and 55%. In particular, the efficiency is between 48% and 49.5%, and thus in the self-locking range.

[0004] German patent application DE 10 2021 210 368 A1 relates to an actuator for a steering system. In this case, a rotation is converted into a linear displacement of a spindle by means of a threaded drive. The device according to DE 10 2021 210 368 A1 has cavities which form a reservoir for lubricant for lubricating the threaded drive.

[0005] CN 1 12 032 267 A discloses a planetary roller screw drive with a multi-part nut, a spindle, and several planetary rollers arranged between the nut and the spindle. The nut comprises a sleeve in which two nut parts are arranged. The sleeve has an axially extending limiting groove on its outer surface.

[0006] The invention is based on the objective of providing a planetary gear unit that is further developed compared to the aforementioned prior art, particularly with regard to lubrication technology, as well as a suitable use of the planetary gear unit.

[0007] This problem is solved according to the invention by a planetary roller gear with the features of claim 1. The planetary roller gear is particularly suitable for use in a steering actuator according to claim 7.

[0008] The planetary gear system, in a basic concept known per se, comprises a threaded spindle, several planets contacting the threaded spindle, and a spindle nut in which the planets roll in contact with a lubricant.

[0009] According to claim 1, a lubricant reservoir with a non-circular cross-sectional contour is formed between the planets and an inner circumferential surface of the spindle nut. Optionally, the cross-sectional area of ​​the lubricant reservoir also varies in the longitudinal direction of the threaded spindle, i.e., in the axial direction of the entire planetary gear train.

[0010] It has been shown that the non-circular cross-sectional contour allows for the fulfillment of static requirements while simultaneously providing a generously dimensioned volume for the storage of lubricant, particularly grease. Furthermore, the non-circular cross-sectional contour offers favorable conditions for the conveyance of lubricant within the planetary roller drive. The cross-section under consideration lies in a plane to which the central axis of the threaded spindle is a surface normal.

[0011] The cross-sectional contour of the lubricant reservoir formed by the inner circumferential surface of the spindle nut describes a non-circular ellipse or a uniform thickness.

[0012] Similarly, embodiments are possible in which the inner circumferential surface of the spindle nut, which defines the lubricant reservoir, is formed as a thread. In these cases as well, the line of intersection between a plane normal to the central axis of the planetary gear set and the inner circumferential surface of the spindle nut has a shape other than a circle. The thread formed on the inner circumferential surface of the spindle nut, which constitutes a wall of the lubricant reservoir, does not interact mechanically with other components of the planetary gear set and can be designed as a single-start or multi-start thread.

[0013] The thread-like structure formed by the spindle nut, which constitutes a wall of the lubricant reservoir, can have a uniform diameter, making it conceivable to imagine a screw being driven into this structure. However, there are also embodiments in which the diameter of the thread, defined by the inner circumferential surface of the spindle nut, varies in the axial direction of the spindle nut. For example, a longitudinal section of the spindle nut may show a continuous increase in the thread diameter in one axial direction. Similarly, an increase in the thread diameter may be observed in one section of the spindle nut, while a decrease in another section may be observed. In this case, apart from the exact shape of the threads, a mirror symmetry with respect to a mirror plane placed centrally between the two end faces of the spindle nut may be present.This mirror symmetry is expressed, for example, in the fact that the spindle nut – viewed in longitudinal section – has a central concave recess which acts as a lubricant reservoir. The concave shape of the recess means that the wall thickness of the spindle nut, i.e., the difference between its outer and inner radii, increases towards the two end faces of the spindle nut. This results in particularly high stability of the spindle nut in those areas where rolling contact exists between the spindle nut and the planets of the planetary gear system.

[0014] The spindle nut of the planetary roller gear can be constructed in one or more parts. In particular, in the case of a multi-part spindle nut, it is possible to adjust the preload.

[0015] One possible further development of the planetary roller gear involves inserting a comb into the base body of the spindle nut, projecting into the lubricant reservoir. The comb interrupts the otherwise circular cross-sectional contour of the lubricant reservoir, ensuring particularly efficient mixing of the lubricant.

[0016] The drive element of the planetary roller gear can be, for example, the threaded spindle or the spindle nut, while the other of the aforementioned elements functions as a rotationally secured, sliding output element.

[0017] If neither the threaded spindle nor the spindle nut serves as the drive element, the corresponding function is performed by a cage guiding the planets, i.e., a planet carrier. In this case, the planetary gear set is designed as a pitch-preserving planetary gear set (SPWG). A less extreme gear ratio is accepted here compared to a planetary gear set with a driven nut or driven spindle.

[0018] If IR denotes the inner radius of the spindle nut, AR the outer radius of the spindle nut, and mR the maximum radius of the lubricant reservoir, the following relationship applies in numerous embodiments: 0.9≤(IR×AR) / mR2≤1.1

[0019] In other words: The dimensionless fraction (IR x AR) / mR 2 deviates from the value 1.0 by no more than 10% above or below.

[0020] All radii IR, AR, mR are to be measured in a cross-sectional plane to which the central axis of the threaded spindle forms a surface normal. If one of the radii IR, AR, mR is to be measured on a surface that is threaded, the flank radius of the thread in question is to be used in the aforementioned quotient.

[0021] The planetary gear is particularly suitable for use in a rear-axle steering actuator or a steer-by-wire steering actuator for steering the front wheels of a motor vehicle. Alternatively, the planetary gear is suitable, for example, as a rotary-linear gearbox in a linear actuator of a stationary industrial machine or system.

[0022] Several embodiments of the invention are explained in more detail below with reference to a drawing. This drawing shows: Fig. 1 in a sectional view a first embodiment of a planetary roller gear which has a lubricant reservoir with a non-circular cross-section, Fig. 2 compared to the embodiment according to Fig. 1. Modified cross-sectional design of a lubricant reservoir in a planetary roller gear, Fig. 3 a section BB through the planetary roller gear according to Fig. 1, Fig. 4 a section AA through the planetary roller gear according to Fig. 1, Fig. 5 a planetary roller gear with a lubricant reservoir which has a thread-like structured wall, in a sectional view analogous Fig. 4, Fig. 6 a planetary roller gear with a lubricant reservoir into which a comb protrudes as an insert, in a sectional view analogous Fig. 4.

[0023] Unless otherwise stated, the following explanations apply to all embodiments. Corresponding or essentially equivalent parts are marked with the same reference numerals in all figures.

[0024] A planetary gear set, designated by reference numeral 1, is intended for use in a steering actuator for the rear axle steering system of a motor vehicle. Regarding the basic design and function of such a steering actuator, reference is made to the prior art cited above.

[0025] The planetary gear set 1 comprises a threaded spindle 2, the threads of which mesh with profiles of several planets 4. These profiles are formed in a central section 5 of each planet 4. The central section 5 transitions into two relatively thin side sections 6, which are also groove-shaped, i.e., without a pitch. Following the two side sections 6, each planet 4 terminates in a smooth cylindrical end section 7. The end sections 7 are mounted in a planet carrier 3, i.e., a cage.

[0026] During operation of the planetary gear 1, the planets 4 contact the threaded spindle 2 exclusively with their central sections 5. The profiles of the side sections 6, on the other hand, mesh with the similarly flat profiles of a spindle nut 9, which is also referred to simply as the nut. The spindle nut 9 can be designed in a single or multi-part manner, in a generally known way, whereby in the case of a multi-part design the nut parts can be preloaded against each other.

[0027] The spindle nut 9 is mounted opposite the planet carrier 3 by means of axial bearings 10, namely axial roller bearings, such that axial forces can be transmitted between the spindle nut 9 and the planet carrier 3. However, the transmission of torque via the spindle nut 9 is not intended in this case. Instead, a cage sleeve 12, which is part of the planet carrier 3, serves this purpose. The cage sleeve 12 is rigidly connected to a pulley 13, which can be used to drive the planet carrier 3 by means of a belt. Thanks to the driven planet carrier 3, the planetary gear set 1 is designed as a pitch-preserving screw drive (SPWG).

[0028] Cage side parts 11 are provided for mounting the planets 4 in the planet carrier 3, in which the end sections 7 of the planets 4 are guided. Furthermore, cage discs 8 are included in the planet carrier 3, which are rigidly connected to both the cage side parts 11 and the cage sleeve 12.

[0029] In all embodiments, a lubricant reservoir 14 is formed by the spindle nut 9. A section cut through the planetary gear set 1, where the central axis of the spindle nut 9, and thus of the entire planetary gear set 1, forms a surface normal to the section plane, shows a cross-sectional contour Q14 of the lubricant reservoir 14. The cross-sectional contour Q14 deviates from a circular shape in all cases. The maximum radius of the lubricant reservoir 14, that is, the maximum distance of a point on the surface of the lubricant reservoir 14 formed by the spindle nut 9 from the central axis of the spindle nut 9, is denoted by mR. AR denotes the outer radius, and IR the inner radius of the spindle nut 9.

[0030] The dimensionless fraction, which is therefore independent of the chosen units. (IR×AR) / mR2 lies in all embodiments according to the Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 to Fig. 6 in the range of 0.9 to 1.1.

[0031] In the case of the Fig. 1, Fig. 3, Fig. Figure 4 describes the cross-sectional contour Q14 as an ellipse deviating from a circular shape. Viewed in the axial direction, i.e., the longitudinal direction of the threaded spindle 2, the lubricant reservoir 14 extends slightly beyond the central sections 5 of the planets 4 on both sides. Each planet 4 rolls on the profiled inner surface of the spindle nut 9 in such a way that it alternately lubricates a radially more extended area of ​​the lubricant reservoir 14, as shown in Figure 4. Fig. 4 shown, and narrow in the radial direction, from Fig. 3 emerging area of ​​the lubricant reservoir 14 is covered.

[0032] A thickness of the lubricant reservoir 14 that varies in a wave-like manner in the circumferential direction of the spindle nut 9 - to be measured in the radial direction of the planetary roller gear 1 - is also present in the variant according to Fig. 2 given. In this, for installation in the planetary roller gear 1 according to Fig. In one suitable variant of the spindle nut 9, the cross-sectional contour Q14 has the form of a constant thickness. A constant thickness is generally characterized by the fact that two tangents parallel to each other, which are applied to the constant thickness, always have the same distance between them.

[0033] In the exemplary embodiment according to the Fig. 5 and Fig. Figure 6 describes the lubricant reservoir 14 as having a lens shape in the longitudinal section shown. The maximum radius mR is given in the section plane that lies midway between the two end faces of the spindle nut 9. At the transition to the groove-shaped profiles of the spindle nut 9, the height of the lubricant reservoir 14, measured in the radial direction of the spindle nut 9, is zero.

[0034] In the Fig. In the design shown in Figure 5, the curved surface of the lubricant reservoir 14, formed by the spindle nut 9, is structured in the manner of a thread 15. Due to the curved shape of the thread 15, it would not be possible to screw an object with an external thread into the thread 15. Therefore, embodiments are also conceivable in which the thread 15 has a non-uniform pitch in the longitudinal direction of the spindle nut 9.

[0035] For example, the pitch of the thread 15 can be greatest in the middle, thickest part of the spindle nut 9, that is, in the area where the maximum radius mR of the lubricant reservoir 14 is measured, and decrease towards the two end faces of the spindle nut 9. Likewise, the thread depth of the thread 15 can decrease from a central plane located midway between the two end faces of the spindle nut 9 towards the end faces of the spindle nut 9. In any case, the thread 15 contributes to the delivery of the lubricant, in particular grease, within the planetary gear set 1.

[0036] In the exemplary embodiment according to Fig.6 The spindle nut 9 is composed of several parts, namely a base body that contacts the planets 4 and a comb 16. The comb 16, generally referred to as the insert, projects into the lubricant reservoir 14 and thus ensures mixing and distribution of the lubricant, which lubricates the rolling contacts between the planets 4, the spindle nut 9, and the threaded spindle 2. In all embodiments, the lubrication of the planetary gear set 1 can be designed as lifetime lubrication. Reference symbol list 1 planetary roller gear 2 threaded spindles 3 planet carriers, cage 4 Planet 5 Middle section Section 6 7 Final Section 8 cage disc 9 Spindle nut 10 axial bearings 11 Cage side panel 12 Cage sleeve 13 Pulley 14 Lubricant reservoir 15 threads 16 Comb, insert piece AR Outer radius of the spindle nut IR inner radius of the spindle nut mR maximum radius of the lubricant reservoir Q14 Cross-sectional contour of the lubricant reservoir

Claims

[1] Planetary roller gear (1), comprising a threaded spindle (2), several planets (4) contacting the threaded spindle (2), and a spindle nut (9) in which the planets (4) roll in contact with a lubricant, characterized by , that between the planets (4) and an inner circumferential surface of the spindle nut (9) a lubricant reservoir (14) with a non-circular cross-sectional contour (Q14) is formed which describes an ellipse or a uniform thickness. [2] Planetary roller gear (1) according to claim 1, characterized by , that the inner circumferential surface of the spindle nut (9) limiting the lubricant reservoir (14) is designed as a thread (15). [3] Planetary roller gear (1) according to claim 1 or 2 characterized by , that a comb (16) projecting into the lubricant reservoir (14) is inserted into a base body of the spindle nut (9). [4] Planetary roller gear (1) according to any one of claims 1 to 3, characterized by, that the spindle nut (9) is intended as a drive element. [5] Planetary roller gear (1) according to any one of claims 1 to 3, characterized by , that a cage (3) guiding the planets (4) is provided as a propulsion element. [6] Planetary roller gear (1) according to claim 5, characterized by , that the quotient (IR x AR) / mR 2 deviates from the value 1.0 by no more than 10%, whereby IR = inner radius of the spindle nut (9), AR = Outer radius of the spindle nut (9), mR = maximum radius of the lubricant reservoir (14). [7] Steering actuator comprising a planetary roller gear (1) according to claim 1.

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