Planetary roller gear and steering actuator

Abstract

A planetary roller gear (1) suitable, among other things, for use in a steering actuator of a rear-axle steering system comprises a threaded spindle (2), a plurality of planets (4) contacting the threaded spindle (2), and a spindle nut (9) in which the planets (4) roll in contact with a lubricant. A lubricant reservoir (14) with a non-circular cross-sectional contour (Q14) is formed between the planets (4) and an inner circumferential surface of the spindle nut (9).

Description

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

[0002] A planetary roller gear of this type is known, for example, from WO 2013 / 004469 A1. The known planetary roller gear is designed for use in a submarine or submersible and is operated, in particular, by means of a brushless electric motor. A lubricant supply system of the device according to WO 2013 / 004469 A1 ensures that the orbiting space of the planets of the gear is always sufficiently filled with lubricant.

[0003] An electromechanical steering actuator that uses 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 lies between 48% and 49.5%, which is in the self-locking range.

[0004] DE 10 2021 210 368 A1 relates to a steering actuator. In this case, a rotation is converted into a linear displacement of a spindle by means of a moving thread. The device according to DE 10 2021 210 368 A1 has cavities that form a reservoir for lubricant to lubricate the moving thread.

[0005] The invention is based on the object of specifying a planetary roller gear which has been further developed compared to the cited prior art, in particular with regard to lubrication aspects, as well as a suitable use of the planetary roller gear.

[0006] This object is achieved according to the invention by a planetary roller gear having the features of claim 1. The planetary roller gear is particularly suitable for use in a steering actuator according to claim 9.

[0007] The planetary roller gear comprises, in a known basic concept, a threaded spindle, several planets contacting the threaded spindle, and a spindle nut in which the planets roll in contact with a lubricant.

[0008] 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 roller gear.

[0009] It has been shown that the non-circular cross-sectional contour allows static requirements to be met while simultaneously providing a generously dimensioned volume for accommodating lubricant, especially grease. Furthermore, the non-circular cross-sectional contour offers good conditions for conveying the lubricant within the planetary roller gear. The cross-section in question lies in a plane to which the center axis of the threaded spindle is a surface normal.

[0010] The cross-sectional contour of the lubricant reservoir formed by the inner circumferential surface of the spindle nut describes, for example, a non-circular ellipse. Alternatively, the cross-sectional contour can be designed as a constant thickness, for example.

[0011] Likewise, embodiments are possible in which the inner circumferential surface of the spindle nut, which defines the lubricant reservoir, is designed as a thread. In these cases, too, the intersection line between a plane normal to the center axis of the planetary roller gear and the inner circumferential surface of the spindle nut has a shape other than circular. The thread formed on the inner circumferential surface of the spindle nut, which forms a wall of the lubricant reservoir, does not interact mechanically with other components of the planetary roller gear and can be designed as a single-start or multi-start thread.

[0012] The thread-like structure formed by the spindle nut and representing a wall of the lubricant reservoir can have a uniform diameter, so that the fictitious screwing of a screw into this structure would be conceivable. Deviating from this, there are also embodiments in which the diameter of the thread, which is described by the inner circumferential surface of the spindle nut, varies in the axial direction of the spindle nut. In a longitudinal section of the spindle nut, for example, there can be a continuous increase in the thread diameter in an axial direction. Likewise, an increase in the thread diameter can be observed in one section of the spindle nut and a decrease in another section of the spindle nut. In this case - apart from the exact shape of the thread turns - a mirror symmetry to a mirror plane located centrally between the two end faces of the spindle nut can 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 that functions 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 radius, increases toward the two end faces of the spindle nut. This ensures particularly high stability of the spindle nut in those areas where rolling contact occurs between the spindle nut and the planets of the planetary roller gear.

[0013] The spindle nut of the planetary roller gear can generally be constructed in one or more parts. Especially in the case of a multi-part spindle nut, it is possible to adjust the preload.

[0014] A possible refinement of the planetary roller gear involves inserting a ridge into the base body of the spindle nut, which extends into the lubricant reservoir. The ridge breaks through the otherwise circular cross-sectional contour of the lubricant reservoir and ensures particularly efficient mixing of the lubricant.

[0015] The drive element of the planetary roller gear can be, for example, the threaded spindle or the spindle nut, while the other of the mentioned elements functions as a rotationally secured, movable output element.

[0016] If neither the lead screw nor the lead screw nut represents the drive element, this function is performed by a cage that guides the planets, i.e., the planet carrier. In this case, the planetary roller gear is designed as a pitch-stable planetary roller gear (SPWG). A less extreme gear ratio is accepted compared to a planetary roller gear with a driven nut or driven lead screw.

[0017] If IR is 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 designs: 0.9≤(IR×AR) / mR2≤1.1

[0018] In other words: The dimensionless fraction (IR × AR) / mR 2 deviates from the value 1.0 by no more than 10% up or down.

[0019] All radii IR, AR, and mR must be measured in a section plane to which the center axis of the threaded spindle is a surface normal. If one of the radii IR, AR, and mR is to be measured on a surface that is a thread, the flank radius of the thread in question must be inserted into the specified quotient.

[0020] 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. Otherwise, the planetary gear is suitable, for example, as a rotary-linear gear for a linear actuator in a stationary industrial machine or system.

[0021] Several embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings: Fig. 1 shows a sectional view of a first embodiment of a planetary roller gear having a lubricant reservoir with a non-circular cross-section, Fig. 2 a 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 to 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.

[0022] 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.

[0023] A planetary roller gear, designated overall by reference numeral 1, is intended for use in a steering actuator of a 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.

[0024] The planetary roller gear 1 comprises a threaded spindle 2, whose thread meshes with the 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 comparatively thin side sections 6, which are also grooved, i.e., have no pitch. Adjacent to the two side sections 6, each planet 4 tapers off in the form of a smooth, cylindrical end section 7. The end sections 7 are mounted in a planet carrier 3, i.e., a cage.

[0025] During operation of the planetary roller gear 1, the planets 4 contact the threaded spindle 2 exclusively with their center sections 5. The profiles of the side sections 6, however, mesh with the profiles of a spindle nut 9, which also have a pitchless design, also referred to as a nut. The nut 9 can be constructed in a single-part or multi-part manner, in a manner known in principle. In the case of a multi-part design, the nut components can be preloaded against each other.

[0026] The nut 9 is mounted relative to the planet carrier 3 by means of axial bearings 10, namely axial roller bearings, such that axial forces can be transmitted between the nut 9 and the planet carrier 3. However, a transmission of torque via the nut 9 is not intended in the present case. Instead, a cage sleeve 12, which is assigned to the planet carrier 3, exists for 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 roller gear 1 is designed as a pitch-stable screw drive (SPWG).

[0027] For supporting the planets 4 in the planet carrier 3, cage side parts 11 are provided, in which the end sections 7 of the planets 4 are guided. Furthermore, cage discs 8 are included in the cage 3, which are rigidly connected to both the cage side parts 11 and the cage sleeve 12.

[0028] In all embodiments, a lubricant reservoir 14 is formed by the spindle nut 9. A section through the planetary roller gear 1, with the center axis of the spindle nut 9 and thus of the entire planetary roller gear 1 representing 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, i.e., the maximum distance of a point on the surface of the lubricant reservoir 14 formed by the spindle nut 9 from the center axis of the spindle nut 9, is designated mR. AR denotes the outer radius, IR the inner radius of the spindle nut 9.

[0029] The dimensionless fraction, which is independent of the selected units (IR×AR) / mR2 is in all embodiments according to the Fig. 1 to 6 in the range of 0.9 to 1.1.

[0030] In the case of Fig. 1, Fig. 3, Fig. 4, the cross-sectional contour Q14 describes an ellipse deviating from a circle. Viewed in the axial direction, i.e., the longitudinal direction of the threaded spindle 2, the lubricant reservoir 14 extends slightly beyond the center 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 covers a radially extended area of ​​the lubricant reservoir 14, as shown in Fig. 4, and a radially narrow, Fig. 3 emerging area of ​​the lubricant reservoir 14.

[0031] A thickness of the lubricant reservoir 14 which 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 possible in the variant according to Fig. 2. In this, for installation in the planetary roller gear 1 according to Fig. In a suitable variant of the spindle nut 9, the cross-sectional contour Q14 has the shape of a constant thickness. A constant thickness is generally characterized by the fact that two parallel tangents applied to the constant thickness are always at the same distance from each other.

[0032] In the example according to the Fig. 5 and Fig. 6, the lubricant reservoir 14 has a lens shape in the longitudinal section shown. The maximum radius mR is defined in the sectional plane located centrally between the two end faces of the spindle nut 9. At the transition to the grooved 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.

[0033] 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. Screwing an object provided with an external thread into the thread 15 would not be possible due to the curved shape of 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. For example, the pitch of the thread 15 can be greatest in the central, thickest area of ​​the spindle nut 9, i.e. in the area in which the maximum radius mR of the lubricant reservoir 14 is to be 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 center plane located centrally 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 conveyance of the lubricant, in particular lubricating grease, within the planetary roller gear 1.

[0034] In the embodiment according to Fig. 6, the spindle nut 9 is constructed in several parts, namely a base body contacting the planets 4 and a comb 16. The comb 16, generally referred to as an insert piece, extends 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 roller gear 1 can be designed as a lifetime lubrication. List of reference symbols 1 planetary roller gear 2 threaded spindles 3 planet carrier, cage 4 Planet 5 Middle section 6 page section 7 Final section 8 cage disc 9 spindle nut 10 thrust bearings 11 Cage side panel 12 cage sleeve 13 Pulley 14 Lubricant reservoir 15 threads 16 Comb, insert 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 QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2013 / 004469 A1

[0002] DE 10 2019 127 328 A1

[0003] DE 10 2021 210 368 A1

[0004]

Claims

[1] Planetary roller gear (1), with 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 a lubricant reservoir (14) with a non-circular cross-sectional contour (Q14) is formed between the planets (4) and an inner circumferential surface of the spindle nut (9). [2] Planetary roller gear (1) according to claim 1, characterized by that the cross-sectional contour (Q14) describes an ellipse. [3] Planetary roller gear (1) according to claim 1, characterized by that the cross-sectional contour (Q14) describes a constant thickness. [4] Planetary roller gear (1) according to claim 1, characterized by that the inner circumferential surface of the spindle nut (9) delimiting the lubricant reservoir (14) is designed as a thread (15). [5] Planetary roller gear (1) according to one of claims 1 to 3, characterized bythat a comb (16) projecting into the lubricant reservoir (14) is inserted into a base body of the spindle nut (9). [6] Planetary roller gear (1) according to one of claims 1 to 5, characterized by that the spindle nut (9) is provided as a drive element. [7] Planetary roller gear (1) according to one of claims 1 to 5, characterized by that a cage (3) guiding the planets (4) is provided as a drive element. [8] Planetary roller gear (1) according to claim 7, characterized by that the quotient (IR × 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). [9] Steering actuator comprising a planetary roller gear (1) according to claim 1.

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