Planetary roller gear and steering actuator

Abstract

A planetary roller gear (1), particularly in a steering actuator (10), comprises a threaded spindle (2), a number of planets (4) rolling on the threaded spindle (2), a cage (17) guiding the planets (4), a nut (11) contacting the planets (4), and a rolling bearing (13) of the nut (11), wherein the cage (17) serves as the drive element. The rolling elements (14) of said rolling bearing (13) are arranged in the cage (17) guiding the planets (4).

Description

[0001] The invention relates to a planetary roller gear, that is to say a planetary roller screw drive, according to the preamble of claim 1. Furthermore, the invention relates to a steering actuator, in particular for a rear axle steering system of a motor vehicle, with a planetary roller gear.

[0002] A planetary roller gear of this type is known, for example, from DE 10 2021 121 736 B4, which relates to a set of components for assembling pitch-accurate planetary roller gears. A cage designed to guide the planets of the gear is provided as the drive element of the planetary roller gear. A spindle nut of the planetary roller gear according to DE 10 2021 121 736 B4 is mounted in the cage by means of inner rolling bearings, which are axial roller bearings. Additional outer rolling bearings, designed as angular contact roller bearings, are provided for supporting the cage in an actuator housing.

[0003] DE 10 2019 112 480 B3 discloses a chassis actuator and a method for operating a gear arrangement of a chassis actuator. A spindle nut as the input-side element of a rotary-linear gear of the arrangement according to DE 10 2019 112 480 B3 is mounted in a housing by means of an axial bearing. The rotary-linear gear is self-locking by means of an adjustable preload device that applies a force to the axial bearing. The rotary-linear gear is a planetary roller gear.

[0004] Another electromechanical chassis actuator, namely a steering actuator, with a planetary roller gear is known, for example, from DE 10 2019 127 328 A1. In this case, the mechanical efficiency of the planetary roller gear is between 40 and 55%, in particular between 45 and 50%.

[0005] A linear actuator described in DE 10 2018 115 787 A1, intended for use in a rear-axle steering system of a motor vehicle, comprises a screw drive designed as a planetary roller screw drive, i.e., a planetary roller gear, and having a plurality of planets arranged with preload between a threaded spindle and a nut. A friction device interacts with the nut, generating a friction torque dependent on an axial load acting on the threaded spindle. In this way, the efficiency of the linear actuator is kept well below 50%, i.e., in the self-locking range, under a wide variety of operating conditions.

[0006] Another screw drive intended for use in a linear actuator, whose efficiency lies in the self-locking range, is disclosed in WO 2020 / 001686 A1. In this case, the rolling elements supporting a nut of the screw drive are designed as friction rolling elements.

[0007] The invention is based on the object of specifying a planetary roller gear which is further developed compared to the cited prior art, has a simple design and is at the same time characterized by high operational reliability, which is particularly suitable for use in a rear axle steering system of a motor vehicle.

[0008] 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 10.

[0009] The planetary roller gear drive, in a well-known basic design, comprises a threaded spindle, a number of planets rolling on the threaded spindle, a cage guiding the planets, and a nut in which the planets roll. The nut is rotatably mounted by means of a roller bearing. The cage serves as the drive element of the planetary roller gear drive. The planetary roller gear drive is thus designed as a pitch-stable planetary roller screw drive (SPWG).

[0010] According to claim 1, the rolling elements of the rolling bearing assembly intended to support the nut are housed in the cage guiding the planets. This means that a single cage is used to guide the planets and the rolling elements. This multifunctional cage can generally be constructed in one or more parts. Likewise, the cage, which also functions as the planet carrier, can be made of the same or different materials. Metallic and non-metallic materials, particularly plastics, can be used for the cage. In all cases, a preload can be applied between the nut, the planets, and the threaded spindle.

[0011] The invention is based on the idea that in a pitch-accurate planetary roller gear, the nut is a rotatable component that functions neither as a drive nor as an output element. Rather, the motion of the nut is of secondary importance for the function of the planetary roller gear, particularly for its transmission ratio. A bearing, such as a rolling or plain bearing, of the nut primarily serves the purpose of transmitting forces, particularly axial forces. If the nut of the planetary roller gear is mounted on a rolling bearing, it is advisable to use a conventional rolling bearing with a cage that guides the rolling elements, particularly balls.

[0012] The solution according to the application deliberately deviates from this approach by using the same cage that guides the planets of the planetary roller gear as the cage that guides the rolling elements of the nut's roller bearing, in particular, keeping them spaced apart from each other in the circumferential direction of the threaded spindle. The cage is also commonly referred to as the planet carrier. Kinematically, the simultaneous use of the planet carrier as a cage for the nut's roller bearing is not ideal. This imperfection is accepted. Depending on the overall design and load condition of the planetary roller gear, this can result in an efficiency of less than 50%, i.e., a self-locking design of the planetary roller gear.

[0013] In applications where self-locking is required, additional components that generate a braking torque and deliberately reduce the efficiency of the planetary roller gear can be dispensed with. In other applications where self-locking properties are not required, a low-friction design of the planetary roller gear is possible. Compared to simpler screw drives, such as simple motion threads, the planetary roller gear according to the application is characterized by a minimal breakaway torque, regardless of whether it is self-locking.

[0014] The rolling elements of the rolling bearing assembly used to support the nut can be in the form of rollers, particularly cylindrical or tapered rollers, needles, or balls. For example, the rolling bearing assembly is designed as an angular contact roller bearing or an angular contact ball bearing. In all cases, a rolling element raceway of the rolling bearing assembly can be formed directly by the nut. Alternatively, separate bearing rings can be connected to the nut.

[0015] According to one possible design of the multifunctional cage, i.e., planetary carrier, it comprises a disc section concentrically surrounding the threaded spindle, in which recesses are formed for guiding the planets, as well as a conical section located radially outside the disc section, in which the rolling elements are guided. At the transition between the disc section and the conical section, the cage can form a cylindrical section surrounded by an inner circumferential surface of an outer ring of the rolling bearing. The inner circumferential surface of the outer ring can thus represent a guide surface for the cage.

[0016] The outer ring can have a stepped end face arranged radially - with respect to the central axis of the threaded spindle - outside the rolling elements, which is opposite a likewise stepped end face of the cage, wherein cage pockets for receiving the rolling elements, in particular balls, border an inner annular disk-shaped partial surface of the stepped end face of the cage.

[0017] Regardless of the exact form of the multifunctional cage, i.e., a planetary carrier with an additional function as part of a rolling bearing arrangement, this rolling bearing arrangement can be designed as a preloaded bearing arrangement. To adjust the preload, a multi-part structure of at least one component of the planetary roller gear can be provided, with individual parts of the respective component being adjustable relative to each other. For example, such an adjustment option can be provided for the cage if it is composed of several individual parts.

[0018] The cage can be completely or approximately mirror-symmetrical to a plane located centrally between the two faces of the nut. The number of rolling elements guided in the cage on one face of the planets can correspond to the number of planets in the planetary roller gear.

[0019] Regarding the drive of the multifunctional cage, numerous design options exist. For example, a gearless electric drive, i.e., a direct drive, can be provided for the cage. Alternatively, the cage can be driven via a planetary gear system designed as a reduction gear, for example. Designs in which the cage is firmly connected to an output element of a belt-driven transmission, i.e., a chain or belt drive, are also possible.

[0020] The planetary roller gear is not only suitable for use in an electromechanical actuator for a rear-axle steering system, but can also be used, for example, in a steer-by-wire steering system designed to steer the front wheels of a vehicle. The planetary roller gear can also be used in stationary industrial systems.

[0021] An exemplary embodiment of the invention and a comparative example are explained in more detail below with reference to a drawing. Shown are: Fig. 1 a planetary roller gear intended for use in a steering actuator in a sectional view, Fig. 2 an outer ring of a rolling bearing of the planetary roller gear in perspective view, Fig. 3 the outer ring in a sectional view, Fig. 4 a cage intended for guiding rolling elements of the said rolling bearing and planets of the planetary rolling gear in a perspective view, Fig. 5 the cage after Fig. 4 in a sectional view, Fig. 6 a non-claimed comparative example in a representation analogous Fig. 1.

[0022] The components of the unclaimed device according to Fig. 6 are, as far as comparability with the Fig. 1 to 5, is marked with the same reference numerals as the components of the planetary roller gear according to the Fig. 1 to 5.

[0023] A planetary roller gear, designated overall by reference numeral 1, is used as a rotary-linear gear of a steering actuator 10 of a rear-axle steering system in a motor vehicle. Regarding the basic design of the planetary roller gear 1 and the entire rear-axle steering system, reference is made to the prior art cited above.

[0024] The planetary roller gear 1 includes a threaded spindle 2, the thread of which is designated 3. The ends of the threaded spindle 2, not visible in the figures, are at least indirectly connected to chassis components that enable a change in the steering angle of the vehicle's rear wheels. As an alternative to a solid design of the threaded spindle 2, a hollow design of the threaded spindle 2, at least along sections of its entire length, is also possible. The thread 3 can be designed as a single-start or multi-start thread.

[0025] Several planets 4 roll on the thread 3, their longitudinal axes being parallel to the central axis of the threaded spindle 2. Each planet 4 has a central section 5 and adjoining, comparatively thin side sections 6. The central section 5 and the side sections 6 are provided with pitchless profiles 8, 9. Only the central section 5 of each planet 4 is in contact with the threaded spindle 2. The ends of the side sections 6 are adjoined by short cylindrical end sections 7, which are guided in a planet carrier designed as a cage 17, which will be discussed in more detail below.

[0026] The profiles 9 of the side sections 6 mesh with profiles 12, which are also pitchless, of a nut 11 of the planetary roller gear 1. The nut 11 is rotatably mounted by means of a rolling bearing 13, with the associated outer rings designated 21. Overall, the rolling bearing 13 is constructed as a double-row angular contact ball bearing, whose rolling elements, i.e., balls, are designated 14 and whose rows of rolling elements are designated 15, 16.

[0027] The rolling elements 14 are guided in the same planet carrier, designed as a cage 17, which also serves to keep the planets 4 at the specified distance from each other. The planet carrier, designed as a cage 17, is thus identical to the cage of the rolling bearing 13. The cage 17 comprises cage parts 18, 19, the shape of which can be seen in detail from the Fig. 4 and Fig. 5. A belt pulley 20 is firmly connected to the cage parts 18, 19, which represents the output-side element of a belt drive connected upstream of the planetary roller gear 1, namely a belt drive.

[0028] Regarding the shape of the outer rings 21, reference is made to the Fig. 1 and the Fig. 2 and Fig. 3. An inner circumferential surface of the outer ring 21 is designated by 29. In the radially outer region of the outer ring 21, a stepped end face 22 is formed on its inner end face, i.e., the end face facing the nut 11. The stepped end face 22 is composed of an outer partial surface 23 and an inner partial surface 24, with the inner partial surface 24 projecting further inward in the axial direction, i.e., toward the pulley 20, than the outer partial surface 23.

[0029] Compared to the outer rings 21, the cage parts 18, 19, as can be seen from the Fig. 4 and Fig. 5, has a much more complex shape. In a disc section 25, which is assigned to the cage 17 and which directly surrounds the threaded spindle 2, recesses 26 are formed, which are intended to guide the end sections 7 of the planets 4. The recesses 26 are, as can be seen in particular from Fig. 4, open radially inwards.

[0030] A conical section 27 of the cage part 18, 19 adjoins the disc section 25 radially outward. Cage pockets designated 30, in each of which a ball 14 is guided, are located in the conical section 27. At the transition between the disc section 25 and the conical section 27 is a cylindrical section 28 directed axially outward, i.e., facing away from the nut 11. The outer peripheral surface of the cylindrical section 28 lies opposite the inner peripheral surface 29 of the outer ring 21. The inner peripheral surface 29 thus provides a contact surface for the cage 17.

[0031] The conical section 27 merges at its outer edge into a sleeve section 31, to which the pulley 20 is fastened with a form fit. On the outer end face of the cage part 18, 19 opposite the sleeve section 31, an end face 32 of the cage 17 is formed, which, corresponding to the stepped end face 22 of the outer ring 21, describes a stepped shape with an outer partial surface 33 and an inner partial surface 34. The cage pockets 30 extend, as can be seen in particular from Fig. 5, up to the inner partial surface 34.

[0032] The Fig. 6 shows a compared to the design according to Fig. 1 considerably more complex, not claimed comparative example of a planetary roller gear 1. In the case of Fig. 6, the nut 11 is mounted in the cage 17 by means of two inner axial bearings 35, namely axial roller bearings. The cage 17, in turn, is mounted in a surrounding structure, in particular a housing, by means of two outer axial bearings 36. In addition, two radial bearings 37, which are also roller bearings, are provided for supporting the cage 17. Each of the total of six bearings 35, 36, 37 of the arrangement according to Fig. 6 has a separate cage. The function of all six bearings 35, 36, 37 is described in the exemplary embodiment according to the Fig. 1 to 5 is taken over by the rolling bearing 13 designed as a double-row angular contact ball bearing, whereby the function of a bearing cage is provided by the planet carrier intended to guide the planets 4. List of reference symbols 1 planetary roller gear 2 threaded spindles 3 threads 4 Planet 5 Middle section 6 page section 7 Final section 8 Profiling of the middle section 9 Profiling of the side section 10 Steering actuator 11 Mother 12 Profiling the mother 13 Rolling bearings 14 rolling elements, ball 15 rolling element rows 16 rolling element rows 17 Cage 18 Cage part 19 Cage part 20 pulley 21 Outer ring 22 stepped end face of the outer ring 23 outer part of the stepped end face 22 24 inner part of the stepped end face 22 25 disc section 26 Recess in the disc section for guiding a planet 27 conical section 28 cylindrical section 29 Inner peripheral surface of the outer ring 30 cage bag 31 Sleeve section 32 stepped end face of the cage 33 outer part of the stepped end face 32 34 inner part of the stepped end face 32 35 inner thrust bearing 36 outer thrust bearing 37 radial bearings

Claims

[1] Planetary roller gear (1), with a threaded spindle (2), a number of planets (4) rolling on the threaded spindle (2), a cage (17) guiding the planets (4), as well as a nut (11) contacting the planets (4) and a rolling bearing (13) of the nut (11), wherein the cage (17) is provided as a drive element, characterized by that the rolling elements (14) of said rolling bearing (13) are placed in the cage (17) guiding the planets (4). [2] Planetary roller gear (1) according to claim 1, characterized by that the rolling bearing (13) is designed as a ball bearing. [3] Planetary roller gear (1) according to claim 2, characterized by that the rolling bearing (13) is designed as an angular contact ball bearing. [4] Planetary roller gear (1) according to one of claims 1 to 3, characterized by that a rolling element raceway of the rolling bearing (13) is formed directly by the nut (11). [5] Planetary roller gear (1) according to claim 3 or 4, characterized byin that the cage (17) comprises a disc section (25) surrounding the threaded spindle (2), in which recesses (26) are formed for guiding the planets (4), and a conical section (27) located radially outside the disc section (25), in which the rolling elements (14) are guided, wherein at the transition between the disc section (25) and the conical section (27) a cylindrical section (28) is formed by the cage (17), which is surrounded by an inner circumferential surface (29) of an outer ring (21) of the rolling bearing (13). [6] Planetary roller gear (1) according to claim 5, characterized bythat the outer ring (21) has a stepped end face (22) arranged radially - with respect to the central axis of the threaded spindle (2) - outside the rolling elements (14), which is opposite a likewise stepped end face (32) of the cage (17), wherein cage pockets (30) for receiving the rolling elements (14) border on an inner annular disk-shaped partial surface (34) of the stepped end face (32) of the cage (17). [7] Planetary roller gear (1) according to one of claims 1 to 6, characterized by that the rolling bearing (13), the cage (17) of which is provided for guiding the rolling elements (14) and is also designed as a planet carrier which guides the planets (4), is designed as a preloaded bearing. [8] Planetary roller gear (1) according to one of claims 1 to 7, characterized by that the number of rolling elements (14) guided on one end face of the planets (4) in the cage (17) corresponds to the number of planets (4). [9] Planetary roller gear (1) according to one of claims 1 to 8, characterized by that the cage (17) is firmly connected to an output element (pulley 20) of a belt transmission. [10] Steering actuator (10) comprising a planetary roller gear (1) according to claim 1.

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