Spindle drive for an actuator of a steer-by-wire steering system, actuator and steer-by-wire steering system

Abstract

Spindle drive (21, 121) for an actuator (20) of a steer-by-wire steering system, comprising a spindle (22, 122) with a spindle thread (122a) and a spindle nut (23, 123) with a thread (123a), wherein the spindle thread (122a) and the thread (123a) are designed as a motion thread and wherein at least one sealing element (27, 28, 128) is arranged between the spindle nut (123) and the spindle thread (122a) in an annular groove (29, 30, 130) in the region of at least one of the end faces (25, 26, 126) of the spindle nut (23, 123), characterized in that the sealing element (128) is floatingly mounted, wherein the sealing element (128) can displace to a limited extent both axially and following the spindle thread.

Description

[0001] The invention relates to a spindle drive for an actuator of a steer-by-wire steering system, an actuator and a steer-by-wire steering system according to the independent claims.

[0002] Spindle drives for steering actuators, particularly steer-by-wire steering systems with spindle drives, are known. The spindle drive comprises a spindle with a spindle thread and a nut with a nut thread that engages with the spindle thread. The spindle nut is rotatable and mounted in an actuator housing, while the spindle is axially displaceable and acts as the actuator. The spindle and nut threads are designed as motion threads, e.g., trapezoidal threads, and are lubricated by a lubricant that can be deposited within the spindle nut. This results in reduced friction and a long, maintenance-free service life in a motor vehicle steering system. For maintenance-free operation, it is important that the lubricant remains within the spindle nut, i.e., in the area of ​​the nut thread, and does not escape due to the constant axial movement of the spindle.For this purpose, sealing elements are used, which are designed as felt wipers or sealing rings, as known, for example, from DE 10 2015 224 781 A1. The sealing element is received in a bore or groove of the spindle nut and is fixed there axially and radially relative to the spindle nut. The felt or sealing lip must be precisely machined, and molds are required to manufacture such sealing elements.

[0003] US patent 4,052,076 discloses a sealing element consisting of a cylindrical carrier ring with a helically arranged sealing ring on its inner surface. The carrier ring has grooves on its outer surface and can therefore only move axially relative to a nut. Another sealing element for a ball screw drive is disclosed in US patent 5,178,029. The sealing element is screwed towards the end face of the spindle nut using a tool until the required seal is achieved. The sealing element is then fixed against rotation relative to the spindle nut using a setscrew. The sealing element can thus only move slightly axially relative to the spindle or spindle nut.

[0004] Against this background, the present invention provides an improved spindle drive, actuator, and steer-by-wire steering system according to the independent claims. Advantageous embodiments are described in the dependent claims and the following description.

[0005] According to a first aspect, the invention relates to a spindle drive for an actuator of a steer-by-wire steering system, comprising a spindle with a spindle thread and a spindle nut with a nut thread. The spindle and the spindle nut share a common longitudinal axis, at least in their installed position. The spindle thread and the nut thread are designed as drive threads. The spindle nut, which can be driven in the direction of rotation and is stationary in a housing of the actuator, preferably by means of at least one rolling bearing, engages with the external thread of the spindle via its internal thread. The drive can be provided, for example, by an electric motor directly or indirectly via a gearbox, preferably a belt drive, most preferably a toothed belt drive. When the spindle nut rotates, it rotates concentrically around the spindle.The spindle is preferably secured against rotation, so that the rotational movement of the spindle via the drive thread causes an axial displacement of the spindle along its longitudinal axis. At least one sealing element is arranged in an annular groove between the spindle nut and the spindle thread in the area of ​​at least one of the end faces of the spindle nut. This sealing element ensures that the required amount of lubricant remains within the confines of the spindle nut. This guarantees low-friction and efficient operation of the spindle drive.

[0006] The invention is characterized in that the sealing element is designed like a round cord and is floatingly mounted within the spindle nut, whereby the sealing element can move both axially and along the spindle thread, limited by the annular groove. The sealing element is arranged in a frictional sliding manner between the bottom of the groove and at least two thread flanks of the spindle thread. The sealing element is designed as a hyperbolic spiral, which is inserted into the thread and held by the bottom of the groove in the thread groove between adjacent tooth flanks. The movement of the sealing element thus occurs, due to the rotational movement of the spindle nut and its internal annular groove, at least temporarily axially relative to the annular groove and also along the thread groove between the at least two thread flanks.Within the confines of the annular groove, the sealing element can slide along the helical path of the spindle thread. In other words, the helical movement of the sealing element allows axial movement relative to both the annular groove and the spindle, whereby the sealing element rotates with the annular groove and the spindle nut thread due to sliding friction. This rotation or axial sliding relative to the groove base is limited by the first or second groove flank, depending on the direction of rotation of the spindle nut. With continued rotation in the same direction, the sealing element rotates only within or following the thread groove. Axial sliding relative to the spindle nut only resumes when the direction of rotation of the spindle nut changes, and even then, only until the adjacent groove flank comes into contact with the sealing element.The annular groove, with its groove flanks, prevents the sealing element from migrating out axially. The sealing element can be manufactured cost-effectively as a round cord and easily cut to the required length. The length is chosen so that the cord follows at least one thread and thus encircles the spindle at least once within its thread. If such a sealing element is used on both ends of the spindle nut, it can be advantageously ensured that sufficient lubricant is available within the spindle nut throughout its entire service life.

[0007] Preferably, the annular groove has a rectangular cross-section with a groove base and a first groove flank, as well as an adjacent second groove flank. The groove base forms a cylindrical, preferably smooth, inner surface extending concentrically and parallel to the longitudinal axis of the spindle nut. The first and second groove flanks adjoin the left and right ends of the groove base and this inner surface, respectively, and extend radially inward from the groove base and perpendicular to the groove base and the longitudinal axis of the spindle nut. The sealing element is held within the axial limits of the annular groove by the groove flanks. Preferably, the groove width of the annular groove covers at least two threads axially. Following the helical path of the spindle thread, one thread corresponds to the pitch of the thread. The width of the annular groove is therefore preferably at least twice the pitch of the thread.

[0008] As mentioned above, the sealing element is rotated by contact with the groove base and two adjacent tooth flanks of the thread groove or spindle thread due to sliding friction between the sealing element and the aforementioned friction partners. In a preferred embodiment, the sealing element rests under preload between the groove base and the at least two adjacent tooth flanks. Depending on the type of thread, its pitch, and the thread diameter, a clearance or the position of the tooth flanks results. The preload can be adjusted as follows: In the clearance between the tooth flanks and the groove base, a circle can be formed tangentially between the groove base and the tooth flanks, thus eliminating play. This circle has a defined diameter, which can be determined during the design of the drive thread, i.e., the spindle nut with annular groove and the spindle thread.The circular cord has only point contact with the annular groove and the tooth flanks. The diameter of the cord is then chosen to be larger than the defined diameter or cross-section in the design position. The desired friction can thus be determined by the preload. The preload also causes the cord to conform to the tooth flanks with more than just point contact. Preferably, the diameter of the cord is chosen to be 5 to 25% larger, preferably 20% larger, than the theoretical circular diameter mentioned above. This results in sufficient sealing performance from the cord sealing element.

[0009] In a preferred embodiment, the sealing element, designed as a round cord, is made of an elastic material. Rubber or, most preferably, PTFE (polytetrafluoroethylene) are preferred materials. Furthermore, HNBR (an acrylonitrile butadiene rubber produced by selective hydrogenation), preferably with a hardness of 85 Shore A, or FKM (a polymer of highly fluorinated hydrocarbons), preferably with a hardness of 85 Shore A, can also be used. The circular cross-section of the cord preferably has a diameter that is at least 65% of the pitch of the spindle thread. It has proven particularly advantageous if the diameter is preferably 66-70% of the pitch of the spindle thread.

[0010] In metric threads, the thread pitch, or pitch, refers to the distance traveled in one revolution (here: the distance traveled by the spindle after one revolution of the spindle nut). In other words, it is the distance between two thread crests. For example, if a trapezoidal thread according to DIN 103 is used with a spindle with TR 21x3, i.e., a spindle pitch of 3 mm, then the nominal diameter of the cord in this case is preferably 2 mm, corresponding to 66% of the pitch. In other words, depending on the pitch of the thread used, a diameter of at least two-thirds, preferably exactly two-thirds, of the thread pitch should be selected.

[0011] In a further preferred embodiment, the sealing element encloses at least one thread of the spindle thread. Preferably, the O-ring encloses 1.3 to 2 threads, most preferably 1.5 threads. As mentioned above, the sealing element or the O-ring lies in the thread groove and follows the thread of the spindle thread. With an enclosure greater than 1, the sealing element is overlapped as shown in [reference to diagram]. Fig. 2 and Fig. Figure 3 is shown. This advantageously results in an improved effect of the sealing element.

[0012] The invention further relates to an actuator for a steer-by-wire steering system with a spindle drive as described above. The invention further relates to a steer-by-wire steering system for a motor vehicle with the aforementioned actuator. The steer-by-wire steering system is preferably configured as a rear-axle steering system for a motor vehicle.

[0013] The advantages achievable with the presented approach are that a simplified sealing element made from a round cord can be manufactured cost-effectively, and the required sealing can still be adequately achieved, so that the required lubricant remains within the confines of the spindle nut and thus the service life of the spindle drive, the actuator and the steer-by-wire steering is not affected.

[0014] A steer-by-wire steering system is a mostly electromechanical unit decoupled from a mechanical steering handle, such as a steering wheel. Based on steering signals and one or more parameters, such as vehicle speed, steering wheel angle, current steering angles at the front and / or rear axles, yaw acceleration, and / or lateral acceleration of the vehicle, etc., steering signals are generated in a control unit. The steering movement is carried out by at least one actuator of the steer-by-wire system, which receives steering signals from the control unit. For example, a spindle or steering rod can be linearly displaced in the actuator by means of a spindle drive. This spindle is directly or indirectly articulated to wheel carriers. By displacing the spindle, the wheel carriers can be pivoted about their vertical axis, so that the wheels, which are rotatably mounted on the wheel carriers, can be subjected to a change in the steering angle of the respective wheel carrier.

[0015] The invention is described below with reference to preferred embodiments and the drawing. The drawing shows: Fig. 1 an actuator according to the state of the art, Fig. 2 a detailed representation of a spindle drive according to the invention Fig. 3 a perspective representation according to Fig. 2

[0016] Fig. Figure 1 shows an actuator 20 of a steer-by-wire steering system according to DE 10 2015 224 781 A1, in which a spindle drive 21, comprising a spindle 22 and a spindle nut 23 with a common longitudinal axis a, is arranged. Identical or analogous components are described in Fig. 2 and Fig. 3 with reference figures increased by 100 compared to Fig. The spindle nut 23 is sealed against the escape of lubricating grease from the spindle 22 by means of sealing elements 27, 28 in the form of a first wiper ring and a second wiper ring, which are axially fixed in a first annular groove 29 and a second annular groove 30, respectively. The actuator 20 has a housing 31 in which the spindle nut 23 is rotatably mounted by means of a first rolling bearing 32 and a second rolling bearing 33 and is fixed in the direction of the longitudinal axis a. A pulley 34 is fixedly arranged on the spindle nut 23 and can be driven by an electric motor 36 via a belt drive 35. The spindle 22 is connected at its two ends to screw-on pins 37, 38, which are guided in the housing 31 and are in turn connected to pivot pins 39, 40. The actuator 20 is preferably used for the rear axle steering of a motor vehicle.The two pivot pins 39, 40 are connected to a steering linkage or wheel carrier(s) of the rear wheels (not shown). The steering angles of the rear wheels can thus be adjusted when the body-mounted actuator 20, upon rotation of the axially fixed spindle nut 23, displaces the spindle 22, which is secured against rotation, axially in the longitudinal direction or along the longitudinal axis a. The actuator 20 shown in the drawing acts simultaneously on both rear wheels and is therefore preferably mounted in the central area of ​​the vehicle. Actuators acting on one rear wheel each are also within the scope of the invention. Such separate actuators have only one pivot pin, so that the actuator is a single component with variable length.

[0017] Fig. Figure 2 shows in detail and in sectional view a partial section of a spindle 122 and a right partial section of spindle nut 123. Fig. 3 is a perspective representation according to. Fig. 2. The spindle nut 123, with its internal thread 123a, engages with the external thread 122a of the spindle 122. Starting from an actuator 20 according to Fig. 1 is also here in Fig. 2. The spindle nut 123 is mounted in a fixed position (not shown) so that when the spindle nut 123 is rotated by means of an electric motor drive, the spindle 122 can be displaced translationally or linearly in one direction or the other, depending on the direction of rotation of the spindle nut 123, via the drive thread. The spindle nut 123 has an end face 126 on its right side, with a circumferential groove provided on the end face as an annular groove 130. The annular groove 130 is formed by a groove base 130a, which is parallel to the common longitudinal axis a of the spindle nut 123 and the spindle 122. Furthermore, the annular groove 130 consists of two groove flanks 130b, 130c, which each adjoin the respective axial end of the groove base 130a. A sealing element 128 in the form of a round cord is arranged in the annular groove 130 or between the groove base 130a and the tooth flanks 150, 151 of the spindle thread 122a.Again . Fig. 2 as well as the Fig. As can be seen in Figure 3, the sealing element 128, or the O-cord, runs in the thread groove of the spindle thread 122a. In the design position shown here, the O-cord, or the sealing element 128, has a circular cross-section with a diameter D. The trapezoidal thread shown here, according to DIN 103, has a pitch P and a flank angle F of 30°, so that the distance between the tooth flanks 150, 151 is the same for each pair of tooth flanks along the entire length of the spindle 122. The O-cord, or its circular cross-section, is enclosed by the two tooth flanks and also held axially in position by the groove base 130a. In the design position shown, the circular cross-section thus has three tangential contact points. Fig. Figure 3 shows three circular cross-sections – two above and one below the longitudinal axis a. This is because the sealing element 128 encloses more than one thread of the spindle thread 122a. In conjunction with Fig. Figure 3 clearly shows that the ends of the round cord of the sealing element 128 overlap. The spindle nut 123 can rotate in one direction or the other, as indicated by the double arrow in Fig. Figure 3 illustrates this. Due to the friction between the cylindrical inner surface of the groove base 130a and the respective tooth flanks 150, 151, the sealing element 128 can rotate to a limited extent, with the O-cord moving in the thread groove of the spindle thread 122a following the helical path of the thread or the thread pitch. Simultaneously, a relative movement occurs between the groove base 130a and the radial outer surface of the O-cord. If the spindle nut were to move out of the groove 122a, the O-cord would move. Fig.When the spindle nut 123 begins to rotate in one direction, the contact between the groove base 130a and the radial outer surface of the sealing element 128 causes the round cord to move in the threaded groove in the direction of rotation of the spindle nut 123. Simultaneously, the spindle 122 begins to shift linearly along its longitudinal axis a (depending on the direction of rotation of the spindle nut 123). This results (from the position shown here) in an initial axial displacement of the groove base 130a relative to the radial outer surface of the sealing element 128 until the groove flank 130b, 130c comes into contact with the sealing element 128. With further rotation of the spindle nut 123 in the same direction, the sealing element 128 only moves or slides within the threaded groove of the spindle thread 122a. Thus, the groove flanks 130b, 130c ensure that the sealing element 128 remains within the annular groove 130 and does not leave the threaded groove.

[0018] In contrast to the design shown here, the diameter D of the round cord of the sealing element 128 is actually chosen to be larger, preferably 20% larger than the circular cross-section in the design position, or corresponds in diameter to 66-70% of the pitch of the selected trapezoidal thread. A pressing action, or compression, of the elastic material of the round cord of the sealing element 128 into the available space between the tooth flanks occurs against the groove base 130a and the tooth flanks 150, 151 of the spindle thread 122a. This results in sufficient sealing effect so that the lubricant remains within the end face of the spindle nut 123 throughout the entire operating time of the spindle drive, actuator, or steer-by-wire system. Simultaneously, low-friction operation is possible. Reference sign 20 actuator 21 Spindle drive 22 Spindle 23 Spindle nut 25 Front 26 Front 27 first sealing element 28 second sealing element 29 first ring groove 30 second ring groove 31 cases 32 first rolling bearing 33 second rolling bearing 34 Pulley 35 Belt drive 36 Electric motor 37 screw-on studs 38 screw-on studs 39 joint pins 40 joint pins 122 Spindle 122a Spindle thread 123 Spindle nut 123a Thread Spindle Nut 126 Front 128 Sealing element 130 Ring groove 130a Nutgrund 130b groove flank 130c groove flank 150 tooth flank 151 Tooth flank a longitudinal axis Diameter of the round cord, sealing element F flank angle P slope

Claims

Spindle drive (21, 121) for an actuator (20) of a steer-by-wire steering system, comprising a spindle (22, 122) with a spindle thread (122a) and a spindle nut (23, 123) with a thread (123a), wherein the spindle thread (122a) and the thread (123a) are designed as a motion thread and wherein at least one sealing element (27, 28, 128) is arranged between the spindle nut (123) and the spindle thread (122a) in an annular groove (29, 30, 130) in the region of at least one of the end faces (25, 26, 126) of the spindle nut (23, 123), characterized in that the sealing element (128) is floatingly mounted, wherein the sealing element (128) can displace to a limited extent both axially and following the spindle thread. Spindle drive according to claim 1, characterized in that the annular groove (130) has a rectangular cross-section with a groove base (130a) and a first groove flank (130b) as well as an adjacent second groove flank (130c), wherein the displacement of the sealing element (128) by means of the groove flanks (130b, 130c) is limited. Spindle drive according to claim 2, characterized in that the sealing element (128) rests between the groove base (130a) and two adjacent tooth flanks (150, 151), preferably under preload. Spindle drive according to one of the preceding claims, characterized in that the sealing element (128) is formed from a round cord made of an elastic material, preferably PTFE or rubber or HNBR or FWK, which has a circular cross-section with a diameter (D) which is at least 65%, preferably 66-70% of the pitch (P) of the spindle thread (122a). Spindle drive according to claim 4, characterized in that the sealing element (128) encloses at least one thread of the spindle thread (122a), preferably 1.3 to 2 threads, most preferably 1.5 threads. Actuator (20) of a steer-by-wire steering system with a spindle drive (121) according to one of the preceding claims. Steer-by-wire steering for a motor vehicle with an actuator (20) according to the preceding claim, preferably configured as a rear axle steering system.

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