Linear actuator and braking system

Abstract

Linear actuator (1), in particular for use in a braking system of a motor vehicle, comprising an electric motor (8), a piston (5), a rotary-translational transmission (2a) with a spindle (2) and a nut (7) for converting the rotary motion of the electric motor (8) into a translational motion of the piston (5), wherein the piston (5) is coupled to the nut (7) for transmitting the torque from the nut (7) to the piston (5), wherein a coupling device (3) is provided, comprising a metallic, elastic coupling element (9) and a motor coupling element (11) which is rotationally fixed to the motor shaft, wherein the elastic coupling element (9) forms a positive-locking and elastic connection with both the spindle (2) and the motor coupling element (11), characterized in thatthat the elastic coupling element (9) comprises at least one outer spring element (41) for connection with the motor coupling element (11) and at least one inner spring element (44) for connection with the spindle (2).

Description

[0001] The invention relates to a linear actuator, particularly for use in a braking system of a motor vehicle, comprising an electric motor, a piston, a rotary-translational transmission with a spindle and a nut for converting the rotary motion of the electric motor into a translational motion of the piston, wherein the piston is coupled to the nut for transmitting the torque from the nut to the piston, and wherein a coupling device is provided comprising a metallic, elastic coupling element and a motor coupling element which is rotationally fixed to the motor shaft, wherein the elastic coupling element forms a positive-locking and elastic connection with both the spindle and the motor coupling element. It further relates to a braking system.

[0002] Linear actuators are known from the prior art which, by means of a ball screw drive with a spindle and a rotatably mounted nut, convert the rotary motion of the electric motor into a translational motion of a piston mounted in a cylinder. The piston, in particular, defines a hydraulic pressure chamber within the cylinder. Such a linear actuator can be advantageously used in hydraulic by-wire braking systems, where it functions as a pressure supply device and builds up brake pressure in hydraulic wheel brakes as required.

[0003] The nut is fixed to the piston both rotationally and translationally, allowing or compensating for a certain degree of inclination or tilting between the nut's axis and the piston's axis. By using an anti-rotation device for the piston, it can be ensured that the piston can only move translationally relative to the cylinder. Rotation between the two components is prevented.

[0004] The electric motor generates a rotary motion and torque, which are transmitted to the spindle via a coupling element. This coupling element also fulfills another function: it compensates for the angular deviation or decentering between the motor axis and the spindle axis.

[0005] The coupling element is usually made of an elastic material, particularly rubber or a rubber compound. The coupling element transmits torque and an axial load between the components between which it is positioned. The rubber material of the coupling element changes its mechanical properties over time. It becomes brittle and, after a few years, is no longer usable due to environmental influences such as temperature changes, water, salt, contamination, vibrations, material fatigue, etc. Furthermore, it can be altered by chemical substances to which it is exposed. The coupling element allows only a small deviation between the two axes and leads to increased wear of the piston assembly. Further disadvantages include the large dimensions of the coupling element and the required installation space, which result from its required functionality of transmitting axial load and torque.

[0006] From CH 415 219 A a linear actuator is known comprising an electric motor, a piston, a rotary-translational gear with a spindle and a nut for converting the rotary motion of the electric motor into a translational motion of the piston, wherein a metallic, elastic coupling element is provided which forms a positive-locking and elastic connection with the spindle and the motor shaft.

[0007] The invention is therefore based on the objective of improving a linear actuator in such a way that the coupling element is more durable and reliable, while simultaneously optimizing the transmission of torque and axial load and compensating for axial displacement. Furthermore, a corresponding braking system is to be specified.

[0008] With regard to the linear actuator, a coupling device is provided comprising a metallic, elastic coupling element and a motor coupling element which is rotationally fixed to the motor shaft, wherein the elastic coupling element forms a positive-locking and elastic connection with both the spindle and the motor coupling element, wherein according to the invention the elastic coupling element comprises at least one outer spring element for connection with the motor coupling element and at least one inner spring element for connection with the spindle.

[0009] Advantageous embodiments of the invention are the subject of the dependent claims.

[0010] The invention is based on the consideration that conventional coupling elements significantly impair the service life and performance of the linear actuator. The commonly used rubber material wears out, and the coupling element, due to the axial misalignment that typically occurs between the motor shaft and the axis of the cylindrical housing, is unable to adequately compensate for the high stress on the components.

[0011] As has now been recognized, these disadvantages can be overcome by using an elastic, metallic coupling element. Since the material itself is not elastic, it forms a positive-locking elastic connection with the spindle and motor coupling element, which is achieved through the design of the coupling element. Due to its metallic construction, the coupling element also wears less quickly, thus increasing the service life of the linear actuator.

[0012] The term "metallic" encompasses metals as well as metal alloys such as steel. The term "elastic" refers to elastic properties in relation to the couplings between the spindle and the motor coupling element.

[0013] The respective spring element is preferably designed as a web that is convexly or concavely curved with respect to an axial axis of the coupling element. This enables elastic contact between the coupling element and the spindle and motor coupling element.

[0014] Preferably, a plurality of outer spring elements are arranged circumferentially on the elastic coupling element.

[0015] Preferably, a plurality of inner spring elements are arranged circumferentially on the elastic coupling element.

[0016] The inner and outer spring elements are preferably arranged alternately around the circumference of the elastic coupling element. This ensures a particularly uniform transmission of torque.

[0017] The spindle advantageously has projections, in particular teeth, between which the respective inner spring element engages with the spindle.

[0018] The motor coupling element advantageously has projections or teeth between which the respective outer spring element engages with the motor coupling element.

[0019] The motor coupling element is preferably rotationally fixed to the motor shaft or motor shaft by means of a press-fit and / or screw connection.

[0020] Advantageously, a locking element is provided which, in the assembled state, secures the elastic coupling element in the motor coupling element.

[0021] With regard to the braking system, the aforementioned problem is solved according to the invention with at least one linear actuator as described above. Preferably, the linear actuator is used in a braking system of a motor vehicle. In particular, the braking system comprises a pressure supply device for actively building up pressure in the wheel brakes, which is formed by the at least one linear actuator.

[0022] Preferably, the braking system comprises at least two hydraulically actuated wheel brakes, which are hydraulically disconnectable from the linear actuator. The braking system is preferably designed as a brake-by-wire system, in which the driver's braking request is detected and, depending on the detected braking request, a system pressure is built up in a hydraulic pressure chamber of the linear actuator. This pressure is then used as brake pressure in the hydraulic wheel brakes to generate a braking torque. The linear actuator can also be used in a steering system.

[0023] The advantages of the invention lie particularly in the fact that the service life of the linear actuator is increased by reducing wear on the coupling element itself and the stress on the components. The coupling element can effectively transmit torque and axial load from the motor shaft to the spindle while simultaneously compensating for axis displacement. The number of required components is reduced; only the elastic coupling element and a locking element are needed. The coupling element can be appropriately coated to make it resistant to environmental influences and / or aggressive chemical compounds.

[0024] An embodiment of the invention is explained in more detail with reference to a drawing. The drawing shows, in a highly schematic representation: Fig. 1 a linear actuator with a housing, a ball screw drive, a coupling device with a coupling element, a locking element, a motor and a motor coupling element in a side section; Fig. 2 the spindle of the ball screw drive, the coupling device, the motor coupling element and the locking element according to Fig. 1 in an exploded view; Fig. 3 the coupling element according to Fig. 2 in a first perspective representation; Fig. 4 the coupling element according to Fig. 2 in a second perspective view; Fig. 5 the coupling element, the spindle, the motor coupling element and the locking element in an exploded view; Fig. 6. A part of the spindle in a perspective view; Fig. 7 the coupling element according to Fig. 2 in a further perspective view; and Fig. 8 a section through spindle and motor coupling element.

[0025] Identical parts are marked with the same reference symbols in all figures.

[0026] A in Fig. The linear actuator 1 shown in Figure 1 comprises a piston 5 movable within a housing 1a, which is essentially designed as a cylinder. The linear actuator 1 includes a ball screw drive 2a with a spindle 2 and a nut 7 rotatably mounted thereon. The rotation of the motor shaft 20 of an electric motor 8 is transmitted to the spindle 2 by means of a coupling device 3. The coupling device 3 serves to transmit the rotation of the motor shaft 20 to the spindle 2.

[0027] The linear actuator 1 is used, for example, as a pressure supply device in a vehicle's braking system. A hydraulic pressure chamber 22 is formed in the housing 1a, preferably bounded by an end face 4 of the piston. The pressure chamber 22 is hydraulically connected to at least two wheel brakes, in particular in a separable manner.

[0028] The coupling device 3 comprises an elastic coupling element 9, a locking element 10, and a motor coupling element 11. In the Fig. Figure 2 shows an exploded view of the spindle 2, the elastic coupling element 9, the locking element 10, and the motor coupling element 11. The elastic coupling element 9 partially receives the spindle 2 at one end 24 and is partially received by the motor coupling element 11. In this way, it forms the interface between the spindle 2 and the motor coupling element 11. The motor coupling element 11 is rotationally secured to the motor shaft 20, preferably by a press-fit connection or a screw connection. The elastic coupling element 9 comprises a coupling area 27, a receiving area 28 for the end 24 of the spindle 2, and a limiting element 29, which preferably serves as a stop when the coupling element 9 is received in the motor coupling element 11.

[0029] The motor coupling element 11 comprises a first cylindrical section 32 and a subsequent cylindrical section 33. In the assembled state, the coupling section 27 is completely or substantially completely received by the first cylindrical section 32, while the receiving section 28 is located in the second section 33. When the piston 5 is moved into the cylinder (in Fig. 1 to the left), especially for a hydraulic pressure build-up, this leads to a change in the opposite direction (in Fig. 1. An axial force acting to the right (1) is exerted on the coupling element 9. The coupling element 9 must counteract this force, which is achieved by means of the design of the coupling element 9.

[0030] If the piston 5 moves in the opposite direction to the pressure build-up direction, particularly for the purpose of drawing in pressure medium (in Fig. 1 to the right), friction arises between a seal 6 (see Fig. 1) and the cylinder, as well as between piston 5 and the cylinder. This friction leads to an axial force opposite to the direction of movement of piston 5, which would cause the coupling element 9 to separate from the motor coupling element 11. The locking element 10, which is preferably designed in a horseshoe-like shape, connects these two components in such a way that this separation does not occur and both retain their relative position to each other in the axial direction. Preferably, it engages with both components 9 and 11 in a positive-locking manner in the axial direction.

[0031] As in Fig. As can be seen in Figure 3, the coupling element 9 has a plurality of outer spring elements 41 and a plurality of inner spring elements 44. The spring elements 41, 44 each preferably extend from an annular region 45, which adjoins the limiting element 29, and from an annular region 47, which adjoins the receiving region 28. The outer spring elements 44 and inner spring elements 44 are arranged alternately along the circumference of the annular region 45 and 47, respectively. The respective outer spring element 41 is convexly bent outwards with respect to a central axis 52 of the coupling element 9. The respective inner spring element 44 is concavely bent inwards. The spring elements 41, 44 have two functions: they transmit the torque from the motor 8 to the spindle 2 and compensate for any axial displacement / tilt difference between the axis of the motor shaft 20 and the axial axis of the spindle 2. Fig. 4 shows the coupling element 9 in another perspective view from below.

[0032] In Fig. Figure 5 shows a perspective exploded view of spindle 2, elastic coupling element 9, motor coupling element 11, and locking element 10. Spindle 2 has a plurality of teeth or projections 55, which are formed in certain areas as elongated projections along the axial direction of spindle 2. The motor coupling element 11 has teeth or projections 58 in area 32 inside the cylinder, which are formed in certain areas as elongated projections along the axial direction of the motor coupling element 11. In the assembled state, the projections 58 are arranged between the outer spring elements 41, while the projections 55 of spindle 2 are arranged between the inner spring elements 44. In this way, a rotationally secured and elastic connection is achieved between spindle 2 and coupling element 9, as well as between coupling element 9 and motor coupling element 11.

[0033] The torque is first transmitted from the motor 8 to the projections 58 of the motor coupling element, from there to the outer spring elements 41 of the coupling element 9, from there to the inner spring elements 44 of the coupling element 9 and from there to the projections 55 of the spindle 2.

[0034] As in Fig. As can be clearly seen in Figure 6, the spindle 2 has an end piece 62 at its end 24, which is essentially hemispherical. The end piece 62 is received by the hemispherical receiving area 28 of the coupling element 9. In this way, during a compression stroke (see Figure 6), the spindle 2 is held in the hemispherical receiving area 28 of the coupling element 9. Fig. 8) transfer the axial load from the end piece 62 to the receiving area 28 (see also Fig. 7).

[0035] The flow of the axial load and the compensation of the inclination direction can be described as in Fig. Figure 8 shows how the axial load is compensated. Arrow 67 indicates the axial load on piston 5 during a suction stroke, when piston 5 retracts and pressurized fluid flows into pressure chamber 22. Arrow 68 indicates the axial load on piston 5 during a compression stroke, when pressure builds up in pressure chamber 22. The axial load during the suction stroke is significantly lower than the axial load or axial force on piston 5 during the compression stroke. The friction between spindle 2 and the internal spring elements 44 prevents spindle 2 from slipping out of the coupling element 9 during the suction stroke. The magnitude of the frictional force depends on the shape and elasticity of the internal spring elements 44.

[0036] If the frictional force is not sufficient to hold the spindle 2 in the coupling element 9, a locking washer can be added to the spindle 2, or the respective gap between the projections 55 of the spindle 2 can be shaped to create additional friction. A curved double arrow 72 symbolizes the inclination of the spindle 2 with respect to the elastic coupling element 9.

[0037] In Fig. Figure 9 shows the most important constraints that must be considered and calculated in a specific application of the linear actuator 1.

[0038] A first distance d1 denotes the influence of the grooves formed between the projections 55 on the inner spring elements 44, whereby an elastic deformation of the inner spring elements 44 must occur. A second distance d2 indicates the gap between the projections 55 of the spindle 2 and the inner diameter of the elastic coupling element 9. An arrow 75 indicates the interaction of the outer spring elements 41, which is caused by an elastic deformation of the outer spring elements 41.

[0039] These geometric constraints have an influence on the elastic deformation of the inner 44 and outer spring elements 42, the maximum inclination deviation that can be compensated, and the stiffness of the coupling element 9.

Claims

[1] Linear actuator (1), in particular for use in a braking system of a motor vehicle, comprising an electric motor (8), a piston (5), a rotary-translational transmission (2a) with a spindle (2) and a nut (7) for converting the rotary motion of the electric motor (8) into a translational motion of the piston (5), wherein the piston (5) is coupled to the nut (7) for transmitting the torque from the nut (7) to the piston (5), wherein a coupling device (3) is provided, comprising a metallic elastic coupling element (9) and a motor coupling element (11) which is rotationally fixed to the motor shaft, wherein the elastic coupling element (9) forms a positive-locking and elastic connection with both the spindle (2) and the motor coupling element (11), characterized by, that the elastic coupling element (9) comprises at least one outer spring element (41) for connection with the motor coupling element (11) and at least one inner spring element (44) for connection with the spindle (2) . [2] Linear actuator according to claim 1, wherein a plurality of outer spring elements (41) are arranged circumferentially on the elastic coupling element (9). [3] Linear actuator according to claim 1 or 2, wherein a plurality of inner spring elements (44) are arranged circumferentially on the elastic coupling element (9). [4] Linear actuator according to one of claims 1 to 3, wherein the inner (44) and outer spring elements (41) are arranged alternately circumferentially on the elastic coupling element (9). [5] Linear actuator according to one of claims 1 to 4, wherein the spindle (2) has projections (55), in particular teeth, between which the respective inner spring element (44) engages with the spindle (2). [6] Linear actuator according to claim 4 or 5, wherein the motor coupling element (11) has projections (58), in particular teeth, between which the respective outer spring element (41) engages with the motor coupling element (11). [7] Linear actuator according to any one of claims 1 to 6, wherein the motor coupling element (11) is rotationally fixed to the motor shaft (20) by a press-fit and / or screw connection. [8] Linear actuator according to one of claims 1 to 7, wherein a locking element (10) is provided which, in the assembled state, secures the elastic coupling element (9) in the motor coupling element (11). [9] Linear actuator according to any one of claims 1 to 8, wherein the spindle has an end piece (62) which is hemispherical, and wherein the elastic coupling element (9) has a hemispherical end piece (28) which is designed to receive the end piece (62) of the spindle (2). [10] Braking system for a motor vehicle, comprising at least one linear actuator according to any of the preceding claims.

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