Brake unit for placement on a wheel of a motor vehicle

Abstract

In a brake unit (42) for placement on a wheel of a motor vehicle, the brake unit (42) includes an axially movable brake element (28) and a drive (64) for selectively moving the brake element (28) axially, the brake element (28) being configured as an axially displaceable brake piston (44) force-transmittingly connected to the drive (64) by means of a hydraulic transmission (82).

Description

[Technical Field]

[0001] The present invention relates to a brake unit for placement on a wheel of a motor vehicle, comprising an axially movable brake element and a drive for selectively axially moving the brake element. The present invention further relates to the use of such a brake unit on a wheel of a motor vehicle. [Background technology]

[0002] It is known to use a central hydraulic brake unit, which is usually located in the engine compartment of a motor vehicle, such as a motorcycle, a car, or a truck. The central brake unit comprises a hydraulic assembly hydraulically connected to a brake pedal actuated by the driver. When the brake pedal is actuated, a piston, conventionally guided in a master brake cylinder, is displaced, thereby transmitting the mechanical pressure of the brake pedal to the hydraulic device of the brake unit. The brake fluid present therein is then hydraulically transferred by volume displacement through brake lines to the wheel brakes of at least one associated wheel, thereby providing braking pressure to the wheel.

[0003] Specifically, the wheel brake of a wheel has at least one piston arranged in a brake caliper as an axially movable braking element. The piston is displaced by the brake pressure applied to the wheel toward a radially movable friction element. Such a friction element can be a brake disc or a brake drum. In this case, the piston presses against the brake pads, which are pressed against the friction element. This generates a friction force that brakes the rotational movement of the friction element and the wheel connected to it. When the pressure is released, the piston returns to the brake caliper, and the braking process is completed.

[0004] In modern brake systems with such a central hydraulic brake unit, the brake pedal is often used solely for detecting the driver's braking request together with the master brake cylinder during control operation. In this case, the master brake cylinder is decoupled from the wheel brakes. The brake pressure of the associated wheel brakes is instead generated using pressure fluid from an electrically controllable pressure generator. In many cases, the pressure fluid from the master brake cylinder can be used to operate the brake system at a hydraulic fallback level. Such a brake system is known from Patent Document 1. In this case, in the case of normal braking, the braking request is transmitted to the electrically controllable pressure generator by an electrical signal. In this case, the brake pedal is no longer directly connected to the brake unit.

[0005] In recent vehicle developments, with increasing automation, the brake pedal is often completely omitted. This places new demands on the braking system and opens up new possibilities. It is known to provide electromechanical brakes, which typically have an electric motor as the converter of electrical to mechanical energy, directly on the wheel as the braking unit. An axially movable braking element in the form of a plunger is pressed against a radially movable friction element by the mechanical energy. In this case, a high transmission ratio between the converter and the braking element is required to achieve the required braking force. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] German Patent Application Publication No. 102014222759 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a brake unit which is force- and space-optimized, particularly for modern vehicle developments, and which further allows for a particularly safe and long-lasting braking operation. [Means for solving the problem]

[0008] The present invention provides a brake unit or wheel brake / brake unit for placement on a wheel of a motor vehicle, comprising an axially movable brake element and a drive device for selectively displacing the brake element axially. The brake element is configured as an axially displaceable brake piston coupled to a drive device or actuator by a hydraulic transmission. In particular, the brake piston is guided in an associated piston or brake housing so as to be axially or translationally displaceable. Furthermore, brake fluid can be or is preferably contained in the brake housing as a pressure medium. The brake piston is guided in the brake housing so as to be axially displaceable by the volumetric displacement of the brake fluid. Such a brake piston can be moved with significantly less force than an axially movable brake element, such as a plunger of a conventional electromechanical brake unit. In particular, significantly less frictional forces of the components involved must be overcome.

[0009] Furthermore, the use of a hydraulic transmission allows for a much higher transmission ratio between the drive and the brake piston compared to conventional electromechanical brake units. This, and in combination with the brake element configured as a brake piston, means that the brake unit and the entire brake system require much lower loads and exert much lower forces. Simpler, cheaper and at the same time longer-lasting components can be used.

[0010] Preferably, the brake piston has an end face that is extended from the brake housing by the drive unit. When extended from the brake housing, the brake piston applies an axial pressure to a friction element attached to the wheel. A pressure element, preferably a brake clip, is then arranged on this outer end face, particularly preferably equipped with brake pads that are pressed against the friction element. When the pressure on the brake piston by the drive unit is relieved, the brake piston returns to the brake housing and is no longer pressed against the friction element. The braking action ends. The friction element is then preferably a brake disc, which is simple in terms of assembly, or preferably a brake drum, which transmits force very well.

[0011] Furthermore, the drive is preferably coupled to an electric control device or electronic controller in a signal-transmitting manner, which is adapted to detect the driver's braking request or, in the case of automated driving, the braking needs of the associated vehicle control system, and transmit the information accordingly to the drive, which in turn causes the brake piston to undergo an associated translational movement in response to the braking request.

[0012] The brake unit according to the invention provides a self-contained hydraulic unit for direct installation on the wheels of a motor vehicle. This closed wheel brake / brake unit with hydraulic transmission is thus a single, distributed brake unit, particularly an electrohydraulic brake, located on the wheel. For particularly uniform, controllable, adaptable, and redundant braking, it is preferred to have at least two such brake units, with one distributed brake unit per wheel being particularly preferred. Compared to conventional central hydraulic brake units, each distributed brake unit takes up less space in the engine compartment. Furthermore, there is no need for brake piping from the engine compartment to each wheel brake. This saves material and valuable structural space.

[0013] According to the invention, the hydrostatic transmission is advantageously configured with a drive piston, which is provided in addition to the brake piston and is axially displaceable in a drive housing that can or contains brake fluid. The drive piston is then hydraulically displaceable by the brake fluid, which also displaces the brake piston. The brake piston, in particular, can be moved out of its brake housing by the brake fluid, while the drive piston can be moved into its drive housing. Such a hydrostatic transmission using two pistons is particularly power-saving. In particular, very few friction losses occur. Overall, therefore, the required load requirements of the involved components are further reduced.

[0014] Furthermore, the drive piston advantageously has a drive piston diameter and the brake piston has a brake piston diameter that is configured differently from the drive piston diameter, so that the hydrostatic transmission can be modified as required and adapted to the respective requirements as required.

[0015] Preferably, the diameter of the brake piston is larger than the diameter of the drive piston. The hydraulic transmission is thus realized such that, due to the larger brake piston diameter, the force that the brake piston can exert on the friction element when the brake fluid pressure is approximately constant is greater than the force transmitted from the transmission to the drive piston. During the braking process, this hydraulic transmission allows a relatively large braking force to be transmitted from the brake piston to the friction element with a relatively small force from the drive piston.

[0016] According to the invention, the drive piston is advantageously configured to be selectively displaceable into and out of the drive housing using a drive device. The drive piston can therefore be actively displaced back and forth by the drive device as needed. To generate pressure through the brake piston, the drive piston can be actively displaced into the drive housing in the drive direction, thereby displacing the brake piston out of the brake housing. To reduce pressure or relieve pressure, the drive piston can be actively retracted by the drive device in the opposite direction. In this case, the brake piston is also retracted by a hydraulic coupling, thereby ending the braking process. In this case, the drive device is preferably configured using a spindle drive connected to the drive piston, which is simple to assemble.

[0017] Alternatively or additionally, the drive piston is advantageously configured according to the invention to be movable in the opposite direction to the drive direction by a spring element. In this case, after the drive piston is displaced in the drive direction into the drive housing, the spring element exerts a restoring effect on the drive piston by its deformation force. When the force acting in the drive direction by the drive unit is no longer present, the drive piston is pushed back in the opposite direction by the spring element. The brake piston is also returned by the hydraulic coupling. That is, the spring element functions as a return spring. In this case, the spring element is preferably an inexpensive mechanical spring element, such as a coil spring or a compression coil spring, particularly preferably a metal spring element. Alternatively or additionally, the spring element is preferably constructed using an elastomer. Even more preferably, the spring element is a single return element acting on the drive piston, which is easy to assemble. Particularly preferably, the spring element is provided in addition to the drive unit that returns the drive piston. As a result, even in the event of an error during the braking process, such as a shutdown of the drive unit due to a power outage, the spring element returns the drive piston and thus the brake piston. Thus, residual pressure due to friction is prevented from remaining in the system.

[0018] Furthermore, the drive device according to the present invention advantageously includes a drive motor or a motor and a transmission that can be driven by the motor. In this case, the motor can preferably be controlled by an electric controller and is particularly preferably configured as an electric motor. Due to the hydraulic transmission, the electric motor can be configured with a particularly low torque. Such a motor requires particularly little construction space and material. In particular, valuable raw materials such as copper and magnets can be saved in this case.

[0019] Preferably, the transmission is configured using a rotary mechanical transmission. In this case, the torque of the rotary motion of a motor element, such as a motor shaft, is first converted by a mechanical transmission into a higher torque of the rotary motion of the transmission. That is, the torque of the motor is first converted into a rotary / rotary motion. For this purpose, the transmission includes a rotating part, which can be configured in various embodiments. Preferably, the rotating part is configured as a planetary gear mechanism, a multi-stage spur gear mechanism, or a worm gear. This is followed by a rotary / translation transmission using a translation part associated with the transmission, which is preferably configured as a ball screw drive or, particularly preferably, as a spindle nut drive. The translational motion generated thereby translates a brake piston, which is hydraulically coupled to the transmission. In this case, a drive piston is preferably positioned between the brake piston and the transmission, and the drive piston is moved axially by the transmission. Such a transmission has a simple and space-saving construction, thereby saving construction space and material. It has been found that combining such a simple mechanical transmission in the drive with a hydraulic transmission between the drive and the brake pistons ensures a power-saving and reliable generation of brake pressure.

[0020] Preferably, the transmission is configured using a ball screw drive. In this case, the spindle is surrounded by a thread provided with at least one nut, in which balls circulate in a closed loop. This achieves rolling friction, which results in less friction loss than sliding friction in the case of a spindle nut drive or spindle drive. However, the transmission is particularly preferably configured using a less expensive spindle drive or spindle drive. In this case, the rotational motion generated by the motor is transmitted to the spindle by a clutch or to a nut or spindle nut that engages with the spindle thread, thereby converting the rotational motion into linear motion via the spindle nut. This provides an inexpensive, precise drive that is very simple in terms of assembly, requiring a relatively small number of parts. The correspondingly small number of parts results in correspondingly less play in the transmission, which improves the precision and service life of the transmission.

[0021] Furthermore, the present invention advantageously provides a pressure compensator, in particular hydraulically connected to the drive. For this purpose, a fluid line is preferably arranged between the drive and the pressure compensator. In particular, the fluid line connects the drive housing to a pressure compensator container. Both the drive housing and the pressure compensator container can contain or are filled with brake fluid. The pressure compensator is thus used to equalize and store brake fluid in the entirely closed hydraulic system of the brake unit. Otherwise, there would be a risk of heating and the associated pressure increase in the closed hydraulic system, resulting in unintended braking forces. In the event of such heating, the pressure compensator allows thermal volume compensation, which prevents such unintended braking forces from occurring.

[0022] Preferably, the fluid line is arranged in the region of the drive housing where the drive piston is in its rest position. In this case, the fluid line is configured as a so-called sniffle hole. The fluid flow connection or separation of the pressure compensator, which is created depending on the position of the drive piston, is preferably provided by a specific geometry of the drive piston. Particularly preferably, the specific geometry is provided on the end face of the drive piston pointing in the drive direction. In particular, the drive piston has a groove on its end face that is arranged in the region of the fluid line in the rest position of the drive piston. This provides a connection between the hydraulic area, in particular the drive housing, and the pressure compensator. Pressure compensation of the brake fluid is possible. When the drive piston is pushed into the drive housing with its end face, the fluid line is preferably closed by the drive piston abutting the drive housing over its entire range. Alternatively, the drive piston preferably does not abut the drive housing over its entire range, but instead has a seal assembly that radially surrounds the drive piston. With the fluid line closed in this way, brake pressure, unaffected by the pressure compensator, is generated in the direction of the brake piston. According to the invention, the pressure compensation device is preferably configured by means of a pressure compensation membrane, which is arranged in a hermetically sealed manner in a pressure compensation container, thereby providing a tight separation between atmospheric pressure and the brake fluid contained in the drive housing, which at the same time reacts particularly flexibly to pressure changes.

[0023] To seal the hydraulic area from the surrounding environment, the brake piston is preferably provided with a seal that radially surrounds the brake piston and is particularly configured as a sealing ring. Particularly preferably, two such seals are arranged axially one behind the other on the brake piston, thereby ensuring reliable prevention of leakage.

[0024] According to the invention, advantageously, a first sensor and a second sensor are further provided, each coupled to the drive. Preferably, the second sensor is coupled to the brake piston. Particularly preferably, the second sensor is coupled to a brake chamber arranged in front of the brake piston in the drive direction in the brake housing. Alternatively, the second sensor is preferably coupled to a pressure chamber associated with the drive in the drive housing. Thus, the required braking force can be detected in various ways. Preferably, the sensors are configured as pressure and / or force sensors. Furthermore, calculations based on the motor current or motor position, which can be determined by a rotor position sensor, are preferred. In particular, the two sensors are coupled in a signal-transmitting manner to a controller that controls the drive and, in particular, the motor therein, in response to the signals.

[0025] Particularly preferably, the first sensor is configured as a rotor position sensor, and the second sensor is configured, in particular, as a pressure sensor. Redundancy for brake force detection is thereby provided by the rotor position sensor in the drive motor and the pressure sensor in the hydraulic transmission. Therefore, various methods for redundancy can be used, which in particular can prevent various faults. Furthermore, due to the hydraulic system, the measurement by the pressure sensor is advantageously very simple to implement. Particularly preferably, the motor is configured with a power pack that forms a connection to the pressure sensor.

[0026] The present invention is further directed to the use of at least one such brake unit on each wheel of a vehicle or as a component of a wheel brake. Preferably, such a brake unit is arranged and used on at least two wheels, and particularly preferably on each wheel of the vehicle. Accordingly, the present invention is also directed to a vehicle braking system in which such a brake unit is arranged on at least one wheel of the vehicle. Preferably, such a brake unit, each with an associated control device, is arranged on each wheel of the vehicle. To brake one or more wheels of the vehicle, an electric signal is sent to a controller of each brake unit. The controller drives an associated motor, which transmits torque to a rotary mechanical transmission, which uses this torque to translate a brake piston via a hydrostatic transmission.

[0027] Thus, according to the present invention, at least one decentralized brake unit is advantageously used, or is used, which does not take up structural space, particularly in the engine compartment of a vehicle. For particularly uniform braking, at least two such decentralized brake units are provided, one for each wheel. It is particularly preferred that such a brake unit is arranged on each wheel of the vehicle. This eliminates space problems in the engine compartment compared to conventional central brake units housed in the engine compartment. Furthermore, brake piping from the engine compartment to each wheel brake is not required. Furthermore, the solution according to the present invention eliminates the need for a second brake unit for autonomous driving, especially since each wheel already has its own brake unit. The required redundancy is preferably provided, or is provided, by at least two brake units according to the present invention, connected to two separate power sources. [Brief explanation of the drawings]

[0028] [Figure 1]FIG. 1 is a circuit diagram of a brake unit according to the prior art. [Figure 2] 2 is a circuit diagram according to FIG. 1 of a first exemplary embodiment of a brake unit according to the invention with a first drive piston variant; [Figure 3] 1 is a schematic longitudinal section of a first exemplary embodiment having a second drive piston variant; FIG. [Figure 4] 1 for a second exemplary embodiment of a brake unit according to the invention with a first drive piston variant; [Figure 5] 4 shows a view according to FIG. 3 of a second exemplary embodiment with a second drive piston variant. [Figure 6] 6A to 6C are views of detail VI according to FIG. 2 in various positions of the drive piston of a third exemplary embodiment of a brake unit according to the invention; [Figure 7] FIG. 7 is a view of detail VII according to FIG. DETAILED DESCRIPTION OF THE INVENTION

[0029] In the following, exemplary embodiments of the solution according to the invention will be explained in detail with reference to the accompanying schematic drawings. [Industrial Applicability]

[0030] 1 shows a simplified circuit diagram of an electromechanical brake 10 configured as a disc brake. The brake 10 comprises a brake unit 12 arranged on a wheel of a motor vehicle (not shown). A brake element 14 or friction element associated with the brake 10 is attached to the wheel and configured as a brake disc 16, which rotates during operation. This attachment is not shown. Only the brake disc 16 is partially shown. When the wheel and brake unit 12 are installed on the vehicle, the brake unit 12 is arranged above the radial extent of the brake disc 16 and surrounds it with a distance 18 therebetween.

[0031] A brake shoe or brake pincer 20, 22 is arranged on each side of the brake disc 16 within a distance 18, with a brake pad 24 located on each side of the brake disc. One brake pincer 20 is mounted on a support element or brake caliper 26, while the other brake pincer 22 is attached to an axially movable brake element 28. The axially movable brake element 28 is configured as a pressure plunger 29 that can be translated back and forth within a housing 32 integrated with the brake caliper 26 by means of a transmission 30. The transmission 30 is configured as a screw drive assembly (not shown in detail) that can be driven by a drive motor or motor 34. The motor 34 and the transmission 30 form a drive 36, which is coupled to an electrical control unit 38 in a signal-transmitting manner. The motor 34 is controlled accordingly in response to the signal, which results in a rotational movement of the motor 34. In this case, the motor 34 is coupled to a transmission 30 such that the rotational movement of the motor 34 is converted into a translational movement of the pressure plunger 29. When controlled, the pressure plunger 29 is moved out of the housing 32 by the transmission 30, and the brake pinchers 20, 22, and therefore the brake pads 24, are pressed against the brake disc 16. In this case, each brake pincher 20, 22, together with its brake pad 24, forms a friction pair that brakes the rotational movement of the wheel and the brake disc 16 as a rotating friction element. When the pressure plunger 29 returns, the brake pads 24 leave the brake disc 16, and braking ends.

[0032] 2 and 3 show an electrohydraulic brake 40, which differs from the electromechanical brake 10. The brake 40 comprises a brake unit 42 having a brake piston 44 as the axially movable brake element 28. The brake piston 44 is guided so as to be translationally displaceable in a brake housing 46 arranged in the brake caliper 26. An end face 48 of the brake piston 44 is located axially outside the brake housing 46 and is force-transmittingly connected to the brake pincher 22 and the brake pad 24 abutting thereon.

[0033] Axial-wise opposite the end face 48, the brake piston 44 has an end face 50 which, together with the brake housing 46, encloses a variable-volume brake chamber 52 containing brake fluid 54. To seal the brake chamber 52 from the ambient environment outside the brake housing 46 or the brake caliper 26, two seals 56 are provided, arranged axially one behind the other, and are configured as sealing rings that radially surround the brake piston 44. The brake chamber 52 is hydraulically connected by a fluid line 60 to a pressure chamber 62 filled with brake fluid 54, axially between the end face 50 and a wall 58 that defines the brake housing 46. The pressure chamber 62 belongs to a drive device 64, which further has a drive housing 66 containing the pressure chamber 62 and a drive piston 68 guided axially displaceably in the drive housing 66.

[0034] The drive piston 68 is guided axially displaceably back and forth by means of a transmission 70 associated with the drive unit 64. The transmission 70 is coupled to a drive motor 72 or motor 72 in a force-transmitting manner, which is also coupled to a control unit 74 in a signal-transmitting manner. The motor 72, configured as an electric motor, is controlled with respect to its rotational movement in response to a signal. When rotating in one direction, a planetary gear mechanism 76 associated with the transmission 70 and connected to the motor 72 is correspondingly rotated. A spindle drive 78 associated with the transmission 70 is coupled to the planetary gear mechanism 76, which converts the rotation into axial movement of the drive piston 68, which is coupled to the spindle drive 78. When the drive piston 68 is guided into the drive housing 66 in the drive direction 80, the brake fluid 54 in the pressure chamber 62 is displaced through the fluid line 60, out of the pressure chamber 62, and into the brake chamber 52. Such volumetric displacement of the brake fluid 54 displaces the brake piston 44 together with its end face 48 out of the brake housing 46. This causes the brake pincer 22 arranged on the end face 48 to be pressed together with its brake pads 24 against the wheel-side rotating braking element 14 configured as a brake disc 16. The rotation of the brake disc 16 and the associated wheel is braked.

[0035] When the motor 72 is driven in the reverse direction by a corresponding signal, the planetary gear mechanism 76 is correspondingly rotated. This reverse rotation is then converted by the spindle drive 78 into a reverse translational movement of the drive piston 68. In this case, the drive piston 68 is guided out of the drive housing 66 in the opposite direction to the drive direction 80 and is thereby actively returned by the drive unit 64. When returned, the outward movement of the drive piston 68 generates a suction force (Sog) in the pressure chamber 62, which draws and displaces the brake fluid 54 from the brake chamber 52 through the fluid line 60 and back into the pressure chamber 62. In this case, a corresponding suction force is generated in the brake chamber 52, which displaces the brake piston 44 into the brake housing 46 in the opposite direction to the drive direction 80. In response, the brake pincer 22, together with its brake pads 24, disposed on the brake piston 44 is pulled back from the brake disc 16, thereby releasing the brake action.

[0036] Thus, a hydraulic transmission 82 is provided in particular by the brake fluid 54, the drive piston 68 guided axially displaceably in the drive housing 66, the brake piston 44 guided axially displaceably in the brake housing 46, and the fluid line 60 connecting the two housings 46, 66. Due to the hydraulic transmission 82, the brake piston 44 further has a brake piston diameter 84 that is larger than a drive piston diameter 86 of the drive piston 68. The same applies to the cross-sectional diameters of the associated brake housing 46 and drive housing 66. This results in a hydraulic transmission 82, by means of which a relatively large braking force of the brake piston 44 is transmitted to the brake disc 16 during the braking process with a relatively small force of the drive piston 68.

[0037] In this case, the brake unit 42 is configured as a closed hydraulic system that allows thermal volume compensation by a pressure compensator 88 connected to the drive housing 66 by a fluid line 90. In this case, the hydraulic line 90 leads out from an area 92 of the drive housing 66 where the drive piston 68 is in its rest position and into a pressure compensator container 94. The brake fluid 54 is contained in the drive housing 66 and in a space 96 adjacent to the fluid line 90 in the pressure compensator container 94. A pressure compensator membrane 98 that defines the space 96 is mounted in the pressure compensator container 94 and hermetically separates the space 96 filled with brake fluid 54 from a space 100 filled with air pressure.

[0038] Depending on the position of the drive piston 68, the fluid line 90 is either closed or fluidly connected to the drive housing 66. The drive piston 68 has a special design for this purpose. A first variant is shown in FIG. 2 (and FIG. 4), and a second variant is shown in FIG. 3 (and FIG. 5). According to the first variant of FIG. 2 (and FIG. 4), and in particular of FIGS. 6 and 7, the drive piston 68 has, on its end face 102 pointing in the drive direction 80, an oblique groove 104 that is arranged in the region 92 of the fluid line 90 in the rest position of the drive piston 68. This creates a connection between the drive housing 66 and the pressure compensation device 88. When the drive piston 68 is pressed into the drive housing 66 with its end face 102 and the groove 104 arranged therein, the drive piston 68 abuts against the inner circumferential surface of the drive housing 66 over its entire range, thereby closing the fluid line 90. On both axial sides of the fluid line 90, seals 106 additionally abut radially about the drive piston 68. In this case, in a variant embodiment not shown, the drive piston 68 does not necessarily abut the drive housing 66, but only the seals 106.

[0039] According to the second variant of FIG. 3 (and FIG. 5), the drive piston 68 has a through opening 107 which leads to an end face 102 of the drive piston 68 which is configured in a cup shape and opens in the drive direction 80. In the rest position, the through opening 107 is located in the fluid line 90.

[0040] For this purpose, a first sensor 108 configured as a rotor position sensor is provided, which is coupled to the motor 72 of the drive 64. The sensor 108 thus detects the rotor position of the motor 72. Redundantly, a second sensor 110 is further provided as a pressure sensor, which detects the pressure of the brake fluid 54 in the brake chamber 52 of the brake housing 46. Both sensors 108, 110 are coupled to the control device 74 in a signal-transmitting manner. Depending on the signal, the motor 72 and thus the drive 64 can be controlled by means of the control device 74.

[0041] 4 and 5, in comparison with the exemplary embodiment shown in FIGS. 2 and 3, an exemplary embodiment of the electrohydraulic brake 40 is shown in which a spring element 112 is arranged in the pressure chamber 62. The spring element 112 is configured as a return spring, here formed as a metal coil spring. Furthermore, the spring element 112 is axially arranged between the end face 102 of the drive piston 68 and an axially opposite wall 114 of the drive housing 66. The spring element 112 deforms when the drive piston 68 retracts into the drive housing 66 and acts on the drive piston 68 to return it against the drive direction 80 when the drive force generated by the motor 72 is no longer present.

[0042] In this exemplary embodiment, spring element 112 acts in addition to transmission 70, which returns drive piston 68 via motor 72. Thus, in the event of an error, such as a power outage, that prevents motor 72 from being driven, drive piston 68 is returned using spring element 112, which acts as an additional return, and brake piston 44 is also returned by hydrostatic transmission 82.

[0043] In another advantageous embodiment, not shown here, the drive piston 68 is configured to be returnable only by the spring element 112 as a single return element.

[0044] FIG. 6 shows various positions of the drive piston 68 within the drive housing 66. Here, a first position 116 is the rest position of the drive piston 68, which is shown in more detail in FIG. 7. The beveled groove 104 in the end face 102 is shown, which provides a fluid communication between the fluid line 90 and the pressure chamber 62 within the drive housing 66. The second position 118 of the drive piston 68 shows how the drive piston 68 is pushed into the drive housing 66 in the drive direction 80 upon the onset of pressure generation. In the third position 120, the drive piston 68 is in its pressure-generation position, while in the fourth position 122, the drive piston 68 is again displaced out of the drive housing 66 in a direction 124 opposite the drive direction 80 to relieve pressure. [Explanation of symbols]

[0045] 10. Brakes 12 Brake unit 14 Braking Elements 16 Brake discs 18 distance 20 Brake clamp 22 Brake clamp 24 brake pads 26 Brake caliper 28 Braking Elements 29 Pressure plunger 30 Transmission Device 32 Housing 34 Motor 36 Drive unit 38 Control Device 40 Electric Hydraulic Brake 42 Brake unit 44 Brake piston 46 Brake housing 48 End face 50 End face 52 Brake chamber 54 Brake fluid 56 Seals 58 Wall 60 fluid line 62 Pressure Chamber 64 Drive unit 66 Drive housing 68 Drive Piston 70 Transmission 72 Drive motor, motor 74 Control Device 76 Planetary gear mechanism 78 Spindle Drive 80 Drive direction 82 Hydrostatic Transmission 84 Brake piston diameter 86 Drive piston diameter 88 Pressure Compensator 90 fluid line 92 Drive housing area 94 Pressure compensation vessel 96 Space 98 Pressure Compensating Membrane 100 space 102 End face 104 Groove 106 Stickers 107 Through opening 108 Sensors 110 Sensors 112 Spring Elements 114 Wall 116 First Position 118 Second Position 120 Third Position 122 Fourth Position 124 opposite direction to driving direction

Claims

1. A brake unit (42) for placement on a wheel of a motor vehicle, comprising an axially movable brake element (28) and a drive (64) for selectively moving said brake element (28) axially, 1. A brake unit, comprising: a brake element (28) configured as an axially displaceable brake piston (44) force-transmittingly coupled to the drive device (64) by a hydraulic transmission (82).

2. 2. The brake unit according to claim 1, wherein the hydraulic transmission (82) is configured using a drive piston (68) that is provided in addition to the brake piston (44) and is guided axially displaceably in a drive housing (66).

3. 3. The brake unit according to claim 2, wherein the drive piston (68) has a drive piston diameter (86) and the brake piston (44) has a brake piston diameter (84) that is configured differently from the drive piston diameter (86), in particular the brake piston diameter (84) is configured larger than the drive piston diameter (86).

4. 4. The brake unit according to claim 2 or 3, wherein the drive piston (68) is configured to be selectively displaceable into and out of the drive housing (66) by the drive device (64).

5. 5. A brake unit according to claim 2, wherein the drive piston (68) is displaceable against the drive direction (80) by means of a spring element (112).

6. 6. A brake unit according to claim 1, wherein the drive device (64) comprises a drive motor (72) and a transmission device (70) drivable by the drive motor (72).

7. 7. Brake unit according to claim 1, characterized in that a pressure compensator (88) is provided, in particular hydraulically connected to the drive (64).

8. 8. A brake unit according to claim 7, characterized in that the pressure compensation device (88) is constituted by a pressure compensation membrane (98).

9. 9. A brake unit according to claim 1, further comprising a first sensor (108) coupled to the drive (64) and a second sensor (110).

10. Use of at least one brake unit (42) according to any one of claims 1 to 9 on a wheel of a motor vehicle, respectively.

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