Brake unit for mounting on a wheel of a motor vehicle

Description

State of the art

[0001] The invention relates to a brake unit for mounting on a wheel of a motor vehicle, comprising an axially movable brake element and a drive device for selectively axially moving the brake element, wherein the brake element is designed as an axially displaceable brake piston which is coupled to the drive device by means of a hydraulic transmission. The invention further relates to the use of such a brake unit on a wheel of a motor vehicle.

[0002] In motor vehicles such as motorcycles, cars, or trucks, it is common practice to use a central hydraulic brake unit, typically located in the engine compartment. This central brake unit comprises a hydraulic power unit connected hydraulically to a brake pedal operated by the driver. When the brake pedal is pressed, a piston, conventionally guided within a master cylinder, is displaced, thus transmitting mechanical pressure from the brake pedal to the hydraulic system within the brake unit. Brake fluid present in the hydraulic system is then transferred via a brake line to the wheel brake of at least one associated wheel, thereby applying braking pressure to the wheel.

[0003] In detail, a wheel brake consists of at least one piston located in a brake caliper, which acts as an axially movable braking element. The brake pressure moves the piston towards a radially movable friction element attached to the wheel. This friction element is either a brake disc or a brake drum. The piston then presses against a brake pad, which is pressed against the friction element. This generates a frictional force that slows the rotation of the friction element and the wheel connected to it. When the pressure is released, the piston retracts into the brake caliper, ending the braking process.

[0004] In modern braking systems with such a central hydraulic brake unit, the brake pedal and master cylinder often serve only to detect the driver's braking request during normal operation. The master cylinder is decoupled from the wheel brakes. Brake pressure at the corresponding wheel brakes is generated by a hydraulic fluid from an electrically controlled pressure build-up device. The braking system can usually be operated in a hydraulic fallback mode using hydraulic fluid from the master cylinder. Such a braking system is known from DE 10 2014 222 759 A1. In this system, a braking request is transmitted to the electrically controlled pressure build-up device via an electrical signal during normal braking. The brake pedal is no longer directly connected to the brake unit.

[0005] Newer vehicle developments with increasing levels of automation are completely eliminating the brake pedal. This places new demands on the braking system and opens up new possibilities for its design. One common approach is to integrate an electromechanical brake directly at the wheel, typically incorporating an electric motor to convert electrical energy into mechanical energy. This mechanical energy is used to press an axially movable brake element, acting as a piston, against a radially movable friction element. A high gear ratio between the converter and the brake element is necessary to achieve the required braking force.

[0006] Furthermore, DE 102 36 686 A1 discloses a brake caliper for engaging a brake disc non-rotatably connected to a vehicle wheel with a housing and a first piston and a second piston slidably guided therein. The first piston is adjustable by means of a service brake system of the vehicle, and the second piston acts on a brake pad intended to bear against the brake disc.

[0007] From WO 03 / 014 588 A1, a brake caliper is known which comprises a hydraulic service brake actuator and an electric parking brake actuator. The service brake actuator has a piston that is slidably guided in a cylinder. Hydraulic fluid enters the cylinder via a suitable hydraulic connection.

[0008] From EP 3 275 741 A1 a brake caliper is known which has a piston pressing against a brake disc, to which a fluid is to be moved from a master cylinder through a brake circuit.

[0009] From EP 3 805 590 A1, a brake caliper is known with a piston slidably mounted in a cylinder body, which presses against a brake pad and in which a brake fluid in the cylinder body is insulated from the environment. The piston is moved by means of a further slidably mounted piston, which is driven by an electrically driven actuator.

[0010] The object of the invention is to create a force- and space-optimized brake unit, especially for newer vehicle developments, which also enables particularly safe and durable brake operation. Disclosure of the invention

[0011] The invention is defined in independent claim 1.

[0012] The wheel brake unit is provided with an axially movable brake element and a drive device that can optionally move the brake element axially.

[0013] The brake element is designed as an axially displaceable brake piston, which is coupled to the drive device or actuator via a hydraulic transmission. Specifically, the brake piston is guided axially or translationally within an associated piston housing or brake housing. Furthermore, the brake housing preferably contains or already contains a brake fluid as a pressure medium. The brake piston is guided axially within the brake housing by means of a volume displacement of the brake fluid. Compared to an axially movable brake element such as a piston in a conventional electromechanical brake unit, such a brake piston requires significantly less force to move. In particular, considerably fewer frictional forces of the components involved need to be overcome.

[0014] Furthermore, the hydraulic transmission significantly increases the transmission ratio between the drive unit and the brake piston compared to a conventional electromechanical brake unit. This, combined with the brake element designed as a brake piston, results in considerably lower loads and forces within the brake unit and the entire brake system. Simpler and more cost-effective components can be used, while simultaneously offering a longer service life.

[0015] Preferably, the brake piston has an end face that extends from the brake housing by means of the drive device. As it extends from the brake housing, the brake piston exerts axial pressure on a friction element attached to the wheel. A pressing element, such as a brake caliper, is preferably arranged on this outer end face, and a brake pad is particularly preferably attached to this pad, which is pressed against the friction element. When the pressure on the brake piston is released by means of the drive device, the brake piston retracts into the brake housing and is no longer pressed against the friction element. The braking effect is then terminated. The friction element is, for structural reasons, preferably a brake disc or, for very efficient force transmission, preferably a brake drum.

[0016] Furthermore, the drive device is preferably coupled to an electrical control device or control unit via signal transmission. The electrical control device is adapted to detect a braking request from the driver or, in the case of automated driving, a braking requirement from an associated vehicle control system and transmit this information to the drive device. The drive device then moves the brake piston into the corresponding translational movement in accordance with the braking request.

[0017] The brake unit according to the invention is a self-contained hydraulic unit for direct mounting on the wheel of a motor vehicle. This closed wheel brake unit with hydraulic transmission is thus a single, decentralized brake unit mounted on the wheel, and in particular represents an electro-hydraulic brake. For particularly smooth, easily controllable, adaptable, and redundant braking, at least two such brake units are preferably used, and preferably one such decentralized brake unit is arranged for each wheel. Compared to a conventional central hydraulic brake unit, none of the decentralized brake units require space in the engine compartment. Furthermore, no brake lines are required from the engine compartment to the respective wheel brakes. This saves material and valuable installation space.

[0018] According to the invention, the hydraulic transmission is advantageously designed by means of a drive piston provided in addition to the brake piston, which is guided axially displaceably in a drive housing in which brake fluid is received or is received. The drive piston is hydraulically displaceable by means of the brake fluid, which also allows the brake piston to be hydraulically displaced. In particular, the brake piston can be moved out of its brake housing by means of the brake fluid, while the drive piston is moved into its drive housing. Such a hydraulic transmission using two pistons is particularly energy-efficient. In particular, only very low friction losses occur. Overall, this further reduces the required loads on the components involved.

[0019] Furthermore, according to the invention, the drive piston advantageously has a drive piston diameter and the brake piston has a brake piston diameter that differs from the drive piston diameter. This allows the hydraulic transmission to be varied as needed and adapted to specific requirements.

[0020] Preferably, the brake piston diameter is larger than the drive piston diameter. This ensures that the hydraulic transmission is such that, with a nearly constant pressure in the brake fluid, the larger brake piston diameter allows a greater force to be exerted on the friction element from the brake piston than can be transmitted from the transmission to the drive piston. During braking, this hydraulic transmission allows a comparatively high braking force to be transferred from the brake piston to the friction element with relatively little force from the drive piston.

[0021] Advantageously, according to the invention, the drive piston is designed to be selectively displaceable into and out of the drive housing by means of the drive device. This allows the drive piston to be actively moved back and forth as required by means of the drive device. To build up pressure using the brake piston, the drive piston is actively moved into the drive housing in one drive direction, thereby moving the brake piston out of the brake housing. To reduce or relieve pressure, the drive piston is actively retracted in the opposite direction of the drive by means of the drive device. The brake piston is also retracted by means of the hydraulic coupling, thus ending the braking process. Preferably, the drive device is designed with a structurally simple spindle drive connected to the drive piston.

[0022] Alternatively or additionally, according to the invention, the drive piston is advantageously designed to be movable against a drive direction by means of a spring element. After the drive piston is displaced in the drive direction into the drive housing, the spring element exerts a restoring force on the drive piston with its deformation force. Once the force acting from the drive device in the drive direction ceases, the drive piston is displaced back against the drive direction by means of the spring element. The brake piston is also reset by means of the hydraulic coupling. The spring element thus acts as a return spring. The spring element is preferably a cost-effective mechanical spring element, and particularly preferably a metallic spring element, such as a coil spring or compression spring. Alternatively or additionally, the spring element is preferably designed with an elastomer.Furthermore, the spring element is preferably a single return element acting on the drive piston. Particularly preferably, the spring element is provided in addition to a drive device that returns the drive piston. This ensures that even in the event of a fault, such as a failure of the drive device during braking (e.g., due to a power outage), the spring element returns the drive piston and thus retracts the brake piston. This prevents residual pressure from remaining in the system due to friction.

[0023] Furthermore, the drive device advantageously comprises a drive motor and a gearbox driven by the motor, according to the invention. The motor is preferably controlled by an electrical control unit and is particularly preferably designed as an electric motor. Thanks to the hydraulic transmission, the electric motor can be designed with a relatively low torque. Such a motor requires very little installation space and material. In particular, valuable raw materials such as copper and magnets can be saved.

[0024] Preferably, the transmission is designed with a rotary-mechanical transmission. In this configuration, the torque of a rotational movement of a motor element, such as a motor shaft, is first converted into a higher torque of rotational movement of the transmission itself by means of a mechanical transmission. Thus, the motor's torque is initially converted rotary-to-rotationally. For this purpose, the transmission includes a rotary component, which can be designed in various configurations. Preferably, the rotary component is designed as a planetary gear set, a spur gear set, or a worm gear set. This is followed by a rotary-to-translational transmission by means of a translation component belonging to the transmission, which is preferably designed as a ball screw drive or, even more preferably, as a spindle-nut drive. The resulting translational movement is used to set the brake piston, which is hydraulically coupled to the transmission, into a translational movement.In this design, the drive piston is preferably positioned between the brake piston and the gearbox, and is set into axial motion by means of the gearbox. Such gearboxes are simple and space-saving in their construction, thus saving installation space and materials. It has been found that such a simple mechanical transmission in the drive unit, in combination with the hydraulic transmission between the drive unit and the brake piston, ensures a power-efficient and reliable build-up of brake pressure.

[0025] Preferably, the gearbox is designed with a ball screw drive. In this design, a spindle is enclosed by a thread on which at least one nut is provided, within which balls circulate in a closed system. This achieves rolling friction, which reduces friction losses compared to sliding friction in a spindle-nut drive. However, the gearbox is particularly preferably designed with the more cost-effective spindle drive. In this design, a rotary motion generated by the motor is transmitted via a coupling to a spindle or to a nut or spindle nut engaging in a thread of the spindle, so that the rotary motion is converted into a linear motion by means of the spindle nut. This results in a structurally very simple, cost-effective, and precise drive that requires comparatively few individual components.With a correspondingly small number of individual components, there is correspondingly little play in the gearbox, which increases its accuracy and service life.

[0026] The pressure equalization device is hydraulically connected to the drive unit. Preferably, a fluid line is arranged between the drive unit and the pressure equalization device. In particular, the fluid line connects the drive housing to a pressure equalization reservoir. Brake fluid is stored in both the drive housing and the pressure equalization reservoir. Thus, the pressure equalization device serves to maintain a balanced supply of brake fluid within the closed hydraulic system of the brake unit. Otherwise, there is a risk that an unwanted braking force could arise due to heating and the associated pressure increase in the closed hydraulic system. The pressure equalization device enables thermal volume compensation in the event of such heating, thereby preventing the occurrence of such an unwanted braking force.Preferably, the fluid line is arranged in a region of the drive housing where the drive piston is in its rest position. The fluid line is designed as a so-called vent bore. A fluid-conducting connection or separation of the pressure equalization device, which is established depending on the position of the drive piston, is preferably created by means of a special geometry on the drive piston. This special geometry is particularly preferably provided on an end face of the drive piston facing in the direction of travel. Specifically, the drive piston has a groove on its end face that, in its rest position, is located in the region of the fluid line. This provides a connection between a hydraulic area, in particular the drive housing, and the pressure equalization device. Pressure equalization in the brake fluid is thus enabled.When the drive piston is inserted into the drive housing with its end face, the fluid line is preferably sealed by ensuring that the drive piston then rests fully against the drive housing. Alternatively, the drive piston preferably does not rest fully against the drive housing and is instead provided with a sealing arrangement that radially surrounds the drive piston. With the fluid line sealed in this way, a brake pressure is built up in the direction of the brake piston, unaffected by the pressure equalization device. According to the invention, the pressure equalization device is preferably designed with a pressure equalization diaphragm, which is arranged to seal, in particular, in the pressure equalization reservoir. This creates a tight separation between atmospheric pressure and the brake fluid located in the drive housing, which also reacts particularly flexibly to pressure changes.

[0027] To seal the hydraulic section from its surroundings, a seal is preferably provided on the brake piston, radially surrounding the piston and in particular designed as a sealing ring. Two such seals are preferably arranged axially one after the other on the brake piston. This ensures increased protection against leakage.

[0028] A further advantage of the invention is the provision of a first sensor coupled to the drive device and a second sensor. Preferably, the second sensor is coupled to the brake piston. Particularly preferably, the second sensor is coupled to a brake chamber arranged in the brake housing in the drive direction upstream of the brake piston. Alternatively, preferably, the second sensor is coupled to a pressure chamber belonging to the drive device in the drive housing. This allows the required braking force to be determined using different methods. Preferably, the sensors are designed as pressure and / or force sensors. Furthermore, a calculation using a motor current or a motor position, which is to be determined via a rotor position sensor, is preferred. In particular, both sensors are coupled to a control unit via signal transmission, which, depending on the signal, controls the drive device and, in particular, the motor.

[0029] The first sensor is preferably designed as a rotor position sensor, and the second as a pressure sensor. This creates redundancy in brake force measurement, using both the rotor position sensor on the drive motor and the pressure sensor in the hydraulic transmission. By employing different redundancy methods, the system can effectively compensate for various failure modes. Furthermore, the ease of implementing pressure sensor measurements due to the hydraulic system is advantageous. The motor is preferably designed with a power pack that provides a connection to the pressure sensor.

[0030] The invention is further directed to the use of at least one such brake unit on each wheel or as a component of a wheel brake of a motor vehicle. Preferably, such a brake unit is arranged and used on at least two wheels, and particularly preferably on each wheel of the motor vehicle. Accordingly, the invention is also directed to a braking system of a motor vehicle in which such a brake unit is arranged on at least one wheel of the motor vehicle. Preferably, such a brake unit is arranged on each wheel of the motor vehicle with an associated control unit. To brake one or more wheels of the motor vehicle, an electrical signal is sent to the control unit of the respective brake unit.The control unit controls the associated motor, which transmits the torque to the rotary-mechanical transmission, with which the brake piston is set into a translational movement by means of the hydraulic transmission.

[0031] According to the invention, at least one decentralized braking unit is advantageously used, which, in particular, does not require any installation space in the engine compartment of the vehicle. For a particularly uniform braking effect, at least two such decentralized braking units are provided, one at each wheel. It is particularly preferred that one such braking unit is arranged at each wheel of the vehicle. This eliminates space constraints in the engine compartment compared to a conventional central braking unit located there. Furthermore, brake lines from the engine compartment to the respective wheel brakes are no longer required. Additionally, a second braking unit is not necessary for autonomous driving, since the solution according to the invention provides a separate braking unit at each wheel. The necessary redundancy is preferably provided by means of at least two braking units according to the invention, which are connected to two separate power sources.

[0032] Exemplary embodiments of the solution according to the invention are explained in more detail below with reference to the accompanying schematic drawings. These show: Fig. 1 shows a circuit diagram of a brake unit according to the prior art, Fig. 2 shows the circuit diagram according to Fig. 1 a first embodiment of a brake unit according to the invention with a first drive piston variant, Fig. 3 a schematic longitudinal section of the first embodiment with a second drive piston variant, Fig. 4 the circuit diagram according to Fig. 1 a second embodiment of a brake unit according to the invention with the first drive piston variant, Fig. 5 the view according to Fig. 3 of the second embodiment with the second drive piston variant, Fig. 6, detail VI according to Fig. 2 in different positions of a drive piston of a third embodiment of a brake unit according to the invention and Fig. 7 detail VII according to Fig. 6 .

[0033] In Fig. 1 Figure 10 shows a simplified circuit diagram of an electromechanical brake designed as a disc brake. The brake 10 comprises a brake unit 12, which is arranged on a wheel of a motor vehicle (not shown). A wheel-side brake element 14, or friction element, belonging to the brake 10 and designed as a brake disc 16, is attached to the wheel and rotates during operation. The mounting is not shown; only the brake disc 16 is partially depicted. When the wheel and brake unit 12 are mounted on the vehicle, the brake unit 12 is positioned at the top of the radial circumference of the brake disc 16 and surrounds it with a gap 18.

[0034] At a distance 18, a brake shoe or brake caliper 20, 22 is arranged on each side of the brake disc 16, with a brake pad 24 on the brake disc side of each caliper. One brake caliper 20 is mounted on a carrier element or brake caliper 26, while the other brake caliper 22 is attached to an axially movable brake element 28. The axially movable brake element 28 is designed as a pressure piston 29, which is moved translationally back and forth in a housing 32 integrated with the brake caliper 26 by means of a transmission 30. The transmission 30 is designed as a threaded drive assembly (not shown in detail) that can be driven by a drive motor 34. The motor 34 and the transmission 30 form a drive unit 36, in which the motor 34 is coupled to an electrical control unit 38 for signal transmission. Depending on the signal, the motor 34 is controlled accordingly, which leads to a rotary movement of the motor 34.The motor 34 is coupled to the gearbox 30 in such a way that the rotary motion of the motor 34 is converted into a translational motion of the pressure piston 29. When the pressure piston 29 is extended from the housing 32 by means of the gearbox 30, the brake calipers 20, 22, and thus the brake pads 24, are pressed against the brake disc 16. Each brake caliper 20, 22, with its brake pad 24, forms a friction pair with the brake disc 16 as a rotating friction element, which decelerates the rotational movement of the wheel. When the pressure piston 29 retracts, the brake pads 24 are released from the brake disc 16, and the braking action is terminated.

[0035] In Fig. 2 and Fig. 3 In contrast to such an electromechanical brake 10, an electrohydraulic brake 40 is shown. The brake 40 comprises a brake unit 42, which has a brake piston 44 as an axially movable brake element 28. The brake piston 44 is guided translationally displaceably in a brake housing 46 arranged in the brake caliper 26. A front face 48 of the brake piston 44 is located axially outside the brake housing 46 and is force-transmittingly coupled to the brake caliper 22 and the brake pad 24 bearing against it.

[0036] An end face 50 is located axially opposite the end face 48 on the brake piston 44. This end face 50, together with the brake housing 46, encloses a brake chamber 52 of variable volume, which contains brake fluid 54. To seal the brake chamber 52 from its environment outside the brake housing 46 or the brake caliper 26, two axially arranged seals 56 are provided. These seals are designed as sealing rings and radially surround the brake piston 44. The brake chamber 52 is hydraulically connected axially between the end face 50 and a wall 58 bounding the brake housing 46 to a pressure chamber 62 filled with brake fluid 54 via a fluid line 60. The pressure chamber 62 belongs to a drive unit 64, which also includes a drive housing 66 encompassing the pressure chamber 62 and a drive piston 68 guided axially displaceably within the drive housing 66.

[0037] The drive piston 68 is guided axially for reciprocating movement by a gearbox 70 belonging to the drive device 64. The gearbox 70 is coupled to a drive motor 72, which in turn is coupled to a control device 74 for signal transmission. Depending on the signal, the rotary motion of the electric motor 72 is controlled. When the motor rotates in one direction, a planetary gearbox 76, belonging to the gearbox 70 and coupled to the motor 72, is set into rotation. A spindle drive 78, also belonging to the gearbox 70, is coupled to the planetary gearbox 76. This spindle drive converts the rotation into an axial movement of the drive piston 68, which is coupled to the spindle drive 78.When the drive piston 68 is inserted into the drive housing 66 in a drive direction 80, the brake fluid 54 located in the pressure chamber 62 is forced out of the pressure chamber 62 and into the brake chamber 52 through the fluid line 60. This volume displacement of the brake fluid 54 pushes the brake piston 44 out of the brake housing 46 with its end face 48. This presses the brake caliper 22, located on the end face 48, with its brake pad 24, against the rotating brake element 14, which is designed as a brake disc 16 on the wheel side. The rotation of the brake disc 16 and the associated wheel is thus braked.

[0038] If the motor 72 is driven to rotate in the opposite direction by a corresponding signal, the planetary gear 76 is set into rotation accordingly. This reverse rotation is then converted into a reciprocal translational movement of the drive piston 68 by means of the spindle drive 78. The drive piston 68 is thus moved out of the drive housing 66 in the opposite direction of the drive 80 and is actively reset by means of the drive device 64. During the reset, the movement of the drive piston 68 creates a suction effect in the pressure chamber 62, which draws brake fluid 54 from the brake chamber 52 back into the pressure chamber 62 through the fluid line 60. This creates a corresponding suction effect in the brake chamber 52, which pushes the brake piston 44 into the brake housing 46 in the opposite direction of the drive 80.Accordingly, the brake caliper 22, arranged on the brake piston 44, is retracted from the brake disc 16 with its brake pad 24. The braking effect is thereby released.

[0039] This creates a hydraulic transmission 82, in particular by means of the brake fluid 54, the drive piston 68 which is axially displaceable within the drive housing 66, the brake piston 44 which is axially displaceable within the brake housing 46, and the fluid line 60 connecting the two housings 46 and 66. For the hydraulic transmission 82, the brake piston 44 also has a brake piston diameter 84 that is larger than the 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. Thus, the hydraulic transmission 82 is realized, which, during a braking process, transmits a relatively high braking force from the brake piston 44 to the brake disc 16 with relatively little force from the drive piston 68.

[0040] The brake unit 42 is designed as a closed hydraulic system in which thermal volume compensation is enabled by means of a pressure equalization device 88, which is connected to the drive housing 66 via a fluid line 90. The fluid line 90 leads from a region 92 of the drive housing 66, where the drive piston 68 is in its rest position, out into a pressure equalization reservoir 94. Brake fluid 54 is contained in the drive housing 66 and in a chamber 96 adjacent to the fluid line 90 in the pressure equalization reservoir 94. A pressure equalization diaphragm 98 is attached to the pressure equalization reservoir 94, delimiting the chamber 96 filled with brake fluid 54 and sealingly separating it from a chamber 100 filled with air pressure.

[0041] 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 in Fig. 3 (and Fig. 5 ). shown. According to the first variant in Fig. 2 (and Fig. 4 ) and in detail in Fig. 6 und Fig. 7 The drive piston 68 has a chamfered groove 104 on its end face 102, which points in the direction of drive 80. In the drive piston 68's rest position, this groove is located in the area 92 of the fluid line 90. This creates a connection between the drive housing 66 and the pressure equalization device 88. When the drive piston 68, with its end face 102 and the groove 104 located therein, is pushed into the drive housing 66, the fluid line 90 is sealed because the drive piston 68 rests completely against the inner surface of the drive housing 66. Additionally, a seal 106 is located radially around the drive piston 68 on both sides of the fluid line 90, providing further sealing. In a variant embodiment (not shown), the drive piston 68 does not necessarily rest against the drive housing 66, but only against the seal 106.

[0042] According to the second variant in Fig. 3 (and Fig. 5 The drive piston 68 has a through-opening 107 that leads into a cup-shaped end face 102 of the drive piston 68, which is open in the drive direction 80. In the rest position, the through-opening 107 is located at the fluid line 90.

[0043] For control purposes, a first sensor 108, coupled to the motor 72 of the drive device 64, is provided and is designed as a rotor position sensor. The sensor 108 thus detects the rotor position of the motor 72. As a redundancy measure, a second sensor 110 is also provided, which, as a pressure sensor, detects the pressure of the brake fluid 54 in the brake chamber 52 of the brake housing 46. Both sensors 108 and 110 are coupled to the control device 74 via signal transmission. Depending on the signal, the control device 74 controls the motor 72 and thus the drive device 64.

[0044] In Fig. 4 and Fig. 5 The electro-hydraulic brake 40 is shown in an exemplary embodiment in which, compared to the one in Fig. 2 and 3 In the illustrated embodiment, a spring element 112 is arranged in the pressure chamber 62. The spring element 112 is designed as a return spring, which in this case is a metallic helical spring. Furthermore, the spring element 112 is arranged axially between the end face 102 of the drive piston 68 and an axially opposite wall 114 of the drive housing 66. When the drive piston 68 moves into the drive housing 66, the spring element 112 deforms and, when the drive force generated by the motor 72 ceases, acts in a restoring manner on the drive piston 68 in the opposite direction 80 to the drive direction.

[0045] In the present embodiment, the spring element 112 acts in addition to the gearbox 70, which resets the drive piston 68 by means of the motor 72. Thus, even in the event of a fault, such as a power failure, in which the motor 72 cannot be driven, the drive piston 68 is reset by means of the additionally resetting spring element 112 and the brake piston 44 is also reset by means of the hydraulic transmission 82.

[0046] In a further advantageous embodiment, not shown here, the drive piston 68 is designed to be reset solely by means of the spring element 112 as the only reset element.

[0047] In Fig. 6 Various positions of the drive piston 68 in the drive housing 66 are shown. A first position 116 shows a rest position of the drive piston 68, which in Fig. 7A detailed section is shown. The chamfered groove 104 provided on the end face 102 is shown, which creates a fluid-conducting connection between the fluid line 90 and the pressure chamber 62 in the drive housing 66. A 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 to begin pressure build-up. In a third position 120, the drive piston 68 is in its pressure build-up position, while in a fourth position 122, the drive piston 68 has been pushed out of the drive housing 66 in a direction 124 opposite to the drive direction 80 to release the pressure.

Claims

1. Brake unit (42) for arranging on a wheel of a motor vehicle with an axially movable brake element (28) and a drive device (64) selectively axially moving the brake element (28), in which brake unit the brake element (28) is designed as an axially displaceable brake piston (44), which is coupled to the drive device (64) in a power-transmitting manner by means of a hydraulic transmission means (82), wherein the brake unit (42) is a self-contained hydraulic unit which is to be arranged individually on the wheel, characterized in that a pressure equalization device (88) is provided, which is hydraulically connected to the drive device (64).

2. Brake unit according to Claim 1, characterized in that the hydraulic transmission means (82) is designed by means of a drive piston (68) which is provided in addition to the brake piston (44) and is guided axially displaceably in a drive housing (66).

3. Brake unit according to Claim 2, characterized in that the drive piston (68) has a drive piston diameter (86), and the brake piston (44) has a brake piston diameter (84) of different design from the drive piston diameter (86), wherein, in particular, the brake piston diameter (84) is of larger design than the drive piston diameter (86).

4. Brake unit according to Claim 2 or 3, characterized in that the drive piston (68) is designed, by means of the drive device (64), selectively to be displaceable into the drive housing (66) and to be displaceable out of the drive housing (66).

5. Brake unit according to one of Claims 2 to 4, characterized in that the drive piston (68) is designed to be displaceable by means of a spring element (112) counter to a drive direction (80).

6. Brake unit according to one of Claims 1 to 5, characterized in that the drive device (64) comprises a drive motor (72) and a gearbox (70) which can be driven by the drive motor (72).

7. Brake unit according to one of Claims 1 to 6, characterized in that the pressure equalization device (88) is designed with a pressure equalization diaphragm (98).

8. Brake unit according to one of Claims 1 to 7, characterized in that a first sensor (108), coupled to the drive device (64), and a second sensor (110) are provided.

9. Use of at least one brake unit (42) according to one of Claims 1 to 8 in each case on a wheel of a motor vehicle.