Patent Publication Number: US-2023150472-A1

Title: Brake actuator unit, electromechanical brake, and spindle drive for a brake actuator unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Priority Application No. 102021129961.6, filed Nov. 17, 2021 and German Patent Application No. 102022119398.5, filed Aug. 2, 2022, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a brake actuator unit for an electromechanical brake, and to an electromechanical brake. The disclosure relates further to a spindle drive for a brake actuator unit according to the disclosure. 
     BACKGROUND 
     A spindle drive that is provided is adapted to ensure linear displaceability of the brake piston relative to the spindle. The brake piston can thus apply a damping force to at least one brake lining of the brake, so that frictional engagement with a brake disc can be generated. The rotation of the spindle about the axis of rotation effects a translatory movement of the brake piston in the axial direction, in order to provide damping forces for at least one brake lining. As a result of the damping forces that are generated, reaction forces occur, which generally have eccentric force components. 
     In electromechanical brakes, the spindle drive is driven by an electric motor. As the force intensity of the damping force increases, the phenomenon of displacement of the force application point at the brake disc or the friction linings occurs. As a result of the elasticity of the brake housing, which bends, of the brake and the friction linings, which are compressed, the force application point of the clamping force is displaced in the radial direction. Likewise, the force application point is displaced in the tangential direction relative to the rotational movement of the brake disc. The force application point of the reaction forces caused by the clamping force, which are absorbed by the brake housing, is thus likewise displaced. As a result, the drive which effects the linear displacement of the brake piston can become unstable in respect of its orientation and mounting. In other words, angular offsets between individual components of the drive occur as a result of eccentric force components of the reaction forces. This causes increased wear and a non-optimal force geometry, so that the clamping forces that are brought about are adversely affected. 
     One approach relates to specially shaped bearing components for the drive spindle, the geometries of which are adapted to absorb and compensate for eccentric force components of the reaction forces (transverse forces). Thus, in the case where the core diameter of a thread of the spindle nut of the spindle drive is smaller than an outside diameter of the bearing component supporting the spindle, the transverse forces can be compensated for. However, such bearing components cause a high outlay in terms of manufacture and have specially shaped contact faces which have comparatively high wear. 
     There is therefore a need to eliminate or at least reduce the disadvantages of the prior art. 
     SUMMARY 
     A brake actuator arrangement of the independent patent claims is disclosed herein. Advantageous exemplary arrangements are indicated in the dependent patent claims and in the following description, each of which can constitute aspects of the disclosure on their own or in (sub)combination. Some aspects are explained in relation to different variants. However, the features are reciprocally applicable. 
     According to one aspect, a brake actuator unit is provided. The brake actuator unit has a spindle, which is driven in rotation by an electric motor, and a brake piston in the form of a spindle nut, which surrounds the spindle and is configured to press against a brake lining. In order to make the brake piston even more compact in its radial construction, the brake piston is preferably formed as a spindle nut in that the spindle thread is formed on its inner side. The circumferential wall of the brake piston having the circumferential face thus merges in one piece into the thread of the spindle in order at the same time to form the spindle nut. The brake piston can consequently be a one-piece component. 
     The brake piston thus combines the functionalities of the spindle nut and the brake piston. Radial installation space is thereby saved in particular in the radial direction relative to the axis of rotation of the spindle. Saving radial installation space allows the core diameter of a thread of the brake piston to be increased in the radial direction. For example, the core diameter can be increased without causing an increase in the total installation space required for the brake in the radial direction. 
     Increasing the core diameter has the result that the force application point for the eccentric force components of the reaction forces which occur as a result of the clamping forces is displaced outwards in the radial direction. The effect of the eccentric force components is thereby weakened. This directly causes a stabilization of the orientation and mounting of the components of the brake actuator unit. Overall, the brake actuator unit so adapted allows improved force application to the brake piston and thus to the brake lining of the brake, and reduced wear. 
     In one exemplary arrangement, the brake actuator unit comprises an axial bearing which mounts the spindle axially, wherein the spindle is supported on the axial bearing on operation of the brake, for example on closing and/or on opening of the brake. 
     “Closing” is to be understood as meaning operation of the brake in which the clamping force is increased at least in stages; “opening” is to be understood as meaning operation of the brake in which the clamping force is withdrawn at least in stages. Operation of the brake can comprise successive stages of “closing” and “opening”, for example within the context of antilock control. 
     The axial bearing can have an axial rolling bearing. 
     In one exemplary arrangement, the axial bearing has rotational symmetry. The axial bearing can thus be designed to be identical on all sides with respect to an axis of rotation of the spindle. 
     The axial bearing makes it possible for a reaction force that occurs as a result of the clamping force generated for operating the brake to be absorbed by the spindle and transmitted at least indirectly to the brake housing of the brake, which serves as a force-supporting device in respect of the reaction force. 
     The thread of the brake piston optionally has a core diameter which is larger than an outside diameter of the axial bearing. The force application point for the eccentric force components of the reaction force is thus displaced so far outwards in the radial direction that it is arranged radially outside the axial bearing. This has the result that the effect of the eccentric force components is reduced and these force components can be compensated for overall, for example by the axial bearing. As a result, the orientation and mounting of the components of the brake actuator unit is improved, which likewise allows the force applied to the brake piston to be optimized. 
     In order to be able to choose such a large core diameter of the thread of the brake piston, sufficient radial installation space must be provided (without the installation space of the brake as a whole thereby being increased), which in the present case is achieved in that the brake piston also combines the functionality of the spindle nut. 
     The spindle drive has a recirculating ball spindle. In a recirculating ball spindle, balls transmit the force between the spindle and the brake piston acting as the spindle nut. Friction and wear are reduced by the rolling movement of the balls. 
     A recirculating ball spindle is not self-locking. This means that the brake piston, owing to elasticities inherent in the system, also moves back into the completely retracted position automatically when it is no longer actively being urged into an extended position by a motor, for example an electric motor. In the completely retracted position of the brake piston, the clamping force is withdrawn completely, so that the brake is fully “open”. 
     The spindle comprises on the brake lining side a shaft portion which is thickened in cross-section and has the transmission thread of the spindle drive on the outer lateral surface. The spindle further has a drive shaft prolongation of comparatively smaller cross-section, and a transition portion between the shaft portion and the drive shaft prolongation. The axial bearing abuts a contact face provided by the transition portion. In other words, the radial extent of the spindle varies in the axial direction, specifically such that the spindle has a comparatively small radial extent at the end opposed to the brake lining and a comparatively large radial extent at the end on the brake lining side. Because the transition portion of the spindle is arranged between these portions and represents a narrowing of the spindle with respect to the radial extent, the outside surface of the transition portion can advantageously be used for contact with the axial bearing. 
     The contact face of the transition portion of the spindle can be rotationally symmetrical. It can also extend perpendicular to the axial direction. The contact face of the transition portion is of planar form. 
     The brake actuator unit comprises a brake housing which has a base and which accommodates the brake piston in its interior. The interior is defined by a free inner volume which is enclosed by the side wall and the base of the brake housing and delimited by the side wall and the base. Because the brake piston is displaceable in the axial direction, it can generally also be arranged only in part in the interior of the brake housing and can extend therebeyond at least in part. 
     In one exemplary arrangement, the axial bearing is supported on the base of the brake housing. The brake housing is oriented such that an open end of the brake housing is arranged in the direction of the brake disc and of the brake lining, that is to say on the brake lining side, and that the base of the brake housing is arranged opposite thereto in the axial direction. This means that the clamping forces that are generated act in the axial direction towards the open end of the brake housing, while the reaction forces that occur as a result thereof act in the direction towards the base. Because the axial bearing is supported on the base, the axial bearing is thus arranged along the direction of action of the reaction forces between the base and the spindle. The axial bearing is thus able to absorb the reaction forces particularly well. 
     Accordingly, the bearing bodies of the axial bearing (for example rolling bearing) then roll on the one hand on the contact face of the transition portion of the spindle and on the base of the brake housing. 
     In one exemplary arrangement, a bearing washer is arranged axially between the base of the brake housing and the rolling bodies of the axial bearing and is pressed in the brake housing by frictional engagement and/or interlocking engagement in such a manner that it is secured against rotation. The brake housing, on account of its geometry, represents a more complex component than the bearing washer. Due to the bearing washer, the base of the brake housing can be protected against damage by the axial bearing (if this were in direct contact with the base), which could generally be caused in the case of a worn axial bearing and under the action of the reaction forces. Thus, only the bearing washer or the entire axial bearing, but not necessarily the brake housing, has to be replaced if required. 
     The bearing washer can have two opposing planar contact faces, of which one is in contact with the base of the brake housing and one is provided for the rolling bodies of the axial bearing to roll on. This gives rise to the additional advantage that the outlay in terms of manufacture for ensuring the quality of the contact face of the base of the brake housing can be lower. 
     The rolling bodies of the axial bearing then roll on the one hand on the contact face of the transition portion of the spindle and on the other hand on a planar contact face of the bearing washer, which likewise forms a bearing ring. 
     In one exemplary arrangement, the brake actuator unit has a cup-shaped bushing. The cup-shaped bushing and the brake piston are then arranged in the brake housing. The cup-shaped bushing can have a base and a side wall, wherein the spindle is supported on the base at least indirectly, for example via the axial bearing and the bearing washer. The cup-shaped bushing has an axial stop with which it is supported on the brake housing on operation of the brake. The reaction forces can thereby be transmitted, starting from the base of the cup-shaped bushing, via the side wall of the cup-shaped bushing to the brake housing. The entire brake housing thus acts as a force-supporting device in respect of the reaction forces that are to be resisted, even though the cup-shaped bushing is integrated. 
     The cup-shaped bushing ensures that the brake actuator unit constitutes a closed subassembly of the brake which can be delimited, for example also sealed, relative to other parts of the brake. The outlay in terms of mounting is thus reduced, because the brake actuator unit can be mounted as a whole. 
     The cup-shaped bushing is optionally arranged in the brake housing so that it is secured against rotation. For example, an interlocking connection can be provided between the brake housing and the cup-shaped bushing, which prevents the cup-shaped bushing from rotating relative to the brake housing. 
     The interlocking connection provided to prevent the cup-shaped bushing from rotating can have a tongue-and-groove connection or a tangential pin connection. 
     The stop is optionally a radial shoulder formed on the cup-shaped bushing or a fastening element attached to the cup-shaped bushing. 
     For example, the radial shoulder can be integral with a side wall of the cup-shaped bushing. 
     The fastening element comprises a snap ring. The snap ring can be arranged, for example, in a radial groove of the cup-shaped bushing. The snap ring can also be multi-layered. 
     The cup-shaped bushing is inserted axially into the brake housing and mounted radially therein. This means that the brake housing has an opening which corresponds at least in part to the outer contour of the cup ring. Manufacturing tolerances and clearances must thereby be taken into consideration. Manufacture of the brake actuator unit is thus facilitated because the cup-shaped bushing fits into the opening of the brake housing. 
     A rotation lock is optionally provided between the brake housing and the brake piston accommodated in a linearly displaceable manner therein. The rotation lock ensures the linear displacement of the brake piston by preventing the brake piston from rotating relative to the brake housing. 
     The rotation lock comprises a slot with which an anti-rotation element is in engagement. 
     A gasket is provided on the brake lining side between the brake piston and the brake housing. The interior provided by the brake housing can thereby be sealed relative to other parts of the brake. For example, the spindle drive arranged in the interior of the brake housing is thus protected against contamination. 
     The brake piston optionally has on the brake lining side an end wall which presses against a brake lining when the brake is operated. The end wall can have an end face which is of annular shape. The force application point of the damping force acting on the brake lining is thereby displaced radially outwards, which allows the force to be applied to the brake lining more uniformly in terms of area. The application of force to the brake lining is thus improved. 
     The brake housing has or forms a brake caliper. 
     According to a further aspect, an electromechanical brake is also provided. The electromechanical brake comprises a brake actuator unit as described hereinbefore, brake linings which can be moved towards one another, and an electric motor which is coupled in a torque-transmitting manner with the spindle. 
     The electromechanical brake can be adapted to serve as a vehicle brake. 
     According to a further aspect, a vehicle having an electromechanical brake as described hereinbefore is also provided. 
     The disclosure also relates further to a spindle drive, in particular a recirculating ball spindle drive, for a brake actuator unit according to the disclosure, wherein the spindle drive has a spindle to be driven and a brake piston in the form of a spindle nut, which surrounds the spindle and is configured to press against a brake lining. 
     For example, the brake piston has an end face which has a depression in the centre, so that only an annular pressure face, which surrounds the depression, is formed for pressing against the brake lining. This design is positive with regard to making the pressing force uniform and transferring heat from the brake lining to the piston. 
     A further addition provides that the spindle has on the brake lining side a shaft portion which is thickened in cross-section and has a transmission thread on the outer lateral surface, and a drive shaft prolongation of comparatively smaller cross-section, and a transition portion between the shaft portion and the drive shaft prolongation therebetween. A spherically shaped bearing contact face for an axial bearing is formed on the transition portion. This bearing contact face points in the axial direction. 
     The vehicle can optionally include a motor vehicle, that is to say a road vehicle. Alternatively, the vehicle can also include other vehicle types, for example aircraft, ships, bicycles, motorcycles or the like. Overall, a vehicle is understood in the present case as being a device which is configured for transporting objects, cargo or people between various destinations. Examples of vehicles are land-based vehicles such as motor vehicles, electric vehicles, hybrid vehicles or the like, rail vehicles, aircraft or water-craft. In the present context, vehicles can be regarded as being road-based vehicles, such as, for example, cars, lorries, buses or the like. 
     All the features explained with regard to the various aspects can be combined with other aspects individually or in (sub)combinations. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure as well as further advantageous exemplary arrangements and further developments thereof will be described and explained in greater detail hereinbelow with reference to the examples shown in the drawings, in which: 
         FIG.  1    is a simplified schematic cross-sectional view of an electromechanical brake according to the disclosure having a brake actuator unit in accordance with an exemplary arrangement of the disclosure with an integrated spindle drive according to the disclosure, 
         FIG.  2    shows a second exemplary arrangement of the electromechanical brake according to the disclosure having a different brake actuator unit according to the disclosure with an integrated spindle drive according to the disclosure, and 
         FIG.  3    shows a spindle drive according to the disclosure provided in the brake actuator unit according to  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION 
     All the features disclosed hereinbelow in relation to the exemplary arrangement and/or the accompanying figure can be combined on their own or in any desired subcombination with features of the aspects of the present disclosure, including features of preferred exemplary arrangements, provided that the resulting feature combination is meaningful to a skilled person in the technical field. 
       FIG.  1    is a simplified schematic cross-sectional view of an electromechanical brake  10  according to the disclosure having a brake actuator unit  12  according to the disclosure in accordance with an exemplary arrangement of the disclosure. 
     The brake  10  comprises a brake housing  14  having a brake caliper  16 . The brake caliper  16 , as part of the brake housing  14 , surrounds a brake disc  18 , such as a brake disc rotor, which is enclosed in the axial direction by two brake linings  20 ,  22 . A clamping force Fz is actively applied by the brake actuator unit  12  to the brake lining  20  that is on the inside along the axis of rotation  24  of the brake actuator unit  12 . The axis of rotation  24  of the brake actuator unit  12  in the present case (in the ideal case of compensated transverse forces) also corresponds to the cylinder axis of the brake housing  54  and to the brake disc axis of rotation of the brake disc  18 . 
     The axially displaceable brake caliper  16  ensures that the clamping force Fz is likewise applied to the brake lining  22  that is on the outside in the axial direction. The clamping force Fz is thereby distributed substantially equally in terms of magnitude between the inner brake lining  20  and the outer brake lining  22 . Frictional engagement with the brake disc  18  can thus be ensured for both brake linings  20 ,  22  as a result of the pressing force that is provided, and this frictional engagement is used for slowing down or stopping a vehicle. 
     The brake  10  further has an electromechanical operating unit  26  which is used, together with the brake actuator unit  12 , for generating the clamping force Fz. Relative to the brake actuator unit  12 , the electromechanical operating unit  26  is arranged along the axis of rotation  24  opposite the brake disc  18 . The electromechanical operating unit  26  comprises at least one electric motor  28  and a reduction gear  30 . 
     The components of the electromechanical operating unit  26  are accommodated in the brake housing  54 , which can be in the form of a skeletal frame of metal or of fibre-reinforced plastics material. The electromechanical operating unit  26  forms a closed subassembly  32  which can be mounted separately. 
     The brake actuator unit  12  and the spindle drive comprise a spindle  34  having a drive shaft prolongation  36 , a thickened shaft portion  38  on the brake lining side, and a transition portion  40  which is arranged along the axis of rotation  24  of the spindle  34  between the drive shaft prolongation  36  and the shaft portion  38 . The diameter of the drive shaft prolongation  36  of the brake actuator unit  12  is smaller in the radial direction than the diameter of the shaft portion  38  in that direction. Accordingly, the spindle  34  narrows in respect of its diameter in the region of the transition portion  40 . 
     The brake actuator unit  12  further has a brake piston  42 , which is in the form of a spindle nut. In the present case, the spindle drive  44  of the brake actuator unit  12  is in the form of a recirculating ball spindle, which is not self-locking. The spindle drive  44  comprises a transmission thread  46 , in which balls  48  are arranged and roll. The spindle  34  and the brake piston  42  have mutually corresponding raceway portions. The balls  48  can make possible a translatory movement of the brake piston  42  along the ball raceways  50  of the transmission thread  46  along the axis of rotation  24  relative to the spindle  34 . For this purpose, the ball raceways  50  are formed at least in part in the shaft portion  38  of the spindle  34  and in the brake piston  42 . 
     The diameter of the ball raceways  50  corresponds to the diameter of the balls  48 , taking into consideration manufacturing tolerances and required clearances. 
     As a result of the translator)/movement of the brake piston  42  in the direction towards the brake disc  18 , the brake piston  42  is moved in the direction towards the inner brake lining  20  and thus ensures that the clamping force Fz is actively applied to the inner brake lining  20 . 
     The brake housing  14  is also part of the brake actuator unit  12 . The brake housing  14  has a side wall  56  and a base  58 . The open end of the brake housing  14  is arranged on the brake lining side along the axis of rotation  24 . This means that the base  58  is provided at the end of the brake housing  14  opposite the brake disc  18 . The base  58  has a through-hole  60  for the drive shaft prolongation  36  of the spindle  34 , which is held therein by means of a radial bearing  62 . 
     The brake housing  14  is coupled with the electromechanical operating unit  26  by means of an interlocking connection which is displaceable along the axis of rotation  24 , such that the reduction gear  30  is centred relative to the brake housing  14 . The displaceable interlocking connection can comprise, for example, a shaft-hub connection with multiple splining or a bolt-and-groove connection. 
     The side wall  56  and the base  58  define an interior  64  of the brake housing  14 , in which at least the spindle  34  and the brake piston  42  are arranged at least in part. Owing to the linear displaceability of the brake piston  42 , the brake piston can also be arranged at least in part outside the interior  64 . 
     The brake housing  54  allows the brake actuator unit  12  to be in the form of a separate subassembly  66 , which can be mounted as a whole in the brake housing, for example with the aid of a cup-shaped bushing  90 , which is shown in  FIG.  2   , in which the spindle drive  44  and the brake piston  42  are arranged. 
     The brake piston  42  is linearly guided and secured against rotation inside the brake actuator unit  12  relative to the brake housing  54  by a rotation lock  70 . For this purpose, the brake piston  42  can have an axial groove which is in engagement with an anti-rotation element. 
     The rotation of the spindle  34  is thereby ensured by the electric motor  28 , which is in engagement with the drive shaft prolongation  36  of the spindle  34  via the reduction gear  30 . The rotation of the spindle  34  in conjunction with the rotational blocking of the brake piston  42  provides for a translatory movement of the brake piston  42 . This movement is transmitted to the brake linings  20 ,  22 . The clamping force Fz which is generated is proportional to the torque which is affected at the drive shaft prolongation  36  by the electric motor  28  and the reduction gear  30 . 
     As a result of the clamping force Fz that is generated, a reaction force Fr opposing the clamping force Fz occurs along the axis of rotation  24 . Owing to the elastic expansion of the components of the brake  10 , an angular offset between the brake disc axis of rotation and the cylinder axis of the brake housing  54  can generally occur, so that the reaction force Fr has eccentric force components. These eccentric force components can lead to an instability of the components of the brake actuator unit  12  in the radial direction, for example when the core diameter DK of the thread of the brake piston  42  is smaller than the outside diameter DL of a bearing which is to absorb the reaction force Fr. 
     Therefore, the saving of an otherwise conventional spindle nut, because the brake piston  42  also performs the function thereof, means that the brake piston  42  can be enlarged in the radial direction. As a result, although the radial installation space of the brake  10  remains the same, it is possible to increase the core diameter DK. 
     For example, the core diameter DK can be increased radially such that it is larger than the outside diameter DL in the radial direction of an axial bearing  72  of the brake actuator unit  12 , which absorbs the reaction force Fr. It is thus possible to displace the force application point of the eccentric force components of the reaction force radially outwards such that the effect of the eccentric force components is weakened, and compensation by the axial bearing  72  is ensured even without particular bearing geometries of the axial bearing  72 . The orientation and mounting of the individual components of the brake actuator unit  12  and the application of force to the brake linings  20 ,  22  are thus improved. 
     In the present case, the axial bearing  72  is rotationally symmetrical and is in contact with the transition portion  40  of the spindle  34 , which for this purpose has a complementary planar contact face  74 . In this exemplary arrangement, the axial bearing  72  is in the form of an axial rolling bearing. 
     Between the axial bearing  72  and the base  58  of the brake housing  14  there is additionally arranged an axial bearing washer  76 , which has opposing planar contact faces along the axis of rotation  24  and is pressed into the brake housing  14  in a rotationally secure manner by frictional and/or interlocking engagement. One of the contact faces of the axial bearing washer  76  is in contact with the base  58  of the brake housing  14 . The bearing bodies of the axial bearing  72  roll on the other of the two contact faces of the axial bearing washer  76  and on the planar contact face  74  of the transition portion  40  of the spindle  34 . 
     The reaction force Fr that occurs is thus transmitted from the shaft portion  38  of the spindle  34  via the transition portion  40  to the axial bearing  72 , and from there is absorbed by the base  58  of the brake housing  14  via the axial bearing washer  76 . 
     In order to protect the spindle drive  44 , the brake housing  14  has a radially internal groove and the brake piston  42  has a radially external groove, in which a gasket  82  is arranged and which acts between the brake housing  14  and the brake piston  42 . 
     In the present case, the brake piston  42  comprises on the brake lining side an end wall  84  having an end face which is of annular form and is provided for applying force to the inner friction lining  20 . The annular form ensures optimized force distribution of the clamping force Fz over the receiving face  86  of the inner friction lining  20 . 
     The spindle drive  44  comprises ball return channels  88  which are integrated in the spindle  34 . 
     In a preliminary mounting step, both the transmission threads  46  of the spindle  34  and the ball return channels  88  integrated in the spindle  34  can be filled completely with balls  48 . The brake piston  42  can then be pushed onto the spindle  34 . 
     The ball return channels  88 , integrated in the spindle  34 , of the spindle drive  44  also ensure that the spindle drive  44 , while having the same stroke, can be made axially shorter than in the case of known spindle drives without integrated ball return channels. The reason for this is the possibility that the axial bearing  72 , on which the spindle drive  44  is supported at the open end of the brake piston  42 , is able to project into the brake piston  42  slightly when the brake piston is in the retracted state, without the coverage of the balls  48  being released. 
       FIG.  2    shows a second exemplary arrangement of the brake actuator unit  12 . Parts which are the same or have the same function thereby bear the reference signs already introduced. 
     Unlike in  FIG.  1   , a cup-shaped bushing  90 , which accommodates and mounts the brake piston  42 , is here provided between the brake housing  12  and the brake piston  42 . The cup-shaped bushing  90  also carries the axial bearing  72  composed of a plurality of parts. 
     The cup-shaped bushing  90  is non-rotatably mounted in the brake housing  12  and has a corresponding groove  94 , which receives an anti-rotation lock  70  on the brake piston  42  in order to prevent it from rotating. 
     The cup-shaped bushing  90  is supported axially on the brake housing  12  via a radial prolongation  96  or other fastening. 
     The spindle drive already shown in  FIG.  2    is shown again on an enlarged scale in  FIG.  3   . As mentioned, the spindle drive comprises a spindle  34  and a brake piston  42 , which at the same time forms the spindle nut. The balls  48  of the spindle drive in the form of a recirculating ball spindle drive run in a ball recirculation track, which is formed by the ball raceways  50  of the transmission thread  46 . 
     The end wall  98  of the cup-shaped brake piston  42  has a central depression  99  and an annular pressing face  100  surrounding it for pressing against the inner brake lining  20 . 
     At the transition portion  40  there is formed on the spindle  34  a spherical bearing contact face  102  on which a bearing ring  104  of the axial bearing  72  abuts with a complementary, concave counter-contact face  106 . 
     The spindle  34  is hollow on the inside, in order to save mass.