Abstract:
A motor vehicle brake, in particular a combined hydraulically and electromechanically actuatable motor vehicle brake, with an actuator assembly comprising: a housing, an actuator which can be moved relative to the housing for hydraulically or electromechanically moving a brake lining, a motor drive, a movement mechanism which is arranged between the motor drive and the movable actuator, a gear arrangement which is paired with the movement mechanism, and a separate self-locking device which is designed to block the movement mechanism when necessary, wherein the movement mechanism has a ball screw drive with a rotatable spindle and a nut which can be moved in the housing in a linear manner. The nut can be moved within the housing in order to move the actuator by rotating the spindle, and the gear arrangement has at least two gear stages. In the motor vehicle brake, the self-locking device is arranged in or on the gear arrangement such that at least two gear stages are arranged between the ball screw drive and the self-locking device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage of International Application No. PCT/EP2014/072819, filed Oct. 24, 2014, the disclosures of which are incorporated herein by reference in entirety, and which claimed priority to German Patent Application No. 10 2013 018 946.2, filed Nov. 12, 2013, the disclosures of which are incorporated herein by reference in entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to an electromechanically and hydraulically actuatable motor-vehicle brake with an actuator assembly comprising:
       a housing,   an actuator, displaceable relative to the housing, for hydraulic or electromechanical displacement of a brake pad,   a motor drive,   a displacement mechanism arranged between the motor drive and the displaceable actuator,   a gear arrangement assigned to the displacement mechanism, and   a separate self-locking device which is designed to block the displacement mechanism as needed,   wherein the displacement mechanism exhibits a ball screw with a spindle and a nut, wherein optionally one component from spindle and nut is capable of being driven in rotation and the respective other component from spindle and nut is capable of being linearly displaced within the housing for the purpose of displacing the actuator by driving in rotation the one component from spindle and nut, and wherein the gear arrangement exhibits at least two gear stages.       
 
         [0010]    For some time, motor-vehicle brakes have been employed in which, in the case of a normal service braking during the travel of the vehicle, the brake pads are displaced hydraulically in the usual way, but in which for the purpose of activating a parking-brake function an electromechanical displacement of the brake pads or at least an electromechanically triggered blocking of the brake pads in a braking position also occurs. Motor-vehicle brakes of such a type offer the advantage that the parking-brake function can be activated or released more conveniently by simple actuating of an activating switch. 
         [0011]    Electromechanically and hydraulically actuatable motor-vehicle brakes of such a type are known from the state of the art. 
         [0012]    For instance, document WO 2008/037738 A1 describes a motor-vehicle brake that can be actuated both hydraulically and electromechanically. In a normal operating situation—that is to say, during the travel of the motor vehicle—this motor-vehicle brake is actuated hydraulically in conventional manner. For the purpose of activating a parking brake, however, the electromechanical actuation function is activated. In this case an electric motor is driven which drives a spindle/nut arrangement via a displacement mechanism with a gear arrangement. The gear arrangement is designed to be self-locking with a worm gear, in order to prevent the parking-brake action from diminishing when the parking brake has been activated. But the self-locking has the disadvantage that only very low efficiencies can be attained, so that the components, in particular the electric motor, have to be designed to be relatively strong and have a high current consumption. It is considered that the overall efficiency of the system is made up of a product of the individual efficiencies of the components. For instance, this overall efficiency results from the product of the efficiency of the engine, the efficiency of the downstream gear arrangement and of the spindle/nut arrangement. In gear arrangements with self-locking effect, overall efficiencies can accordingly only be attained within the range of 30% and less. 
         [0013]    Document DE 10 2012 208 294 A1 describes a vehicle brake in which a separate self-locking device is arranged directly on the spindle/nut arrangement. In particular, reference is made to FIG. 8 of this document. Therein it is shown that the self-locking device designated as coupling device 41 is arranged between the rolling-element ramp transmission and the brake piston. Such a direct spatial assignment of the self-locking device to the nut/spindle arrangement of a ball screw has the disadvantage that the reaction forces arising in the course of the activation of the brake and in the course of the maintenance of a (parking) braking state have to be borne in their full magnitude by the self-locking device. The self-locking device has to be constructed to be correspondingly sturdy, having the consequence that it takes up considerable construction space. This has the result that the brake has to be dimensioned to be relatively large, particularly in the axial direction with respect to the threaded spindle. 
         [0014]    Furthermore, document DE 10 2011 102 860 A1 presents a vehicle brake which has been improved in comparison with this state of the art, in which the self-locking device is in fact arranged close to the nut/spindle arrangement but is integrated into the gear arrangement, so that construction space can be saved in the axial direction with respect to the threaded spindle. This brake can be configured to be more compact. 
         [0015]    Document DE 196 54 729 A1 discloses a motor-vehicle brake having the features of the precharacterising portion of Claim  1 . Further state of the art is to be found in documents U.S. Pat. No. 5,829,845 A and DE 103 49 078 A1. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    A feature of the present invention is to make available a motor-vehicle brake that can be utilised both as service brake and as parking brake and that has been further optimised in comparison with the state of the art with regard to flux of force and dimensioning. 
         [0017]    This feature is achieved by a motor-vehicle brake of the type characterised in the introduction, in which there is provision that the self-locking device is arranged in or on the gear arrangement in such a manner that at least two gear stages are arranged between the ball screw and the self-locking device. 
         [0018]    The invention has the advantage that the at least two gear stages with their transmission ratio or reduction ratio can be exploited in order to reduce the reaction forces stemming from the brake pads, so that the self-locking device can be made smaller. Furthermore, the arrangement, according to the invention, of the self-locking device at a distance from the threaded spindle gives more freedom to make the entire motor-vehicle brake smaller, since the self-locking device can be arranged at a point where it requires less construction space. 
         [0019]    A further advantage according to the invention consists in the fact that the overall efficiency of the vehicle brake can be improved in comparison with the state of the art, in particular because the gear arrangement or the ball screw itself no longer has to be constructed to be self-locking. Consequently, efficiencies of clearly above 50% can be attained for the transmission of force, which also contributes to the fact that the motor drive can be designed to have lower current consumption, to be weaker and hence also of smaller construction. 
         [0020]    A preferred embodiment of the invention provides that the gear arrangement exhibits three or four gear stages. The choice of three or four gear stages makes it possible to achieve a relatively large transmission ratio to the benefit of a relatively small dimensioning of the motor drive, in which case the displacement mechanism can nevertheless be designed to be relatively compact. This number of gear stages also allows a favourable positioning of the self-locking device. 
         [0021]    In this context, according to a practical variant of the invention there may be provision that the self-locking device is arranged between the motor drive and the gear arrangement. This means that the entire gear arrangement with all the gear stages is exploited for the purpose of reducing the reaction forces stemming from the brake pads. Accordingly, the self-locking device can be dimensioned to be very weak and space-saving, and yet performs its action reliably. 
         [0022]    Almost similarly good effects can be achieved in a modification of this practical variant, in which there is provision that the self-locking device is positioned downstream of the gear stage of the gear arrangement nearest to the motor drive. In this practical variant, the self-locking device is capable of being integrated into the gear arrangement and utilises all the other gear stages, with the exception of the drive-side gear stage, for the purpose of reducing the reaction forces exerted by the brake pads in the braking state. 
         [0023]    In accordance with the invention there is preferentially provision that the self-locking device is arranged in such a manner that the reaction forces arising in the course of blocking the displacement mechanism can be indirectly or directly diverted into the housing. In this context, a further development of the invention provides that the self-locking device diverts the reaction forces arising in the course of blocking the displacement mechanism into a housing portion assigned to the gear arrangement, said housing portion being connected in force-transmitting manner to a housing portion bearing the brake-pad arrangement. The housing portion bearing the brake-pad arrangement is, as a rule, designed to be somewhat sturdier, which is advantageous for absorbing the reaction forces from the self-locking device. Furthermore, there may be provision that the self-locking device diverts the reaction forces arising in the course of blocking the displacement mechanism directly into the sturdier housing portion bearing the brake-pad arrangement. 
         [0024]    A preferred practical variant of the invention provides that the self-locking device is designed with a wrap-spring clutch which permits a transmission of torque from the motor drive to the spindle and which is designed for blocking a transmission of torque from the spindle to the motor drive. With such a wrap-spring clutch, high losses of efficiency can be avoided, since the wrap-spring clutch permits a largely direct and virtually loss-free transmission in its released state but reliably blocks in the closed state. In this context, in accordance with the invention there may be provision that a switching element is assigned to the wrap-spring clutch, which switching element in a first switching position permits a transmission of torque from the spindle to the motor drive and upon reaching a second switching position causes the wrap-spring clutch to block a transmission of torque from the spindle to the motor drive. Such a switching element may be, for example, a meandering spring. 
         [0025]    In this context there may furthermore be provision that the wrap-spring clutch as a wrap spring exhibits a helical spring, wound around a stator which is fixed in relation to the housing, with at least one winding and with two ends, which with its one end can be coupled in torque-transmitting manner with a first gearwheel, and with its second end can be coupled in torque-transmitting manner with a second gearwheel. In this case the at least one winding is in contact with a surface of the stator. In this practical variant of the invention there is preferably provision that each of the gearwheels exhibits at least one pawl which can be brought into torque-transmitting engagement with the end of the helical spring respectively assigned to the gearwheel. The pawls serve, so to speak, for force-transmitting and torque-transmitting coupling of the respective gearwheel with the helical spring arranged between said gearwheels. The helical spring is the force-transmitting element which permits a transmission of force from the motor drive towards the spindle and which blocks in the opposite direction. According to one practical variant of the invention, this is obtained, in particular, in that in the course of a transmission of torque from the motor drive to the spindle the helical spring expands radially somewhat with respect to the stator which is fixed in relation to the housing, so that it slides on the stator securely in the course of the transmission of torque, and in that in the case of a transmission of torque from the spindle to the motor drive the helical spring contracts radially with respect to the stator which is fixed in relation to the housing, acting radially on the stator, fixed in relation to the housing, in such a manner that it blocks a transmission of torque. In the course of blocking, the helical spring accordingly tightens around the stator which is fixed in relation to the housing, and in this way provides for strong friction forces, as a consequence of the wrapping, which ultimately prevent a further movement and hence transmission of torque via the helical spring. In this state, said spring blocks the transmission. 
         [0026]    If in a practical variant the motor-vehicle brake is employed both as service brake and as parking brake, for the function as service brake it may be necessary to permit a transmission of torque, within certain bounds, from the spindle towards the motor drive. But, for the stated reasons, such a transmission of torque should be prohibited within the scope of the parking-brake function. This means that opposing requirements, so to speak, exist for the function as service brake and for the function as parking brake. In one practical variant, the present invention removes the differentiation between service-brake function and parking-brake function by exploiting the fact that for the purpose of activating the parking brake significantly higher torques or tensional forces are ordinarily employed than within the scope of the service-brake function. Depending on the magnitude or absolute value of the tensional forces employed, ‘switching’ between service-brake function and parking-brake function can accordingly take place. Structurally, this switching is obtained by virtue of the fact that in this practical variant a switching element is assigned to the wrap-spring clutch, which switching element in a first switching position permits a transmission of torque from the spindle to the motor drive and upon reaching a second switching position causes the wrap-spring clutch to block a transmission of torque from the spindle to the motor drive. This switching element switches in a manner depending on the tensional force acting. In accordance with the invention, it is possible in this connection that the switching element includes at least one elastic deformation element. The at least one deformation element allows a transmission of force, within the bounds of a certain degree of deformation, in both directions between motor drive and spindle. Starting from a certain deformation induced by tensional force, the at least one deformation element reaches the second switching position in which a transmission of force then continues to be possible from the motor drive towards the spindle but is blocked in the opposite direction. In particular, in this case there may be provision that the switching element assumes its first or second switching position in accordance with a tensional force acting on the wrap spring between the first gearwheel and the second gearwheel. 
         [0027]    The invention provides that the gear arrangement exhibits a planetary gear mechanism. Furthermore, there is provision that the motor drive exhibits an electric motor, the output shaft of which exhibits a sun wheel of the planetary gear mechanism, in which connection a ring gear of the planetary gear mechanism is arranged to be fixed in relation to the housing, and in which connection planet wheels of the planetary gear mechanism are rotatably supported on a planet carrier which is rotatably supported in the housing. In this case it is in accordance with the invention, that the self-locking device is integrated into a sun wheel of the planetary gear mechanism. 
         [0028]    Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a spatial exterior view of an actuator assembly for elucidating the technical background of the motor-vehicle brake according to the invention; 
           [0030]      FIG. 2  is an axis-including sectional view through the actuator assembly of the motor-vehicle brake according to  FIG. 1 ; 
           [0031]      FIG. 3  is a cut-away view of the electric motor, the gear arrangement and the spindle; 
           [0032]      FIG. 4  is an exploded view for elucidating the wrap-spring clutch; 
           [0033]      FIG. 5  is an axially orthogonal sectional view of the arrangement consisting of two gearwheels with wrap-spring clutch; 
           [0034]      FIG. 6  is a schematic view of a first practical variant of the invention, in which the self-locking device is arranged between the first and second gear stages; and 
           [0035]      FIG. 7  is a schematic view of a second practical variant of the invention, in which the self-locking device is arranged between the motor drive and the first gear stage. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    In  FIG. 1  an actuator assembly of a motor-vehicle brake is shown in spatial representation and denoted generally by  10 .  FIGS. 1-5  serve for elucidation of the technical background of the invention. The actual exemplary embodiments will be described with reference to  FIGS. 6 and 7 . 
         [0037]      FIG. 1  shows a housing  12 , in which a gear arrangement is arranged, as well as a partial housing  14  for accommodating a drive motor, and a further partial housing  16  in which a displaceable piston  18  is arranged, with which a brake pad, not shown, in a brake unit of a motor-vehicle brake can be displaced so as to be effective as a brake. The actuator assembly  10 , shown in  FIG. 1 , of the motor-vehicle brake according to the invention can be installed in conventional manner, for example in a floating-calliper brake. In this regard, reference is made to document WO 2009/046899 A1, where such an installation situation is shown by way of example. This document is an application from the present applicant. Since in the following it is a matter of the actuator assembly consisting of motor drive and displacement mechanism for displacing the brake pad, the following description concentrates on these components. Also in the Figures, only corresponding components are shown. 
         [0038]      FIG. 2  shows an axis-including cross-sectional view through the drive unit  10  shown in  FIG. 1 . It will be discerned that the housing  12  has been constructed from many parts and is composed of a housing cover  20 , the partial housing  14  for reception of an electric motor  22 , and the partial housing  16  for displaceable reception of the piston  18 . The piston  18  is hydraulically and electromechanically displaceable along the longitudinal axis A in a manner known as such. Its exposed surface  24  cooperates in conventional manner with a brake-pad arrangement, not shown, for the purpose of achieving a braking action. A hydraulic displacement occurs in known manner during service braking. An electromechanical displacement occurs for the purpose of activating and releasing the parking-brake function. 
         [0039]    The electromechanical displacement mechanism will be discussed in detail in the following. 
         [0040]    The displacement mechanism includes a gear arrangement  26  and also a nut/spindle arrangement  28 . The two basic components of the displacement mechanism have been represented, cut away, in  FIG. 3  together with the electric motor  22 . The gear arrangement  26  serves to convert a rotary motion of the electric motor  22  into a corresponding linear motion of the piston  18  along the longitudinal axis A. In detail, the electric motor  22  exhibits a motor output shaft  30  extending along the longitudinal axis B of the motor. Said shaft is coupled in torsion-resistant manner with a toothed wheel  32 . The toothed wheel  32  serves as sun wheel of a planetary gear  34 . The toothed wheel  32  exhibits a journal  36  in its end remote from the motor. On this journal  36  a planet carrier  38  is rotatably supported which, in turn, again exhibits several journals  40 . The journals  40  serve to support planet wheels  42  which are in meshing engagement with the toothed wheel  32 . 
         [0041]    Radially outside the planet wheels  42  a ring gear  44  is arranged which is fixed in relation to the housing. A further housing part  46  serves for this purpose. The planet carrier  38  is rotatably supported in this housing part  46  with a journal  50 . Said planet carrier exhibits, between the journals  40  and its journal  50 , an external tooth gearing  52 . This external tooth gearing  52  meshes with a first gearwheel  54  which is rotatably supported on a stator  56 , fixed in relation to the housing, via a radially inner bearing portion  58 , and exhibits on its outer periphery an external tooth gearing  60 . The gearwheel  54  is pot-shaped. In the interior of said gearwheel there extends a friction-wheel portion  62  which has been formed on the stator  56  and hence has likewise been fitted in torsion-resistant manner in the housing  12 , in particular fitted in torsion-resistant manner to housing part  46  and to a further housing part  64 , for example by being pressed in. On the stator  56  a second gearwheel  66  is rotatably supported. This second gearwheel  66  can be coupled with the first gearwheel  54  via a wrap-spring clutch  70  for transmission of torque. This will be discussed in detail in the following. The second gearwheel  66  exhibits an external tooth gearing  72  on its portion facing away from the first gearwheel  54 . Said tooth gearing meshes with an external tooth gearing  73  of an output toothed wheel  74  which is supported in torsion-resistant manner on a spindle  76  of the nut/spindle arrangement  28 . 
         [0042]    It will be discerned in  FIG. 2  that the spindle  76  is supported in the housing part  16  via a radial needle bearing  78  and a thrust bearing. On its outer periphery the spindle  76  exhibits a thread formation  80  in which rolling elements  82  are received. The rolling elements  82  are held in a rolling-element cage  84  which via a helical spring  85  is biased into its initial position shown in  FIGS. 2 and 3 . Supported on the rolling elements  82  is a nut  86  which in conventional manner executes a linear movement within the housing part  16  in the course of a rotary motion of the spindle  76 . The nut  86  is firmly connected to a coupling element  88  which is displaced correspondingly in the course of the motion of the nut  86 . At its free end the coupling element  88  exhibits a conical coupling surface  90  which is capable of being brought into engagement with a corresponding piston surface  92  in the interior of the piston  18  and is displaceable together with the piston  18  for the purpose of displacing the same and hence for the purpose of displacing the brake pad, which is not shown. 
         [0043]    In  FIG. 2  there will further be discerned a connecting cable  94  as well as various seals  96  in order to seal, or to guide in sealed manner, the housing parts in relation to one another and components linked thereto. Finally, it will also be discerned that the piston  18  has been sealed with a bellows  98 . 
         [0044]    Particulars of the displacement mechanism will be discerned in  FIGS. 2 and 3 . 
         [0045]    Turning now to  FIGS. 4 and 5 , the structure and the individual components of the wrap-spring clutch  70  will be discerned therein. The stator  56  with its friction-wheel portion  62  is provided in the interior of the wrap-spring clutch  70 . As already stated, the stator  56  is supported in the housing so as to be fixed in relation to the housing and hence so as to be incapable of twisting via two journals  102 ,  104 . The first gearwheel  54  is supported on the stator  56 . Said gearwheel exhibits the external tooth gearing  60  on its outer periphery and is provided in its interior with a cavity  106 . The cavity  106  is provided with a side pocket  108  with abutment surface  110 . Furthermore, a circular-segment-shaped pawl  112  extends from a side surface of the first gearwheel  54 . The second gearwheel  66  exhibits, besides its external tooth gearing  72 , a plate-like structure  114  on which a first pawl  116  and a second pawl  118  are arranged which extend in the axial direction. Furthermore, in  FIG. 4  a helical spring  120  with cranked ends  122  and  124  will be discerned. The helical spring  120  is dimensioned in such a manner that in the relaxed state it bears snugly against the outer peripheral surface of the friction-wheel portion  62  but is able to slide on said surface. For the purpose of assembly, the helical spring  120  is placed onto the friction-wheel portion  62 . This arrangement is received in the cavity  106 . The two cranked ends  122  and  124  extend outwards in the radial direction so far that they are capable, as will be elucidated in detail in the following, of being brought into engagement with the pawls  112 ,  116 ,  118  without rubbing along or scratching the surfaces bounding the cavity  106 . 
         [0046]    In  FIG. 4  there will further be discerned a shaped spring  130  with several wound loops  132  which extend along a base strand  134 . The two ends  136 ,  138  engage one another. The wound loops  132  can be compressed out of their relaxed position into a compressed position when a force F is acting. The shaped spring  130  is arranged in the pocket  108  and with its last loop bears against the contact surface  110 . 
         [0047]    The assembled state of the wrap-spring clutch  70  will be discerned in  FIG. 5 , in which connection it should be added that the cranked spring end  122  is received between pawl  116  and pawl  118  in the intermediate space  140 . 
         [0048]    Generally, the motor-vehicle brake described above is employed with its actuator assembly described above primarily for the purpose of activating a parking-brake function. This means that in the service-brake case the piston  18  is ordinarily displaced hydraulically in such a manner that it is displaced out of the housing part  16 . Equally, it has to be possible that upon enabling of the motor  22  for the purpose of releasing the brake in the service-brake case the piston  18  is partly or entirely displaced back again into its initial position according to  FIG. 2 . As a rule, this happens solely by virtue of the effect of a relaxation on the brake pad, which is not shown, and also through the agency of the return spring  85 , without a motor drive by the electric motor  42  being required for this. For the service-braking case, let it be noted that relatively small tensional forces are required. 
         [0049]    In a parking-brake situation, the piston  18  is displaced with relatively large tensional force for the purpose of generating a parking-brake action and is intended to be held durably in this position in order to immobilise the motor vehicle securely. The piston position that has been reached should be maintained unconditionally for the purpose of retaining the parking-brake action, and the situation should be avoided where the piston  18  is displaced back in the course of time by virtue of settling processes in the gear arrangement  26 . Only when an active drive via the motor  22  occurs, with which the parking brake is to be actively released, can the piston  18  be displaced back again into its initial position according to  FIG. 2 . 
         [0050]    Accordingly, a distinction has to be made between the service-brake situation and the parking-brake situation, in which connection a flux of force from the piston  16  towards the motor  22  is to be permitted or prohibited, depending on the current state of the brake. In order to comply with this requirement, the wrap-spring clutch  70  is employed in the manner shown. The wrap-spring clutch  70  operates in interaction with the two gearwheels  54  and  66  as follows:
       Firstly, let the flux of force from the motor-side gearwheel  54  be considered, i.e. the case in which the motor  22  is driven and drives the gearwheel  54  in rotation through the agency of the planetary gear  34 . Here there are two directions of rotation, namely the direction of rotation of the gearwheel  54  in the anti-clockwise direction for tensioning the brake (tensioning the service brake and the parking brake) and the direction of rotation of the gearwheel  54  in the clockwise direction for active releasing of the brake (releasing of the parking brake).   In the course of a rotation of the first gearwheel  54  in the anti-clockwise direction corresponding to arrow P 1 —this corresponds to a motion for displacing the piston  18  out of the housing  16 , i.e. for activating the brake via the motor  22  both in a service-brake situation and in a parking-brake situation—the shaped spring  130  is displaced towards pawl  116  via the contact surface  110 . The greater the resistance becomes that pawl  116  sets against such a displacement (i.e. in the case of increasing tensional forces), the more strongly is the shaped spring  130  compressed. The helical spring  120  remains ineffective in the course of this actuation in the anti-clockwise direction corresponding to arrow P 1 , because its spring ends are able to move freely in the clearance between the pawls  112 ,  116  and  118 , and it slides on the friction-wheel portion  62 .   If greater tensional forces arise, as is the case in the course of the activation of the parking brake when the brake pads have to be pressed strongly against the brake disc via the piston  18 , the shaped spring  130  is compressed more strongly. If the shaped spring  130  is compressed more strongly in consequence of a rotation corresponding to arrow P 1 , this means that the first gearwheel  54  has now been displaced relative to the second gearwheel  56  corresponding to the compression of the shaped spring  130 . The maximum relative displacement is determined by virtue of the fact that pawl  112  bears with its end face  142 , through the agency of the cranked end  124 , against pawl  118  and the end face  144  thereof. Once this state has been reached, the shaped spring  130  has been maximally compressed, and any further rotation of the electric motor  22  is transmitted via the gear arrangement  26  to the nut/spindle arrangement  28  for further tensioning of the parking brake.   Once the parking brake has been tensioned, reaction forces resulting from the tensional force react on the transmission. These reaction forces, which are transmitted to pawls  116  and  118  via the nut/spindle arrangement  28 , the output toothed wheel  74  and the external tooth gearing  72 , try to push pawls  116 ,  118  back into their initial position—that is to say, to move them in the anti-clockwise direction. But as soon as pawl  116  tries to return to its initial state—that is to say, to move in the anti-clockwise direction—it comes into engagement with the cranked end  122  of the helical spring  120 . In consequence of this mutual engagement, due to the tendency of pawl  116  to move in the anti-clockwise direction the helical spring  120  is tightened around the friction-wheel portion  62 , so that the loops thereof narrow and act more strongly on the outer peripheral surface of the friction-wheel portion  62 . This wrapping of the outer peripheral surface of the friction-wheel portion  62  has the result that the helical spring  120  cannot slide any further onto this external peripheral surface but, so to speak, clings to it. Accordingly, pawl  116  cannot move back into its initial position. The parking brake remains activated. A settling process is therefore ruled out.   For the purpose of releasing the parking brake, once again a drive of the motor  22  is required. This is effected in such a manner that the gearwheel  54  is rotated in the clockwise direction. If the gearwheel  54  moves in the clockwise direction corresponding to arrow P 2 —this corresponds to a motion for displacing the piston  18  into the housing  16 , i.e. for active releasing of the brake via the motor  22  in the course of deactivating the parking brake—pawl  112  moves correspondingly likewise in the clockwise direction and releases the tension on the shaped spring  130 . In addition, the helical spring  120  is released and sets pawl  118  free. Said pawl can—following pawl  112 —move back, this occurring under the action of the return spring  85 .       
 
         [0056]    The wrap-spring clutch  70  accordingly has the general effect that for the parking-brake function it ensures that a transmission of torque from the motor to the nut/spindle arrangement also occurs in the case of high tensional forces and that settling effects or an inadvertent releasing of the parking brake is prevented by the action of the wrap-spring clutch  70 , in particular by the action of the then tightening helical spring  120  acting firmly on the outer peripheral surface of the friction-wheel portion  62 . 
         [0057]    If merely a parking-brake function is to be realised, in which case the actuation in the service-brake case is effected purely hydraulically, the shaped spring  130  is an option and may be omitted. 
         [0058]    But if with the brake both a service-brake function and a parking-brake function with electromechanical activation are to have been provided, additionally the shaped spring  130  is employed which deforms more or less strongly, depending on the current state of tension. In the case of low tensional forces such as ordinarily arise during service braking, the shaped spring  130  is deformed not at all or only to a slight degree, so that it, so to speak, keeps the helical spring  120  (wrap spring) inactive. As a result, a self-locking in the transmission can be prohibited for the service-brake function. A flux of force through the transmission in both directions between motor drive and nut/spindle arrangement is then possible. Only when the shaped spring  130  has been compressed sufficiently does the helical spring  120  (wrap spring) become active, so to speak, in its function and block a flux of force through the transmission from the nut/spindle arrangement towards the electric motor. The shaped spring  130  is accordingly required in the practical example shown, because both brake functions—service-brake function and parking-brake function—are to be made available. 
         [0059]    Proceeding from this understanding of the embodiment according to  FIGS. 1 to 5 , the invention sets about an optimisation of this embodiment. A first practical example of the invention is shown schematically in the block diagram according to  FIG. 6 . 
         [0060]    In  FIG. 6  it can be discerned that the electric motor EM driven by an electrical supply source EV delivers a motor torque MM to a first gear stage GS 1  of the gear arrangement via an output shaft and at an angular velocity of the motor ω M . Arranged between the first gear stage GS 1  and the second gear stage GS 2  is the self-locking device SHV, with which reaction forces R are diverted to the housing portion AG which is assigned to the gear arrangement with the individual gear stages GS 1  to GS 3  and to the electric motor EM. This housing portion may also be designated as the actuator housing portion AG. The reaction forces R can then be diverted to the housing portion of the brake BG by suitable coupling of the actuator housing portion AG and of the housing portion of the brake BG, elucidated in more detail in the following, corresponding to the dashed arrow R′. 
         [0061]    Positioned downstream of the second gear stage GS 2  is a further gear stage GS 3 , starting from which, and at an angular velocity of the actuator ω A , a torque M A  is transmitted to the brake. The brake includes a spindle/nut arrangement SM of a ball screw and a brake piston BK, proceeding from which the brake-actuating force F B  is transmitted to the brake pads at an actuation speed v B . Reaction forces are retroactive from this brake piston BK in the braking case. 
         [0062]    The peculiarity of this embodiment lies in the fact that the self-locking device SHV is arranged in such a manner that the two gear stages GS 2  and GS 3  are arranged between the ball screw with its spindle/nut arrangement SM and the self-locking device SHV. This makes it possible to exploit the transmission ratio of the two gear stages GS 2  and GS 3  as gear reduction for the retroactive reaction forces from the brake piston BK in a parking-brake situation, so that the reaction forces acting ultimately on the self-locking device SHV have been distinctly reduced. As a result, the self-locking device SHV can be dimensioned to be relatively small, thus saving construction space in particular. Furthermore, this arrangement of the self-locking device SHV offers freedom in the design and accommodation of the self-locking device SHV within the gear arrangement. 
         [0063]    A further embodiment of the invention is shown in  FIG. 7 . Therein it will be discerned that the self-locking device SHV is arranged between the electric motor EM and the first gear stage GS 1 , so that the transmission ratio of all three gear stages GS 1  to GS 3  can be exploited as reduction gear for the retroactive reaction forces from the brake piston BK. 
         [0064]    The advantages of the invention accordingly lie in the arrangement of the self-locking device SHV within the gear arrangement with the individual gear stages GS 1  to GS 3  relative to the actual components of the brake, namely the ball screw (spindle/nut arrangement SM). By virtue of this arrangement, transmission ratios of the gear stages GS 1  to GS 3  can advantageously be exploited in order to reduce the reaction forces. Furthermore, this arrangement offers greater variability relating to the saving of construction space. 
         [0065]    In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.