Abstract:
The present invention relates to a hydraulic vehicle brake equipped with a parking brake device, in particular for motor vehicles, including a brake housing in which a hydraulic service pressure chamber is delimited by a brake piston, wherein the parking brake device acts on the brake piston and, in the applied condition, can be locked by means of a locking device, and an energy accumulator cooperating with the brake piston is equipped with at least one integrated spring element. In order to realize a simple and low-cost parking brake function that complies with legal requirements, according to the invention, the parking brake device is operable by a pressure that is introduced into the service pressure chamber and enables charging the energy accumulator.

Description:
TECHNICAL FIELD  
       [0001]     The present invention generally relates to hydraulic vehicle brakes equipped with parking brake devices and more particularly relates to a hydraulic vehicle brake equipped with a parking brake device, in particular for motor vehicles, including a brake housing in which a hydraulic service pressure chamber is delimited by a brake piston, wherein the parking brake device acts on the brake piston.  
       BACKGROUND OF THE INVENTION  
       [0002]     DE 197 11 382 C2 discloses a hydraulic vehicle brake of this type.  
         [0003]     In the prior art brake, the brake is applied in the parking brake mode by means of a two-stage gear driven by an electric motor. A parking brake operation is effected by way of energizing the electric motor, while the spring element of the energy accumulator is preloaded in addition. A reversed rotation of the electric motor is necessary for the release operation of the parking brake.  
         [0004]     Also, combined service and parking brakes are known in the art, wherein there is provision of a hydraulic actuating device for service brake operations and mechanically actuated locking devices as a parking brake. However, these assemblies suffer from the shortcoming that an additional brake cable must be provided apart from the hydraulic supply line for each brake. The result is an increased expenditure in material and manufacture. In addition, the brake cable can be mounted to the vehicle only after installation of the brake. This assembly is usually not carried out by a particularly competent brake manufacturer, but is generally done by the vehicle manufacturer. Beside the disadvantage incurred by an increase assembly effort, the vehicle manufacturer must additionally bear the risk of a faulty assembly.  
         [0005]     Further, German published application DE 42 05 590 C1 discloses a brake system wherein a service brake operation is carried out hydraulically and a parking brake operation is assisted by an electromotive adjusting apparatus. An electromotive adjusting apparatus of this type, which acts directly on the position of the brake pedal, admittedly, relieves the driver in terms of the force effort to be made in a parking brake operation, yet entails additional structural effort. The result is that additional costs are incurred.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     In view of the above, an object of the invention is to improve a hydraulic vehicle brake with a parking brake device of the type mentioned hereinabove to such effect that the parking brake function is realized in a simple and low-cost manner, while legal requirements are complied with.  
         [0007]     According to the invention, this object is achieved in that the parking brake device is operable by a pressure that is introduced into the service pressure chamber and enables charging the energy accumulator.  
         [0008]     In a favorably straightforward embodiment of the invention, the energy accumulator is formed of a recess in the brake piston accommodating the spring element, with said spring element being supported on a plate that is in a force-transmitting connection with the brake piston and cooperates with a first friction lining.  
         [0009]     A favorable improvement of the subject matter of the invention provides that the locking device is a threaded-nut/spindle assembly, the threaded nut thereof being supported on the brake piston or being integrally designed with the brake piston, while the spindle includes a first friction surface interacting, in the locked condition, with a second friction surface that is arranged in an unrotatable manner in the brake housing.  
         [0010]     Besides, the spindle is preferably equipped with an axial central bearing, which cooperates with a hydraulic or electromechanical device for activating the locking device.  
         [0011]     It is particularly advantageous that the hydraulic device is formed of a closable pressure chamber being in connection with the service pressure chamber, and of an operating piston delimiting the pressure chamber and being preloaded by means of a spring assembly. Said operating piston cooperates with the spindle in such a fashion that, after decrease of the pressure prevailing in the pressure chamber, the force produced by the spring assembly is transmitted to the spindle in order to thereby cause disengagement of the friction surfaces of the locking device. The operating piston can have a two-part design, consisting of a first operating piston part and a second operating piston part, and a spring assembly is interposed between the two operating piston parts so as to allow a movement of the first operating piston part relative to the second operating piston part.  
         [0012]     The force generated by the spring assembly is transmitted to the spindle preferably by means of the first operating piston part, while the second operating piston part has an annular design and radially encompasses the first operating piston part.  
         [0013]     In a favorable improvement of the subject matter of the invention, the electromechanical device is formed of an actuating element that is in a force-transmitting connection with the spindle, and of a second actuating element operable by means of an electromagnetic arrangement, with both actuating elements including interacting slopes or ramps, respectively.  
         [0014]     In another favorable design of the invention, there is provision of another spring that is supported on the brake housing and biases the spindle in the direction of the second friction surface.  
         [0015]     At least part of the locking device is arranged in the energy accumulator in another favorable improvement of the subject matter of the invention.  
         [0016]     The energy accumulator in this embodiment is formed of a closable accumulator pressure chamber being in connection with the service pressure chamber, and of an accumulator piston delimiting the accumulator pressure chamber and being in a force-transmitting connection with an adjusting ring that is arranged so as to be rotatable within limits. The spring element is supported on the adjusting ring, which is movable into engagement with a spindle that is connected to the brake piston by means of a non-self-locking thread, with the locking device being formed of the adjusting ring and the spindle.  
         [0017]     The adjusting ring includes guiding projections being guided in differently long guiding grooves that are provided in the wall of a housing accommodating the energy accumulator offset in its circumferential direction, and the length of the short guiding groove defines the released position of the locking device, and the length of the longer guiding groove defines the locked position of the locking device.  
         [0018]     In another especially favorable design variant of the subject matter of the invention, the service pressure chamber is delimited by the brake piston on one side and by an accumulator piston on the other side, on which piston a spring assembly is supported. A first contact or friction surface and a second contact or friction surface are moved into engagement with each other upon actuation of the locking device, while they are disengaged during release.  
         [0019]     The mentioned locking device is formed of a threaded-nut/spindle assembly whose spindle is connected to the brake piston, while the threaded nut is provided with a first friction surface cooperating, in the locked condition, with a second friction surface provided in the accumulator piston, and wherein an electromagnetic device is arranged exerting a tension force on a tension-force transmitting element rigidly connected to the threaded nut and, thus, causing a rigid locking engagement between the brake piston and the accumulator piston.  
         [0020]     A non-self-locking thread is interposed between the threaded nut and the spindle.  
         [0021]     In another favorable improvement of the invention, the energy accumulator is formed of a closable accumulator pressure chamber connected to the service pressure chamber and of an accumulator piston delimiting the accumulator pressure chamber, with the connection between the service pressure chamber and the accumulator pressure chamber being closable by means of a mechanically operable separating valve.  
         [0022]     The locking device in this design is provided by a threaded-nut/spindle assembly, whose spindle is driven by an electric motor, while the threaded nut includes the first contact surface that cooperates, in the locked condition, with a second contact surface designed in the brake piston.  
         [0023]     A self-locking thread is provided between the threaded nut and the threaded spindle in these embodiments.  
         [0024]     In another favorable improvement of the invention, the separating valve is operable by the threaded nut.  
         [0025]     Besides, a second connection is provided between the service pressure chamber and the accumulator pressure chamber, in which a non-return valve opening towards the service pressure chamber is inserted.  
         [0026]     It is especially favorable in this respect when a means is provided to release the parking brake in a case of emergency, said means cooperating with the operating piston or the accumulator piston, respectively. The hydraulic pressure chamber or the accumulator pressure chamber, respectively, is closable by means of an electrically operable valve.  
         [0027]     It is arranged for in another favorable embodiment that an arresting unit maintains the energy accumulator in its charged condition during service brake operations. The arresting unit is then formed of at least one electromagnet whose coil fulfils the function of a sensor for sensing the position of a slide actuated by the armature of the electromagnet.  
         [0028]     Furthermore, the coil fulfils the function of a sensor for monitoring the pressure introduced into the service pressure chamber or for detecting the condition of the vehicle brake or the parking brake device.  
         [0029]     In a particularly advantageous embodiment, the arresting unit is formed of at least two electromagnets, whose armatures are connected to the slide, with the coil of the first electromagnet actuating the slide, while the coil of the second electromagnet performs the function of a sensor for detecting the slide position. As this occurs, the coils assume the function of a sensor for detecting the slide position, unless they perform the function of an actuator for actuating the slide.  
         [0030]     In an alternative embodiment, the arresting unit is a piezoelectrically actuated actuator that actuates a slide and performs the function of a sensor for detecting the slide position. Besides, the piezoelectric actuator fulfils the function of a sensor for monitoring the pressure introduced into the service pressure chamber and for detecting the condition of the vehicle brake or the parking brake device.  
         [0031]     Further favorable embodiments provide that the pressure buildup both in the service pressure chamber and in the hydraulic pressure chamber takes place by means of a hydraulic pump which is used as an independent pressure source of an electrohydraulic brake system, or by means of a pressure generator operable by the vehicle operator.  
         [0032]     Expediently, the actuation of the vehicle brake of the invention can also make use of other electrically operable energy sources that are already provided in the brake system (such as an independently operable brake booster, plunger drive, high-pressure accumulator with electric valves, etc.). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is an axial cross-sectional view of a first design of the hydraulic vehicle brake of the invention in the released condition.  
         [0034]      FIGS. 2   a, b  are simplified views of a second design of the hydraulic brake of the invention in the released and locked conditions.  
         [0035]      FIGS. 3   a, b  are axial cross-sectional views of a third design of the hydraulic brake of the invention in a representation corresponding to  FIGS. 2   a, b.    
         [0036]      FIG. 4  is an axial cross-sectional view of a fourth design of the hydraulic brake of the invention.  
         [0037]      FIG. 5  is a partial view of a fifth design of the hydraulic brake of the invention.  
         [0038]      FIG. 6  is an axial cross-sectional view of a sixth design of the subject matter of the invention.  
         [0039]      FIG. 7  is an axial cross-sectional view of a seventh design of the hydraulic brake of the invention, shown in part only.  
         [0040]      FIGS. 8   a  to c show lateral views of designs of an electromagnetically or piezoelectrically actuated arresting unit employable in the seventh embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     The hydraulic vehicle brake of the invention shown in the drawings includes a brake housing  1  straddling the outside edge of a brake disc  2  and two brake pads  3 ,  4  arranged on either side of the brake disc  2 . The brake housing  1  forms on its inside a brake cylinder  5  receiving a brake piston  6  in an axially slidable manner. By way of a hydraulic port  8 , brake fluid can be fed into the service pressure chamber  7  formed between brake cylinder  5  and brake piston  6 , whereby braking pressure develops that displaces the brake piston  6  axially towards the brake disc  2 . This will urge the brake pad  3  facing the brake piston  6  against the brake disc  2 , whereupon the brake housing  1 , as a reaction, displaces in an opposite direction and thereby urges also the other brake pad  4  against the brake disc  2 .  
         [0042]     As can be taken from  FIG. 1  in particular, an energy accumulator designated with reference numeral  10  is provided in the area of the brake piston  6  close to the first friction lining  3 . Said energy accumulator  10  is mainly comprised of an annular recess  11  designed in the brake piston  6  and a spring element  12  arranged in the recess  11  and resting against a plate  12 , which latter is movable within limits in relation to the brake piston  6  and connected to it. It is achieved by the mentioned provisions that the application force acting on the brake pads  3 ,  4  is almost independent of thermally induced changes in length in the area of the brake caliper.  
         [0043]     A spindle drive or a threaded-nut/spindle assembly, respectively, provided with reference numeral  14 , forms a locking device, which is necessary for realizing a parking brake function in the first design illustrated in  FIG. 1 . The mentioned threaded-nut/spindle assembly  14  comprises a threaded nut  15  and a spindle  16  being in connection with each other by means of a non-self-locking thread. In this arrangement, the threaded nut  15  is rigidly connected to the brake piston  6 , while the spindle  16  at its end remote from the brake piston  6  includes a preferably conical first friction surface  17 , which can be moved into and out of engagement with a second friction surface  18  arranged in the brake housing  1  in an unrotatable fashion. A spring  19  supported on the brake housing  1  biases the spindle  16  in the direction of the second friction surface  18  by the intermediary of an axial bearing  20 . Besides, the spindle  16  includes a central bearing  21  at its end remote from the brake piston  6 . The function of the central bearing will be explained in the following.  
         [0044]     Also, the brake housing  1  houses a cylindrical pressure chamber  22  which is connected to the hydraulic port  8  and the service pressure chamber  7 , respectively. Inserted into the connection  23  of the cylindrical pressure chamber  22  to the port  8  is an electrically operable valve, preferably a normally closed (NC) valve  24  that allows connecting the two pressure chambers  7  and  22  by way of connection  23 . A non-return valve  26  closing towards the pressure chamber  22  is inserted into another connection  25  between the cylindrical pressure chamber  22  and the service pressure chamber  7 . Along with the pressure chamber  22 , an operating piston  27  delimiting the pressure chamber  22  forms a hydraulic device for activating the above-mentioned locking device. The operating piston  27  that is biased in the direction of the locking device by means of a spring assembly  28  preferably has a two-part design and comprises a first operating piston part  27   a  that is axially supported on the central bearing  21  and an annular second operating piston part  27   b  which radially encompasses the first operating piston part  27   a . A spring assembly  29  is interposed between the two operating piston parts  27   a ,  27   b  in such a way that a limited movement of the first operating piston part ( 27   a ) relative to the second operating piston part  27   b  can occur upon release of the parking brake. Besides, a mechanical means  30  for releasing the locking device is provided, which can be designed as a threaded pin cooperating with the first operating piston part  27   a , for example.  
         [0045]     The first design of the hydraulic vehicle brake of the invention is illustrated in  FIG. 1  in the released condition of the parking brake. The parking brake is locked after buildup of hydraulic pressure in the service pressure chamber  7 , by changing over the normally closed valve  24  into its open operating position or by applying hydraulic pressure to the operating piston  27 . When the operating piston  27  is moved to the right in the drawing, the spindle  16  is moved until engagement of the friction surfaces  17 ,  18 . To release the parking brake, hydraulic pressure is built up in the service pressure chamber  7 , causing relief of the spindle  16  so that the friction surfaces  17 ,  18  are disengaged by way of the effect the spring assembly  28  takes on the operating piston  27 .  
         [0046]     In the second design of the brake of the invention as shown in  FIGS. 2   a , b, the device for activating the locking device mentioned with respect to the first design shown in  FIG. 1  is configured as an electromechanically or electromagnetically operable device  35  which is, essentially composed of an actuating element  31  that is in a force-transmitting connection with the spindle  16 , and of a second actuating element  32  operable by means of an electromagnetic assembly  36  and being preferably formed of the armature of the electromagnetic assembly  36 . The first actuating element  31  includes a first slope or ramp  33  cooperating with a second slope or ramp  34  designed at the second actuating element  32 . To minimize the friction occurring between the slopes  33 ,  34 , schematically shown rolling elements designated by reference numerals  37  are provided. Further, an elastic means  38  is provided for retaining the second actuating element  32  in its end positions, said means being preferably configured as a compression spring having two supporting end pieces. As can be seen in the drawing,  FIG. 2   a  shows the second design in the released condition of the locking device, while  FIG. 2   b  illustrates the locking device in the locked condition.  
         [0047]     In the third design, which is exhibited in  FIGS. 3   a, b  in the released or locked condition of the locking device, respectively, the energy accumulator  10  is arranged at the end of the spindle  16  remote from the brake piston  6  and essentially contains a hydraulic accumulator pressure chamber  40 , an accumulator piston  41  delimiting the accumulator chamber  40  and the above-mentioned spring element  12  that is configured as a cup spring in the example shown. The spring element  12  is supported through an axial bearing  43  on an adjusting ring  42  that is in a force-transmitting connection with the control piston  41  and is arranged so as to be rotatable within limits. The accumulator pressure chamber  40  is connected by way of a hydraulic connection  44  to the hydraulic port  8  mentioned with respect to the first design according to  FIG. 1 , and an electromagnetically operable, preferably normally closed (NC) valve  45  is inserted into the connection  44 , said valve being used to close or open the connection  44 . In this arrangement, the brake piston  6  forms a threaded nut cooperating with the spindle  16  by way of a non-self-locking thread. Spindle  16  is supported on an axial bearing  51  by means of a radial bead  50 . The end of the spindle  16  remote from the brake piston  6  that extends through the control piston  41  in an axial direction has fine teeth being movable into engagement with a mating design of a medium recess  46  in the adjusting ring  42 . The fine teeth along with the adjusting ring  42  form the locking device, said adjusting ring  42  including guiding projections  47  that cooperate with guiding grooves  48 ,  49  of different length being designed in the wall of a cylindrical housing  52  in which the energy accumulator  10  and the locking device are incorporated. The length of the short guiding groove  48  determines the end position of the adjusting ring  42  that corresponds to the release condition of the locking device, while the boundary of the longer guiding groove defines the end position of the adjusting ring  42  corresponding to the locked condition of the locking device. Further, the long guiding groove  49  accommodates a projection  53  provided at the control piston  41  and serving as a rotation prevention mechanism of the control piston  41 . When the parking brake device is being locked, hydraulic pressure is applied to the control piston  41 , displacing it to the right in the drawing so that the adjusting ring  42  is lifted from its locked position shown in  FIG. 3   a  and starts rotating. As this occurs, it proves particularly favorable that the effective diameter of the control piston  41  is selected to be larger than the effective diameter of the brake piston  6  in order to reduce the activating pressure of the parking brake device. When the pressure acting on the control piston  41  is subsequently reduced, the adjusting ring  42  is shifted to the left by the force of the cup spring  12  so that its internally toothed recess  46  is moved into engagement with the finely toothed end of spindle  16 . As shown in  FIG. 3   b , the spindle  16  is locked against rotation by way of engagement with the adjusting ring  42  so that the force generated by the cup spring  12  is transmitted onto the brake piston  6 . The above-mentioned radial bead  50  of the spindle  16  is at a distance a from the axial bearing  51  in the locked condition of the parking brake device.  
         [0048]     To release the parking brake device, the valve  45  is first of all opened by means of a corresponding actuating signal, the control piston  41  is again acted upon by hydraulic pressure and displaced to the right in the drawing so that the adjusting ring  42  is lifted from its position shown in  FIG. 3   b , continues rotating and, in a subsequent reduction of the pressure acting on the control piston  41 , adopts the end position illustrated in  FIG. 3   a  which is predetermined by the stop of its guiding projection  47  at the boundary of the short guiding groove  48 .  
         [0049]     The arrangement of the energy accumulator  10  in the fourth embodiment of the subject matter of the invention illustrated in  FIG. 4  corresponds to the third design according to  FIGS. 3   a, b . In this design the service pressure chamber  7  is additionally used as an accumulator pressure chamber, which is delimited by an accumulator piston  54  on the right-hand side in the drawing. The above-mentioned spring element is formed of a spring assembly  60  being supported on the accumulator piston  54 . The locking device is again configured as a threaded-nut/spindle assembly, having its spindle  56  rigidly connected to the brake piston  6 . A threaded nut  55  coupled with the spindle  56  by way of a non-self-locking thread includes a first friction surface  57 , which cooperates with or is moved into engagement with a second friction surface  58  when the parking brake device is locked. In this arrangement, the second friction surface  58  is preferably designed at the end of an axial extension  59  of the accumulator piston  54 , said extension extending in the direction of the brake piston  6 . Moreover, there is provision of an electromagnetic device  61 ,  62  being received by the extension  59 . Armature  61  of the electromagnetic device  61 ,  62  forms a tension force transmitting element which is rigidly connected to the threaded nut  55  so that, upon energization of the coil  62 , the armature  61  is tightened and closes the slot between the two friction surfaces  57 ,  58 , with the result that a rigid coupling between the brake piston  6  and the accumulator piston  54  is safeguarded.  
         [0050]     Likewise the fifth embodiment of the invention illustrated in  FIG. 5  has a similar design of the locking device. The accumulator piston  64  is additionally used as a radial guide of the spindle  66  of the threaded-nut/spindle assembly, with a cardan-type joint  67  being designed at the end of the spindle  66 . Said joint  67  cooperates with a toothed wheel  68  of an angular gear  70  that is used to transmit the rotation of the output shaft of an electric motor  69  onto the spindle  66 . A threaded nut  65  coupled to the spindle  66  by means of a self-locking thread includes a first contact surface  71 , which cooperates with or is moved into engagement with a second contact surface  72  when the parking brake device is locked. Said second contact surface  72  is preferably designed in the brake piston  6 .  
         [0051]     When the parking brake is applied, hydraulic pressure is built up in the service pressure chamber  7 , exactly as in the design according to  FIG. 4 , said pressure causing displacement of the brake piston  6  to the left in the drawing, on the one hand, and movement of the accumulator piston  64  to the right, on the other hand, so that the spring assembly designated by reference numeral  63  is biased to a still greater extent. A subsequent actuation of the electric motor  69  and a rotation of the spindle  66 , respectively, will cause a translational movement of the threaded nut  65  in the direction of the brake piston  6  until the contact surfaces  71 ,  72  are moved into engagement with each other. This action causes the parking brake to assume a locked condition. When the parking brake is released, hydraulic pressure is again built up in the service pressure chamber  7  so that the spindle  66  is relieved and, due to actuation of the electric motor  69 , turned back in the direction opposite to the actuating direction so that the contact surfaces  71 ,  72  are disengaged and the brake piston  6  can be passed over into a non-actuated position.  
         [0052]     In the sixth design of the subject matter of the invention depicted in  FIG. 6 , the service pressure chamber  7  is isolated from the accumulator pressure chamber  83 , and a separating valve  77  that is operable preferably mechanically is inserted into the connection of the two chambers  7  and  83 . Like in the previous design, the locking device is again configured as a spindle/nut assembly, the spindle being designated by reference numeral  76 , the threaded nut by reference numeral  75 , and the corresponding contact surfaces being designated by reference numerals  81 ,  82 . Further, a non-return valve  78  opening towards the service pressure chamber  7  is provided in a second connection between the service pressure chamber  7  and the accumulator pressure chamber  83 . An electric motor  79  driving the spindle  76  is arranged so that its longitudinal axis extends in parallel to the longitudinal axis of the spindle/nut assembly. The threaded nut  75  is of course guided in the brake piston  7  in an unrotatable fashion. Because the mode of functioning of the sixth design basically corresponds to the fifth design, it need not be explained in detail.  
         [0053]     In a seventh design which is illustrated in the released condition of the locking device in  FIG. 7 , the energy accumulator  10  is arranged at the side of the brake housing  1  remote from the brake piston  6  and mainly comprises an accumulator piston  94  delimiting the service pressure chamber  7  and a spring element  90 . Similar to the embodiment shown in  FIG. 4 , the service pressure chamber  7  in this design is additionally used as an accumulator pressure chamber. Again the locking device is configured as a threaded-nut/spindle assembly having its threaded nut  15  integrally designed with the brake piston  6 . A spindle  16  coupled to the threaded nut  15  by means of a non-self-locking thread includes a first friction surface  97 , which cooperates with a second friction surface  98  at the accumulator piston  94  when the parking brake device is locked.  
         [0054]     Further, an arresting unit  91  is provided being shown in a lateral view in  FIG. 8   a . The arresting unit  91  is arranged outside the energy accumulator  10  and formed by an electromagnet  95  whose armature  92  is rigidly connected to a slide  93 . The accumulator piston  94  is blocked by the slide  93  during service brake operations by way of preventing a translational movement of a force-transmitting part  96  that is rigidly connected to the accumulator piston  94  in the direction of the brake piston  6 . For this purpose, the force-transmitting part  96  is so designed that it includes two projections being received in notches in the slide  93 . As can be taken from  FIG. 8   a  in particular, the force-transmitting part  96  is inhibited in moving in the direction of the brake piston  6  by the projections taking support on the slide  93 . When the slide  93  is moved by the electromagnet  95  in such a fashion that the projections of the force-transmitting part  96  align with the notches at the slide  93 , the force-transmitting part  96  and the accumulator piston  94 , respectively, can move in the direction of the brake piston  6 . This position of the slide  93  and the armature  92  is shown in dotted lines in  FIG. 8   a . A translational movement of the force-transmitting part  96  in the opposite direction beyond the position illustrated in  FIG. 8   a  is likewise suppressed because the accumulator piston  94  in this direction comes into abutment with respect to the brake housing  1 . This prevents an additional volume intake in the service pressure chamber  7  during service brake operations, which volume intake would be caused by an accumulator piston  94  moving to the right in the drawing.  
         [0055]     When the parking brake device is applied, hydraulic pressure is built up in the service pressure chamber  7  causing both displacement of the brake piston  6  to the left in the drawing and relief of the accumulator piston  94  in opposition to the effect of force of the spring element  90 . The slide  93  is operable after this relief and can release the force-transmitting part  96  because the notches of the slide  93  are brought into coincidence with the projections of the force-transmitting part  96 . After the actuation of slide  93 , the hydraulically biased spring element  90  induces a translational movement of the accumulator piston  94  in the direction of the brake piston  6  until the friction surfaces  97 ,  98  are engaged, with the result that the parking brake device is set into a locked condition. As this occurs, the spindle  16  lifts from the central bearing  21 , and the spring assembly  90  acts by means of the closed flux of force from the accumulator piston  94  through the threaded-nut/spindle assembly to the brake piston  6  and generates a component of the application force that is necessary to perform the parking brake operation. To release the parking brake device, again hydraulic pressure is built up in the service pressure chamber  7  and the accumulator piston  94  is displaced to the right in  FIG. 7 , with the spring element  90  being hydraulically biased. It is favorable in this arrangement that the effective diameter of the accumulator piston  94  is chosen to exceed the effective diameter of he brake piston  6 , with the result that the activation pressure of the parking brake device is reduced. The accumulator piston  94  is blocked again by the slide  93  by means of the force-transmitting part  96  in the released condition of the parking brake device. Further, a mechanical means  30  can be provided to release the locking device.  
         [0056]     The coil  89  of the electromagnet  95  fulfills the function of a sensor for detecting the position of the slide  93 , where it can be detected whether the force-transmitting part  96  is freed or blocked, or whether the parking brake device is released or locked, respectively. This can prevent an inadvertent locking of the parking brake device during the service brake operations. Besides, the condition of the vehicle brake or of the parking brake device can be found out by determining the slide position. When e.g. the brake pads are worn out or the spring  19  (whose function has been described already by way of  FIG. 1 ) preloading the spindle  16  is defective, the brake piston  6  will move during actuation further to the left in the drawing than in the normal case. The result is that the accumulator piston  94  freed by the slide  93  for locking the parking brake device will also move further to the left. The end of the force-transmitting part  96  remote from the brake piston  6  is thereby positioned outside the range of motion of the slide  93 . When the slide  93  is actuated in this situation, it will move further away from the electromagnet  95  than in the previously explained arresting positions. This condition is detected by a determination of the slide position.  
         [0057]     To reliably determine the slide position, the self-inductance variation of the coil  89  of the electromagnet  95  induced by the armature movements is determined. This is done by means of applying square-wave voltage pulses to the coil  89 . The variation of the current flowing through the coil  89  is additionally determined. This current variation leads to conclude the position of the armature  92  and, hence, the position of the slide  93 . When the position of the armature  92  changes, the variation of the current flowing through the coil  89  will change as well.  
         [0058]      FIG. 8   b  depicts an arresting unit  91  having two electromagnets  95 ,  105 . Both armatures  92 ,  102  are rigidly connected to the slide  93  in this design. This is advantageous because it renders possible a mutual control by means of a comparison of the two armature positions, because the self-inductance variations of the two coils  89 ,  109  can be separately determined. In addition, the position can be determined continuously because the first coil  89  actuates the armature  92  and the slide  93  connected to it as well as the second armature  102 , while the position of the two armatures  92 ,  102  and the slide  93  is determined by means of the change of the current variation in the second coil  109 . Also, this arrangement can monitor the pressure introduced into the service pressure chamber  7 , When e.g. during pressure buildup which is necessary for the parking brake operation, the first electromagnet  95  is actuated in such a fashion that it would actuate the two armatures  92 ,  102  and the slide  93  as soon as the accumulator piston  92  is adequately relieved due to the pressure buildup, this pressure value can be detected by way of the method with the second electromagnet  105  described already. A corresponding operation may, of course, also be performed during pressure buildup for terminating a parking brake operation.  
         [0059]      FIG. 8   c  illustrates another embodiment of the arresting unit  91 . Arresting unit  91  is herein configured as a piezoelectric actuator  103 , which is basically formed of a piezoelectrically actuated transverse beam  101 . Said transverse beam  101  is received in a recess at slide  93  and adopts an intermediate actuating position in the voltageless condition. By applying a voltage to the transverse beam  101 , the transverse beam  101  is deformed and actuates the slide  93 , whereby the force-transmitting part  96  is blocked or freed. To sense the slide position, the current consumption and/or the capacity of the piezoelectrically actuated transverse beam  101  is measured in this design. An embodiment with two piezoelectric actuators (not shown), similar to the design in  FIG. 8   b  with two electromagnets ( 95 ,  105 ), is also part of the invention.  
         [0060]     Instead of the transversely moving slide  93 , the slide  93  may also be designed in such a fashion that it turns about its own axis while actuated by the electromagnets  95 ,  105  and, thus, blocks or frees the force-transmitting part  96 .  
         [0061]     As has been mentioned already, various preferably independently actuatable pressure-generating aggregates are employed for pressure development both in the service pressure chamber  7  and in the accumulator pressure chamber  22 ,  40 . Thus, it is e.g. possible to use a hydraulic pump, which has the purpose of an independent energy source of an electrohydraulic brake system. It is also feasible to utilize an actuating unit with an independently actuatable brake booster and a master brake cylinder connected downstream of the brake booster. Alternatively, however, a pressure generator operable by the vehicle operable may also be used.