Abstract:
The invention is a electromechanical locking device for a brake piston of a hydraulically actuated wheel brake. The locking device is disposed at least in some portions in the interior of a hollow-cylindrical brake piston. This interior of the brake piston is filled with hydraulic pressure fluid and in the event of brake actuation is subjected to pressure. The invention includes a filler piece and a coupling element, which reduce the pressure-fluid-filled volume of the brake piston, prevent blocking of the locking device in its terminal positions, and make it possible for the two terminal positions of the locking device to be detected and evaluated by an electronic control unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on German Patent Application No. 10 2007 053 278.6 filed on Nov. 8, 2007, upon which priority is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention is based on an electromechanical locking device for a brake piston of a hydraulically actuated wheel brake. 
         [0004]    2. Description of the Prior Art 
         [0005]    One such electromechanical locking device is know for instance from German Patent Disclosure DE 10 2005 055 084 A1. The wheel brake disclosed in this reference is hydraulically actuatable. The wheel brake has a hollow-cylindrical brake piston, whose interior is filled with hydraulic pressure fluid. In the event of brake actuation, a hydraulic pressure is built up by the driver of a vehicle. A pressure force causes a displacement of the brake piston relative to a brake cylinder, in which this brake piston is movably guided. The brake piston presses a brake lining against a rotating brake body, such as a brake disk, and the magnitude of a frictional force that comes to be established between the rotating brake body and the brake lining is proportional to the brake pressure. The frictional force brakes the brake body. 
         [0006]    In order to brake a stopped vehicle firmly on the spot in a parking situation, the known wheel brake additionally has an electromechanically actuatable locking device. This locking device includes a spindle that can be driven to execute a rotary motion by an electric drive motor and an intervening gear. The spindle in some portions protrudes into the interior of the brake piston and has a spindle nut. This spindle nut is disposed in a manner fixed against relative rotation, and as a result, a rotary motion of the spindle trips a translational motion of the spindle nut. In one of its terminal positions, the spindle nut is in contact with the brake piston and acts upon it with an axial force. With this axial force, the brake piston acts on the brake lining and presses it against the stationary brake body and thus prevents the latter from being able to execute a rotary motion. 
         [0007]    The spindle and the spindle nut occupy a considerable portion of the internal volume of the hollow-cylindrical brake piston. Nevertheless, the remaining space inside the brake piston, which is filled with hydraulic pressure fluid, is relatively large. This has the disadvantages that a comparatively large amount of pressure fluid has to be positively displaced by the driver in order to build up brake pressure, and the response performance of such a wheel brake is therefore not fully satisfactory. In professional circles, such a response performance of a wheel brake is called indirect. 
         [0008]    Another disadvantage is that hydraulic pressure fluid does not behave ideally incompressibly, and that therefore with increasing pressure fluid volume in the brake piston, the hydraulic elasticity of a brake circuit also increases. This elasticity is expressed as an unwanted, soft actuation of the service brake. 
         [0009]    Moreover, the spindle nut received in a manner fixed against relative rotation is urged axially by the rotationally driven spindle, in its terminal positions, in such a way that mechanical warping can occur between the individual components. This warping makes an ensuing release of the spindle nut from its terminal positions more difficult or even impossible, so that an actuated locking device might still remain blocked, and then the vehicle cannot be moved from that spot. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0010]    The object of the present invention is to overcome the aforementioned disadvantages. 
         [0011]    A locking device according to the invention for a brake piston of a hydraulically actuated wheel brake has the advantage over the prior art that the pressure-fluid-filled interior of the hollow-cylindrical brake piston is reduced to a minimum, and that thus less pressure fluid has to be positively displaced by the driver in order to generate a defined brake pressure. A brake circuit with wheel brakes that have a locking device according to the invention includes less hydraulic pressure fluid in total and thus has less hydraulic elasticity. All in all, the response performance of a brake system can be approved by the invention in the direction of being “more direct.” 
         [0012]    The filler piece proposed according to the invention is guided axially displaceably in the interior of the hollow-cylindrical brake piston and is operatively connected mechanically to the spindle nut by a coupling element. The coupling element limits a torque that can be transmitted from the spindle to the spindle nut and thus prevents mechanical warping among the components when these components assume one of their terminal positions. Thus the locking device is readily reversible at any time by the drive motor. 
         [0013]    Because of the coupling element, the torque that can be transmitted by the drive motor in the terminal positions of the spindle nut is variably high. By monitoring the intensity of the current then flowing to the drive motor, an electronic detection of these terminal positions is detectable by an electronic control unit and can be used to control the locking device. For instance, with the detection of an attained terminal position, the control unit can discontinue flirter delivery of current to the drive motor and can thus secure the locking device against overloads that would reduce its service life. 
         [0014]    Securing the spindle nut against joint rotation on the spindle is effected indirectly accordingly to the invention by the filler piece. A rotationally symmetrical component, which is especially inexpensive to manufacture, can therefore be used as the spindle nut. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawings. In which: 
           [0016]      FIG. 1  shows a three-dimensional view of a locking device of the invention in longitudinal section while in a locked position; and 
           [0017]      FIG. 2  shows a three-dimensional view in longitudinal section of the locking device in its released position. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    In  FIG. 1 , a brake piston  10  is shown, with a locking device  12  built into it. The brake piston  10  is a hollow-cylindrical component, which is closed in pressure-fluid-tight fashion on one of its ends by a lining contact plate  14 . The lining contact plate  14  and a piston shaft  16  are embodied in one piece with one another, and on the circumference of the brake piston  10 , between the lining contact plate  14  and the piston shaft  16 , a recess  18  is provided, which has a single perpendicular graduation from the outside inward. The interior of the brake piston  10  is divided into two portions  20 ,  22  of different inside diameters. A first portion  20 , located at the open end of the brake piston  10 , has a larger inside diameter than a second portion  22 , which is oriented toward the lining contact plate  14  and is connected to the first portion  20 . The transition from the first portion  20  to the second portion  22  is embodied as a conical indentation  24 . This conical indentation is located approximately below the end, remote from the lining contact plate  14 , of the recess  18  on the circumference of the brake piston  10 . 
         [0019]    With one face end  28  shaped as a universal ball joint, a spindle nut  26  rests on the conical indentation  24  of the brake piston  10 . The spindle nut  26  is a rotationally symmetrical sleevelike component and is divided into a spindle nut head  30  of thickened outside diameter and a spindle nut neck  32  of reduced outside diameter by comparison. This spindle nut neck  32  is remote from the universal ball joint-shaped end  28  and is provided with a female thread  34 . By comparison, the spindle nut head  30  is penetrated by a bore  36 , extending coaxially to the female thread  34 , and the inside diameter of this bore  36  is greater than the flank diameter of the female thread  34 . 
         [0020]    With its female thread  34 , the spindle nut  26  cooperates with a male thread  42  of a drive spindle  40  that protrudes in some portions into the interior of the brake piston  10 . This drive spindle  40  can be driven to a rotary motion by means of an electric drive motor, not shown. The male thread  42  extends from a first end of the drive spindle  40  to a radially protruding, one-piece stop flange  44 . This stop flange  44  is located outside the brake piston  10 . On the side of the stop flange  44  remote from the male thread  42 , the drive spindle  40  continues in the form of a drive peg  46 . This drive peg  46  has a cylindrical outer contour and an inner contour that is not apparent, which makes a positive engagement possible between a power takeoff shaft (not shown) of the drive motor and of the drive spindle  40 . 
         [0021]    Over nearly its entire length, the spindle nut  26  is surrounded by a sleevelike filler piece  50 , which largely fills up the existing space between the outer contour of the spindle nut  26  and the inner contour of the brake piston  10 . The filler piece  50  is received in the interior of the brake piston  10  in a manner fixed against relative rotation and is axially movably guided in the direction of its longitudinal axis. For the sake of disposition in a manner fixed against relative rotation, radial flattened faces or protrusions (not visible in the drawing) may for instance be embodied on the circumference of the filler piece  50  and cooperate with corresponding protuberances or recesses (also not visible) on the inside surface of the brake piston  10 . The filler piece  50  is recessed once at a right angle in its interior, and as a result an encompassing stop shoulder  52  is created. With this stop shoulder  52 , the filler piece  50  rests on an outer flank  29  of the spindle nut  26 . This outer flank  29  is formed by the transition from the spindle nut head  30  to the spindle nut neck  32 . 
         [0022]    Coupling of the spindle nut  26  to the filler piece  50  is effected by a coupling element  60 . This coupling element  60  is a spiral spring with a plurality of spring windings  64 . A spring winding located closest to the filler piece  50  is bent on its end perpendicular to the winding direction and forms a protruding mandrel  62 , which extends axially parallel to the longitudinal axis of the coupling element  60 . With this mandrel  62 , the coupling element  60  engages a recess  51 , intended for that purpose, of the filler piece  50 . The recess  51  and the mandrel  62  are adapted to one another dimensionally and/or in their shaping in such a way that a clamping connection can be established between the filler piece  50  and the coupling element  60 . 
         [0023]    With its spring windings  64 , the coupling element  60  embraces the head  30  of the spindle nut  26 . As a result of spreading a part of the spring windings  64  upon installation of the coupling element  60 , an initial spring tension is generated, which urges the spindle nut  26  in the radial direction in space. 
         [0024]    By axial stretching of the spring windings  64  upon installation of the coupling element  60 , an axial force can furthermore be furnished by the coupling element  60 , and this force presses the filler piece  50  with its stop shoulder  52  against the outer flank  29  of the spindle nut  26 . 
         [0025]    Hereinafter the mode of operation will be described. When the drive spindle  40  is driven counterclockwise, as indicated by the directional arrow R, by a drive motor (not shown), the initial spring tension of the coupling element  60  prevents the spindle nut  26  from rotating jointly with it. The coupling element  60  accordingly assumes its locking position and effects a quasi-rigid connection between the spindle nut  26  and the filler piece  50 . A continued counterclockwise rotation of the drive spindle  40  thus trips an axial motion of the spindle nut  26  in the direction of the arrow A. The spindle nut moves to the left in terms of  FIG. 1 , until its ball-joint-shaped end  28  comes into contact, as shown, with the conical indentation  24  in the interior of the brake piston  10 . Once this positive engagement is reached, the brake piston  10  follows along with the axial motion of the spindle nut  26  as well, until finally, a brake lining, braced on the lining contact plate  14 , rests on a brake body not shown in the drawing. The torque generated by the drive motor is now converted, by the threaded connection between the drive spindle  40  and the spindle nut  26 , into an axial force that acts on the brake lining and thus prevents an incipient rotation of the brake body. 
         [0026]      FIG. 1  shows the locking device  12  in its locked position. In it, the locking device  12 , with its drive spindle  40 , the spindle nut  26 , the coupling element  60 , and the filler piece  50 , fills the interior of the brake piston  10  virtually completely. The remaining voids, which upon a hydraulic actuation of the brake piston  10  are filled with hydraulic pressure fluid that is at high pressure, occupy only minimal volume. 
         [0027]      FIG. 2  shows the locking device  12  of the brake piston  10  of  FIG. 1  in the released position. Components corresponding to one another are identified by the same reference numerals in both drawings. 
         [0028]    To put the locking device  12  into the released position shown, the electric drive motor must be driven clockwise (directional arrow R′). The drive motor drives the drive spindle  40  accordingly. Since the direction of rotation of the spindle nut  26  is oriented counter to the winding direction of the spring windings  64  of the coupling element  60 , these individual spring windings  64  open, and as a result the effective radial initial tension force of the coupling element  60  on the spindle nut  26  drops. The remaining initial spring tension force of the coupling element  60  nevertheless suffices to prevent joint rotation of the spindle nut  26  with the drive spindle  40 . The spindle nut  26 , prevented from rotating jointly, thereupon executes an axial motion to the right (directional arrow A′) and, because of the existing positive engagement between the spindle head  30  and the filler piece  50 , carries this filler piece  50  along with it. 
         [0029]    A terminal position is reached when the spindle nut  26 , with the end face of its spindle nut neck  32 , strikes as show against the stop flange  44  of the drive spindle  40 , so that a further axial motion of the spindle nut  26  is prevented. Because of the reduced initial tension force of the coupling element  60 , a continued clockwise rotation of the drive spindle  40  enables the spindle nut  26  to begin a clockwise rotary motion as well, and the quasi-rigid connection between the spindle nut  26  and the filler piece  50  is rescinded. The coupling element  60  accordingly assumes a released position, as a result of which the spindle nut  26  can revolve virtually unhindered with the rotating drive spindle  40 . 
         [0030]    The axial forces with which the spindle nut  26 , in the event of an actuation of the locking device  12 , is pressed against its two end stops are variably strong, as explained. They are due to the design of the toothing between the drive spindle  40  and the spindle nut  26  and to the initial spring tension of the coupling element  60 . For design reasons, the maximally attainable axial forces are small enough for warping among the individual components to be rescinded again by merely reversing the direction of rotation of the drive motor. A locking device  12  according to the invention can therefore be released again from its terminal positions at any time. 
         [0031]    As soon as the spindle nut  26  has assumed one of its two terminal positions and can no longer execute any further axial motion, the current consumption of the drive motor increases markedly. This increase in current can be detected and evaluated with the aid of an electronic control unit. In this way, the terminal positions of the spindle nut  26  can be detected relatively simply, and the locking device  12  can be controlled accordingly. 
         [0032]    The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.