Abstract:
The invention relates to a selective freewheeling mechanism for developing an electromechanical vehicle brake (service brake) into a parking brake. The freewheeling mechanism according to the invention has an outer ring and a locking element cage which are configured as an electric motor with a rotor and a stator. The rotor and stator can be swiveled in relation to each other when a coil is supplied with current. Due to a symmetric design, the freewheeling mechanism is advantageously insensitive to acceleration forces.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/070201 filed on Dec. 22, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a selective freewheeling mechanism. The selective freewheeling mechanism is intended in particular for an electromechanical vehicle brake, in order to expand it into a parking brake. The invention also relates to an electromechanical vehicle brake having the selective freewheeling mechanism. 
     2. Description of the Prior Art 
     Freewheeling mechanisms are known per se and are also known as directionally-shifted clutches. A freewheeling mechanism has a shaft and an outer ring, and the shaft can also be embodied as a hollow shaft or inner ring. Between the shaft and the outer ring, there are locking elements, which allow a rotation of the shaft relative to the outer ring in one direction, the so-called freewheeling direction, and lock against a rotation of the shaft relative to the outer ring in the opposite direction, the so-called locking direction. 
     In a selective freewheeling mechanism, the locking elements can be put in a disengagement position, in which they are out of action, or in other words do not lock against a rotation of the shaft relative to the outer ring in any direction. In the disengagement position, the shaft is rotatable in both directions relative to the outer ring. In an engagement position, a selective freewheeling mechanism has the usual function of a freewheeling mechanism; that is, the shaft is rotatable relative to the outer ring in the freewheeling direction and is locked against rotation in the opposite locking direction. 
     Electromechanical vehicle brakes as wheel brakes for motor vehicles are also known. An electromechanical vehicle brake has an electromechanical actuation device, with which a fiction brake lining can be pressed for braking against a brake body that is fixed against relative rotation to a vehicle wheel. The brake body is typically a brake disk or a brake drum. The actuation device typically has an electric motor and a rotation-to-translation conversion gear that converts a rotary driving motion of the electric motor into a translational motion for pressing the friction brake lining against the brake body. Worm gears, such as spindle gears or roller worm drives, are often used as rotation-to-translation conversion gears. It is also possible to convert the rotary motion into a translational motion by means of a pivotable cam, for instance. A step-down gear, for instance in the form of a planetary gear, is often placed between the electric motor and the rotation-to-translation conversion gear. Self-boosting electromechanical vehicle brakes are also known, which have a self booster that converts a frictional force, exerted by the rotating brake body against the friction brake lining that is pressed for braking against the brake body, into a contact pressure, which presses the friction brake lining against the brake body in addition to a contact pressure that is exerted by the actuation device. Wedge, ramp, and lever mechanisms are known for the self boosting. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The selective freewheeling mechanism of the invention has a locking element cage, which by pivoting relative to the outer ring into an engagement position puts the locking elements in the engagement position and by pivoting relative to the outer ring into a disengagement position puts the locking elements in the disengagement position. The locking element cage, which may be embodied in a manner comparable to a roller cage of a roller bearing, puts the locking elements of the freewheeling mechanism of the invention collectively into the engagement position or the disengagement position. It is defined by this function, not by its embodiment as a “cage”. According to the invention, the locking element cage and the outer ring are embodied on the order of an electric motor, as a rotor and a stator, and have at least one coil. The locking element cage may also form the stator, and the outer ring may form the rotor. By provision of current to the at least one coil, the locking element cage can be pivoted relative to the outer ring into the engagement position and/or into the disengagement position, and as a result the locking elements can be put in the engagement position and/or the disengagement position; that is, by the provision of current to the at least one coil, the freewheeling mechanism of the invention is engaged or disengaged. Pivoting means a rotation of the locking element cage relative to the outer ring about a limited angle. In addition to a monostable embodiment, in which the freewheeling mechanism remains disengaged or engaged when without current, a bistable embodiment is also conceivable, in which when without current the freewheeling mechanism remains both in the engaged and in the disengaged position and for shifting from the engagement position to the disengagement position and vice versa, the at least one coil merely has to be supplied with current. Moreover, it is preferably provided that the at least one coil generates an axially symmetrical magnetic field. 
     The freewheeling mechanism of the invention has the advantage that it can be embodied rotationally symmetrically. Hence, forces acting on the freewheeling mechanism do not exert any torque on parts of the freewheeling mechanism that could cause shifting of the freewheeling mechanism. Such forces may be due to acceleration acting on the freewheeling mechanism that acts on a freewheeling mechanism of an electromechanical wheel brake of a motor vehicle while the vehicle is in motion, for instance. The invention avoids unintended shifting of the freewheeling mechanism, even at high accelerations acting on the freewheeling mechanism. Moreover, given a rotationally symmetrical magnetic field, a shifting torque for engaging and/or disengaging the freewheeling mechanism acts rotationally symmetrically on the locking element cage or the outer ring in every case. As a result, a uniform action on all the locking elements is possible; all the locking elements contribute approximately equally to locking the freewheeling mechanism in the locking direction. An additional advantage of the invention is that a separate actuator is not needed for pivoting the locking element cage for engaging or disengaging the freewheeling mechanism; the function of the actuation device is integrated with the locking element cage and the outer ring as a result of the embodiment of these parts on the order of an electric motor. This economizes in terms of installation space and makes a compact embodiment of the freewheeling mechanism of the invention possible. 
     Advantageously, an electromechanical vehicle brake has a selective freewheeling mechanism of the type described above. As a result, the vehicle brake is expanded to a parking brake. The freewheeling mechanism acts for instance on a motor shaft of an electric motor of an actuation device of the vehicle brake. When the freewheeling mechanism is disengaged, the motor shaft of the electric motor is freely rotatable in both rotary directions; the vehicle brake can be used as a service brake, in the same way as without a freewheeling mechanism. For locking the vehicle brake, the brake is actuated and the freewheeling mechanism is engaged; the freewheeling mechanism locks against a reverse rotation of the actuation device and thus against a release of the vehicle brake. An actuating and braking force exerted by the vehicle brake is maintained unchanged even if the vehicle brake is without current. Since the engaged freewheeling mechanism is rotatable in the freewheeling direction, an actuation or in other words tightening of the vehicle brake is possible even when the freewheeling mechanism is engaged. Because of mechanical tensing of the actuated vehicle brake, the engaged freewheeling mechanism is kept in the engagement position, in which it locks against a release of the vehicle brake. As a result of the mechanical tensing of the actuated vehicle brake, the freewheeling mechanism does not release on its own, even if it is embodied in monostable form. Not until the mechanical tensing of the vehicle brake is released by the provision of current in the actuation direction does the freewheeling mechanism release, so that the vehicle brake can then be released. It is furthermore possible for an air clearance of the vehicle brake to be adjusted, and thus to make a readjustment for wear by engaging the freewheeling mechanism upon release of the vehicle brake. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in further detail below in conjunction with exemplary embodiments shown in the drawings, in which: 
         FIG. 1  shows a freewheeling mechanism of the invention, seen in perspective; 
         FIG. 2  is a cross section through the freewheeling mechanism of  FIG. 1 ; 
         FIG. 3  illustrates a detail marked III in  FIG. 2 ; 
         FIG. 4  is an axial section through a modified embodiment of a freewheeling mechanism of the invention; and 
         FIG. 5  shows a schematic illustration of an electromechanical vehicle brake of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The freewheeling mechanism  1  shown in  FIG. 1  has a tubular outer ring  2  and a locking element cage  3  disposed coaxially in the outer ring. For the sake of clarity in the drawing, a shaft that is disposed coaxially in the locking element cage  3  is not shown. The locking element cage  3  is tubular and has axially parallel, essentially rectangular openings, which are known as pockets  5  and in which cylindrical roller bodies are received that form locking elements  4  of the freewheeling mechanism  1 . 
     As can be seen in cross section in  FIG. 2 , the shaft  6 , not shown in  FIG. 1 , of the freewheeling mechanism  1 , the locking element cage  3 , and the outer ring  2  are disposed concentrically to one another; the locking element cage  3  and the locking elements  4  are located in an annular interstice between the outer ring  2  and the shaft  6 . On its inner circumference, the outer ring  2  has indentations, which are also known as pockets  7 . The locking elements  4  roll on bottom faces  8  of the pockets  7 . In the circumferential direction, the bottom faces  8  extend spirally at a wedge angle to the circumferential direction. A spacing of the bottom faces  8  of the pockets  7  of the outer ring  2  from the surface of the shaft  6  therefore decreases in a circumferential direction hereinafter called the locking direction. 
     For engaging the freewheeling mechanism  1 , the locking element cage  3  is pivoted about its axis in the locking direction, which is counterclockwise in terms of the drawing; that is, it is rotated far enough that the locking elements  4  come into contact with the shaft  6 . It moves the locking elements  4  in the circumferential direction; the locking elements  4  rolls on the bottom faces  8  of the pockets  7  of the outer ring  2  of the freewheeling mechanism  1 . Since the bottom faces  8  spirally approach the shaft  6  in the locking direction, the locking elements  4  come into contact with the shaft  6  and are pressed against it. The locking element cage  3  and the locking elements  4  are moved into an engagement position, not shown, and the freewheeling mechanism  1  is engaged. A rotation of the shaft  6  in the locking direction urges the locking elements  4 , pressed against it, in the locking direction, or in other words in the direction of the increasingly narrower wedge gap between the bottom faces  8  of the pockets  7  of the outer ring  2  and the shaft  6 . The locking elements  4  lock the shaft  6  by nonpositive engagement against rotation in the locking direction. In the opposite rotary direction of the shaft  6 , known as the freewheeling direction, the shaft  6  urges the locking elements  4 , pressed against it, in the direction of the increasingly larger wedge gap between the bottom faces  8  of the pockets  7  of the outer ring  2  and the shaft  6 . In this rotary direction, the shaft  6  is rotatable, with the freewheeling mechanism  1  engaged. 
     For disengaging the freewheeling mechanism  1 , the locking element cage  3  is pivoting in the freewheeling direction about its axis into the disengagement position shown in  FIG. 2 . It moves the locking elements  4  in the circumferential direction, specifically in the freewheeling direction, up to the ends of the pockets  7 . Here, the spacing of the bottom faces  8  of the pockets  7  of the outer ring  2  from the shaft  6  is greater than a diameter of the cylindrical locking elements  4  that are embodied as roller bodies; the locking elements  4  are out of action, and the shaft  6  is freely rotatable in both rotary directions. 
     The locking element cage  3  is supported rotatably or pivotably in the outer ring  2 . The support is a slide bearing of the tubular locking element cage  3  in the outer ring  2 . Other bearings are possible. The rotary bearing centers the locking element cage  3  in the outer ring  2 , so that over the entire circumference, a uniform gap exists between the locking element cage  3  and the shaft  6 , and the locking element cage  3  does not scrape the shaft  6 . A transfer of torque from the shaft  6  to the locking element cage  3  and unintended engagement or disengagement of the freewheeling mechanism  1  are thereby avoided. 
     Spring elements  9 , which are suspended from the locking element cage  3  and engage the locking elements  4 , urge the locking elements  4  in the locking direction and into contact with one edge  10  of the pockets  5  of the locking element cage  3 , this edge being located out of sight of the locking elements  4  in the locking direction. This edge  10  is oriented obliquely outward according to the invention, so that it urges the locking elements  4  outward against the bottom faces  8  of the pockets  7  of the outer ring  2 . In the disengagement position shown in  FIG. 2 , the locking elements  4  have thus been lifted from the shat  6 . The edges  10  of the pockets  5  of the locking element cage  3  may also be called contact faces for the locking elements  4 . The locking elements  4  touch the shaft  6  only when the freewheeling mechanism  1  is engaged; when the freewheeling mechanism  1  is disengaged, there is a gap between the locking elements  4  and the shaft  6 . 
     The enlargement shown in  FIG. 3  in the region of one of the pockets  7  of the outer ring  2  shows that the bottom  8  of the pockets  7  is not at a constant wedge angle to the circumferential direction; instead, the wedge angle is greater on the end of the pockets  7  in the freewheeling direction, or in other words the right-hand end in  FIG. 3 , and becomes constant or gradually more-acute in the locking direction. The initially greater wedge angle of the bottom faces  8  of the pockets  7  brings about greater lifting of the locking elements  4  away from the shaft  6  upon disengagement of the freewheeling mechanism  1 , at a predetermined pivot angle of the locking element cage  3 . When the freewheeling mechanism  1  is disengaged, the gap between the locking elements  4  and the shaft  6  is greater. Moreover, the initially greater wedge angle enables engaging the freewheeling mechanism  1  with a smaller pivot angle of the locking element cage  3  and thus makes it possible to shorten the shifting time. 
     The acute wedge angle in an end region, in terms of the locking direction, of the bottom faces  8  of the pockets  7  of the outer ring  2  leads to a high clamping force and thus a good locking action. In the aforementioned end region of the bottom faces  8 , the wedge angle is selected to be so acute that self-locking ensues. When the locking element cage  3  is pivoted far enough in the locking direction that the locking elements  4  are located at the acute wedge angle in the end regions, located in the locking direction, of the bottom faces  8  of the pockets  7 , then they and the locking element cage  3 , because of the self-locking, remain on their own in the locking position; that is, the freewheeling mechanism  1  in this case does not release from its engaged position. For release, either the locking element cage  3  or the shaft  6  must be pivoted in the freewheeling direction. To improve the self-locking action, an outside of the outer ring  2  is provided with recesses  11  on the end regions, located in the locking direction, of the bottom faces  8  of the pockets  7 . In those regions, a wall thickness of the outer ring  2  is weakened; in those regions, the outer ring  2  is elastic and resilient in the radial direction. As a result of the elasticity of the outer ring  2  in the aforementioned end regions of the bottom faces  8  of the pockets  7 , the locking elements  4  are pressed resiliently against the shaft  6 . Unintentional release, for instance from the pivoting motion of the shaft  6  with a small pivot angle relative to the outer ring  2 , or from vibration, is counteracted; the hold of the freewheeling mechanism  1  in the self-locking, locking engagement position is improved. The desired elasticity of the outer ring  2  in the radial direction in the region of the pockets  7  can also be attained in some other way. For instance, a housing, not shown in  FIGS. 1 through 3 , and to which the outer ring  2  is pressed can have recesses on the inside in the aforementioned regions. In that case, bracing of the outer ring  2  from outside in those regions is omitted, and free spaces are created into which the outer ring  2  can yield radially outward. 
     In a middle region between the end regions having the large wedge angle and with the acute wedge angle that brings about the self-locking of the locking elements  4  and of the freewheeling mechanism  1 , the bottom face  8  of the pockets  7  of the outer ring  2  has a wedge angle that is so acute that self-locking ensues; that is, with the freewheeling mechanism  1  engaged, reliable locking against rotation of the shaft  6  in the locking direction is ensured under all operating conditions. However, in the aforementioned middle region of the bottom face  8  of the pockets  7 , the wedge angle is larger, or in other words more-obtuse, than in the end region, so that the freewheeling mechanism  1  does not seize in the engaged position if the shaft  6  is rotated in the locking direction. 
     The locking element cage  3  protrudes from the outer ring  2  on one side; in this region, it is located in a stator  12  ( FIG. 1 ) that is rigidly connected to the outer ring  2 . The stator  12  has two coils  13  located diametrically opposite one another or in other words rotationally symmetrically. The locking element cage  3  forms a rotor. The outer ring  2  with its stator  12  and the locking element cage  3  are thus embodied as a rotor and a stator on the order of an electric motor. Providing current to the coils  13  generates a rotationally symmetrical magnetic field, which exerts a torque in the locking direction on the locking element cage  3 . The freewheeling mechanism  1  can thus be engaged by the provision of current to the coils  13 . The torque engaging the locking element cage  3  is rotationally symmetrical; the locking element cage  3  is not urged transversely to the shaft  6 . 
     For restoration, or in other words for disengaging the freewheeling mechanism  1 , two tangentially disposed spring elements  14  are provided, which engage the stator  12  and the locking element cage  3  that forms the rotor. The restoring spring elements  14  are disposed symmetrically and likewise urge the locking element cage  3  solely rotationally symmetrically, or in other words without a resultant transverse force. In the exemplary embodiment shown of the invention the restoring spring elements  14  are embodied as helical tension springs. 
     The freewheeling mechanism  1  has an overall rotationally symmetrical construction, without eccentricities. Transverse forces from accelerations and jarring of the freewheeling mechanism  1  therefore do not exert any torque on parts of the freewheeling mechanism  1  that urge it in the engagement position or the disengagement position. Unintentional shifting of the freewheeling mechanism  1  is thus avoided. 
     In the exemplary embodiment shown, the locking element cage  3  is in one piece with the rotor; it itself forms the rotor. According to the invention, it is also possible to connect the rotor rigidly to the locking element cage  3 , for instance with a pin, rivet, and/or screw connection (not shown). As a result, the locking element cage  3  can be produced from a magnetically nonconductive material, to avoid magnetic effects on the locking elements  4  when current is supplied to the coils  13 . 
     On the side away from the stator  12 , the outer ring  2  protrudes past the locking element cage  3 . There, it has a shaft bearing  15  for the shaft  6 , not shown in  FIG. 1 , of the freewheeling mechanism  1 . In the exemplary embodiment shown of the invention, a ball bearing has been selected as the shaft bearing  15 . Still other roller bearings or a slide bearing can also be used as the shaft bearing  15  (these options are not shown). As a result, the shaft  6  is rotatably supported and radially braced axially quite close to the freewheeling mechanism  1 , or in other words to the locking elements  4 . As a result, good coaxially of the shaft  6  in the outer ring  2  is attained, which is important for the function of the freewheeling mechanism  1 . Preferably, the shaft  6  not shown in  FIG. 1  is rotatably supported and radially braced on both sides of the locking elements  4  and close to the locking elements  6 , to ensure good coaxially of the shaft  6  in the outer ring  2  even if a load is put on the shaft  6 . As a result, a locking action distributed uniformly over all the locking elements  4  is attained, which is important for proper function of the freewheeling mechanism  1 . 
     The locking element cage  3  has a pivot angle limitation. In the exemplary embodiment, this is formed by tabs  30  protruding radially outward from the locking element cage  3 , from which tabs the spring elements  14  for restoring the locking element cage  3  and for disengaging the freewheeling mechanism  1  are suspended. The tabs  30  cooperate with stops  31  on the stator  12 , which limit the pivot angle of the locking element cage  3  in both directions. 
     In the embodiment of the invention shown in  FIG. 4 , the freewheeling mechanism  1  has a shiftable friction clutch  16 . The friction clutch  16  is formed by the locking element cage  3  and an annular shoulder  17  on a diameter graduation of the shaft  6 , against which the locking element cage  3  is pressed in the axial direction and by its face end upon engagement of the friction clutch  16 . When the friction clutch  16  is engaged, the shaft  6 , when it rotates, subjects the locking element cage  3  to a torque. Depending on the rotary direction of the shaft  6 , the freewheeling mechanism  1  can be engaged and disengaged as a result. 
     Engaging of the shiftable friction clutch  16  is done magnetically by the provision of current to a coil  18 , which is inserted into a housing  19  into which the outer ring  2  of the freewheeling mechanism  1  is pressed. The housing  19  may be a component of a housing of an electromechanical friction brake, not shown in  FIG. 4 . Providing current to the coil  18  creates a magnetic field, which pulls the locking element cage  3  axially against the annular shoulder  17  of the shaft  6  and thus engages the friction clutch  16 . A magnetic circuit is closed by the housing  19 , the outer ring  2 , the locking element cage  3 , and the shaft  6 . The locking element cage  3  may also be conceived of as an armature, and the annular shoulder  17  of the shaft  6  as a pole piece, of an electromagnet that also includes the coil  18 , for engaging and shifting the friction clutch  16 . The shiftable friction clutch  16  has the advantage that the freewheeling mechanism  1  can be shifted continuously variably into any angular position. 
     For disengagement, the friction clutch  16  has a spring element  20 , which in the exemplary embodiment shown of the invention is embodied as a helical tension spring. The spring element  20  is screwed by one end onto a helical groove  21  of the locking element cage  3  and by the other end onto a helical groove  22  of a restoring element  23 . The restoring element  23  is tubular and is disposed coaxially to the outer ring  2  and to the shaft  6 . The restoring element  23  is pressed into the outer ring  2  and thus held axially and in a manner fixed against relative rotation. After the supply of current to the coil  18  is switched off the spring element  20 , by its spring force, pulls the locking element cage  3  axially away from the annular shoulder  17  of the shaft  6  and disengages the friction clutch  16 . Since upon engagement of the friction clutch  16  and engagement of the freewheeling mechanism  1 , the locking element cage  3  is pivoted by rotation of the shaft  6  in the rotary direction of the shaft  6 , the spring element  20  is rotated elastically. When the coil  18  is switched off, the previously elastically rotated spring element  20  exerts a restoring torque on the locking element cage  3  in the freewheeling direction, which reinforces the disengagement of the freewheeling mechanism  1 . Moreover, face ends, facing one another, of the locking element cage  3  and of the restoring element  23  have complementary sawtooth-like teeth  24 , whose direction is selected such that they likewise exert a restoring torque in the freewheeling direction and thus in the disengagement direction of the freewheeling mechanism  1 , when the locking element cage  3  is pulled by the spring element  20  against the restoring element  23 . 
     Otherwise, the freewheeling mechanism  1  shown in  FIG. 4  is embodied identically to the freewheeling mechanism  1  shown in  FIGS. 1 through 3  and described above, and functions in the same way. To avoid repetition, reference is therefore made to the aforementioned explanations of  FIGS. 1 through 3 . For the same components, the same reference numerals are used. 
     The electromechanical vehicle brake  25  according to the invention, shown in  FIG. 5  in the form of a mechanical circuit diagram, is intended as a wheel brake for a motor vehicle. It has an electromechanical actuation device  26 , with which a friction brake lining, not shown individually, can be pressed for braking against a friction clutch, such as a brake disk  27 . Such vehicle brakes are known per se in various constructions and will therefore not be described in further detail here. The actuation device  26  has an electric motor  28 , with which a rotation-to-translation conversion gear  32 , for instance in the form of a worm drive, can be driven via a step-down gear  29 . The rotation-to-translation conversion gear  32  converts a rotary driving motion of the electric motor  28  into a translational motion for pressing the friction brake lining against the brake disk  27 . To the extent described thus far, the electromechanical vehicle brake  25  is a service brake. 
     The selective freewheeling mechanism  1  described above is disposed on a motor shaft of the electric motor  2 . In other words, the motor shaft is the freewheeling mechanism&#39;s shaft  6 , or the two shafts are joined together in a manner fixed against relative rotation. The freewheeling mechanism  1  may also act at some other point on the actuation device  26  of the vehicle brake  25 ; for instance, it may (not shown) be disposed on a gear shaft of the step-down gear  29 . The freewheeling direction of the freewheeling mechanism  1  is selected to be in the actuation direction of the vehicle brake  25 , and the locking direction of the freewheeling mechanism  1  is selected to be in the release direction of the vehicle brake  25 . In the disengagement position of the freewheeling mechanism  1 , the vehicle brake  25  forms an actuatable and releasable service brake. Once the freewheeling mechanism  1  is engaged, the vehicle brake  25  can only be actuated, or in other words tightened, but cannot be released. A braking force once exerted is preserved. In that case, the vehicle brake  25  forms a parking brake. Since the actuated vehicle brake  25 , by mechanical tension, urges the motor shaft of the electric motor  28  in the release direction of the vehicle brake  25  and thus in the locking direction of the freewheeling mechanism  1 , the freewheeling mechanism  1  remains engaged and locked, as described further above, even when it is not supplied with current. The parking brake force, once built up, is thus maintained even when the vehicle brake  25  is without energy. Furthermore, for locking the actuated vehicle brake  25 , the locking element cage  3  can be pivoted so markedly in the locking direction that the aforementioned self-locking ensues, which keeps the freewheeling mechanism  1  in the engaged locking position. For release, the mechanical tension of the motor shaft must be reversed by supplying current to the electric motor  28  in the actuation direction, thereby disengaging the freewheeling mechanism  1 . 
     The foregoing relates to the 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.