Abstract:
The invention relates to a friction brake ( 10 ) for a motor vehicle. The friction brake ( 10 ) for instance has a disk brake ( 12 ), which is actuatable by means of an actuating device, for instance in the form of a ball thread drive ( 22 ). The invention proposes embodying the friction brake ( 10 ) with a band brake ( 34 ), whose band ( 36 ) is mounted on the circumference of a nut ( 26 ) of the ball thread drive ( 22 ). If the brake band ( 36 ) is put under tensile stress in order to actuate the band brake ( 34 ), then the brake band ( 36 ) rotates the nut ( 26 ) of the ball thread drive ( 22 ) in a tensing direction and in this way actuates the disk brake ( 12 ). The invention has the advantage that the friction brake ( 10 ) has high brake boosting and therefore requires only little actuating energy. It is accordingly suitable for embodiment as an electromechanical friction brake ( 10 ) with an electric motor ( 56 ) for its actuation.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates generally to a friction brake for a motor vehicle. 
   From German Patent Disclosure DE 42 07 640 A1, a friction brake embodied as a disk brake is known. The known friction brake has a brake disk as its brake body, against which a friction brake lining can be pressed to generate a braking moment or braking force. For pressing the friction brake lining against the brake body, the known friction brake has a piston, which is displaceable perpendicular to the brake disk by means of a wedge drive mechanism, by rotation about a piston axis, and which presses the friction brake lining against the brake disk. 
   A driving energy for rotating and displacing the brake piston is drawn from the rotatable brake body by a controllable, electromagnetic friction coupling. The friction coupling is embodied annularly and is disposed coaxially to a rotary axis of the brake body. A circular-disk-shaped coupling lining is mounted fixedly and coaxially to the brake body and rotates along with the brake body. An electromagnetic part, having a winding, of the friction coupling is embodied annularly and is disposed rotatably and coaxially to the brake body. Supplying electric current to the winding causes the electromagnetic part of the friction coupling to achieve frictional engagement with a coupling lining that is solidly attached to the brake body and rotates along with it, so that the electromagnetic part of the friction coupling is driven to a rotary motion by the brake body. A driving moment exerted on the electromagnetic part of the friction coupling by the brake body is dependent on an intensity of the current with which the winding of the electromagnetic friction coupling is supplied. The electromagnetic part of the friction coupling has a set of teeth, which meshes with a set of teeth of the piston of the actuating device, so that supplying current to the winding of the friction coupling drives the piston to rotate, thus pressing the friction brake lining against the brake body. 
   SUMMARY OF THE INVENTION 
   The friction brake of the invention having the characteristics of claim  1  has a band brake with a brake band that wraps around a rotatable brake body. When the friction brake is not actuated, the brake band wraps loosely around the brake body. To generate a braking moment or braking force, the brake band is tensed and thereby brakes the brake body. At the same time, a tensile stress acts on the brake band, which in the friction brake of the invention is utilized to press a friction brake lining against a rotatable brake body. The brake body can be the same brake body that has the brake band of the band brake wrapped around it, or a different brake body. The brake band of the band brake is operatively connected to the actuating device with which the friction brake lining can be pressed against the brake body. A tensile stress on the brake band drives the actuating device in such a way that the latter presses the friction brake lining against the brake body and thus generates a braking moment or braking force. The actuating device can be driven solely by the brake band of the band brake, or the tensile stress on the brake band acts in addition to a driving force exerted by external force and/or muscle power on the actuating device. The external force can for instance be applied by means of an electromagnet, while the muscle power is transmitted in a manner known per se, for instance hydraulically, to the actuating device of the friction brake. 
   The friction brake of the invention has the advantage that some of the energy required to generate a braking moment or braking force is drawn from the rotatable brake body to be braked. The friction brake of the invention has brake boosting in the sense that with a comparatively slight force exerted on the brake band, a high braking moment or major braking force can be generated. The band brake has a dual function: First, the band brake directly brakes the brake body that it wraps around. Second, the tensile stress exerted on the brake band is utilized to drive the actuating device and thus to press the friction brake lining against the brake body. Because the attainable brake boosting is high, the friction brake of the invention can be actuated with little energy. It can be embodied with low weight and a small size and has high dynamics upon its actuation. Another advantage of the friction brake of the invention is its good meterability. 
   Because of the low actuating energy that is possible and the high dynamics, the friction brake of the invention is especially well suited for embodiment as an electromechanical friction brake. Claim  8  therefore contemplates an electric motor for actuating the friction brake. The electric motor, preferably via a gear, exerts the requisite tensile force for actuating the friction brake on the brake band of the band brake of the friction brake of the invention. Because only little energy is required to actuate the friction brake, a small, lightweight electric motor of low current consumption suffices; this puts only a slight load on an on-board electrical system of a motor vehicle equipped with the friction brake of the invention. Nevertheless, because of the high dynamics and because of the brake boosting, a high braking moment or major braking force can be brought to bear quickly. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described in further detail below in terms of an exemplary embodiment shown in the drawing. Shown are: 
       FIG. 1 , an elevation view of a friction brake of the invention; 
       FIG. 2 , an axial section of the friction brake taken along the line II—II in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The friction brake of the invention, shown in the drawing and identified overall by reference numeral  10 , has a disk brake  12  with a brake disk  14 . The brake disk  14  is integral with a drum  10 , which forms a hub of the brake disk  14  for connecting the brake disk  14  to a vehicle wheel, not shown, in a manner fixed against relative rotation. The brake disk  14  forms a brake body of the disk brake  12 . For braking the brake disk  14 , the disk brake  12  has two friction brake linings  18 , which are received in a brake caliper  20 . The brake caliper  20  is embodied as a floating caliper, so that by pressing the friction brake lining  18 , shown on the right in  FIG. 2 , against one side of the brake disk  14 , the other friction brake lining  18 , shown on the left in  FIG. 2 , is pressed against the other side of the brake disk  14 . 
   For pressing the friction brake linings  18  against the brake disk  14 , the friction brake  10  has an actuating device in the form of a ball thread drive  22 . The ball thread drive  22  has a spindle  24 , a nut  26 , and balls  28 , which connect the nut  26  to the spindle  24  in a manner known per se, on the order of a screw thread. One friction brake lining  18  is mounted on one end of the spindle  24 . The nut  26  is supported rotatably in the brake caliper  20  by a bearing bush  30  and is braced rotatably and axially on the brake caliper  20  via an axial roller bearing  32 . Driving the nut  26  to rotate in a tensing direction displaces the spindle  24  axially in the direction of the brake disk  14  and presses the one friction brake lining  18  against the brake disk  14 . A reaction force displaces the brake caliper  20  embodied as a floating caliper, so that the brake caliper  20  presses the other friction brake lining  18  against the other side of the brake disk  14 . In this way, a braking moment is exerted on the brake disk  14  and the brake disk  14  is braked. For release, the nut  26  of the ball thread drive  22  is driven to rotate in the opposite direction (restoring direction), as a result of which the spindle  24  is displaced away from the brake disk  14 , the brake linings  18  are lifted from the brake disk  14 , and the disk brake  12  is released. 
   Besides the disk brake  12 , the friction brake  10  of the invention, shown in the drawing, also has a band brake  34 . The drum  16 , which is integral with the brake disk  14  and also forms the hub of the brake disk  14 , forms a brake body of the band brake  34 . The band brake  34  has a brake band  36 , which is wrapped around the drum  16 . Two ends  38  of the brake band  36  are secured jointly to one point of a circumference of the nut  26  of the ball thread drive  22 , which forms the actuating device of the disk brake  12 . The two ends  38  of the brake band  36 , coming from the drum  16  of the brake disk  14 , are wrapped in the same circumferential direction around a portion of the circumference of the nut  26 . If the brake band  36  is put under a tensile stress for the sake of braking, then the two ends  38  of the brake band  36  exert a torque in the tensing direction on the nut  26  of the ball thread drive  22 . 
   To exert a tensile stress on the brake band  36 , the band brake  34  has a tensing device  40 . The tensing device  40  has a spindle  42 , with a clockwise thread  44  and a counterclockwise thread  46 . As can be seen from  FIG. 1 , the spindle  42  is disposed between the drum  16  of the brake disk  14  and the ball thread drive  22 . As can be seen from  FIG. 2 , the spindle  42  is disposed laterally beside the brake band  36 . One end of the spindle  42  is supported rotatably and axially displaceably in a bearing bush  48  that is inserted into a bore in the brake caliper  20 . Another end of the threaded spindle  42  is connected axially displaceably but in a manner fixed against relative rotation to a (nonshiftable) shaft coupling  52  by means of a sliding spring  50 . The sliding spring  50  rests displaceably in a groove of the spindle  42  and a groove in a bore  54  of the shaft coupling  52 . The shaft coupling  52  is fixed against relative rotation to a shaft of an electric motor  56 . With the electric motor  56 , the spindle  42  of the tensing device  40  of the band brake  34  can be driven to rotate. 
   One nut  58  each is spaced on the clockwise thread  44  and the counterclockwise thread  46  of the spindle  42 . The nuts  58  are shown partly cut away in  FIG. 1 , to make the course of the brake band  36  visible. Driving the spindle  42  to rotate by means of the electric motor  56  causes the two nuts  58  to move toward or away from one another, because of the clockwise and counterclockwise threads  44 ,  46  of the spindle  42 . Two pegs  60  protrude to the side from the two nuts  58  and contact outer sides, facing away from one another, of the brake band  36 . The two pegs  60  rest on two portions  64 ,  66  of the brake band  36 , which lead from a circumference of the drum  16  of the brake disk  14  away to the circumference of the nut  26  of the ball thread drive  22 . If the nuts  58  of the tensing device  40  are moved toward one another in a tensing direction by the fact that the spindle  42  is driven to rotate, then the pegs  60  press together the portions  64 ,  66  of the brake band  36 , which lead from the drum  16  of the brake disk  14  to the nut  26  of the ball thread drive  22 , and as a result put the brake band  36  under a tensile stress. The tensile stress causes friction between the brake band  36  and the drum  16  of the brake disk  14 , and the brake band  36  exerts a braking moment on the drum  16 . Assuming that the direction of rotation of the brake disk  14  with the drum  16  is as indicated by the arrow  26  in  FIG. 1 , the drum  16  by friction exerts a tension on the portion  64  of the brake band  36  on the right in  FIG. 1 , leading from the drum  16  to the nut  26  of the ball thread drive  22 . Via the peg  60 , this portion  64  of the brake band  36  braces the nut  58 , on the right in  FIG. 1 , of the tensing device  40  against further displacement. Further rotation of the spindle  42  of the tensing device  40  displaces the spindle  42  axially as a consequence. With the spindle  42 , the nut  58  shown on the left in  FIG. 1  is displaced, and with its peg  60  it presses the portion  66  of the brake band  36 , shown on the left in  FIG. 1  and leading from the drum  16  of the brake disk  14  to the nut  26  of the ball thread drive  22 , in the direction of the portion  64  of the brake band  36  shown on the right in  FIG. 1 . In this way, the tensile stress on the brake band  36  and thus the braking moment exerted by the brake band  36  on the drum  16  of the brake disk  14  can be increased in a metered way. 
   Since the two ends  38  of the brake band  36  engage the nut  26  of the ball thread drive  22  in the same circumferential direction, the two ends  38  of the brake band  36 , put under tensile stress by the tensing device  40 , exert a torque in the tensing direction on the nut  46  of the ball thread drive  22 . The brake band  36  of the band brake  34 , put under tensile stress by the tensing device  40  for the sake of braking, accordingly not only exerts a braking moment on the drum  16  of the brake disk  14 ; in addition, the brake band  36  put under tensile stress also drives the nut  26  of the ball thread drive  22  to execute a rotary motion in a tensing direction of the disk brake  12 . When the brake band  36  is put under tensile stress, it presses the two friction brake linings  18  against the brake disk  14  via the ball thread drive  22  and thus also exerts a braking moment on the brake disk  14 . The band brake  34  of the friction brake  10  of the invention, with its brake band  36 , brakes the drum  16  of the brake disk  14  directly and, via the disk brake  12 , it brakes the brake disk  14  indirectly. The result is major brake boosting, so that with a low torque exerted on the spindle  42  of the tensing device  40  of the band brake  34 , a high braking moment can be exerted on the brake disk  14 . As a result, a small electric motor  56  with little consumption of electric power is sufficient to actuate the friction brake  10  of the invention. 
   For releasing the friction brake  10 , the electric motor  56  is driven in a restoring direction, so that the two nuts  58  of the tensing device  40  move apart from one another and release the tensile stress on the brake band  36 . The brake band  36  wraps loosely around the drum  16  of the brake disk  14 . With the disappearance of the tensile stress on the brake band  36 , the disk brake  12  is released, so that the brake disk  14  is again freely rotatable. 
   It is possible for the ball thread drive  22  and the spindle  42  with the nuts  58  of the tensing device  40  of the band brake  34  to be made free of self-locking, so that the friction brake  10  of the invention releases automatically when the electric motor  56  is without current. In the exemplary embodiment of the invention shown and described, however, the spindle  42  with the nuts  58  of the tensing device  40  of the band brake  34  is embodied as self-locking, so that a braking moment of the friction brake  10  exerted in the tensing direction as a result of a supply of electric current to the electric motor  56  is maintained when the electric motor  56  is without electric current. This has the advantage that the electric motor  56  need not be supplied with electric current during braking in order to keep the braking moment constant; current is supplied to the electric motor  56  only in order to vary the braking moment. This lessens the current consumption of the electric motor  56  and also lessens the load on an on-board electrical system of a motor vehicle equipped with the friction brake  10 . Heating of the electric motor  56  is also reduced, which makes a small-sized electric motor  56  possible. Another advantage of a self-locking embodiment of the spindle  42  with the nuts  58  is that the friction brake  10  can be used as a parking brake, which keeps a braking moment, created by supplying current to the electric motor  56 , in force even if current does not continue to be supplied to the electric motor  56 .