Abstract:
The invention relates to a drum brake having an electromechanical actuating device. The invention proposes to convert a rotating drive movement of the actuating device by use of a gearwheel and two toothed racks. The toothed racks mesh with the gearwheel on opposite sides thereof and are driven in opposite directions by the gearwheel into translational movements for pressing attached brake shoes against a brake drum. The toothed racks are configured in such a way that the two toothed racks have a common line of action, with resulting symmetrical actuating forces. According to one embodiment of the invention, the lines of action of the toothed racks run tangentially with respect to the gearwheel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/067917 filed on Oct. 30, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a drum brake having an electromechanical actuating device. 
     2. Description of the Prior Art 
     Drum brakes are known per se. They have two brake shoes, which for actuation of the drum brake can be pressed by an actuating device against a brake drum. The actuating device may for instance be hydraulic or mechanical; for instance, it may have a pivotable double cam that presses the two brake shoes apart on one end and against the brake drum. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     The actuating device of the drum brake of the invention has one rotationally drivable gear wheel and two toothed racks, which mesh on diametrically opposite sides with the gear wheel and each of which is coupled to one of the two brake shoes of the drum brake. The two toothed racks need not necessarily mesh with the gear wheel while being exactly diametrically opposite one another and need not extend parallel to one another; an angular deviation is possible. By rotation of the gear wheel, the toothed racks are moved in contrary directions and press the brake shoes against the brake drum, or in the opposite direction of rotation of the gear wheel, the toothed racks lift the brake shoes from the brake drum. Hence one advantage of the invention is the possibility that the brake shoes can be actively lifted from the brake drum; that is, the drum brake can be actively released, using the actuating device. A further advantage, compared to cam actuation, is reduced friction within the actuating device and reduced hysteresis upon actuation and release of the drum brake. Still another advantage of the invention, that is, continuously variable wear readjustment, is simplified by rotating the gear wheel about a certain angle of rotation upon release of the drum brake almost back to its outset position. 
     The actuating device of the drum brake of the invention has only linear transmission members. As a consequence, a drive torque of the actuating device is proportional to a contact pressure of the brake shoes against the brake drum, if the effects of friction and hysteresis are ignored. A braking force of the drum brake of the invention is therefore simpler to control or regulate. The contact pressure of the brake shoes against the brake drum need not be measured directly; instead, a drive torque of an electric motor that drives the gear wheel can for instance be used to control or regulate a braking force of the drum brake. 
     The invention provides a locking device, with which the actuating device can be locked in its position at the time. It may for instance be a magnet brake or a shiftable freewheel, which in the engaged state blocks the actuating device from rotating the gear wheel, which meshes with the toothed racks, in reverse in the release direction. The locking device can be monostable or bistable. With it, first, the air play can be readjusted to compensate for wear, by locking the actuating device upon release or after release of the drum brake once a predetermined air play is reached. Second, the locking device of the actuating device embodies the drum brake as a parking brake; it can be locked in the actuated position without current or energy. 
     One embodiment of the invention provides that the brake shoes of the drum brake of the invention are displaceably guided radially to the brake drum. As a result, the drum brake has no leading and trailing brake shoes that bring about self-boosting or self-weakening and that lead to variable lining wear of the leading and trailing brake shoes as well as variable contact pressures. The radially displaceable guidance of the brake shoes has the advantage of uniform, symmetrical stress on the actuating device and uniform lining wear of the brake linings of the two brake shoes. A further, major advantage of the radially displaceably guided brake shoes is reduced vulnerability of the braking parameter C* of the drum brake of the invention upon fluctuations in the coefficient of friction. The braking parameter C* is the ratio of the braking force (circumferential force) that is operative at the brake drum and the contact pressure of the brake shoes against the brake drum. Any change in the braking parameter upon a change in the coefficient of friction is less; that is, the ratio ΔC*/Δμ is more constant than in conventional drum brakes that have one leading and one trailing brake shoe, or even in duplex brakes that have two leading brake shoes. As a result, control or regulation of a braking force of the drum brake of the invention is simplified. In particular, anti-lack and slip control systems, such as anti-lock brakes (ABS), traction control systems (TCS), and vehicle dynamics control (ESP, or electronic stability program) are simplified considerably, compared to major nonlinearity, with increased constancy of the braking parameter C*. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in further detail below in terms of exemplary embodiments shown in the drawings. In the drawings: 
         FIG. 1  is an elevation of a drum brake of the invention; 
         FIG. 2  shows an actuating device of the drum brake of  FIG. 1 ; 
         FIG. 3  is an enlarged view of part of the actuating device of the drum brake in the direction of the arrow III in  FIG. 1 ; 
         FIG. 4 , in a view corresponding to  FIG. 3 , shows a modified actuating device of the drum brake according to the invention; and 
         FIG. 5 , in a view corresponding to  FIG. 3 , shows a further modified actuating device of the drum brake according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drum brake  1  according to the invention shown in  FIG. 1  has two brake shoes  2 , which are disposed diametrically opposite one another and which for actuation of the drum brake  1  can be pressed outward against an inside circumference of a brake drum  3 . The brake shoes  2  are guided displaceably, radially to the brake drum  3 , on a holder plate  4 , in the form of a circular perforated disk, of the drum brake  1 . To that end, the holder plate  4  has bearing blocks  5 , which are disposed on ends of the brake shoes  2  and which guide the brake shoes  2  radially displaceably. 
     In order to press the brake shoes  2  outward against the brake drum  3 , the drum brake  1  has actuating levers  6 , which are disposed radially inside the brake shoes  2 . The actuating levers  6  are supported pivotably on one end thereof on the holder plate  4 . The other ends of the actuating levers  6  are pivotably connected each to a respective toothed rack  7 ; that is, the actuating levers  6  are coupled with the toothed racks  7 . The toothed racks  7  are disposed diametrically opposite one another with respect to a gear wheel  8  disposed between them and are parallel to one another. At diametrically opposed circumferential points, the toothed racks  7  mesh with the gear wheel  8 . By driving the gear wheel  8  to rotate, the toothed racks  7  are displaced in contrary directions and press the two actuating levers  6  apart. The actuating levers  6  press the brake shoes  2  outward against the brake drum  3 , so that the brake drum  3  is braked. The drum brake  1  is actuated. In the opposite direction of rotation of the gear wheel  8 , the toothed racks  7  pull the actuating levers  6  inward back into an outset position. The drum brake  1  is released as a result. In addition, a spring element  9  in the form of a helical tension spring is provided, which is suspended from the two actuating levers  6  and pulls them together or in other words inward. 
     For the rotational drive of the gear wheel  8 , the drum brake  1  has the electric motor  10 , shown in  FIG. 2 , which drives the gear wheel  8  to rotate via a step-down gear  11 . The step-down gear  11  is a two-stage spur gear train having the four gear wheels  12  through  15 . The gear wheel  8  that meshes with the toothed racks  7  is fixed against relative rotation with respect to a driven gear wheel  15  of the step-down gear  11 . The gear wheels  12  through  15  of the step-down gear  11  have helical toothings  16 . This has the advantage of smoother operation, less noise, and more-uniform transmission of the torque. In contrast to this, both the gear wheel, meshing with the toothed racks  7 , and the toothed racks  7  themselves have spur toothings  17 . As a result, a transverse force component upon driving of the toothed racks  7  by the gear wheel  8  is avoided. The toothed racks  7  need not be braced against transverse forces. 
     A shiftable freewheel  18  is flanged to the electric motor  10  and acts on a shaft  19  of the electric motor  10 . In the engaged position, the freewheel  18  blocks the motor shaft  19  against rotating in reverse in the direction of releasing the drum brake  1 . A braking force exerted is maintained while the electric motor  10  is without current. As a result, the drum brake  1  can be used not only as a service brake but also as a parking brake. The freewheel  18  forms a locking device of the drum brake  1 . An air play between the brake shoes  2  and the brake drum  3  can also be set by means of the freewheel  18 , in that upon release of the drum brake  1 , the motor shaft  19  is blocked by the freewheel  18  against further reverse rotation in the direction of releasing the drum brake  1  once a desired air play is reached. For disengagement, the freewheel  18  has a lifting magnet  20 . When current is supplied to the lifting magnet  20 , the freewheel  18  is disengaged; that is, the motor shaft  19  is freely rotatable in both directions of rotation. When the freewheel  18  is engaged, or in other words the lifting magnet  20  is not supplied with current, the motor shaft  19  is rotatable in only a tightening direction, in which the brake shoe  2  is pressed outward and the drum brake  1  is actuated. The freewheel  18  is monostable; its engaged position is stable. A bistable freewheel  18  may also be used, in which the lifting magnet  20  need merely be supplied with current to switch from one position to the other. Such freewheels  18  are known per se; as an example, see German Patent Disclosure DE 102 55 192 A1. 
     The electric motor  10 , with the freewheel  18  flanged to it; the step-down gear  11 ; the toothed racks  7 ; and the gear wheel  8  meshing with the toothed racks all form an electromechanical actuating device  21  of the drum brake  1 . 
     As can be seen in  FIG. 3 , the two toothed racks  7  are bent at right angles in such a way that they have a common imaginary line of action  22 . The line of action  22  is a straight line that passes through pivot bearings  23 , which connect the toothed racks  7  in articulated fashion to the actuating levers  6  of the drum brake  1 . The line of action  22  indicates the direction of the actuation forces that the toothed racks  7  exert on the actuating levers  6  upon actuation of the drum brake  1 . As a result, the actuation forces exerted on the actuating levers  6  by the toothed racks  7  are not offset from one another but instead have a common line of action  22 ; if the actuation forces exerted on the actuating levers  6  and, because of the symmetrical construction of the actuating levers  6  and brake shoes  2 , the contact pressures exerted on the brake shoes  2  are of the same magnitude, then both brake shoes  2  are pressed against the brake drum  3  with the same contact pressure. The right-angle bend of the toothed racks  7  is selected in such a way in  FIG. 3  that the line of action  22  of the actuation forces exerted by the toothed racks  7  on the actuating levers  6  intersects an axis of rotation  24  of the gear wheel  8  that meshes with the toothed racks  7 . As a result, a symmetrical construction of the actuating device is attained, including in the region of the toothed racks  7  and the gear wheel  8  meshing with them. Because of the right-angle bend of the toothed racks  7 , actuating the drum brake  1  exerts a torque on the toothed racks  7 . This torque is due to an offset among the actuation forces, which engage the pivot bearings  23  of the toothed racks  7 , and drive forces, which the gear wheel  8  exerts on the toothed racks  7 . The drive forces engage a rolling circle  25  of the gear wheel  8  and a rolling line  26  of the toothed racks  7 . 
     The drum brake  1  has a roller bearing  27  for the toothed racks  7 , which is offset toward a side remote from the actuating lever  6  relative to the gear wheel  8  which meshes with the toothed racks  7  and with which the respective toothed rack  7  is coupled. The offset a of the roller bearing  27  from an engagement point  28  of the toothed rack  7  on the gear wheel  8  causes the torque, exerted by the actuation force on the toothed rack  7 , to keep the toothed rack  7  in engagement with the gear wheel  8 . The torque causes the toothed rack  7  to be pressed toward the gear wheel  8 . The offset  a  is selected to be great enough that the drive force, which is exerted by the gear wheel  8  on the toothed rack  7  and which has a component that, because of the shape of the teeth of the gear wheel  8  and of the toothed rack  7 , points away from the gear wheel  8 , is compensated for or overcompensated for. For rectilinear guidance of the toothed rack  7 , the one roller bearing  27  is thus at least theoretically sufficient. This does not preclude additional structural guide elements or bearing points of the toothed rack  7  (although these are not shown). The engagement point  28  of the gear wheel  8  on the toothed rack  7  specifically means the geometric contact point of a rolling circle  25  of the gear wheel  8  and a rolling line  26  of the toothed rack  7 . 
     In a modification of the invention shown in  FIG. 4 , the right-angle bend of the toothed racks  7  is changed. The right-angle bend of the toothed rack  7  shown at the top in  FIG. 4  is selected such that the line of action  22  of the actuation force exerted by the toothed rack  7  on the actuating lever  6  extends at a tangent to the rolling circle  25  of the gear wheel  8  that meshes with the two toothed racks  7 . As a result, this toothed rack  7  is torque-free; it is subjected solely to tensile or compressive stress and not bending stress and in order to be guided linearly it theoretically needs merely to be kept in engagement with the gear wheel  8 . For that purpose, a roller bearing  29  is provided, which braces the toothed rack  7  on a side remote from the gear wheel  8 . The roller bearing  29  is disposed on an imaginary radial of the gear wheel  8 , meshing with the toothed rack  7 , through the engagement point  28  of the toothed rack  7  and the gear wheel  8 . Once again, additional guide elements for linear guidance of the toothed rack  7  are possible, but are not shown. 
     The right-angle bend of the second toothed rack  7 , shown at the bottom in  FIG. 4 , is enlarged in  FIG. 4  compared to  FIG. 3 , so that the lines of action  22  of the actuation forces, exerted by the toothed racks  7  on the actuating levers  6 , are once again located on the same line. This line or straight line is the tangent, mentioned above in connection with the toothed rack  7  shown at the top in  FIG. 4 , to the rolling circle  25  of the gear wheel  8  that meshes with the two toothed racks  7 . The result is the symmetrical conditions, already explained in conjunction with  FIG. 3 , with respect to both the gear wheel  8  and the actuating levers  6 . The drive forces exerted by the gear wheel  8  on the two toothed racks  7 , like the actuation forces exerted by the toothed racks  7  on the actuating levers  6  coupled with them, are of the same magnitude. As a result, what acts on the gear wheel  8  is solely torque, and not a force transverse to its axis of rotation  24  that would have to be braced. The forces exerted on the actuating levers  6 , and thus also the contact pressures of the brake shoes  2  against the brake drum  3 , are likewise of equal magnitude. 
     A roller bearing  27  of the toothed rack  7  shown at the bottom in  FIG. 4 , having the larger right-angle bend, is, as explained in conjunction with  FIG. 3 , offset by the distance  a  away from the actuating lever  6  with respect to the engagement point  28  of the toothed rack  7  and the gear wheel  8 . The offset  a  is again selected to be large enough that the torque which the actuation force, exerted by the toothed rack  7  on the actuating lever  6 , and the drive force of the gear wheel  8  on the toothed rack  7 , which force engages in offset fashion in accordance with the right-angle bend of the toothed rack  7 , exert on the toothed rack  7  keeps the toothed rack  7  in engagement on the gear wheel  8 . Once again, additional roller bearings and/or sliding guides for the toothed rack  7  may in practice be useful (not shown). Otherwise, with regard to  FIG. 4 , the description of  FIG. 3  applies, which is referred to for the sake of avoiding repetition. For identical components, the same reference numerals are used. 
       FIG. 5  shows a modification of  FIG. 4 . The toothed rack  7  shown at the top is embodied identically to what is shown in  FIG. 4 ; its right-angle bend is selected such that the line of action  22  of the actuation force exerted by the toothed rack  7  on the actuating lever  6  is at a tangent to the rolling circle  25  of the gear wheel  8 . In  FIG. 5  as in  FIG. 4 , the roller bearing  29  of the toothed rack  7  is disposed on an imaginary radial of the gear wheel  8  through it point  28  of engagement with the toothed rack  7 . 
     The right-angle bend of the toothed rack  7  shown at the bottom in  FIG. 5  is also so large, as in  FIG. 4 , that the lines of action  22  of the actuation forces, exerted by the toothed racks  7  on the actuating levers  6 , are located on a common straight line, specifically a tangent to the rolling circle  25  of the gear wheel  8 . Solely the rectilinear guidance of the toothed rack  7  shown at the bottom in  FIG. 5  has changed in comparison with  FIGS. 3 and 4 : The toothed rack  7  is embodied as a frame in which two roller bearings  30 ,  31  are provided, which guide the toothed rack  7  rectilinearly and at a tangent to the gear wheel  8  in compulsory fashion and keep the toothed rack  7  in engagement with the gear wheel  8 . Otherwise, for  FIG. 5 , see the descriptions of  FIGS. 3 and 4 . The roller bearings  30 ,  31  are disposed on both sides of the engagement point  28  of the gear wheel  8  and the toothed rack  7 ; that is, they are offset in both directions with respect to the engagement point  28 . 
     Instead of the roller bearing  27 ,  29 ,  30 ,  31 , slide bearings are intrinsically also possible (not shown) as guides of the toothed racks  7 , which brace the toothed racks  7  and keep them in engagement with the gear wheel  8 . However, because they have less friction, the roller bearings  27 ,  28 ,  30 ,  31  are preferred. 
     The actuating device  21  of the drum brake  1 , with the gear train  11  and the toothed racks  7  that mesh with the gear wheel  8 , has solely linear transmission members. A drive torque of the electric motor  10  is proportional to the contact pressure of the brake shoes  2  against the brake drum  3 . Control or regulation of the braking force of the drum brake  1  is thus simplified, in particular with a view to an anti-lock brake system or traction control system. 
     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.