Abstract:
In a disc brake ( 10 ) having a disc ( 16 ) and a brake shoe ( 82 ) movable towards and away from a friction surface ( 86 ) provided on one face of the disc ( 16 ), there is provided a parking brake ( 22   a   /22   b ) having a pushing member ( 44   a   /44   b ) for moving the brake shoe ( 82 ) towards and away from the friction surface ( 86 ) of the disc ( 16 ). The parking brake ( 22   a   /22   b ) further includes a cam ( 36   a   /36   b ) displaceable between a first position in which the cam ( 36   a   /36   b ) forces the pushing member ( 44   a   /44   b ) against a biasing force acting thereon to maintain the brake shoe ( 82 ) in friction engagement with the disc ( 16 ) and a second position in which the pushing member ( 44   a   /44   b ) is free to move in a direction away from the disc ( 16 ) to release the brake shoe ( 82 ) from the friction surface ( 86 ) of the disc ( 16 ). A piston and cylinder arrangement ( 28 ) is provided to displace the cam ( 36   a   /36   b ) between its first and second positions.

Description:
RELATED APPLICATIONS 
   This is a continuation of International Patent Application No. PCT/CA01/01448 filed Oct. 18, 2001, which claims benefit of Canadian Patent Application No. 2,323,817 filed on Oct. 18, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a vehicle brake system and, more particularly, to disc brakes for heavy road vehicles. 
   2. Description of the Prior Art 
   U.S. Pat. No. 5,205,380 issued to Paquet et al. on Apr. 27, 1993 discloses a disc brake assembly for heavy road vehicles. The disc brake assembly includes a parking or safety brake which is automatically activated when the road vehicle is parked. The parking brake comprises spring acting on a movable plate to urge a brake shoe against a friction surface provided on one face of a disc. A fluid bladder is provided to overcome, when expanded, the force of the spring in order to release the brake shoe from the friction surface of the disc. 
   Although the parking brake described in the above-mentioned patent is effective, it has been found that there is a need for a new parking brake which is more compact. 
   SUMMARY OF THE INVENTION 
   It is an aim of the present invention to provide a new parking brake for a disc brake assembly. 
   It is also an aim of the present invention to provide a new disc brake assembly having a system for automatically repositioning a brake shoe to compensate for wear thereof. 
   It is a further aim of the present invention to provide a compact parking brake which is integrated with a disc brake assembly. 
   Therefore, in accordance with the present invention, there is provided a disc brake assembly having a disc and a brake shoe movable towards and away from a friction surface provided on one face of the disc, and a parking brake comprising a first pushing member for moving the brake shoe towards and away from the friction surface of the disc, a first movement transmitting member displaceable between a first position wherein said first movement transmitting member forces said first pushing member against a biasing force acting thereon to maintain the brake shoe in friction engagement with the disc and a second position wherein said first pushing member is free to move in a direction away from the disc to release the brake shoe from the friction surface of the disc, and a motive means to displace said first movement transmitting member between said first and second positions thereof. 
   In accordance with a further general aspect of the present invention, there is provided a parking brake for mechanical connection to a wheel of a vehicle for maintaining the vehicle stationary, comprising a disc adapted to be mounted to the wheel and having a friction surface on a face thereof, a brake shoe movable towards and away from said friction surface of said disc, and a brake actuator for normally maintaining said brake shoe against said friction surface, said brake actuator comprising a first pushing member biased in a direction away from said friction surface, said brake shoe being movable by said first pushing member, and a first cam displaceable by a motive means between a first position wherein said first pushing member is pushed against a biasing force thereof by said first cam and a second position wherein said first pushing member is allowed to return to a rest position thereof under the biasing force acting thereon, and wherein said brake shoe is applied against said friction surface as long as said first pushing member is pushed by said first cam against said biasing force thereof. 
   In accordance with a further general aspect of the present invention, there is provided a self-adjusting brake for a wheel on a vehicle, comprising at least one disc adapted to be mounted to the wheel and having a friction surface on one face thereof, at least one brake shoe movable axially towards and away from said friction surface for friction engagement therewith and release thereof, and a brake actuator for displacing the brake shoe from an idle position to a functional position in which said brake shoe is urged against said friction surface of said disc, a wear compensating mechanism for automatically readjusting said idle position of said brake shoe to accommodate wear thereof, at least two pivotally mounted ratchet arms biased towards a closed position wherein said ratchet arms are urged in toothed engagement with a pawl member, said ratchet arms having a number of axially spaced-apart level of notches, said pawl member being loosely mounted for limited axial movement along an axially extending brake shoe projection so that when the stroke of the brake shoe becomes greater than a permitted distance of travel of said pawl member on said brake shoe projection, said ratchet arms are pivoted to an open position thereof by said pawl member to allow said pawl member to fall into a next level of notches on said ratchet arms. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
       FIG. 1  is a top view of a disc brake assembly for heavy road vehicles in accordance with a first embodiment of the present invention; 
       FIG. 2  is a rear plan view of the disc brake assembly of  FIG. 1 ; 
       FIG. 3  is an enlarged perspective view, partly in section, of the disc brake assembly illustrated in an idle position thereof; 
       FIG. 4  is an enlarged cross-sectional view of a parking brake spring biased in an idle position thereof; and 
       FIG. 5  is an exploded perspective view of a pair of parking brake forming part of the disc brake assembly of  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Now referring to the drawings, an in particular to  FIGS. 1 and 3 , a disc brake assembly  10  suited for heavy road vehicles, such as trucks, busses, tractors or trailers, will be described. 
   As illustrated in  FIG. 1 , the disc brake assembly  10  comprises a housing  12  adapted to be mounted on an axle  14  of a vehicle for housing a pair of axially spaced-apart ventilated discs  16  and  18  adapted to be connected to the hub  20  of a wheel (not shown) for rotative movement therewith, as described in U.S. Pat. No. 5,205,380 issued on Apr. 27, 1993 to Paquet et al. 
   A pair of mechanically linked identical parking brakes  22   a ,  22   b  are housed in respective cylindrical shells  24   a  and  24   b  secured on opposed sides of the housing  12 . The security or parking brakes  22   a  and  22   b  are mechanically connected with a disc brake sub-assembly  26  ( FIG. 3 ) which is, in turn, operatively connected to the pedal brake (not shown) of the vehicle to act as the main brake of the vehicle to control the speed thereof when the latter is in operation. 
   As shown in  FIG. 2 , the parking brakes  22   a  and  22   b  are mechanically linked and operated by a brake actuator including a pneumatic cylinder  28  extending therebetween. The pneumatic cylinder  28  includes a cylindrical housing  30  and a piston rod  32  normally biased in a retracted position by a spring (not shown) provided within the cylindrical housing  30 . As shown in  FIG. 5 , the piston rod  32  is pivotally connected at  34  to a cam  36   a  which is, in turn, pivotally mounted at  35  to a bracket  38   a  secured onto the shell  24   a . Likewise, the housing  30  is pivotally mounted at  40  to a cam  36   b  which is, in turn, pivotally mounted to a bracket  38   b  secured onto the shell  24   b . Therefore, when the biasing force of the spring (not shown) of the pneumatic cylinder  28  is overcome by the air pressure directed into the housing  30  via conventional fluid lines (not shown), the piston rod  32  will slide axially out of the housing  30  to an extended position thereof, thereby causing the cams  36   a  and  36   b  to rotate in opposed directions, as depicted by arrows  39   a  and  39   b  in  FIG. 1 , respectively. As seen in  FIG. 5 , a brace member  42  extends between the brackets  38   a  and  38   b  to structurally unify the same and increase the rigidity of the assembly. The brackets  38   a  and  38   b  have respective bottom through bore  41   a  and  41   b  for receiving corresponding tubular necks  43   a  and  43   b  formed on respective top surfaces of the shells  24   a  and  24   b . The term cam is herein intended to encompass any rotating or sliding piece of any definite shape for imparting a desired movement to the pushing members  44   a  and  44   b . For instance, a sliding wedge defining an inclined surface could also be used to displace the pushing members  44   a  and  44   b . It is also contemplated to use a pantograph linkage or a pair of scissor links in lieu of a cam to transmit a movement to the pushing members  44   a  and  44   b.    
   Referring now to  FIG. 4 , the action of the cam  36   b  on the parking brake  22   b , as well as the structural details of the latter will now be described. The interaction between the cam  36   a  and the parking brake  22   a  is similar to that of the cam  36   b  and the parking brake  22   b  and, thus, the duplicate description thereof will be omitted. The structural details of the parking brake  22   a , which are identical to those of the parking brake  22   b , will not be repeated for brevity. 
   As seen in  FIG. 4 , the parking brake  22   b  includes a pushing member  44   b  mounted for axial movement within the shell  24   b  and having a cylindrical stem portion  46   b  extending outwardly of the shell  24   b  through a cylindrical passage  48   b  defined by the tubular neck  43   b  thereof. The cam  36   b  has a curved cam surface  50   b  for engaging a domed-shaped terminal distal end  52   b  of the cylindrical stem portion  46   b . Upon rotation of the cam  36   b  in the direction indicated by arrow  54 , the pushing member  44   b  will be pushed axially into the shell  24   b  due to the curvature of the cam surface  50   b.    
   The pushing member  44   b  has three circumferentially spaced-apart ratchet arms  55   b ,  57   b ,  59   b  ( FIG. 5 ) pivotally mounted thereto for engagement with a pawl provided in the form of an annular ring  61   b  loosely fitted about a piston head  56   b  securely mounted to a spring-loaded pusher or piston  58   b . More particularly, the annular ring  61   b  has a beveled bottom rim  63   b  for mating engagement into axially spaced-apart interdental spaces or notches  65   b  defined on respective inner surfaces of the ratchet arms  55   b ,  57   b  and  59   b . The ratchet arms  55   b ,  57   b  and  59   b  are normally biased radially inwardly to a closed position thereof against the annular ring  61   b  by an annular spring member  66   b  encircling the lower ends of the arms  55   b ,  57   b  and  59   b . The piston  58   b  has a stem  68   b  having a radially enlarged end portion  70   b  from the periphery of which depends a cylindrical skirt  72   b  defining an annular seat  74   b  about the stem  68   b  for receiving one end of a compression spring  76   b . The other end of the spring  76   b  is abutted against a spider  77   b  mounted on the axle  14  to support the housing  12  and receive the actuator of the disc brake-sub-assembly  26 . An annular dish member  78   b  extends about the skirt  72   b  and is urged against the free terminal ends of the arms  55   b ,  57   b  and  59   b  by a second compression spring  80   b  concentrically disposed about the first spring  76   b  and having a first end abutting against the dish member  78   b  and a second opposed end received in an annular seat  79   b  defined in the spider  77   b . The second spring  80   b  normally urges the dish member  78   b  against the arms  55   b ,  57   b  and  59   b  to resist the axial displacement of the pushing member  44   b  and, thus, allow the radial deployment of the arms  55   b ,  57   b  and  59   b  when the piston  58   b  is drawn against the spring  76   a  in response to the activation of the disc brake sub-assembly  26  to brake or control the speed of the vehicle, as will be explained hereinafter. 
   The piston  58   a  and  58   b  are structurally connected to an annular pressure plate  82  (see  FIG. 3 ) by conventional fastening elements (not shown). Therefore, the axial displacement imparted to the pushing members  44   a  and  44   b  by the rotational movement of the cams  36   a  and  36   b  and transferred from the pushing members  44   a  and  44   b  to the pistons  58   a  and  58   b  via the ratchet arms  55   a ,  55   b ,  57   a ,  57   b ,  59   a ,  59   b  and the annular rings  61   a  and  61   b , will be communicated to the pressure plate  82  which forms part of the main brake, herein referred to as the disc brake sub-assembly  26 . 
   As seen in  FIG. 3 , a plurality of brake shoe lining segments  84  forming a lining ring or, alternatively, a one-piece lining ring are/is mounted to the front surface of the pressure plate  82  adjacent a radial friction surface  86  of the disc  16 . A second brake shoe lining ring  88  is mounted to an axially movable intermediate annular plate  90  adjacent a second radial friction surface  92  of the disc  16  opposite the first friction surface  86  thereof. The intermediate plate  90  is slidably mounted to the pressure plate  82 . As seen in  FIG. 3 , the intermediate plate  90  includes a plurality of axially extending fingers  94  which are slidably receive in corresponding channels  96  formed on an axially extending portion of the pressure plate  82 . A third brake shoe lining  98  ( FIG. 1 ) is mounted to the intermediate plate  90  opposite the second brake shoe lining  88  adjacent a radial friction surface (not shown) of the second disc  18 . A fourth stationary brake lining (not shown) is mounted within the housing  12  adjacent a second friction surface (not shown) of the second disc  18  opposite the first friction surface thereof. 
   When the vehicle is not in operation, the pneumatic cylinder  28  is depressurized so as to retract the piston rod  32  and cause the rotation of the cams  36   a  and  36   b  in the direction indicated by arrows  39   a  and  39   b  in  FIG. 1 . The rotational movement of the cams  36   a  and  36   b  will cause the pushing members  44   a  and  44   b  to be pushed within respective shells  24   a  and  24   b , thereby pushing the pistons  58   a  and  58   b  and the annular dish members  78   a  and  78   b  against the springs  76   a ,  76   b  and  80   a ,  80   b , respectively. The pistons  58   a  and  58   b  will then push on the pressure plate  82  which will, in turn, press the movable brake shoe lining  84  against the friction surface  86  of the first disc  16  which is mounted for limited axial movement on the axle  14  via a spline arrangement (not shown), as described in U.S. Pat. No. 5,205,380 issued to Paquet et al. Therefore, the first disc  16  will also be pushed against the second brake shoe lining  88  which will, in turn, push the intermediate plate  90 , and the third brake shoe lining  98  against the second disc  18  which will move axially against the stationary brake shoe lining (not shown). 
   When the pneumatic cylinder  28  is pressurized, the springs  76   a  and  76   b  act on the pistons  58   a  and  58   b  to maintain the brake shoe linings  84 ,  88  and  98  out of engagement with the discs  16  and  18 , thereby allowing the discs  16  and  18  to rotate freely with the associated wheel (not shown). 
   When the vehicle is operated, the parking brakes  22   a  and  22   b  are disabled, i.e. the pneumatic cylinder  28  is pressurized, and the speed of the vehicle is controlled by a pneumatic brake actuator  100  ( FIG. 3 ) mounted within the spider  77   b  for selectively pushing the pressure plate  82  towards the discs  16  and  18  to engage the movable brake shoe linings  84 ,  88  and  98  and the stationary brake shoe lining (not shown) with the radial friction surfaces of the discs  16  and  18 , as described hereinbefore with respect to the parking brakes  22   a  and  22   b . As the pressure plate  82  is pushed by the pneumatic brake actuator  100 , the pistons  58   a  and  58   b  are pulled against the springs  76   a  and  76   b  thereof. As seen in  FIG. 4 , the piston head  56   b  has a flange  102   b  which is axially spaced from the annular ring  61   b  to define therewith a play  104   b  when the piston  58   b  is at rest, i.e. when the piston  58   b  is not solicited by external axial forces. It is understood that a similar play exist between the piston head  56   a  and the annular ring  61   a . These plays correspond to the play existing between the brake shoe linings  84 ,  88  and  98  and the discs  16  and  18  when the brake assembly  10  is not operated and the discs  16  and  18  are free to rotate. 
   Therefore, when the pneumatic actuator  100  is activated to displace the pressure plate  82 , the pistons  58   a  and  58   b  will travel with the pressure plate  82  over an axial distance corresponding to the play  104   b . Accordingly, the annular rings  61   a  and  61   b  will remain trapped in the first level of notches  65   a  and  65   b . However, when the brake shoe linings  84 ,  88  and  98  will become worn, the thickness thereof will reduce and consequently the displacement of the pressure plate  82  and the pistons  58   a  and  58   b  necessary to effect braking will increase. At a certain level of wear of the brake shoe linings  84 ,  88  and  98 , the displacement of the pressure plate  82  and the pistons  58   a  and  58   b  under the governed of the pneumatic operator  100  will be such that the annular rings  61   a  and  61   b  will be drawn by the piston heads  56   a  and  56   b , thereby causing the radial deployment of the arms  55   a ,  55   b ,  57   a ,  57   b ,  59   a  and  59   b  which are retained against axial movement by the spring loaded dish members  78  and  78   b , to allow the annular rings  61   a  and  61   b  to move axially relative to the arms  55   a ,  55   b ,  57   a ,  57   b ,  59   a  and  59   b  beyond the first level of notches  65   a  and  65   b  thereof. When the pressure exerted by the pneumatic actuator  100  is released, the springs  76   a  and  76   b  will urge the pistons  58   a  and  58   b  and the annular rings  61   a  and  61   b  towards their original position but the respective beveled rims  63   a  and  63   b  of the annular rings  61   a  and  61   b  will fall into the second level of notches  65   a  and  65   b  of the arms  55   a ,  55   b ,  57   a ,  57   b ,  59   a  and  59   b , which tend to return to their original closed position under the biasing force of the annular spring  66   a  and  66   b , thereby preventing the pistons  58   a  and  58   b  from returning to their original resting 
   When the brake shoe linings  84 ,  88  and  98  will become further worn, the annular rings  61  and  61   b  will automatically fall in the next level of notches  65   a  and  65   b  and so on. This mechanism allows to automatically compensating for the wear of the brake shoe linings  84 ,  88  and  98  to maintain the original adjustment of the parking brakes  22   a  and  22   b  irrespectively of the condition of the brake shoe linings  84 ,  88  and  98 .