Abstract:
The present invention concerns a brake mechanism of a disc brake received in a caliper ( 16 ) and a method for controlling the force amplification for the brake mechanism. The brake mechanism comprises a lever ( 26, 31 ) actuated by a brake actuator. The lever is acting on a cross bar ( 34, 37 ) by means of an intermediate part ( 27 ). The lever ( 26, 31 ) and the intermediate part ( 27 ) have a line of contact moving along a circular arc during an application stroke of the brake mechanism. The cross bar ( 34, 37 ) of the brake mechanism is closely guided and is only moveable in the thrust direction and in the tangential direction of the brake disc. The force amplification of the brake mechanism is adapted to the force characteristics of the brake actuator.

Description:
CROSS REFERENCE OF PENDING APPLICATIONS  
         [0001]    This application claims priority from pending Swedish Patent Application Numbers 0002058-6 filed on May 31, 2000 and 0003547-7 filed on Oct. 3, 2000.  
         TECHNICAL FIELD  
         [0002]    The present invention concerns a brake mechanism for a disc brake and a method of controlling the transmission of force from the brake actuator. Especially the invention concerns the lever of the brake mechanism and the force amplification from the brake actuator to the brake pads.  
           [0003]    The brake mechanism according to the present invention is primarily intended for a heavy road vehicle but may quite as well be used for a lighter road vehicle or a rail vehicle.  
         PRIOR ART  
         [0004]    Different types of levers acting in different ways on cross bars are previously known. In some embodiments the cross bar or equivalent part is more or less floating while in other embodiments the cross bar is closely guided. Furthermore, the levers acts directly or by means of an intermediate part on the cross bar giving a more or less extended contact surface between the lever or intermediate part and the cross bar. The force application systems of the prior art normally form some kind of eccentric giving a nearly constant transmission of the force of the brake actuator in the form of a pneumatic cylinder or the like.  
         SUMMARY OF THE INVENTION  
         [0005]    The brake actuators, e.g. pneumatic cylinders used to actuate the lever of a brake mechanism normally have a force which will vary depending on the stroke length of the pneumatic cylinder. If the lever and the cross bar of the brake mechanism have a nearly constant amplification of the force of the brake actuator, the brake as such will have a varying effect. Thus, the performance of the brakes will vary depending on the stroke length of the brake actuator and the wear of the brake pads (change of elasticity). Furthermore the different brakes may give different brake force which in extreme cases may lead to further problems. An object of the present invention is to have brakes which always give the same breaking power of both the right and left brakes.  
           [0006]    The brake mechanism may have a modular form, which makes it fairly easy to change different parts to adapt the brake mechanism to the actual actuator. Thus, it is possible to store different levers etc. to be combined with different brake actuators.  
           [0007]    By the invention the contact between the lever and the intermediate part is linear. The linear contact between the lever and the intermediate part of the invention reduces friction, wear and hysteresis.  
           [0008]    The cross bar and the thrust units will always move in the direction of the normal of the brake disc. In a conventional brake according to the prior art there will be a change of angle. Which leads to a greater risk for inclined wear. The brake mechanism of the present invention makes it possible to control the break characteristics by choosing a suitable cam curve.  
           [0009]    The brake mechanism of the present invention is preferably pneumatically actuated, but it may also be hydraulically or electrically actuated.  
           [0010]    Further objects and advantages of the invention will be obvious for a person skilled in the art from reading the description below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Embodiments of the invention will be more closely described below as a way of example and by reference to the enclosed Figs., in which  
         [0012]    [0012]FIG. 1 is a perspective view of a first embodiment of a brake mechanism according to the invention,  
         [0013]    [0013]FIG. 2 is a cross section of the brake mechanism of FIG. 1 taken along the line II-II of FIG. 3,  
         [0014]    [0014]FIG. 3 is a section of the brake mechanism of FIG. 2 taken along the lines III-III of FIG. 2,  
         [0015]    [0015]FIG. 4 is a perspective view of a second embodiment of a brake mechanism according to the invention,  
         [0016]    [0016]FIG. 5 is a cross section of the brake mechanism of FIG. 4 placed in a caliper, taken along the lines V-V of FIG. 6,  
         [0017]    [0017]FIG. 6 is a cross section taken along the lines VI-VI of FIG. 5,  
         [0018]    [0018]FIG. 7 is a cross section taken along the line VII-VII of FIG. 6, and  
         [0019]    [0019]FIG. 8 shows schematically three graphs relating to the force versus stroke of the brake actuator and the lever of the disc brake, respectively, and the aggregate of the two. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    A first embodiment of a brake mechanism according to the invention is shown in FIGS.  1  to  3 . In this embodiment a lever  26  is mounted in a bearing bracket  22 , to be received in an opening of the caliper  16 . The caliper  16  is not shown in FIGS.  1  to  3  but it corresponds to the caliper  16  shown in section in FIGS.  5  to  7 . The lever  26  acts on a cross bar  34  by means of an intermediate part  27 . The lever  26  is supported by a bearing placed in the bearing bracket  22 . The bearing of the bearing bracket  22  is a roller bearing or a plain bearing. The cross bar  34  is guided in the caliper  16  in such a way that the cross bar  34  may move in the thrust direction and in the tangential direction of the brake disc (cf. FIG. 7). The cross bar  34  is not movable in a direction perpendicular to these directions. The lever  26  is not fixed to the cross bar  34 .  
         [0021]    The cross bar  34  has two threaded openings each receiving an adjustment screw  4 . Thus, the adjustment screws  4  are rotatably mounted in the cross bar  34 . Each adjustment screw  4  is provided with a thrust plate  5 , which is to act on a brake pad holder (not shown) or the like. The brake pad will go into contact with the brake disc during activation of the brake. As is known a further brake pad is arranged on the opposite side of the brake disc. The further brake pad is in a known way brought into contact with the brake disc in that the open caliper  16  is moved at the activation of the brake. In the bearing bracket  22  two adjustment and reset shafts  6  are mounted. When the brake mechanism is assembled the adjustment and reset shafts  6  will be received inside the adjustment screws  4 . The adjustment and reset shafts  6  are axially moveable but non-rotatably connected to the adjustment screws  4 . Furthermore, the brake mechanism comprises a cover  7  fixed to the open caliper  16  at assembly.  
         [0022]    The brake force from the piston rod of the brake cylinder (not shown) is transmitted to the mechanism, especially the cross bar  34 , by means of the lever  26 . The brake cylinder is normally referred to as the brake actuator throughout this description. The lever  26  consists in essence of an arm extending out from what could be called a curved wedge  8  having inner and outer surfaces in engagement with cylindrical surfaces of the intermediate part  27  and the bearing bracket  22 , respectively. The inner surface of the curved wedge  8  is cylindrical and co-operates with the bearing bracket  22 , whereas the outer surface co-operates with the intermediate part  27 . The outer surface of the wedge  8  may not be cylindrical and in the shown embodiment the outer surface of the wedge  8  has a non-cylindrical cam profile. The centres for the inner and outer surfaces are offset in relation to each other. When the lever  26  moves downwards, as shown in FIG. 3, the curved wedge  8  will be forced in between the bearing bracket  22  and the intermediate part  27  pushing the cross bar  34  downwards, accomplishing a brake application. The cross bar  34  will move in a plane essentially perpendicular to the plane of the brake disc.  
         [0023]    The lever  26  acts on the cross bar  3  by means of an intermediate part  27 . The intermediate part has the form of a rocker  29  in this embodiment but may have other forms in another embodiments. The lever  26  rotates around a first point of rotation  1 . The intermediate part  27  rotates around a second point of rotation  2 .  
         [0024]    The bearing bracket  22  has a protruding part  32  with a cylindrical surface for co-operation with the cylindrical inner surface of the lever  26 . A bearing may be placed between the lever  26  and the protruding part  32  of the bearing bracket  22 .  
         [0025]    The rocker  29  has a cylindrical surface in contact with the outer surface of the wedge  8  of the lever  26 . The rocker  29  is received in a groove  35  of the cross bar  34 . In the end positions of the movement of the rocker  29  it will abut the sides of the groove  35 . The contact surfaces of the groove  35  and the rocker  29  are both cylindrical.  
         [0026]    In an alternative embodiment (not shown) a cylindrical stud is placed between the rocker  29  and the bottom of the groove  35 .  
         [0027]    A return spring  11  is positioned between the cover  7  and the cross bar  34  in order to bring the brake mechanism back to its rest position.  
         [0028]    The bearing bracket  22  is received in an opening of the caliper  16 . The opening is placed in the wall of the caliper  16  furthest from the brake disc. Thus, the caliper is of an open design. The bearing bracket  22  has a shoulder  18  abutting the inside of the caliper  16  and thus, the bearing bracket  22  is placed in the opening of the caliper  16  from the inside. The reaction of the clamp force of the brake is transmitted by means of the bearing bracket  22  to the open caliper  16 . The force of reaction is transmitted via the shoulder  18  of the bearing bracket  22 . As the force of reaction is transmitted by means of the shoulder  18  of the bearing bracket  22  the force is transmitted in an area surrounding the opening of the caliper  16 . The bearing bracket  22  is a loaded part of the brake mechanism and, thus, the bearing bracket  22  and its shoulder  18  should have enough strength to transmit the force of reaction. A person skilled in the art realises that the shoulder may be placed in the caliper in stead of the bearing bracket, in which case the force of reaction will be transmitted via the edge area of the bearing bracket to the shoulder of the caliper.  
         [0029]    A sealing is placed between the bearing bracket  22  and the open caliper  16 . The sealing between the bearing bracket  22  and the open caliper  16  is received in a groove of the bearing bracket  22 . The groove and thus the sealing may be placed in any position axially or radially in the bearing bracket  22 . The inside of the open caliper  16  may be machined via said opening of the caliper  16 .  
         [0030]    An adjuster mechanism  23  of known construction is placed on top of one of the adjustment and reset shafts  6 . When the brake is applied a lever pin  43  of the lever  26  will act on the adjuster mechanism  23 . In the bearing bracket  22  a number of gear wheels  24  are placed between gear wheels of the adjustment and reset shafts  6  to transfer and synchronize the movement between the shafts. Each gear wheel is placed on a pin  25  fixed to the bearing bracket  22 . The gear wheels  24  are placed under a cover  42  received in the bearing bracket  22 . In FIG. 1 the cover  42  is shown partially broken away for clarity. In the shown example there are four gear wheels  24  between the gear wheels of the adjustment and reset shafts  6 . A person skilled in the art realises that other numbers of gear wheels may be used. As the adjuster mechanism forms no part of the present invention it will not be described further here.  
         [0031]    At least one of the adjustment and reset shafts  6  is furnished with a suitable head to receive a tool used to reset the thrust units when the brake pads are to be replaced. This movement will be transferred to the other adjustment and reset shaft  6  by means of the set of gear wheels  24 . The adjustment and reset shaft  6  is rotated in the normal way until the distance between the thrust plates  5  and the brake disc is sufficient to receive the new brake pads. Then the adjustment and reset shaft  6  is rotated in such a way that the distance between the thrust plates  5  and the brake disc corresponds to the desired running clearance. The at least one adjustment and reset shaft  6  is received in a sealed opening of the bearing bracket  22 .  
         [0032]    The cross bar  34  is supported by four protruding parts  36  of the cross bar and the return spring  11  in the cover  7 . The protruding parts  36  of the cross bar  34  abut the inside of the open caliper  16 . The part of the caliper  16  in contact with the protruding parts of the cross bar  34  is machined to give a smooth surface. The machining is done via the opening of the open caliper  16 . The return spring  11  is guided in an opening in the cross bar  34  and acts between the cross bar  34  and the cover  7 . The return spring  11  is received in a holder of the cover  7 . The cross bar  34  is free to move in the thrust direction along the machined part and in one direction perpendicular to the thrust direction. The latter direction is the tangential direction of the brake discs. In an alternative embodiment the cross bar  34  is guided on guide sleeves (not shown) placed around the screws that are used to fix the cover  7  to the open caliper  16 .  
         [0033]    When the brake is activated the lever  26  will press the intermediate part  27 , the cross bar  34  and thus the thrust plates  5 , via the adjustment screws  4  and the brake pads in direction towards the brake disc (not shown). When the brake pads hits the brake disc, the pads will move in the tangential direction of the brake disc a short distance before the brake pads hit a support (not shown). The movement in the tangential direction of the brake disc is normally not more than a few millimeters. The lever  26 , the intermediate part  27 , the cross bar  34  and the thrust units of the brake mechanism will follow the brake pads in their movement. During this movement the lever  26  will slide in the bearing bracket  22 . The cross bar  34  will move in the tangential direction of the brake disc guided by the protruding parts  36  of the cross bar  34  or the guide sleeves. The cover  7  is fixed to the caliper  16  and will not move. The movement between the cross bar  34  and the cover  7  is taken up by the return spring  11 . When the brake is released the return spring  11  will bring the cross bar  34  back to its centred position. The lever  26 , the intermediate part  27  and the thrust units will move with the cross bar  34  into the centred position. Thus, the return spring  11  resets the brake mechanism both in the thrust direction and sideways.  
         [0034]    The cover  7  is fixed to the open caliper  16  by means of screws. The cover  7  has openings to receive the thrust units. There is a clearance between the cover  7  and the thrust units allowing the thrust units to move in any direction in relation to the cover  7 .  
         [0035]    The second embodiment of FIGS.  4  to  7  correspond to the above embodiment in many ways and the corresponding parts will not be described further.  
         [0036]    In the embodiment of FIGS.  4  to  7  the lever  31  has a protruding part received in a slide bearing  33  of the bearing bracket  30 . The lever  31  acts on an intermediate part  27  in form of a stud  28 , which acts on the cross bar  37 . The stud  28  is received in a groove of the cross bar  37  by means of a bearing. The surface of the lever  31  in contact with the intermediate part  27  is shown as cylindrical but may have other curve forms. In this embodiment the lever  31  has no wedge form.  
         [0037]    As indicated in FIG. 7 the cross bar  37  is guided in the caliper  16  in such a way that it is not moveable in a direction perpendicular to the thrust direction or the tangential direction of the brake disc.  
         [0038]    In the second embodiment the thrust units are connected with a single, common thrust plate  38 , which thrust plate  38  acts on a brake pad. The thrust units each comprises a thrust screw  39 . The thrust screws  39  have an opening at the bottom, which receives a stud on the thrust plate  38 . The studs of the thrust plate  38  and the openings of the thrust screws  39  are formed to lock the thrust screws  39 , thus hinder them from rotating. This is important for control of the distance between the brake pads and the brake disc.  
         [0039]    One of the thrust units is further furnished with the adjuster mechanism  23  as stated above and an adjuster shaft  40 . The adjuster mechanism  23  is placed on top of the thrust screw  39 . The other thrust unit is furnished with a reset shaft  41 . The shafts  40 , 41  are drivingly connected by means of a set of gear wheels  24 . The set of gear wheels  24  are placed in the area between the cover  7  and the thrust plate  38 . The function of the gear wheels is the same as for the set of gear wheels  24  of the first embodiment.  
         [0040]    The adjuster mechanism  23  co-operates with the lever  31  by means of a lever pin  43 . The shafts  40 , 41  and the screws  39  of the thrust units may rotate relative each other, which is of importance for adjustment of slack in the disc brake. The shafts  40 , 41  have the form of sleeves placed on the outside of the thrust screws  39 .  
         [0041]    To protect the brake mechanism from road dirt bellows  15  are placed between the thrust plates  5  and the cover  7 . In the shown embodiment the bellows  15  are placed in a heat protection ring. The bellows  15  are normally also present in the embodiment of FIGS.  1  to  3  but are not shown in said FIGS.  
         [0042]    As shown in FIG. 4 the brake mechanism may have a modular form. In FIG. 4 one module consists of the bearing bracket  30  and the lever  31 . The lever  31  is attached to the bearing bracket by means of a clip  13 . A second module is formed by the rest of the brake mechanism and is held together by a brace  14 . The modular form may be advantageously concerning assembly and maintenance.  
         [0043]    A person skilled in the art realises that the lever and intermediate part according to the present invention may be used independently of if the brake mechanism is of modular form or not. He also realises that the lever and intermediate part may be used even if the brake mechanism has no bearing bracket.  
         [0044]    In a further embodiment (not shown) the lever is turned around compared to the previous embodiments. The turning is made in such a way that the intermediate part  27  and the lever change places. Thus, in this embodiment the intermediate part  27  will be received in the bearing bracket and the lever will be in direct contact with the cross bar.  
         [0045]    The lever and the intermediate part function in practice in the same way for all the above embodiments. The lever  26 , 31  and the intermediate part  27  are in contact with each other along a line. Thus, they have a line of contact, which will reduce friction and wear compared to the normal extended area of contact between the lever and the cross bar or an intermediate part. The line of contact between the lever  26 , 31  and the intermediate part  27  moves along a cam profile which may be a circular arc. The cam profile has a distance R 1  in relation to the point of rotation  1  for the lever and a radius R 2  in relation to the point of rotation  2  for the intermediate part  27 . The distance R 1  varies as the lever  26  of the first embodiment moves if the outer surface of the wedge  8  has a non-cylindrical curve form. The distance R 1  is a radius if the surface of the lever in contact with the intermediate part is cylindrical. The intermediate part  27  is supported in the cross bar  37  via a roller bearing. The intermediate part  27  has two flanges between which the lever  26 , 31  is received. The surface of the intermediate part  27  in contact with the lever  26 , 31  is cylindrical having a radius R 2 . Thus, also the part of the intermediate part  27  in contact with the lever  26 , 31  moves along a circular arc having the radius R 2  in relation to the point of rotation  2  of the intermediate part  27 .  
         [0046]    The lever  26 , 31  has an elongated part the upper part of which co-operates with the actuator. At the opposite end the lever  26 , 31  has a protruding part or a recess received in a recess or protruding part, respectively of the bearing bracket  22 , 30 . Thus, the lever  26 , 31  is supported between the bearing bracket  22 , 30  and the intermediate part  27 . The lever  26 , 31  has an active length L between a position  3  where the brake actuator acts on the lever and the point of rotation  1  for the lever. The positions of the points of rotation  1 , 2  for the lever  26 , 31  and the intermediate part  27 , respectively are offset a distance D from each other.  
         [0047]    When the brake is applied the lever  26 , 31  will rotate in the bearing  33  of the bearing bracket  22 , 30 . By varying the radius R 2 , the offset distance D, the curve form of the surface of the part of the lever  26 , 31  in contact with the intermediate part and the length of the lever  26 , 31  it is possible to vary the force amplification characteristics of the brake mechanism. In an ideal situation the force should be the same independent of the stroke length of the brake actuator. If a pneumatic cylinder is used to actuate the brake lever  26 , 31 , such a cylinder normally gives a force dependant on stroke length as indicated in the middle graph of FIG. 8. As shown in the graph, the force follows a sloping curve, having a stronger inclination at the beginning and end of the stroke. By making the variations as stated above it is possible to form a characteristic for the lever ratio versus stroke length of the brake actuator, which is a mirror image of the characteristic of the pneumatic cylinder, as shown in the middle graph. A suitable characteristic for the lever ratio for the lever  26 , 31  versus stroke length of the brake actuator is shown in the upper graph of FIG. 8. The result of the two characteristics will be a force executed on the brake pads which is independent of the stroke length of the brake actuator as indicated in the lower graph of FIG. 8.  
         [0048]    Thanks to the module form of the brake mechanism it is easy to change lever  26 , 31 , bearing bracket  22 , 30  and/or intermediate part  27  in order to make the alterations of the force amplification characteristics as stated above.  
         [0049]    A person skilled in the art realises that the lever and intermediate part may be used in many different types of brake mechanisms.