Abstract:
A disc brake assembly includes a brake pad having a first pad abutment spaced from a second pad abutment and a third pad abutment positioned between the first pad abutment and the second pad abutment. An actuator is operable to apply a force to the brake pad to apply the brake. The disc brake assembly has a first reaction abutment facing the first pad abutment, a second reaction abutment facing the second pad abutment, and a third reaction abutment facing the third pad abutment. The first, second and third pad abutments are operable to transfer brake pad drag torque to the first, second and third reaction abutments respectively. The disc brake is configured such that under no load conditions with the third pad abutment engaging the third reaction abutment, the first pad abutment is spaced from the first reaction abutment.

Description:
RELATED APPLICATION 
     This application claims priority to GB Application 1016013.3, which was filed Sep. 24, 2010. 
     TECHNICAL FIELD 
     The present invention relates to a disc brake assembly, in particular a disc brake assembly for use with a heavy land vehicle, such as a lorry or a truck. 
     BACKGROUND OF THE INVENTION 
     Known trucks typically use drum brakes or disc brakes, or a combination of drum and disc brakes, to slow the vehicle when required. 
     EP1473481 shows a known disc brake assembly that has a carrier with just a leading edge abutment and a trailing edge abutment for each brake pad. When the associated vehicle is travelling in a forward direction, the brake is applied and the brake pad drag torque on the inboard brake pad is reacted entirely by the abutment on the trailing edge of the carrier. When the vehicle is travelling in a reverse direction, the brake is applied and the brake pad drag torque on the inboard pad is reacted entirely by the abutment on the leading edge of the carrier. This is the case when the brakes are applied relatively lightly or relatively heavily. 
     US2006/0060431 shows a light vehicle caliper which is hydraulically operated. The arm of the caliper passes over the top of the brake pads, and in order to remove and replace the brake pads the caliper must be removed from the vehicle. 
     Trucks have a primary direction of movement, i.e. they primarily travel in a forward direction, and as such the brakes operate almost exclusively when the vehicle is travelling in a forward direction. While the vehicle can travel in a reverse direction, this typically occurs only occasionally and when it does so the vehicle will be travelling at a low speed thereby causing very little wear on the brake pads/shoes. 
     When a disc brake assembly is fitted to such a truck, a brake disc rotates within a space created between two brake pads. Because the vehicle has a primary direction of travel, each brake pad has a leading edge and a trailing edge, i.e. when the vehicle is travelling in the forward direction a point on the brake disc friction surface will approach the leading edge of the pad first, then pass under the brake pad and depart from the trailing edge of the pad. 
     The brake pad typically includes a friction material attached to a brake pad backplate. Clearly, when the brake is applied, a drag torque is created at the interface between the disc friction surface and the pad friction surface. This drag force is typically reacted by the brake pad backplate near the trailing edge of the pad. 
     When new, typically the brake pad backplate is flat and the brake pad friction material will be of constant thickness. In some brakes the friction material can tend to wear unevenly, in particular it can wear in a taper fashion with the leading edge wearing faster than the trailing edge, i.e. in a tangentially tapered fashion. This results in a brake pad becoming unusable (i.e. worn out) earlier than it might otherwise become had it worn in a parallel, i.e. non taper fashion. 
     The present invention seeks to mitigate or at least partially mitigate the problem of taper wear on brake pads. 
     SUMMARY OF THE INVENTION 
     A disc brake assembly includes a brake pad having a first pad abutment spaced from a second pad abutment, and a third pad abutment positioned between the first pad abutment and the second pad abutment. 
     An actuator is operable to apply a force to the brake pad to apply the brake. The disc brake assembly has a first reaction abutment facing the first pad abutment, a second reaction abutment facing the second pad abutment, and a third reaction abutment facing the third pad abutment. The first, second, and third pad abutments are operable to transfer brake pad drag torque to the first, second, and third reaction abutments respectively. The disc brake is configured such that under no load conditions, with third pad abutment engaging the third reaction abutment, the first pad abutment is spaced from the first reaction abutment. 
     Advantageously, by providing three pad abutments for transferring brake pad drag torque, it allows each pad abutment to transfer brake pad drag torque under certain circumstances. By positioning the third pad abutment between the first and second pad abutments, the third pad abutment can be utilized to transfer drag torque under relatively low braking forces when the vehicle is travelling in a forward direction. The third and first pad abutments can be utilized to transfer brake torque when the brakes are being applied relatively heavily with the vehicle travelling in the forward direction. The second pad abutment can be used to transfer brake torque when the vehicle is travelling in a reverse direction. Such an arrangement reduces leading edge taper wear when the brakes are applied relatively lightly (due to the relative position of the first, second, and third pad abutments, i.e. the third pad abutment being positioned between the first and second pad abutments), and hence reduces leading edge taper wear over the life of the brake pad. By providing the third pad abutment between the first pad abutment and the second pad abutment, it creates a relatively short brake pad which is generally advantageous. When such a brake pad is used in conjunction with a slideably mounted brake caliper having circumferentially spaced arms, which connect an inboard part of the brake caliper to an outboard part of the brake caliper, and when the brake pad is installed in or removed from the disc brake assembly by inserting/removing from between the brake arms a relatively short brake pad, it allows the arms to be relatively circumferentially close together. This is advantageous since it allows for a compact caliper. 
     The third pad abutment may be circumferentially between the first and second pad abutments. 
     The disc brake assembly may include a brake caliper slideably mounted on a brake carrier and in which the first reaction abutment and/or the second reaction abutment and/or the third reaction abutment is provided on the brake carrier. 
     The brake caliper may include a first side portion connected to a second side portion by two bridge arms, the bridge arms being circumferentially spaced apart by a distance sufficient to allow the brake pad to be installed in or removed from the disc brake assembly. 
     In one embodiment two pins are provided to allow the brake caliper to be slideably mounted on the brake carrier. Each pin may be mounted on the caliper and be slideable in a hole in the carrier, or may be mounted on the carrier and be slideable in a hole in the caliper. 
     A method of operating a disc brake assembly includes operating the brake such that the force is relatively low, thereby generating a relatively low circumferential drag force on the pad. Further steps include: reacting the relatively low circumferential drag force substantially entirely at the third reaction abutment alone; operating the brake such that the force is relatively high, thereby generating a relatively high circumferential drag force on the brake pad; and reacting the relatively high circumferential drag force substantially entirely at the third reaction abutment and at the first reaction abutment. 
     The method may include mounting the brake assembly on a vehicle having a primary direction of movement, applying the brake when the vehicle is travelling in the primary direction of movement such that the relatively low circumferential drag force is reacted substantially entirely at the third reaction abutment and the relatively high circumferential drag force is reacted substantially entirely at the third reaction abutment and at the first reaction abutment. 
     Vehicles having a primary direction of movement define a leading edge and a trailing edge of the brake pads, which in turn defines a leading edge side and a trailing edge side of the actuator (and any associated piston). In one embodiment, the third reaction abutment may be offset on the leading edge side of the piston. In another embodiment, the third reaction abutment may be offset on the trailing edge side of the piston. 
     The actuator may be mounted on the first side. The brake pad may be mounted on the first side. 
     Advantageously, the third abutment may be relatively circumferentially close to the line of action of the actuator when it applies the force to the brake pad. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a plan view of a known disc brake assembly, 
         FIG. 2  is a plan view of part of a disc brake assembly according to the present invention, 
         FIG. 3  is a view taken in the direction of arrow A of  FIG. 2 , and 
         FIG. 4  is a schematic view of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1  there is shown a known brake assembly  10  having a carrier  12  mounted on a vehicle suspension component  14  (shown schematically). A caliper  16  is slideably mounted on the carrier for movement in the direction of arrow B. A brake disc (also known as brake rotor)  18  is rotatably mounted on the suspension component and rotates with an associated wheel. When the vehicle is travelling in the forward direction the brake disc rotates in the direction of arrow C. 
     An actuator mechanism is mounted in the brake assembly. 
     The brake assembly includes an inboard brake pad  22  and an outboard brake pad  24 . The inboard brake pad  22  includes an inboard brake pad backplate  30  having a trailing edge abutment  32  and a leading edge abutment  34 . 
     Attached to the inboard brake pad backplate is an inboard brake pad friction material  36  having a trailing edge  38  and a leading edge  40 . 
     When the brake is applied the actuator mechanism causes the piston  20  to move in the direction of arrow D. This forces the inboard brake pad  22  in the direction of arrow D whereupon it engages the inboard friction surface  18 A of the brake disc  18 . Continued application of the brake then causes the caliper  16  to move in the direction of arrow H which causes the outboard brake pad  24  to also move in the direction of arrow H resulting in the outboard brake pad  24  engaging the outboard friction surface  18 B. Continued application of the brake clamps the brake disc  18  between the inboard and outboard brake pads thereby slowing the rotational movement of the brake disc  18 , and hence slowing the vehicle. 
     As the brake is applied, the actuator mechanism applies a force to the piston  20  in the direction of arrow G and a circumferential brake pad drag torque is created between the brake disc  18  and inboard brake pad  22 . This brake pad drag torque acts in the direction of arrow E on the brake pad. This brake pad drag torque is reacted at the trailing edge abutment  32  in the direction of arrow F. 
     The center line of the piston is offset from the trailing edge abutment  32  by distance M 1 . As the brake is progressively applied, the inboard brake pad moves in the direction of arrow D and the brake pad drag torque E creates friction at the interface between the trailing edge abutment  32  and the associated reaction abutment on the carrier  12 . Continued application of the brake results in the piston applying a turning moment in an counterclockwise direction when viewing  FIG. 1 , the moment arm being distance M 1 . It is this turning moment that creates leading edge taper wear. 
     With reference to  FIGS. 2 and 3  there is shown a brake assembly  110  including a carrier  112  (shown schematically), a vehicle suspension component  114  (shown schematically), a caliper  116  (shown schematically), and a brake disc  118  (shown schematically). 
     The brake assembly  110  includes an actuator mechanism  121  (shown schematically). The actuator mechanism includes a piston  120  (only part of which is shown). The brake assembly includes an inboard brake pad  122  having an inboard brake pad backplate  130  having a trailing edge abutment  132  and a leading edge abutment  134 . The inboard brake pad includes an inboard brake pad friction material  136  having a leading edge  140  and a trailing edge  138 . Brake assembly  110  includes outboard brake pad  123  (see  FIG. 4 ). When the associated vehicle is travelling in a forward direction the brake disc  118  rotates in the direction of arrow C (see  FIGS. 2 and 3 ). 
     The inboard brake pad backplate also includes a further abutment (also known as a third abutment)  142 . 
     The trailing edge abutment  132  (also known as first abutment) faces a first reaction abutment  150  of the carrier  112 . The leading edge abutment  134  (also known as a second abutment) of the inboard brake pad backplate faces a second reaction abutment  151  of the carrier  112 . The further abutment  142  of the inboard brake pad backplate faces a third reaction abutment  152  of the carrier  112 . 
     Both the trailing edge abutment  132  and the further abutment  142  face in opposite direction to the leading edge abutment  134 . Thus, trailing edge abutment  132  and further abutment  142  face to the right hand side when viewing  FIG. 3  and the leading edge abutment  134  faces to the left when viewing  FIG. 3 . It will also be appreciated that the trailing edge abutment  132  and the further abutment  142  face away from the direction in which the leading edge abutment  134  faces. Thus, as shown in  FIG. 3 , the trailing edge abutment  132  and further abutment  142  both face right when viewing  FIG. 3  and are both positioned on the right hand side of the leading edge abutment  134 . 
     The trailing edge abutment  132  and leading edge abutment  134  are positioned on circumferential ends of the brake pad. 
     The distance X 1  between the further abutment  142  and the trailing edge abutment  132  is slightly smaller than the distance X 2  between the third reaction abutment  152  and the first reaction abutment  150 . This is best seen in  FIG. 3 , and note that the difference in the distance has been exaggerated so as to aid explanation. The difference in distances X 1  and X 2  will depend upon the particular circumstances but examples may be 0.5 mm and 1.0 mm as a difference between the distances X 1  and X 2 . 
     The further abutment  142  is formed in a notch on the inboard brake pad backplate  130 . 
     As will be appreciated from  FIG. 3 , the trailing edge abutment  132  is offset by distance M 1  from the center of the piston  120  to the trailing side of the brake pad. However, the further abutment  142  is offset from the center line of the piston by distance by M 2 , a distance smaller than M 1 . Furthermore, the further abutment  142  has been offset on the leading edge side of the pad relative to the piston. 
     The third reaction abutment  152  is formed on the carrier  112 . In this case the third reaction abutment  152  is formed on a surface of a canter lever arm or peg  154 . This case arm  154  is integral with the carrier, though in further embodiments this need not be the case. 
     Operation of the brake is as follows: 
     General operation of the brake is as described with regard to the prior art. However, the brake pad drag torque is reacted differently, depending upon how hard the brake is applied. 
     Vehicle Travelling in Forwards Direction, Brakes Applied Relatively Lightly 
     If the brakes are applied relatively lightly, then relatively little amount of deflection occurs in the carrier and caliper. In particular, a relatively small amount of deflection occurs on arm  54 . Thus, when the brakes are applied relatively lightly, the brake pad drag torque applied to the inboard brake pad is reacted by the third reaction abutment  152  alone. In the rest position, because, as described above, distance X 1  is smaller than distance X 2 , the trailing edge abutment  132  is separated from the first reaction abutment  150  by distance T. If the brake is applied relatively lightly, while the arm  154  may deflect slightly to the right when viewing  FIG. 3 , nevertheless it will deflect less than distance T, and under these circumstances the trailing edge abutment  132  will not come into engagement with the first reaction abutment  150 . Under these circumstances the brake drag torque will be reacted by the third reaction abutment  152  alone. Because the third reaction abutment  152  is offset from the piston center line by distance M 2 , which is smaller than distance M 1 , then the turning moment arm is smaller. Furthermore, because the third reaction abutment  152  is offset on the leading edge side of the piston, the turning moment is in the opposite sense to the prior art, i.e. when the brake is applied relatively lightly, the turning moment tends to cause the inboard brake pad to rotate clockwise when viewing  FIG. 2 . 
     Vehicle Travelling in Forwards Direction, Brakes Applied Relatively Heavily 
     When the vehicle is travelling in the forward direction and the brakes are applied relatively heavily, then a higher brake pad drag torque is created sufficiently high to cause arm  154  to deflect at least by a distance equivalent to distance T. Under these circumstances the gap between trailing edge abutment  132  and the first reaction abutment  150  will close and the brake pad drag torque will be reacted in two places, namely at the third reaction abutment  152  and also at the first reaction abutment  150 . The turning moment will then be a combination of clockwise moment generated by the further abutment  142  and the counterclockwise moment generated by the trailing edge abutment  132 . Depending upon how hard the brakes are applied, the gross moment on the pad may be clockwise, neutral, or counterclockwise. 
     Vehicle Travelling in Reverse, Brakes Applied 
     When the vehicle is travelling in reverse and the brakes are applied, either relatively hard or relatively soft, the brake pad drag torque is reacted at the second reaction abutment  151 . 
     For ease of explanation, in the description above it has been assumed that all of the brake pad drag torque is reacted at the first reaction abutment, second reaction abutment, and third reaction abutment as the particular circumstances dictate. In practice, relatively small amounts of brake pad drag torque can be reacted elsewhere. Thus, small amounts of brake pad drag torque can be reacted at the interface between the piston and the brake pad backplate, at the interface between radially inner edge of the brake pad backplate and the carrier, and at the interface between the brake pad retaining spring (not shown) and the brake pad retaining strap (not shown). However, these alternative reaction points only amount to a very small percentage of brake pad drag torque, and as such, brake pad drag torque is substantially entirely reacted by the first reaction abutment, second reaction abutment, and third reaction abutment as circumstances dictate. By analogy with regard to the prior art, the brake pad drag torque of the inboard brake pad  22  is reacted substantially entirely by that part of the carrier facing trailing edge abutment  32  when the vehicle is travelling in the forward direction and the brake pad drag torque is substantially entirely reacted by that part of the carrier facing the leading edge abutment  34  when the vehicle is travelling in reverse. 
     As described above, the third reaction abutment is offset on the leading edge side of the piston, though in other circumstances it may be offset on the trailing edge side of the piston. As described above the actuator mechanism includes a single piston. In further embodiments, two or more pistons may be utilized. As described above, the third reaction abutment is provided on the inboard side of the brake pad, i.e. on the actuator side of the brake assembly. In further embodiments, a third reaction abutment may be provided on the outboard side of the brake assembly (when considering the rotor) for use with the outboard brake pad. As described above, the third reaction abutment has been provided on the carrier. In embodiments where the brake torque may be transferred directly to the caliper, then the third reaction abutment may be provided on the caliper. As described above, the third abutment  142  is closer to the leading edge abutment  134  (see distance Y) than it is to the trailing edge abutment  132  (see distance X 1 ). In further embodiments, the third abutment could be closer to the trailing edge abutment than the leading edge abutment. In further embodiments, the third abutment may be equidistant from the trailing edge abutment and the leading edge abutment. 
     Attention is drawn to features 20A and 20B of EP1473481. Under no circumstances do these features transfer brake pad drag torque to the carrier, rather they simply reduce the likelihood of misassembly of the brake pads. 
     Replacement of Brake Pads 
       FIG. 4  shows a schematic view of the brake disc assembly of  FIG. 2 . The caliper  116  is slideably mounted via pins  117  on the carrier  112 . The carrier is mounted on a suspension component  114  of a vehicle  200 . The caliper has a first side portion  116 A connected to a second side portion  116 B by bridge arms  116 C and  116 D. The bridge arms  116 C and  116 D are spaced apart by a distance D which is slightly larger than the distance d between the ends of the brake pads  122  and  123 . 
     Thus, when the brake pads  122  and  123  have become worn, they can be removed radially outwardly (in the direction of the viewer when viewing  FIG. 4 ), and can be replaced with newer brake pads which are inserted radially inwardly (into the paper when viewing  FIG. 4 ) without having to remove either the caliper  116  or the carrier  112  from around the brake disc  118 . In other words, the brake pads can be removed and replaced while the caliper and carrier are correctly positioned around the brake disc  118 . This is significant when dealing with heavy vehicle brake assemblies since the calipers and carriers of such assemblies are relatively heavy and cannot be manipulated by a single operator. For these reasons brake assemblies of heavy vehicles are designed such that the brake pads can be removed and replaced without having to disturb the carrier or caliper. Accordingly, by providing a relatively short brake pad, it means that the carrier arms  116 C and  116 D can be spaced apart by a relatively short distance, and this in turn means that arms  112 A and  112 B of the carrier can also be spaced apart by a relatively short distance thereby providing a compact brake assembly. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.