Abstract:
A caliper for a fixed twin rotor disc brake includes a relatively fixed body, two yokes movable axially with respect to the body, formations defined on the yokes for receiving four brake pads for respective application to rotor sides, and an actuator adapted to urge the yokes relatively axially. One of the yokes is operable to urge two of the four brake pads in a first axial direction, the other of the yokes is operable to urge the other two of the four brake pads in a second, opposite axial direction. The four brake pads are positioned on a first side of the actuator in the first axial direction.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom Patent Application GB 0610960.7 filed on Jun. 2, 2006. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a twin rotor disc brake primarily, but not exclusively, for heavy commercial vehicles. 
     Disc brakes include a rotor having opposite annular faces to which brake pads are applied by a caliper. Hydraulic actuation is generally provided in passenger and light commercial vehicles, and the brake pads are applied to the rotor by opposed pistons of the caliper, or by a single piston caliper having a sliding yoke. Heavy commercial vehicles rely upon air actuation, which necessarily requires a relatively large air actuator. 
     Disc brakes are preferred over drum brakes because of their increased performance and ease of inspection and maintenance. However, a rotor diameter is generally restricted by a vehicle wheel diameter, and in some cases a swept area of the rotor may be insufficient for the weight of the vehicle. Accordingly, twin rotor disc brakes have been developed, but in order to use a single caliper, one or both of the rotors must slide axially relative to the axle, for example on splines. This arrangement is not wholly satisfactory because sliding splines are relatively expensive to manufacture and have a tendency to stick due to corrosion or an accumulation of dirt and brake pad residues. Multiple calipers are not practicable. It would be desirable to provide a single caliper, twin rotor disc brake in which the rotors are relatively fixed, but the caliper has a single actuator. 
     A further problem of twin rotor disc brakes is that they are generally mounted at axle ends, and in use at least partially shrouded by the wheel. Accordingly, removal of the wheel is necessary for inspection and replacement of the brake pads and/or the rotors. Frequently brake pad wear is uneven so that semi-worn brake pads are replaced at the same time as fully worn pads in order to avoid an intermediate inspection. It would be desirable to equalize brake pad wear and to provide a way of actuation suitable for mid-axle location, where appropriate. Such an arrangement would be especially useful in undriven rigid axles of e.g., semi-trailers, so that unnecessary wheel removal is avoided. Generally speaking, wheel removal should be avoided if possible because of the well known problem of settling of freshly tightened wheel nuts, which can lead to loosening of the nuts and wheel shedding. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a caliper for a fixed twin rotor disc brake. The caliper includes a relatively fixed body, two yokes movable axially with respect to the body, formations defined on the yokes for receiving four brake pads for respective application to rotor sides and an actuator adapted to urge the yokes relatively axially. One of the yokes is operable to urge two of the four brake pads in a first axial direction, and the other of the yokes is operable to urge the other two of the four brake pads in a second, opposite axial direction. The four brake pads are positioned on a first side of the actuator in the first axial direction. 
     According to a second aspect of the invention, there is provided a caliper for a fixed twin rotor disc brake. The caliper includes a relatively fixed body, a first yoke moveable axially with respect to the body, and a second yoke moveable axially with respect to the body. The first yoke has formations for receiving a first pair of brake pads for application to rotor sides. The first yoke further includes a first leg connecting the formations for receiving each of the first pair of brake pads. The second yoke has formations defined thereon for receiving a second pair of brake pads for application to the rotor sides and a second leg connecting the formations for receiving each of the second pair of brake pads. The first yoke is operable to urge the first pair of brake pads in a first axial direction, and the second yoke is operable to move the second pair of brake pads in a second, opposite axial direction. The first leg and the second leg move in substantially the same plane. 
     Such an arrangement combines the reliability of a fixed rotor installation with the simplicity of a single actuator operable on both yokes at the same time. This latter feature ensures equalization of brake pad wear between pad pairs. The actuator acts directly on each yoke, and in one embodiment in a common plane. 
     In one example, the actuator floats with respect to the body, thus permitting the wear of the two brake pad pairs to be equalized. In one embodiment, the actuator is substantially midway between the rotors, and the yokes are identical. In another embodiment, the actuator is inboard of the rotor pair. 
     In one embodiment, the yokes overlap. The yokes include oppositely facing ‘U’ shaped members defining an aperture between the members, the actuator being provided in the aperture and operable to urge the members apart. The yokes may have mutually engaging sliding surfaces such that one yoke may be supported in part on the body via the other. Each yoke may include both a supporting portion for the other yoke, and a portion which is supported on the other yoke. 
     The actuator may be a cam pivotable about a radius of the axis of rotation of the axle. The cam is preferably symmetrical and in one embodiment is oval. Cams pivotable about the other axes are possible, provided that the effect is to urge the members equally apart. 
     In place of a cam, the actuator may include a pneumatic jack or any other feature capable of extension on operation thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One feature of the invention will be apparent from the following description of preferred embodiments shown by way of example only in the accompanying drawings in which: 
         FIG. 1  is a plan schematic view of a first embodiment of the invention; and 
         FIG. 2  is a plan schematic view of a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to  FIG. 1 , a caliper body  10  is mounted by any suitable manner on a fixed element of a vehicle axle having an axis of rotation  11 . Twin rotors  12  and  13  are attached to a rotating element of the axle, such as a hub, by any suitable conventional method. The rotors  12  and  13  may include a single component or a unitary assembly. In the embodiment shown, the rotors  12  and  13  are ventilated, however ventilation of the rotors  12  and  13  may not always be necessary. 
     Two overlapping yokes  14  and  15  are provided, one yoke  14  and  15  straddling a respective rotor  12  and  13 . The yoke  14  has two depending arms  16  and  17 , and the yoke  15  has two depending arms  18  and  19 . The four depending arms  16 ,  17 ,  18  and  19  are each applied to the backplate of the four brake pads  21 ,  23 ,  22  and  24 , respectively, as illustrated. The depending arms  16  and  17  are connected by a pair of connecting legs  30  and  31  to hold them a set distance apart. Similarly, the depending arms  18  and  19  are connected by a pair of connecting legs  32  and  33 . The connecting legs  30 ,  31 ,  32 ,  33  are offset relative to each other and move relative to each other in the same plane such that the yokes  14  and  15 , when assembled, only have the height of a single yoke; i.e., they are side-by-side and co-planar. This arrangement is possible by ensuring that the depending arm  18  is positioned below the parallel connecting legs  30  and  31 , and the depending arm  17  is positioned below the parallel connecting legs  32  and  33 . Alternatively, grooves or indentations may be provided on the depending arms  17  and  18  to slideably receive the connecting legs  30 ,  31 ,  32  and  33 . 
     The arrangement provides that the depending arms  16 ,  17 ,  18  and  19  of each yoke  14  and  15  are associated with one brake pad  21 ,  22 ,  23  and  24  of each rotor  12  and  13 . Thus, the depending arms  16  and  17  are associated with the brake pads  21  and  23 , and the depending arms  18  and  19  are associated with the brake pads  22  and  24 . 
     The yokes  14  and  15  are slidable in the direction of the axis of rotation  11  with respect to each other and to the caliper body  10 . Suitable sliding arrangements are provided, such as key and keyway elements  25 , but any suitable conventional method may be used, including cylindrical pins and corresponding apertures. As with all kinds of sliding calipers, the intent is that only relative movement in the direction of the axis of rotation  11  is permitted. Relative arcuate movement is, of course, prevented to allow grounding of the braking torque via the caliper body  10 . 
     The yokes  14  and  15  are generally rectangular in plan and overlap to define an aperture  26  centered between the rotors  12  and  13 . An oval cam  27  is provided in the aperture  26  and is rotatable about the caliper body  10  to increase the yoke  14  and  15  overlap, or permit a reduction in the yoke  14  and  15  overlap. As illustrated, the cam  27  is rotatable about an axis radial of the axis of rotation  11 , but a cam  27  rotatable about an axis orthogonal to a radius could also be employed. 
     In use, rotation of the cam  27  is effected by any suitable actuation mechanism, including, for example, mechanical, electrical, hydraulic or pneumatic systems. Rotation in either direction causes the yokes  14  and  15  to be urged, respectively, in the direction of arrows  28 , thus applying the brake pads  21 ,  22 ,  23  and  24  to the rotors  12  and  13 . The yoke  14  applies the brake pads  21  and  23  in one axial direction, whereas the yoke  15  applies the brake pads  22  and  24  in the other axial direction. 
     The brake pads  21 ,  22 ,  23  and  24  associated with each yoke  14  and  15  will wear equally, and any uneven wear of one brake pad is immediately compensated by a greater braking effort being generated at the other brake pad. 
     The cam  27  may be fixed with respect to its rotational axis or be allowed to float relative to the rotary axis. The latter is preferable to equalize braking effort exerted by each yoke  14  and  15 . 
     Reverse rotation of the cam  27  allows the brake pads  21 ,  22 ,  23  and  24  to push the yokes  14  and  15  back to the released condition. Return springs are not usually necessary because a light rubbing contact of the brake pad  21 ,  22 ,  23  and  24  and the disc is generally beneficial. However, such spring(s) can be provided if required in any convenient location. 
     In the embodiment shown in  FIG. 1 , the yokes  14  and  15  are identical and reversible. This reduces manufacturing and stockholding costs. Furthermore, the yokes  14  and  15  may be adapted to engage each other via sliding overlapping surfaces to provide mutual support. The yoke  14  and  15  pair of  FIG. 1  can be manufactured for assembly by slotting the outboard arm of one or both yokes  14  and  15 . Alternatively, one or both yokes  14  and  15  may be assembled from a number of subassemblies. 
     An alternative asymmetric form of the invention is illustrated in  FIG. 2 . The method of operation is identical, but one of the yokes  14   a  is extended axially to overlap the other yoke  14   b  at both sides. The additional overlap allows the cam  27   a  to be located at an inboard side rather than in the center. An advantage of this arrangement is that the rotors  12  and  13  are closer together and can in fact be much closer than illustrated if a central aperture  26   a  is minimized for unworn brake pads. This arrangement is especially suitable for an axle end location and permits the wheel(s) to overlap both of the rotors  12  and  13 , while allowing the actuator to protrude at the inboard side where connection to the actuation system may be facilitated. 
     The invention is especially suitable for undriven rigid axles, such as those of semi-trailers. The twin rotor arrangement can be mounted inboard of the wheels and may even be provided at the center of the axle if respective rotors provide the braking to respective axle ends. The arrangement equalizes pad wear and allows for rapid routine inspection, while leaving the wheels in place. 
     In an alternative embodiment, the caliper body  10  of  FIG. 1  could extend to one or both of the depending arms  19  and  16 , with an abutment defined thereon to act as a load path for the drag load experienced by the depending arms  19  and  16 . This will advantageously reduce the torque produced at the position where the yokes  14  and  15  are mounted to the caliper body  10 . 
     The brake pads  21 ,  22 ,  23  and  24  may be mounted to the yokes  14  and  15  in a number of ways, for example by mechanical fasteners, or by abutment with sidewalls of a recess in the yoke surface facing the rotor  12  and  13 . Additionally, the connecting legs  30 ,  31 ,  32  and  33  may be positioned sufficiently apart such that the brake pads  21 ,  22 ,  23  and  24  can be inserted and removed therebetween for replacement. 
     The foregoing description is only exemplary of the principles of the invention. Many modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than using the example embodiments which have been specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.