Abstract:
A force transmission device for a disc brake is operable to move a friction element of the brake into engagement with a rotary brake disc along a first axis in response to a loading from a thrust member. The device is restrained from movement transverse to the first axis near a first end engageable with the thrust member, but is unguided near a second end engageable with the friction element.

Description:
REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority to United Kingdom Patent Application GB 0301798.5 filed on Jan. 25, 2003.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to a disc brake incorporating a force transmission device.  
           [0003]    Force transmission devices of disc brakes, particularly air actuated disc brakes, are mounted within a caliper of a brake and transmit an actuation force from an actuator to friction elements of the brake via tappets or pistons. Typically, the force transmission devices includes an adjuster mechanism that compensates for wear of the friction elements.  
           [0004]    There are several drawbacks to known force transmission devices. For one, the high number of metal-to-metal sliding surfaces reduce efficiency. Additionally, the machining process used to provide a close fit between the tappet shafts and the housings is relatively expensive. The seals located near to the rear of the friction element may also be damaged in extreme high temperature applications. Furthermore, since the tappet heads are individual components, they must be securely attached to the tappet shafts, usually by a retaining ring or a circlip, increasing the number of parts. Finally, although cast iron backplates for friction elements are becoming more common, machining the backplate to ensure that the backplate is flat and the load from the force transmission device is adequately distributed over the backplate is expensive.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention seeks to overcome, or at least mitigate, the problems of the prior art.  
           [0006]    The present invention provides a force transmission device for a disc brake operable to move a friction element of the brake along a first axis into engagement with a rotary brake disc in response to a loading from a thrust member. A first end of the device is engageable with the thrust member, and a second end of the device is engageable with the friction element. The device is restrained near the first end from movement transverse to the first axis. The device is unguided near the second end of the device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0008]    [0008]FIG. 1 schematically illustrates a cross-sectional view through a portion of a prior art disc brake incorporating a prior art force transmission device;  
         [0009]    [0009]FIG. 2 schematically illustrates an exploded perspective view of a prior art friction element and a portion of the force transmission device;  
         [0010]    [0010]FIG. 3 schematically illustrates a cross-sectional view through a disc brake incorporating a force transmission device according to one embodiment of the present invention;  
         [0011]    [0011]FIG. 4 schematically illustrates an exploded perspective view of a portion of the force transmission device and a friction element;  
         [0012]    [0012]FIG. 5 schematically illustrates a cross-sectional view of the backplate of the friction element of FIG. 4;  
         [0013]    [0013]FIG. 6 schematically illustrates a second method of securing a tappet and backplate;  
         [0014]    [0014]FIG. 7 schematically illustrates a third method of securing a tappet and backplate; and  
         [0015]    [0015]FIG. 8 schematically illustrates a fourth method of securing a tappet and backplate. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    [0016]FIGS. 1 and 2 illustrates a prior art force transmission device  5  mounted within a chamber  9  of a housing  7  of a disc brake caliper. The housing  7  is adapted to mount a conventional air or other power actuator (not shown) on an external face of the housing  7 . An actuating lever (not shown) may perform an angular reciprocal swinging movement under the action of a thrust member of the power actuator. The lever can be integral or attached to a rotary actuating member  13  rotatably supported within the caliper. The rotary actuating member  13  is recessed to house respective cylindrical rollers  15 ,  16  having axes offset from the rotary axis of the rotary actuating member  13  TO form an eccentric, actuating arrangement. The rollers  15 ,  16  bear against respective thrust assemblies  17 ,  18  (shown as adjustable tappet assemblies) of the force transmission device  5 .  
         [0017]    Rotation of the lever and the rotary actuation member  13  causes the tappet assemblies  17 ,  18  to apply an actuating thrust to a directly actuated friction element  2  and, by reaction via the caliper, to an indirectly actuated friction element (not shown). The friction elements  2  are mounted to face respective sides of a brake disc or rotor  4 . The friction elements  2  are received within openings in a brake carrier  3  and fixed to a non-rotatable portion of a vehicle (e.g., the suspension) to which the brake is mounted. The friction elements  2  are restrained from circumferential and radial inward movement. Radial outward movement is restricted by pad springs  30  and a pad strap  32 .  
         [0018]    An adjuster assembly  19  may be of any appropriate conventional type and needs no detailed description. The adjuster responds to excessive movement of the friction element  2  (e.g., due to wear of the friction material  40 ) and produces resultant rotation of an adjuster shaft  21  via a gear  52 , which in turn rotates a pair of adjuster shafts  22 ,  23  of the restrictive adjustable tappet assemblies  17 ,  18 .  
         [0019]    The tappet assemblies  17 ,  18  are of identical construction and operation, and therefore only the tappet assembly  17  will be described herein. The tappet assembly  17  includes a tappet housing  24  including a trilobular bore  24 A. A tappet head  26  is mounted to a closed end portion of a tappet shaft  25  having a trilobular external surface received in and axially guided by the bore  24 A. The tappet shaft  25  further includes a threaded internal bore arranged to receive the adjuster shaft  22  having a corresponding external thread. The tappet assemblies  17 ,  18  are disposed with tappet heads  26  adjacent to the friction element  2 .  
         [0020]    When the brake actuator applies a force from the right as shown in FIG. 1, the entire tappet assembly  17  slides along the bore  24 A to transmit the braking force to the friction element  2  via the tappet head  26 . Because the friction element  2  is able to move radially outwardly and circumferentially, there may be some relative movement between the tappet head  26  and the friction element  2  in operation. To adjust the length of the tappet, the adjuster shaft  22  rotates to cause relative axial movement between the adjuster shaft  22  and the tappet shaft  25  by the mating threads. The trilobular configuration of the outer surface of the tappet shaft  25  within the bore  24 A prevents rotation of the tappet shaft  25  relative to the housing  7 , ensuring that rotation of the adjuster shaft  22  results in actual lengthening of the tappet assembly  17 . A seal  29  between the tappet head  26  and housing  24  prevents particles of dirt and debris from entering into the sliding interface between the triblobular outer surface of the tappet shaft  25  and bore  24 A.  
         [0021]    [0021]FIGS. 3 and 4 illustrate the force transmission device  105  of the present invention fitted within the chamber  9  of the housing  7  of a disc brake of the type disclosed above. The remaining parts of the disc brake identical with the prior art are designated by the same reference numerals and operate in the same way. The rotary actuating member  13  applies an actuating thrust via the tappet assemblies  117 ,  118  to a directly actuated friction element  102 . The tappet assemblies  117 ,  118  are adjusted by an adjuster mechanism  19 .  
         [0022]    The tappet housing  124  of the present invention does not include a trilobular bore arranged to receive and guide a tappet shaft along its axial length, and the prior art guidance for the tappet assemblies  117  and  118  is not needed. Instead, the end  150  of each tappet assembly  117 ,  118  near the actuating member  13  is guided by the interface of the gear ring  152  of the adjuster shaft  122  with the housing  124 . Therefore, no contact between the housing  124  and the tappet shaft is necessary. The opposite end of each tappet assembly  117 ,  118  is guided by a formation on the backplate  138  of the friction element  102 .  
         [0023]    As shown in FIGS. 4 and 5, the brake pad backplate  138  is generally planar and has a friction material surface  162  on one side and a loading surface  164  on an opposite side. The loading surface  164  includes two identical local load spreading features in the form of raised bosses  166 ,  168 . Only boss  166  will be described, although it is to be understood that boss  168  is identical. The top of the boss  166  (as shown in FIG. 5) has a diameter d′ which is smaller than the diameter D of the lower portion of the boss  166 . Therefore, the boss  166  can act as a load spreading feature.  
         [0024]    The boss  166  is circular and has a raised abutment and an edge  170  tapering towards the loading face  164 . The raised abutment  172  is annular and has a diameter d′ slightly larger than the diameter d of the end  174  of the tappet shafts  125  and defines a surface  166 A having a recessed square locating feature  173  at its center.  
         [0025]    The tappet ends  174  have a substantially uniform circular cross section with a diameter d and a square end projection  175  dimensioned to fit within the recessed square locating feature  173 . The tappet end  174  of the tappet shaft  125  includes an annular groove  174 B. A resilient member, such as a wavy spring  176 , is positioned in the groove  174 B. The tappet shaft  125  and the wavy spring  176  are then inserted into the recessed square locating features  173  formed by the surface  166 A and the raised abutment  172  such that the projection  175  fits within the recessed square locating feature  173 . The assembly process can be achieved either by moving the brake pad toward the tappet shaft  125  or by moving the tappet shaft  125  toward the brake pad, as further described below.  
         [0026]    The inner and outer friction element  102  of the prior art can be removed during servicing by simply removing the pad strap  32  and then moving the inner and outer friction element  102  in a radial direction relative to the brake disc.  
         [0027]    The force transmission device of the present invention provides an alternative method of assembling the friction element backplates  138 , either during initial assembly or during reassembly following servicing. To assemble the friction element backplate  138  in the outer position of the caliper, i.e., remote from the force transmission device, the friction element  102  has to be inserted in a radial direction and then subsequently moved in an axial direction, away from the brake disc  4 , to engage the features of the backplate  138  with the features on the inner face of the bridge.  
         [0028]    The wavy spring  176  is interference fitted in the recess formed by the surface  166 A and ensures that the end  174  of the tappet shaft  125  remains in contact with the surface  166 A when the vehicle is in use. The wavy spring  176  also allows a limited amount of radial movement of the brake pad relative to the tappet shaft  125  (provided some clearance is provided between the location feature  173  and the projection  175 ). The resilient nature of the wavy spring  176  can damp the transmission of vibrations induced in the friction element  102 .  
         [0029]    The wavy spring  176  is similar to proprietary components known as “tolerance rings.” It is possible to design the piston and the brake pad backplate  138  to utilize “off the shelf” tolerance rings, rather than having a specific unique wavy spring  176 . The wavy spring  176  is received in the groove  174 B of the tappet shaft  125 . However, in other embodiments, the groove  174 B could be provided on the raised abutment  172 . Since the end  174  of the tappet shaft  125  is radially and circumferentially restrained by engagement with the backplate  138 , the guidance provided by the bore  24 A of prior art is no longer needed.  
         [0030]    The backplate  138  is generally arcuate and has circumferential ends  178 ,  180  that may be locally thicker (T) than the thickness (t) of the main portion of the backplate  138 . The local thickening (T) ensures adequate engagement of the circumferential end  178 ,  180  with the associated carrier  3 , even at the extremes of friction element  102  and disc wear. Similar local thickening (not shown) can be provided on a radially inner edge  182  of the backplate  138  where it abuts the associated carrier  3 .  
         [0031]    When the backplate  138  is assembled in an inner position, i.e., in the position shown in FIG. 3, it is first moved in a radially inward direction and then in an axial direction away from the brake disc so that the location features of the backplate  138  engage with the tappet shaft  125 .  
         [0032]    An alternative method of assembling the backplate  138  into a caliper is to first move the backplates  138  in a radial direction such that the friction material  140  is near the brake disc surface and then to apply the brake so that the tappet shafts  125  and the inner face of the bridge approach and engage with the location features on the backplate  138  to prevent escape of the pads during subsequent normal use of the vehicle.  
         [0033]    FIGS.  7  to  9  illustrate an alternative tappet shaft  225  having an end  274  and grooves  274 B. The brake pad backplate  238  includes cordially orientated holes  284 . The tappet shaft  225  is inserted into a recess  273 , and a spring clip  286  is then inserted through the holes  284  in the direction of arrows A such that the regions  287  are received within the grooves  274 B, retaining the piston to the brake pad. The grooves  274 B preferably do not extend around the entire circumference of the shaft  225  so that relative rotation of the shaft  225  and the backplate  238  is prevented when the clip  286  is inserted.  
         [0034]    Although the abutments  172  and the tappet ends  174  have been disclosed as circular, it is to be understood that non-circular formations can be employed. These formations would be a substitute for the projections  175  and the grooves  274 B in enabling the wear adjustment mechanism  19  to function properly. Any other suitable form of engagement that prevents relative rotation between the backplate  138  and the tappet shaft  125  may also be employed.  
         [0035]    It is possible to rely solely on the interface between the tappet shafts  125  and the backplate  138  to ensure that the tappet assemblies  117 ,  118  extend in response to wear of the friction material  140 . However, it is also possible that the tappet shafts  125  may rotate once the backplate  138  is removed from the ends of the tappet shafts  125  and go out of mutual synchronism during pad replacement, potentially leading to difficulties in fitting a replacement pad and in achieving even wear of the friction material  102  during use of the disc brake. To prevent this, a further anti-rotation feature, such as teeth  190 , may be provided near the actuating member  13  at the end of the tappet shaft  125  to engage a complementary formation (not shown) on the housing  124  when the tappet shaft  125  is in the retracted position with respect to the adjuster shaft  122 , as illustrated in FIG. 3. Because the brake pad can only be replaced once the adjustment mechanism has been retracted, the teeth  190  are need to operate only in this position.  
         [0036]    The force transmission device of the present invention also includes a secondary seal  192  extending between the housing  124  and the end of the tappet shaft  125  near the actuating member  13 . The secondary seal  192  prevents the ingress of certain debris into the force transmission device if the primary seal  129  fails.  
         [0037]    The force transmission device of the present invention provides a number of advantages over the prior art. For example, the force transmission device provides for improved operational efficiency due to the reduced number of sliding contact points. Additionally, the cost of manufacturing the device may be reduced because there are fewer parts and because the need to machine both the tappet shaft and the bushing for the tappet shaft is eliminated. Furthermore, the force transmission device of the present invention provides additional savings in the manufacturing of cast backplates because only the contact area for locating the ends of the tappets require machining, rather than a large area of the backplate that accommodates prior art tappet heads or spreader plates.  
         [0038]    It should be appreciated that numerous changes may be made within the scope of the present invention. For example, the force transmission device may be adapted for use with other adjuster mechanisms, such as those disclosed in WO 99/49232 and WO 97/01045. The present invention may further be applied to disc brakes that are fluid or mechanically actuated rather than air actuated and to non-floating caliper type disc brakes.  
         [0039]    The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.