Patent Publication Number: US-8540061-B1

Title: Disc brake pad mounting and retention system and method

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to disc brakes for vehicles, and in particular to a system and method for mounting, removing and retaining brake pads in disc brakes, such as air-operated disc brakes utilized on commercial vehicles. 
     Pneumatically-operated disc brakes have been undergoing development and deployment on commercial vehicles since at least the 1970&#39;s, and are beginning to replace drum-style brakes due to advantages in areas such as cooling, fade resistance and serviceability. German Patent Publication No. DE 40 32 886 A1, and in particular FIG. 1 of this document, discloses an example of such an air disc brake. In this design, a pneumatic diaphragm chamber (pneumatic actuator) is attached to a rear face of the disc brake caliper housing, and applies a brake actuation force through a linear actuator rod to a brake actuator lever within the caliper. The brake&#39;s actuator lever in turn transfers and multiplies the force applied by the actuator rod to one or more spindles, which force brake pads against a brake disc or rotor. The terms “brake disc,” “rotor” and “brake rotor” are used interchangeably herein. 
     As shown in FIG. 1 of DE 40 32 886 A1, the actuator is located inboard of the brake caliper, in large part because commercial vehicle wheel rims are sized to only provide adequate clearance for the drum-type brakes historically employed on such vehicles. Because the resulting space envelope between the wheel and its axle is limited, the actuator must be located into the space adjacent to the wheel. For the same reason, brake pads must be configured to conform to the limited available radial space, and thus have typically been located and retained on one of the brake caliper or brake caliper carrier/mount using transverse suspension pins and/or using leaf spring-type metal strips disposed over the outer radius of the brake pads. Brake pads have also been retained by capturing the brake pads between the caliper mounting frame and the portion of the brake caliper which straddles the brake disc. (As one of ordinary skill will recognize, the same brake pad support function may be provided by a brake caliper carrier/mount designed to support the brake pads or by a brake pad carrier which is separate from the caliper mounting structure. For convenience in this description, the terms caliper carrier, caliper mount and brake pad carrier may be interchanged without intending to limit the brake pad supporting structure to any specific brake pad and brake caliper carrying structure.) 
     Conventional commercial vehicle air disc brakes have typically required the installation of ancillary brake pad retention mechanisms, and/or use of the brake caliper itself to retain the brake pads during service. Both of these approaches, and in particular use of the brake caliper as the retention means, requires disassembly of the pad retention mechanism and/or removal of the brake caliper in order to replace worn brake pads and install new brake pads. As a result, brake pad replacement in previous air-operated disc brake designs has been a labor-intensive, and therefore costly, process. 
     A further problem with prior art brake pads is the tendency for the brake pad to rotate and/or vibrate during brake operation. As illustrated in  FIG. 6 , when a brake pad  101  is applied against a friction surface of a brake disc (not illustrated) which is rotating in direction DR, the brake disc&#39;s rotation induces motion and reaction forces between the brake pad  101  and its adjacent mount abutment surfaces (not illustrated for clarity). Specifically, at the leading edge  102  of the brake pad the brake pad attempts to move upward in direction LU in response to the friction forces along the face of the brake pad (illustrated here by force arrows across the face of brake pad  101 ). At the trailing edge  103  of the brake pad, the brake pad attempts to move downward in direction TD. However, because the brake pad  101  is constrained by adjacent mount abutment surfaces, the overall motion of the brake pad is generally a rotation about an axis parallel to the brake disc rotation axis. This motion may be unilateral during the brake application, or may manifest itself as a moderate-to-severe oscillation of the brake pad in its mount, significantly increasing wear of the abutting brake pad and mount surfaces. 
     In order to prevent undesired rotation and/or vibration of the brake pad within its mounting (for example, rotation about the brake application direction), the brake pad backing plate and the adjacent mounting bracket horns supporting the brake pads in the circumferential direction required a relatively tall radial height to minimize the amount of brake pad rotation before a corner of the backing plate contacted the adjacent mount horn (a motion referred to as “pad kick” or “pad turnout”). This relatively tall structure in turn would require the brake caliper, which is installed over the brake pads and mounting bracket, to have its corresponding opposing inner surfaces radially outboard of the mount horns be relieved enough to accommodate the outer corners of the brake pad and/or mount horns. A problem with this thinning is that because the maximum outer radius of the brake caliper is typically constrained by very tight clearance to the inside of the adjacent wheel rim, the brake caliper arms straddling the brake disc between the application side and the reaction side of the caliper may end up being thinner than desired in this region in order to accommodate both the tall carrier mount horn and the close-fitting wheel rim. This can lead to very high tensile and bending stresses in the thin region, and an undesired reduction in fatigue life and service life. 
     The present invention addresses these and other problems by providing a brake pad mounting and retention arrangement and method of installation and removal which provides greater ease in in-situ brake pad installation and removal, without brake caliper removal or other significant brake disassembly work. This solution is of particular significance in highly space-constrained commercial vehicle air disc brake applications, where brake pad servicing without significant brake disassembly work caliper was not previously believed to be commercially practical. 
     In one embodiment of the present invention the brake caliper mount pad support horns are provided with a narrow vertical groove or slot on the side of the horns facing away from the brake disc. This groove is arranged to permit the brake pad, having features of corresponding thickness on its lateral sides, to slide in the radially-inward direction down the groove, until the brake pad reaches the installed position. The horns further have lateral grooves at or near their bases extending parallel to the brake disc rotation axis, arranged to receive the side features of the brake pad, such that the brake pad may advance toward the brake disc when pushed forward by the brake caliper&#39;s actuator. During service, the brake pad is positively retained in the brake by the combination of: (i) the lateral grooves in the mount horns, preventing lifting of the brake pad out of the caliper and undue rotation of the brake pad (i.e., twisting or “tipping” of the pad in the caliper as the rotating brake disc attempts to raise one end of the brake pad while pushing down on the other end of the brake pad); (ii) the brake disc, which prevents the brake pad from advancing so far as to emerge from the mount horn lateral grooves; and (iii) by the brake application device, which prevents the brake pad from retreating so far as to reach the mount horn vertical grooves and slide out the top of the caliper. 
     This arrangement also facilitates easy and rapid brake pad changes, as all that is required to remove the brake pads after wheel removal is to fully withdraw the brake application device, slide the brake pad backing plate axially to reach the mount horn vertical groove, and then simply lift the brake pad out of an opening in the top of the still-installed brake caliper. A new brake pad may then be inserted into the vertical groove until the backing plate&#39;s lateral features are aligned with the lateral grooves, followed by advancing the brake application device a sufficient distance behind the brake pad to prevent its backing out of the lateral grooves. 
     The present invention also provides the ability to significantly lower the height of the mount horns, thereby permitting the brake caliper thickness in high-stress regions to be made thicker to increase strength and caliper service life. The geometry of the relatively compact grooves and corresponding engaging features on the brake pad backing plates ensures the amount of brake pad rotation within the carrier is significantly limited as compared to prior art arrangements. Accordingly, because there is no longer a need to provide relatively tall mount horns to limit pad rotation (as the grooves now limit pad rotation), the horns may be made shorter. With the reduction in horn height, the newly-found additional clearance between the horn and the inner surface of the brake caliper in the regions over the horns allows the caliper to be made thicker in these highly-loaded areas. The addition of caliper material in these regions provides additional load-bearing cross-sectional area, with corresponding reduction in local stress levels and increase in caliper fatigue life. 
     Preferably the brake pad, mount and/or brake caliper may be provided with vibration-damping features, such as spring-loaded brackets on the backing plate lateral extensions or a spring member engaging tabs on the upper surface of the backing plate and on the caliper or caliper mount. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view of a disc brake in accordance with an embodiment of the present invention. 
         FIG. 2  is an oblique view of the caliper mounting carrier and brake pad of  FIG. 1 . 
         FIGS. 3   a ,  3   b  and  3   c  are side, front and top views, respectively, of the brake pad of  FIGS. 1 and 2 .  FIG. 3   d  is a detail view of an embodiment of a pad retention feature shown in  FIG. 3   b.    
         FIGS. 4   a  and  4   b  are elevation and front views, respectively, of the caliper mounting carrier of  FIGS. 1 and 2 .  FIG. 4   c  is a cross-section view of the caliper mounting carrier of  FIGS. 1 and 2  showing an embodiment of mount horn brake pad guiding and retaining grooves. 
         FIG. 5  is a detailed view of a pad retention feature including a pre-loading device in accordance with an embodiment of the present invention. 
         FIG. 6  is a schematic view of motions and forces resulting from application of a brake pad to a rotating brake disc. 
         FIGS. 7   a  and  7   b  are views of an alternative embodiment of a brake pad with pad retention features at a lower surface of the brake pad. 
         FIGS. 8   a  and  8   b  are views of an alternative embodiment of a brake pad with asymmetrical and rotation-accommodating pad retention features. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In the embodiment shown  FIG. 1 , a disc brake  1  of a commercial vehicle includes a brake disc  2 , a brake caliper  3  straddling the brake disc  2 . The caliper  3  is affixed to a caliper mounting carrier  4  which in turn is fixed to a vehicle axle, typically via a torque plate or a brake spider (not illustrated). The caliper  3  is actuated by an actuator  5 , in this embodiment a spring brake actuator which is controlled by pneumatic pressure from the vehicle. The actuator  5  acts upon a brake pad application mechanism contained within caliper  3  to press brake pads  6  against the brake disc  2  to slow the vehicle. The present invention is not restricted to a pneumatic actuator (for example, an electrically-driven actuator may be used) or to a particular type of brake caliper/mount arrangement (for example, a fixed carrier and a sliding caliper with a single-side brake pad application mechanism, or a fixed carrier and fixed caliper with two-sided brake pad application mechanisms). In this embodiment the caliper  3  is provided with an aperture  7  which is wide enough in the circumferential direction A and the axial direction B to permit the brake pads  6  to be withdrawn and inserted without removal of the caliper  3  from the carrier mount  4 . 
       FIG. 2  shows an oblique view of the carrier and brake application-side brake pad of  FIG. 1 , with the brake disc  2 , caliper  3 , actuator  5  and opposing brake pad  6  removed for clarity. In this view and in  FIGS. 4   a - 4   c , there are illustrated the holes  8  in mount  4  for receiving caliper and axle mounting bolts, as well as the lateral brake pad engagement surfaces  9  on mount horns  10 . It will be appreciated that the carrier need not be a caliper-supporting carrier, i.e., the caliper may be mounted on a component separate from the carrier, with only the brake pads being carried by the carrier. 
     Each of the mount horns  10  in this embodiment has a vertical brake pad installation/removal groove  11 , and a horizontal brake pad guiding groove  12  at the bottom of each mount horn  10 . It will be appreciated that the grooves or slots need not be located at the farthest axial locations on the carrier mount  4  away from the brake disc  2 , or precisely at the bottom of the mount horns  10 , as long as the brake pad  6  to be used in the brake is blocked from escape from the carrier mount  4  during in-service use and has pad retaining features with a vertical height corresponding to the height of the horizontal grooves  12 . In an alternative embodiment, the vertical installation/removal groove  11  may be omitted, as long as sufficient space is provided in the caliper aperture in the direction of the brake disc rotation axis to permit the brake pad to be inserted radially inward into the brake, such that the retention features of the brake pad may reach and enter the horizontal grooves  12 . 
       FIG. 2  and  FIGS. 3   a - 3   d  show the brake pad  6 , comprising a brake pad backing plate  14  and brake pad material  15  affixed to the brake disc side of the brake pad  6 . The brake pad material  15  is arranged to fit between the mount horns without engaging the grooves  11  or  12  to ensure free movement toward and away from the brake disc  2  and vertically in and out of the caliper during brake pad installation or removal. The brake pad  6  is also provided with pad retention features  16  at the lower corners of the lateral sides of the backing plate  14 . In this embodiment the retaining features  16  are arranged as tab-like extensions of the backing plate  14 . Preferably the tabs have a thickness in the brake application direction corresponding to the thickness of the backing plate  14 , so that no special machining or other forming for width reduction are needed. In any event, whatever the thickness of the pad retaining features  16 , the vertical grooves  11  must have an axial thickness and a circumferential width large enough to accommodate vertical passage of the pad retaining features  16  during insertion and removal of brake pad  6 . Similarly, the lateral or axial grooves  12  must have a vertical height and a circumferential width large enough to accommodate horizontal passage of the pad retaining features  16  during in-service brake application and release and during movement away from the brake disc  2  during removal of brake pad  6 . Preferably, the lateral grooves  12  are located toward the radially-inner portion of the caliper mount  4 , where the carrier is more stiff. 
     In addition to the preferred arrangement with male pad retaining features extending laterally into female lateral receiving grooves, female slots in the lateral sides of the brake pad backing plate may be arranged to receive corresponding male projections from the caliper mount after the brake pad is inserted into the disc brake along the radially-oriented grooves. Alternatively, the female lateral receiving grooves may be placed directly on the caliper body, with or without corresponding features on the caliper mount. Additional example pad retention feature arrangements are discussed further, below. 
     Also visible in  FIGS. 3   b  and  3   c  is a groove  18  on the brake pad  6  for receiving a brake pad wear indication sensor (not illustrated). 
       FIG. 3   d  shows a detailed view of the region of brake pad  6  highlighted in area C of  FIG. 3   b . In this region the brake pad retaining feature  16  is provided with a bevel, chamfer or rounded contour  19  at its lower edge to facilitate jam-free insertion and removal of the brake pad and to avoid developing a stress-concentrating contact point between the retaining feature  16  and the carrier mount  4  within groove  12 . Preferably, a corresponding mating feature is provided on the carrier to reduce stress concentration. The pad retaining feature  16  is also provided with a bevel  20 , preferably set at an angle corresponding to a similar angle at the top of horizontal groove  12 . The upper bevel  20  is provided to present a larger bearing surface, and therefore lower contact pressures and stresses, as brake application forces are transferred from the brake pad  6  to the carrier mount  4  during brake application, a time when the brake disc  2  drives the brake pad  6  to rotate in the carrier mount  4 . Inclusion of bevel  20  provides a planar surface to absorb forces over a broader contact area to minimize component wear, thereby preferably avoiding point or edge contact between the brake pad  6  and the carrier mount  4  which may occur with opposing rectilinear surfaces. 
     The angle of the bevel may be set to ensure that the contact pressure generated during maximum brake pad loading is below the yield strengths of the brake pad backing plate and the carrier mount materials. If the loads to be supported during brake application are relatively low, a shallow bevel angle on the order of 95-110 degrees may be sufficient to provide enough contact area on top of the bevel so that the stress level (force/area) is below the yield strength of the components. In the case of higher loadings or use of materials with lower yield strengths, a larger angle, on the order of 110-165 degrees may be required to provide a large enough contact area over which the load is distributed to remain below yield strength limits. An angle between 140-160 degrees, preferably 150 degrees, provides a large contact area while still minimizing the amount of pad “lift” or rotation during brake application. 
     A significant benefit of the pad retention arrangements of the present invention is evident in this embodiment. In the prior art, due to the need to maintain at least minimal clearances between the flat opposing faces of conventional straight-edged brake pads and their opposing mount horn abutment surfaces, the mount horns and brake pad mounting plate faces had to be relatively tall in order to minimize the extent to which the brake pad could rotate about the axial direction B when the brake was applied. With the current state of the art, due to the distance from the diagonally opposite corners of the brake pad, the typical tolerances between the brake pad backing plate and the mount horns would result a relatively large degree of undesirable pad rotation within the mount unless tall mount horns were provided to minimize the angular displacement of the brake pad. In the present invention, because the retaining features interact with the relatively small horizontal grooves  12 , even with the same minimum brake pad/mount horn clearance requirements as in a conventional prior brake, the brake pad  6  cannot rotate nearly as far as a conventional brake pad before the upper surfaces of the pad retaining features  16  meet the top of horizontal groove  12  and halt the angular displacement of the brake pad  6 . 
     For example, a typical prior art brake pad and retention arrangement would allow on the order of 4 mm of movement, leading to large displacement vibrations and accelerated wear of abutting surfaces of the pad backing plate and the mount horns. With the present invention, these movements may be reduced by 75% or more. Experiments with sample configurations have demonstrated movements being limited to as little as 0.75 mm. As a consequence, because pad rotation management has been transferred from the top edges of the mount horns to the horizontal grooves  12 , the mount horns do not need to be as tall as previously known in order to control brake pad rotation to a desired extent. This in turn permits avoidance of undesired thinning of the brake caliper in the regions above the tops of the mount horns, thereby allowing these very highly loaded and stressed regions to have a greater cross-sectional area to absorb these loads. The lower stresses directly correspond to increase caliper strength and increased caliper fatigue life, despite the very constrained wheel rim environment of a commercial vehicle. The improvement in stress-bearing volume of material may also enable replacement of costly specialty alloys used to obtain sufficient strength in the thin sections of the caliper with common cast iron, significantly reducing material and production costs. It should be noted that although the amount of “pad kick” may be lower in the present invention, when the bevel angles of the brake pad retention features and the corresponding lateral grooves are selected, the angles may differ slightly to accommodate the pad rotation under braking, e.g., the pad retention feature angle may be slightly more obtuse than the mount&#39;s lateral groove bevel, so that when the pad rotates in the mount, the contact faces make contact across a planar area, rather than alone a single line of contact. 
     A further advantage of the present invention is that it permits the elimination of retaining leaf spring interlocking features on the top of the brake pad backing plate, as well as associated structures radially above the brake pads to receive and retain the leaf springs (e.g., a pad hold down bar), thereby increasing radial clearance above the brake pad and potentially permitting either a decrease in the greatest radial extent of the caliper within the wheel envelope, or increase in the diameter of the brake disc and pad height to increase the size of the brake swept area where the brake pad material interacts with the brake disc surface. 
     The present invention&#39;s pad retaining features may also be provided with spring elements at or near the pad retaining features to desirably pre-load or dampen the pad relative to the mount, to reduce pad motion and reduce vibrations which may induce undesired noise and component wear during brake application.  FIG. 5  shows a detailed view of an example arrangement, wherein a spring element  22  is located between the brake pad backing plate  14 &#39;s pad retaining feature  16  and the outer wall of lateral groove  12 . Such a pre-loading element may be affixed to either the mount  4  or the brake pad backing plate  14 , for example by riveting or clipped on, or may be a separate element inserted between the mount  4  and the backing plate  14  when the pad  6  is inserted into the brake. Alternatively, the pre-loading element may have any of a number of suitable arrangements, such as a hinged member provided with a spring element which biases the hinged member laterally outward to engage the opposing surface. 
     A further embodiment of the present invention is illustrated in  FIGS. 7   a - 7   b . In this embodiment, the pad retention is achieved with tabs  16  located on a lower edge or surface of the brake pad backing plate  14 , with an undercut groove on at least one lateral side of the tab which engages a corresponding undercut groove in horizontal receiving groove  12 . In this embodiment groove  12  is machined into the lower surface of caliper mount adjacent to the vertical pad abutment  10 , rather than laterally across the face of the abutment. As with the foregoing example embodiments, this bottom-tab embodiment is inserted vertically (i.e., radially inward) along the outsides of the abutments  10 , then advanced into engagement with grooves  12  toward the brake disc to positively retain the brake pad. 
       FIGS. 8   a  and  8   b  illustrate another embodiment of the present invention, in which the pad retention features are asymmetrical, and are also configured to accommodate the brake disc-induced brake pad rotation with minimal pad movement and minimum pad and abutment surface wear.  FIG. 8   a  shows a brake pad backing plate  14  with a beveled tab  16  on the leading edge side of the brake pad, and a curved tab  16 A on the trailing edge side of the brake pad. This configuration provides a number of further advantages, including a configuration which inherently prevents the brake pads from being installed on the wrong sides of the brake disc due to the incompatibility of the corresponding grooves  12  in which they slide. The curved surface also forms a “ball-and-socket”- or “cylinder-and-socket”-like bearing arrangement with its corresponding groove  12 , thereby providing a broad surface area for contact between the brake pad tab  16 A and groove  12  which lowers contact face stresses and resulting component wear. The curved surface may also further reduce the amount of brake pad motion (“pad kick”) by more tightly limiting the range of motion of the trailing edge of the brake pad, with the curved tab  16 A potentially acting as a nearly fixed point of rotation about which the leading edge tab  16  rotates. This nearly-fixed trailing edge configuration, acting essentially as a pivot, also provides additional resistance against brake pad motion becoming so great as to induce large-amplitude vibrations. 
     As shown in  FIG. 8   b , the trailing edge tab  16 B may be a large radius curve, further spreading reaction force loads over a broader contact area. The curvature of the trailing edge tab is not limited to curves of constant radius. For example, the curves may be configured with decreasing or increasing radii, for example, to further define the motion of the trailing edge of the brake pad during brake application. Alternatively the curves maybe polygonal in shape. As with all of the embodiments of the present invention, these shapes may be economically formed, for example by lateral machining of the grooves  12  across the faces of the mount abutment surfaces with suitable tooling. 
     As with the previous embodiments, it may be preferable to account for pad rotation under braking in the brake pad backing plate design and/or the mount arrangements. For example, where the curved retention feature is on the trailing edge of the brake pad, in order to accommodate the “lift” of the leading edge of the brake pad during brake application (due to the leading edge&#39;s retention feature taking up the clearance in its lateral groove as its beveled surface meets the beveled top of the lateral groove) it may be desirable to provide a very corresponding very small relief or taper angle on the straight portion of the trailing edge of the brake pad. Preferably the amount of clearance in the leading edge lateral groove is maintained large enough to ensure free pad movement toward/away from the brake disc during all operating conditions, but small enough that when the pad rotates about the curved retaining feature on the trailing edge the backing plate only rotates on the order of three degrees or less (i.e., the trailing edge relief angle would be less than approximately 10 degrees, and preferably would be maintained at three degrees or less, very preferably approximately one degree). 
     A time- and labor-saving method of installation of brake pads in a new brake embodiment of the present invention would include a first step of sliding the as-yet pad-less brake caliper axially outward to position the brake for receipt of the outboard brake pad. A second step would involve inserting radially inwardly the outboard brake pad until the brake pad&#39;s lateral retention features are aligned with the lateral receiving features in the outboard side of the brake pad carrier. A third step would involve sliding the caliper in the axially inward direction to both advance the outboard brake pad into engagement with the carrier&#39;s outboard side pad receiving features, and to position the caliper for receiving the inboard brake pad. A fourth step would involve inserting radially inwardly the inboard brake pad until the brake pad&#39;s lateral retention features are aligned with the lateral receiving features in the inboard side of the brake pad carrier. A fifth step would involve advancing the brake application and/or adjusting mechanisms in the brake application direction to both advance the inboard brake pad into engagement with the carrier&#39;s inboard side pad receiving features, and to position the caliper for brake operation. The thickness of the brake pad retention features and the arrangements of the corresponding receiving features must be such that sufficient brake pad retention engagement is always ensured, even when the brake pad and the brake rotor are new, i.e., at their maximum thicknesses before use. 
     In the case of replacing brake pads in, for example, an in-service brake in which brake pads are already installed, the foregoing brake pad loading method would be prefaced with a corresponding brake pad removal operation, in which: the brake application and/or adjustment mechanisms are withdrawn far enough to permit the inboard brake pad to be moved away from the brake disc to reach the brake pad removal position, the inboard brake pad would be extracted radially outward through the caliper aperture, the caliper would be moved in the radially outward direction far enough to permit the outboard brake pad to be moved away from the brake disc to reach its brake pad removal position, and the outboard brake pad would be extracted radially outward through the caliper aperture. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. For example, rather than machining vertical and lateral grooves  11 ,  12  in the mounting horns  10 , a replaceable pad abutment surface may be provided on the carrier mount  4 , with a width and height corresponding to the geometry of the retaining features on the brake pad backing plate  14 . As another example, more than one pad retaining feature and corresponding receiving feature on the caliper may be provided on each of the lateral sides of the brake pad, and/or the features may be provided at different heights. Because other such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.