Abstract:
A brake shoe ( 1; 2 ) comprising an arcuate platform ( 5 ) carrying a lining ( 6, 7, 8; 12, 13 ) of friction material, the lining ( 6, 7, 12, 13 ) having, at or adjacent its outer ends, localised portions ( 9, 10; 14, 15 ) projecting radially outwardly beyond the remainder of the lining ( 6, 7, 12, 13 ).

Description:
The present application claims priority to the international application published under the Patent Cooperation Treaty PCT/GB99/01908 filed on Jun. 16, 1999, which in turn claims priority to the Great Britain patent GB 9812869.7 dated Jun. 16, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a brake shoe having an arcuate platform carrying a lining of friction material and intended for use in an internal shoe drum brake, primarily for a motor vehicle, and of the kind having a pair of brake shoes mounted on a back plate and separable by an actuator into braking engagement with a rotary brake drum. The invention also embraces an internal shoe drum brake incorporating a plurality of brake shoes. 
     Problems during bedding in new brake linings against the braking surface of a rotary brake drum are often due to the occurrence of random high spots on the linings which can give rise, during the bedding in process, to local areas of excessive loading, leading potentially to uneven braking, and/or drum distortion and its resultant brake judder. These problems can occur despite the fact that new linings are often ground to match generally the drum curvature, because production tolerances result in the contact areas of the linings being ill-defined or spaced unevenly around the drum circumference. 
     SUMMARY OF THE INVENTION 
     The invention is a brake shoe for an internal shoe drum brake, and a drum brake containing a plurality of such shoes, in which the aforesaid problems are alleviated or avoided, by controlling accurately the formation of the lining to drum contact areas and spacing these areas as evenly as possible around the drum circumference. 
     According to the invention, a brake shoe comprises an arcuate platform carrying a lining of friction material, the lining having, at or adjacent its outer ends, localised portions projecting radially outwardly beyond the remainder of the lining. 
     The lining may conveniently be in three separate sections, of which the two outermost ones preferably have convex projecting surfaces having radii of curvature which are, typically, smaller than that of the remaining lining. 
     With the arrangement of the invention, the sections of lining providing the initial areas of contact between the lining and drum are spaced in a generally balanced manner around the drum, which greatly reduces the risk of drum distortion and resultant brake judder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
     FIG. 1 is plane view of the brake shoe and internal shoe drum brake. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention will now be described, by way of example, with reference to the accompanying single figure which is a schematic end view, with inessential parts omitted, of one form of the internal shoe drum brake of the invention, two alternative forms of brake shoe being illustrated respectively at either side of a brake centreline. 
     The brake partially illustrated in FIG. 1 has a pair of brake shoes  1 ,  2  mounted on a back plate  3  within a rotatable brake drum  4 . The usual ancillary equipment, such as actuator and torque sustaining devices, is omitted from the illustrated brake, for simplicity. Moreover, it will be understood that the brake would, in practice, be provided with two identical or similar shoes. However, alternative forms of the shoe of the invention are illustrated respectively at either side of the brake centreline C, for convenience. 
     The right-hand shoe  1  will be seen to include an arcuate platform represented at  5 , which carries three separate sections of lining material, the outer sections  6 ,  7  extending over a relatively small part of the platform circumference, whereas the central lining section  8  covers a major part of that circumference. The lining sections  6  and  7  have curved operative surfaces  9 ,  10 , of which the radii of curvature are smaller than that of the central section  8 , the operative surface  11  of which initially lies slightly radially inwardly of the surfaces  9 ,  10 . 
     The drawing illustrates the shoe  1  in new condition and applied against the drum braking surface. During braking, the relatively small area braking surfaces  9 ,  10  will wear and bed in rapidly, increasing their effective areas and the surface  11  of the section  8  will be applied against the drum, either after a short time, following the rapid bedding in of the sections  6  and  7  or possibly simultaneously with those sections under heavy braking loads. It will be seen that the positioning of the sections  6  and  7  effectively spreads the braking load across the drum and prevents any untoward force concentration at other locations. Even when local high spots on the section surface  11  contact the drum, the spreading of the load between such a high spot and the sections  6  and  7  wholly or largely avoids undesireable drum distortion. 
     The shoe  2  is provided with only two lining sections  12 ,  13 , each of which is formed or ground to provide a respective domed portion  14 ,  15  projecting radially beyond the remaining surface portions  16 ,  17  of the sections. The operation of this arrangement is similar to that described previously and the rapid bedding in and location of the projections  14 ,  15  avoid, or substantially so, untoward drum distortion. 
     It will be understood that the lining of each shoe may be in a single piece, with appropriate projecting portions formed at or adjacent the free ends of the shoe by a forming or grinding operation. The projecting portions may be of any suitable form other than arcuate, such as will be benefical in promoting rapid bedding in of those portions during initial braking. 
     In a further alternative arrangement, the lining on each shoe may again be in two parts, one of which constitutes a small section, such as one of those illustrated on the shoe  1 , with the other lining part incorporating a further small section and a larger section having the major braking area of the shoe in a single piece. It may alternatively or additionally be possible to form the bedding in surfaces of the projecting portions with ribs or serrations in order further to facilitate the bedding in process. 
     The foregoing description is exemplary, not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. 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, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is 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.