When the brake shoes in the drum brakes of heavy vehicles become worn, it is common to reaffix new linings of friction material. The main means of reaffixing are by riveting new linings to the backing plates and by bonding using an appropriate adhesive.

Linings which are riveted on to the backing plates are drilled through their operating face and this causes a reduction in the surface area of friction material able to contact the drum, usually in the order of a 3-5% reduction. This reduces the performance of the braking system because less friction material is available to transfer the energy.

Also, the blind holes in the working surface of the friction material can collect foreign material which can greatly increase wear of the brake drums.

Adhesive bonding of the linings to the shoes means such holes are not present, but bonding of linings requires a more sophisticated manufacturing operation than riveting and so the worn shoes must be transported between where the vehicle is being repaired and a central factory. This adds substantially to costs. As the bonding operation takes much longer than riveting, this means that in practice a range of adhesively relined shoes are usually kept in stock for use on a changeover basis and a substantial capital investment is required to maintain adequate stock. This is not required as much for the more simple riveting operation which can be done on-site

with the vehicle using a limited range of replacement linings and the shoes from the vehicle.

A further problem with riveted attachment is that it is not uncommon for operators to use inferior rivets which are easier to affix but which can fail under the high temperatures and loadings experienced in braking systems.

There is a need for an alternative means of attaching a brake lining to a brake shoe which has the advantages of : 1. being able to be simply done in the workshop with the vehicle, 2. the lining does not have holes through the operating face, and 3. there is a reduced chance of operators using unsuitable materials to attach linings to shoes.

One attachment technique is proposed in International Patent Application PCT/AU97/00857 but in use this could suffer from the inherent difficulty of satisfactorily attaching its screws to the keying elements. It would be difficult to satisfactorily control the tension in the screws by way of measuring torque applied to the screw during tightening because friction in the fastening system could introduce large errors in the measurement. Thus stripping of the thread in the keying elements could be a problem.

Another attachment system is proposed in US Patent 2,541,978. This is similar to the system of PCT/AU97/00857 except the screws pass through the backing plate in the opposite direction. But there are two major limitations with the system of US 2,541,978. The first is that the clamping strips (keying elements) must be fitted to the backing plate with the screws before the brake lining is slid onto the strips. The second limitation is that if the linings are worn down to the point where the channels are exposed, the segments of lining between the channels would be prone to coming away from the backing plate.

The attachment technique in PCT/AU97/00857 does not suffer the above second limitation because the channels do not pass fully across the lining. But the first limitation is incompatible with a design where the grooves enter from each side without passing fully across the lining.

If the channels were enlarged sufficiently to allow a conventional bolt head (or a bolt head as shown in US 2,541,978) to move sufficiently to accommodate the swing necessary to allow assembly of a significantly curved lining and brake shoe, then the groove would need to have such a large width and depth that the life of the lining would be greatly diminished.

There is a need for a fastening system that provides the benefits of both the above described systems without their incompatibility limitations.

Accordingly, in one aspect the invention provides an attachment between a lining of friction material and a backing plate, the backing plate having attached to it a plurality of elongated dished washers, each washer having a base and upwardly extending side walls, the lining having formed in it a plurality of blind-ended grooves, the grooves having inwardly directed side shoulders engaged with the side walls of the washers, each dished washer having a threaded clamping bolt and associated nut for attachment to and for drawing the dished washer toward the backing plate, the head of said bolt being positioned between said side walls and bearing upon the base of the dished washer but not upon the side walls.

In another aspect the invention provides an assembly of a washer on a bolt, the washer being elongated and dished to form a bathtub shape having a flat base, upwardly extending side walls and a central hole, the bolt having a head elongated in opposite directions laterally to its shank to form a T-shape, the head of the bolt having a flat base substantially covering the base of the washer and adapted to tighten onto the washer without bearing directly upon said side walls.

In another aspect the invention provides a method of attaching a lining of friction material to a backing plate, said method comprising: -providing a plurality of blind-ended grooves in said lining, said grooves having inwardly directed side shoulders; -providing a plurality of elongated dished washers, each washer having a base and upwardly and outwardly extending side walls; -inserting through each of said washers a threaded clamping bolt, each bolt having a head elongated in opposite directions laterally to its shank to form a T- shape, such that the head of the bolt is received within and aligned with the dish in its associated washer; -sliding the washers and bolt heads into respective said grooves such that the shanks extend from the slot; -inserting the shanks through holes in the backing plate and tightening nuts onto the shanks to thereby clamp the friction material to the backing plate.

Brief Description of the Drawings One embodiment of the invention will now be described with reference to the attached drawings where: Figure 1 is a partially exploded perspective view showing the attachment of friction linings to the backing plate portion of a brake shoe incorporating the present invention; Figures 2,3 and 4 are orthogonal views of a dished washer shown in Figure 1; Figure 5 is a section view along line A-A in Figure 2; Figures 6 and 7 are respectively front and side views of the bolt shown in Figure 1; Figures 8 and 9 are respectively top and bottom views of the bolt of Figure 6; Figure 10 is a cross-section detail through a backing plate, lining, bolt, dished washer and groove when the lining shown in Figure 1 is fully installed; and Figure 11 is the same view as Figure 10 but with the lining worn down to beyond the normal level for replacement.

Detailed Description of the Invention Referring to Figure 1, the system is shown for attaching two segments of a brake lining 12 and 13 to the backing plate 10 of a brake shoe. In practice the backing plate would be in turn attached, usually by welding or integrally casting or forging, to appropriate reinforcement webs and bearing means for actuation, but for clarity such webs and bearings have not been shown in Figure 1.

Also deleted from Figure 1 for clarity are many of the duplicates of some components such as nuts and bolts which form part of the assembly, although representative examples have been retained.

The linings 12 and 13 have dovetail grooves 18 machined into them by a rotary cutter. These grooves extend in from, and at right angles to, the arcuate edge 19 of the linings. The grooves 18 have inwardly directed shoulders 20 and, because the grooves extend only part-way across the shoe, a blind end 22 is created at the end of the groove. This blind end is rounded due to the action of the rotary cutter used. The shoulders 20 slope at about 45° to the face of the lining.

A bolt 16 is engaged with a respective washer 14 and the combination is slid into a groove 18 from its opening on the arcuate edge 19. The washer 14 has a generally bathtub shape, being approximately twice as long as it is wide, with rounded ends 38 and being dished to a depth of about half its width. The washer is seen in more detail in Figures 2 to 5. As seen in Figure 5, the dishing is angular with flat flanges 32 forming side walls extending from a flat base 36. The side flanges 32 mate to the shoulders 20 of the groove with the peripheral edge 33 of the shoulders lying only a small distance from the seat 24 of the groove. Each rounded end 38 is shaped so that it might nest closely with the blind end 22 of the groove and the washer is the same length as the groove so that when installed fully in the groove the outboard end of the washer sits flush with the arcuate edge 19 of the lining. Advantages of having both ends of the washer rounded include that it cannot be inserted into the groove the wrong way around and that it also increases washer stiffness.

As seen from Figures 6 to 9, the bolt 16 has an elongated head 44 extending as wings 45 to each side of the shank 42 such that the bolt overall has the shape of a T. As best seen in Figures 7 and 10, the head 44 has a generally pentagonal cross-section with a flat base 46, a pair of sloping underside faces 50 and a pair of top faces 52. The ends 54 of the head 44 are rounded. The top faces join at a peak ridge 53 which is lightly radiused and extends along the centre of the top of the bolt head. The underside faces 50 and top faces 52 meet at shoulder ridges 47.

The illustration in Figure 1 shows one such T-shaped bolt 16 on the far left which is aligned for insertion into a washer 14 which is in turn aligned for insertion into a groove 18. On mating of the lining 12 with the backing plate 10, the bolt shank 42 would pass through hole 27 and be tightened with nut 17. The upper lining segment 13 is shown already aligned with the backing plate with one nut 17 shown fully tightened, while another nut 15 is engaged with a bolt 16 ready for tightening. Each segment is attached by six bolts with associated washers and nuts.

The arcuate edge 19 of the lining has marked thereon a line 21 indicating the minimum thickness to which the lining should be allowed to reach before replacement. When worn down to this line, the thickness of the friction material is still greater than the depth of the grooves 18, so there is still some friction material covering the seat 24 of the groove. However it must be appreciated that from time to time vehicle operators do not perform proper maintenance procedures, even to the point of not properly checking brake systems, and the preferred embodiment now described takes this into account. To allow for poor maintenance procedures, the grooves do not pass fully across the lining. This means that even if the lining is worn down to expose the grooves, the segments of lining are not then prone to breaking away from the backing plate as might otherwise have been the case.

Another feature is illustrated in Figures 10 and 11. Although the shoulder ridges 47 lie within the dishing of the washer, the peak ridge 53 on the head of the bolt stands proud of the surrounding peripheral edge 33 of the washer. This means that as the

working face 61 of a lining 13 which is overdue for replacement wears down into the groove, the peak ridge 53 of the bolt head 44 commences rubbing on the brake drum 63 before the washer does. This is the point shown in Figure 11. The feature is important because the preferred material for the washer is a spring steel which is much harder than the grade 8.8 steel of the bolt head. There is a good chance that the operator would hear noise from the brakes when the bolt head commences to wear against the brake drums and take appropriate measures before the spring steel washers make contact with greater consequent damage to the brake drum. It is also possible to engineer the shape of the bolt to produce a squeal louder than otherwise expected if it rubs against the brake drum, and this would be a significant warning measure.

The shank 42 of the bolt carries a thread 56 which mates to the thread on a corresponding nut 17. The nut carries an integral collar and a locking arrangement to prevent the nut working loose and this can be by one of many known systems. Between the thread 56 and the head 44 of the bolt, is a short unthreaded section 58 of the shank which forms a neck. The thread 56 is rolled onto the shank, so its outside diameter is greater than that of the neck 58.

The outside diameter of the thread may be larger than the diameter of the hole 34 in the washer. This could be achieved by rolling the thread after the washer is engaged with the bolt. The washer is then captive on the bolt and this provides a number of advantages. The components are more easily handled during the brake relining operation but, more importantly, it provides protection from substitute bolts of inadequate properties being inadvertently used in field servicing of what is important safety apparatus.

In a preferred embodiment of the invention the central hole 34 in the washer is oval, being elongated transverse to the longer axis of the washer, and has its smaller diameter only a loose clearance fit larger than the diameter of the shank 42 before the thread 56 is rolled upon it. The washer is in this way held captive on the bolt while there is substantial freedom for the bolt to swing upon the flat base 36 of the washer

in the direction of elongation of the hole i. e. in the direction of curvature of the backing plate.

One of the major potential problems in the assembly of a lining to a backing plate using the system described above with reference to Figure 1 is that when causing the backing plate 10 to contact the lining 12 while all the bolts 16 are protruding from the lining, in order for the bolts 16 to be fed into their corresponding holes 26 the bolts need to be able to pivot around the groove 18 in the direction of curvature of the arcuate backing plate and lining.

It can be seen from Figure 10 that when the bolt is tensioned, there is no load transferred from the tapered faces 50 on the head directly to the side flanges 32 on the washer. Instead the tensile load in the bolt is transferred from the bolt head 44 through the flat base 46 on the head wings 45 to the flat base 36 of the washer.

The clearance between the side flanges 32 on the washer and the faces 50 on the bolt head allows the bolt to rock about the base 45 of its head while still touching the flat base 36 of the washer. The slope of the top faces 52 means such rocking can also occur without the head touching the lining at the seat 24 of the groove even though the seat is relatively close to the peak ridge 53 in the tensioned position. This is important because it is most desirable to have the grooves 18 as shallow as possible in order to provide as great a depth of friction material as possible before wear cuts into the grooves. Therefore anything that reduces the need for a greater groove depth is beneficial. The sloped faces 52 on the top of the bolt head provide a similar benefit in that the bolt may rock over much further than with a flat headed bolt of the same head depth. The ridged centre of the bolt head provides a significant strength increase without affecting clearance difficulties.

The particular alignment of the bolt and nut in the present invention, whereby the nut and portion of the bolt thread are visible to the installer, enables better control of torque and tension when compared with the disclosure of PCT/AU97/00857. There is a visual check on possible thread damage, so allowing a damaged part to be more

easily identified and the part replaced. The degree of thread engagement can also be visually checked. There is also an ability to lubricate the nut without getting lubricants near the working face of the friction lining; this allows control of thread and under-head friction and thus tension in the bolt.

Although the invention has been described with particular emphasis on its use for relining the shoes of drum brakes of heavy vehicles, it will be appreciated that its use is much wider and it can be adapted to disc brake pads, clutch plates, band brakes and the like for other vehicles and machinery. Also it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

For example although the T-bolt described with reference to the Figures has a head with a pentagonal cross section, which produces on the bolt head a vaulted top formed by a pair of sloping top faces joining at a peak ridge running along the bolt head in the direction of its elongation, the invention in its broadest sense encompasses the use of bolts having the tops of their heads vaulted differently by use of other cross sectional shapes. This can be by way of semi-circular or semi-elliptical cross section shapes for example so that a curved surface is formed with its top ridge running along the bolt head in the direction of the head's elongation. The term"vaulted"is used here to mean that along the top of the elongated bolt head the centre is higher than the edges.

However it is important that the base of the head has a face which is shaped in order to properly mate with the base of the washer. While the base of the head and the base of the washer are preferably flat in order to maximise bolt head strength, they could be rounded or peaked in some manner if desired.

In relation to the bolt described with reference to Figures 6 to 9, the sloping faces 50 on the underside of the head preferably subtend between them an angle in the range of 50° to 60°. The faces on the top of the head are preferably sloped in the range of 65° to 80°, more preferably 70° to 75°, to the axis of the shank 42.

The shoulders 20 of the groove described in relation to Figure 1 slope at about 45° to the seat 24. This angle may be varied as required, for example between 30° and 60°.

It is preferred that the shoulders be straight in cross section, but they may be curved to give a different clamping action if wished, although it should be kept in mind that the side flanges 32 on the washer need to be matched to the chosen groove shoulder form.

Figure 1 shows each segment of the brake lining attaching to the backing plate by use of six sets of dished washer, bolt and nut. It will be understood that the number and placement of these fastenings may be chosen to be whatever is suitable to provide adequate fastening in any given circumstances. Also it may be that in order to utilize existing brake shoe components, where the backing plate is manufactured with the number and spacing of holes appropriate for riveted attachment of the linings, that more dished washers, bolts and nuts will be used than would be necessary for reasons of strength in order to achieve an appropriate spacing.

The materials chosen for the componentry may vary from that described in the above embodiments. For example, the bolt may be of brass and the dished washer of steel, and each component may be independently optimised for strength, fatigue and wear characteristics.

It will be understood that where the word"comprise", and variations such as "comprises"and"comprising", are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features.