GB2073834A - Disc Brake Pad and Method of Manufacture - Google Patents

Abstract

A disc brake pad (D) comprises a metal shoe plate (P) and a friction lining (L); the shoe plate (P) has integral hollow rivets (6) which extend into the friction lining (L) and are expanded within flared rivet holes (20) in the friction lining (L) so that the latter is wedged in position. The walls (8) of rivets (6) are formed by extrusion i.e. the substance of the walls (8) is largely scooped from hole in plate (P)-plate (P) may be of mild steel. Rivet holes (20) are formed during moulding of the lining (L) to be left with a coating or skin, the holes are shown conically flared at an angle of about 10 DEG to the axis of the rivet holes-but flaring need not be continuous. <IMAGE>

Description

SPECIFICATION Disc Brake Pad and Method of Manufacture The present invention relates to improvements in friction pads for disc brakes as well as in the method of making such articles.

The disc brake pad of present interest is of the riveted type comprised of a backing plate or shoe of plate metal usually about .135 to .250 inches in thickness and a relatively thick lining of friction material secured thereto by riveting.

In the past, the friction lining was secured to the shoe by rivets (usually part hollow) installed in registering bores or rivet holes provided in the shoe and in the friction lining. Each bore had a quite narrow internal shoulder for respective engagement either by the rivet head or by the swaged shank of the rivet staked thereover.

The large shear stresses sustained by disc brake pads in use have been known to cause the rivets to work loose therein and thus ultimately hasten the deterioration and destruction of the friction lining. Such lining is generally composed of asbestos and resin admixed with possibly ten or more other ingredients for imparting to it the requisite frictional properties. Depending upon the specific composition thereof, friction linings may therefore vary in strength and resistance to shearing forces within a significant range.

To a degree, problems engendered by the loosening of the rivets in the shoe have been met by the novel shoe disclosed in co-pending Application S. N. 06/0714,75 filed August 31st, 1979 and assigned to the same Assignee as the present invention such shoe being characterized by integral rivets extruded therefrom which are received within rivet holes in the friction lining and are headed or staked over shoulders formed within said rivet holes. While the shoe with integral rivets has contributed to a significant improvement in disc brake pads, it has not appreciably minimized the tendency of the shoulders in the friction lining to disintegrate under shear stresses; it being significant that the said shoulders normally measure approximately 0.060 inches (1.5 mm) in thickness.

It is further significant that it was not heretofore practicable to mold the rivet holes in the friction lining contemporaneously with the molding of the friction lining itself; said friction lining being formed of a plurality of ingredients as previously indicated with differing characteristics which predispose it to uneven shrinkage during the cooling thereof and, hence, to distortion of the shoulders thus creating irregular seating for the rivets in the rivet holes. In consequence, the rivet holes in the friction linings had heretofore preferably to be drilled rather than molded which of course left the rivet holes with raw walls.

The general objects of the invention are to provide a disc brake pad and a method of making a disc brake pad which, on the whole, is not only more economical than in the past but is also more strongly resistant to the destructive effects of the shear stresses encountered in use.

The invention further contemplates a disc brake pad comprised of an interfacing shoe and friction lining joined together by rivets which are integral with the shoe and extend into bores in the friction lining and are fastened therein in a manner reducing the liability of the rivets to loosening therein.

More specifically, the invention seeks to provide a disc brake pad as described with a lining having bores for rivets which are free of shoulders as in the described prior art.

The invention still further seeks to provide a method of constructing a disc brake pad which is not only in keeping with the foregoing objects but is, in particular, so constructed that the rivet holes in the friction lining need not be drilled but may be molded therein thereby further strengthening the friction lining while, at the same time, eliminating the machining heretofore required for providing the rivet holes.

As has already been explained, the invention makes use of a shoe with hollow, integral, rivets of the type and as formed, for example, in accordance with the method disclosed in Application S. N. aforesaid.

The invention further visualizes a friction lining which is, of course, appropriately matched to the shoe and, hence, provided with registering holes for the integral, hollow, rivets projecting from the shoe.

The shoe and the friction lining have abutting interface surfaces while the rivet holes in the friction lining, into which the hollow rivets project, are provided with zones which widen progressively, or flare, from the interface surface of the friction lining towards the opposite surface thereof. Quite obviously, the flaring need not be continuous to said opposite surface.

The friction lining is assembled in superimposition on the shoe so that the respective interface surfaces are in close abutment with each other and the hollow rivets of the shoe are accommodated in the rivet holes of the friction lining with portions thereof extending well into the flared zones.

After the assembly of the friction lining and the shoe as described, the shanks of the hollow rivets extending into the flared zones are expanded or flared to spread into closely conforming, abutting and wedging relation to the flared zones of the rivet holes.

Preferably, the zones in the friction lining are flared at a level commencing at/or closely adjacent its interface surface.

Preferably, also the flaring aforesaid is conical and, in accordance with the best form of the invention presently known to the inventors, the walls thereof are sheer and inclined at an angle of about 1 0o to the axis of the rivet holes.

Known to the inventors is the prior art hereinafter set forth in which there is disclosed vehicle brakes assembled with rivets with tapered or conical heads bearing a distant relationship to the present invention.

Kindler-US 1711 ,602-discloses, in Fig. 5, a :iisc c of a substance broadly equivalent to the present friction lining which is "fixed" by countersunk rivets c1 within a cup c2 of sheet metal, in turn bolted to a brake shoe c4.

Although the reference is silent on the point, said rivets c' are clearly separate from (rather than integral with) the plate c2 and, while illustrated as having conical heads, there is nothing in the reference to indicate that the head portion of rivet c1 is other than solid.

Structure substantially similar to the foregoing is shown in Tack-US 2438,483, wherein rivets 1 6 with conical heads at each end thereof are shown joining brake block 14 and brake head 2.

As in the previous reference, this Patent is silent as to the specific nature of the rivets 1 6 and the only logical inference to be drawn from the drawing and the meagre description is that such rivets are solid.

Murphy-US 2067,677, and Gordon-US 3767,018 show hollow rivets with wide, bevelled, heads which are molded into the friction lining material; the hollow portion then being -spun or swaged over the inside surface of the brake shoe, as indicated at 1 6 in Fig. 3.-" (Col. 2 L-8;;-US 2067,677) In Can. 658,959-Dubois-, there are disclosed brake elements in which centrally perforated discs of friction material 22 are encased in containers 18 and 20 which are fastened to an annular carrier by means of hollow rivets whose respective ends are flared essentially, to retain the containers 18 and 20 on carrier 12 and, as well, to bear against the friction material as indicated in modifications of Figs. 3 and 4.

One real distinction between the rivets of the present invention and those disclosed in the reference will be apparent from the text of Col. 1, between L. 17 and L. 25 wherein it is emphasized that the invention is concerned with friction material which does not require to be self supporting; hence, the necessity for the containers described in the Patent.

This structure differs from that of the present invention, of course, in that the friction lining of the present invention is most certainly of the self supporting type capable of being riveted to a backing plate or shoe.

The invention will now be described merely by way of illustration with reference to the accompanying drawings, in which Fig. 1 is an isometric view of a disc brake pad according to the invention with parts of the friction lining broken away to reveal concealed details.

Fig. 2 an exploded and enlarged sectional view of a fragment of friction lining and a matching fragment of a shoe plate in disassembly, and Fig. 3 is a view of the parts shown in Fig. 2 in assembly with each other.

In the following description of this invention, method and product aspects thereof have been intermingled for better comprehension whereby the method will sometimes be obvious from the description of the product, and, conversely, the product at times will be better understood from the description of the method.

As shown in the drawing, disc brake pad D according to the invention is comprised of a backing plate or shoe plate P and a matching block of friction lining L mounted on the shoe plate P with the respective interface surfaces 2 and 4 thereof in te close and intimate abutment which is essential to the proper functioning and longevity of the disc brake pad D.

Shoe plate P as taught by co-pending U.S.

Application S.N. has hollow rivets 6 extending integrally from its interface surface 2; one such rivet 6 being shown in detail in cross-section in Fig's 2 and 3 of the drawing, in generally enlarged form.

Generally stated, the shoe plate P is customarily formed of mild steel of having an average thickness of about .135 to .250 inches.

In the context of this submission, the term "integral" is intended to indicate that the rivets 6 are formed of the material constituting shoe plate P and are preferably formed by extruding a sufficient amount of the shoe material (through a pre-formed pilot hole) to form such rivets in the manner described in the co-pending application identified above.

For the sake of clarification, it is deemed useful and important to describe in some detail, at this point, what is herein contemplated by the term "extruded rivet", the term "extrusion" being used, sometimes erroneously, to describe what is, in actuality, a simple drawing or flanging procedure whereby the rim zone of a hole formed in thin sheet metal is -turned perpendicularly to the plane of the sheet to form a flange around the hole.

While the extruded rivet 6 has walls 8 which also form a flange around a hole, the fact is that the substance of the walls 8 is largely scooped from the sides of the hole as will be evident from the drawing; the rivet 6 thus effectively extending well into and merging integrally with the shoe plate P and being thereby provided with strong rooting therein.

The advantages of an extruded rivet over an ordinary drawn flange as above described are many, including: Firstly, and apart from the strong rooting above described, it is manifestly difficult, if not practically impossible, to flange steel of the thickness of a shoe plate as herein contemplated without contemporaneous "hilling" of the metal around the base of the flange or rivet which, in the present context, would deleteriously prevent the flat seating of the friction lining L on the shoe plate P.

Secondly, true extrusion is ordinarily best suited to vest the rivet 6 with walls of uniform thickness which is also essential to the invention as will appear, and Finally, extrusion, as herein described and contemplated can ordinarily produce a sturdy rivet with walls of uniform thickness and of the dimension best suited to serve the fairly robust requirements of the present invention.

Each of the hollow rivets 6 has a right cylindrical wall 8 projecting perpendicularly from interface surface 2 of shoe plate P; the rivet 6 being terminally defined by an annular shoulder 10 within a cylindrical recess 12 extending into the shoe plate P from its outer surface 14.

For orientation purposes, it is noted that the annular shoulder 10 is closer to the interface surface 2 of the shoe plate P than it is to its opposite surface 14; the depth of recess 12 between said surface 14 and annular shoulder 10 thus representing a major portion of the total thickness of the shoe plate P.

As a general indication and not by way of limitation, the rivet 6 may extend approximately 3 mm. above interface surface 2 of a shoe plate P having a thickness as described thereby corresponding to 60 per cent, more or less, ofthe shoe plate thickness.

In Fig. 1, the shoe plate P is shown in assembly with friction lining L which is provided with rivet holes 20 into which the integral hollow rivets 6 of the shoe plate P project.

In accordance with the invention, the rivet holes 20 in the friction lining L are flared away from its interface surface 4 which is necessarily in close and intimate abutment with the interface surface 2 of the shoe plate P.

In summary, it will thus be noted that the hollow integral rivets 6 of shoe plate P project into rivet holes 20 in friction lining L which are flared towards its opposite surface 22 imparting a somewhat V-shaped cross section to said rivet holes 20. As has been disclosed hereinabove, the penetration of rivet 6 into rivet hole 20 approximates 3 mm. more or less. Preferably, as has also been stated, the walls 24 of rivet hole 20 are inclined at an angle of about 10 degrees to its axis and are smooth-i.e. free of shoulders.

The friction lining L of the invention may be formed of a variety of materials in various proportions including asbestos, binder, resins, graphite, oxides, metallic particles, barytes and frictional elements.

In accordance with the customary practice, the block forming the friction lining L is molded; the rivet holes 20 being also molded contemporaneously therewith and, of course, positioned to receive the hollow rivets 6 of the shoe plate P when the disc brake pad D is assembled as in Fig. 1 and Fig. 3.

One very significant advantage over the prior art flowing from the molding of the rivet holes 20 in the friction lining L is that each of said rivet holes 20 is left with a coating or skin which is more strongly resistant to the destructive effects of shear stresses that the "green" or raw core of the friction lining L which would be exposed if said rivet holes 20 were drilled or otherwise cut into friction lining L instead of being molded.

Molding of the rivet holes 20 is, of course, rendered possible by the flaring thereof as above described; eliminating the shoulders or ledges which provided seating for the rivets of the prior art. That is to say, it was essential in the past, that the said shoulders or ledges provide truly flat surfaces for engagement by prior rivets. This requirement made it impractical for the shoulders or rivets to be molded into prior friction linings because of the uneven shrinkage of the various components thereof after molding. This disability made it necessary, in turn, for said shoulders to be formed by drilling into the friction lining which, for the reasons indicated above, tended to weaken the friction lining at that point.

The ledge or shoulder of the prior art, of course, suffered from another disability in that it was ordinarily very thin-usually about .060 inches in thickness and hence prone to crumble under stresses sustained by the disc brake pad in use.

The shoe plate P and the friction lining L are assembled by superimposing one on the other with their interface surfaces 2 and 4 in close abutment and with the rivets 6 extending from shoe plate P well into the smooth, flared zones of rivet holes 20 in friction lining L.

Walls 8 of rivets 6 are spread by a tool, not shown, to expand and wedge them substantially over their entire length into close, conforming, and frictional engagement with the flared walls 24 of the rivet holes 20.

Under ordinary circumstances it should not be necessary to provide an anvil or like counter pressure devices behind the shoe plate P on spreading the rivet walls 8.

It will be appreciated that the ductility of the shoe plate material will normally be a factor in the spreading of the hollow rivets 6 and, hence, in the flaring of the rivet holes 20 in friction lining L.

In general, however, when the shoe plate P is formed of mild steel as herein contemplated, its rivets 6 are readily expandable into close frictional engagement with walls 24 of rivet holes 20 inclined at an angle of about 100 to the axis of the rivet holes.

On the other hand, it has been determined that the walls 8 of the rivet 6 of a mild steel backing plate would tend to crack on being expanded into rivet holes which have walls inclined at an angle of more than 200 to the axis of the rivet hole.

The invention is predicated to a large extent on the fact that wedging of a hollow rivet on an inclined surface as herein contemplated provides beneficial results in the enhanced retentivity achieved thereby as will be well understood in the art.

Thus, the performance of a disc brake pad in accordance with the present invention has been evaluated in comparative tests with a standard riveted disc brake pad.

In such evaluation it was found that a disc brake pad constructed and assembled in accordance with the present invention was capable of withstanding stresses due to shear up to about 700 p.s.i.

In comparison, it has been found that when a friction lining was secured by separate rivets to a shoe plate, the assembly was capable of withstanding stress due to shear only to an upper limit of approximately 450 p.s.i.

Thus, it will be appreciated, that the disc brake pad in accordance with the invention provides an improvement of approximately 70% in the ability to resist destruction by shear stresses.

In prior art disc brake pads, it was also noted that the rivet shank invariably had play to a limited extent, however minimal, in the rivet holes; frequently leading to substantial wear, with ultimate untimely destruction of the assembly.

Conversely, a disc brake pad in accordance with the present invention is formed into a relatively homogeneous assembly since each rivet is not only integral with the shoe plate P but, being flared, spread within the rivet hole of the friction lining L, is wedged therein relatively immovably vesting the shoe plate P with a tenacious hold on friction lining L as rendered possible, obviously by the smoothness of the flared rivet hole walls 24 and the consequent absence of promontories such as the prior shoulders. In accordance with the present invention, there is thus provided a disc brake pad and a method for making it with a number of important advantages partly summarized as follows.

For one, the method of forming the friction pad does not require drilling of holes in the friction material. Instead, the required holes can be provided by molding at the time when the entire pad is molded. Moreover, it does dispense with the internal rivet seats in the rivet holes and provides a larger area of engagement.

Furthermore, because separate rivets are not involved, the actual fastening operation can be carried out at a faster rate and with less equipment than was required in the past.

Finally, it is most noteworthy that the presently conceived expedient of expanding the hollow rivets into wedging engagement with the flaring in the rivet holes of the friction lining, at once eliminates therefrom one common objection namely, the vulnerable rivet seat of the prior art and, co-incidentally, the rivet of the invention engages the friction lining by its entire height and not merely by a small rim such as engaged the shoulder or rivet seat of the prior art.

It is noteworthy, however, that apart from engaging the friction lining by its entire height, the manner in which the present rivet effects such engagement is particularly advantageous.

That is to say, the flaring or expansion of the present rivet in the flared rivet hole to fit snugly and conformingly therein vests it with great resistance not only to axial forces but, as well, to radial forces which are operative to prevent lateral play of the rivet hole. Moreover, it will be further appreciated that spreading the rivet into wedging engagement with the flared walls of the rivet hole effectively cams the lining close to the shoe plate and very strongly resists any separation thereof.

Contradistinctively, the familiar heading or staking of prior rivets did little or nothing to prevent radial displacement of the parts of the disc brake pad relative to each other and, in fact, provided none of the camming action characterizing the present invention.

It will be appreciated that the use of the word "smooth" in relation to the rivet holes 20 in friction lining L has been adopted to define a reivet hole which is free of promontories such as the rivet seats of the prior art or any other promontories--or depressions-which would frustrate relatively complete and close mating engagement of an expanded rivet wall 8 with the flared walls 24 of a rivet hole 205 such complete and close mating engagement being an important element in the advantages afforded by the present invention.

The foregoing description relates to the best form of the invention presently known to the inventors and it is recognized that the invention may be embodied in other forms as well within the scope of the invention as defined in the hereinafter following claims.

Claims (12)

Claims

1. A method of making a disc brake pad comprising the steps of: forming a shoe plate material having an interface surface with integral hollow rivets extending above said interface surface; forming a matching friction lining with a interface surface and smooth-walled holes for said rivets in said friction lining between its said interface surface and its opposite surface; incorporating in the rivet holes of said friction lining zones flaring from its interface surface towards its opposite surface;; assembling said shoe plate and friction lining with their respective interface surfaces in closed abutment and with the hollow rivets of the shoe plate intalled in the rivet holes and extending into said flared zones of said friction lining, and spreading the hollow rivets in said flared zones to expand and conform them to the contours of such zones and to wedge them into close frictional engagement with the walls of said zones.

2. The method as claimed in Claim 1 , wherein said zones are flared conically.

3. The method as claimed in Claim 2, wherein the walls of said flared zones are inclined at an angle of about 100 to the axis of the rivet hole.

4. The method as claimed in Claim 1,2 or 3 wherein the flaring of said zones is commenced at a level closely adjacent the interface surface of said friction lining.

5. A disc brake pad comprising: a shoe plate formed of plate metal; a matching friction lining, and rivets interfastening the shoe plate and friction lining with their interface surfaces in close abutting relation; said rivets being hollow and being expanded in rivet holes in the friction lining wedging the friction lining against the shoe plate.

6. A disc brake pad as claimed in Claim 5 wherein said rivets are integral with the shoe plate.

7. A disc brake pad as claimed in Claim 5 or Claim 6 wherein said rivet holes in the friction lining are smooth walled and are flared away from the interface surface of the friction lining.

8. A disc brake pad as claimed in Claim 7 wherein said rivet holes are conicaliy flared.

9. A disc brake pad as claimed in Claim 8 wherein said rivet holes are flared at an angle of about 10 to the vertical axis of the rivet hole.

10. A disc brake pad as claimed in any one of Claims 5 to 9 wherein said rivets are constituted by extrusions of said shoe plate.

11. A method of making a disc brake pad substantially as herein described with particular reference to the accompanying drawings.

12. A disc brake pad, constructed and arranged substantially as herein particularly described with reference to the accompanying drawings.