DRUM BRAKE ASSEMBLY, BRAKE SHOES FOR SAME, AND METHODS

Abstract: A drum brake assembly has a brake shoe lining friction surface subtending a total effective friction surface arc length between the shoe heel (36,38) and toe (28,32) ends of about 49° on the first, primary shoe (30), disposed symmetrically about the shoe horizontal centre line (48), and about 57° on the second, secondary shoe (34), similarly symmetrically disposed. In some instances the first, primary shoe may have about the same friction surface arc length as the second, secondary shoe, with each preferably being about 50% of the friction surface arc length of the prior-art typical secondary shoe lining of about 117°. Different methods of reducing the effective friction surface arc lengths are disclosed, as well as the effect the invention has on the brake effectiveness variations of brake assemblies, both with the same lining material composition and with different lining material compositions. Background and test graphs are provided, as well as a review of accepted brake design considerations of the past leading the brake designer away from the invention.

Description: This invention relates to drum brake shoes, drum brake assemblies, and methods of modifying a drum brake assembly on an existing vehicle and of decreasing the variation in brake effectiveness of a drum brake assembly. Automotive vehicles have for many years had drum brake assemblies fitted on either the front or the rear axles, or on both. In recent years it has also become common to equip automotive vehicles with disc-type front brake assemblies and drum-type rear brake assemblies. The type of drum brake assembly commonly known as a duo-servo drum brake has been used since the 1920's on millions of automotive vehicles including passenger cars, light trucks, heavy-duty trucks, buses and trailers. Duo-servo drum brake assemblies are characterised by having a primary brake shoe and a secondary brake shoe with one set of adjacent shoe ends acted upon by a wheel cylinder or cam to selectively expand the brake shoes for braking engagement with a brake drum, and the other set of adjacent shoe ends typically being connected by way of an adjuster strut or link mechanism for maintaining the brake shoes in adjustment as the brake linings wear. The shoes are mounted on a backing plate, and are capable of limited sliding movement in a generally arcuate direction as well as expanding outwardly to engage the internal friction surface of the brake drum. Other types of brake shoe assembly have also been used at various times. These include, for example, leading-trailing brake shoe assemblies and leading-leading brake shoe assemblies. However, the duo-servo type of brake shoe assembly has been most commonly used, particularly in passenger and light truck automotive vehicles. It is a characteristic of the duo-servo drum brake that the hydraulic servo action obtained by pressurising the wheel cylinder is augmented by a mechanical servo action of the brake shoes. In normal forward rotation of the brake drum, the primary brake shoe engages the rotating drum, and the rotation of the drum with the primary shoe in engagement therewith mechanically drives the primary shoe in an arcuate direction towards the adjuster strut or similar, so generating additional brake actuating force, that is transmitted through the adjusting strut and applied to the secondary brake shoe. Thus the hydraulic servo action and the mechanical servo action co-operate to provide a higher rate of increase in the brake torque than the gain which would be obtained without the mechanical servo action. At times it has been desirable to reduce the output variation of drum brakes, or in other words the variation in brake effectiveness of a drum brake assembly, this being a well-known condition that has existed for many years. Efforts to accomplish this have usually focused upon modifying both the shape and the location of the brake anchor pin. It has been common in recent years, particularly with front wheel drive vehicles having disc brakes on the front wheels, to employ a wide oval-shaped anchor pin, inasmuch as this form of anchor pin reduces the gain of the foundation brake (that is, the brake structure in the region of the wheel to be braked) by permitting the anchor pin to be mounted closer to the friction surface of the drum, and thereby reducing the leverage of the friction force reaction at the anchor pin. Another approach has been to locate the primary brake shoe lining below the centre of the shoe, which produces a tendency to reduce the mechanical servo action of the primary brake shoe on the secondary brake shoe, thus reducing the gain of the duo-servo brake design. Studies of vehicles employing leading-trailing drum brakes and duo-servo drum brakes have shown that neither type of drum brake is completely stable throughout the useful life of the brake in normal service. Drum brakes tend to produce an increase in specific torque with use. However, in duo-servo brakes this trend is somewhat offset by a tendency for the hold-off pressure to increase, hold-off pressure being the hydraulic pressure applied to the wheel cylinder that is required to overcome the force of the shoe retractor springs, the drag forces on the shoe exerted between the shoe and the backing plate, and wheel cylinder efficiency losses, all of which factors tend to increase with use. Various approaches intended to accommodate the brake system to output variations have been made, including the use of lower-gain brake designs, such as the disc brake or the leading-trailing drum brake. Disc brakes are known to experience some decrease in their brake effectiveness with use. Existing leading-trailing brakes have been known to increase in brake output, to decrease in brake output, or to remain relatively unchanged in brake output during their service life. This approach does not however address the issue of variation in brake output during the useful life of the brakes, but rather it attempts to provide some reduction in the effect of such variations by the use of such lower-gain brake designs. A disadvantage of the approach is the need for major re-tooling and validation, so adding to the cost. Contact pressure distribution between the friction material forming the brake lining and the friction surface of the drum brake results from a complex relationship between the anchor pin location, the wheel cylinder location, the magnitude of the hydraulic brake actuating pressure applied, the primary and secondary shoe and lining stiffnesses, and the brake drum stiffness and deflection under loading. This contact pressure distribution may also be affected by the work history of the brake as the brake lining wears. The history of the analysis, theory and understanding of drum brakes has indicated that such analysis and understanding has not kept pace with the practical design and production of drum brakes. The following represents a moderately comprehensive listing of literature and source documents relating to such analysis, theory and understanding: (1) Acres, "Some Problems in the Design of Braking Systems", The Journal of the Institution of Automobile Engineers, London, Vol XV, No 1, October 1946, pp 19-49; (2) Fazekas, "Some Basic Properties of Shoe Brakes", Journal of Applied Mechanics, published by the American Society of Mechanical Engineers, Vol 25, March 1958, pp 7-10; (3) Robinson, "Brake Design Considerations -Some Notes on the Calculation of Shoe Factor", Automobile Engineer, September 1959, pp 340-348; (4) Oldershaw and Prestidge, "Brake Design Considerations - An Approach Based on the Concept of a Centre of Pressure for the Reaction Between the Shoe and the Drum", Automobile Engineer, April 1960, pp 157-159; (5) Steeds, "Brake Geometry - Theory of Expanding Rigid Types", Automobile Engineer, June 1960, pp 261-262; (6) Newcomb, "Determination of the Area of Friction Surfaces of Automotive Vehicles", Journal of Mechanical Engineering Science, Vol 2, No 4, December 1960, pp 312-324; (7) Ferodo Ltd, "Friction Lining Calculations -The Sizing of Brake and Clutch Facings", Automotive Design Engineering, Vol 4, July 1965, pp 93-94; (8) "Motor Vehicle Performance - Measurement and Prediction", Section 6.0, "Braking Systems and Braking Performance", University of Michigan Engineering Summer Conferences, Conference Proceedings, July 9-13, 1972, pp 157-208; (9) Millner and Parsons, "Effect of Contact Geometry and Elastic Deformations on the Torque Characteristics of a Drum Brake", The Institution of Mechanical Engineers Proceedings 1973, Vol 187, pp 317-331, and Discussion, pp D105-D108; (10) "Engineering Design Handbook - Analysis and Design of Automotive Brake Systems", DARCOM Pamphlet DARCOM-P 706-358, HQ, U.S. Army Material Development and Readiness Command, December 1976, Chapters 1, 2 and 14; (11) Orthwein, "Estimating Torque and Lining Pressure for Bendix-Type Drum Brakes", SAE Paper No. 841234, October 1984; (12) USG 2456 dated January 13, 1986, entitled "General Motors Corporation Response to Proposed FMYSS 135 - Passenger Car Brake System", Docket No 85-06, Notice 1, Appendices Nos 1 (Glossary of Terms) and 12 (Variability). This report is a public record filed with the Associate Administrator for Rule Making, NHTSA, 400 Seventh Street SW, Washington, D.C. 20590, USA. In various ones of these documents, analyses may be found of many different drum brake configurations including leading-leading, trailing-trailing, leading-trailing, and duo-servo brakes. Document (12) above provides in its Appendix 1 an excellent glossary of brake terms and in its Appendix 12 a thorough discussion of brake variability. Early problems which the analysis of drum brake designs tried to solve included developing sufficiently powerful designs and predicting the output of such brakes prior to the construction of test parts. The development of the duo-servo drum brake, where the primary brake shoe is used to increase the applied load to the secondary brake shoe to increase the torque output of the drum brake, yielded a very powerful brake in a relatively small physical package which did not require large-displacement hydraulic systems. These advantages are still found in this type of drum brake as it is used today. The early modelling efforts assumed a graphical approach to calculate "shoe factors", a term meaning the specific torque attributable to each individual brake shoe. These early efforts dealt primarily with the geometrical leverage ratios associated with wheel cylinder location, anchor pin location, and the like, and assumed uniform pressure distribution along the friction interface. It appears that uniform pressure distribution was assumed because such an assumption was then convenient for analytical solution of the brake design. Examples are found in the Acres, Fazekas, Steeds and Oldershaw references noted above, as well as in other treatises. Later design and theory efforts recognized that the uniform-distribution assumption did not correctly predict the measured specific torque of drum brakes. This led to analysis based on the assumption of a sinusoidal pressure distribution. All brake design efforts prior to the early 1970's assumed that both the drum and the brake shoe as well as the lining on the shoe were rigid and did not undergo elastic deformation under load, even though the theorists realised that this was not in fact the case. This assumption was made in order to simplify the analytical efforts. It was only with the advent of high-speed digital mainframe computers and the development of finite element analysis that the shoe and drum deflection has been able to receive any theoretical effort. The 1973 article by Millner and Parsons, noted above, dealt with these deflections. That article also contains the observation that the pressure distribution may change due to work history of the brake. Heel-toe contact was analysed for a leading-leading drum brake and shown to cause an increase in brake output. That article and the discussions appended thereto also discussed variations with changes in coefficients of friction of different lining materials. The Millner and Parsons article touched on the effect of lining arc length in relation to the "shoe factor", but did not include any discussion or other indication which would point to the desirability of short lining arc lengths as now proposed. Historically, brake system engineers have held a sceptical viewpoint with regard to the ability of drum brake mathematical models to accurately predict the specific torque characteristics of practical drum brake designs. This has apparently resulted from their experience in seeking to accurately employ the model predictions in the designing of vehicle brake systems. For example, minute changes in the dimensions of the component parts of the typical drum brakes in use for many years, or changes in lining composition and thus the friction coefficient of the lining, may result in changes in drum brake specific torque, and thus similar changes in the torque output of a brake. A missing piece of the puzzle has always been the knowledge of the actual lining-drum friction interface pressure distribution. The classical assumptions of rigidity and consistent pressure distributions have been coupled with the practical experience of the brake system engineers to produce a significant body of sceptics as regards the usefulness of models and drum brake analysis. Several of the publications noted above touched on the arc length of the brake lining material. There is a common thread of thought throughout them that there should be as much arc length as reasonably practical, considering the need for cooling. The unit load on the friction material, as well as its deformation and wear characteristics, further led to the broad adoption in duo-servo brakes of primary and secondary shoes with arc lining lengths in the ranges of 90° to 100° and 110° to 120° respectively, and particularly lining arc lengths of about 97° and 117° respectively. The Acres rference proposed the use of four shoes each with 75° to 80° arc length, but found that there was insufficient heat dissipation, probably because the linings covered most of the drum friction surface at all times and hindered cooling. He expressed a thought that perhaps he should have tried only two such shoes, with double duty on the lining, just as a check, but he did not pursue this further. Instead, Acres cited with approval an article by Super in the May 1946 issue of the SAE Journal that the total arc length should be reduced to 240° because of the heat dissipation problem. On the assumption of even division between two shoes, each shoe lining would then have an arc length of 120° Acres also discussed the effects of various friction lining materials, noting the changes which occur with them, and he made it obvious that different materials yielded very different results. The Steeds paper made assumptions relating to rigidity and full contact of the lining with the brake drum along its whole lining arc. Steeds gives no indication that he was considering lining arc lengths other than those in common use at the time and about the same as in the other prior art. The Oldershaw and Prestidge article, contemporaneous with Steeds, touched on lining arc lengths, and they concluded that the limits of lining arc length for a shoe were 80° to 135°, giving graphs clearly showing their position that shoe lining arc lengths of 100° to 120° were the most practical. Some typical patent disclosures discussed below indicate various developments in brake shoes and linings which generally relate to the problems solved by the use of the invention as herein disclosed and claimed. US-A-2 750 006 (issued 1956) dealt with improvements in brake lining wear and chatter. The invention was that of a brake lining of a pivoted-shoe assembly in which the brake lining has a cylindrical curvature at least at both ends greater than the cylindrical curvature of the internal surface of the brake drum with which the shoe is brought into contact during rotation of the wheel being braked. US-A-2 818 941 (issued 1958) proposed that the primary lining of a duo-servo brake assembly be made considerably narrower in width than the secondary lining. By proper proportioning of these widths, substantially uniform unit pressures between each of the linings and the drum was considered to be obtained. It was felt that this would reduce brake fade under severe braking conditions. US-A-2 848 073 (issued 1958) proposed the use of a plurality of short lining elements on each brake shoe. The lining elements on both shoes together occupied a total of about 60° of the total circumference of 360°. Thus each brake shoe has, as shown and described, a total of about 30° of arc length, divided into two sections apparently of about 15° each, for each shoe. The inventor recognised that additional cooling area contributed to the satisfactory performance of his brake. However, he had no recognition of the pressure distribution or the deflection characteristics of shoes and drums. He also did not recognise any advantages with regard to the different coefficients of friction of different materials, even though he suggested the use of non-metallic or composition aligning elements. He arranged these shoes in a leading-trailing shoe assembly. The inventor limited his friction devices to having a plurality of short pads on each shoe, the pads being spaced from the horizontal axis of the shoe and therefore extending to be more closely adjacent each of the shoe ends than the linings herein disclosed and claimed. US-A-2 910 145 (issued 1959) and US-A-3 013 637 (issued 1961) show other configurations using small multiple friction lining pad units in a duo-servo brake context. In a similar fashion, US-A-3 007 549 (issued 1961) showed gaps between various friction lining sectors on the secondary brake shoe of a duo-servo brake arrangement, or on both brake shoes of a leading-trailing brake arrangement. US-A-3 029 901 (issued 1962) concerns a duo-servo drum brake with a modified anchor arrangement to limit the self-energising force on the secondary shoe. The disclosure also shows two sectors of brake lining materials provided on the primary shoe and the secondary shoe, the sectors being mounted at the heel and toe of each shoe and being spaced considerably apart and well away from the horizontal axis of the assembly. It will be noted that the patentee utilised the brake analysis of Acres as set forth in the article referred to above. The technology of asbestos-containing friction materials for drum brakes has evolved over a period of more than 70 years, with improvements in wear, fade and friction stability continuing over this period. Some of the above-noted articles and papers touch on this. The technological development of non-asbestos friction materials, particularly for drum brake applications, has occurred to a major extent only in about the last 10 years. Although the non-asbestos technology has been successful in the development of metal fibre-reinforced materials for disc brake applications, attempts to use a wide range of mineral, metal and man-made fibres in drum brake applications has met with only limited success. Although some limited activity occurred 25 to 30 years ago, the results were never widely adopted, at least in part because of wear and noise problems. Among the issues faced in drum brake applications were and are wear, friction, braking effort variations, and noise performance of non-asbestos materials. Because it is desirable to avoid the use of asbestos products, and because the more suitable non-asbestos friction materials generally have higher friction coefficients than the currently used asbestos friction materials, the brake industry has tended to plan to meet a proposed government ban on asbestos by designing future vehicles with lower-gain rear brake systems, such as leading-trailing drum brakes. To produce equivalent brake torque, these alternative brake systems require greater hydraulic displacement, which has a design impact on master cylinder size and brake booster response as well as on brake pedal ratio. The brake engineer also has to consider the desired stopping distances and the effects of such changes thereon, as well as modulation by either manual or anti-lock brake control systems, particularly when displacements are increased. Even if the various factors so noted can be treated through innovative system designs, the concern of variability in brake output with higher friction coefficient materials must be addressed to ensure acceptable brake performance throughout a reasonable useful life of the brakes on the vehicle. If the use of asbestos products is prohibited in brake linings manufactured for service use, an issue that is also in need of consideration is the use of non-asbestos lining substitutes in the servicing of existing vehicles when the brakes were designed for use with asbestos-type friction materials. The present invention is based on new studies and analyses (feasible only recently because of the availability of computer power) which have challenged the standard assumptions that have been made and used by brake designers for many years in relation to rigidity and pressure distribution over the friction-surface faces of drum brake linings. Unexpected and advantageous results that can be obtained (as compared with expected results using the earlier standard rigidity and pressure distribution assumptions) include: (A) substantially decreased change in output or brake effectiveness, with changes in friction coefficient; (B) substantially decreased deviation from the braking effectiveness or effort of new drum brake linings with use, giving more consistent braking effort over the life of the brake linings; (C) reduced variations arising from the use of different lining formulations, from organic asbestos-containing formulations to inorganic semi-metallic or other formulations; (D) more consistent brake adjuster performance; (E) a lower lining piece cost, but improved performance; (F) improved match between front disc and rear drum brake lining life on a vehicle; and (G) improved mass and volumetric efficiency. Because of (C) above, it appears that inorganic, non-asbestos linings may now be substituted for organic linings having asbestos in them, to provide positive benefits instead of some negative results which have until now been a severe block to the elimination of asbestos-containing linings. Leading-trailing brakes have in recent years been seriously considered by brake designers and manufacturers as a replacement for duo-servo brakes because of factors (A), (B) and (F) above, involving high tooling and other costs, as well as the scrapping of duo-servo brake manufacturing facilities which are currently efficiently used. Short-arc linings in conformity with the present invention will permit the continued use of duo-servo drum brakes. The present invention is concerned with the control of output variation produced in drum brakes, and particularly in duo-servo drum brakes, as a result of the variations in contact pressure distribution between the friction material and the drum brake friction surface. The prior art as specified in the preamble of claim 1 is disclosed in GB-A-2,087,995. A drum brake shoe assembly in accordance with the present invention is characterised over GB-A-2,087,995 by the features specified in the characterising portion of claim 1. By the use of the present invention, the effect of the contact pressure distribution variations can be substantially reduced. Recent analytical and vehicle studies of production vehicles have shown that the drum brake output variations are largest when the contact pressure distributions are concentrated at the largest deviations from the horizontal centre line of the drum brake assembly. A "worst-case" condition of heel-toe contact, in which pressure distribution contact of the lining with the drum occurs only at the top and bottom of each shoe, produces an output variation of approximately twice the nominal brake output efficiency. This has been found to occur in brakes using prior-art shoe assemblies. However, for test purposes, to ensure the occurrence of this worst-case condition, a brake shoe was provided with lining material at each end location of a standard 117° arc lining, each such lining end being about 25.4 mm (one inch) long along the arc and therefore covering about 10% of the normal 117° arc length. Nominal brake output efficiency is expressed as a specific torque, or the slope of the torque-pressure plot with full lining contact. Therefore for the worst-case condition of heel-toe contact, the drum brake output can be as much as, or even more than, twice the nominal output. The present invention is based on the appreciation that the effect of contact pressure variation can be reduced by limiting the lining arc length, and placing the short arc length lining on the shoe so that it is symmetrical about the horizontal axis of the brake assembly, without changing any other factors in the drum brake assembly such as the location of the anchor pin, the wheel cylinder displacement requirements, or other factors discussed above. This reduces the concentrations of contact pressure distributions from large deviations from the horizontal centre line of the drum brake assembly to short variations from the horizontal centre line, particularly with the shorter-arc brake lining positioned symmetrically about the horizontal centre line of the drum brake assembly. It has also been observed that the limitation of lining arc length to the ranges more specifically prescribed will substantially reduce the variable effects of different coefficients of friction caused by the use of different lining material compositions, while providing sufficient lining area for the useful life of the linings in vehicle rear drum brakes to be commensurate with the useful life of vehicle front disc brakes. The invention from this aspect has the potential to permit the use of non-asbestos lining materials in duo-servo brake systems without any need for changing other portions of the system, such as for example the displacement of the wheel cylinder, or the master cylinder. There is accordingly the added benefit of being able to provide drum brake shoes with linings extending through arc lengths and located in conformity with the present invention as herein disclosed and claimed for use on vehicles that have been produced in past years (and thus made to previous designs and based on previous assumptions) as well as vehicles yet to be produced. There is thus no requirement for reconfiguration of the brake system of vehicles already in use. Brake shoes provided with linings in conformity with the present invention as herein disclosed and claimed will substantially decrease the tendency to large variations in brake effectiveness of the drum brakes with use, and can therefore maintain more constant vehicle brake performance throughout the useful life of the brakes than would be obtained with drum brakes having wide changes in effectiveness with use. This decrease has been found to be typically a decrease by a factor of four. Even though shorter-arc-length linings are likely to wear somewhat more quickly than the full arc length linings that have been in common use for many years, those made in accordance with the invention will typically wear at a rate such that when installed as rear brakes in vehicles fitted with front disc brakes, for example, they would normally be replaced at about the same time that the disc brake pads of the front disc brake assemblies would be replaced. Typically, current production vehicles with front disc brakes and rear drum brakes have been found to wear the disc brake pads so as to require their replacement at about one-half the mileage at which the rear drum brake linings require replacement due to wear. The use of approximately one-half of the amount of lining on each drum brake shoe as compared with current production brake shoes, and the ability to continue the manufacture of drum brake designs for which brake manufacturers are already tooled, usually requiring only a change in the lining arc lengths and the lining compositions, will also result in increased economy of manufacture. In the drawings: Figure 1 is a simplified elevational view, with parts in section, illustrating a typical prior-art duo-servo drum brake, and showing particularly the typical brake lining arc lengths of such a brake; Figure 2 is a view similar to Figure 1 but illustrating a drum brake assembly having a primary shoe with a 70° lining arc length and a secondary shoe with a 90° lining arc length, the shoe lining arc lengths being symmetrical about the horizontal centre line of the brake shoe assembly; Figure 3 is a view similar to Figure 2, illustrating a drum brake assembly in accordance with the invention wherein the primary shoe lining arc length is about one-half of the primary shoe lining arc length of the prior-art assembly and the secondary shoe lining arc length is about one-half of the secondary shoe lining arc length of the prior-art assembly, the shoe lining arc lengths being symmetrical about the horizontal centre line of the brake shoe assembly; Figure 4 is a view similar to Figure 3 and illustrating a drum brake assembly wherein the effective arc lengths of the primary and secondary brake shoe linings are the same as in Figure 3, but the linings of the prior-art linings of Figure 1 have been altered to create the desired effective lining arc lengths, the shoe lining effective arc lengths being symmetrical about the horizontal centre line of the brake shoe assembly; Figure 5 is a schematic illustration of a typical prior-art brake shoe and lining, illustrating the assumed uniform pressure distribution throughout the entire arc length of the lining in earlier common use by brake designers when designing typical prior-art drum brakes, there also being the assumption that the entire arc length of the lining is in contact with the brake drum; Figure 6 is a view similar to Figure 5 but illustrating the assumed sinusoidal pressure distribution in more recent common use by brake designers when designing typical prior-art drum brakes; Figure 7 is a view similar to Figures 5 and 6 but illustrating the potential "worst-case" pressure distribution on a prior-art duo-servo secondary brake shoe lining as it is in engagement with the brake drum; Figure 8 is a graph showing data for a duo-servo drum brake assembly using the shoe and lining assembly of Figure 1, plotting the deviation from the ideal brake balance in a vehicle having front disc brakes and rear duo-servo drum brakes. The vehicle was a 1986 model year vehicle identified as vehicle "Q". One curve of the graph is plotted for full-area contact of the brake shoe linings with the brake drum, whereas the other curve of the graph is plotted for the same brake assembly but with only heel-toe contact; Figure 9 is a graph similar to Figure 8, on which similar data are plotted, but using data pertaining to a 1980 production vehicle identified as vehicle "R"; Figure 10 is a graph similar to Figures 8 and 9 but plotting data obtained from tests on a 1979 vehicle identified as vehicle "S", using the production rear duo-servo drum brakes of that vehicle, which are similar to those of the drum brake assembly shown in Figure 1; Figure 11 is a graph similar to Figures 8, 9 and 10, but plotting data obtained from tests on a 1981 production vehicle identified as vehicle "T", using the production rear duo-servo drum brakes of that vehicle, which are similar to those of the drum brake assembly shown in Figure 1; Figure 12 is a graph illustrating the output sensitivity of a production prior-art type duo-servo drum brake like that of Figure 1, showing one curve plotted from data obtained with brake linings in full contact with the friction surface of the drum and another curve plotted from data obtained with brake linings having only heel and toe contact with the brake drum as defined herein. The data plotted are coefficient of friction plotted against specific torque; Figure 13 is a graph similar to Figure 12, plotted from similar data obtained with a duo-servo drum brake like that of Figure 2; Figure 14 is a graph similar to Figure 8, plotted from data obtained with brakes on the same 1986 production vehicle identified as vehicle "Q" but with the lining arc lengths being those illustrated in Figure 3; Figure 15 is a graph similar to Figures 12 and 13 but plotted from data using 50 per cent of the typical current production prior-art lining arc lengths of the type illustrated in Figure 3; Figure 16 is a graph similar to Figure 15 and is a graph plotted from data obtained using 58° lining arc lengths on both the primary shoe and the secondary shoe, arranged in a similar manner to the shoes and linings of Figure 3, the shoe lining arc lengths being symmetrical about the horizontal centre line of the brake shoe assembly; Figure 17 is a graph plotted from data obtained using the same vehicle brakes as those used for the data obtained to plot Figure 12 above, the data plotted being the percentage change in specific torque output with changes in the coefficient of friction. One curve relates to that particular test brake lining arrangement, another curve is a plot of data using the 70°/90° symmetrically arranged lining arc lengths illustrated in Figure 2, a third curve is a plot of data using a primary shoe and a secondary shoe each having a 58° arc lining length which is symmetrical about the horizontal centre line of the brake shoe assembly, and a fourth curve is a plot of data obtained using the 50 per cent of prior-art and typical production lining arc length brake linings of the type illustrated in Figure 3; and Figure 18 is a graph plotting the specific torque output of a duo-servo drum brake assembly, using data obtained from the same various-arc-length configurations as were used for obtaining the data for the curves of Figure 17. With reference now to the various drawings, the prior-art brake shoe assembly shown in Figure 1 is of the type found in production duo-servo drum brake assemblies in use for many years on vehicles manufactured throughout the world. The simplified drum brake assembly 10 illustrated includes a rotatable brake drum 12 which is suitably secured to a vehicle wheel for rotation therewith as is well known in the art. The assembly has a backing plate 14, a wheel cylinder 16, a primary brake shoe assembly 18, a secondary brake shoe assembly 20 and a brake adjuster strut 22. As is well known in the brake art, the wheel cylinder 16 is attached either to the backing plate or to a fixed flange, and is arranged to receive hydraulic brake actuating pressure therein to actuate the drum brake. The wheel cylinder is illustrated as having oppositely disposed plungers 24 and 26 that are connected to wheel cylinder pistons inside the wheel cylinder housing. A plunger 24 engages an end 28 of the brake shoe 30 of the primary brake shoe assembly 18, and a plunger 26 engages an end 32 of the brake shoe 34 of the secondary brake shoe assembly 20. The shoe ends 28 and 32 are considered to be adjacent ends of the brake shoes, and the opposite adjacent ends 36 and 38 of the shoes 30 and 34 respectively are arranged to engage the adjuster strut 22 as is well known in the art. The shoe assemblies 18 and 20 are appropriately mounted to the backing plate 14 (by means not shown) so that the shoes can move in various directions parallel to the backing plate as the brake is selectively actuated and released. This overall arrangement is well known in the drum brake art. The brake shoe assembly 18 has a brake lining 40 secured to the shoe rim 42 in a suitable manner well known in the art. Similarly, the brake shoe assembly 20 has a brake lining 44 appropriately secured to the shoe rim 46 of the shoe 34. In the typical prior-art brake assembly illustrated, the primary brake shoe lining 40 subtends a lining arc length of about 97.5° as illustrated, and has a greater portion thereof positioned below the horizontal centre line 48 of the brake shoe assembly than the portion that is positioned above that horizontal centre line, and the secondary brake shoe lining 44 subtends a lining arc length of about 117° as illustrated, and may be symmetrical about the horizontal centre line 48. As can be seen, this arrangement shown in Figure 1 follows the brake design teachings which have been set forth consistently in brake design treatises on the subject over the years, including the location of the primary shoe lining in an arcuate direction nearer the adjuster strut 22 so that more of the lining is below the centre line 48 than above that centre line. This type of drum brake assembly is typical of those used for many years on vehicles produced in the United States as well as in many other parts of the world. Similar brake shoe assemblies have been used on front wheels as well as on rear wheels. With the common advent of disc brake assemblies on the front wheels of many passenger and light-truck vehicles, drum brake assemblies have continued to be used on the rear wheels. It has been common practice to use duo-servo drum brake assemblies, with various arrangements having been made to provide a somewhat lower gain for the drum brake assembly than would be the case if the maximum duo-servo effect were used. Figure 2 illustrates a modification of the drum brake assembly shown in Figure 1, and shows an intermediate stage in the development of the concept of the invention as herein disclosed and claimed in a range of brake lining arc lengths which approach the upper limits of desirability to obtain advantages of the invention. The parts of the drum brake assembly of Figure 2 are identified by the same reference characters as are used in Figure 1 except in relation to the primary and secondary brake shoe assemblies and the brake linings thereon. In Figure 2 the primary brake shoe assembly identified as 218 has a primary brake lining 240 thereon, and the secondary brake shoe assembly identified as 220 has a secondary brake lining 224 appropriately secured thereto. In this brake assembly the primary brake lining 240 subtends a lining arc length of 70°, and is symmetrically positioned relative to the horizontal centre line 48 of the brake shoe assembly so that the lining arc length on either side of that centre line is the same. In a similar fashion, the secondary lining 224 subtends a lining arc length of 90°, and is likewise symmetrically positioned about the centre line 48. The shoe linings 240 and 224 of the brake shoe assembly 210 of Figure 2 are preferably of the same width as the shoe linings 40 and 44 of the brake shoe assembly 10 of Figure 1, and the lining material merely extends through a shorter arc length for the purposes and reasons discussed above and below, for the purpose of obtaining the unexpected results neither found nor taught in the prior art. Although the brake linings 240 and 224 as well as the linings of Figures 3 and 4 are each considered to be a single continuous brake lining, this does not preclude the possibility of the brake linings having minor gaps or spacings if the linings are provided in lining segments, as long as there is no substantial change in the effective arc lining lengths provided and in the lining locations. Similarly, if it becomes desirable to provide a groove in the friction braking surface, either lengthwise or transversely of the brake lining, the linings may still be considered to be a single lining extending continuously through the illustrated arc length. It can be seen that by the shortening of the arc lining lengths, the materials that constituted the heels and toes of the linings 40 and 44 have effectively been eliminated, so that the effective heel and toe of the friction braking surface of each of the linings 240 and 224 are now positioned much closer to the centre line 48 than heretofore. Furthermore, the ends of each of the linings 240 and 224 are evenly and symmetrically spaced from the centre line 48. The shorter lining arc lengths of Figure 2 having been tried, it became apparent that some important advantages were obtainable, and this then led to the development of shorter-arc-length linings which were found to be extremely advantageous when each of the linings occupies approximately 50 per cent of the lining arc length of the prior-art linings illustrated in Figure 1. Therefore, the drum brake assembly of Figure 3 has such shorter-arc-length linings, which are still symmetrically positioned about the horizontal brake assembly centre line 48. In the drum brake assembly 310 of Figure 3, the primary lining 340 subtends a lining arc length of about 49°, and is positioned symmetrically about the horizontal centre line 48, and the secondary lining 344 subtends an arc lining length of about 57°, and is also symmetrically positioned about the brake assembly horizontal centre line 48. Another modification, on which some graphic data is plotted but which is not otherwise illustrated, had the primary and secondary brake lining arc lengths about 58° in each instance. In other respects the drum brake assembly is the same as that of Figure 3, and the same hydraulic system (not fully shown) is used. The drum brake assembly 410 of Figure 4 is quite similar to the drum brake assembly 310 of Figure 3. The effective arc length of the primary lining 440 subtends a lining arc length of about 49°, and is positioned symmetrically about the horizontal centre line 48. In this instance the effective lining arc length is generated by modifying the primary brake shoe assembly 18 of Figure 1. This modification is accomplished by removing a substantial portion of the brake lining 40, leaving only a portion 441 as illustrated in Figure 4 at one end, and a similar portion 443 at the other end, sufficient to accommodate suitable fastening means holding the lining to the brake shoe rim. In instances in which the prior-art lining is cemented to the shoe rim, the entire lining in the areas 441 and 443 may be removed, as desired. The removal is accomplished such that the effective lining arc length 440 subtends the desired lining arc length of approximately one-half of the arc length of the prior-art lining 40. The secondary lining 444 shown in Figure 4 has an effective lining arc length of about 57°, as does the secondary lining 344 in Figure 3. This lining is likewise illustrated as having been modified from the prior-art brake shoe assembly 20, involving also the removal of some lining material from either end of the shoe 44, as shown at 445 and 447 in Figure 4. As is the case with the primary shoe assembly 418, the secondary shoe assembly 420 has its effective lining arc length subtending the same arc as the secondary shoe illustrated in Figure 3, and likewise is positioned symmetrically about the brake assembly horizontal centre line 48. The shoes may have the lining ends bevelled, with the friction surfaces extending through the requisite arc lengths when new as the linings wear through the bevelled area. Also, the effective arc lengths may be increased slightly but still remain within the scope of the invention. The minor modifications discussed above in relation to the lining segments 440 and 444 are also applicable to the other short-arc-length lining segments embodying the invention, for example the lining segments 240 and 244 of Figure 2 and the lining arc lengths 340 and 344 of Figure 3. Figure 5 illustrates a typical brake designer's assumption over the years of a uniform pressure distribution across the entire surface and arc length of a brake lining, with the lining always considered to be in full contact with the brake drum when the brake is actuated. Figure 6 is similar to Figure 5, but illustrates the later assumption of brake designers that the pressure distribution across the entire brake lining is sinusoidal, being less at the heel and toe and at a maximum at the centre of the brake shoe. These discussions relate particularly to the secondary brake shoe of a duo-servo drum brake assembly, and the brake shoe illustrated in each of Figures 5, 6 and 7 is the secondary brake shoe 20 of Figure 1 with the 117° arc length lining 44 installed thereon. Thus the arrows 60 of Figure 5 illustrate the assumed uniform pressure distribution acting along the entire arc length and surface of the lining 44. Similarly, the arrows 70 of Figure 6 indicate the pressure distribution along the entire arc length of the lining 44, assuming the sinusoidal pressure distribution concept. Recent analytical and vehicle studies, including the test results supporting the actual "worst-case" pressure distribution pattern illustrated in Figure 7, support the present contention that the drum brake output variations are largest when the contact pressure distributions are concentrated at the largest deviations from the horizontal centre line 48 of the drum brake assembly. Subsequent to such analyses, studies and tests, tests were performed in which the "worst-case" pattern was positively generated. About 80% of the arc length and area of the lining material was removed so that it could not contact the brake drum except for small areas at the heel and toe, substantially in the areas of the pressure distribution arrows 80 and 82 of Figure 7. Typically, each such area covered up to about 10% of the area of the 117° arc length lining. As shown in later graphs bearing on heel-toe contact, such a shoe arrangement produces an output variation of brake effectiveness of approximately twice the nominal drum brake output. Instrumented torque wheel tests were conducted on a vehicle using a semi-metallic friction lining material in duo-servo drum brakes mounted on the rear axle of the test vehicle. The vehicle was first tested with full lining arc lengths of the type illustrated in Figure 1. A nominal rear brake specific torque of 0.1 Nm/kPa (0.5 lb-ft/psi) was determined to exist after burnishing of the rear brakes. The rear lining was then modified to permit lining contact with the brake drum at only about 25.4 mm (one inch), measured arcuately, at each of the heel and toe of both the primary and secondary linings. This essentially provided brake lining material only in the areas of the pressure distribution arrows 80 and 82 of Figure 7, by removal of the lining material between the two adjacent ones of the arrows 80 and 82. These linings were re-burnished, and the rear brake specific torque was again determined with instrumented torque wheels on the vehicle. The rear brake specific torque was found to be approximately 0.2 Nm/kPa (1.0 lb-ft/psi). The same test was repeated with the same vehicle equipped with rear linings having an arc length limited to about 60° on each of the primary and secondary brake shoes, with the linings positioned symmetrically about the horizontal centre line of the drum brake assembly. These tests resulted in the finding that the nominal rear brake specific torque was then about 0.07 Nm/kPa (0.35 lb-ft/psi). These linings were then modified to provide only heel-toe contact in a similar manner to that described above, and the rear brake specific torque was found to be about 0.08 Nm/kPa (0.38 lb-ft/psi). These test findings were confirmed by analytical results generated using a current state-of-the-art duo-servo drum brake mathematical model having modelling capability to treat non-uniform pressure distributions along the friction interface. Analyses of both duo-servo and leading-trailing brake arrangements show a similar tendency for the specific torque to be influenced by the contact interface pressure distribution. The contact pressure distribution at the ends of the full-arc-length leading-trailing brake indicated an increase in drum brake specific torque of about the same magnitude as that found in the equivalent duo-servo drum brake assemblies. Because of the generally higher friction coefficients of non-asbestos friction materials, the successful use of such non-asbestos friction materials in drum brake applications is particularly dependent upon the recognition of the teaching of this invention whereby the brake specific torque can be controlled so that it remains about the same under various pressure distributions along the friction interface between the lining and the brake drum. This is accomplished by control and positioning of the lining arc length, and in particular the short lining arc lengths disclosed and claimed herein. Without such control and positioning, the tests have clearly shown that drum brake specific torques can be expected to vary beyond acceptable limits. With such control and positioning, on the other hand, the variation of specific torque, or brake effectiveness factor, has been reduced by a factor of about four over the useful life of the lining, independently of the friction braking material of which the brake lining is made. The establishment of particular lining arc lengths in accordance with the above is in part a trade-off (compromise) between the wear rate of the drum brake lining and the decrease in variability of brake output. From the variability point of view, the shorter are the the lining arc lengths, when located symmetrically on the horizontal axis of the drum brake assembly, the less can the variation of brake specific torque be expected to occur as the result of changes in the normal friction interface pressure distribution that occurs as the brake lining wears, as well as changes in coefficient of friction due to the use of different lining material compositions. However, the shorter the lining arc lengths, the shorter the lining life, and the sooner the linings must be replaced. It has been found that a desirable compromise to obtain the overall best results is for the arc lining lengths to be about one-half of the prior-art and recent production arc lining lengths exemplified by Figure 1 of the drawings, such shorter lining arc lengths and lining positioning being illustrated in Figures 3 and 4 of the drawings. Only slightly shorter arc lining lengths, such as those illustrated in Figure 2, yielded results that were somewhat better than those of prior-art linings. Once the teaching of the invention herein disclosed and claimed is fully understood, however, it is preferable that the arc lining lengths be in the general area of those shown in Figure 3. It is within the overall scope of the invention for the arc lining lengths of the primary and secondary shoe linings be slightly different from one another, as is illustrated in Figure 3, or alternatively for them to be the same, as discussed above, when arc lining lengths of 58° or 60° were used for test purposes in some instances. When existing brake assemblies are to be modified, it may at times be more economical to modify an already completed brake shoe assembly having the brake linings extending through an arc length of about 97° for the primary brake shoe or 117° for the secondary brake shoe, without removing the linings themselves. Such a modification is illustrated in Figure 4. It may be effected by an appropriate operation such as grinding, to remove brake lining portions at either end of the brake lining, as shown at 441, 443, 445 and 447 in Figure 4, so reducing the brake lining thickness in the region of those portions so that those portions no longer effectively contribute to the friction braking surfaces. Appropriate safety precautions should of course be practised during the grinding operation, and for disposal of the removed materials. For example, standards corresponding to those of the United States Occupational Safety and Health Administration requirements regarding exposure to absestos should be met when asbestos-containing material is being removed. Such a removal operation would then leave a brake lining with a friction braking surface having an effective lining arc length in conformity with the invention, even though (non-active) portions of the brake lining extend for a greater arc length. The friction braking surface remains symmetrical about the horizontal centre line of the brake assembly, which may also be identified as a centre line located at the mid-point between the brake shoe ends. This may be particularly desirable when the linings are riveted to the shoe. The lining material could be removed at the lining ends to a depth flush with the rivet ends, for example. In other instances, the original brake lining may be completely removed and replaced by a shorter-arc-length brake lining in conformity with the invention, as illustrated in Figure 3. For the manufacture of a new drum brake assembly, it would of course be the normal practice to provide the shorter arc length brake lining initially. However, in some instances it may still be desired to provide a full-arc-length brake lining with the end portions decreased in thickness as described above. Such modifications are clearly within the scope of the present invention. Once the significance of the background against which the present invention was developed has been recognised, it becomes clear that the attempts by brake designers for many years to maintain more constant drum brake effectiveness throughout the useful life of the brake linings is much better accomplished by the use of the present invention than by the use of the prior-art production brake lining such as is shown in Figure 1. The prior-art teachings did not recognise the real causes of their various problems, and therefore they followed different design directions, resulting in solutions which did not solve the real problems. It is also an important result of the present invention that different brake lining compositions having quite different friction braking characteristics, including differing coefficients of friction, may be used without giving rise to untoward changes in the brake output effectiveness (referred to above as the brake specific torque). In some instances, within the context of the same overall brake system design, minor changes may be appropriate, including, by way of example, substitution of a different wheel cylinder size within the same wheel cylinder family. Linings containing no asbestos may accordingly be substituted in used vehicles originally having asbestos-containing linings. Whereas only a short time before this invention the brake industry was indicating that it would be some years before asbestos lining materials could be eliminated, the present invention makes it possible to use brake lining materials containing no asbestos without substantial modification of the brake effectiveness in new vehicles, with the use of the same current production brake arrangements, as well as in older vehicles as the brake linings on the drum brakes of those vehicles fall due to be replaced. In some instances, modification of the parking brake actuating mechanism may be required if the brake lining compliance requires such modification. At times, also, it may be necessary to strengthen the shoe web and rim. The unexpected result obtainable by the use of the present invention is extremely important in meeting the requirement as set forth for example by the United States Government and the governments of some other countries to eliminate asbestos in brake lining materials as soon as possible. Application of the present invention has the outstanding advantage that the brake systems themselves do not require overall modification or re-design. For instance (by way of example), wheel cylinder sizes may not need to be changed, anchor pins need not be re-designed or relocated, master cylinder capacities need not be changed, booster outputs need not be changed, and service brake pedal ratios need not be changed. Furthermore, the use of the short-arc-length brake linings in existing vehicles or new vehicles will not affect the modulation of anti-lock brake control systems on such vehicles, because it does not change the hydraulic capacities of the systems. The use of the short-arc linings proposed as the best compromise will allow the needed benefits to be obtained, and will potentially result in the drum brake linings wearing out in the course of their useful life at about the same time and mileage as the front disc brake linings, such that the front and rear linings both require replacement at the same time.

Claims: A brake shoe assembly for use in a drum brake, comprising a pair of brake shoes (30,34) each having a heel end (36,38) and a toe end (28,32), and a friction brake lining (240,224,340,344) secured to each brake shoe (30,34) and having a friction surface adapted in operation to engage a brake drum (12) for braking action, each friction surface being substantially continuous and arcuately spaced from the heel end (36,38) and the toe end (28,32), and each brake lining (240,224,340,344) being secured to its respective brake shoe with the transverse mid-point of the friction surface located at the transverse mid-point of the brake shoe between the heel end (36,38) and the toe end (28,32); characterised in that the friction surface of the brake lining (240,340) of one brake shoe (30) subtends a total arc in the range of 49° to 70°, and the friction surface of the brake lining (224,344) of the other brake shoe (34) subtends a total arc in the range of 57° to 90°. A brake shoe assembly as claimed in claim 1, wherein the friction surface of the brake lining (340) of said one brake shoe (30) subtends a total arc of about 49°, and the friction surface of the brake lining (344) of said other brake shoe (34) subtends a total arc of about 57°. A brake shoe assembly as claimed in claim 1, wherein the friction surface of each brake lining subtends a total arc of about 58°. A brake shoe assembly as claimed in claim 1, wherein the brake lining (440,444) of each brake shoe (30,34) subtends a total arc in the range of about 97° to about 117°, and wherein a portion (441,443,445,447) of the brake lining adjacent the heel end (36,38) and the toe end (28,32) of each brake shoe has a reduced thickness to leave the friction surface between the said portions. A drum brake comprising a brake drum (12) to be braked, a brake shoe assembly (218,220, 318,320,418,420) as claimed in any one of claims 1 to 4 comprising a pair of brake shoes (30,34) movable to engage the brake drum (12) in braking relation, and actuator means (16) arranged to selectively act on the brake shoes to move the same to engage the brake drum (12).

Inventors: FLAIM, THOMAS A.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: F16D65
IPC codes: F16D51, F16D65, F16D69