Document 2q44ZkrwEEx03qO8jnNzp131N

FILE NAME: AMMCO (AMC) DATE: 1950 June DOC#: AMC025 DOCUMENT DESCRIPTION: Trade Journal Article - Brake Lining Material THE varied properties required of a brake lining material calls for careful selection and blending of its ingredients. Lining requirements are: , A utomotive I. Correct coefficient of friction, which is influ enced by temperature reaction, aging qualities, water reaction, and oil and grease reaction; 2. Durability; 3. Relative freedom from any tendency to score drums; 4. Quietness in operation; and 5. Nonoffensive odor. . Lining friction coefficients generally run between 0.20 and 0.40. Manufacturers usually describe lin ings as having high, medium, and low friction co BASED ON PAPEP* BY efficients without specifying friction value numeri cally. It's not hard to compound a lining with an A. J. Carter initial specific friction coefficient; but to produce a Assistant Department Head lining with uniform braking performance under various operating conditions takes much compound Rubber 0 P asties Laboratory Chrysler Corp. ing study and laboratory and road testing. The ideal lining would have a constant friction coefficient at high and low temperatures, under wet and dry conditions, throughout the lining's life. There is no such lining. Ad linings disintegrate under high braking temperatures. Chemical and physical changes either increase friction coefficient (build-up) or decrease it (fade). With fade, if friction characteristics return to their initial condition, the lining is said to have good recovery properties. Satisfactory linings fade slightly with each brake application, but recover immediately after cooling. Linings producing build up in friction coefficient are not satisfactory. Friction coefficient may increase or decrease with age. When it decreases, it develops a hard pedal. Some linings harden and tend to score drums and be noisy. Other linings are sensitive to water on the fric tion face, with effects as pronounced as those from temperature. Moisture also causes ``morning sick ness." Iron oxide forms on the drum and gives high friction reaction during the first two or three stops made after the car has been parked over night. Be- * Paper "Automotive Brake Lining M a*era!s," was presented at SAE Annual Meeting. Detroit, Jan. 3 . 1950. paper is available in full In multilithographed form from SAE Soecfil Publications Department P ric e : 254 to members, 504 to norm errcers) cause of contact with oil and grease in service, lin ings should have some resistance to rhzze materials. Linings also should wear slowly and uniformly. This insures more consistent braking action by con tinually renewing the friction surface. Negligible wear may produce a glazed friction surface. Certain ingredients tend to score drums. Steel drums score more readily than cast-iron ones. Brake system quietness is a function of the lining as well as other factor a Ingredients must not pro duce offensive odors a: high braking *emperatures. Good compounding ingredients may oe discarded because of this limitation. Brake lining materials derive their properties from fillers, binders, and wear-enhancing ingredi ents, such as those in Table 1. Selection and per centages of ingredients used varies with the type of lining. Chief lining constituent is Chrysotile asbestos, used as the primary reinforcing material. Chemi cally, it is an hydrous magnesium silicate (H4 Mg3 Si2 0 9) . The mineral fibers are to 8 in. long. Under high magnification the fiber looks like many finer crystalline threads bundled together. Diam- Binder* Elastom ers Rubber G R-S Buna N Neoprene Phenolic resin Oil m odified phenolic re sin s Cashew nut oil resins Drying oils Sulfurhsd oil* Table 1-- Brake Lining ingredients Reinfwcmg Chrysotile Asbestos Fillers Nonreinforcing Barium sulphate Calcium sulphate W hite lead Lead carbonate Clay Asbestine Friction Modifying and Wear Enhancing Agents Cashew n u t liquid products (powders) Rubber and synthetic rubber Ground rubber tire scrap Iron oxide M etals--lead, zinc, brass Lead salts Talc G raphite Bituminous m aterials Abrasives Curing Agent* and Accelerators S tandard rubber and resin prim ary and secondary curing agents and accel erators 20 * SAE JOURNAL 04 i BRAKE LINING MATERIAL Ingredients Hold Key To Service Behavior eter of the smallest fiber which can be separated Is about 0.00003 in. Asbestos makes a good friction material because of its heat resistance, chemical resistance, flexibil ity, low therm al conductivity, and hardness. Its reaction to heat is part.rzlarly important. The asbestos fibers i tart losing their water of crystallization at about ioO F. The loss rate in creases with temperature and become rapid at 1000 F. When the water is driven off, asbestos looses its crystalline properties zrd. becomes a powder. As bestos fiber breakdown to powder with heat makes possible rejuvenation of the lining's friction surface. Today's brake linings trzuld be impossible If heat generated in braking a car decomposed only organic materials and changed asbestos into a hard, organic fused layer of abrasive material. Poor heat-conducting properties of asbestos help keep heat from penetrating deeply into the lining. This would produce chemical changes in binder materials and would harm lining friction character istics. Asbestos fabrics have a friction coefficient of about 0.35. This is within the 0.2 to 0.4 range around which satisfactory braking systems have been designed. Brass, lead, or lead alloy wires used in woven ma terials strengthen the yam. Some claim lead sur passes other metals became it stabilizes the friction coefficient, acts as a dry lubricant to prevent drum scoring, and inhibits formation of abrasive particles on the friction surface. Metallic powders, such as zinc and lead, improve performance at high temperatures. Limitation with fine lead is th a t it oxidizes easily to litharge, which promotes oxidation in unsaturated organic compounds. Some believe these powders help the continuity of the friction surface film during braking action. Large amounts of metal (40% ), such as brass chips, are added to linings for very high temperature requirements. Brake lining compounders also add lead to com positions In the form eff organic salt. High tem perature liberates it as finely divided lead in a reducing or inert atmosphere. Thu prevents oxida tion of the metal to an oxide and permits it to func tion as a friction stabilizer. Graphite in lining compounds irzzarts a lubricat ing effect for smoother stopping 1: ran be incor porated in the hard rubber o' trie d separately. Some compounders see two adv..r tires for graphite encased in rubber. First, it doer ' interfere with flow of the resin binder durir r . _r.n g. Second, graphite is released for its labr.mvug action only after the rubber is softened by :_rn making tem perature. Iron oxide in small amounts sorm'imes is used as a friction-controlling element. 1: rends to have a self-polishing action which part:; :-:r.:rc!s surface frictional properties. The compound usually requires amounts of inorganic fillers to produce frictirt.il effects. This necessitates an improved fricrim itabiiizer that functions over a wide temperature range. Organic modifiers--such as rubber, g r utz rubber scrap, pitches, and gilsonite--function over narrow temperature ranges. A powdered product made from nut liquid is one of the better friction-st:.:n_.zir,g and wear enhancing agents used today. Tr__- material works satisfactorily up to temperatures :f ''00 to e50 F. Tire scrap particles function up t: trout only 500 F. About 6 to 8% of dust is needed :: .improve wearing qualities. Ground-rubber tire scrap has bier., and will con tinue to be, widely used because u :s a cheap raw material. Other friction modifiers--such as pitches, gilsonite, coal, and petroleum com?--can be used in iimlted quantities only because :f their low tem perature resistance. Braking temperatures destruc tively distill these materials to form tarry or pitchy residues a t the friction surface. . ztese increase the friction coefficient at low tempera; ur-s. But at high temperatures, volatile materials r..v; be driven oft too rapidly'before formation of tarry products, los ing their effectiveness. Research today is aimed at gitzrtg binding ma terials with high heat resistance. Currently the : JUNE, 19S0 21 * .1 fi8- 1-- As this brake shoe shows, bonded lining makes available twice the usual lining and virtually eliminates arum score Lists New Need With Bonding Lining Add bondabiiity as a sixth lining requirement, advises S. G. Tiiden, The Permafuse Co. The advent of bonded brake linings makes this a must. Tilden's organization has set up an arbitrary minimum bond strength requirement of 600 psi in shear. This gives a total shear strength of 11,500 lb on a 134 x 11-in. segment. That is about five times the maximum shear force ex erted on linings by a simulated emergency stop with a 1 g deceleration. All lining factors producing good bondabiiity have not been established. But tests have been made to find the effect of porosity. Strangely enough, denser and less absorbent linings bond better. Bonding suitability also calls for availability of the entire lining thickness for use. T hat's why wire-back linings are not suited for bond ing. They can be used only down to the wire backing; after that they -cere 'he drums. With wire backing, the promLcd double wear from bonded linings cannot be red. Typical of the extra --e..r avail.'.bi-: v.uth bonded lining is the hnir.g b 1. Tins brake shoe with bonded lining w n ; ,?d until the seg ment had worn completely through to the shoe at the central tangential .uvu. Mon the lark of drum score, except for the lower edge portion; this came from actual metal-to-metal contact between brake shoe and drum. Tilden points out that 'oor.uetl linings also im prove the path for conducting heat into the brake shoes, backing plate, on;: axls. it pro vides a bridge v/ithout the m. mating air layer usually present in riveted linings. main binding agents are synthetic resins, drying oils, rubber, and bituminous materials. Most im portant group is the synthetic resins. Oil modified phenolics are mostly used. Synthetic resins hold much promise because they can be synthetized in the laboratory to meet desired binding material requirements. - Available resins vary in their properties. Some can be used alone, others must be used together with natural, synthetic, or reclaimed rubber. Oil modified types also are commonly used with these rubbers. Used a3 a binder, rubber or GR-S must be vulcan ized to function properly. In lining compositions they usually are cured to a hard rubber with 25 to 40% sulfur as the vulcanizing agent. Their rela tively low softening and decomposition tempera tures limit rubbers alone as binding agents. In the future it may be possible tc synthetize rubber polymers that will make satisfactory brake lining binding materials. Some day special rubbers may be expressly made for this use. Drying oils, used for many years In brake lining formulations, are limited because of control of polymerization (hardening). They are used in air curing type and woven linings. Wide use also is made of them as a binding component in modifying phenolic resins. (The paper also tells how fabric and molded lin ings are made.) 22 SAE JOURNAL ,- L !*PPIWN^