Document LggQqd6Ko45K0x3B7owgVpe05

FILE NAME: AMMCO (AMC) DATE: 1946 Jan DOC#: AMC023 DOCUMENT DESCRIPTION: Trade Journal Article - Friction Brake Linings Types, Uses and Design Data In this issue: PLASTIC RESINS AS PROTECTIVE COATINGS i Friction Brake Linings Types, Uses and Design Data Six general types of brake linings are described briefly; some applications of the types are mentioned and illustrated; and the use o f coefficients of friction, brake band or drum temperatures and wear data in selecting a lining during the design of brakes is dis cussed. The performance basis of selecting a lining in automobile service is explained. BRAKE LININGS can be classified as (1) woven, (2) fabric, (3) dry mixed, (4) wire back, (5) extruded and (6) sheeter type. None of the types meets all the require ments for linings in today's brakes. Each type, though adaptable to a wide variety of operating conditions, has to be designed to function with the other members of the brake, particularly the shoe and the drum, if good performance would be assured. W oven L inings are one of the oldest types. In construction, this type is a solid tape, woven from asbestos and cotton yarn hav ing a wire core. A soft brass wire is gen erally used in the core but alloy wires of lead and zinc are also common. The tape is impregnated with oils and heat-resisting compounds, cured by baking, and formed to size between rolls. Linseed oil, thermal setting resins, gilsonite, artificial asphalts, rubber cement, mineral oils, pitches and coal tars are some of the other constituents of brake linings. in a press under pressure of the magnitude of 2,000 lb. per sq. in. and heated. The heat softens the binder compounds. Con tinued heat and pressure harden the mate rial, which upon cooling can be removed from the press in finished form. Generally the material is cooled and removed from the press in an uncured state, cut to size and cured in heated molds. W ire Back Linings contain a wire screen as reinforcement. The wire screen can be readily built into dry mixed, wet mixed and dough mixed linings. One method of ap plying the friction materials to the screen rises a pair of heated rolls of unequal diam eters. The wire screen passes over the larger roll. The friction materials flow from a hopper to keep filled the space between the rolls as they revolve together. The lin ing in the pliable state is cut to size and cured in molds. E xtruded L inings get the name from the process of molding them. W et mixed ma terials can be fed into the hopper of a standard extruding machine where they are forced by the revolving spiral blade through an orifice, which forms them to the desired width and thickness. A hydraulic ram is an Fig. |-- An inertia brake dynamometer is often used in test ing airplane wheel brakes under simulated landing conditions. Hayes Iudusti ies, Inc. Fabric L in ing s, also one of the oldest types, have a cloth base. The cloth is woven from asbestos and cotton covered wire. Pieces of the cloth 40 to 60 in. wide and a 100 yd. or more in length are com mon. After dipping in a light impregnating fluid, the doth is passed between heated rolls where rubber or Buna S friction com pounds are pressed into the spaces of the fabric. Brake linings of the fabric type are made of layers of the cloth, which can be built up by successive layers of equal width, by folding pieces of the cloth one or more times and by rolls that can be cut in seg ments. The lining is compressed during curing at pressures around 2,000 lb. per sq. in., which causes the built-up thickness to diminish by 25 percent. A curing temper ature of 300 deg. F. or slightly higher is customary. i D ry M ixed L in in g s a re so n a m e d because I the materials Composing them are mixed in the dry state. The rubber and cotton in gredients are finely pulverized. Short as bestos fiber, almost a powder, is used. Sul phur, thermal setting resins and drying com pounds are added in the powdered state. The thoroughly mixed materials are placed l ^nam OTwa.- . - _________ . C'q m e r o N M a c h in e Co. i& is S S S a a ^ pjg 2 __A heavy duty w oven lining is used on band brakes of paper winding machines. pjg_ 3 _ v / oven linings are also used on ihe disk brakes of paper machinery. C a m ir c n M a ch in e Go. 1 p p jf: . , f ia.ji E. W. Bltsa Co. - i i i i r cja 4__ fabric and woven linings are used on the band brakes ot presses. pj-^ 5__Blocks of molded lining are used on expander-tube brakes of aircraft wheels. Hayes Industrie*, Inc. alternate method of forcing the wet mate rials through the orifice. The extruded material is deposited on a slowly moving belt. W ire can be pressed into the putiaUy dried lining at this stage- if desired. Tue lining is then cut to size and cured. Sheetf.r Lin ing is m ade on a `Miceter machine, which is a modified rubber mid. The larger roll, which is 3 to 5 ft. in diam eter and 10 to 15 ft. long, is heated Mixed material is fed into the opening between 40 them. The material clings to the heated roll. The thickness between the rolls is in creased as material accumulates on the heated roll until the desired lining thickness is attained. Then the sheet is slit, removed from th e roll, c u t to size and cured. The different types of linings are avail able in a great variety of styles, sizes, thick nesses, shapes and coefficients of friction. These data are available in lining manufac turers' catalogs, to which the designer is referred because they are too voluminous for coserage here. ' Friction blocks differ mainly from the other molded linings in that they are thicker, wider, and shorter. General Application Woven linings are widely u:ed on the contracting members of external band brakes. The brakes of hours, cranes, presses, winches, tractors and oil-drilling machinery are familiar examples. One reason for the choice of woven linings on P roduct E ngineering -- January. 1946 e band bieats is the flexibility of the woven type of construction. The lining bends with the band without cracking. The flexibilitv allows the lining to ad]ust itself to irregularities in both the dium and the band, thereby attaining gradual engage ment of the rubbing surfaces over large areas. Another reason is the high coefficient of friction. Asbestos fiber, which constitutes from 50 to 80 percent of the volume, is a high friction material. It is a hard material too, being harder than the drum in some instances. The harder compounds, such as iron oxide, emery, cement and others, which are sometimes found in linings, can also be incorporated into woven linings. A heavy duty woven lining is used on the band brake of the paper winder, Fig. 2. Fig. 3 shows a water-cooled disk brake having a woven lining. The lining is secured to both sides of the rotating steel disk, which is compressed between the water-cooled jack ets during braking action. Woven linings have been relegated to a secondary place in the automobile industry. They are used still on propeller shaft brakes and on some light types of war vehicles that are designed for operation in mud, sand and dirt. _ The fabric linings are used on many in dustrial applications in types of sendee simi Tar to those for woven linings. Fig. 4 shows an external band brake on a press. The purpose of the brake is twofold: It arrests -the upward movement of the ram at the top of the stroke; it keeps the ram from descending except when tripped for a work The torque lugs in the frame not only psi- . tion the brake blocks but also prevent them from rotating. The disk-type railroad-cat brake uses a molded block. Fig. 6 is a recent develop ment in a small motor brake. The drum is a molded friction material. The brake band, when the motor switch is shut off, grips the drum from either direction of ro tation, the motor being reversible. Coefficients of Friction Manufacturers and suppliers of brake lin ings describe linings as having high, medium and low' coefficients of friction, frequently without designating a numerical range of the three classifications. Linings having friction values of 0.40, 0.30 and 0.z0 are generally recognized as belonging respec tively to the three ranges. Although opin ions differ over boundaries of the three ranges, the values 0.35 and 0.25 represent convenient limits between them. Thus O.dO plus or minus 0.05 would establish a range within which a medium friction material could reasonably be expected to fall. Although manufacturers of brake lin ings are reluctant to limit themselves to a range of this magnitude, they have devel oped friction materials to the point where a designer can specify such a range of fric tion coefficients, supplemented by the con ditions of the brake tests and service, and reasonably expect the manufacturers to furnish a product to meet the requirements. The importance of designing the hand or the drum, whichever is the other member of the rubbing pair, to suit the brake lin ing should not be overlooked. Temperature of Operation Changes in temperature affect the be havior of a brake lining perhaps more than any other single condition, despite the fact that measurements of temperature are seldom made. Friction brakes convert energy of motion, usually rotation, into heat energy'. The band or drum absorbs and dissipates the heat. The friction ma terial absorbs practically none of the heat, but its surface temperature approximates that of the contacting band. Incandescence can he observed along the contacting edges under severe braking action. Any lining subjected to braking action beyond its capacity will either disintegrate and fade or build-up and wear rapidly. Linings can disintegrate through softening and pulverizing of the bonding con stituents. Vulcanized rubber softens at 300 deg. F. and thermal setting resins be gins to carbonize at 600 deg. F. Asbestos fiber begins to lose w'ater of crystallization at 700 deg. F. At 1,200 deg. F. all the water of crystallization is driven off, the fibers are close to glowing, and when cooled, ing stroke. Fabric linings are used also on the ex panding shoes of internal brakes. They are used to some extent on trucks and on war vehicles where operation in mud and grit are encountered. Fabric linings can be de signed to have a wide range of friction co efficients and as either semi-flexible or rigid linings. They contain lower asbestos con tent than woven linings and consequently -arc easier on brake drums. The dry mixed, wire back and extruded types, often designated collectively as molded linings, find wide application in automobile passenger cars. The wire back type is used on many trucks; the block type is common on the heavier trucks and on buses of medium and large sizes. Molded linings can be designed with fric tion coefficients in the high, medium or low ranges, but linings in the medium friction range are the most frequent. Molded lin ings are dense, hard and rigid. The doughmixed vvireback h pc are slightly the more porous and have a little flexibility. All molded tvpes should be ground to fit both the drums and the shoes. Airplane wheel brakes of the expander tube t\pe carry thin blocks of molded lin ing; the disk type of wheel uses segments. Fig. 3 shows the friction blocks assembled on a large expander-tube type of wheel. 41 P roduct E ngineering -- January, 1946 operation; hence brake designs represent compiomises. As a consequence the selec tion of the lining is based on that condition of greatest moment in the predicted or known service of the lining. For example, some brakes must function in grease or oil. It would be foil/ not to place this condi tion foremost in designing the brake. The coefficient of friction, the maximum temperature attained during braking service greater reserve capacity Decause me rorecasts of maximum brake loads are less re liable and also because they are more likely to be used while already hot from previous braking action. Some linings will stand higher tempera tures than others. Table 1 contains drum temperatures that apply to average speci mens of the various tjpes of linings. The data are suitable for design calculations. and empirical data often guide the designer through the drawing board stages of the They should be confirmed by data from lin ing manufacturers m the final selection of design. Important brakes are proved under actual service conditions before the design is considered closed. _ Whenever the coefficient of friction is the lining. Many brake designs are based on empiri cal data. In the automobile industry for example, rules like 20 lb. of car weigh per used in computations, the values at the sq. in. of lining are criticized because too limits of the range are preferred to the mean value. The coefficient of friction is bound to fluctuate under service conditions few of the factors, such as speed, weight, deceleration, operating temperatures and others, are reflected in them. The rule of and the effect of fluctuations should be lip. per sq. in. of brake lining contains studied and allowed for in the brake design. Brakes to absorb moderate or large, amounts more of the factors, but operating tempera ture is not among them. Some simple of energy seldom are designed on a basis of method of determining and evaluating the o i-- 1-- 1--1-- 1-1-- --- 1-- 1 0 100 200 300 400 500 600 700 800 Temperature, deg. F the coefficient of friction alone. Small brakes and braktN that prevent movement, but do not arrest motion, can be. Such brakes may be larger than necessary. rates of heat generation and dissipation would be a great help in brake design. Deceleration-Pedal-Pressure Tests Fig. 7 -- Curves showing the relative wear of lining materials at different temperatures. In calculations to determine temperature rises, all the energv of the arrested motion is treated as being converted into heat. Heat is dissipated while stoppage of the Automobile manufacturers buv brake lin ings on a performance basis. Fig. 8 is a tvpical performance specification. The motion is in progress. Ten percent repre the asbestos has been reduced to a fine sents an average estimate of the amount of white powder. A lining is said to fade when its co efficient of friction diminishes under brak heat dissipated during generation so that 90 percent of the energy accumulates momentarilv in the drum or band. The Table I-- Drum Temperatures Suitable for Different Types of Linings ing action. Sometimes the fade during a single application of the brake is meant; at other times the word denotes the falling oft of brake effectiveness upon successive applications of the brake. corresponding rise in drum temperature can be computed. As an example suppose 6,000 ft.-lb. of energy is absorbed per sq. in. of swept area in an airplane wheel brake. This is the T y p e o f lin in '; D rum tem p eratu re after norm al brake sendee, deg. F. M axim um . d ru m tem p eratu re, deg. F. oO ;1 If the coefficient of friction increases equivalent of 6,000/778 or 7.7 B.t.u. per when the brake is applied, the lining is sq. in. The disk is 5/16 in. thick and both W o v e n ....................... 300 W oven, heavv said to build-up. Some linings soften- and sides are rubbing surfaces. Assuming ten d u t v ....................... 450 750 become sticky at moderate temperatures to per cent of the heat is dissipated immedi F a b r i c ........................ 350 COO exhibit an increase in friction coefficient. ately, 13.8 B .tu would be stored in a 5/16 D r y m i x e d ............. 450 Abrasive action of either rubbing surface cu. m of the metal at the end of braking D r v m ix e d b lo c k s, heavy d u ty .. . 700 on the other hand also can cause build-up. action. A temperature rise of about 1,300 W ire b a c k , d o u g h Generally abrasive action is undesirable be deg. F. would be attained in a steel disk. m i x e d ............... 300 750 1 ,100 500 cause the rapid wear associated with it The amount of heat generated in stop E x t r u d e d .................. 400 700 shortens the life of the lining. ' ping a 3.000 lb. automobile from 60 m.p.h. S h e e t e r ...................... 400 700 The curves of Fig. 7 indicate the rela at a deceleration of 20 ft. per sec. per sec. tive wear of linings at different tempera tures. They were drawn from data obtained from laboratory tests in which the wear was measured at constant temperature. Since is about 475 B.t.u. This is enough heat to ' raise the temperature of an ordinary 5/32 in. drum about 300 deg. F. A 3/16 in. drum would undergo a rise in temperature Table I I -- Time in Seconds to Stop Inertia Dynamometer Wheel During Fade and Recovery Test brakes seldom operate at constant tempera tures, the curves have little design signifi cance; nevertheless they serve as a warning of 240 deg. F. upon absorbing an equal quantitv of heat. " The two examples illustrate in a quanti S top N o. Fade, sec. R ecovery, sec. concerning what happens to brake linings tative wav the manner of using drum tem if brakes are operated at high temperatures peratures in computations. A comparison 1 S .2 13 6 2 9 4 12 6 for prolonged periods. of some of the conditions of operation in 3 1 1 .0 14 .0 the two kinds of service shows why the 4 1 3 .0 16 0 Selecting a Brake Lining higher temperatures are tolerated in the 5 brakes of airplane wheels. The brakes in 6 7 The specification or selection of a lining automobiles are used oftener than those 8 in the design of a brake is not a simple mat on aircraft; automobile brakes would be 9 ter. Long life and constant friction are in unsatisfactory to the customer if the linings 10 compatible with elevated temperatures of wore as fast. Automobile brakes need 14 0 13 2 14 0 14 0 14 0 14 6 19 0 2 1 .2 19 0 18 0 20 0 20 .2 1 42 ' P ro d u ct E n g in e e r in g -- January, 1946 Fig. 8-- Typical deceleration-pedal pressure performance of automobile brakes. lines establish limits on the effectiveness of the entire braking system. The mean slope of the lines is determined by the co efficient of friction of the lining, and fluc tuations in the coefficient of friction are a major cause for departures of test results from the mean slope. The two curves within the limit lines, Fig. 8, represent tests of specimens of the same lining in two passenger vehicles. The tests were made from an initial speed of 50 m p.h. It is customary to make such tests from two and sometimes from three speeds. The pedal pressure required to overcome the retracting springs in both cars w'as 20 lb. as indicated by the horizontal portions of the test curves, Fig. 8. The resultant deceleration from bearing friction, wind resistance and other items was slightly less than one ft. per sec. per sec. for the test in car No. 397 and a little more than one ft. per sec. per sec. for the other car. Frequent attempts haye been made to devise laboratory tests that w'ould prose a brake lining for automobile service. Dy namometer tests provide a good index on lining performance; nevertheless the indus try generally admits that road testing in test cars is still necessary. Lining fade, its property to recover after fade, deceleration curves, lining build-up and wear tests are among the qualities that can be determined by inertia dynamometer tests. Table II contains data from a fade and recovery test. The fade test w'as con ducted in the following manner. The inertia wheel, weighted proportionally to a fully loaded automobile, was brought up to a speed equivalent to 50 m.p.h. and then stopped by the brake. The time in sec. to $!op the loaded rotating wheel was 8.2 sec. Twenty-five sec. later the wheel was stopped again from the same speed in 9.4 sec. and so on for ten stops. The lining was cold at the beginning of the test. The pressure on the brake was kept constant through out all stops so that the increase in the time of stopping the rotating wheel is indicative of lining fade. The recoserv test started 72 sec. after the tenth stop and from the same speed, i.e. the equivalent of 50 m.p h. The time icquired to stop the wheel was 13.6 sec., which is less than 14.6 of the tenth stop and indicates a recovery of nearly 16 per cent of the frictional loss. Further recovery takes place during the second 72-sec. in terval. As noted from the times in sec., Table II, the lining exhibits a wavering fade and recovery during the other eight stops. Some investigators measure wear as diminished thickness of the lining before and after a specified number of stops; others by the loss of weight. Still others prefer to measure wear in units that are radicalise of the amount of energy absorbed, such as oz. per million ft.-lc. or cu. in. per hp.-hr. Ease of measurement is one reason W'hy the different methods arc in use. The change in weight of a lining mounted in some brakes is readily obtained whereas dismount ing of the lining might be required to measure the change in thickness. On the contrary the change in thickness is some times easier to get than the change in weight. Changes in weight can easily be converted into average changes in thickness if the density of the lining is known. The effect of temperature on lining wear has already been mentioned. The metal against which the lining rubs is another factor that influences wear. Steel and cast iron riormally cause about the same amount of wear of the friction element, although steel is more likely to score under severe service and as a consequence cause the lin ing to wrear faster. On the other hand a comparison between the rates of lining wear against steel and aluminum show a marked difference. Bronze and other alloys also exhibit differences in rates of wear of the friction material. Normally a friction material that wears little is desired. Yet, brake effectiveness may be impaired through a glazing of the friction surface if the lining does not wear at all. Fig. 9 is a wear curve obtained from dynamometer tests. The wear _s nearly constant. The more rapid wear at the be ginning of the te 't is characteiistic of some linings. E ditor's N oth: The assistance and the contributions of matter for this article from more than thirty lining manufacturers, ma chine builders and makers of automobile and aiicraft-w heel brakes are gratefully acknowledged. P roduct E ngineering -- January, 1946 43