Document 6wwdbNrp8wvJb0VoKV0806gdE

FILE NAME: AMMCO (AMC) DATE: 1941 Jan DOC#: AMC022 DOCUMENT DESCRIPTION: Trade Journal Article - Friction Materials  XS'Zfi'' - ? -7 j#$*,.* - !iy y(r * ' ^ .> Wy 4 <**^*Wy 1m> ` **'' * / ^ l r .;t. ,Sfi*/ -f^* ' jl ..^?*.S-J - Weaving thick asbestos lining on a giant loom, after uhich the lining is impregnated and molded to size by heat and pressure FRICTION MATERIALS-I Linings and Blocks for Brakes and Clutches Fu n d a m e n t a l l y any type of material during deceleration or accel temperatures, loads, drum mateiial band, shoe, disk brake, or clutch eration. Factors which must be con and finishes, deceleration rates, and fie- is designed with the objective of sidered in determining the operating quencies of application encounteted in retarding rotation in order to decrease characteristics and capacity of a brake the various services. In the industrial or to stop the relax:'-- motion between or clutch are the effects on friction field some services require flexible lin brake or clutch m eau-rs. by convert values caused by temperature rise, ing, others require dense rigid blocks. ing the kinetic energy of the moving maximum operating pressure, moisture, The quantity of h e a t g e n e r a t e d a t I system to heat energ-.. rubbing speed, wear, condition of the contacting surfaces of the brake A major consider..:, n in the solu brake drum surface, and length of time lining and the drum foi any given tion of braking or ..'..oiling problems between operations. braking operation has a definite value is concerned with t:i- variation of the No single friction material can meet dependent upon the difference in the coefficient of friction of the friction the widely different conditions of brake kinetic energy in the moving system i 2 P roduct E ngineering ! hef'jit' and after deceleration, regard less of the time taken to stop or decel erate the moving load. The brake factor' on which converting the kinetic energy into heat energy depend are the eoetheient of friction between the lining or block and the drum, the total nor mal contact pressure, and the rubbing speed. Friction linings are good heat insa latoli, metal drums are good conduct or' and therefore teadily absorb the heat produced by the frictional force' with an accompanying rise in tem perature. Naturally the maximum tem perature developed in the drum will depend upon the frequency of opera tion. the mass and type of material of which it is made, the exposed area of the drum and its ability to radiate heat. The high resistance to heat posses'ed bv friction materials made with an as bestos base, as compared with 'infilar juoperties of wood, paper, fiber and leather, are the chief reasons for the preference shown asbestos friction ma terials in the automotive, industrial ami construction machinery field'. The tvpical values of operating factors shown in Table I are for asbestos com position linings and blocks. Effect of Temperature Coefficients of friction for asbestos imposition materials at temperatures ninne 500 deg. F. become erratic, r i ing to high values or dropping to low \alues with consequent lack of control oi the rate of deceleration. Therefore, fur satisfactory operation brake d iu m ' must lune the ability to dissipate heat rapidly enough to prevent the tem perature of tile drum from becoming excessive. The temperatures at which variation in the friction value occur; are tho'e at which the binder in the friction material undergoes a physical change of state, and varies greatly with different types of brake lining depend ing upon the heat resisting qualities of the binder or fillers after curing. The curves shown in Fig. 1 were estab lished by dynamometer tests with various linings and indicate how with speed and pressure constant an in crease of temperature may affect the coefficient of friction. \lthough -the binding materials are heat-treated in manufacture, the tem perature on the surface of the friction material in service is often higher than that employed in manufacturing, with the result that the impregnant or binder may soften or even approach liquification thus reducing the coefficient of friction to low values and then to in crease it as the binding materials are driven off developing a possibility of running on dry a'bestos with a friction value of approximately 0.8. If a car bonaceous condition develops, the fric tion value may be higher or lower than normal, depending upon the char acter of the surface formed. For satisfactory operation and long life of lining, brake drum tempera tures should never exceed 400 deg. F. for rubber bonded friction materials, nor 600 deg. F. for resinous materials. With the latter the higher temperature is permissible since the majority of the synthetic resins used in the manufac ture of friction materials are thermo setting and are rendered infusible by the curing temperature. High temperatures also break down the mechanical strength of asbestos fibers and destroy their structure. As bestos occurs in nature in many differ ent forms and chemical compositions. The form used in the manufacture of friction material' is a magnesium sili cate containing about 14 per cent water of crystalli/.ation. At tempera tures at or above 700 deg. I . some of the water of crystallization is driven off, accompanied by a decided loss in strength cf the fiber, in addition there may be a loss of organic impregnant through destructive distillation. At temperatures of about 1.000 deg. 1. the magnesium silicate is rapidly dehv- Table I -- Perform ance Characteristics of Asbestos Linings and Blocks These data cover only the best types of materials when used in dry service conditions. S tyt.b CoF.Frn hint OF III NMNC F riction R ibbing Speed of FT. PFH MIN. M\X. D iu m T em p. .Max. D eg . F.j S hout P eriod' Constant ' P ressure SO. IN. 1 j R esistance . TO 1 Shock R ate OF We aR Folded and compressed rubber ( em pound .................................... !loxdiW* N't o\ f*n . . - H*avy-dut\ woven . Molded l i n i n g s ............... . Blocks ....................... . it 3(1 -- 0.10 o:r> -- o r> 0.83 -- 0 50 0 30 -- 0.10 0 30 -- 0.10 3,000 500 250 5,000 500 250 7,500 750 500 5,000 i 750 500 5,000 . 1.000 750 100 Excellent hvvIU 'nt 50 Pair 1 Fair 100 Good 1 Good 100 Good ( loot! 150 Excellent ( u)od 3 January. 1941 drated. resulting in a chalk-like sub stance without mechanical stiength. Therefore, it can be readily understood why it is necessary to keep drum tem peratures below 700 deg. F. in order to assure satisfactory operation. Increasing the normal braking pres sure will, ol course, increase the decel erating force applied to the drum, howewr. this i- accompanied with slight tendencies either to build up or to de crease tilt! coefficient of friction in most materials. Hence tor any application the increase in decelerating force is generally in direct proportion to the increase in applied pressure. The curve in Fig. 2 was established from dynamometer tests on samples of lin ings at constant speed and temperature, and indicates how an increase in unit contact pressure may influence the co efficient of friction. Examples of the effect of increasing normal braking pressure on deceleration rate are shown in Fig. 3 for various coefficients of friction. Dry. unlubricated friction is independent of the area in contact for constant conditions of velocity, tem perature and surfaces. Increased area, however, is obviously highly beneficial in reducing wear. At low speeds of a few feet per hr. most friction materials have a coeffi cient of friction which is less than their values at high operating speeds. When the rate of sliding approaches zero just before stopping, it is thought that the coefficient of friction rises sharply pos sibly because of a fade at higher speed and an antifade on cooling of the con tact surfaces. A drop in frictional re sistance as the velocity approaches zero is desirable. If the low speed fric tional resistance is considerably below the normal friction value, however, the holding ability of the brake may be come uncertain. Since if for any rea son a slight movement or vibration of the drum were induced in a standing or suspended load, the frictional 'grip of the brake would immediately drop and the load might start and continue to creep indefinitely. Some tests have shown that with increases of rubbing speeds up to about 500 ft. per min. there is a gradual increase in the coefficient of friction, and that at rubbing speeds between 500 to 5,000 ft. per min. there is a small dropping off in the friction value as the velocity increases, however, this reduction is small. The rate of wear o: asbestos friction materials varies considerably in differ ent types. In order :) maintain a friction level within dependable limits some wear is desirable remove the smooth glazed surface .v-iich forms on the lining because oi accumulations which take place. Oil or grease may Fig. 2-- Increase in unit contact pressure is accompanied with slight tendency either to build up or to decrease the coefficient of friction in most materials Fig. 4--Asbestos linings. Left, folded tyi>e. Right, flexible woven type 4 P roduct E ngineering cause the lining to glaze. As glazing develops the coefficient of friction fades. Rough drum surfaces naturally increase wear and develop an apparent high friction value. The effect of water on asbestos fric tion materials is to form a lubricating film, lienee with some linings dangeious slipping occurs when the material is wet. This film during braking oper ations. however, is quickly squeezed out if not renewed. The rate of recoverv in friction value depends upon the amount of water absorbed by the as bestos fibers. Asbestos is a livgroscopic material, it will absorb from two to four per cent moisture from the atmosphere. During manufacture the asbestos fibers are sometimes water proofed as well as covered with the impregnating material. But as the friction lining or block wears in sendee, the fibers at the surface be come uncovered and absorb water in addition to that held in the inter stices of the lining structure and in the brake dust, which may when the lin ing lias not worn uniformly account for tile grabbing effects of asbestos friction materials when damp. The curves in Fig. 5, taken from Prony brake tests, indicate the effect of water on the frictional value of linings and their ability to recover from the lubri cating effect of the water film. The ideal lining, of course, would be unaf fected by water. Some linings, in re covering from the lubricating effect uf the water film, become very sensitive and remain so for a considerable time. Others recover their prior effectivenc-and remain at this level. Some linings have a tendency to swell with the absorption of water so that they drag on the drum surface jf the clearance is small. Oil may al-o cause linings, particularly rubberbonded linings, to swell and soften. Some synthetic resin-bonded linings are not physically affected by a thorough soaking in oil; their fric tional value, however, drops consider ably. Oil on a rubber lining, on the other hand, usually increases the fric tional value after the consequent softening and disintegrating of the rubber. For satisfactory braking operations lining materials should be fitted tightly to bands with full bearing on the b a th ing surface to pi event wrinkling and consequent crowding. All rivet- or bolts u-ed for fastening the lining should be body tight in the band in order to prevent any `"play"' that might cause a rivet failure by -hear if ~ubjeeted to shock or impact. There should be no cleat anee on the bolts in line with the force on thr blocks. as the blocks may slip ami January, 1941 shear the holts. All bolts and screws should have heavy steel Lock washers to prevent loosening under vibration. For flexible industrial linings larger than y2 in. thick brass bolts are gen erally employed beeau-e the shearing effect increases with the difference in radius of the outer and inner lining surface. Flexible lining-, le-s than % in. thick are usually secured to the bands by rivets. In Fig. 6 are shown various methods for fastening. Fastenings should never be per mitted to come in contact with the brake drum. The -heads should be countersunk below the friction ma terial surface as far as can conven iently be done without weakening the holding power of the fastening. In flexible materials, it is customary to counterbore at least hall the thickness; in rigid materials about two-thirds the thickness. Because of wear of the friction ma terial. it is sometimes impossible to prevent occasional contact of the fastenings with the drums. Every effort should be made, therefoie. to prevent the pos-ible cutting, scoring, or wear ing away of the drum by the proper selection of the fastening material. Rivets and bolts for fastening friction material to bands are usually made of brass, copper, or aluminum. Brass rivets are superior to copper because they abrade rather than drag or plate the drum suifaee and show a minimum tendency to score under nor mal operating conditions. As a genera] rule, solid flat head foo ow oA ion < 0.40 0.30 0.20 0.10 n -- ID E A L LiN ir* G h l o t set 7s itiv e to m o is t u r e POOR LI NI N G sns/th'e , fc m ois tu re ' Loss f r om 14^a t e r /u b ric a tffon OOOOO Coefficient of Friction ooo 3000 poo p E X C E : l l e t viT L 1N1NG f -- \ '~Scm e sensdtivit'yr to m o is tu re 1 -- ' Loss fr om w ater Itib ric a iLion "0 10 20 30 40 50 Number of Stops Fig. .5--Curves from Prony brake tests indicating the effect of water on the frictional value of linings and their ability to recover 5 rV : H e. 6 -Methods of fastening friction materials to brake bands. (A) Correct and imorrect methods for riieting Unin. bands. (B) Methods of fastening blocks to bands It) brake m e t' are flic most satisfactory type for Rigid friction materials in. thick trying to design brake lining- that i iiulu-trial purposes. Semi-tubuku rivets and -mailer are termed molded lin have definite frictiun value- and to in are better tor this work than the full tubular kind a.- they can be spread ings: and thicker pieces ate known as friction blocks. Because of their hi cell clude a graded range of the-e valm in their line. It ha- been found, how mme easily when upsetting to make a tighter grip. Split rivets should not be mechanii a! strength, ligid molded fric tion materials are often Used where ever. that some btakes require an overall frictional value, where there used on industrial equipment under any considerations. high stie-ses would break down the softer and more flexible types of ma are two or more shoes, and this value i- not satisfied by any one lininu. The minimum length of rivet should be the thickness of lining under the terials. Rigid molded materials being formed under pressure and the binder Some brake designers use diffennt types or styles of lining for different rivet head, plus the thickness of the set to a rigid and infusible condition biake band or shoe, plus the diameter usually are more resistant to heat and brakes in order to select the friction and of the rivet body. The maximum length more constant in frictional values over wear characteristics that are most satis factory. should be not more than one and a quarter times the minimum length. a relatively wide range of temperature To provide in-between fricti m l'or fabric linings it is suggested than are the flexible types. Rigid molded linings are also less compres values, two different lining- mav be used on a given brake. One tvpe of that rivet heads be made twice the diameter of the body or shank. Molded sible and so require less frequent ad lining may be used on an energiriug linings are dense and mechanically justment when high pressures are em ployed. shoe and another kind on an auxiliary strong and do not require rivets with as large heads to hold the lining in Molded linings in roll form are usually made with an integral backing shoe. In the two-shoe type of brake, the energizing shoe does more of the position. The number of rivets to use in a material to increase their mechanical strength, and are suitable for either braking than the non-energizing shoe and wears faster, fo compensate tor lining will depend upon the maximum pressure of engagement, the maximum internal expanding or external con tracting wrap band or shoe type this increased wear, one automobile manufacturer furnished a thicker piece coefficient of friction, ami the safe shear strength of the rivets. The area brakes. These linings have the fric tion facing on one side only and have of lining on the energizing shoe. An other type of two-shoe brake has a A of brake lining in sq. in. per rivet, may be expressed in terms of maxi to be curved to suit design with the hacking on the,proper surface, that is, longer piece of lining on the energizing shoe than on the non-energi/ing shoe. mum ptessure of engagement p in lb. per sq. in., maximum coefficient of internally for expanding and exter nally for contracting brakes. Coeffi Some lining segments are so tapered that they are thicker at the areas of friction /, and safe shear strength S cients of friction range from 0.20 to more rapid wear yet fit the drum. in lb. per rivet, as 0.50. The linings are highly resistant E ditors N o t e: For contributing data A -- S tpf to wear and are suitable for rubbing and helpful criticism during the prep Safe shear strenatu of brass rivets based on a unit -irei mill in sheai of 12.000 lb. per sq. in . can be listed as 5/16 in. did. tivrt. ...........920 lb. Vt in. dia. rive:. . ...........600 lb. 3/16 in. dia. rivet. Vs in. dia. r iv e t ... ...........150 lb. speeds up to 5.000 ft. per min., drum temperatures up to 500 deg. F. con stant service, and up to 100 lb. per sq. in. brake pressure. Flexibility de pends on incompletely curing the or ganic binder and this type is moie in clined to fade at high temperatures. in lecent vears manufacturers are aration of this article, the editors acknowledge their appreciation of the valuable cooperation of ,f. V . Ilempbill. Jr.. Johns-Manville Sales Coipoiation; T. R. Stenberg. Mat-hall-Felip-e Div.. Bendix Aviation Coi position: and Dr. F. C. Stanley. Ravbestns Div.. Raybestos-Manhattan, Incut porated. 6 P roduct E ngineering