Asbestos in friction materials such as brake linings, clutches & transmissions: history, manufacture, visual identification: how to recognize asbestos-based or asbestos containing electrical insulation materials in products - a visual guide to identifying asbestos.
This articles series about the manufacture & use of asbestos-containing products includes detailed information on the production methods, asbestos content, and the identity and use of asbestos-containing materials.
Page top photo shows asbestos friction material in a Schwitzer clutch plate. In this application the asbestos-based clutch plate protected the opposing alulminum clutch plates from scoring damage, operated at high temperatures, and maintained torque while immersed in silicone.
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The following text is Adapted from Rosato (1959) p. 113-129  © 2013 InspectApedia.com
Asbestos is an important and major constituent of many types of friction materials. Modern industry makes use of friction materials in a wide variety of applications—automobiles, trucks, hoists, washing machines, and other moving or rotating equipment. See Figures 7.1 and 7.2 below.
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Photo at left illustrates use of asbestos within a butterfly valve observed in Australia in 2013 - discussed further under Bearing Compounds
On the basis of volume, the automobile and heavy building equipment industries are the largest users of friction materials. Brake linings, automatic transmission braking, and clutch facings are of primary interest.
Braking means converting kinetic energy into heat. It is accomplished by bringing such friction material as a brake lining or clutch facing into contact with a moving member, a brake drum. The performance of the brake is proportional to the relative coefficient of friction between friction material employed and the moving part.
Different types of brakes differ with respect to their ability to dissipate heat. Such special operating conditions must be observed as ease and speed of installing the friction material or special resistance to shear at the rivet or adhesive bonded locations. In order to satisfy all requirements, a variety of different types of friction materials are in use.
Friction materials with asbestos as a basic material have properties which cannot be obtained with other materials. They are not apt to be hot. By virtue of the natural slipperiness of the fiber, suitable braking action occurs. Such parts as brake drums, when exposed to frictional action, are not subjected to excessive rates of wear.
Chrysotile asbestos is used universally. It is much easier to process than the crocidolite asbestos and it is more abundant. The chrysotile fibers have better heat resistance than the crocidolite in these applications. Inasmuch as they are also softer, it has been reported that they have les tendency to cause scoring of metal. The molded and extruded products are the major types produced.
Figure 7.1. Schwitzer clutch plate. Asbestos-base friction material selected by Schwitzer Corp. because of its ability to avoid scoring opposite aluminum plates, to resist damage from 250°F operating temperatures, and to maintain sufficient torque capacity to engage while immersed in silicone. The thermo-modulated fan-drive unit matches fan speed to engine cooling demands in trucks and buses. - Courtesy Armstrong Cork Company.
Asbestos-binder products have a normal dry coefficient of friction on steel or cast iron drums of approximately .0.35. That coefficient may be as high as 0.65. Cotton and brass wire in the yarn may change these values slightly.
It is interesting to note that cotton, wood, or leather will char against hot iron or steel at approximately 300°F.
When mixed with asbestos fiber in a relatively small amount, cotton fibers do not burn as readily.
The frictional heat normally reported in braking may be between 400 and 600°F for approximately one-half the lining thickness. This temperature will reduce the cotton to carbon particles and tend to produce a highly polished or glazed surface.
Figure 7.2. Automatic transmission clutch plates consisting of wafers of asbestos-metallic material, cork, and treated paper material, paper material or steel material bonded with thermosetting adhesives to steel plate. - Courtesy Raybestos Manhattan, Inc.
Extremely high temperatures are theoretically obtained on the surface of friction material, or on the surface of brake drums during the braking action of standard automobiles and trucks.
Actual temperature readings are generally difficult to record. Literature records temperatures from 2,000 to 2,500°F for short periods of time. The actual temperature of the friction material varies with the distance from the friction surface.
During severe braking action, the surface temperatures at the interface between lining and drum are invariably higher than any recorded drum temperature. These flash temperatures are of extremely short durations, which in turn is the basic reason that organically bonded brake linings perform satisfactorily in these high-temperature cycles. The actual temperature of the adhesive bonds normally does not exceed 300 to 400°F because of the low thermal conductivity of the friction-plastic materials.
Some of the friction materials may not soften but they do disintegrate. For these types of products, the generated heat goes into the brake drum. The friction materials are good heat insulators.
In classifying friction materials, the general procedure is to consider the basic structure or construction of each type. Predominate forms of asbestos applicable to frictional materials follow. See Figure 7.3.
Figure 7.3. Asbestos woven and molded brake linings, clutch facings, cone facings and segments. Courtesy Johns Manville, Inc.,
Molded or fiber linings With or without metallic wire (staple, continuous, etc.) Millboard Papers Woven or fabric linings With or without metallic wire Single or plied sheets Others, such as braiding
Fabrication of Asbestos-Based Friction Materials
The general basic materials used to impregnate or mix molded and woven linings are asphalts, drying oils, synthetic resins, and rubber. The specific materials include linseed oil, phenolic resins, furan resins, gilsonite, artificial asphalts, pitches, Chinawood oil, mineral oil, natural and synthetic rubber, silicates, casein, and coal tar.
Combinations of these materials are also made—asphalts and drying oils, asphalts and phenolic resins, asphalts and sulfur, synthetic resins and gums, drying oils and silicates, and asphalts with gums. However, these materials can be used with or without solvents. During solvent impregnation of woven asbestos products, it is important to study the depth of penetration which actually occurs. A certain amount of filtering action of the fibers can occur. In addition, capillary attraction during the drying cycles to remove solvent can cause the binders to concentrate on the surface of the woven material. The openness of the yarn and weave can control depth of saturation.
Other controls—the solids content of binder bath, speed of treatment, and type of saturating ingredients—exist too. With high binder content on the surface of lining, the brakes can "grab" during high temperature operation. Sulfur has been used to eliminate this problem substantially.
Gilsonite as well as other asphalts can be applied as dry powders, in water dispersion and with solvent. Asphalts can be emulsified in water with the aid of colloidal clay, casein, starch, glue and soaps. Coal tar, wood tar, water gas tar, and oil gas tar are the types of tars used. Water gas tar is most commonly used inasmuch as it acts as a vehicle for other materials as well as colors the lining.
Rubberized linings are used extensively. Rubber compounds can be varied in regard to thermal stability. Certain materials can be added to the compounds to provide for special chemical or physical requirements. Available literature and patents disclose that a general brake lining rubber compound contains the following materials with percentage limits by weight: 10 to 70 per cent rubber, 5 to 32 per cent sulfur, 1 to 5 per cent zinc oxide, 2 to 20 per cent litharge, 1 to 5 per cent lime, 15 to 70 per cent clay or whiting, 1 to 15 per cent carbon black, 5 to 15 per cent graphite, 1 to 5 per cent softeners, 1/2 to 1 1/2 per cent accelerators, and approximately 1 per cent age resistors. Curing of rubber stocks is generally accomplished at 250 to 325°F.
Latex is used in the treatment of asbestos woven and fiber molded friction materials. The fact that latex contains rubber particles in a finely divided condition suggests that it would be an ideal material for applying rubber to asbestos in the manufacture of rubber-bonded friction parts. Processes have been installed to handle various types of compounds. A mixture or compound applicable to the manufacture of linings follows: 40 per cent latex solids, 10 per cent sulfur, 7 per cent carbon black, 8 per cent zinc oxide, and 2 per cent graphite.
Impregnating Processes for Asbestos Brake Linings, Clutches, and Similar Friction Materials
In order to provide suitable or complete saturation of asbestos woven lining materials, the untreated asbestos is sometimes preheated prior to compounding or treating. Preheating temperatures are in the range of 250 to 300°F in order to remove moisture which can collect during storage.
For tapes and fabrics of woven linings, heating can be accomplished either in ovens where material remains in fixed positions, or in continuous heating ovens. Preheating can also be accomplished in the first step of a binder dip tank operation. For example, material can be sent through the heating chamber, then dipped in the binder, redried to remove solvent, and redipped.
Some manufacturers immerse rolls of raw tape in open tanks and allow them to soak in the impregnating solutions. The rolls are withdrawn and placed in an oven to dry. During this procedure, there is a tendency for the solvent in drying to draw the binder material to the edges of the tape on the side of the roll. To provide better distribution of binder, vacuum and pressure processes are also used.
Processes include putting the untreated tape in loose rolls and placing them in a large autoclave. A vacuum is created and the temperature is raised above 200°F in the vat. This operation removes air and moisture which may be trapped in the tape. Then, the impregnating fluid is made to flow into the vat. Pressure is applied which in turn forces the solution into all parts of the tape. After saturation, the rolls are removed and dried thoroughly in drying ovens.
Another popular method of impregnating woven material is to feed material under a tank which contains a row of orifices in the bottom which are regulated to allow a prescribed amount of liquid binder to drip on the fabric. In this drip process, the fabric moves under the row of orifices.
An impregnating material in powder form, such as powdered synthetic resin or pulverized gilsonite, provides another process. The powder is sprinkled on the layer of asbestos and cotton fiber as it comes from the carding machine and before it is made into rovings. As this layer is rubbed into the roving during the calendering process, the powder is interspersed with the fiber and remains there until treated by heat.
Methods of treating fabric for producing friction materials have included treating the yarns with the impregnating solution, drying the resin treated yarns and then weaving the fabric or tape.
Asbestos-Based Molded Lining Friction Material Production
Molded friction materials are identified as all types of products which are cured in molds under pressure. These materials include both dry and wet mixed molding corn- pounds as well as impregnated or nonimpregnated fabrics and millboard. Molding compounds are the most popular.
Molded dry or wet mixed types of brake linings permit the use of extremely short fibers as low as group 7F. Basically, the processes consist of special mixes of asbestos fibers with organic resins, inorganic fillers, and in the majority of applications, metallic reinforcements. The linings consist of unoriented asbestos fibers without pattern or direction. A random arrangement of the fibers in the lining is produced. With other methods the material is combed in such a manner that fibers assume a parallel or sheet arrangement. Molded linings may have a smooth surface coating which should be removed. In addition to the skin of the bonding material, the surface may have a mold lubricant which is undesirable. Grinding or sand blasting is generally used to remove the surface.
Composition of the molded friction products is generally similar to that of the impregnated fabrics; they are referred to as rubber-asbestos mixes. The average composition of rubber compounds includes 40 to 70 per cent asbestos with 30 to 60 per cent rubber compounds. The fibers used can include asbestos, cotton, mineral wool, flax, jute, and rayon. Asbestos fiber is the most useful of these, as it is heatresistant and tends to knit the compound together. Carbon black, zinc oxide, magnesium carbonate and clay are commonly used to strengthen or reinforce brake lining rubber compounds. Fillers used to provide bulk include gypsum, whiting, iron oxide, barium sulfate, fuller's earth, pumice and cement.
Dry Mixing Asbestos Based Friction Materials
The materials which are used to make the finished lining can be made in dry mixes. Standard procedures of weighing out a dry mix stock, introducing it into a suitable hot mold, and curing it under pressure loads ranging from 1,000 to 4,000 psi are used to mass produce friction materials. The mixed compounds are transferred to deep cavities in compression molds.
The high bulk factor of regular dry compounds makes it necessary to use heavy and deep molds. In order to reduce cost and to provide smaller sized molds, the practice of preforming the loose stock is widely used. In addition, the dry compounds are made dense by passing them through calenders, retumbling for mixing, etc.
Dry compounds are also made into linings by using equipment consisting of a hopper and a pair of rolls revolving in opposite directions. The two rolls are set at an opening in the lowest part of the hopper. As they revolve, the dry - material passes between them and produces a continuous strip of compressed material. A wire screen backing may be carried around one of the rolls and incorporated with the dry compressed friction material. The compressed sheet material is later cut into desired sizes and cured in molds under heat and pressure.
Dry mix linings are considered the most heat stable friction materials in general use. They are particularly suitable for brake blocks which are used in heavy duty service.
Wet Mixing Asbestos Based Friction Materials
Pulp beaters, roller mills, and extruder mills can be used to produce finished products in wet mixes. For extremely wet mixtures with a nonvolatile liquid, a paper-making pulp beater is used. The asbestos fiber and compounding ingredients are added and mixed in the beater. The pulp is fed into a paper making type of machine and formed into a thin sheet which is wound on a large cylinder and laminated or built up to required thicknesses. This type of product is also identified as asbestos millboard. The cylinder may be steam heated so that the sheet is dried before removal. However, the wet sheet can be removed from the cylinder, pressed in order to remove any excess liquid or solvent, and hung up to dry.
Instead of removing a layer of paper from only one roll, a number of rolls can be placed in series. The layers from each roll can be stacked together during manufacture so that a thick plied mat is formed.
The pulp can also be drawn to the surface of a suction roll which can revolve in the vats. An endless moving blanket removes the pulp covering the suction roll and carries it into a continuous sheet. The large cylinder or winding roll takes the pulp sheet from the blanket and accumulates a thicker laminated sheet. The finished sheet or mat is similar to millboard construction except that it is fully loaded with rubber or plastic friction compound.
Many different processes are used to fabricate friction products with wet mixes. Special preform shapes in exceptionally thick sections can be made by using a preform or perforated metal screen located next to the pulp beaters. Suction applied to the outer side of the perforated screen will permit a deposit of compound within the form. This type of filtering process is similar to conventional paper pulp and plastic preforming equipment which uses screens to make preform shapes of flat or complex shapes.
Millboard Lining Asbestos Based Brake Linings
One of the oldest methods of making brake lining involves the use of millboard. This type of lining is built up from a sheet of pulp; it is impregnated either before or after plying. Then it is cured by heat and pressure.
Millboard can be cut to desired size and treated with rubber or resin friction compounds. After press or oven curing the impregnated sheets, the product can be machined or ground to the required dimensions. The basic problem in this type of construction is to produce a compound which will readily penetrate the millboard in order to provide the maximum amount of solids impregnation. Penetrating qualities of phenol and cresol resins are very useful in miliboardfriction products.
A thin layer of other materials can be bonded to the back of molded millboard. Hard rubber, fabric or wire screen is applied as a reinforcing backing. -
Rubber Mill Lining Asbestos Based Friction Materials
Varied types of rubber roll mills such as Banbury and two roll dough mixers are used to prepare rubber or plastic friction compounds. The general mixing procedure is similar to conventional rubber compounding. The crude rubber is fed into the V-opening between the rolls where the tearing action of the faster roll gradually breaks it down and softens the material. As it softens, the compound clings to the slower roll and forms a sheet on it. Asbestos fiber, sulfur and other compounds are gradually fed into th rubber and mixed with it. A smaller amount of fiber and compounding materials can only be added to the rubber in this type of process as compared with the pulp method. The mixed compound can be formed into a sheet on a rubber mill or it can be made to flow through a calender and more accurately rolled into sheet form. Later, it is cut to size and cured.
Rubber itself breaks down and softens readily on a rubber mill. As the fibrous and powdered materials are added, the batch becomes stiff and unworkable. In order to mix the compounds satisfactorily, materials known as softeners are added to soften the batch. Softeners include mineral hydrocarbons, vegetable oils, tars, pitches, waxes, resins, gums, and soaps.
Generally, in order to obtain higher frictional values in rubber compounds, these materials are added: iron oxide, emery, silica, ground glass, pumice and iron filings. To produce lower frictional values these materials are added: grapilite, mica, talc, waxes, and soaps.
Sheeter Lining for Asbestos Based Friction Materials
Another method of preparing sheet material is in a "sheeter" machine which is a modified rubber mill. It is made up of one large heated roll from 10 to 15 ft long and 3 to 5 ft in diameter. A smaller cold roll in front of it revolves in the opposite direction. The soft solvent-containing compound or stock is introduced between the two rotating rolls. A thin film of stock, ranging in thickness from one to two mils, is formed and adheres to the hot roll. Subsequent rotations result in additional layers or laminations being deposited on the hot roll. As the thickness of the brake lining stock is built up, the distance between the adjacent rolls increases automatically. In this way, a sheet for use as brake lining is gradually built up to the desired thickness.
During the operation, the solvents are removed. During the sheeting operation, metallic wire and fabric can be introduced and wound around a large roll with each successive accumulation of material. When the sheet has reached the desired thickness, it is slit and removed from the roll. Lining which has been built up on the sheeter machine can be fully cured before removing from the large roll. Segments can be cut from the cylinder and finished as desired.
Uncured or partially cured molded linings which have been manufactured by this as well as other methods, can be bent around a steam heated mandrel of specified curvature and held in place with a clamping jacket. Molded linings of the sheeter type are relatively expensive to manufacture because of the method of fabrication.
Asbestos-Based Extruded Lining Friction Material Production Methods
Linings are also made by the extrusion method. This method is very popular. It involves the use of wet rubber or plastic stock. After the stock has been compounded in a rubber mill, it is fed into the hopper of a standard extruding or tubing machine and forced by revolving spiral blade through an orifice of the desired shape and size. The formed but uncured extruded brake lining is dried to remove any solvent present. Then it is cut into predetermined lengths and cured in heated molds.
Woven Asbestos Brake Lining Production Methods
Woven brake lining generally includes wire reinforced long fiber asbestos yarns woven to produce a basic high mechanical strength product. The resinated bonding materials used with the fabric incorporate many different types of resin systems with such different types of fillers as graphite and aluminum oxide.
The woven asbestos fabrics are generally dipped in resin or varnish solutions by various dip-pan procedures. The solvents used with the resin systems are evaporated by means of heating chambers either in the horizontal or vertical direction. After preparing resin impregnated fabrics, the materials can either be calendered or hot pressed in molds to form the brake linings.
For heavy-duty lining applications, asbestos woven fabrics are manufactured in large looms. Special attention has to be given to the resinated binder solutions in order to properly and accurately penetrate woven fabrics which may be approximately '/ 2 -in. thick. These particular types of linings are tailored for such heavy duty units as logging trucks.
The use of all asbestos yarn, tape or fabric is rare, inasmuch as the cost of the finished product would be extremely high. Such organic fibers as cotton and such synthetic fibers as rayon are blended with the asbestos fibers. ASTM Grade A yarn is the one generally used in brake lining. If the ratio of asbestos to cotton by weight is 80 per cent to 20 per cent, the ratio by volume is approximately two-thirds asbestos to one-third cotton.
Cotton is absorbent and must be treated against the action of water, oil and grease. During the impregnation process to which the woven lining is subjected, the cotton fibers furnish a channel for moving the saturant into the center of the lining. Water proofing agents include asphalt or phenolic resins.
Wire-Woven Asbestos Friction Lining Materials
The wire reinforcement used in woven linings adds to the strength of the lining; it also contributes to the friction and wear properties of the product. There are some woven linings available with wire reinforcement as a backing for the lining; it is not subjected to direct friction contact. See Figure 7.4.
The wire can be used as a core for such asbestos yarn as ASTM Grade A or stitched through woven asbestos fabrics [ASBESTOS TEXTILES].
Linings in this category are among the most popular on the market. They possess good frictional properties and have a generally acceptable rate of wear.
Wire reinforced linings aid in conducting heat away from the friction material.
Woven linings generally contain, on a weight basis, from 40 to 60 per cent asbestos, 10 to 20 per cent cotton, 20 to 40 per cent wire, and 5 to 20 per cent binder material.
Figure 7.4 Profile, calendering equipment for making wire-clad brake linings. Courtesy Johns Manville Corp.
Calendered Asbestos-Based Woven Lining Friction Materials
The friction material generally used is some form of rubber compound and the processes of mixing are similar to those used in the rubber industry. The mixed compound meets the impregnating cloth at a calender. This operation is usually performed on a heavy machine with three large rolls. The rolls can be heated or cooled. The top and bottom rolls are slightly crowned to counteract the tendency to bend at the middle under the tremendous pressure which is exerted.
The friction compound is fed into the opening between the two top rolls. The cloth is fed in from the opposite side between the two bottom rolls and picks up the compound which has been carried around the back side of the middle roll. If the middle and the bottom roll run at the same speed, the operation is called "skimming." The compound is pressed into the spaces in the fabric and the final thickness is determined by the opening between the rolls.
After this operation, the fabric may be coated on both sides, although one side usually has a heavier covering. Skimming is a much faster operation than the more popular friction method. In the friction method, the rolls run at different speeds and the action is similar to scraping the compound into the fabric. This type of operation is generally used. The calender rolls operate so that the first and third rolls turn in the same direction and the middle roll turns in the opposite direction.
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