Friction materials types

Biake linings and clutch facings consist of friction materials. Friction materials technology encompasses friction material types, their appHcations, friction and wear characteristics, raw materials, manufacturing methods, and evaluation and test methods. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

Prior to the mid-1970s, the most common type of friction materials in use in brakes and clutches for normal duty for original equipment installations and for the aftermarket were termed organics. These materials usually contained about 30—40 wt % of organic components and were asbestos-based (3). 

Carbon Composites. Cermet friction materials tend to be heavy, thus making the brake system less energy-efficient. Compared with cermets, carbon (or graphite) is a thermally stable material of low density and reasonably high specific heat. A combination of these properties makes carbon attractive as a brake material and several companies are manufacturing carbon fiber—reinforced carbon-matrix composites, which ate used primarily for aircraft brakes and race cats (16). Carbon composites usually consist of three types of carbon carbon in the fibrous form (see Carbon fibers), carbon resulting from the controlled pyrolysis of the resin (usually phenoHc-based), and carbon from chemical vapor deposition (CVD) filling the pores (16). 

In order to define the extent of emissions from automotive brakes and clutches, a study was carried out in which specially designed wear debris collectors were built for the dmm brake, the disk brake, and the clutch of a popular U.S. vehicle (1). The vehicle was driven through various test cycles to determine the extent and type of brake emissions generated under all driving conditions. Typical original equipment and aftermarket friction materials were evaluated. Brake relines were made to simulate consumer practices. The wear debris was analyzed by a combination of optical and electron microscopy to ascertain the asbestos content and its particle size distribution. It was found that more than 99.7% of the asbestos was converted to a nonfibrous form and... 

A. E. Anderson, "Brake Systems Performance—Effects of Eiber Types and Concentrations", Proceedings from Fibers in Friction Materials Symposium, Asbestos Institute, Atiantic City, N.J., Oct. 1987. 

All of the unique properties imparted by fluorocarbons can be traced back to a single origin the nature of the C—F bond. These properties include low surface tension, excellent thermal and chemical stability, low coefficient of friction, and low dielectric constant. However, not all of these properties are possessed by the entire inventory of available fluorocarbons. The fluorocarbons can be assigned to two major categories (1) fluoropolymers, which are materials that are comprised mainly of C—F bonds and include such examples as PTFE, and (2) fluorochemicals (FA) based on the perfluoroalkyl group, which are materials that generally have fewer C—F bonds and often exist as derivatives of other classes of molecules (e.g., acrylates, alcohols, esters). In addition, the properties that dictate the uses of fluorocarbons can be classified into (1) bulk properties (e.g., thermal and chemical stability, dielectric constant) and (2) surface properties (e.g., low surface tension, low coefficient of friction). The types of materials available and properties imparted are not exclusive and overlap substantially. From this array of fluorocarbons and attributes, a large variety of unique materials can be constructed. 

Abrasive modifiers are used in several types of friction materials. Very hard materials such as alumina, silicon carbide, and kyanite [1302-76-7] are used in fine particle sizes in organic, semimetallic, and cermet materials that are generally less than 74 im (200 mesh). Particle size is limited by the fact that large particles of such hard materials would groove cast-iron mating surfaces. Larger particle sizes are possible for harder mating surfaces in the special steel rotors used with sintered materials. 

As an initial step, the reports of l80 in-process accidents were viewed. A sianmary of the type of information obtained is shown in Fig 22. The process operation and the probable causative stimuli which led to the accident are given in terms of the number of accidents and the percentage of the total number. Thus the most probable causes of an accident were identified in an accident analysis. The causes varied by process operation and material type. However friction, impact, electrostatic discharge (ESD), and heating were the most commonly identified causative stimuli. 

This paper will discuss the requirements for shaft sinking operation—where effective braking with minimal run back is required, the type of disc brake imits to be selected, the selection of disc pad friction materials and the selection of the brake disc material. 

The ATF chanistry required to achieve a positive p-v curve to prevent shudder and maintain a sufficiently high friction level to provide adequate torque capacity will depend on the chemistry of the friction materials and the type of materials used to construct the transmission clutches [20]. A typical transmission clutch uses a cellulosic friction material in combination with steel plates. Other friction materials like carbon fiber are also used. Instead of steel plates, aluminum, copper, bronze, tin, and other metallic materials can also be used. In addition to these materials, plastic, nylon, and a variety of elastomeric sealant materials are used to construct other components of the automatic transmission. More recently, ATFs are required to be compatible with electronic circuitry and sensors [21]. Thus, changes in transmission design and materials have a direct effect on the... 

The terms cohesionless and cohesive are sometime mistakenly used as representing soils whose shear strengths are characterized with friction and cohesion only, respectively. This could lead to a problem in the foundation design. In the long term following the application of all types of permanent loads, most soils are likely to behave as purely frictional materials, and inclusion of any cohesion in the long-term stability evaluation can lead to dangerous consequences. 

In addition to aircraft applications, adhesives of this type are employed in the bonding of friction materials (e.g. grinding wheels), brake lining to brake shoes, clutch facings, etc. The cement shown in Table 22 is suitable for such diverse jobs as metal-to-metal bonding or brake lining bonding. 

The five tables of friction coefficient values in this Chapter contain both static and kinetic friction coefficients. They are arranged by material type as follows ... 

State parameter y/, the preferred measure of in situ state within a critical state framework, can be measured (interpreted) from cone penetration tests with reasonable precision. Without measuring the critical state properties of the sand (e.g. M, A), shear modulus G or plastic hardening, interpretation relies on the friction ratio and parameters as indicators of material type and a precision in of 0.08 is possible. With supporting laboratory testing to determine the critical state parameters and measurement of with a seismic CPT, a precision in Xffof 0.02 can be achieved. Plewes et al. (1992), Been and Jefferies (1992) and... 

Material Type Sliding Condition Friction Coefficient Relative Error (%) Wear Rate [10" mmV(N m)] Relative Error (%)... 

Cold Welding (CW) Ductile metals such as carbon steels, aluminum, copper and precious metals. Friction Welding (FRW) can weld many material types and dissimilar metals effectively, including aluminum to steel. Also thermoplastics and refractory metals. 

In the absence of skidding, the coefficient of static friction applies at each instant, the portion of the tire that is in contact with the pavement has zero velocity. Rolling tire friction is more of the type discussed in Section XII-2E. If, however, skidding occurs, then since rubber is the softer material, the coefficient of friction as given by Eq. XII-5 is determined mainly by the properties of the rubber used and will be nearly the same for various types of pavement. Actual values of p, turn out to be about unity. 

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