Brakes and clutches

Carbon-carbon is used for the construction of brakes and clutches (Section 23.8.1 and 23.8.2). 

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Brakes and clutches operate both dry and wet. In dry friction couples, the heat is removed by conduction to the surrounding air and stmctural members. Wet friction couples operate within a fluid, usually an oil, which absorbs the heat and maintains the couple at relatively low (below 200°C) temperatures. The fluid also traps the wear debris. 

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). 

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... 

Brake Linings. Substantial amounts of crystalline flake, lump, and amorphous graphite are used in brake and clutch finings, mostly in heavier duty nonautomotive situations. The graphite has been substituted for asbestos because of health considerations. The graphite proportion of the part has risen from 2 to 15% in some instances. The graphite lubricates, transfers the heat of friction away from the fining, and lowers the rate of wear. 

Ho, T.L., Role of Solid Lubrication in High-Energy Braking, I.Mech.E. Conf. on Oil-Immersed Brakes and Clutches, Paper No. C34/77, I.Mech.E., London, 1977. 

NIOSH. 1975. Asbestos exposure during servicing of motor vehicle brake and clutch assemblies. Current intelligence bulletin 5. Rockville, MD U.S. Department of Health, Education, and Welfare, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health. 

The high frictional coefficient (0.4 to 0.5 compared with < 0.1 for glass fibers) of asbestos fibers is crucial to its utilization in the frictional lining sector. In the manufacture of brake and clutch linings 20 to 60% asbestos is incorporated together with fillers, metal chips and preferably phenol resins and rubber into a composite material, which has to satisfy many requirements. Currently there are asbestos-free so-called semimetallic brake linings, which consist of mixtures of metal fibers, metal powders, cellulose fibers, aluminum silicate fibers and mineral wool bonded with synthetic resins. 

Whiskers can be easily mixed with powders and processed as such. Composites are formed with metal (utilization in automobile construction e.g. pi.stons of whisker-reinforced aluminum), ceramic or polymer matrices for frictional applications (e.g. brake and clutch linings). Small quantities command prices of 1500 to 2000 DM/kg. For larger quantities prices of 100 to 150 DM/kg are expected. 

In brake and clutch linings graphite has largely supplanted the toxicologically suspect asbestos. 

Knecht U, Elliehausen HJ, Jusas W, et al. 1987. Polycyclic aromatic hydrocarbons (PAH) in abraded particles of brake and clutch linings. Int J Environ Occup Soc Med 28 227-236. 

UTTOs are specifically formulated for driveline performance, including transmissions, final drives, wet brakes and clutches and also hydraulic systems, typically from a common sump on the tractor. This unique combination of performance characteristics makes tractor fluids acceptable for use in both on and off-highway commercial transmissions and high-pressure hydraulic systems. They can also be used in mining equipment. Tractor Fluid specifications are dominated by four manufacturers ... 

Sometimes the operation of machine elements such as brakes and clutches may result in audible sound, ranging from high-pitched squeals to low-pitched growls. It is often assumed that such sounds are manifestations of stick-slip sliding, but in reality they are the result of frictionally induced quasiharmonic vibration. This aspect of friction has been analyzed and studied experimentally by Brockley and Ko [16, 17],... 

The phenolic lipids of Anacardieum occidentale have been commercially exploited (ref. 174) and those in Rhus vernicifera to a lesser extent. Most of the technical cashew nut shell liquid (CNSL) which results from industrial processing is and has been employed as a phenolic source for formaldehyde polymerisation the products from which in compounded form have been the basis for friction dusts widely used throughout the world in vehicle brake and clutch linings (ref.175). Urushiol has had use over many centuries in the art of Japanese lacquering (ref. 176) and in more recent years has been sometimes supplemented with CNSL. Chemical uses are referred to later. 

Traditionally, asbestos has been used in friction materials such as vehicle brake and clutch components because of its toughness and non-flammability. The use of asbestos has been restricted in many countries. Aramid, as an alternative material, has been increasingly used instead of asbestos. Aramid is a tough, synthetic fibrous material which is beUeved to be safer to health than asbestos. 

For reasons related to industrial safety and environmental protection, considerable effort has been invested for decades now in replacing asbestos in such items as automobile brake and clutch plate linings with metal, glass, synthetic mineral and organic fiber materials. Asbestos-free disk brakes and clutch linings have now been on the market, and on original equipment lines, for some time. The phenolic resin... 

Uses NBR latex for oil- and petrol-resistant proofings and coatings for fabrics and paper dipped goods binders for nonwoven fabric, brake and clutch linings, gasket materials, etc. 

The reasonable mechanical strength and good thermal conductivity make grey iron a suitable material for cylinder heads, and brake and clutch applications in motor vehicles. 

Toxicology Nuisance dust Uses Constituent of cement in refractories anticaking agent in table salt, vanilla powd. filler-reinforcement for plastics, phenolic, epoxy, melamine, nylon, polyimide, PBT, PVC, PC, DAP, polybutadiene, thermoset polyester, polyolefin, and urethane polymers provides friction and wear props, in brake and clutch iTidtoridls... 

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