The manufacture of friction materials is a highly specialized process, and it is
advisable to consult manufacturers’ catalogs and handbooks, as well as manufacturers
directly, in selecting friction materials for specific applications. Selection involves a
consideration of the many characteristics as well as the standard sizes available.
The woven-cotton lining is produced as a fabric belt that is impregnated with resins
and polymerized. It is used mostly in heavy machinery and is usually supplied in rolls
up to 50 ft in length. Thicknesses available range from 1
8 to 1 in, in widths up to about
12 in.
A woven-asbestos lining is made in a similar manner to the cotton lining and may
also contain metal particles. It is not quite as flexible as the cotton lining and comes in
a smaller range of sizes. Along with the cotton lining, the asbestos lining was widely
used as a brake material in heavy machinery.
Molded-asbestos linings contain asbestos fiber and friction modifiers; a thermoset
polymer is used, with heat, to form a rigid or semirigid molding. The principal use was
in drum brakes.
Molded-asbestos pads are similar to molded linings but have no flexibility; they
were used for both clutches and brakes.
Sintered-metal pads are made of a mixture of copper and/or iron particles with
friction modifiers, molded under high pressure and then heated to a high temperature
to fuse the material. These pads are used in both brakes and clutches for heavy-duty
applications.
Cermet pads are similar to the sintered-metal pads and have a substantial ceramic
content.
Table 16–4 lists properties of typical brake linings. The linings may consist of a
mixture of fibers to provide strength and ability to withstand high temperatures, various
friction particles to obtain a degree of wear resistance as well as a higher coefficient of
friction, and bonding materials.
Table 16–5 includes a wider variety of clutch friction materials, together with some
of their properties. Some of these materials may be run wet by allowing them to dip in
oil or to be sprayed by oil. This reduces the coefficient of friction somewhat but carries
away more heat and permits higher pressures to be used.