Improvement in mechanical properties,
rigidity, and wear resistance is obtained by adding fillers such as graphite and
molybdenum disulfide to nylon. While the maximum recommended continuous service
temperature for ordinary nylon is 170°F, and 250°F for heat-stabilized compositions,
filled- nylon parts resist distortion at temperatures up to 300°F.
PTFE: Has an exceptionally low coefficient
of friction and high self-lubricating characteristics, resistance to attack by almost any
chemical, and an ability to operate under a wide temperature range. High cost combined
with low toad capacity has frequently caused FIFE resin to be selected only In some
modified form. PTFE Is used as a bearing material in automotive knuckle and ball joints,
chemical and (cod processing equipment, aircraft accessories, textile machinery, and
business machines.
Although unmodified PTFE can be used to a PV value of only 1,000,
PTFE filled with glass fiber, graphite, or other inert materials, can be used at PV values
up to 10,000 or more. In general, higher PV values can be used with PTFE bearings at low
speeds where its coefficient of friction may be as low as 0.05 to 0.1.
One bearing material combines the low friction and good wear
resistance of lead-filled PTFE with the strength and thermal conductivity of a bronze and
steel supporting structure. A plated steel backing is covered with a thin layer of
sintered, spherical, bronze particle8. The porous bronze is then impregnated with a
mixture of PTFE and lead to provide a thin surface layer. Service temperatures of
—330 to +536°F are possible.
Woven PTFE fabrics are often readily handled and applied.
With their resistance to cold flow, they are used as bearings in a wide variety of
high-load applications as automotive thrust washers, ball- and-socket joints, aircraft
controls and accessories, bridge bearings, and electrical switch gear. To provide a strong
bond to either steel or other rigid backing material, a secondary fiber such as poiyester,
cotton, or glass is commonly Interwoven with the PTFE. The woven fabric then Is bonded to
a steel backing.
Improved versions of this type of bearing have woven or
braided “socks” (of PTFE and a bondable material). The bearing sleeve is then
filament wound with a fiberglass-epoxy shell. These bearings have been rer rted to carry
dynamic loads as high as 50,000 psi.
Acetal: Has been used for inexpensive
bearings in a wide variety of automotive, appliance, and industrial applications. It is
particularly useful in wet environments because of its stability and resistance to wet
abrasion.
Polyimide, Polysultone, Potyptienylene sulfide:
High-temperature materials with excellent resistance to both chemical attack and burning.
With suitable fillers, these moldable plastics are useful for PV factors to 20,000 and
30,000. Polyimide molding compounds employing graphite as a self- lubricating filler show
promise in bearing, seal, and piston ring applications at temperatures to 500°F.
Polyphenylene sulfide cart be applied as a coating through use of a slurry spray, dry
powder, or fluidized bed; These coating techniquee require a final bake at about 700° F.
Ultrehigh-Molecular-Weight,
Polyethylene: Resists abrasion and has a smooth, low-friction surface. Often an
ideal material for parts commonly made from acetal, nylon, or PTFE materials.
Carbon-Graphite:The self-lubricating
properties of carbon bearings, their stability at temperatures up to 750°F, and their
resistance to attack by chemicals and solvents, give them important advantages in
fieldswhere other bearing materials are unsatisfactory. Carbon-graphite bearings are used
where contamination by oil or grease is undesirable, as In textile machinery, food
handling machinery, and pharmaceutical processing equipment. They are used as bearings in
and around ovens, furnaces, boilers, and jet engines where temperatures are too high for
conventional lubricants. They are also used with low-viscosity and corrosive liquids in
such applications as
metering devices or pumps for gasoline, kerosene, hot and cold-water, sea water, chemical
process streams, acids, alkalis, and solvents.
The composition and processing used with carbon
bearings can be varied to provide characteristict required for particular applications.
Carbon-graphite has from 5% to 20% porosity. These pores can be filled with a phenolic or
epoxy resin for Improved strength and hardness, or with oil or metals (such as silver,
copper, bronze, cadmium, or babbitt) to improve compatibility properties.