BEARING METALS
Babbitt metal is a general term used for soft tin and lead-base alloys
which are cast as bearing surfaces on steel, bronze, or cast-iron shells.
Babbits have excellent embedability (ability to embed foreign particles
in itself) and conformability (ability to deform plastically to compensate
for irregularities in bearing assembly) characteristics. These alloys may
be run satisfactorily against a soft-steel shaft. The limitations of Babbit
alloys are the tendency to spread under high, steady loads and to fatigue
under high, fluctuating loads. These limitations apply more particularly
at higher temperatures, for increase in temperature between 68 and
212°F (20 and 100°C) reduces the metal’s strength by 50 percent. These
limitations can be overcome by properly designing the thickness and
rigidity of the backing material, properly choosing the Babbitt alloy for
good mechanical characteristics, and ensuring a good bond between
backing and bearing materials.
The important tin- and lead-base (Babbitt) bearing alloys are listed in
Table 6.4.9. Alloys 1 and 15 are used in internal-combustion engines.
Alloy 1 performs satisfactorily at low temperatures, but alloy 15, an
arsenic alloy, provides superior performance at elevated temperatures
by virtue of its better high-temperature hardness, ability to support
higher loads, and longer fatigue life. Alloys 2 and 3 contain more antimony,
are harder, and are less likely to pound out. Alloys 7 and 8 are
lead-base Babbitts which will function satisfactorily under moderate
conditions of load and speed. Alloy B is used for diesel engine bearings.
In general, increasing the lead content in tin-based Babbitt provides
higher hardness, greater ease of casting, but lower strength values.
Silver lined bearings have an excellent record in heavy-duty applications
in aircraft engines and diesels. For reciprocating engines, silver
bearings normally consist of electrodeposited silver on a steel backing
with an overlay of 0.001 to 0.005 in of lead. An indium flash on top of
the lead overlay is used to increase corrosion resistance of the material.
Aluminum and zinc alloys are used for high-load, low-speed applications
but have not replaced Babbits for equipment operating under a
steady high-speed load. The Al 20 to 30, Sn 3 copper alloy is bonded to
a steel bearing shell. The Al 6.25, Sn 1, Ni 1, copper alloy can be either
used as-cast or bonded to a bearing shell. The Al 3 cadmium alloy with
varying amounts of Si, Cu, and Ni can also be used in either of these two
ways. The Zn 11, Al 1, Cu 0.02, magnesium (ZA-12) and Zn 27, Al 2,
Cu 0.01 magnesium (ZA-27) alloys are used in cast form, especially in
continuous-cast hollow-bar form.
Mention should be made of cast iron as a bearing material. The flake
graphite in cast iron develops a glazed surface which is useful at surface
speeds up to 130 ft/min and at loads up to 150 lb/in2 approx. Because of
the poor conformability of cast iron, good alignment and freedom from
dirt are essential.
Copper-base bearing alloys have a wide range of bearing properties
that fit them for many applications. Used alone or in combination with
steel, Babbitt (white metal), and graphite, the bronzes and copper-leads
meet the conditions of load and speed given in Table 6.4.10. Copper-lead
alloys are cast onto steel backing strips in very thin layers (0.02 in) to
provide bearing surfaces.
Three families of copper alloys are used for bearing and wear-resistant
alloys in cast form: phosphor bronzes (Cu-Sn), Cu-Sn-Pb alloys, and
manganese bronze, aluminum bronze, and silicon bronze. Typical compositions
and applications are listed in Table 6.4.10. Phosphor bronzes
have residual phosphorus ranging from 0.1 to 1 percent. Hardness increases
with phosphorus content. Cu-Sn-Pb alloys have high resistance
to wear, high hardness, and moderate strength. The high lead compositions
are well suited for applications where lubricant may be deficient.
The Mn, Al, and Si bronzes have high tensile strength, high hardness,
and good resistance to shock. They are suitable for a wide range of
bearing applications.
Porous bearing materials are used in light- and medium-duty applications
as small-sized bearings and bushings. Since they can operate for
long periods without an additional supply of lubricant, such bearings are
useful in inaccessible or inconvenient places where lubrication would
be difficult. Porous bearings are made by pressing mixtures of copper
and tin (bronze), and often graphite, Teflon, or iron and graphite, and
sintering these in a reducing atmosphere without melting. Iron-based,
oil-impregnated sintered bearings are often used. By controlling the
conditions under which the bearings are made, porosity may be adjusted
so that interconnecting voids of up to 35 percent of the total volume may
be available for impregnation by lubricants. Applicable specifications
for these bearings are given in Tables 6.4.11 and 6.4.12.
Miscellaneous A great variety of materials, e.g., rubber, wood,
phenolic, carbon-graphite, ceramets, ceramics, and plastics, are in use
for special applications. Carbon-graphite is used where contamination
by oil or grease lubricants is undesirable (e.g., textile machinery, pharmaceutical
equipment, milk and food processing) and for elevated-temperature
applications. Notable among plastic materials are Teflon and
nylon, the polycarbonate Lexan, and the acetal Delrin. Since Lexan and
Delrin can be injection-molded easily, bearings can be formed quite
economically from these materials.

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