BEARING MATERIALS-Trimetal bearings,Bearing
A large variety of metals and nonmetallic materials in monolithic and composite (laminate) form are used for bearings. Monolithic ferrous bearings are made of gray cast iron, pressed and sintered iron and steel powder, and many wrought steels, including low- and high-carbon plain-carbon steels, low-alloy steels, alloy steels, stainless steels, and tool steels. Most cast-iron bearings are made of gray iron because it combines strength with the lubricity of graphitic carbon. Pressed and sintered bearings can be made to controlled porosity and impregnated with oil for lubricity. Because of its wide use in ball and roller bearings, one of the best-known bearing steels is AISI 52100 steel, a through-hardening 1% carbon and 1.3 to 1.6% chromium alloy steel. Many steels, however, are simply surface-hardened for bearing applications. In recent years, the performance of bearing steels has been markedly improved by special melting practices that reduce the presence of nonmetallic inclusions.
Monolithic nonferrous bearings include copper-zinc bronze, leaded bronzes, unleaded bronzes, and an aluminum-tin alloy, containing about 6% tin as the principal alloying element. The bronze and aluminum alloy provide similar load-bearing capacity and fatigue resistance, but the bronze is somewhat better in resistance to corrosion by fatty acids that can form with petroleum-based oils. It is also less prone to seizure and abrasion from mating shafts; more able to embed foreign matter and thus prevent shaft wear; and more tolerant of shaft misalignment. The load-bearing capacity of tin bronzes depends on the lead content. Low-lead and lead-free tin bronzes have the highest load capacity, about 5,000 lb/in2 (34 MPa), and fatigue strength. Applications include auto engine starter-motor bearings, or bushings, for the copper-zinc bronze; auto engine connecting-rod bearings for the aluminum alloy; and various bearings in motors, machine tools, and earthmoving equipment for the tin bronzes. An aluminum-tin-silicon alloy (Al-8Sn-2.5Si-2Pb-0.8Cu-0.2Cr), developed by Federal Mogul Corp., features high resistance to wear, seizure, and fatigue at an optimal hardness of Vickers 50.
Monolithic bearings are also made of cemented tungsten and chromium carbides, plastics, carbon-graphite, wood, and rubber. Plastics provide good combinations of inherent lubricity, corrosion resistance, and adequate strength at room to moderately elevated temperatures. Thermal conductivity and other performance features that may be required can be provided by metal and other fillers. Plastic
bearings can be made of acetal, nylon, polyester, ultrahigh-molecular-weight polyethylene, polytetrafluoroethylene, polysulfone, polypheny-lene sulfide, polyimide, polybenzimidazole, and polyamide-imide. Carbon-graphite bearings are more heat-resistant but rather brittle, thus limited to nonimpact applications. Wood bearings are made of maple and the hard lignum vitae. Rubber bearings, usually steel-backed, are used for applications requiring resilience.
Nonferrous metals are widely used in dual- or trimetal systems. Dual-metal bearings comprise a soft, thin, inner liner metallurgi-cally bonded to stronger backing metal. Steel lined with bronze containing 4 to 10% lead provides the highest load-bearing capacity—8,000 lb/in2 (55 MPa), or about twice that of the bronze alone—and fatigue strength. However, the aluminum alloy with a steel backing provides the best corrosion resistance and only moderately less load-bearing capacity. Tin and lead babbitt linings excel in surface qualities conducive to free-sliding conditions and are used with steel, bronze, or aluminum-alloy backings; load-bearing capacities range from 1,500 to 7,000 lb/in2 (10 to 48 MPa). Dual-metal systems cover a gamut of bearings for motors, pumps, piston pins, camshafts, and connecting rods.
Trimetal bearings, all with steel backings, have an inner liner of tin or lead babbitt and an intermediate layer of a more fatigue-resistant metal, such as leaded bronze, copper-lead, aluminum-tin, tin-free aluminum alloys, silver, or silver-lead. Load-bearing capacity ranges from 1,500 to 12,000 lb/in2 (10 to 83 MPa). The silver bearing systems provide the best combination of load-bearing capacity, fatigue and corrosion resistance, and compatibility to mating materials; but a lead babbitt, medium-lead bronze and steel system is a close second, sacrificing only a moderate reduction in corrosion resistance but at a reduction in cost. Applications include connecting-rod, camshaft, and main bearings in auto engines and reciprocating aircraft engines.
Hybrid bearings comprise silicon-nitride balls, a fiber-reinforced polyimide separator, and a steel race. The balls are much more lightweight than steel balls, markedly reducing centrifugal force, and ride on the low-friction separator, increasing wear life and running speed. They also have high fatigue resistance, increasing service life. These bearings are used in medical instruments and machine-tool spindles.

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