Machinery's Handbook, 31st Edition
2438 Plain Bearing Materials In small bushings for fractional-horsepower motors and in automotive engine bearings, babbitt is generally used as a thin coating over a flat steel strip. After forming oil distribu tion grooves and drilling required holes, the strip is cut to size, rolled, and shaped into fin ished bearing. These bearings are available for shaft diameters from 0.5 to 5 inches (12.7– 127 mm). Strip bearings are turned out by the millions yearly in highly automated factories and offer an excellent combination of low cost with good bearing properties. For larger bearings in heavy-duty equipment, a thicker babbitt is cast on a rigid backing of steel or cast iron. Chemical and electrolytic cleaning of the bearing shell, thorough rins ing, tinning, and then centrifugal casting of the babbitt are desirable for sound bonding of the babbitt to the bearing shell. After machining, the babbitt layer is usually 1 ⁄ 2 to 1 ⁄ 4 inch (12.7–6.35 mm) thick. Compared to other bearing materials, babbitts generally have lower load-carrying capacity and fatigue strength, are a little higher in cost, and require a more complicated design. Also, their strength decreases rapidly with increasing temperature. These short- comings can be avoided by using an intermediate layer of high-strength, fatigue-resistant material that is placed between a steel backing and a thin babbitt surface layer. Such com- posite bearings frequently eliminate any need for using alternate materials having poorer bearing characteristics. Tin babbitt is composed of 80 to 90 percent tin to which is added about 3 to 8 percent cop- per and 4 to 14 percent antimony. An increase in copper or antimony produces increased hardness and tensile strength and decreased ductility. However, if the percentages of these alloys are increased above those shown in Table 2, the resulting alloy will have decreased fatigue resistance. These alloys have very little tendency to cause wear to their journals because of their ability to embed dirt. They resist the corrosive effects of acids, are not prone to oil-film failure, and are easily bonded and cast. Two drawbacks are encountered from use of these alloys because they have low fatigue resistance and their hardness and strength drop appreciably at low temperatures. Lead babbitt compositions generally range from 10 to 15 percent antimony and up to 10 percent tin in combination with the lead. Like tin-base babbitts, these alloys have little tendency to cause wear to their journals, embed dirt well, resist the corrosive effects of acids, are not prone to oil-film failure and are easily bonded and cast. Their chief disad - vantages when compared with tin-base alloys are a rather lower strength and a suscepti- bility to corrosion. Cadmium Base.— Cadmium alloy bearings have a greater resistance to fatigue than bab bitt bearings, but their use is very limited due to their poor corrosion resistance. These alloys contain 1 to 15 percent nickel, or 0.4 to 0.75 percent copper, and 0.5 to 2.0 per- cent silver. Their prime attribute is their high-temperature capability. The load-carrying capacity and relative basic bearing properties are shown in Table 3. Copper-Lead.— Copper-lead bearings are a binary mixture of copper and lead contain- ing from 20 to 40 percent lead. Lead is practically insoluble in copper, so a cast micro- structure consists of lead pockets in a copper matrix. A steel backing is commonly used with this material and high volume is achieved either by continuous casting or by powder metallurgy techniques. This material is very often used with an overplate such as lead-tin and lead-tin-copper to increase basic bearing properties. Table 3 provides comparisons of material properties. The combination of good fatigue strength, high-load capacity, and high-temperature performance has resulted in extensive use of this material for heavy-duty main and connecting-rod bearings as well as moderate-load and speed applications in turbines and electric motors. Leaded Bronze and Tin-Bronze.— Leaded and tin-bronzes contain up to 25 percent lead or approximately 10 percent tin, respectively. Cast leaded bronze bearings offer good compatibility, excellent casting, and easy machining characteristics, low cost, good structural properties and high-load capacity, usefulness as a single material that requires
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