Machinery's Handbook, 31st Edition
Plain Bearing Materials 2439 Table 3. Properties of Bearing Alloys and Bearing Characteristics Ratings
Recommended Shaft Hardness (Brinell Hardness Number)
Maximum Operating Temp., ° F b
Load-Carrying Capacity, psi a
Material
Tin-Base Babbitt Lead-Base Babbitt Cadmium Base Copper-Lead Tin-Bronze Lead-Bronze Aluminum Silver-Overplate Trimetal-Overplate
150 or less 150 or less 200–250
800–1500 800–1200 1200–2000 1500–2500
300 300 500 350
1 1 1 2 3
1 1 2 2 5 4 3 3 2
1 3 5 5 2 4 1 1 2
5 5 4 3 1 2 2 1 3
300
300–400
4000+
500+
300 300 300
3000–4500
450–500 3 225–300 5
4000+ 4000+
500
2
230 or less
2000–4000+
225–300 1
a 1 psi = 6.8947 kPa b Temp. in °C = (°F – 32)/1.8. c Note: 1 is best; 5 is worst.
neither a separate overlay nor a steel backing. Bronzes are available in standard bar stock, sand or permanent molds, investment, centrifugal or continuous casting. Leaded bronzes have better compatibility than tin-bronzes because the spheroids of lead smear over the bearing surface under conditions of inadequate lubrication. These alloys are generally a first choice at intermediate loads and speeds. Table 3 provides comparisons of basic bear- ing properties of these materials. Aluminum.— Aluminum bearings are either cast solid aluminum, aluminum with a steel backing, or aluminum with a suitable overlay. The aluminum is usually alloyed with small amounts of tin, silicon, cadmium, nickel, or copper, as shown in Table 1. An aluminum bearing alloy with 20 to 30 percent tin alloy and up to 3 percent copper has shown promise as a substitute for bronzes in some industrial applications. These bearings are best suited for operation with hard journals. Owing to the high ther mal expansion of the metal (resulting in diametral contraction when it is confined as a bearing in a rigid housing), large clearances are required, which tend to make the bearing noisy, especially on starting. Overlays of lead-tin, lead, or lead-tin-copper may be applied to aluminum bearings to facilitate their use with soft shafts. Aluminum alloys are available with properties specifically designed for bearing applications, such as high load-carrying capacity, fatigue strength, and thermal conduc- tivity, in addition to excellent corrosion resistance and low cost. Silver.— Silver bearings were developed for and have an excellent record in heavy-duty applications such as aircraft master rod and diesel engine main bearings. Silver has a higher fatigue rating than any of the other bearing materials; the steel backing used with this material may show evidence of fatigue before the silver. The advent of overlays, or more commonly called overplates, made it possible for silver to be used as a bearing mate rial. Silver by itself does not possess any of the desirable bearing qualities except high fatigue resistance and high thermal conductivity. The overlays such as lead, lead-tin, or lead-indium improve the embeddability and antiscoring properties of silver. The relative basic properties of this material, when used as an overplate, are shown in Table 3. Cast Iron.— Cast iron is an inexpensive bearing material capable of operation at light loads and low speeds, i.e., up to 130 ft/min (40 m/min) and 150 lb/in 2 (1.03 MPa). These bearings must be well lubricated and have a rather large clearance so as to avoid scoring from particles torn from the cast iron that ride between bearing and journal. A journal hardness of between 150–250 BHN (Brinell Hardness Number) has been found to be best when using cast-iron bearings.
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