Basics of sliding metallic-bearing materials: Part 3

Aug. 1, 2000
This is the third of a series on principles and characteristics of metallic plain bearing materials and how to optimize them in given uses. Part 1 appeared in June 1994 and Part 2 in July.

Part 2 concluded a discussion of bearing- material properties in general, including both surface and bulk properties. Here, we begin coverage of bearing-material systems, in this part covering only single-metal systems. Discussion of multilayer systems will begin in Part 4.

Bearing-material systems

Because of widely varying operating conditions for bearings, commercial bearing materials evolved as specialized engineering-material systems rather than commodity products. Total production is in small tonnages produced by just a few manufacturers. Alloy formulation and processing methods involve much proprietary technology. Successful bearing material selection often requires close user-producer cooperation.

Single-metal systems. Most singlemetal sliding bearings are made of either copper alloy or aluminum alloy. Cast zinc-base (Zn) alloys sometimes serve as lower-cost substitutes for solid bronze. Table 1 lists commercially significant alloys used as single-metal bearings.

Wide ranges of compositions and properties are available in the older copper group, some dating to the 19th Century. World War II metal shortages stimulated interest in aluminum alloys, which accelerated with the commercial introduction of aluminum-tin bearing alloys in 1946. Since then, metal economics have encouraged aluminum alloy bearings, but many designers of heavy and special-purpose machinery continue to prefer brasses and bronzes.

Single-metal systems have no outstanding surface properties, and tolerance of boundary and thin-film lubrication conditions is limited. Therefore, the load-capacity rating for a single-metal bearing is usually low relative to the fatigue strength of the material from which it was made. Because of metallurgical simplicity, single metals suit small-lot manufacturing from cast tubes or bars, by conventional machining.

• Copper alloys. Except for commercial bronze and low-lead tin bronze, copper alloys in single-metal systems are almost always in cast form. This provides thick bearing walls (0.125 in. and greater), strong enough so that the bearing is retained when press-fitted into the housing.

Commercial bronze and medium-lead tin bronze alloys C83420 and C83520 serve extensively in wrought strip for thin-wall bushings. They are made in large volumes by high-speed press forming. Poor compatibility of these alloys can be improved by embedding a graphiteresin paste in rolled or pressed-in indentations, so that the running surface of the bushing consists of interspersed areas of graphite and bronze. Such bushings serve in automotive-engine starting motors.

The lead in leaded tin bronze is free lead, dispersed throughout a copper-tin matrix so the bearing surface consists of interspersed areas of lead and bronze. In general, the best selection from this group of materials for a given application is the highest-lead composition you can use without risking excessive wear, plastic deformation, or fatigue damage.

• Aluminum alloys. Most solid aluminum bearings in the United States are of alloys containing 5.5 to 7% tin, plus smaller amounts of copper, nickel, silicon, and magnesium. Starting forms for fabrication include cast tubes as well as rolled plate and strip, which can be press-formed into half-round shapes. As with solid bronze bearings, bearing walls are thick.

Tin in these alloys is free tin, dispersed throughout an aluminum matrix so that the bearing surface consists of interspersed areas of aluminum and tin. Free tin enhances surface properties much like free lead improves those of bronze.

Aluminum’s high thermal expansion coefficient poses special problems in maintaining press-fit and running clearances. Various methods are used to increase yield strength, through heat treatment and cold work, to overcome plastic flow and permanent deformation at service temperatures and loads.

• Zinc alloys. Over the past two decades, zinc-aluminum-copper casting alloys replaced cast bronze alloys in some low-speed machinery bearings. The practice has advanced most in Europe.

These alloys contain no soft microconstituents that correspond to lead used in bearing bronzes and to tin in cast aluminum bearing alloys. Compatibility of the zinc-base alloys seems to derive mostly from their chemical behavior with hydrocarbon lubricants. Formation of a stable, low-shear-strength film of zinc-base soap seems important.

• Porous metal bushings. You can also consider oil-impregnated porous metal bushings as single-metal systems. Compositions include unleaded and leaded tin-bronze, bronze-graphite, iron-carbon, iron-copper, and iron-bronze-graphite. Oil content is 8 to 30% of total volume.

George R. Kingsbury, P.E., recently retired as Senior Engineer from Glacier Vandervell Inc., a major producer of metal plain bearings, is principal of his own metallurgical engineering consulting practice in Lyndhurst (Cleveland), Ohio. He is well known in the bearing materials field as an author, lecturer, inventor, and consultant.

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