Motion System Design

Basics of sliding metallic-bearing materials — Part 7

This is the seventh and last 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, p. 36; Part 2, July 1994, p. 32; Part 3, Aug. 1994, p. 90; Part 4, Nov. 1994, p. 56; Part 5, Aug. 1995, p. 62; Part 6, Apr. 1996, p. 51

In Part 6, we discussed electroplating processes along with tin alloys, lead alloys, overlays, and copper alloys. Here in this last part, we cover aluminum alloys, silver alloys, zinc alloys, other metallic bearing materials, nonmetallic bearing materials, and bearing-material selection.

Aluminum alloys

Successful commercial use of aluminum alloys in plain bearings dates to about 1940, when low-tin aluminum alloy castings began replacing solid bronze bearings for heavy machinery. Production of steel-backed strip materials by roll bonding became commercially successful about 1950, allowing development of practical bimetal and trimetal systems using aluminum alloys in place of babbitts and copper alloys.

Ready availability of aluminum and its stable cost have been incentives for broadening its use in plain bearings. Aluminum single-metal, bimetal, and trimetal systems now can be used in the same load ranges as babbitts, copperlead alloys, and high-lead tin bronzes. Moreover, aluminum’s outstanding corrosion resistance has gained importance and led to widespread preference of aluminum alloy materials in auto engine bearings over copper-lead alloys and leaded bronzes.

In these alloys, additions of silicon, copper, nickel, magnesium, and zinc strengthen the aluminum through solidsolution and precipitation mechanisms. These elements largely control fatigue resistance and opposing properties of conformability and embeddability.

Tin and lead help upgrade the inherently poor compatibility of aluminum. Cadmium also serves as an alloy addition for this reason. Silicon helps compatibility besides its moderate strengthening effect. Although not well understood, this compatibility-enhancing mechanism is of much practical value. Silicon serves effectively in many alloys for this reason, usually along with tin, lead, or cadmium.

Conventional mechanical properties are more valuable in understanding fabrication behavior of aluminum-base bearing alloys than in predicting bearing performance. Except for solid aluminum alloy bearings — in which there is no steel back and where press-fit retention depends entirely on aluminum-alloy strength — mechanical properties of finished bearings are rarely specified, and then usually for control purposes only.

Most current commercial applications of aluminum-base bearing alloys involve steel-backed bimetal or trimetal bearings. To determine the most cost-effective aluminum material for any given application, consider the economic advantages of bimetal vs. trimetal systems. The higher cost of high-tin and high-lead alloys usually is offset by eliminating the cost of the lead alloy overlay plate. Here is a clear demonstration of the cost-effectiveness of aluminum bimetal materials: About 75% of U.S.-built passenger car engines use high-lead aluminum alloy bimetals for main and connecting rod bearings. In Europe and Japan, intermediate and high-tin aluminum bimetals are likewise preferred.

If you need the higher load capacity of a trimetal material, then select an aluminum liner alloy that provides adequate — but not excessive — strength. That way, you don’t sacrifice conformability and embeddability unnecessarily. The tin-free alloy group offers a wide range of strength properties, and the most economical choice usually is in this group.

Silver alloys

Use of silver in bearings is largely confined to unalloyed silver electroplated on steel shells, which then are machined to close tolerances and finally precisionplated to size with a thin soft-metal overlay. The overlay may be lead-tin, lead-tincopper, or lead-indium. As a bearing material, plated silver is invariably used with an overlay. Silver on steel with an overlay is the ultimate fatigue-resistant bearing material.

Silver was widely used during and after World War II in aircraft, where its high cost was justified. As piston engines phased out, however, the use of silver in bearings greatly declined. Current applications are special, chiefly in the aircraft and locomotive industries. In view of the high cost of silver, any increase in demand for it would stimulate a search for a comparable, less costly substitute.

Zinc alloys

The zinc-base alloys that have served successfully for machinery bearings are standard zinc foundry alloys of the Zn-Al- Cu-Mg high-performance type. Tubular shapes made by conventional sand, permanent- mold, and pressure die casting methods are machined into bearings much like solid bronze bearings are. Most applications have been direct substitutions for solid bronze bearings, made mostly to reduce cost.

High compressive strength and hardness of these materials suggest greater load capacities than those of solid bronze and solid aluminum bearing materials. This is not so in practice however, largely because of the high rate at which the zinc alloys soften with increasing temperature. Maximum recommended running temperatures of 200 to 250 F for zincbase alloys are more than 200 F less than temperature limits for copper and aluminum- base bearing alloys.

Because of low cost, zinc-base alloys will probably continue to replace copperbase alloys in some applications in construction, earthmoving, mining, and millmachinery markets. However, technical limits with respect to high-temperature strength and corrosion resistance will prevent massive movement away from bronzes to zinc alloys.

Other metallic bearing materials

Gray cast irons. Cast irons are standard materials for some applications involving friction and wear, such as brake drums, piston rings, cylinder liners, and gears. Cast irons do well in such applications, and thus should be considered bearing materials. Gray iron bearings have been successful in refrigeration compressors where bearing pressures seldom exceed 650 psi for main bearings and 800 psi for connecting-rod bearings. Normally, journals in refrigeration compressors are either of steel, carburized and hardened to 55 to 60 Rockwell C (Rc), or of pearlitic malleable or ductile iron, hardened to 44 to 48 Rc and having a surface finish of 12 min. rms or better. Because of occasional dilution of oil with liquid refrigerant and heavy foaming of the oil, lubrication may become marginal for short periods. Fine-grain iron with uniformly distributed graphite flakes usually does well during these periods. Often, the bearings are phosphate-coated to improve seizure resistance. Such coating also creates a sponge-like surface that promotes oil retention.

For good wear resistance, gray cast iron should be pearlitic with randomly distributed graphite flakes. Cast irons have been heat treated to martensitic structures for use as cylinder liners, but benefits of such heat treatment have not been economically justifiable. Hardened cast iron has been successful in machine-tool ways.

Cemented carbides. Extremely hard materials, including cemented tungsten and titanium carbides and combinations, have been successful for various special bearing and seal applications. They have essentially no conformability and embeddability, but rank high in strength, hardness, corrosion resistance, and compatibility. They have been of most interest in high-temperature aerospace uses, but have also been useful in some machinery and machine-tool applications.

Nonmetallic bearing materials

Today, nonmetallic bearing materials are widely used. They have many inherent advantages over metals, including better corrosion resistance, lighter weight, better mechanical-shock resistance, and the ability to function with little or no lubricant. The major disadvantages of most nonmetallics are their high coefficients of thermal expansion and low thermal conductivity. For many years, carbon-graphites, wood, rubber, and laminated phenolics dominated nonmetallic bearing materials. In the early 1940s, development of nylon and PTFE (Teflon) gave designers two new nonmetallics with unique characteristics, especially the ability to operate dry.

A variety of polymer composites now serves bearings. Adding fiber reinforcements and fillers such as solid lubricants and metal powders to the resin matrix can significantly improve physical, thermal, and tribological properties.

Bearing-material selection

Bearing-material system selection for a given application and mechanical design for the bearing itself are interrelated processes. Neither is entirely straightforward, neither can be approached independently, and both require a good understanding of other interacting components of the machine system.

This article has considered the principles involved in bearing operation, but it hasn’t tried to discuss mechanical design factors in detail. Don’t expect to make final decisions on materials for specific applications based on this text alone.

Most plain-bearing manufacturers can help with mechanical design and with material selection. Because of the wide material selection most of these specialized producers offer and their broad experience in practical applications, you should take advantage of the engineering services they can provide.

*Material in this series is condensed from the chapter “Friction and Wear of Sliding Bearing Materials,” by George R. Kingsbury, ASM HANDBOOK, Friction, Lubrication and Wear Technology, ASM International, Materials Park, Ohio, 1992, pages 741-757. For ordering information about the entire book, contact ASM International, Materials Park, OH 44073-0002, ph. (216)-338-4634.

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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