This is the fourth of a series on principles and characteristics of metallic plain bearing materials and how to optimize them in given uses. See August 1994 PTD for Part 3.
In Part 3, we discussed single-metal bearing-material systems. Now, we cover multilayer systems.
Bimetal systems. Every bimetal system uses a strong bearing back to which a softer, weaker, thin layer of a bearing alloy is metallurgically bonded. Low-carbon steel is by far the most common bearing- back material. Alloy steels, bronzes, brasses, and (to lesser extent) aluminum alloys are also used. Table 2 classifies bimetal bearing material systems in significant commercial use.
Classes 3 and 4 in Table 2 show the strengthening effect of a steel bearing back. Compare that with the aluminum and copper alloy single-metal systems in Table 1. With steel bearing backs, loadcapacity ratings for copper and aluminum alloys increase sharply over those of the corresponding single metals, without degrading any other property. Similarly, in classes 1, 2, 5, 6, and 7, strong bearingback materials permit use of lead and tin alloys that have extremely good surface properties but such low strength that they can serve as single-metal bodies only at very light load.
Classes 1 and 2 show the strengthening effect of thin-layer construction on lead and tin alloys, where a 25% load-capacity increase is gained by reducing babbitt layer thickness. Although similar behavior has been seen with aluminum and copper alloys, the thin liner effects are less pronounced. Liner thicknesses for these stronger alloys are established by metal economics and manufacturing process considerations rather than by strength-and-thickness relationships.
In Table 2, you can see deterioration in surface properties with increasing lineralloy fatigue strength by comparing classes 1 and 2 with classes 3 and 4, and by comparison within classes 3 and 4. In practice, only systems with surface property ratings of D or better succeed in boundary and thin-film lubrication.
Bronze-back bearings of classes 5, 6, and 7, Table 2, do not show performancecharacteristic combinations much different from those of steel-back bearings. The practical advantages of bronze as a bearing-back material lie partly in the economics of small-lot manufacturing and partly in the ease with which rebabbitting and remachining can salvage worn bronze-back bearings. Regarding performance, bronze beats steel as a bearing-back material in the protection it offers against catastrophic bearing seizure with severe liner wear or fatigue. The aluminum alloy bearing back in class 8 provides similar protection.
The surface properties of bronze bearing- back materials are not impressive, but they exceed those of steel, and these “reserve” properties can be important in some large, expensive machines.
Trimetal systems. Nearly every trimetal system uses a steel bearing back, a high-strength intermediate layer, and a tin alloy or lead alloy surface layer. Table 3 lists systems in commercial use. Most are derived from the bimetal systems of Table 2, classes 3 and 4, by adding a lead or tin-base surface layer.
Strengthening effects of thin-layer construction are notable in systems that have electroplated lead alloy surface layers no more than about 0.001 in. thick, Table 3, classes 4 through 11. In comparing fatigue-strength and load-capacity ratings of these systems with those of corresponding bimetal systems, Table 2, you see that the thin lead alloy surface layer upgrades not only surface properties, but also fatigue strength. The gain in fatigue strength is at least partly due to elimination of stress raisers from which fatigue cracks can propagate.
Class 1 and class 2 trimetal systems comprise leaded bronze intermediate layers with relatively thick tin alloy surface layers. They are an evolution from bronze-back babbitt construction wherein steel has replaced most of the bronze. This produces the expected economy and bearing-back yield strength, but keeps the desirable “reserve” bearing properties of bronze-back construction.
Class 11 trimetal systems, which have silver intermediate layers, are too costly for most commercial uses. However, they offer an unequaled combination of high load capacity and corrosion resistance. They are still in limited use in aircraft radial- piston engines.
Trimetal systems with electroplated lead-base surface layers and copper or aluminum alloy intermediate layers offer the best combinations of cost, fatigue strength, and surface properties. They tolerate boundary and thin-film lubrication well, and thus can be used at higher loads than any bimetal system. Although more costly than corresponding steelback bimetal systems, they serve in some high-volume automotive uses as well as larger mobile and stationary engines. A highly developed body of mechanical, metallurgical, and chemical manufacturing technology is established in plain bearings, and it permits mass production of precision trimetal bearings without severe cost penalty.
*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.