Volume 18 - Friction, Lubrication, and Wear Technology Part 20 ppsx

Fig. 8 Compressive yield strength levels typically obtained in the individual layers of a trilayer construction bearing material Corrosion Resistance. Bearing failure due to corrosion alone is rare. Corrosion usually interacts with mechanical and thermal factors to produce failure by fatigue or seizure under conditions the bearing normally would be able to tolerate. To a considerable extent, bearing corrosion can be avoided by use of oxidation inhibitors in commercial lubrication oils, and by periodic oil changes. There are, however, many situations in which neither of these practices is dependable and where bearing materials with inherently high corrosion resistance should be used. Commercially pure lead is susceptible to corrosion by fatty acids. Lead-base and copper-lead bearing alloys can suffer severe corrosion damage in acidic lubricating oils. Tin additions in excess of 5% provide effective protection against this kind of corrosion, and for this reason tin is used extensively in lead-base bearing alloys. Both copper and lead are attacked by acidic oils that contain sulfur. This is of particular concern with copper-lead and leaded bronze bearing alloys. Effective protection can be obtained by employing layered construction, with a surface layer of either a lead alloy containing tin or a tin alloy. As long as the corrosion-resistant surface layer is intact, the underlying copper-lead alloy will not suffer damage by corrosion. Tin and aluminum bearing alloys are substantially impervious to corrosion by the products of oil oxidation, and they are used extensively in applications where the potential for lubricating oil corrosion is known to be high. Although lubricating oil oxidation and contamination are the most common causes of bearing damage by corrosion, other sources of bearing corrosion, such as seawater, animal and vegetable oils, and corrosive gas, should be recognized. Selection and specification of a bearing material for a specific application should take into account the anticipated service conditions under which the bearings will have to operate, and the potential for corrosion that these conditions may stimulate. Heat and Temperature Effects. The reduced mechanical strength of bearing liner materials at elevated temperatures is an important consideration in the selection of a bearing material for a given application. Fatigue strength, compressive yield strength, and hardness decrease significantly with increased bearing operating temperature. As shown by the softening curves in Fig. 9, lead-base and tin-base bearing alloys are most severely limited in this respect, and copper alloys the least. Fig. 9 Strength retention at elevated temperatures for selected bearing alloys. (a) Copper- base alloys. (b) Aluminum-base alloys. (c) Zinc-base alloys. (d) Lead-base alloys and tin-base alloys Load Capacity. The load capacity of a bearing material is defined as the maximum unit pressure under which the material can operate without excessive friction or wear damage. Load capacity ratings published as guides for machinery designers generally represent upper limits, which may be safely employed only under very good conditions of lubricant film integrity, counterface finish, mechanical alignment, and temperature control. In cyclic-load service (for example, in crankshaft bearings), load capacity is primarily limited by fatigue strength. In steady-load service, it depends more strongly on compressive yield strength, reflected in indentation hardness. In all cases, the strength of the material at operating temperature will be the determining factor that governs the choice of bearing material. Temperature and its control are therefore critically important to the successful operation of sliding bearings. Although useful to the designer as reference values, load capacity ratings must be recognized as imprecise and somewhat judgmental approximations. They are not guaranteed or directly measurable material properties. Bearing Material Systems Because of the widely varying conditions under which bearings must operate, commercial bearing materials have evolved as specialized engineering materials systems rather than as commodity products. They are used in relatively small tonnages and are produced by a relatively small number of manufacturers. Much proprietary technology is involved in alloy formulation and processing methods. Successful selection of a bearing material for a specific application often requires close technical cooperation between the user and the bearing producer. Single-Metal Systems Most single-metal sliding bearing are made from either copper alloys or aluminum alloys. Some use is also made of cast zinc-base alloys which serve as lower-cost substitutes for solid bronze. Commercially significant alloys that are used as single-metal bearings are listed in Table 2. Table 2 Single-metal bearing material systems Bearing performance characteristics (a) Load capacity rating (c) Class Material Compatibility Conformability Embeddability Fatigue strength Corrosion resistance (b) MPa ksi Typical applications 1 Commercial bronze (10% Zn) F E F D B 28 4 Bushings, washers Tin bronze High lead (16-25% Pb) D D D D E 21 3 Mill-machinery bearings, pump bearings, railroad-car bearings Medium lead (4- 10% Pb) E E E C D 28 4 Wrist pin bushings, pump bushings, electric-motor bushings, track- roller bushings, farm-equipment gear bushings, mill-machinery bearings, machine-tool bearings Low lead (1-4% Pb) F F F B B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool bearings, earth-moving machinery bearings, farm-equipment gear bushings 2 Unleaded F F F A B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool bearings, railroad-car wheel bearings 3 Aluminum alloy, low tin D D D D A 28 4 Connecting-rod main bearings, bushings, mill-machinery bearings Zinc alloy 12% Al E E F B E 28 4 Compressor bearings, pump bushings, mill-machinery bearings, earth-moving machinery bearings 4 27% Al E F F A E 34 5 Compressor bearings, pump bushings, mill-machinery bearings, earth-moving machinery bearings Porous metal Bronze C C C D B 14 2 Electric motor bushings, home appliance bearings, agricultural equipment bushings Iron D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural equipment bushings 5 Iron-bronze D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural equipment bushings (a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest). (b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils. (c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication. Wide ranges of compositions and properties are available in the older copper group. Brasses and bronzes have been widely used in bearing applications since the mid-1800s. Interest in the use of aluminum alloys was stimulated by World War II metal shortages and greatly accelerated by the commercial introduction of aluminum-tin bearing alloys in 1946. Since then, metal economics have dictated the use of aluminum alloy bearings, but brasses and bronzes continue to be preferred by many designers of heavy and special-purpose machinery. Single-metal systems do not exhibit outstandingly good surface properties, and their tolerance of boundary and thin-film lubrication conditions is limited. As a result, the load capacity rating for a single-metal bearing usually is low relative to the fatigue strength of the material from which it was made. Because of their metallurgical simplicity, these materials are well suited for small-lot manufacturing from cast tubes or bars, using conventional machine shop processes. Copper Alloys. Except for commercial bronze and low-lead tin bronze, copper alloys in single-metal systems are almost always used in cast form. This provides thick bearing walls ( 3.20 mm, or 0.125 in.) that are strong enough so that the bearing is retained in place when press fitted into the housing. Commercial bronze and medium-lead tin bronze alloys C83420 and C83520 are used extensively in the form of wrought strip for thin-wall bushings, which are made in large volumes by high-speed press forming. The relatively poor compatibility of these alloys can be improved by embedding a graphite-resin 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 are widely used in automotive engine starting motors. The lead in leaded tin bronzes is present in the form of free lead that is dispersed throughout a copper-tin matrix so that the bearing surface consists of interspersed areas of lead and bronze. In general, the best selection of materials from this group for a given application will be the highest-lead composition that can be used without risking excessive wear, plastic deformation, or fatigue damage. Aluminum Alloys. Virtually all solid aluminum bearings used in the United States are made from alloys containing from 5.5 to 7% Sn, plus smaller amounts of copper, nickel, silicon, and magnesium. Starting forms for bearing fabrication include cast tubes as well as rolled plate and strip, which can be press formed into half-round shapes. As is the case with solid bronze bearings, relatively thick bearing walls are employed in solid aluminum alloy bearings. The tin in these alloys is present in the form of free tin that is dispersed throughout an aluminum matrix so that the bearing surfaces consist of interspersed areas of aluminum and tin. Surface properties are enhanced by the free tin in much the same way that those of bronze are improved by the presence of free lead. The high thermal expansion of aluminum poses special problems in maintaining press fit and running clearances. Various methods are employed for increasing yield strength (for example, heat treatment and cold work) to overcome plastic flow and permanent deformation under service temperatures and loads. Zinc Alloys. During the past 20 years, zinc-aluminum-copper casting alloys have been used to replace cast bronze alloys in certain low-speed machinery bearing applications. This practice has advanced farthest in Europe, as an outgrowth of World War II material substitution efforts. These alloys do not contain any soft microconstituents that correspond to the lead used in bearing bronzes and to the tin in cast aluminum bearing alloys. To a considerable degree, compatibility of the zinc-base alloys seems to derive from their chemical behavior with hydrocarbon lubricants. Formation of a stable low shear strength film of zinc-base soap appears to be an important factor. Porous Metal Bushings. Oil-impregnated porous metal bushings can also be included in the single-metal systems category. The materials used for these bushings include unleaded and leaded tin-bronze, bronze-graphite, iron-carbon, iron-copper, and iron-bronze-graphite compositions. Oil content of these materials constitutes 8 to 30% of total volume. Bimetal Systems All bimetal systems employ a strong bearing back to which a softer, weaker, relatively thin layer of a bearing alloy is metallurgically bonded. Low-carbon steel is by far the most widely used bearing-back material, although alloy steels, bronzes, brasses, and (to a limited extent) aluminum alloys are also used. The bimetal bearing material systems currently in significant commercial use are classified in Table 3. Table 3 Bimetal bearing material systems Bearing performance characteristics (a) Load capacity rating (c) Class Backing layer Surface layer Compatibility Conformability Embeddability Fatigue strength Corrosion resistance (b) MPa ksi Typical applications Tin babbitt: 0.25-0.50 mm (0.010-0.020 in.) A A A F A 14 2 1 Steel 0.102 mm (0.004 in.) A B B E A 17 2.5 Connecting-rod and main bearings, camshaft bearings, electric-motor bushings, pump bushings, thrust washers Lead babbitt: 0.25-0.50 mm (0.010-0.020 in.) A A A F B 14 2 2 Steel 0.102 mm (0.004 in.) A B B E B 17 2.5 Connecting-rod and main bearings, camshaft bearings, transmission bushings, pump bushings, thrust washers Aluminum alloy: High tin B C C D A 41 6 Connecting-rod and main bearings, camshaft bearings, transmission bushings, pump bushings, thrust washers Medium-tin B C C C A 55 8 Connecting-rod and main bearings, camshaft bearings, transmission bushings, pump bushings, thrust washers High-lead B C C C A 55 8 Connecting-rod and main bearings, camshaft bearings, transmission bushings, pump bushings, trust washers Low-tin D D D C A 55 8 Camshaft bearings, transmission bushings, thrust washers 3 Steel Tin-free D D D C A 55 8 Camshaft bearings, transmission bushings, thrust washers Copper alloy: Copper-lead C C C C F 38 5.5 Connecting-rod and main bearings, camshaft bearings High-lead bronze D D D C E 45 6.5 Camshaft bearings, turbine bearings, pump bushings, thrust washers 4 Steel Medium-lead bronze E E E B D 55 8 Piston pin bushings, rocker-arm bushings, wear plates, steering-knuckle bushings, guide bushings, thrust washers 5 Medium-lead bronze Tin babbitt, 0.25-0.50 mm (0.010-0.020 in.) A A A F B 14 2 Connecting-rod and main bearings, thrust washers, railroad-car journal bearings, mill- machinery bearings 6 Medium-lead bronze Lead babbitt: 0.25- 0.50 mm (0.010- 0.020 in.) A A A F C 14 2 Connecting-rod and main bearings 7 Medium-lead- bronze Lead babbitt: 0.025 mm (0.001 in.) A C B C C 48 7 Connecting-rod and main bearings 8 Aluminum alloy, low tin Lead babbitt, 0.025 mm (0.001 in.) A C B D C 41 6 Connecting-rod and main bearings (a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest). (b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils. (c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication. The strengthening effect of a steel bearing back is illustrated clearly for classes 3 and 4 in Table 3; these ratings can be compared with those for the aluminum and copper alloy single-metal systems in Table 2. When steel bearing backs are employed, load-capacity ratings for both copper and aluminum alloys are sharply increased above those of the corresponding single metals without degrading any other properties. Similarly, in classes 1, 2, 5, 6, and 7, the strong bearing-back materials permit use of lead and tin alloys that have extremely good surface properties but that are so low in strength that they can be used as single-metal bodies only under very light loads. The strengthening effect of thin-layer construction on lead and tin alloys is illustrated in Table 3 (classes 1 and 2), where a 50% increase in load capacity is achieved by reducing babbitt layer thickness. Although similar behavior has been observed with aluminum and copper alloys, the thin-liner effects are less dramatic. Liner thicknesses employed with these stronger alloys are established by metal economics and manufacturing process considerations, rather than by strength/thickness relationships. Deterioration in surface properties with increasing liner alloy fatigue strength is clearly seen by the comparison of classes 1 and 2 with classes 3 and 4, and by comparisons within classes 3 and 4 (Table 3). In practice, only those systems with surface properties rated "D" or better are successful under boundary and thin-film lubrication conditions. This restricts the use of bimetal materials in connecting-rod and main bearings to loads of 55 MPa ( 8 ksi). Bronze-back bearings (see Table 3, classes 5, 6, and 7) do not exhibit combinations of performance characteristics substantially 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 relative ease with which worn bronze-back bearings can be salvaged by rebabbitting and remachining. From the standpoint of performance, the advantage of bronze over steel as a bearing-back material is the protection bronze affords against catastrophic bearing seizure in case of severe liner wear or fatigue. Similar protection is provided by the aluminum alloy bearing back in class 8. Although the surface properties of bronze bearing-back materials are not impressive, they are superior to those of steel, and these "reserve" bearing properties can be of considerable practical importance in large expensive machinery used in certain critical applications. Trimetal Systems Virtually all trimetal systems employ a steel bearing back, an intermediate layer of relatively high strength, and a tin alloy or lead alloy surface layer. The systems in current commercial use are listed by classes in Table 4. Most of these systems are derived from the bimetal systems of Table 3 (classes 3 and 4) by the addition of a lead-base or tin-base surface layer. Table 4 Trimetal bearing material systems Bearing performance characteristics (a) Load capacity rating (c) Class Backing layer Intermediate layer Surface layer Compatibility Conformability Embeddability Fatigue strength Corrosion resistance (b) MPa ksi Typical applications 1 Steel Medium-lead bronze Tin babbitt, 0.25- 0.50 mm (0.010- 0.020 in.) A A A F B 14 2 Large connecting-rod and main bearings, bushings 2 Steel High-lead bronze Tin babbitt, 0.25- 0.50 mm (0.010- 0.020 in.) A A A F B 14 2 Large connecting-rod and main bearings, bushings 3 Steel Copper-lead Lead babbitt, 0.075 mm (0.003 in.) A B B E C 21 3 Connecting-rod and main bearings, camshaft bearings 4 Steel Copper-lead Lead babbitt, 0.025 mm (0.001 in.) A C C B C 59 8.5 Connecting-rod and main bearings, bushings 5 Steel High-lead bronze Lead babbitt, 0.025 mm (0.001 in.) A C C B C 83 12 Connecting-rod and main bearings, thrust washers 6 Steel Medium-lead bronze Lead babbitt, 0.025 mm (0.001 in.) A D D A C 83 12 Connecting-rod and main bearings 7 Steel Aluminum, low tin Lead babbitt, 0.025 mm (0.001 in.) A C C B B 55 8 Connecting-rod and main bearings 8 Steel Aluminum, tin free, low alloy Lead babbitt, 0.025 mm (0.001 in.) A C C B B 55 8 Connecting-rod and main bearings 9 Steel Aluminum, tin free, low alloy, precipitation hardened Lead babbitt, 0.025 mm (0.001 in.) A C C B B 76 11 Connecting-rod and main bearings 10 Steel Aluminum, tin free, high alloy Lead babbitt, 0.025 mm (0.001 in.) A C C B B 76 11 Connecting-rod and main bearings 11 Steel Silver Lead babbitt, 0.025 mm (0.001 in.) A D D A B 83 12 Connecting-rod and main bearings for aircraft reciprocating engines (a) Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest). (b) Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils. (c) Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication. [...]... 9 0-1 00 1 3-1 4 7 5-8 5 1 1-1 2 Ultimate strength MPa 31 0-4 40 40 0-5 80 24 0-3 10 27 5-2 90 31 0-4 40 24 0-2 55 185 -2 10 tensile Hardness, HB ksi 4 5-6 4 5 8-8 4 3 5-4 5 4 0-4 2 4 5-6 4 3 5-3 7 2 7-3 0 7 8-1 15 8 0-1 60 7 0-1 70 6 5-7 7 7 8-1 15 6 0-6 5 5 0-1 30 4 8-5 5 5 5-9 0 3 0-8 0 0.1% offset Test information of this kind is helpful in the material selection process as a supplement to information generated in dynamic rig tests and. .. ksi 7 0-1 40 1 0-2 0 8 0-1 40 1 2-2 0 Ultimate strength MPa 10 0-1 30 12 5-2 20 14 0-1 70 tensile Hardness, HB ksi 1 5-1 9 1 8- 32 2 0-2 5 2 5-4 0 4 0-5 0 5 0-6 0 4 5-6 5 4 0-5 5 3 5-4 5 3 5-6 5 0.2% offset Product Applications The majority of the current commercial applications of aluminum-base bearing alloys involve steel-backed bimetal or steel-backed trimetal bearings To determine the most cost-effective aluminum... content, and chromium-rich M23C6 is common in the low-carbon alloys Table 5 Composition of selected nonferrous alloys Alloy Composition, wt% Fe Cr Mo W Cobalt-base/carbide type ERCoCr-A 28 5 ERCoCr-B 29 8 ERCoCr-C 31 13 ERCoCr-E 27 6 Cobalt- and nickel-base/Laves type 9 29 T-400 16 33 T-700 18 29 T-800 Nickel-base/boride type 1.5 7.5 Alloy 40 11 ERNiCr-B 3 16... 50, 55, and 60 wt% tungsten carbide, with the carbon steel tube making up the balance For each composition, several carbide size ranges are available As an example, for the 60% WC oxyacetylene welding consumable, four mesh size ranges are available: AWS designation RWC-12 /20 RWC -2 0/ 30 RWC-30/40 RWC-40/ 120 Mesh size range 1 2-2 0 2 0-3 0 3 0-4 0 4 0-1 20 The same composition is also available in flux-coated... carbon content and the resistance to low-stress abrasion for the cobalt-base/carbidetype alloys (ERCoCr-A, -B, -C, and -E) From Fig 5 and 6, it is evident that the cobalt-base matrix (solid solution) is responsible for the excellent self-mated sliding properties and cavitation erosion resistance of these alloys Additional information is available in the article "Friction and Wear of Cobalt-Base Wrought... mill hot-work rolls (which demand considerable hot hardness, resistance to oxidation, and resistance to thermal fatigue) both ER 420 and EFe3 have been found suitable Other applications for the metal-to-metal wear alloys in Table 2 include tractor rollers and crane wheels (EFe2), pincer guide shoes (EFe3), and blast furnace bells (ER 420) Metal-to-Earth Abrasion Alloys The high-chromium irons encompass a... hardfaced with two-layer open arc deposit welding process (a) ERFeCr-A3 (b) ERFeCr-A4(Mod) (c) ERFeCr-A2 300× In addition to M7C3, deposits of ERFeCr-A2 contain small quantities of M6C and deposits of ERFeCr-A4(Mod) contain small quantities of both M6C and M3C With regard to the matrix, both ERFeCr-A3 and ERFeCr-A4(Mod) exhibit a facecentered cubic (fcc) austenitic structure as deposited ERFeCr-A2 is largely... 1990 "Standard Designations for Copper and Copper Alloys," Application Data Sheet, Copper Development Association Inc., 1990 "Standards Handbook, Part 2 Alloy Data, Wrought Copper and Copper Alloy Mill Products," Copper Development Association, 1985 "Standards Handbook, Cast Copper and Copper Alloy Products, Part 7 Alloy Data, Copper Development Association, 1978 "Standard Specification for Car and Tender... Alloy Composition, wt% Fe Cr C Build-up weld overlay bal 2 0.1 EFe 1(a) bal 4 0.8 EFeMn-C(a) EFeMn-Cr(a) bal 15 0.5 Metal-to-metal weld overlay bal 3 0.2 EFe2(a) bal 6 0.7 EFe3(a) bal 12 0.3 ER 420( b) (a) (b) (c) (d) Hardness, HRC Abrasion, volume loss Low-stress(c) High-stress(d) 3 3 -3 mm in × 10 mm3 in.3 × 10 -3 Si Mn Mo Ni 1.0 1.3 1.3 1 14 15 1.5 2.0 4 1 37 18 24 88 65 93 5.4 4.0 5.7 49 57 46... Bearing and Bushing Alloys, SAE J460e, SAE Information Report, SAE Handbook 1990, Part 1, Society of Automotive Engineers, 1990 E.R Booser, Bearing Materials and Properties, Mach Des., 10 Mar 1966, p 2 2-2 8 K.G Budinski, Surface Engineering for Wear Resistance, Prentice-Hall, 1988, p 1 5-4 2 Bushing and Thrust Washer Design Manual, Clevite Engine Parts Div., Gould Inc., 1973, p 2 3-3 2 T Calayag and D Ferres, . . . . 5 0-6 0 Cast tubes 7 0-1 40 1 0-2 0 12 5-2 20 1 8- 32 4 5-6 5 Wrought plate 8 0-1 40 1 2-2 0 14 0-1 70 2 0-2 5 4 0-5 5 Low-tin alloys Steel backed . . . . . . . . . . . . 3 5-4 5 Tin-free alloys. . . . . . . 31 0-4 40 4 5-6 4 7 8-1 15 Wrought strip . . . . . . 40 0-5 80 5 8-8 4 8 0-1 60 Unleaded tin bronzes Cast tubes 9 0-1 25 13 -1 8 24 0-3 10 3 5-4 5 7 0-1 70 Low-lead tin bronzes. Cast tubes . . . . . . 27 5-2 90 4 0-4 2 6 5-7 7 Wrought strip . . . . . . 31 0-4 40 4 5-6 4 7 8-1 15 Cast tubes 9 0-1 00 1 3-1 4 24 0-2 55 3 5-3 7 6 0-6 5 Medium-lead tin bronzes Steel