Selection of bearing materials for specific applications involves a scrutiny of the following characteristics:
(1) Compatibility
(2) Embeddability and conformability (3) Corrosion resistance
(4) Compressive and fatigue strength.
TABLE 7.3 List of Synthetic Lubricantsa
Flash point
◦F
Pour point
◦F
Approx.
cost per
gallon Typical uses Kinematic viscosity, cSt
Type 210◦F 100◦F −65◦F Diester
Turbo oil 15 3.6 14.2 12,600 430 −90 $10.00 MIL-L-7808 high-load capacity, high-temperature jet engine oil MIL-0-6085 3.5 13.5 10,000 450 −90 10.00 Low volatility aircraft hydraulic
and instrument oil MIL-0-6387 4.6 15.8 5,000 410 <−80 — Aircraft hydraulic fluid for
alternator drives Phosphate
Tricresyl phosphate 3.8 30.7 — 465 — 3.60 Low flammability hydraulic fluid for diecasting machines Skydrole 3.85 15.5 >20,000 355 70 12.00 Nonflammable aircraft
hydraulic oil
Pydraul F-9 5.8 54 — 430 +5 3.75 Nonflammable hydraulic oil for diecasting machines, punch pressures etc.
Silicon
SF-96 (40) 16 40 850 600 <−100 30.0 Low-torque aircraft oil bearings, air craft hydraulic, and damping fluid
SF-96 (300) 122 300 7,000 605 <−55 30.00 Heat transfer, hydraulic, and damping applications SF-96 (1,000) 401 1,000 20,000 605 <−55 30.00 Heat transfer, hydraulic, and
damping application DC-710 40 275 — 575 −10 40.00 Heat transfer, high-temperature
trolley bearings Silicate
OS-45 3.95 12.4 2,400 — <−85 20.00 Wide-temperature-range aircraft Orsil BF-1 2.4 6.8 1,400 395 <−100 hydraulic fluid
Polyglycol
LB-140X 5.7 29.8 — 345 −50 2.40 Water-insoluble oils used for LB-300X 11.0 65.0 — 490 −40 2.40 internal combustion engines LB-650X 21.9 141.0 — 490 −20 2.40 (Prestone Motor Oil),
high-temperature hearings in ovens and furnaces and gears 50-HB-55 2.4 8.9 — 260 −85 2.40 Water-soluble oils used in wire 50-HB-280X 11.5 60.6 — 500 −35 2.40 drawing, metal forming, and 50-HB-2000 72 433 — 545 −25 3.00 some machine tools
Hydrolube — 666.3 — None −55 2.50 Water–polyglycol mixture used as
300N nonflammable hydraulic fluid
in die easting and machine tool work
Chlorinated aromatics
Aroclor 1248 3.1 48 — 380 20 2.30 Die-casting machines and Aroclor 1254 6.1 470 — None 50 2.30 high-pressure compressors Polybutenes
No. 8 7.9 72 — 310 −40 — Electrical oils, hydralic and shock No. 20 106 3,600 — 410 10 1.05 absorbing fluids, kilns and No. 128 4,000 — — 450 70 1.40 ovens, refrigerator compressors aFor a more detailed discussion, the interested reader is directed to Zaretsky, E.V., Ed. “Tribology for Aerospace Application,”
STLE SP-37, 1997, pages 137–168.
Principles of Gas Turbine Bearing Lubrication and Design 7-9
TABLE 7.4 General Types of Additives With Typical Chemical Compositions Function Typical Chemical Type Oxidation inhibitor Phenolics, dithiophosphate Detergent Calcium petroleum sulfonate
Rust inhibitor Organic acids, sodium petroleum sulfonate Wear preventive Trieresyl phosphate
Bounday lubrication Chlorinated naphthalene, sulfurized hydrocarbon Viscosity index improver Polyisobutylene
Pour-point depressant Polymethnerylate Defoaming agent Silicone oil
Source: Walton, J.F., and Heshmat, H., “Complaint Foil Bearings For Use in Cryogene Turbopumps,” Proceedings of Advanced Earth-to-Orbit Propulsion Technology Conference. Held at NASA/MSFC May 17–19, 1994, NASA CP3282, Vol. 1, Sept. 19, 1994, pp. 372–381.
In hydrodynamic bearings, the most relevant items are the allowable maximum pressures before the material begins to deform or flow, and the value of Tmax it can endure. For compressive strength, an alloy with intermediate strength is desirable; an alloy too low in strength is prone to extrude under load, while too strong a metal, being brittle, may crumble under impact loading. Fatigue strength is particularly important in applications with dynamic loading in order to prevent the formation of cracks or surface pits. The use of a thin soft layer bonded to a hard backing metal often gives the desired combination of fatigue and compressive strength; in such cases, however, the fatigue strength of the bond itself requires attention. When a material has low corrosion resistance, difficulties can be minimized by using oils with good oxidation inhibitors and by maintaining low bearing temperatures.
7.2.2.1 Babbitts
The most common bearing materials are babbitts, either tin based or lead based. The detailed properties of babbitts are given in Table 7.5. Babbitts can operate under conditions of boundary lubrication or dirty operation. They have excellent compatibility and nonscoring characteristics and are outstanding in tolerating errors in construction and operation. Their deficiencies with regard to fatigue strength can be improved by using an intermediate layer of high-strength material between a steel backing and a thin babbitt layer. Many of these, known under the name of trimedal bearings, use the following construction (1) a low-carbon–steel back, (2) an intermediate layer of copper or bronze, and (3) an overlay of lead-base babbitt from 0.001 to 0.020 in. thick. The intermediate layers increase the mechanical strength of the babbitt bearing and also provide reasonable good bearing surfaces in cases where the thin babbitt surface layer is destroyed in operation.
7.2.2.2 Nonbabbitt Bearing Materials
Other common bearing materials used, whenever babbitt cannot be employed are:
Bronze: Bearing bronzes may be grouped into lead bronzes, tin bronzes, and high-strength bronzes.
The strength and high-temperature properties generally improve as one proceeds from high-lead to high- tin to various high-strength bronzes. However, there is a loss in the compatibility properties as the amount of lead decreases. For this reason it is generally advisable to use the highest lead content and the softest bronzes while still retaining the necessary strength and load-carrying capacity.
Silver: Silver bearings normally consist of electro-deposited silver on steel backings with an overlay of 0.001 to 0.005 in. of lead. Indium is usually flashed on top of the lead overlay for corrosion protection.
They have outstanding metallurgical uniformity, excellent fatigue resistance and thermal conductivity, can carry very high loads, and can be operated at high temperatures. Although the lead coating helps to relieve problems of poor embeddability and conformability, silver bearings are not recommended for applications where misalignment and dirt are present [30].
HandbookofLubricationandTribology TABLE 7.5 Composition and Physical Properties of Babbitts
Tin-base babbitts
Yield pointa
Ultimate strengtha
Composition, % psi psi
Brinell hardness
Melting point
Complete liquefaction
Alloy
Specific
gravity Cu Sn Sb Pb 66◦F 212◦F 66◦F 212◦F 68◦F 212◦F ◦F ◦F
1 7.34 4.56 90.9 4.52 None 4400 2680 12,850 6050 17.0 8.0 433 700
2b 7.39 3.1 39.2 7.6 0.03 6100 3000 14,900 8700 24.5 12.0 466 669
3b 7.46 8.3 83.4 8.3 0.03 6800 3100 17,600 9900 27.0 14.5 464 792
4 7.52 3.0 75.0 11.6 10.2 5550 2150 18,150 8900 34.5 12.0 363 583
5 7.75 2.0 65.5 14.1 18.3 2150 2150 18,060 8750 22.5 10.0 358 565
Lead-base babbitts
Yield pointa
Ultimate strengtha
Composition, % psi psi
Brinell hardness
Melting point
Complete liquefaction
Alloy
Specific
gravity Cu Sn Sb Pb As max 66◦F 212◦F 66◦F 212◦F 68◦F 212◦F ◦F ◦F
6(e) 9.33 1.5 20 15 63.5 0.15 3,800 2,050 14,550 8,060 21.0 10.6 358 581
7(f) 9.73 0.50 10 15 75 0.60 3,550 1,600 15,650 6,150 22.5 10.5 464 514
8 10.04 0.50 5 15 80 0.20 3,400 1,760 15,600 6,150 20.5 9.5 459 522
10 10.07 0.50 5 15 83 0.60 3,550 1,850 15,450 5,450 17.6 9.0 468 507
11 10.28 0.50 — 15 85 0.25 3,050 1,400 12,800 5,100 15.0 7.0 471 504
12 10.67 0.50 — 10 90 0.25 2,800 1,250 12,900 5,100 14.5 6.5 473 498
15(g) 10.05 0.5 1 15 82 1.40 21.0 13.0 479 538
16(f) 9.88 0.5 10 12.5 77 0.20 27.5 13.6 471 495
19 10.50 0.50 5 9 95 0.20 15,600 6,100 17.7 8.0 462 495
Note:ain composites.
bBabbitts predominantely used by electric utilities (ASTM alloy B23).
© 2006 by Taylor & Francis Group, LLC
Principles of Gas Turbine Bearing Lubrication and Design 7-11
Table 7.6 Approximate Temperature Limitations of Various Bearing Materials
Babbits Lead base Tin base Sintered Metals Bronze Composites Iron
Aluminum alloys Copper–lead Bronzes Leaded Tin Aluminum Cast Iron Hardened steels Tool steels
Carbongraphites (Untreated) Carbongraphites (Treated) Stellites
Nickel-based superalloys Metal-bonded carbides Metal-bonded oxides Ceramics
Temperature °F
Temperature °C
0 200 400 600 800 1000 1200
0 200 400 600
1400
Source: Walton, J.F. and Heshmat, W., “Complaint Foil Bearnags For Use in Cryogenic Turbopumps,” Proceedings of Advanced Earth-to-Orbit Propulsion Technology Conference. Held at NASA/MSFC May 17–19, 1994, NASA CP 3282, Vol.1, Sept. 19, 1994, pp. 372–381.
Aluminum: Aluminum bearing alloys offer excellent resistance to corrosion by acidic oils, good load- carrying capacity, superior fatigue resistance, and good thermal conductivity. A smooth machine finish of the running surface is recommended along with a clean lubricant, a shaft hardness of 300 Brinell or higher, and a large enough clearance to allow for the high thermal expansion of the aluminum. Sometime the aluminum is overlaid with a thin coating of lead babbitt. This overlay assists in making up for the otherwise poor embeddability and conformability characteristics of the aluminum.
The range of temperatures that these various bearing materials, as well as some other materials, can endure is given in Table 7.6.