Bearings for Extreme Special Environments-Part 1 Bearing Materials and Solid Lubricants

Transcription

1 Bearings for Extreme Special Environments-Part 1 Bearing Materials and Solid Lubricants H. TAKEBAYASHI * *Product Engineering Center, EXSEV Project Office Koyo was the first company to develop and produce EXSEV bearings (bearings for extreme special environments) on a commercial scale. EXSEV bearings are used in such environments as high temperatures, vacuums, corrosive environments and clean rooms. This paper is the first in a series of reports on EXSEV bearings to be presented in the Koyo Engineering Journal. Herein characteristics of bearing materials and solid lubricants used for the EXSEV bearing series are shown. Materials: ferrous alloys, nonferrous alloys and ceramics Solid lubricants: the soft metals, layer lattice substances and polymers Life: service life formulas of solid lubricated bearings 1. Introduction Rolling bearings have come to be used in environments and under conditions much severer than in the past as a result of recent advances in technology, and ordinary high-carbon steel rolling bearings (hereafter referred to as "bearings") using grease or oil no longer suffice. Requests for advanced bearings from the customers in advanced-technology fields, such as electronic parts, equipment for manufacturing medical supplies, etc., semiconductors, liquid crystal displays, vacuum and space instruments, continue to increase, with customers stating that they want to use grease but the conditions are severe; the scattering of oil and dust generation from normal bearings is a problem; conventional bearings can't be used because of the corrosive environment: or there are no bearings that meet nonmagnetism and insulation requirements. Perceiving this trend at an early stage, Koyo developed the Koyo EXSEV (Extreme Special Environment) bearings series that can be used in extreme environments. Koyo's series of EXSEV bearings includes bearings for use in vacuums, ceramic bearings, low-dust-generation bearings, etc. and many technologies are applied to EXSEV bearings. The aim of this paper is to introduce Koyo EXSEV bearings and explain their suitability for use in a wide range of products and operating conditions. Part 1 of this paper introduces the bearing materials and solid lubricants used in Koyo EXSEV bearings. 2. General Explanation of Koyo EXSEV Bearings Fig. 1 shows Koyo's lineup of EXSEV bearings. Koyo developed these bearings for extreme special environments, defined as vacuums, corrosive environments, clean rooms, magnetic environments and high-temperature environments, and for uses requiring high-speed performance. For example, oil or grease cannot be used in many cases if bearings are used in a vacuum environment. This necessitates the use of a solid lubricant. The type of solid lubricant is important and must be selected in accordance with the vacuum pressure. Also, if the bearings are to be used in a corrosive environment, a material with superior corrosion resistance must be used as the bearing material, since conventional bearing steel will rust. In other words, when selecting bearings for use in extreme special environments, careful consideration must be given to the bearing material and type of solid lubricant. Koyo succeeded in commercializing the bearings for extreme special environments ahead of other makers, and as a result, it is able to choose the most appropriate bearing material and solid lubricant for each application based on the wealth of technical know-how it has accumulated through many years of development and supply. 3. Materials Used in EXSEV Bearings Materials used in EXSEV bearings suitable for vacuums, corrosive environments, high-temperature environments, etc. are introduced here. Materials mainly used are metal materials (ferrous alloys, nonferrous alloys) and ceramics Ferrous Alloys Table 1 gives the representative ferrous alloys used for the bearing rings and rolling elements of bearings for extreme special environments 1). The main ferrous alloys are high-carbon chrome bearing steel (SUJ2), martensite stainless steel (SUS440C), precipitation-hardening stainless steel (SUS630), and highspeed tool steels (M50, SKH49). 52 KOYO Engineering Journal English Edition No.156E (2000)

2 Usable in vacuums Corrosion-resistant Low dust generation Nonmagnetic Usable in high-temp. environments High-speed rotation Fig. 1 Koyo EXSEV (extreme special environment) bearings Table 1 Typical ferrous alloys for EXSEV bearings " : Excellent # : Good Hardness Modulus of longitudinal elasticity, GPa Linear expansion coefficient, 10 6 /; Load capacity Abrasion resistance Gas emission Remarks High-carbon chrome bearing steel (SUJ2) 61 HRC " # " Martensite stainless steel (SUS440C) 60 HRC " # " Precipitation-hardening stainless steel (SUS630) 40 HRC # # " High-speed tool steel (M50) 61 HRC " # " High-speed tool steel (SKH4) 64 HRC " # " Standard material for ordinary bearings Standard material for EXSEV bearings High-carbon chrome bearing steel (SUJ2) is a standard bearing material, and it is used for extreme special environment bearings in some cases. However, Koyo decided to make martensite stainless steel (SUS440C) the standard material for EXSEV bearings because of its performance in vacuums, corrosive environments, high-temperature environments, etc. Martensite stainless steel (SUS440C) and precipitationhardening stainless steel (SUS630) are corrosion-resistant bearing materials. Because the hardness of martensite stainless steel (SUS440C) is minimum HRC60, this material has sufficient load capacity as a bearing, but its corrosion resistance is not sufficient for highly corrosive environments. For applications requiring high corrosion resistance, precipitation-hardened stainless steel (SUS630) is used, although its load capacity is inferior to that of martensite stainless steel (SUS440C) at a hardness of about 40HRC. High-speed tool steels M50 and SKH4 are used as hightemperature bearing materials. Fig. 2 shows the hightemperature hardness of ferrous alloys 2). The hardness of ordinary-bearing standard material SUJ2 drops dramatically when the temperature becomes high, and load capacity and life become a problem. On the other hand, the hardness of high-speed tool steels M50 and SKH4 is about 60HRC even when the temperature is 400;~500;. High-speed tool steels M50 and SKH4 are therefore used as high-temperature bearing materials because the hardness doesn't drop even at high temperatures. Also, because the heat resistance of martensite stainless steel (SUS440C) is comparatively superior, it is used for medium-high-temperature applications. KOYO Engineering Journal English Edition No.156E (2000) 53

3 Hardness, HRC SUJ2 440C SKH4 M50 Beryllium copper alloy and titanium alloy are nonmagnetic bearing materials. Because beryllium copper alloy in particular can be hardened to HRC40 or more by solution heat treatment and aging, it is widely used as a nonmagnetic bearing material. Recently, however, there is a trend against the use of beryllium copper because it contains beryllium, environmental load substances. (Although beryllium copper is nontoxic, beryllium by itself is toxic.) Temperature, : Fig. 2 Hardness of ferrous alloys at high temperatures 3. 2 Nonferrous Materials Table 2 shows representative nonferrous materials used for corrosion-resistant bearings, heat-resistant bearings, or nonmagnetic bearings and other bearings for extreme special environments 2). All nonferrous materials shown in Table 2 have corrosion resistance equal to or better than that of austenite stainless steel. The nickel-based alloy (hastelloy C) and titanium alloy show particularly good corrosion resistance. The cobalt-based alloy (stellite 1) has superior roomtemperature hardness and high-temperature hardness, and in the high-temperature range of 600; or more in particular, it has high-temperature hardness superior to that of high-speed tool steel Ceramic Materials Table 3 shows the features of various ceramic materials 1). Ceramic materials include silicon nitride (Si 3 N 4 ), zirconia (ZrO 2 ), silicon carbide (SiC) and alumina (Al 2 O 3 ). Mechanical strength is in the order alumina < silicon carbide < silicon nitride < zirconia, and corrosion resistance is in the order silicon nitride < zirconia < silicon carbide = alumina. Fig. 3 shows the results of rolling life testing, in which a thrust type bearing tester was used to evaluate the applicability of various types of ceramic materials for bearings 1). Life is indicated in the figure as that point in time when damage occurred on the ceramic flat plate as test load was increased each stress cycles. Test results showed load resistance and rolling life to be in the following order (best to worst) : silicon nitride, zirconia, silicon carbide, and then alumina. Table 2 Typical nonferrous alloys for EXSEV bearings Material Cobalt-based alloy Type Base metal Main additives (%) Hardness (HRC) Remarks Stellite 1 Co 30Cr, 12W, 2.5C 57 Good high-temperature hardness Good abrasion resistance Stellite 6 Co 28Cr, 4W, 1C 47 High toughness than stellite 1 Nickel-based alloy Hastelloy C Ni 17Mo, 16Cr, 5Fe, 4.5W 29 Copper alloy Beryllium copper alloy Cu 2Be, 0.2Co 40 Titanium alloy 6A1/4V alloy Ti 6Al, 4V 38 Good corrosion resistance Non-magnetic Good electrical conductivity Non-magnetic, good corrosion resistance Light weight (specific gravity half of steel) Table 3 Characteristics of fine ceramics Ceramic material Silicon nitride Zirconia Silicon carbide Alumina Item Si 3 N 4 ZrO 2 Sic Al 2 O 3 Density, g/cm Linear expansion coefficient, 1/; Vickers hardness, HV Modulus of longitudinal elasticity, GPa Poisson's ratio point bending strength, MPa Fracture toughness, Mpa m 1/ KOYO Engineering Journal English Edition No.156E (2000)

4 1 250 Oil lubrication Zirconia (ZrO2) Silicon nitride (Si3N4) Lubricant Ball (3/8") Load (per ball), N Alumina (Al2O3) Silicon carbide (Sic) Flat plate test-piece (ceramic) Load Number of stress cycles, 10 7 Fig. 3 Life test results of ceramics Table 4 shows the suitability of each type of ceramic for use in bearings based on the characteristics of each type of ceramic materials and the results of the rolling life test shown in Fig. 3 1). This table is Koyo standard when selecting ceramic materials for bearings. Because the load capacity and rolling life of silicon nitride are equal to or better than those of highcarbon chrome bearing steel, silicon nitride is commonly used as a bearing material. Silicon nitride, however, has a problem with corrosion resistance in high-temperature corrosive environments such as acid or alkali, and therefore zirconia or silicon carbide is the best material for such applications. However, it is necessary in such cases to limit bearing load because the load capacity and rolling life of zirconia and silicon carbide are inferior to those of silicon nitride. Because the load capacity (strength) of alumina is low, it cannot be used basically as a bearing material. 4. Solid Lubricants Used in Extreme Special Environments In this section various types of solid lubricants are introduced and an equation for predicting the life of bearings using solid lubricants presented Various Types of Solid Lubricants In many cases greases and oils cannot be used in extreme special environments such as vacuums and high-temperature and low-temperature environments. However, unlike greases and oils, solid lubricants show lubricating properties that change according to specific bearing usage conditions and environment. It is therefore necessary to select the solid lubricant most suitable for the specific usage condition or environment. Table 4 Applicability of ceramics to bearings " : Suitable # : Suitable for some applications : Not suitable Application to rolling bearings Judgment Performance/application Features Load capacity and life equal to or better than those of High-speed rotation Suitable for high vacuum Silicon nitride " bearing steels Corrosion resistance Heat resistance Si 3 N 4 Suitable for applications requiring high performance Nonmagnetic High rigidity Zirconia Load must be limited # ZrO 2 Usable in strong corrosive chemicals High corrosion resistance Silicon carbide Load must be limited High corrosion resistance # SiC Usable in strong corrosive chemicals Usable in ultrahigh temperatures Alumina Al 2 O 3 Not suitable for rolling bearings KOYO Engineering Journal English Edition No.156E (2000) 55

5 Table 5 shows representative solid lubricants used for bearings for extreme special environments 1). The table gives crystal structure, thermal stability, friction coefficient, load capacity, dust generation, and gas emission for various types of solid lubricants such as soft metals, layer lattice structure materials, high-polymer materials, etc. This information is used as a basis for selecting the solid lubricant most suitable for the particular usage and environment conditions. Because soft metals silver (Ag) and lead (Pb) have superior lubrication film self-restoration and an extremely low vapor pressure, they are used as lubricants for ultrahigh-vacuum bearings, high-temperature bearings, etc. In many cases the rolling elements (balls) are coated by ion plating method or sputtering method for the use in bearings. However because silver (Ag) has the drawback of being easily oxidized, caution must be exercised regarding usage in air. Molybdenum disulfide (MoS 2 ) of layer lattice structure is well known as a solid lubricant. The retainer and bearing ring are coated using the sputtering method or baking method. Molybdenum disulfide is mainly used for vacuum bearings. Tungsten disulfide (WS 2 ) and graphite (C) are sometimes used as retainer materials (composite materials) and are mainly used as lubricants for high-temperature bearings. Furthermore, the high-polymer materials such as polytetrafluoroethylene (PTFE) and polyimide are sometimes used as coatings for rolling elements, retainers, etc. and are sometimes used as a retainer material. Because these materials have superior chemical resistance and good adhesion and relatively unaffected by the environment, they are used for bearings requiring low dust generation and bearings repeatedly moved between air and vacuum environments Life Equations Bearings using solid lubrication, such as those for extreme special environments, are generally used for comparatively light loads of 10% or less of the static load rating. In the case of bearings using solid lubrication, stable rotational performance can be obtained as long as solid lubricant remains on the rolling surfaces, but once this solid lubrication is gone, metal-to-metal contact occurs, increasing rotational torque drastically and ending the life of the bearing quickly. Because life differs according to the type of solid lubrication and usage conditions, it is necessary to predict life of solid lubrication bearings based on various life test results Life of Bearing for High Vacuums with Silver (Ag) Coated Balls In 1983 Koyo announced a life test equation for bearings with silver (Ag) ion plated balls 3). This life test equation was determined on the basis of many life test data. Thus the life of high-vacuum bearings with silver (Ag) ion plated balls can be predicted using the following equation a. Lvh = b 1 b 2 b 3 (Cv/P) q /n a Lvh: Life (reliability: 90%), h Cv : Basic dynamic load rating of ball bearing for vacuums (1/13 of basic dynamic load rating for steel bearing of same dimensions), N P : Dynamic equivalent load, N q : Exponent, q = 1 Table 5 Typical solid lubricants for EXSEV Bearings " : Excellent # : Good % : Acceptable Solid lubricant Crystal structure Thermal stability, ; Friction coefficient Load capacity Atmosphere Vacuum Atmosphere Vacuum MPa Dust generation Gas emission Remarks Silver (Ag) Face centered cubic 600 or more 0.2~0.3 Max % " Unsuitable in the atmosphere Lead (Pb) Face centered cubic 300 or more 0.05~ ~0.15 Max % " Molybdenum disulfide (M 0 S 2 ) Hexagonal lattice ~ ~0.25 Max % # Tungsten disulfide (WS 2 ) Hexagonal lattice ~ ~0.2 Max % # Graphite (C) Hexagonal lattice ~ ~1.0 Max % # Unsuitable in vacuum Polytetrafluoroethylene (PTFE) Long chain structure ~ ~0.2 Max " % Polyimide Long chain structure or more 0.05~ ~0.6 Max # % 56 KOYO Engineering Journal English Edition No.156E (2000)

6 Axial load, N Bearing A Bearing B Bearing C 0 1.E+04 1.E+05 1.E+06 Total number of rotations up to life, rev Fig. 4 Relationship between bearing life and axial load 1.E+07 n : Rotational speed, min 1 ; here 10 n b 1 : Speed factor b 1 = n + 1 b 2 : Material coefficient b 2 = 1 (in case when silver was plated with special ion plating method) b 3 : Coefficient for atmospheric pressure and temperature b 3 = 1 (10 3 Pa, in case of room temperature) Life of Bearings Coated with Molybdenum Disulfide (MoS 2 ), Polytetrafluoroethylene (PTFE) and Special Fluorocarbon Film (Koyo Clean Pro) The life equation for solid lubrication bearings using molybdenum disulfide (MoS 2 ), polytetrafluoroethylene (PTFE) and special fluorocarbon film (Koyo Clean Pro) is introduced in this section. Fig. 4 shows the relationship between bearing life and axial load 4). Test bearings were type 608 (φ8 φ22 7) stainless steel bearings(inner ring and balls SUS440C). Bearing A was equipped with a stainless steel retainer coated with MoS 2, bearing B was equipped with a stainless steel retainer coated with PTFE, and the entire bearing surface of bearing C was treated with special fluorocarbon film (Koyo Clean Pro). Each bearing type was tested under four load conditions and the regression line sought. Test results showed that, while bearings A and B exhibited approximately the same life, bearing C exhibited approximately seven times the life of bearings A and B. As a result, the following equation s of life equation can be obtained from Fig. 4. Ce : Basic dynamic load rating of steel bearing of same dimensions 0.85, N P : Dynamic equivalent load, N 5. Conclusion This paper introduced bearing materials and solid lubricants used in bearings for extreme special environments (Koyo EXSEV bearings). In the future, bearings for extreme special environments will likely be required to perform in increasingly harsh environments and under increasingly severe conditions. It is therefore important to continue basic research regarding bearing materials and solid lubricants in order to meet these demands and put the results of such research to practical use in the development of new bearings. References 1) "EXSEV Bearings Series: Ceramics Bearings and EXSEV Bearing", CAT. 208, Koyo Seiko Co.Ltd. 2) Koyo Engineering Journal, 145 (1994) 39. 3) K. Kakumoto: Kikaisekkei, 29, 2 (1983) ) A. Fujiwara, K. Hayashida, Y. Toyota: Proceeding of JAST Tribology Conference (1998-5, Tokyo) 216. Lav = b 2 (Ce/P) 3 s Lav: Life (reliability: 50%), total number of rotations b 2 : Material coefficient MoS 2 coating: 6 PTFE coating: 6 Special fluorocarbon film: 42 KOYO Engineering Journal English Edition No.156E (2000) 57