Bearings for Extreme Special Environment (4) Application of Ceramic Bearings

Transcription

1 Bearings for Extreme Special Environment (4) Application of Ceramic Bearings H. TAKEBAYASHI Outlines of Koyo EXSEV series ( for extreme special environments) have been explained in the Koyo Engineering Journal No.156E, 157E, 158E. Herein comparison of characteristics between ceramic material (silicon nitride) and high carbon chrome steel (SUJ2), configurations of ceramic s, fundamental performance, and application examples of ceramic s are described. Key Words: ceramic rolling, silicone nitride, extreme special environment, application 1. Introduction The environment and conditions under which rolling s (hereinafter referred to as "s") are used become harsher and more diversified year by year. For this reason Koyo has progressed in commercializing its Koyo EXSEV (Extreme Special Environment) s for extreme special environment that can be used in special environments and under extreme conditions. This journal has introduced Koyo EXSEV series s for extreme special environment three times in the past. This time, the fourth time, applications are introduced for the ceramic, which is regarded as the main type for extreme special environment. In concrete items, the paper gives a comparison of characteristics for ceramics (silicon nitride) and steel, configuration and applications of ceramic s, basic performance concerning applications of ceramic s, and recent practical examples of ceramic s. 2. Characteristics of Ceramic Bearings Table 1 gives a comparison of characteristics of steel used for ordinary s and silicon nitride (Si 3 N 4 ) primarily used as the material for ceramic s 1). The superior characteristics of silicon nitride, when compared with those of steel have become the merits of ceramic s. Ceramic Bearing steel, No. Item Unit Advantage of Ceramic s material, Si 3 N 4 SUJ2 1 Heat resistance ; 8 18 Higher load durability maintained in high-temperature range 2 Density g/ml Reduction of centrifugal force induced by rolling elements (balls or rollers) eincreased service life and restricted increase in temperature Linear expansion Smaller change of internal clearance caused by temperature rise 3 1/; coefficient ereduced vibration, small change of preload 4 Vickers hardness MPa 13 72~ ~ Module of longitudinal elasticity MPa Poisson's ratio Corrosion resistance Good Not good 8 Magnetism Non-magnetic material 9 Conductivity Bonding of raw material Table 1 Comparison between silicon nitride and high carbon chromium steel Insulating material Covalent bonding Smaller change of deformation at rolling contact point ehigh rigidity Can be used in acid solutions, alkali solutions, and other special environments Ferromagnetic Smaller speed fluctuation caused by magnetism in intense magnetic material field Conductive material Metallic bonding Eliminates electric pitting (applicable to electric motors, etc.) Minimized seizure (or cohesion) at contact points, usually resulting from discontinued oil film 56 KOYO Engineering Journal English Edition No.16E (22)

2 For an example, the superior heat resistance characteristics of silicon nitride are exhibited when applied to s used in high temperature atmosphere, and the low density characteristics of silicon nitride reduces the mass of the s and contributes to the reduction in centrifugal force generated by the rolling elements (balls, rollers) when the runs at high speed. The high rigidity characteristics of silicon nitride are responsible for providing s with high rigidity and superior corrosion resistance, non-magnetism, and insulation characteristics are used as material for special applications. In addition, the bonding form of silicon nitride (covalent bond) gives higher anti-resistance characteristics caused by discontinued oil film during highspeed rotation. 3. Configurations of Ceramic Bearings Table 2 gives main configurations and application examples of ceramic s 2). There are basically two types of ceramic configurations: all-ceramic (inner/outer rings and rolling elements made of silicon nitride) and hybrid ceramic (rolling elements made of silicon nitride). In the case of hybrid ceramic s, special steel (heat-resistant steel, corrosion-resistant steel, non-magnetic steel) is used for inner/outer rings depending upon application conditions. Surface treatment (especially for the purpose of corrosion resistance) is provided in some cases. Retainer materials are selected according to usage conditions of the. In the case of high-speed rotation applications, inner ring and rolling elements are sometimes made of silicon nitride. This is because the inner ring expands due to centrifugal force when the rotates at high speed, and hence the interference between the inner ring and shaft will loosen. 4. Basic Performance Concerning Application of Ceramic Bearings This section describes the basic performance for considering the application of ceramic s, such as highspeed performance of ceramic s, life in water, and operating performance at high temperatures. Table 2 Structure of ceramic s and application examples : Ceramics a For high-speed rotation s For use in a vacuum environment d For corrosive environment Specific gravity 4% of steel Suitable for high-speed rotation because lower centrifugal force is produced by rolling elements. Can be used in a vacuum of 1 to Pa Lubricating method should be selected according to conditions. Can be used in acids, alkalis, salt water, and molten metals. Main spindle of machine tools, turbo chargers for automobiles, and industrial Semi-conductor production facilities and vacuum equipment (turbo molecular Chemical equipment, steel production facilities, and textile machinery equipment (spin tester, etc.) pump, etc.) f For high-temperature g For non-magnetism h For insulation Ceramic is heat resistant up to 8;. Lubricating method should be selected according to the temperature. Can be used in magnetic fields. Ceramics are insulating materials and can be used in applications where electric leakage may occur. Steel production facilities, industrial equipment, and automotive diesel engines. Semi-conductor production facilities, superconductivity-related equipment, and nuclear power generators Railway rolling stock and electric motors KOYO Engineering Journal English Edition No.16E (22) 57

3 4. 1 High Speed Performance 3) Ceramic s using silicon nitride for rolling elements are used as s for high-speed rotation. The reason for this is because, by using silicon nitride with its low density for rolling elements of s for high speed rotation, slipping due to gyro moment and centrifugal force produced in the rolling elements can be reduced and high speed rotation can be possible. Here, the superior performance of hybrid ceramic s in comparison to steel s at high-speed rotation is demonstrated by studying the power loss of the. Figure 1 shows the structure of the test machine, Table 3 gives the test conditions, and Fig. 2 and Table 4 give the dimensions and configuration of the test s. The test equipment can measure power loss of the test by the three methods of rotation torque of the, temperature rise of lubricating oil, and electric power consumption of the motor. Quill shaft To traction drive Support Measuring rod Test Fig. 1 Test equipment configuration 3) Item Axial load Rotational speed (max.) Lubricating oil Ambient temperature Table 3 Test conditions 3) u41 12 Chamber Heated air Condition 2 N rpm ASTO#5 Room temperature u2 Fig. 2 Test dimensions 3) Inner/outer rings Material Ball Retainer Hybrid ceramic Tool steel (AISI-M5) HIP silicon nitride Polyimide resin Steel Tool steel (AISI-M5) Tool steel (AISI-M5) Diameter 1/4" (6.35mm) 1/4" (6.35mm) Number 9 9 Polyimide resin Figure 3 gives a comparison of three types of power loss measured based on the power consumption of the motor, the temperature rise of lubricating oil, and the rotation torque of the. The values of power loss determined by the three methods indicate almost the same values and tendencies. A comparison of power loss between the hybrid ceramic and steel shows that at a rotational speed of 8 rpm or more, there was less power loss for the hybrid ceramic. Power loss, kw Table 4 Test compositions 3) # % ' : Electric power consumption & : Rotational torque ( : Oil temperature rise & ( Steel Hybrid ceramic &( & ( Rotational speed, rpm Fig. 3 Comparison of power loss between hybrid ceramic and steel 3) Figure 4 shows the results of a study of maximum speed of anti-seizure when the amount of lubricating oil is reduced in the hybrid ceramic and steel. The results show that, compared with steel s, hybrid ceramic s require a smaller amount of lubricating oil for running when they run at the same speed, or run up to higher speed rotation with the same amount of lubrication oil. Table 5 gives the results of a comparison of power loss of hybrid ceramic s and steel s in relation to the amount of oil. Power loss is determined from the electric power consumption, and power loss of the steel at each speed rotation expresses being 1. With twice the amount of lubricating oil of the maximum speed of anti-seizure for &( ( & & ( 58 KOYO Engineering Journal English Edition No.16E (22)

4 each rotation speed as the optimal amount of oil, power loss is compared based on the optimal amount of oil. As a result, it was found that power loss for the hybrid ceramic was reduced approximately 3% in comparison with that of the steel at 8 rpm, and that approximately 55% can be reduced at rpm. In other words, in high speed rotation applications, reducing lubricating oil amount using a hybrid ceramic enables dramatic reduction of power loss of the section. The test equipment was configured so that one test could be totally immersed in a tank filled with tap water, and only a radial load applied. The test s were equivalent of deep groove ball s 626. There were three types: q all-ceramic (inner/outer rings and balls made of silicon nitride), w hybrid ceramic 1 (inner/outer rings made of SUS44C, balls made of silicon nitride), e hybrid ceramic 2 (inner/outer rings made of SUS63, balls made of silicon nitride)..6.5 %# No seizure & Seizure Hybrid ceramic Steel Coil spring for loading Vibration pick-up Amount of lubricating oil, R/min Steel Seizure Coupling Water Water tank Test.1 Hybrid ceramic Seizure Fig. 5 Test equipment 16 Rotational speed rpm Fig. 4 Maximum anti-seizure rotational speed 3) 4. 2 Life in Water 4) Because silicon nitride has superior corrosion resistance, ceramic s are sometimes used in solutions such as seawater and chemicals. When using s in solutions, the solution itself is often used as the lubricant. In order to use ceramic s in solutions, therefore, it is more important to get good understanding of the life of ceramic s if used in low viscosity solutions compared with ordinary lubricating oils. Figure 5 shows the test equipment, Fig. 6 shows the test and Table 6 gives the configurations of the s. u62 u3 Fig. 6 Test Table 5 Power loss in relation to the amount of oil 3) Power loss ratio (taking the loss in steel as 1) Same amount of lubricating oil When amount of oil is considered Smallest amount of oil for (amount of oil : 1.r/min) (amount of oil : twice the smallest preventing seizure, r/min allowable amount of oil to prevent seizure) 8 rpm rpm 8 rpm rpm 8 rpm rpm Steel Hybrid ceramic KOYO Engineering Journal English Edition No.16E (22) 59

5 Table 6 Configurations of test s Hybrid ceramic Hybrid ceramic Bearing type All-ceramic 1 2 Bearing number NC626 3NC626ST4 3NC626MD4 Outer ring Silicon nitride SUS44C SUS63 Inner ring Silicon nitride SUS44C SUS63 Balls Silicon nitride Silicon nitride Silicon nitride (Ball diameter number of balls) (3/8 inch 9) (3/8 inch 9) (3/8 inch 9) Retainer Fluorocarbon resin Fluorocarbon resin Fluorocarbon resin Table 7 Test conditions Hybrid ceramic Hybrid ceramic Bearing type All-ceramic 1 2 Bearing number NC626 3NC626ST4 3NC626MD4 Radial load, N Lubricating conditions In room temperature water Table 7 gives the test conditions and Fig. 7 shows the results of the life test. The test was conducted with the load of 1 47 N for the all-ceramic and 196 N for hybrid ceramic 1 and 2. The results of the life test are plotted on the Weibull probability sheet, with results of the all-ceramic represented by #, hybrid ceramic 1 by %, and hybrid ceramic 2 by '. The damage form for the all-ceramic in this test was the flaking of outer/inner rings and balls, the same as observed under oil lubrication. The damage form for both hybrid ceramic s 1 and 2, on the other hand, was wear of raceway (SUS44C is harder than SUS63 and therefore has longer life). Flaking and wear did not occur for the silicon nitride balls. The results show that if all-ceramic s are used in water, they are used at 1/ or less of the load rating (if used at more than 1/ the load rating, flaking occurs in extremely short time), and life at that time is approximately 3/ of the calculated life. Next, results show that in the case where hybrid ceramic s are used in water, the inner/outer rings start to wear if used as 1/ or less of the load rating. If hybrid ceramic s are used in water, therefore, it is necessary to get a good understanding of the relationship between load and amount of wear Operating Performance at High Temperatures 5) Because hardness and strength of silicon nitride do not deteriorate at high temperatures when compared to those of steel, silicon nitride holds a lot of promise as a material for high temperatures. Of more relevance is the fact that the s are used in a high-temperature atmosphere, and then oils or greaces can not be used for lubrication. It is therefore necessary to use solid lubricants. Molybdenum disulfide, tungsten disulfide and graphites are well known as solid lubricants. These solid lubricants are used for retainer materials, or coatings formed on the retainers or rolling surfaces. Cumulative failure probability, % Bearing type NC626 3NC626ST4 3NC626MD4 3NC626MD4 (Load: 196N) 3NC626ST4 (Load: 196N) B (h) B5 (h) Weibull coefficient NC626 (Load: 1 47N) u37.14 u3.99 B b a Life, h Fig. 7 Life test results e t d D u36.1 u3.38 Fig. 8 Test s Calculated life.3 times Flaking Suspended a B b e t d D a) Outer ring guide b) Inner ring guide Bearing inner bore diameter d: 2mm, outer diameter D: 47mm, width B : 14mm, contact angle a: 15, ball diameter: 6.35mm, number of balls: 8 6 KOYO Engineering Journal English Edition No.16E (22)

6 The results of studying the dividing method and guide type of the retainer when graphite retainer is used as a solid lubricant for an all-ceramic are introduced here. The test is a 724 equivalent angular contact ball that uses graphite material for the retainer and silicon nitride for the inner/outer rings and balls. Figure 8 shows test s for which guide form of the retainer are different. Both retainers were machined cages made of graphite material. The width b was 13mm, thickness t was 3.mm and guide clearance e was.3mm. Figure 9 shows the dividing method of the retainer. The retainer is the outer ring guide type, so this is cut into four at the cage pocket or cage bar. The machining allowance was.7mm per a place. Cut position Friction torque, N m a) Outer ring guide b) Inner ring guide When increasing speed When decreasing speed u3 u37 u Fig. Friction torque 6. a) Outer ring guide 5. a) Division at cage pocket b) Division at cage bar Fig. 9 Retainer division method and dimensions The test conditions were 5; for temperature, 392 N for load, and 1 ~15 rpm for rotational speed. Figures and 11 give a comparison of vibration and friction torque of s using retainers with different types of guides. Figures 12 and 13 give a comparison of vibration and friction torque of s using retainers with different dividing methods. With the comparison of retainer guide types, compared to the containing an inner ring guide retainer, the containing an outer ring guide retainer showed less friction torque and friction torque fluctuation. It was also discovered that with the containing the divided retainer, the one divided at the cage bar showed better operating performance than the one divided at the cage pocket. A comparison of the one-piece retainer and divided retainer shows that the with the outer ring guide one-piece retainer and the with the retainer divided at the cage bar exhibits about the same operating performance. Considering these test results, therefore, it is necessary to study the shape of retainers according to conditions under which the s are used. Friction torque, N m Vibration, G b) Inner ring guide When increasing speed When decreasing speed Fig. 11 Vibration Division at cage bar Division at cage pocket 5 15 Fig. 12 Friction torque KOYO Engineering Journal English Edition No.16E (22) 61

7 Vibration, G 5 Division at cage bar Division at cage pocket 5. 2 Bearings for HDD (Hard Disk Drive) Spindle 7) Bearings for HDD spindles require higher speed, lower torque, improved anti-seizure properties (under insufficient lubrication) and higher rigidity. Use of hybrid ceramic s using ceramic balls have been considered for HDD spindles. Figure 15 gives a comparison of life for hybrid ceramic s and steel s under very little oil lubrication. It was determined that hybrid ceramic s under very little oil lubrication conditions have a life at least 3 times longer than that of steel s. As a result, hybrid ceramic s have been made practical for some types of HDD requiring higher speed and higher capacity Fig. 13 Vibration 5. Application Examples of Ceramic Bearings Recent application examples of ceramic s are introduced here. In concrete terms, this section describes ceramic parts for fuel injection systems and s for semiconductor manufacturing equipment, s for machine tool main shafts, s for HDD spindles, and s for turbochargers Bearings for Turbochargers 6) Bearings for automobile turbochargers require improved acceleration response and lower torque when rotating at high speeds. Manufacturers have, therefore, considered using rolling s in place of floating bush s (full float s) traditionally used. In addition to that, Koyo has promoted to commercialize hybrid ceramic s as s for turbochargers because hybrid ceramic s using ceramic balls for rolling elements exhibit longer life in contaminated oils or in used oils than that of steel s. Figure 14 gives a comparison of life of hybrid ceramic s and steel s in a contaminated oil and in a used oil. The figure shows that hybrid ceramic s exhibit longer life than that of steel s. As a result, Koyo was the first in the world to realize practical use of hybrid ceramic s for automobile turbochargers. <Test conditions> Bearing : 798 (u8 u19 6) Rotational speed : rpm Oil type : W-3CD Used oil : Used for 2 km Used oil Contaminated oil Steel ball Ceramic ball Steel ball Ceramic ball Load cycle Foreign matter :.27mm high-speed steel Load : 59 N (repeated load) Oil temperature : 8: Condition Strong vibration Suspended Flaking on ball Suspended Vibration, mg <Test conditions> Bearing : 695 (u5 u13 4) Rotational speed : 7 2 rpm n = 3 all lock 1 h Steel ball Lock 3 4h 3 6h n = 3 average 4 6h Time, h Lubrication : Oil 1mmg drip Load : Preload 1.5 kgf Ceramic ball Fig. 15 Oil lubrication durability test results (very little oil lubrication) 5. 3 Bearing for Machine Tool Spindle 8) Bearings for machine tool spindles require low temperature, high rigidity, and improved anti-seizure properties. Hybrid ceramic s using ceramic balls and ceramic s using ceramic balls and inner ring have, therefore, been prepared for practical use. Figure 16 gives a comparison of temperature rise for hybrid ceramic s and steel s, and Fig. 17 a comparison of temperature rise for ceramic s using ceramic balls and inner ring (6 NC type) and hybrid ceramic s using ceramic balls (3 NC type). The figure shows that temperature rise at high-speed rotation is lower in the order of steel s, 3 NC type and 6 NC type. Fig. 14 Durability test results 62 KOYO Engineering Journal English Edition No.16E (22)

8 Temperature rise on outer ring, deg Bearing : 6NCACH2CPA 3NCACH2CPA Oil/air lubrication : Oil : ISO VG equivalent.5 ml/2 min Air : 6 NR/min Speed : 17 rpm (dn value 17 4 ) 6 h Cooling : Jacket oil cooling (R/min, room temperature control) 3NCACH2CPA Exhaust temperature rice, : ACH18CDBD (preload 588N) O/A temperature rise comparison (ceramic VS SUJ2) VG.4ml/min Brg Air: 6 N R/min Brg 2 Bearing steel ball Silicon nitride ball No cooling Cooling Fig. 16 Temperature rise for steel and hybrid ceramic 6NCACH2CPA 3 NC type Preload # 49 N { 5 kgf} ' ( 245 N {25 kgf} % & 441 N {45 kgf} 6 NC type Fig. 17 Temperature rise for 3 NC and 6 NC type s 5. 4 Bearings for Semi-conductor Production Facility 9) Bearings for semi-conductor manufacturing equipment require low particle generation, high rigidity, and improvement of anti-seizure property. Figure 18 shows a comparison of particle generation characteristics in air and in vacuum for stainless steel s, hybrid ceramic s and all ceramic s. The results show that the s have superior particle generation characteristics in both air and vacuum in the order of stainless steel s, hybrid ceramic s and all ceramic s. Thus more and more ceramic s for semi-conductor manufacturing equipment are being made widely practical. KOYO Engineering Journal English Edition No.16E (22) <Test conditions> Bearing Rotational speed Temperature Ambient atmosphere Axial load Measured particle size Total number of generated particles Bearing A Stainless steel : 68 equivalent : 2 rpm : Room temperature : Air and vacuum ( 4 Pa) : 2 N : (Air) min..3lm (Vacuum) min..38lm Bearing B Hybrid ceramic Bearing C All ceramic In air Fig. 18 Particle generation characteristics In vacuum 5. 5 Ceramic Parts for Fuel Injection System ) Turning the attention to high hardness of ceramic materials, high strength and superior wear resistance, ceramic balls and rollers are used for fuel injection systems of diesel engines. Figure 19 and Fig. 2 give the operation system of the injector of a fuel injection system and the structure of the control valve. The u2mm ceramic ball (for ) with flat area is used for this control valve. High pressurization of recent diesel engine fuel injection systems is advancing, and this is the result of the high strength and wear resistance of ceramics being required. Oil pressure Control chamber 135MPa Inlet orifice No injection Solenoid valve Spring force Outlet orifice Control piston Nozzle needle Suction force 135MPa Fig. 19 Operation system of injector ) Injection Leak 63

9 Solenoid valve Inlet orifice Valve u2. ceramics Low pressure Pressure distribution groove Control chamber Control piston Outlet orifice 6. Conclusion Fig. 2 Structure of control valve ) This paper has described the characteristics of ceramic s, configuration of and applications for ceramic s, basic performance concerning application of ceramic s and practical examples of ceramic s. Because ceramic s are used under severe conditions than conventional steel s, it is necessary to consider the lubrication method and configuration suitable for each usage condition when considering wide applications of ceramic s. In other words, it is important to proceed with research and development of retainer materials, lubrication methods and materials for inner/outer rings in the case of hybrid ceramic s corresponding to ambient temperature, load, rotational speeds, degree of corrosive atmosphere and with or without lubricants. The need for higher performance for rolling s will probably become more and more severe in the future. The demand for ceramic s is therefore expected to grow in the future. References 1) K. Rokkaku, H. Takebayashi, K. Nishida: Koyo Engineering journal, 133 (1988) 63. 2) Koyo Seiko Co., Ltd.: EXSEV Bearing Series Ceramic Bearings and EXSEV Bearings, CAT. no28, 11. 3) H. Takebayashi, K. Tanimoto, T. Hattori: Journal of the Gas Turbine Society of Japan, 26, 2 (1998) 61. 4) H. Takebayashi: Fundamental and Applied Study of Silicon Nitride Rolling Bearing, A doctoral Dissertation (1998) 83. 5) H. Takebayashi, Y. Yuine: Tribologist (Journal of Japanese Society of Tribologist), 38, 12 (1993) 77. 6) K. Tanimoto, K. kajiwara, K. Yanai: Koyo Engineering Journal, 157E (2) 21. 7) M. Mukasa: Gekkan Toraibologi (The Tribology) 136 (1998) 25. 8) Koyo Seiko Co., Ltd.: EXSEV Bearings Series Ceramic Bearings and EXSEV Bearings, CAT. No ) H. Toyota: Koyo Engineering Journal, 15 (1996) 53. ) T. Aida, T. Fujimura, H. Suzuki, M. Fujii, S. Yasunishi, A.Negishi: TOYOTA Technical Review 49, 2, Dec. (1999). H. TAKEBAYASHI * * Aerospace & Super Precision Engineering Department, Bearing Engineering Center, Dr. 64 KOYO Engineering Journal English Edition No.16E (22)