Bearings for Extreme Special Environments-Part 2 Bearings for Vacuum and Clean Environments

Transcription

1 Bearings for Extreme Special Environments-Part 2 Bearings for Vacuum and Clean Environments H. TAKEBAYASHI * *Bearing Engineering Center, EXSEV Engineering Department Outlines of Koyo EXSEV bearing series (bearings for extreme special environment) have been explained serially from the previous issues of Koyo Engineering Journal no.156e. In no.156e characteristics of bearing materials and solid lubricants used for EXSEV bearing series were explained. Herein, examples of EXSEV bearings, especially bearings for clean environment, and bearings for vacuum environment, are shown. Key Words: special environment, clean, vacuum, bearing 1. Introduction The environments and conditions under which rolling bearings (hereinafter referred to as bearings) are used have become severer and more diversified in recent years. In other words, there is a demand for applying bearings to special environments such as ultra-high vacuum, clean, corrosive and high-temperature atmospheres. A series of articles concerning Koyo EXSEV bearings has been introduced from the previous issues of this publication, in which solid lubricants and bearing materials used for bearings for extreme special environments were described. Here, typical configurations of EXSEV bearings are shown and bearings for vacuum environments and bearings for clean environments, which are the main products in Koyo EXSEV bearing series, are explained. 2. Typical Configurations of EXSEV Bearings Table 1 shows the typical configurations of EXSEV bearings 1). The table gives materials of outer rings, inner rings, rolling elements and cages and solid lubricants corresponding to application and environment conditions For High-speed Rotation Hybrid ceramic bearings that use silicon nitride for rolling elements and bearing steel for outer /inner rings are generally used for high-speed rotation. This is because, sliding caused by centrifugal force and gyro moment on the rolling elements can be reduced due to low density of silicone nitride rolling elements and superior high-speed rotation is possible when compared with bearing steel rolling elements 2), 3). If even higher rotation is required and centrifugal expansion of the inner ring becomes a problem (centrifugal expansion of the steel inner ring occurs, resulting in bearing clearance reduction, or lack of interference between shaft and inner ring), a ceramic bearing is used with bearings steel outer ring and silicon nitride rolling elements and inner ring For Clean Environments Beginning with semiconductor and liquid crystal manufacturing, which places maximum emphasis on cleanliness of the work environment, various applications and needs for bearings for clean environments are rapidly expanding. Because oil scattering is a problem for such applications, users are averse to using oil and grease for bearing lubrication. Therefore stainless steel () bearings that use solid lubricants, such as special flurocarbon polymer, PTFE, highhardness carbon material, or fluorocarbon resin, are used for clean environments. Specifically, bearings whose entire surface is coated with special fluorocarbon polymer, bearings that use high-hardness carbon material for rolling elements (mainly for balls), bearings whose cage is made of fluorocarbon resin, and bearings whose cage is coated with PTFE, are used for clean environments For Vacuum Environments Even if referred to simply as bearings for vacuum environments, these bearings are used under a variety of conditions, from low vacuum to ultra-high vacuum, or repeatedly switching from air to vacuum atmospheres. Therefore, in the case of bearings for use in vacuum environments, bearings made of stainless steel () use grease or solid lubricants according to the conditions under which they are used. These are specifically bearings that use grease for vacuum, those that use molybdenum disulfide coating for the cage, or those that use ion plating of lead (Pb) or silver (Ag) for their rolling elements (mainly for balls) For Corrosive Environments For applications where bearings are immersed in various types of chemicals, such as acids or alkalis, corrosion resistant bearings are required. All ceramic bearings whose inner /outer rings and rolling elements are made of ceramics, and cages are made of fluorocarbon resin are mainly used as bearings for corrosive environments. (sintering aids: Al 2 O 3-58 KOYO Engineering Journal English Edition No.157E (2000)

2 Table 1 Configurations of Koyo EXSEV bearings Application and environment Outer ring, inner ring Material and lubricant Rolling elements Retainer (or cage) Bearing series a For high-speed rotation SUJ2 Outer ring : SUJ2 Inner ring : (Si3N4) (Si3N4) Reinforce polyamide resin (FG) 3NC FG 6NC FG s For clean environments + special fluorocarbon polymer coating + special fluorocarbon polymer coating + special fluorocarbon polymer coating SE STPR YS High-hardness carbon material Fluorocarbon resin (FA PT) SE ST FA (PT) SL ST4 FA (PT) SUS304 + high-temperature resistant PTFE coating SE STMPD7 YS High-hardness carbon material SUS304 SL ST4 YS SUS630 High-hardness carbon material (FA PT) 3NC MD4 FA (PT) SL MD4 FA (PT) d For vacuum environments SUS304 with vacuum grease SV ST YS SUS304 + MoS2 coating SE STMSA7 YS PEEK resin (PG) SE ST PG + lead coating + silver coating SUS304 SE STMB3 YS SE STMG3 YS f For corrosive environments (Si3N4) (Si3N4) NC FA (PT) Corrosion resistant silicon nitride Corrosion resistant silicon nitride (FA PT) NCT FA (PT) Silicon carbide (SiC) Silicon carbide (SiC) NCZ FA (PT) g For high temperatures SKH4 (full complement ball type) Graphite (GF) 3NC HT4 GF NC V h For non- magnetism Nonmagnetic (FA PT) 3NC YH4 FA (PT) NC FA (PT) j For insulation (FA PT) 3NC ST4 FA (PT) NC FA (PT) KOYO Engineering Journal English Edition No.157E (2000) 59

3 Y 2 O 3 ) is mainly used as ceramic material, but according to the degree of the corrosive environment, silicon nitride with enhanced corrosion resistance (sintering aids: spinel) or silicon carbide with its superior corrosion resistance properties is used 4) For High Temperatures When bearings are used in high-temperature atmospheres such as food manufacturing ovens or heat treatment furnaces of metals, the biggest problem is the lubricant. Here, greases or oils can be used up to 300;, but solid lubricants must be applied for much higher temperatures. Also, stainless steel () can be used as a bearing material in temperatures up to 300;. For high-temperature bearings used in temperatures over 300;, hybrid ceramic bearings whose inner /outer rings are made of SKH4 and whose rolling elements (balls) are made of silicon nitride and cages are made of graphite, or all-ceramic bearings whose inner /outer rings and rolling elements (balls) are made of silicon nitride are used. Bearings that use graphite for their cages are generally used for temperatures less than 500;. And, there are examples where all-ceramic bearings are used in high-temperature atmospheres of 500; ~ 800; without solid lubricants 5) For Nonmagnetism Among bearings used for superconductivity, semiconductor manufacturing equipment or various types of inspecting equipment, some are used in magnetic fields. If ordinary steel bearings are used in magnetic fields, they could disturb the magnetic field or the rotation torque of the bearing could fluctuate largely. Therefore nonmagnetic bearings are required in these applications. Therefore for nonmagnetism bearings, hybrid ceramic bearings that use nonmagnetic steel (nonmagnetic stainless steel, nonmagnetic cemented carbide) for their inner /outer rings, and silicon nitride for their rolling elements, or allceramic bearings that use silicon nitride for their inner /outer rings and rolling elements are used. Also, the cage for this type of application is typically made of fluorocarbon resin 6) For Insulation One type of bearing damages used for traction motors for railway and various types of motors is the phenomenon called electrical pitting. Electrical pitting is a damage whereby a parts of the surface that contact with each other is fused when a spark is produced through an extremely thin oil film on the rolling contact zone when an electric current is conducted inside the bearing during rotation. For this reason a bypass is provided on the outside of the bearing (to be accurate, outside the raceway) so that current does not pass through the bearing, or in some cases insulation bearings that do not allow current to pass inside the bearing may be used. For insulation bearings, hybrid ceramic bearings whose inner /outer rings are made of metal and rolling elements are silicon nitride as an insulation material, or all-ceramic bearings that use silicon nitride for their inner /outer rings and rolling elements are used. In addition to these, there are also bearings that have a insulation-material coating like ceramic on the bore of the inner ring or the outside of the outer ring. 3. Bearings for Clean Environments Because oil scattering from bearings is most undesirable for semiconductor /liquid crystal manufacturing equipment, electronic component manufacturing equipment, and pharmaceutical and food product manufacturing equipment, degreased bearings without oil or grease are used. So, if a bearing is used for some time, wear particles (bearing material) are generated, resulting in a problem. Therefore, bearings for clean environments that minimize wear particles from the bearing were developed at Koyo. Along with clarifying the performance, the configuration and features of various types of bearings for clean environments are introduced here Comparison of Dust Generation Performance of Bearings for Clean Environments Fig. 1 shows the test method, and Table 2 gives the test conditions 1). The test equipment shown in Fig. 1 was placed on a clean bench (cleanliness: class 10) set up in a clean room. The amount of dust generation was determined by measuring particles 0.3µm in diameter or larger from two bearings using particle counter for a period of 20 hours. Bearing Test bearings Clean bench (class 10) Item Load Rotation speed Ambient atmosphere Test time Particle size to be measured Sealing with magnetic fluid Fig. 1 Test method Table 2 Test conditions Condition Number : ML6012 Dimensions : φ6 φ12 3mm Radial 2.9N / two bearings 200r/m Particle counter Recorder In a class 10 clean bench at room temperature 20h 0.3µm and larger 60 KOYO Engineering Journal English Edition No.157E (2000)

4 Table 3 Test bearings Bearing Material for bearing components and surface treatment Surface treatment method symbol Outer ring, inner ring Rolling elements Retainer and other notes A No lubricant B SUS304 No lubricant C Fluorine grease D +Ag SUS304 Ion plating E SUS304+MoS2 MoS2-baking F +PTFE SUS304+PTFE PTFE-baking G SUS304+PTFE PTFE-baking H (FA) SUS304+high-temperature Baking with hightemperature-resistant PTFE I resistant PTFE J SUS304+high-temperature Baking with hightemperature-resistant PTFE resistant PTFE K + + SUS304+ Baking with special fluorocarbon special fluorocarbon special fluorocarbon polymer special fluorocarbon polymer Amount of generated particles 0.3 µm diameter or larger, particles / 2 bearings Bearing A Bearing B Bearing C Bearing D Bearing E Bearing F Bearing G Bearing H Bearing I Bearing J Bearing K Fig. 2 Test results (comparison of total mount of dust generation during 20 hours of testing) Table 3 gives the configuration of the test bearings 1). Eleven types were used as test bearings. Martensite stainless steel () or silicon nitride ceramics were used for the inner /outer rings and rolling elements. Bearings were either unlubricated, used molybdenum disulfide or were coated with PTFE or special fluorocarbon polymer film as a solid lubricant. Fig. 2 shows the test results 1). Amount of generated particles 0.3µm in diameter or larger was measured when 20 hours of testing was carried out for each of the 11 types of bearings for comparison. As a result, among bearings that used solid lubricants, dust generation of bearing (K), whose surface was coated with special fluorocarbon polymer, bearing (H), with fluorocarbon resin cage, and bearings (F, G, I, J), with PTFE coating cage, was less than bearing (D), with Ag ionplating balls, and bearing (E), with molybdenum disulfide coating cage. This clearly indicates that these bearings (K, F, G, I, J, H) show superior dust-generation performance for use in clean environments. In other words, whereas the amount of dust generation for bearing (D) with Ag ion-plating ball and bearing (E) with molybdenum disulfide coating cage was of the order of 10 4 ~ 10 5 particles, the amount of generated dust for the bearing whose entire surface was coated with special fluorocarbon polymer, the bearing with the fluorocarbon resin cage, and the bearings (F, G, I, J) with PTFE coating cage were of the order of 10 ~ 10 2 particles. Also, the amount of dust generation for the stainless steel bearing without a lubricant was of the order of 10 6 particles and the hybrid ceramic bearing was of the order of 10 4 particles. Also, the amount of dust generation for bearing (C), which used fluorine contained grease, was of the order of 10 particles, but because there is evaporation and scattering of oil, etc., it is not used as a bearing for clean environments. KOYO Engineering Journal English Edition No.157E (2000) 61

5 Table 4 Configurations Koyo EXSEV bearings for clean environments " : Superior # : Good Application Components Outer ring, inner ring Rolling elements Retainer Pressure Pa Temperature C Low dust generation Corrosion resistance Low dust generating bearing for wide use Low dust generating bearing for use in high-temp conditions Low dust generating bearings for use in corrosive environments SUS304+PTFE coating to 10 5 to 200 " + special fluorocarbon polymer coating SUS304+ special fluorocarbon polymer coating SUS304+ # high-temperature resistant to 10 5 to 300 PTFE coating " " SUS630 to 10 5 to 200 " " SUS304+PTFE coating 3. 2 Configuration and Features of Bearings for Clean Environments With semiconductor /liquid crystal manufacturing equipment requiring bearings for clean environments, there are cases where the bearings are used in a high-temperature atmosphere and cases where the bearings are used in a corrosive atmosphere. Therefore, three types of bearings are required: wide-purpose bearing for clean environments, hightemperature bearings for clean environments, and corrosionresistant bearings for clean environments. The configurations and features of bearings for clean environments are given in Table 4 1). These are the commercialized products whose configurations are based on the previous test results of Fig. 2. As for wide-purpose bearings for clean environments, there are stainless steel bearings whose cages (SUS304) are coated with PTFE, and stainless steel bearings whose entire surfaces are coated with special fluorocarbon polymer, either of which can be used at temperatures up to 200;. Next, as for high-temperature bearings for clean environments, there are stainless steel bearings whose cages (SUS304) are coated with PTFE for high-temperatures and allceramic bearings whose cages (SUS304) are coated with PTFE as a high-temperature coating, either of which can be used at temperatures up to 300;. In other words, hightemperature bearings for clean environments are the bearings for applications used at temperatures from 200; to 300; and requiring cleanliness. And, there are three types of corrosion resistant bearings for clean environments: hybrid ceramic bearings with inner /outer rings of precipitation hardened stainless steel (SUS630) whose rolling elements (balls) are made of silicon nitride and whose cages are made of fluorocarbon resin, all-ceramic bearings whose inner /outer rings and rolling elements (balls) are made of silicon nitride and whose cages are made of fluorocarbon resin, and similarly all-ceramic bearings whose cages are coated with PTFE. These bearings are used for applications requiring cleanliness in a corrosive environment in temperatures up to 200;. 4. Bearings for Vacuum Environments Bearings made of martensitic stainless steel (SUS440) are generally used as bearings for vacuum environments. Concerning lubrication, for vacuum environments, because there is no lubricant that can be used in all applications from atmosphere to ultra-high vacuum atmospheres, it is important to select the lubricant (mainly solid lubricants) according to the conditions under which the bearing is used Configuration and Features of Bearings for Vacuum Environments Table 5 gives the configuration and features of bearings for vacuum environments 1). The features, performance and application of the bearings are summarized by classifying the lubricant (mainly solid lubricants) used for bearings for vacuum environments. Bearings with vacuum grease can be used up to a vacuum of about 10 4 Pa. Because these bearings can be used at highspeed rotation and have superior load capacity, as bearings for vacuum environments, it is desirable to use bearings with vacuum grease as much as possible until grease contaminates vacuum environment. 62 KOYO Engineering Journal English Edition No.157E (2000)

6 Table 5 Configurations Koyo EXSEV bearings for vacuum environments " : Superior # : Good Packed with vacuum grease Coated with MoS 2 PEEK resin Special fluorocarbon polymer Ion-plating of lead Ion-plating of silver Structure Features In extremely high vacuums " In atmospheric conditions " Cleanliness " Performance Applicable temp. range 40 to 200 C 100 to 300 C 100 to 200 C 200 to 300 C max. 550 C High-speed rotation " # # (dn value 1) : Up to ) Load " # # Conductivity "(for semiconductors) " <Vacuum pump> T.M.P Cryopump Other devices <Semiconductor production facilities> Bearings for transferring waters (spattering device) Bearings for transferring large trays (P-CVD) Bearings for door opening /closing mechanisms (in etching machines) <Semiconductor production facilities> Bearings for supporting turning mechanisms in various types of transfer machines Applications Bearings for supporting ball screws (in vertical type diffusing furnaces) (Spattering devices, CVD, MBE devices) Bearing for supporting robot arms <Medical facility> Other equipment Bearings for X-ray tubes <Various types of measuring instruments and analyzers> Other equipment and devices Note : 1) dn is defined by "inside diameter of bearing (d ) rotation speed (n )". Next, MoS 2 coating bearings (coating for cage), PEEK resin-cage ones and special fluorocarbon polymer coating ones for entire bearing are mainly used for semiconductor manufacturing equipment. Compared with PEEK resin ones and special fluorocarbon polymer film ones, MoS 2 coating bearings can be used for higher speed rotation, and have superior load capacity. Also, because special fluorocarbon polymer film ones have superior cleanliness, they are used for applications requiring cleanliness among semiconductor manufacturing equipment. Furthermore, because bearings of lead ion-plating bearings (Pb-IP for balls) and silver ion-plating ones (Ag-IP for balls) use soft metal for lubrication, almost no emission gas is generated from the lubricant, and the bearings are therefore used for ultra-high vacuums. Because these bearings furthermore exhibit comparatively superior characteristics for heat resistance, high-speed rotation, and load capacity, they are used for rotation anode of X-ray tubes for medical KOYO Engineering Journal English Edition No.157E (2000) purposes Gas Emission Characteristics of Bearings for Ultra-High Vacuum Environments In the previous section, it was stated that bearings whose rolling elements (balls) are ion-plated with lead or silver are used as bearings for ultra-high vacuum environments. Here, gas emission characteristics for bearings whose balls are ionplated with lead and bearings whose balls are ion-plated with silver are examined. Fig. 3 shows the ultra-high vacuum tester used here 7). The vacuum chamber is made of aluminum alloy, and its internal dimensions are The exhaust system consists of a rotary pump, turbo molecular pump, sputter ion pump, titanium sublimation pump, etc. Fig. 4 shows the test method and Table 6 gives the test conditions 7). 63

7 Extraction type ionization vacuum gauge Quadruple-pole type mass analyzer Table 6 Test conditions Pressure Load Rotation speed Temperature Pa F r = 3N Room 140r/m ( Torr) F a = 98N temperature Bearing testing unit Sputter ion pump Titanium sublimation pump Rotary pump Fig. 3 Ultra-high vacuum tester (Vibration measuring point) Vacuum isolating partition Vacuum chamber Turbo molecular pump Internal rotor (magnetic stainless steel) External rotor (permanent magnet) Two test bearings (#608) are used and axial load is applied by springs. Pressure in the vacuum chamber is Pa, and the bearings are rotated from outside the vacuum chamber by a magnet coupling type rotation implementation machine. Fig. 5 shows the test results 7). Fig. 5 (1) shows pressure change inside the vacuum chamber for the bearings whose balls were ion-plated with lead (Pb-IP), and Fig. 5 (2) shows pressure change in the vacuum chamber for the bearings whose balls were ion-plated with silver (Ag-IP). The figures show that pressure increased simultaneously with the start of rotation for both the bearings whose balls were ion-plated with lead and the bearings whose balls were ion-plated with silver, and recovered to about the same pressure as before testing when approximately 10 hours had elapsed. Also, it has been confirmed that even if rotation of the bearing is stopped within 10 hours, pressure quickly returns to the level of that prior to rotation. Thus, from the results of the bearing rotation test in an ultra-high vacuum of Pa, it has been made clear that bearings whose balls are ion-plated with lead or silver can be used in an ultrahigh vacuum. Coil spring Vacuum chamber Fig.4 Test method Test bearings 5. Conclusion Here, the typical configuration of Koyo EXSEV bearings for extreme special environments have been introduced, followed by a detailed description of bearings for vacuums and bearings for clean environments, which are the main products among the bearings for extreme special environments. Bearings for vacuums and bearings for clean Start of rotation Pressure in vacuum chamber, Pa Pressure in vacuum chamber, Pa Start of rotation Time, h Time, h (1) Pb-IP bearing (2) Ag-IP bearing Fig. 5 Test results (Pressure change in vacuum chamber) 64 KOYO Engineering Journal English Edition No.157E (2000)

8 environments are for applications where grease and oil are disliked, and are used mainly for semiconductor and liquid crystal manufacturing equipment. Technology concerning semiconductors and liquid crystal progresses year by year, and it is believed that usage environments and performance requirements along with this progress will also become severer year by year. In order to improve the performance of bearings for vacuums and bearings for clean environments, therefore, Koyo has to continue to make progress for developing bearing materials and solid lubricants. References 1) Koyo Seiko Co., Ltd: EXSEV Bearings Series Ceramic Bearings and EXSEV Bearings, CAT. No ) H. Takebayashi, K. Tanimoto, T. Hattori: Journal of the Gas Turbine Society of Japan, 26, 102 (1998) 55. 3) J.R. Miner, W.A. Grace, R. Valori: ASLE Preprint No. 80- AM-3C-3 (1980). 4) H. Takebayashi, K. Hayashida: Koyo Engineering Journal, 145 (1994) ) H. Takebayashi, T. Yuine, T. Yoshioka: Tribologist (Journal of Japanese Society of Tribologist), 38, 10 (1993) ) T. Hattori, K. Kitamura, K. Hayashida: Koyo Engineering Journal, 156E (1999) 30. 7) Y. Fujii, H. Ishimaru: J. Vac. Technol. A9 (3). KOYO Engineering Journal English Edition No.157E (2000) 65