Bearings with Solid Lubricants

Transcription

1 SURVEY Bearings with Solid Lubricants Y. FUJII H. ONO With recent progress in technology, s have been used in severe environments and in extreme conditions. Especially, highly advanced technology requires s to be applicable in special environments, such as vacuum, cleanliness and high-temperature. As for conventional s lubricated with oil and grease, it is extremely difficult to meet these requirements. Koyo has developed and marketed various types of s with solid lubricants, which meet these various needs. In this paper, the development of s with solid lubricants is presented. Key Words: EXSEV, solid lubricant, special environment 1. Introduction With recent progress in technology, the requirements on performance have become more and more severe and diversified year after year. Particularly in advanced technological fields, such as semiconductor, liquid crystal, space and medicine, s are used in extreme environments such as vacuum, cleanliness and high-temperature. In such special environments, there are many cases where it is impossible to use oil and grease, and therefore solid lubricants must be applied. Koyo has developed various types of s with solid lubricants in order to meet the diversified needs. This paper presents Koyo solid lubricant s. 2. Classification of Solid Lubricants and Their Application to Bearings There are mainly three types of solid lubricant; soft metals, layer lattice s and high polymers. Typical usage, their characteristic and applications to s are shown in Table 1 1). Solid lubricants of soft metals like silver (Ag) and lead (Pb) are used in the extreme high vacuum condition where gas emission from s may cause problems. It must be noted that Ag-lubricated drastically reduced its durability due to effects of oxygen etc. when used in the atmosphere. High polymers are often used where cleanliness is required, or in corrosive environments. Due to their inertness in the atmosphere, high polymers can also be used for s operated in repeatedly changing environments of atmosphere and vacuum. Layer lattice lubricants are used either under high temperature or when longer life is required with a solid lubricant. Moreover, both MoS 2 and WS 2 can be used even in vacuum. Configurations of s with these three types of solid lubricants commercialized according to their applications are shown in Table 2. Soft metal Ag is coated on balls. Layer lattice and high polymer lubricants are either coated on itself or used as retainer s. Table 1 Typical solid lubricants for rolling Principal characteristics Application to s Classification Lubricant Heat stability, ; Cleanliness Gas emission Typical usage Main applications, Atmosphere Vacuum purposes Soft metal Silver (Ag) 500 % " Lead (Pb) 300 % " Coating to balls Extreme high vacuum MoS % # Layer lattice High temperature WS % # Coating to retainers Longer life Graphite 500 % # and raceways High polymer PTFE " % Retainer Cleanliness Polyimide # % Corrosive environment 24 Koyo Engineering Journal English Edition No.164E (2004)

2 Table 2 Configurations and features of Koyo s with solid lubricants Types of lubricants High polymer Layer lattice Soft metal Classification for hightemperature PTFE coating MoS 2 Coating Ag ion plating Coating type Bearings configuration (Thick line shows coated part) Whole surface Coated parts Details of (except shields) coating film Fluorine high Film polymer Atmospheric Performance pressure, Pa Inner and outer raceways, balls, retainer inside face Fluorine resin Ambient pressure ~ 10 5 Retainer Retainer PTFE (organic binder) MoS 2 (inorganic binder) Balls Ag (with special undercoating) 10 3 ~10 10 Temperature, ; * 100~ ~ ~ ~ ~500 Cleanliness Excellent Excellent Excellent Combination of high Types of lubricants High polymer polymer & layer lattice Layer lattice Classification FA retainer PN retainer WS 2 separator Graphite retainer Retainer type Retainer components Performance Bearing structure Fluorine resin layer Lubricants Fluorine resin WS 2 Graphite lattice lubricant Others Reinforced fiber PEEK resin, reinforced Metallic sintered fiber Atmospheric pressure, Pa Ambient pressure ~ 10 5 Temperature, ; * 100~200 30~ ~350 Cleanliness Excellent Inorganic additives Ambient pressure 100~500 * The temperature is applicable only to lubricants and lubricating retainers. According to the operating temperature, s for inner and outer rings, balls must be considered additionally. 3. Life of Bearings with Solid Lubricants Life of s with solid lubricants differs depending on application method of the lubricants to the s. In case of coating type, when the film wears out and the supply of lubricants is exhausted, the reaches the service life. On the other hand, when the retainers or separators themselves are made of solid lubricants, it is not necessary to consider lubricant supply like the case of film. Therefore longer life can be expected under proper rotational conditions. In this case, the reaches the life after longer operation when the torque increases due to biting of lubricant particles from excessive lubricant supply or when vibration of s increases due to wear of retainers or separators. Koyo Engineering Journal English Edition No.164E (2004) 3. 1 Life of Coating Type The followings are estimated life calculation formulas for several lubrication film types of s obtained through 1), 3) tests. a), PTFE and MoS 2 coating Lav = b 2 (Ce / P) / n Lav : life (reliability: 50%), h b 2 : coefficient PTFE, MoS 2 coating = 6 = 42 Ce : basic dynamic load rating of the steel with the same dimensions 0.85, N P : dynamic equivalent load, N n : rotational speed, min 1 25

3 b) Ag ion plated balls Lvh = b 1 b 2 b 3 (CV/ P) / n Lvh : life (reliability: 90%), h Cv : basic dynamic load rating of the steel with the same dimensions 1 / 13 P : dynamic equivalent load, N n : rotational speed, min 1 However, speed range 10 n b 1 : rotational-speed dependent coefficient b 1 = n + 1 b 2 : coefficient, = 1 (when silver is plated by means of special ion plating method) b 3 : coefficient by ambient atmosphere temperature = 1 (10 3 Pa, in case of room temperature) 3. 2 Life of Retainer Material Type As described above, the life with retainers or separators made of lubricant is generally longer than that of coating type. The life of WS 2 s with solid lubricants developed for longer life is described below 6). Figure 1 shows the life test results of this type of. The results shown here were gained when the test was suspended. There was no abnormality and the test has continued. The life of this is estimated to be over 100 times as long as that of, which has the longest life among the coating type s. Total number of rotation Life of WS 2 separator s * room temperature ) 300: : Suspended data (Continuous operation is possible) <Reference> Estimated life curve of coating type s MoS2 coating Equivalent load / dynamic load rating (stainless steel ), % Fig. 1 Life test results 4. Bearing Configurations Applicable to Each Special Environment 4. 1 For Vacuum Environment Figure 2 shows the application range of lubricants by ambient pressure and temperature. Fluorine type grease can be used even in vacuum environment till 10 5 Pa, when the temperature is around room temperature. However, it is necessary to consider the contamination due to fluorinated oil. To avoid contamination by oil, solid lubricants should be used in high temperature range even if the pressure is the same. According to the temperature range, each lubricant as shown in Fig. 2 can be used. In the extremely high vacuum with pressure lower than 10 5 Pa, the gas emission from s becomes more important and soft metal, mainly silver (Ag) has to be used. Temperature, : Test WS2 separator PN retainer, MoS 2 coating for high temperature 1 FA retainer { Fluorine type grease } Ag ion plating (heat-resistant is used) Ag ion plating Pressure, Pa Fig. 2 Application range of each lubricant for vacuum environment A test result of gas emission characteristics of s with Ag ion-plated balls is introduced 2). Figure 3, Table 3, and Fig. 4 show the test method, the test conditions and the test results respectively. Vacuum chamber Test conditions Table 3 Test conditions Bearing type number 608 (u8 u22 7) Material specifications Ambient pressure Load Rotational speed Ambient temperature Test s Spring for loading Magnet {Driven by magnet coupling from outside of the vacuum chamber} Fig. 3 Test method Inner/outer rings, balls: stainless steel Retainer: stainless steel plate * Ag ion plating on balls Pa Axial : 98N Radial : 3N 140min 1 Room temperature 26 Koyo Engineering Journal English Edition No.164E (2004)

4 Ion current intensity Ion current intensity Ion current intensity H2 H2 H2 H2O H2O H2O N2, CO N2, CO N2, CO q : Before rotation CO2 w : 2 minutes after rotation Ar CO2 e : 10 hours after rotation CO Mass spectra Fig. 4 Test results (mass spectra of residual gas) For this test, a test was placed in a vacuum chamber and was driven by a magnetic coupling from outside of the vacuum chamber. The gas was detected by a gas analyzer (mass spectrometer) installed in the vacuum chamber. Emission of various kinds of gas most likely adsorbed to the, and Ar gas used for the treatment of ion plating was observed immediately after rotation. However, after 10 hour-rotation, the condition returned to the same as before the rotation, and gas emission from the could not be detected. In the same method, s with balls treated with MoS 2 sputtering were tested. As a result, gas emission was observed even after 60 hours of rotation. Sulfur (S) forming film was observed as a gas emission constituent 2). Also, a with retainers coated with PTFE (baking treatment) was tested at higher pressure of around 10 6 Pa. Gas emission of fluorocarbon (FC) and hydrocarbon (HC) was observed here. 1) From the above observation, it can be concluded that Ag is suitable as a lubricant at extremely high vacuum ranges of less than 10 5 Pa. Particle emission, particles/ 2 s Bearing code Inner/ outer rings Balls Retainers Temperature, : Test s Clean bench (Class 10) Ambient air Magnetic fluid seal SUS304 Fig. 5 Test method Particle counter A B C D E F Si3N4 38 Recorder Test conditions Bearing : ML6012(u6 u12 3) Load : radial 2.9N / 2 s Rotational speed: 200 min 1 Atmosphere : Class 10 Clean bench room temperature Test time : 20 h Particle diameter: over 0.3lm Ag ion plating SUS304 + MoS2 coating Fig. 6 Test results for high temperature FA retainer FA retainer 7 Layer lattice lubricants 4. 2 For Clean Environment Particle emission test using various kinds of lubricants was conducted. Figure 5 shows the test apparatus and Fig. 6 shows the results 1), 4). While the particle amount of Ag or MoS 2 lubricated s (C and D in Fig. 6) is over , that of s with a solid lubricant of fluorine high polymer (E and F in Fig. 6) is much smaller by three digits figures. It shows that s with solid lubricants of fluorine high polymer are applicable for clean environment. Figure 7 shows the application range of each solid lubricant for clean environment. Koyo Engineering Journal English Edition No.164E (2004) Cleanliness class Fig. 7 Application range of each solid lubricant for clean environments 27

5 4. 3 For High Temperature Environment Figure 8 shows the application range of each solid lubricant for high-temperature environments.,,,,,,, Temperature, : Graphite retainer WS2 separator PN retainer, MoS2 coating for high temperature Ambient pressure Vacuum Fig. 8 Application range of each solid lubricant for high-temperature environments Up to 350;, for high temperature, PN retainer or WS 2 separator can be applied both in ambient pressure and vacuum according to the temperature. For higher temperature over 350;, graphite retainer can be applied up to 500;. However, the application is possible only in ambient pressure, as there is no lubrication effect in vacuum. In the extreme high temperature range over 500;, there is no applicable lubricant today. In this temperature range full ceramic s (both inner/ outer rings and rolling elements are made of ceramic) could be used without lubricant since there is no other solution. 5. Application of Ceramic Ceramic is not a lubricant. But its bond configuration is covalent bond and has characteristics not to cause adhesion easily as metals do. Consequently, there are some cases where ceramic s are used without lubricant under special environments. In the preceding Fig. 6 where particle emission was tested under no lubrication condition, s with ceramic balls (Si 3 N 4 ) reduced the particle emission by two digits compared with all other steel s. From this viewpoint, the effect of ceramic can be easily recognized. (Comparison of A and B in Fig. 6) Moreover, when solid lubricants are used with ceramic s, superior performance can be achieved in many cases. A test using the combination of solid lubricants and ceramic has been conducted 5). Table 4 shows the test s and the test conditions. Figure 9 shows the particle life result of the s consisting of some s and PTFE coated retainers until the particle amount from the s reaches a certain point. In this case, PTFE film on the retainers functioned as a lubricant. It can be seen that the particle life dramatically increases by changing the from stainless steel to ceramic. From this test, it was confirmed that PTFE film has different adhesiveness characteristics depending on retainer s, and that ceramic s could be lubricated with smaller amount of lubricant compared with other s. Test Test conditions Particle life, h Table 4 Test and conditions Bearing A Bearing B Bearing C Stainless Hybrid ceramic Ceramic steel Bearing No. 608 (u8 u22 7) Inner/ outer rings Silicon nitride Balls Silicon nitride Silicon nitride Retainer SUS304 + PTFE coating (baking) 1.00E E E E+01 Rotational speed Atmosphere Measurement Judgment of life 200 min 1 In Class 10 clean bench, room temperature Number of particles with particle diameter over 0.3 lm Time for over particles/0.1cf Stainless steel Hybrid ceramic 1.00E Axial load, N Ceramic Bearing A Bearing B Bearing C Fig. 9 Particle life of ceramic coated with PTFE film 28 Koyo Engineering Journal English Edition No.164E (2004)

6 6. Conclusion This paper presented the outline of Koyo s with solid lubricants. In the future, the environments where s are used will become more and more severe and diversified. Further development of s with solid lubricants for special environments will be significantly important in order to comply with diversifying customer-needs and satisfy still growing performance requirements. References 1) Koyo Seiko Co., Ltd.: CERAMIC BEARINGS AND EXSEV BEARINGS, CAT. no. 208E. 2) H.Yasui, K. Kakumoto, Y. Fujii : Koyo Engineering Journal, 139 (1991) ) H. Takebayashi: Koyo Engineering Journal, 156E (2000) 52. 4) H. Takebayashi: Koyo Engineering Journal, 157E (2000) 58. 5) H. Toyota : Koyo Engineering Journal, 150 (1996) 53. 6) Koyo Seiko Co., Ltd.: WS Bearing, CAT. no. 184E. Y. FUJII * H. ONO ** * Analysis Engineering Department, Bearing Business Operations Headquarters ** Industrial Machinery Application Engineering Department, Bearing Business Operations Headquarters Koyo Engineering Journal English Edition No.164E (2004) 29