Recent Trends in Research and Development of Rolling Bearings at NSK

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1 Recent Trends in Research and Development of Rolling Bearings at NSK Kyozaburo Furumura Managing Director, Research and Development Center. Introduction Rolling bearings have an extremely long life cycle, but their design details are under continual evolution though their basic shape remains the same. For example, general trends in bearings are toward increasing compactness, lightness of weight, thinness, and reliability. These developments are in line with the progress of automotive technology. Other trends are toward ultra-compactness and higher accuracy along with function sophistication and size reduction for computers and related products. Furthermore, new bearings are being developed as industries related to the environment, semiconductor manufacture, and space equipment emerge. NSK, and specifically, its Research and Development Center, has been engaged energetically both day and night in research to push forward fundamental technologies that support the advancement of bearings. 2. Recent Trends in Computer-aided Bearing Analysis As the computational power of computers has progressed, the design of rolling bearings relies more and more on computers. In addition to modern technology to calculate the load and deflection on the bearings as well as the structural analysis of the bearing surrounding, EHL (Elasto-hydrodynamic Lubrication) theory has provided a clear explanation of the lubrication mechanism at the contact section of rolling bearings. Since then, the technology to analyze and to theoretically predict the performance of rolling bearings has progressed remarkably. With the emphasis on compact and lightweight designs, a concern has emerged that the deformation of housing and shaft may affect the bearing life substantially. This requires elasticity analysis of the bearing-shaft-housing system. In the case of roller bearings, in particular, inclination of the shaft causes the contact pressure between the rolling element and bearing ring to result in stress concentrations near the edges of the contact area. In this case, the contact analysis is indispensable for determining the optimum crowning shape. Dynamic analysis of bearing behavior during rotation is now possible thanks to progress in computer and theoretical analysis methods. By fully utilizing these technologies, NSK has developed an original bearing performance analysis software called BRAIN that stands for "BeaRing Analysis In Nsk." Though sophisticated in its analytical features, BRAIN can calculate results extremely quickly (a few minutes on a PC), and is thus frequently used as a design tool for bearing optimization. With BRAIN, analysis can be made for all of NSK's bearing products, including ball bearings, cylindrical roller bearings, tapered roller bearings, spherical roller bearings, etc. Also with this software, the user can calculate the attitude of the rolling elements (skew, tilt) or revolving/rotating slide of rolling elements as well as the PV value even in the case that the inner/outer ring is deformed. Bearing performance analysis includes simulation of the motion of bearing elements on the basis of the equilibilium of forces and moments, acting on the inner/outer ring, rolling elements, and cage considering frictions. By solving the dynamic equation for each bearing element while eliminating the assumption of pure rolling used in a conventional bearing calculation, a more reliable analysis becomes possible. In this way, this bearing performance analysis software enables the user to predict the bearing heat generation, rolling element attitude (skew, tilt), slide of rolling element and bearing ring, and the PV value, thereby promoting optimization of the bearing's internal design dimensions to meet the customer's needs. This Frictional loss, W Fig BTX Steel ball Rolling resistance Gyroscopic slip Spin slip Others (X) Speed, rpm Friction loss analysis of angular contact ball bearings by BRAIN Motion & Control No. 996

2 Attitude angle, degree Fig BTX Si3N4 ball Steel ball (x) Speed, rpm Attitude angle analysis of angular contact ball bearings by BRAIN analysis can also be used for the prediction of bearing damage, such as seizure, smearing, abnormal wear, and premature flaking. As an example of a calculation with BRAIN, Fig. shows the calculated heat generation of an angular contact ball bearing ). This calculation shows not only the total heat generation of the bearing, but also a breakdown of the heat generation into its different types. It is therefore possible to study how bearing design affects heat generation. Fig. 2 shows the ball attitude angle obtained through calculation. The "Jones control theory" applied widely up to now does not include the gyro moment. However, as the speed rises, depending on whether the gyro moment is included or ignored, leads to a substantial difference in the result. This in turn means that the conventional analysis based on control theory and the assumption of pure rolling is not rigorous enough for accurate analysis of high-speed bearings. NSK is aiming at analysis with improved reliability which allows the bearing to demonstrate its fullest performance. Also, NSK will continue to make its best efforts to support its customers by means of fast and accurate software. Computer-aided bearing performance analysis is indispensable, since NSK has to satisfy increasingly stricter demands on bearings including speed increase, torque reduction, life extension, and price reduction. Also, automotive bearings and large industrial bearings increasingly require elasticity analysis of the bearing and its surroundings as an essential part of the design process. High-level know-how and extensive expertise are necessary for such analysis. In NSK, a dedicated group of computer specialists and engineers makes highly reliable analysis within a short period of time by fully using appropriate FEM software and BRAIN. 3. Recent Development of Tribology Technology The practical achievements of EHL theory, which may be traced to around 96, are closely connected to the tremendous computational power of computers. NSK is pushing forward research to improve EHL theory further. This includes EHL analysis of high pressure on the one hand and the so-called starved EHL problem with an extremely limited lubricating oil amount on the other hand. In research relating to EHL analysis, the absolute value of the contact pressure was around GPa at maximum. For actual rolling bearings, however, a contact pressure of GPa is classified as a light load condition, since a higher pressure of 2 GPa occurs more frequently in general use (and even a contact pressure of 3 GPa is not uncommon in severe cases). Since the Dowson-Higginson's formula that is applied widely for calculation of the oil film thickness was derived from the results of analysis under low contact pressure conditions, there are disputes concerning its applicability to high contact pressure conditions. Recent computer development and calculation software have proved their worth by producing accurate calculations for applications under high contact pressure. Fig. 3 shows a comparison between the calculation results of oil film thickness made systematically up to 3 GPa by Natsumeda and the results obtained by the Dowson-Higginson's equation 2). The results agree well with each other. In short, this figure indicates the reliability of the Dowson- Higginson's equation. Hc x, H min x 2 p H, GPa G= G=2 : Dowson-Toyoda : Dowson-Higginson : Hc : Hmin Wx Fig. 3 Minimum and central film thickness for varying W (U= - ) Fig. 4 shows the calculation of the pressure distribution. According to this figure, the pressure distribution at high pressure is approximately equivalent to the Hertzian pressure distribution. A pressure spike, which is commonly observed with EHL, does not appear. This is also a reasonable result from the viewpoint of practical experience. Motion & Control No

3 p/p H p/p H.. W=3.7x -4 (p H =.6GPa) W=.8x -3 (p H =3.GPa) Film thickness, µm Fig VG32 VG68 VG Experiment Theory Fa = 9.6N n = 36 rpm Initial amount: mm 3 2 Running time, hrs Film thickness during long term test the oil amount decreased, as shown in Fig., and that the oil film did not often become thick even when the speed increased. Further, they showed that such a thin oil film might survive for a considerably long period as shown in Fig. 6. This is considered to indicate a new lubrication mechanism (parched EHL, etc.). Fig. 4 Film thickness, µm Fig Pressure distribution and film shape for varying Load (U = -, G = ) Amount of oil mm Rotational speed, rpm The second topic, starved EHL, has been the subject of systematic experimental study by Spikes, et al 3). An optical interference method was used to show the formation of the EHL oil film. Its thickness was on the order of nm. In another study, Akagami and Aihara 4) lubricated an actual rolling bearing with an extremely small amount of oil. By measuring the EHL oil film thickness with a displacement method, they showed that the oil film became thin when 2. Oil-film thickness measurement of ball bearing with trace oil lubrication 4. Technology to Enhance the Performance of Bearings Along with increased efficiency and labor savings as well as size reduction and increased output, the operating conditions of bearings have become considerably more severe. NSK relentlessly develops assorted technologies to enhance bearing performance to satisfy diverse customers' needs. 4. Trends toward speed-increasing technology A bearing for an airplane gas turbine spindle must demonstrate its high reliability under severe conditions, including high speed, high temperature, and heavy load. Recently, a new performance demand emerged relating to the survival of the bearing during and after a lubricating oil shut-off test as a means to ensure safe high-speed operation. In other words, the bearing must not suffer damage even though the lubricating oil supply becomes temporarily insufficient during operation. NSK has developed a chemical surface modification technology that is effective at improving the above-mentioned performance ). Using such surface modification for a threepoint contact ball bearing, the lubricating oil was shut off at the dn value of 2.2 million to confirm its superior seizure resistance. As shown in Fig. 7, the above dynamic analysis of bearings has shown that the improved bearing can have its seizure resistance enhanced through both reduction in the friction heat generation between the ball and bearing ring during shut-off of the lubricating oil supply and inhibition of the local temperature rise between the ball and groove 6). NSK is also proceeding with development of ceramic 7 Motion & Control No. 996

4 Inner ring temperature rise, C Fig Bearing Normal Surface Surface Treated Si3N4 Ball Rev. speed (rpm) Normal surface Axial load (kn) Surface treated Seizure Si3N4 Ball 2 3 Time after oil shut-off, s Bearing temperature rise after oil shut-off Oil re-supply (3.6 s) performance gas turbines for the 2st century. NSK s responsibility is to create a new generation of high-speed and high-temperature bearings. 4.2 Improvement of axial load capacity Higher axial load capacity cylindrical roller bearings are required as substitute bearings to achieve compact yet lowcost machines, thereby eliminating the original requirement for thrust bearings in certain applications. NSK is undertaking basic research on the lubrication characteristics between the collar that supports the axial load and the roller end surface in order to further enhance the thrust load capacity of the above cylindrical roller bearing. According to model tests and to analysis by partial EHL, the seizure of the cylindrical roller bearing collar depends on the contact condition between the roller end surface and collar. Local seizure is found to grow until seizure of the whole collar 7). Currently, a model test is under way to check for change in the roughness due to run-in and to investigate the progress of seizure. Friction loss at inner raceway, W Chemically modified Non-modified 4.3 Trends toward lower torque Nowadays, tapered roller bearings used in automobiles must have dynamic frictional loss reduced, or torque reduced, in order to save energy. It is essential for the reduction of the torque of the tapered roller bearing that a stable oil film be formed over the contact area between the roller and rib. A few years ago NSK achieved operation at low torque on the basis of EHL theory (Fig. 9) 8). To reduce the dynamic friction further in a low-torque bearing, decrease in the viscosity of the lubricating oil used and reduction of the lubricating oil amount proved to be effective measures. However, these measures cause an increased possibility of seizure. Consequently, NSK is proceeding with the research and development of a tapered roller bearing with low torque and superior seizure resistance. Fig Time after oil shut-off, s Friction loss due to slip between inner ring raceway and rolling element (Silicone Nitride) bearings for airplane gas turbines, since they can satisfy the demand for light weight and high speed. A hybrid bearing that is equivalent to an M material bearing (except for its balls which were made from Silicone Nitride) was tested at the dn value of 2. million. The test results confirmed its superior survival performance after the lubricating oil supply shut-off. Experimental results are shown in Fig. 8. The next generation of gas turbines requires bearings that can run reliably at temperature and speed conditions far exceeding the current limits. NSK participates in the AMG project which is sponsored by a consortium that is conducting long-term research and development of fundamental technologies necessary for ultra-high Running torque, kgf cm Fig. 9 Conventional bearing Low-torque bearing 2 3 Rotational speed, rpm Development of low-torque tapered roller bearings based on analysis Motion & Control No

5 . Vibration and Noise Reduction Technology of Bearings Recently, quietness has been highlighted as one of the sales points for electric home appliances (air conditioners, washing machines, vacuum cleaners, etc.) and automobiles. This means that the bearings used in these machines must meet increasingly stricter demands calling for lower vibration and less noise. Research and development projects are under way to satisfy such demands. The low-vibration characteristic is of vital importance for AV and OA equipment, such as VCR, HDD (hard disk drive), etc. In particular, the vibration of a bearing in an HDD can adversely affect the memory, so such vibration must be reduced by all means. As years go by, HDD spindle motor bearings are becoming smaller and have better accuracy. The accuracy of a bearing considered as an independent unit is. µm or less in terms of NRRO (Non Repetitive Runout). The capacity of HDD will be enlarged further in the future to reach GB to 2 GB. It is presumed that NRRO required for the bearing will be.2 µm or less in the year 2, as shown in Fig. 9). Therefore, the NSK R&D Center is actively researching how to improve the performance of HDD spindle motor bearings. (µm) Track pitch Motor NRRO Bearing Noise is a problem related to human senses, and thus, the noise needs to be reduced especially in household appliances that are used daily such as air conditioners, VCRs, etc. When vibration is considered, the bearing noise contains a frequency component in the higher frequency range and is distributed over a wide bandwidth. This makes it difficult to adopt a countermeasure specifically against noise, but it is possible to take countermeasures 2..6 year Fig. Improvements in track pitch and motor NRRO of hard disk drives 2.. (µm) against vibration. Factors responsible for bearing noise include the shape error (waviness, roughness of bearing components) and the grease. To solve such problems, low-noise greases with optimum base oil viscosity, consistency, and soap fiber structure have been developed specifically for this purpose. These greases are designed not only to achieve satisfactory acoustic performance during the initial period, but also to offer low-noise performance over a long period of use. 6. Lubricating Grease Lubricating grease for rolling bearings plays an important role in assuring a long bearing service life. Approximately 8% of bearings are lubricated with grease because it helps to simplify the surrounding parts of the bearing. To meet increasing demands for maintenance-free operation, about 7% of the above-mentioned bearings are sealed or shielded ball bearings. The grease used affects the performances of the bearing, including long life (flaking, seizure, sound), low friction torque, low temperature rise, low noise, low vibration, and high rustpreventive capacity. For example, a sealed ball bearing, without any subsequent grease supply, eventually suffers damage (seizure, etc.) due to degradation of the grease, which results in a shorter service life. It goes without saying that the grease performance is one of the key factors controlling the performance of bearings ). 6. Present grease development in NSK Standard greases for rolling bearings till the middle of the 97s were lithium soap/mineral oil, natrium soap/mineral oil, and lithium soap/silicone oil greases. The demands of customers, which were growing increasingly stricter concerning enhanced performance, life, and reliability, seemed not to be fully satisfied with the improvements brought by the above greases. Against that background, NSK decided to develop its own grease inhouse, instead of relying on greases developed by outside suppliers. NSK development has focused on ball bearings for electric machinery and for information fields as well as for automotive electric devices and engine auxiliary equipment fields, in which the grease contributes considerably to the bearing performance. To meet the needs for heat resistance and low-temperature performance under application conditions, a synthetic oilbased grease is frequently used. Over the years, NSK has formulated many dedicated greases and now offers about 3 different greases. NSK is ready to recommend an appropriate grease for your application. In electric machinery and information processing fields, several types of lithium soap/ester oil grease have been developed to meet the demands for low noise, low torque, and satisfactory acoustic life performance. Moreover, in electric devices and engine auxiliary equipment fields, more than ten types of urea/synthetic oil (ester, polyalpha- 9 Motion & Control No. 996

6 olefin, ether) greases have been developed to satisfy the demands for superior high-temperature resistance, high speed rotation, and rust prevention. Since the middle of 98, the poly V belt has been used to drive the electric devices and auxiliary equipment. This in turn caused frequent flaking with structural change in an alternator. Though the frequency could be reduced through improvement of the bearing material and design, this problem is not yet fully eliminated. However, the urea/ether oil-type MA7 grease has been developed and proved effective in eliminating the occurrence of any flaking for the last five years ). Fig. shows the effects of MA7 grease. Percent failed, % 99 9 Grease E Grease MA7 L cal Life, hrs Fig. Rolling contact fatigue life of actual engine test under updown cyclic revolution (E grease vs. MA7 grease) Rotational speed: 3 rpm (top) rpm (bottom) Load ratio (P/Cr):.4 Currently, more greases are being developed. Greases thus developed include a grease for roller bearings, a lowsplashing grease, a fretting resistant grease, and a fluoric grease for high temperature use. 6.2 Prediction and equation for grease life NSK has supplied grease life equations in bearing catalogs. These equations were formulated through accumulation of extensive data from grease life tests in the laboratory. For the mixed use of lithium soap and urea greases in the bearing, NSK has studied the degradation level, establishing the degradation limit value for estimating the remaining service life 2). Subsequently, experiments were made to classify general greases into five types according to their composition. Based on this classification, a life equation has been prepared to enable calculation of the average life of grease when the bearing number and bearing conditions are known. Fig. 2 shows a summary of experimental results which provided a basis for the grease life equation 3). The ability to estimate the grease life more precisely contributes to better machine design and maintenance scheduling. 7. Long Life and High Reliability Given satisfactory lubricating conditions with sufficient oil film formation and successful prevention of mixing-in of foreign particles, then the fatigue strength of materials and its variance are key factors governing the durability and reliability of a bearing. Based on joint research with steel makers conducted over a long period of time, NSK has succeeded in co-developing the world's cleanest bearing steel. As shown in Fig. 3, this extra purified steel, or EP steel, offers an over five-fold longer life and provides higher reliability than existing highly clean steels. In actual practice, however, lubricating conditions are often less than satisfactory. In response, NSK's research 99 9 Grease life, hrs Percent failed, % L cal. SAE2 (N=3) Existing steel SAE2 (N = 3) EP steel Life, hrs Temperature, C Fig. 2 Effect of temperature on grease life Fig. 3 Results of fatigue life test for two high-cleanliness steel bearings (626 deep groove ball bearing) Motion & Control No. 996

7 has focused on long-life and high-reliability under tough lubricating conditions where mixing-in of foreign particles is expected. HTF and STF bearings of NSK have been developed to provide long life under contaminated conditions with foreign particles. With contaminated lubrication, the life of the bearing is determined by the stress concentration at the edge of the dent. The aim is, therefore, to reduce such stress concentration. A study of material properties that alleviate stress concentration at the edge of the dent showed that a hard material can reduce such concentration provided the retained austenite content is kept high. Retained austenite, however, is traditionally associated with a lower hardness material. Therefore, raising the hardness while simultaneously maintaining the retained austenite appears to be a conflicting demand. To meet this contradictory requirement, a new steel was developed. It is an SAC steel that is combined with a newly developed carbonitriding treatment. Material A, which is hard yet contains a large amount of retained austenite, was developed by dispersing extremely fine carbon nitrides uniformly. This newly invented technology is covered by patents issued in the USA, UK, and Japan. As shown in Fig. 4, HTF and STF bearings are expected to attain a life that is three to five times as long as existing composition bearings under contaminated conditions 4). The HTF bearing is made of an economical Mn-Cr material and is applied mainly for automobile applications. The STF bearing uses a Cr-Mo material that satisfies extremely strict performance requirements. This material is employed in increasing quantities in the steel, machine, and automobile industries. The values obtained from calculations with the currently available equation suffer a drawback. Namely, they are shorter than the actual life under clean lubricating conditions but longer than the actual life under conditions where mixing-in of foreign materials is expected. Therefore, NSK proposes a new life equation derived from theoretical analysis of life-test data from research labs and practical life-data gathered from customers ) : L nas = a a 2 a 3 a 4 a (C/P) p a 4 : Environmental coefficient indicating the state of mixing-in of foreign materials a : Fatigue limit factor based on consideration of the fatigue limit of the bearing material Introduction of a 4 has enabled prediction of the bearing life used in the lubricating oil contaminated by foreign materials. Concerning the rolling fatigue, it has been said that there is no fatigue limit. As the cleanness of the bearing steel has been enhanced considerably, data has emerged indicating the possibility of the existence of the fatigue limit. In this equation, a has been introduced for the term C/P. This equation offers the advantage of being familiar, since it resembles the ISO calculation equation. Therefore, the relative relationships can be clearly and easily understood since the past experience of technicians and engineers serves as a foundation. 8. Conclusions NSK persistently pursues research and development in areas that are the foundation of bearing technology. Based on its long history and extensive achievements in making rolling bearings, NSK constantly invents and innovates products and technology. NSK contributes to the progress of industries throughout the world by offering its bearing technology expertise, since bearings are considered to be essential elements of machines. Bearings are, like the air we breathe, something you take for granted, so you rarely think about it. Bearings are, nevertheless, vital for the smooth and proper functioning of machines. Percent failed, % 99 9 Conventional bearing Life, hrs HTF STF Fig. 4 Fatigue life of bearings made of different materials under contaminated lubrication (deep groove ball bearings) References ) Aramaki, H. et al, The Performance of Ball Bearings with Silicone Nitride Ceramic Balls in High Speed Spindles for Machine Tools, ASME, JoT, Vol. (988) ) Natsumeda, NSK Technical Journal No. 64 (992) 9, [In Japanese]. 3) Johnson, G., Wayte, R. & Spikes, H., The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts, Trib. Trans., Vol. 34 (99) ) Akagami, K. and Aihara, S., Experimental Study of Starved EHL in a Small Ball Bearing Lubricated with Minute Amount of Oil, Proc. ITC Yokohama 9 (JAST), [to be published] ) Hachiya, K. et al, Preprint of JAST Tribology Conference Tokyo (992) [In Japanese]. 6) Ijuin, S. et al, Improving the Survivability of High Speed Ball Bearings under Oil Shut-off Conditions by Chemical Modification of the Surface, Tribology Trans., Vol. 38, No. 2 (99) Motion & Control No. 996

8 7) Aramaki, H. et al, NSK Technical Journal, No. 69 (99) 4 [In Japanese]. 8) Aihara, S., A New Running Torque Formula for Tapered Roller Bearings under Axial Load, Trans. ASME, JoT, Vol. 9 (987) ) Ichiyama, Y., J. JSPE, Vol. 6, No. 9 (99) [In Japanese]. ) Naka, M., Koizumi, H., Ishihara, S., Touma, K., High-speed Ball Bearings with Grease Lubrication, Proc. Japan Int. Trib. Conf., Nagoya (99) ) Murakami, Y., Naka, M., Iwamoto, A., Long Life Bearings for Automotive Alternator Applications, SAE Paper 9944 (99). 2) Ito, H., Tomaru, M., Suzuki, T., Physical and Chemical Aspects of Grease Deterioration in Sealed Ball Bearings, Lubrication Engineering, Vol. 44, No. (988) ) Ito, H., Koizumi, H., Naka, M., Grease Life Equations for Ball Bearings, Proc. ITC Yokohama 9 (JAST) [to be published]. 4) Furumura, K., Murakami, Y., Abe, T., The Development of Bearing Steels for Long Life Rolling Bearings under Clean Lubrication and Contaminated Lubrication, Creative Use of Bearing Steels, ASTM (994) ) Takata, H., Furumura, K., Murakami, Y., Development of a New Method for Estimating the Fatigue Life of Rolling Bearings, Proc. ASME Symposium, Orlando (99) -8. Kyozaburo Furumura Motion & Control No