Ball Bearings for Hard Disk Drives

Transcription

1 Ball Bearings for Hard Disk Drives M. MUKASA Performance of hard disk drives (HDD) has been improved remarkably in the past three or four years. Memory capacity of HDD has increased from 1GB to 2 ~ 3GB. Rotational speed has also increased and HDDs with 15 rpm speeds are now readily available. The more HDD capability has been improved, the higher the requirement for miniature/small size bearings for HDD are changing. In this report, recent developments in HDD spindle motor bearings are introduced. Key Words: miniature ball bearing, hard disk drive, spindle motor 1. Introduction As worldwide economic recovery continues to stall, companies are busy with structural reforms where reduced productivity is replaced by IT (information technology) innovations. In the field of information equipment relating to IT, the switch to semiconductor-driven electronic components that enable lower cost, compactness and offer improvements over conventional information equipment is progressing at a rapid pace. Progress and technical improvement in this field are extremely fast and product cycle is short. Amidst this wave of IT innovation, hard disk drives (hereinafter referred to as "HDD") are the mainstream equipment that store information. Miniature and small-diameter ball bearings (hereinafter referred to as "ball bearings") are used for supporting rotation. The major requirements of these bearings are high speed rotation capability, high precision, low noise and low torque. This paper introduces technical problems involving bearings for HDD used in information storage equipment and what is being done to cope with those problems. 2. Higher Expectations for Information Equipment The switch to digital technology is proceeding in various fields, and has spread to AV equipment, information-related equipment, household electrical appliances and office equipment. Digital satellite broadcasting began in December 2, and it is estimated that in the future various types of information will be accessible via a digital television. Use of HDD has been proposed to store information concerning a home network 1) such as the one shown in Fig. 1. Because the memory capacity of HDD has increased dramatically, memory unit is extremely low price and is thought to be the ideal place to store an enormous amount of information. KOYO Engineering Journal English Edition No.16E (22) Digital satellite broadcasting Digital network VTR Living room Telephone/fax Audio Digital set-top box Digital TV Home servers Library Video camera Home PC Printer Child room Mobile computers TV Headphone stereo set Mini component Fig. 1 Conception of digital electric household appliance network 1) 3. Technologies for Coping with Bearings for HDD 3. 1 Technical Trends Figure 2 2) shows an ever-increasing number of HDD units produced. Figure 3 shows the dramatic increase in memory density of HDD. Figure 4 shows appearance of a HDD unit and Fig. 5 shows a spindle motor construction. The number of HDD units produced in the world exceeds 18 million and is expected to continue to grow by 1% or more per year. Quantitatively, the 3.5-inch design is expected to be the most common. Memory density approximately doubled from 1994 to 1996, but since then it has dramatically accelerated and the capacity in 22 is expected to be five times that of 2. Almost complete adoption of The Giant Magneto Resistive (GMR) head and the next generation Tunnel Magneto Resistive (TMR) head have greatly contributed to this increase. Recent mass-produced products have 2 G-bit/(inch) 2 capacity, and the future level is going to be 1 G/(inch) 2. In order to go to this high surface density, the track density or linear recording density must be increased. Of these two, track density will be the most influenced by bearing performance. Track width has recently become.4lm, and is expected to be.8lm by the end of 22 3). The Non Repeatable Runout (NRRO) of the bearings must be reduced to increase the narrowness of track width. 15

2 Units, 1 units inch 3.5-inch Fig. 2 Market change of HDD production Provided by Electronic Information Technology Industry Association Main components of the spindle motor include the shaft, hub, flange, magnetic fluid seal, two ball bearings and the motor parts. There are some examples where fluid dynamic bearings (FDB) have been adopted in place of the ball bearings. The performance of FDB will be reported at another opportunity. This paper introduces technical improvements in HDD ball bearings Bearing Size and HDD Applications Table 1 gives applications and operating conditions for HDD, Table 2 gives dimensions of bearings used for HDD and Fig. 6 gives typical bearing examples. Table 1 Application and operating conditions Gbit/in Disc size 3.5" 2.5" 1." Main application Desk-top PC, digital electric household Mobile computers Mobile instruments Fig. 3 Change in HDD storage capacity appliance, servers Bearing bore diameter, mm 5~ ~2 Rotation speed, 5 4~15 4 5~ ~5 4 min -1 Temperature, ; ~7 ~7 ~7 Spindle motor Preload, N 1~2 5~1 2~4 Table 2 Dimensions of ball bearings for HDD Disc 3.5-inch 2.5-inch 1-inch Bore, mm Outside diameter, mm Width, mm Ball diameter 2. 1/16" /16.6 Ball bearings Number of balls Fig. 4 HDD & ball bearings 2.6 Magnetic fluid seal Hub Shaft Upper bearing Disc 1.15 Stator Lower bearing Motor base u13 u5 u1 u4 u4 u1.5 Fig. 5 Structure of HDD spindle motor 3.5-inch 2.5-inch 1-inch Fig. 6 Ball bearings for HDD 16 KOYO Engineering Journal English Edition No.16E (22)

3 There are three classifications of HDD, the 3.5", 2.5" and 1.". The 3.5" HDD are used for desk-top PCs, digital electrical appliances and servers. The 2.5" are used for laptop computers, and manufacturers are thinking about equipping information equipment with 1." HDD in the future. The bearings used in 3.5" HDD have an inner diameter of u5 ~ 6mm, 2.5" uses u4mm and 1." uses u1.5 ~ 2mm. HDD size differs somewhat according to usage conditions and the rotational speed of the 3.5" are getting faster to catch up with the ever-increasing processor speeds. Additionally, there are demands for HDD to be thinner and more compact, which results in demands for ball bearings to be narrower and have more variations. Because ball bearings have a fixed rolling vibration, they resonate within the HDD motor when turning, which makes the NRRO worse in some cases. In order to avoid this, the problem is solved by redesigning the internal geometry, including the ball configuration, of the bearings while maintaining the same envelope dimensions Function Required for Bearings Table 3 gives a comparison of the performance required for HDD and for the bearings. Table 3 Comparison of bearings for HDD Required Requirement Steel Ceramic ball Fluid dynamic performance for HDD Ball (silicon nitride) bearing (FDB) for bearing High capacity NRRO # # " Low noise Noise level # # " Low current consumption Torque # " % High speed High speed performance performance % " " Shock Shock resistance resistance # # " Durability Reliability # " % Low cost Cost " # % Where higher capacity, higher speed, lower noise, lower current consumption, better shock resistance and durability are required for HDD, low NRRO, high-speed rotation, low noise, low starting and running torques, shock resistance and high reliability are required for the bearings. The bearings with steel balls are mainly used due to relative low cost. Because bearings using ceramic balls are superior to that of steel ball bearings in high speed performance and reliability, they are primarily used for servers. FDB are superior to that of ball bearings in low noise, low NRRO and shock resistance, and are being adopted for some HDD (primarily 2.5 inch). There are problems with the aspects of cost, reliability and torque for FDB, and these problems will have to be addressed in the future. Improvements in NRRO, high-speed performance, noise level and shock resistance, are introduced in the following section Improvement in NRRO Figure 7 shows NRRO required value transition for ball bearings, Fig. 8 shows the NRRO measurement method for ball bearing itself, Fig. 9 shows transition of NRRO reduction for bore u5 outer diameter u13 width 3mm (narrow width type of 695), which is the typical type used for 3.5" HDD, and Fig. 1 shows NRRO reduction when the number of balls is changed. NRRO, nm Sample bearing Fig. 7 Change of NRRO requirement FFT Fig. 8 NRRO measurement system and an example of NRRO NRRO, nm N number Air bearing Motor Sensor Amplifier Personal computer Air pressure Runout Time 8 NRRO 695 u2 8 balls Fig. 9 Change in NRRO balls 1 balls NRRO, lm Fig. 1 NRRO of bearings with various number of balls 5 KOYO Engineering Journal English Edition No.16E (22) 17

4 Table 4 Characteristics of silicon nitride and high carbon chromium bearing steel Item Ceramics (Si 3 N 4 ) Bearing steel (SUJ2) Advantage and characteristics according to application of ceramics High temperature performance, ; 8 18 High loading capacity up to high temperature Density, g/ml Reduction of centrifugal force Coefficient of linear expansion, K Small change of internal clearance due to temperature rise Vickers hardness, Hv 1 4~1 7 7~8 Young' modulus, GPa Smaller change of deformation at rolling contact zone Poisson ratio.26.3 Corrosion resistance Satisfactory Unsatisfactory Can be used in acids, alkalis, and other liquids Magnetism Non-magnetism material Ferromagnetic material Small speed fluctuation due to intense magnetic field Electric conductivity Insulator Electric conductor Eliminates electric pitting (motor, etc.) Material bonding state Covalent bond Metallic bond Minimized seizure under oil-film breaking NRRO value required for ball bearings tends to decrease year by year. The current requirements are 5 nm or less. The main factors influencing NRRO are as follows: q Precision of inner and outer ring raceway surfaces w Ball precision e Cage precision r Bearing mounting condition As the result of the improvements listed above, the average value of NRRO has dropped to 2 nm for an 8 ball design of the 695 mentioned earlier. To further reduce NRRO, the number of balls has been increased to 1, and an NRRO of 15 nm has been achieved. Recently 1 balls per bearing has become the mainstream bearing design High Speed Performance Figures 11 and 12 show the transition to high-speed HDD 4). Rotational speed differs for desk top PCs and servers. 5 4 min 1 is primarily used for desk top PCs, but is gradually shifting to 7 2 min 1. For servers, 1 min 1 is the mainstream. Recently a 15 min 1 has been introduced, and further speed increases are being planned. Bearings using ceramic balls are being adopted to accommodate the shift to higher speeds. Ceramics have superior characteristics to bearing steel such as light weight and high stiffness. Using ceramics for balls is effective for improving bearing rigidity, reduction of temperature rise, improvement of grease life, and bearing seizure life. Table 4 compares characteristics of ceramics (silicon nitride Si 3 N 4 ) and bearing steel. 5) As shown in Table 4, ceramics have the following advantages: 1) High heat resistance 2) Small density 3) Small linear-expansion coefficient 4) High hardness Composition ratio, % 4 2 Composition ratio, % 1% 8% 6% 4% 2% % 1% 8% 6% 4% 2% % min Fig. 11 Rotational speed change of PC min Fig. 12 Rotational speed change of server Bearing: 695 (8 balls, width 3mm) Lubrication condition: Oil, 1 mg drip Vibration, mg Test conditions: Outer ring rotational speed Temperature Preload n = 3, all locked Locked 1 h 3 4 h 3 6 h 4 6 h Steel balls Ceramic balls Time, h Fig. 13 Least oil lubrication endurance test : 7 2 min 1 : 7: : 15 N 18 KOYO Engineering Journal English Edition No.16E (22)

5 Bearing: 696 (7 balls) Lubricant: Multemp SRL grease Vibration, mg 1 5 Steel balls, n = 5 All stop due to excessive vibration Test conditions: Outer ring rotational speed Temperature Preload Ceramic balls, n =1 : 15 min 1 : 7: : 15 N 3. 6 Noise Figure 15 shows trends in bearing vibration. Along with the increased HDD rotational speed, demands for lower noise are becoming more and more severe. The response to these demands has resulted in improved bearing raceway surface precision and ball surface precision and have resulted in a 4% reduction in bearing vibration when compared to Shock Resistance Figure 16 shows performance of bearing shock resistance Time, h Fig. 14 Grease lubrication endurance test Figures 13 and 14 show the test results when ceramic balls are used for ball bearings. Ceramic ball bearings have shown a durability three times or greater than that of steel ball bearings when lubricated with a small quantity of oil. With grease lubrication, the vibration in the steel ball bearing increased dramatically at 8 h in comparison to the initial values. In the case of ceramics, there was almost no change in the vibration levels compared to the initial values even at 16 h and it was confirmed that lubrication conditions do not deteriorate due to extended operation. The reason ceramic ball bearings have superior lubrication life is thought to be because unsatisfactory conditions such as surface roughening, seizure, adhesive wear due to contact with different type materials (metal and non-metal) are minimized. Cost reduction is also a point of issue for expanding use of ceramic ball bearings. Bearing vibration, mg Amount of vibration change, mg Conventional bearing Improved bearing Axial load, N 695 (8 balls) Fig. 16 Shock resistance test results Demands for shock resistance for the 2.5" HDD differ from that for the 3.5" HDD. The 2.5" HDD are typically used in mobile computers, and receive shock loads as high as 6 ~ 7 G. On the other hand, the 3.5" HDD are used in desk top computers and receive considerably lower shock loads (15 ~ 2 G). For bearings, shock resistance of which value is multiplied by acceleration G value to the total weight of disk and motor parts are required. For an example, the total weight for a 3.5" HDD was 1.2 N and the acceleration were 2 G, the bearing load would be at least 25 N. To improve bearing load capacity, load carrying capacity must be increased. To achieve this, new heat treatment processes are being developed and bearing internal designs are continuously being optimized Fig. 15 Tendency of ball bearing vibration KOYO Engineering Journal English Edition No.16E (22) 19

6 4. Conclusion Ball bearings for HDD are very diverse as shown by the performance improvements summary in Table 5. HDD usage is expected to expand to information / household electrical appliances, and Koyo will continue to make progress in further improving low NRRO / low vibration, in order to enhance precision and improve reliability of the entire rolling bearing. Table 5 Requirements of HDD and improvement of ball bearings Required items for HDD Required items for spindle motor Improvement in ball bearing High capacity Low NRRO Bearing (inner ring, outer ring) finishing precision, parts (ball, cage) precision, NRRO measurement improvement Low noise Low noise Bearing ring, ball surface precision improvement Avoidance of resonance Bearing size, variety of ball component Low current consumption Low running torque Optimization of grease, cage design and bearing size High speed High speed rotation Adoption of ceramic ball High Long life Adoption of ceramic ball reliability Development of long life grease Compactness Shock resistance Thin ultrasmall motor Optimization of internal design and heat treatment Variety of bearing size, Thin ultra-small bearing machining References 1) "5 chouen shijyou, jyouhou kaden ni kakeru HDD", NIKKEI MECHANICAL, 553 (2. 1) 44. 2) Japan Electronics and Information Technology Industries Association: 1999 nen, 2 nen jyouhou tanmatsu kanrenkiki no sekaishijyou, kokunaishijyou jisseki oyobi jyuyouyosoku. 3) Mark E. Re: "Areal density of 6Gbits/inch 2 is confirmed by both perpendicular and longitudinal recording", NIKKEI ELECTRONICS, 786 ( ) ) "HDDs stand at the crossroads of the 1G byte age" NIKKEI ELECTRONICS, 788 ( ) ) Koyo Seiko Co., Ltd.: EXSEV Bearings, CAT. No (Reprinted with permission of GEKKAN TORAIBOLOGI (21. 4)). M. MUKASA * * Electrical Engineering Department, Bearing Engineering Center 2 KOYO Engineering Journal English Edition No.16E (22)