Characterization of NRRO in a HDD Spindle System Due to Ball Bearing Excitation

Transcription

1 IEEE TRANSACTION ON MAGNETICS, VOL. 37, NO. 2, MARCH Characterization of NRRO in a HDD Spindle System Due to Ball Bearing Excitation G. H. Jang, Member, IEEE, D. K. Kim, and J. H. Han Abstract This research characterizes the source of NRRO by analyzing NRRO of the outer-race rotating spindle and the inner-race rotating spindle, respectively. It also characterizes the transmission of NRRO from the ball bearing to the disk by investigating NRRO and the frequency response function before and after mounting the disks on the spindle, respectively. Finally, it investigates the effect of the visco-elastic damping material to reduce NRRO by inserting it in the transmission path of NRRO. This research shows that visco-elastic damping material reduces NRRO by damping out ball bearing excitation, and it improves the frequency response function in the frequency region where the loss factor of the damping material is effective. Index Terms Ball bearing, HDD, NRRO, visco-elastic damping material. Fig. 1. Outer-race rotating spindle motor. I. INTRODUCTION NON-REPEATABLE runnout (NRRO) is one of the main sources of track misregistration which prevents high track density. It is reported that NRRO is mainly generated by the ball bearing. Over the last couple of years, NRRO has been reduced substantially to meet the high track density in the HDD industry, but most of the NRRO reduction has been achieved through the tight inspection of ball bearings. By the end of 2000, magnetic track density is expected to increase up to TPI that requires a NRRO smaller than 5% of track pitch, i.e., 0.03 m. It is getting more and more difficult, not only to measure and analyze NRRO, but also to reduce NRRO only through the inspection of ball bearings. Many researchers have analyzed its characteristics in several ways [1] [3], and identified its sources as the ball bearing [3], [4]. Ohmi [5] discussed the effect of NRRO on head tracking error and Noguchi et al. [6] proposed the optimum number of balls from the theoretical analysis of a ball bearing. Bouchard et al. [7] reported that a fluid bearing spindle has negligible NRRO, but it has some problems to solve such as small load capacity and leakage in order to be used in HDD. Recently, Jang et al. [8] developed a new design for a ball bearing spindle motor using visco-elastic damping material to reduce NRRO. But most of the prior research did not fully explain the source of NRRO in terms of the components of ball bearings, i.e., inner-race, ball and outer-race, and they did not explain how the excitation of the ball bearing is transmitted to the disk through the various components of spindle system. Manuscript received July 15, G. H. Jang is with the Department of Precision Mechanical Engineering, Hanyang University, Seoul, , Korea ( ghjang@ .hanyang.ac.kr). Publisher Item Identifier S (01) Fig. 2. Inner-race rotating spindle motor. This research rigorously characterizes the source of NRRO by analyzing the NRRO of the outer-race rotating spindle and the inner-race rotating spindle, respectively. It also characterizes the transmission of NRRO from the ball bearing to the disk by investigating NRRO and the frequency response function before and after mounting the disks on the spindle, respectively. Finally, it investigates the effect of the visco-elastic damping material to reduce NRRO by inserting it in the transmission path of NRRO. II. CHARACTERIZATION OF THE SOURCE OF NRRO A ball bearing is composed of the inner-race, outer-race and balls, and the spindle motor is often classified into the outer-race rotating type and inner-race rotating type depending on the rotating race as shown in Figs. 1 and 2. The outer-race rotating type has been popular, and it is mechanically rigid due to the fixed shaft. On the other hand, inner-race rotating type is mostly used in a small HDD because it can effectively use the space for coil winding. The ball bearing supports the rotor through the direct contact between the ball and the race. Waviness and other form errors in ball or race are reported to generate the vibration of a rotor with the combination of fundamental frequencies of a ball bearing. They are the cage rotational frequency ( ), the frequency of /01$ IEEE

2 816 IEEE TRANSACTION ON MAGNETICS, VOL. 37, NO. 2, MARCH 2001 TABLE I FUNDAMENTAL FREQUENCIES OF BALL BEARING TABLE II BEARING FREQUENCIES AND THEIR SOURCES Fig. 4. Campbell diagram of NRRO in the outer-race rotating spindle motor. Fig. 5. Campbell diagram of NRRO in the inner-race rotating spindle motor. Fig. 3. Runout measurement setup. TABLE III SPECIFICATIONS OF BALL BEARINGS IN THE OUTER-RACE AND INNER-RACE ROTATING SPINDLE MOTOR rotating race with respect to the cage ( or ) and the rotational frequency of the ball ( ). They are theoretically derived from the kinematic analysis of the ball bearing as shown in Table I [9]. The bearing frequencies, due to the form errors of each component, are reported as shown in Table II [9], [6]. Fig. 3 shows the experimental setup to measure the runout of a HDD spindle system. Axial capacitance probe is used with a resolution of 12.7 nm. Total indicated runout (TIR) is measured during 128 revolutions, and 128 data points per revolution are sampled by the electrical index signal generated from a motor drive. RRO and NRRO are determined by the frequency domain method [3]. Figs. 4 and 5 show Campbell diagrams of the NRRO of the outer-race rotating spindle motor in a 3.5 HDD and the inner-race rotating spindle motor in a 2.5 HDD, respectively. Abscissa shows the speed of motors and ordinate shows the frequency contents of NRRO. The radius of circles represents the magnitude of NRRO corresponding to the frequency in the ordinate. Solid lines show bearing frequencies of a ball bearing theoretically determined by the equations in Table II. Dotted lines show the natural frequencies experimentally determined by modal testing. The specifications of the ball bearings in each case are shown in Table III. Figs. 4 and 5 show that most of the NRRO in each case of the spindle motors can be correlated with the bearing frequencies in Table II. Table IV shows that the sources of NRRO are mostly the form errors of the rotating race and ball. In the outer-race rotating spindle motor, the frequency components due to the form error in the inner-race are not observed. And in the inner-race rotating spindle motor,

3 JANG et al.: CHARACTERIZATION OF NRRO IN A HDD SPINDLE SYSTEM DUE TO BALL BEARING EXCITATION 817 TABLE IV BEARING FREQUENCIES AND THEIR SOURCES Fig. 7. Campbell diagram of NRRO on the upper disk. TABLE V COMPARISION OF NRRO SOURCES Fig. 6. Experimental setup to measure the frequency response function. only one frequency component due to the form error in the outer-race is observed, i.e., in Fig. 5, but its contribution is small compared with other frequencies due to the rotating race and ball. The excitation due to the contact between the rotating race and ball is directly transmitted to the hub. However, the excitation due to the contact between the stationary race and ball is absorbed in the ball to some extent so that form errors of stationary race may have very small effect on the frequency components of NRRO. III. TRANSMISSION OF NRRO DUE TO BALL BEARING EXCITATION In order to investigate how NRRO is transmitted from the ball bearing to the disk, runout is compared before and after mounting the disks on the inner-race rotating spindle motor, respectively. NRRO in both cases cannot be measured at the same location due to the complex structure of the model so that quantitative comparison is meaningless, but the characteristics of transmission of the NRRO can be explained through the comparison of frequency components of NRRO and the frequency response functions in both cases. Fig. 6 shows the experimental setup to measure the frequency response function of a HDD spindle system. While the HDD rotates at the operating speed of 70 Hz, the excitation force in the form of swept sine is applied to the HDD. The force transducer measures the excitation force and the laser doppler vibrometer (LDV) measures the axial velocity of the HDD to produce the frequency response function. Fig. 7 shows a Campbell diagram of the NRRO on the upper disk after two disks are mounted on the spindle motor. As compared with the NRRO on the spindle motor in Fig. 5, the mass of the disk lowers the first three natural frequencies to the low frequency region between 500 and 700 Hz so that NRRO is amplified in this region especially when bearing frequencies match with the natural frequencies of the spindle system. Table V shows the sources of NRRO before and after two disks are mounted on the spindle motor. It shows that almost every component of NRRO in the spindle motor appears after the disks are mounted. In addition, there are new frequency components such as,,,,, and, and there is also a disappeared frequency such as. They can be explained by the frequency response function as shown in Fig. 8. The former frequencies components are too small to be detected in the spindle

4 818 IEEE TRANSACTION ON MAGNETICS, VOL. 37, NO. 2, MARCH 2001 Fig. 10. Monograph of visco-elastic damping material. Fig. 8. Frequency response functions (a) before disks are mounted (b) after disks are mounted. TABLE VI COMPARISON OF STATISTICAL PROPERTIES OF NRRO IN SPINDLE MOTOR Fig. 9. Design of a spindle motor with bearing spacer damping. motor before disks are mounted, and they are amplified and observed after disks are mounted due to the increase of the frequency response function in that region. After mounting two disks on the motor with the rotation speed of 70 Hz, the magnitude of the frequency response function at 823 Hz ( ), is lowered from 0.97 [mm/s/n] to 0.61 [mm/s/n]. It decreases the NRRO at this frequency, which makes NRRO at this frequency unobservable. After disks are mounted, the magnitude of the frequency response function is decreased above 800 Hz, compared with the frequency response function before mounting disks. Due to the small transmissibility in that frequency region after mounting disks, other frequency components of NRRO are not observed above this frequency. IV. EFFECT OF DAMPING MATERIAL ON NRRO Visco-elastic damping material may be one of the possible design methods to damp out NRRO. Jang et al. [8] developed a new design of a spindle motor that can reduce NRRO effectively by inserting visco-elastic damping material in the transmission path of NRRO, i.e., where the strain energy is highly concentrated. Fig. 9 shows the proposed spindle motor by Jang et al. [8] that has the damping material between the outer race and bearing spacer (Bearing Spacer Damping: B.S.D.). Fig. 10 shows the monograph of the applied visco-elastic damping material. In the operating temperature range between 20 C and 50 C, the loss factor of this visco-elastic damping material has the maximum value at the frequency region between 40 Hz and 700 Hz. As shown in Table VI, B.S.D. reduces the NRRO of the Fig. 11. Campbell diagram of NRRO in the spindle motor with B.S.D. spindle motor by 14% and 24% in terms of the peak-to-peak and standard deviation, respectively. Fig. 11 shows a Campbell diagram of NRRO in the spindle motor with B.S.D. and it shows that the visco-elastic damping material effectively reduces almost every frequency component of NRRO, compared with NRRO in the spindle motor with no damping in Fig. 5. Fig. 12 shows the frequency response function of the spindle motor with no damping and B.S.D. Visco-elastic damping material does not improve the frequency response function corresponding to the first natural frequency because its loss factor is not effective in that frequency range. However, it damps out NRRO below 700 Hz where its loss factor is effective as shown in Fig. 10. B.S.D also reduces NRRO on the disk after two disks are mounted on the spindle motor. As shown in Table VII, B.S.D. reduces NRRO on the disk by 26.6% and 19.8% in terms of the peak-to-peak and standard deviation, respectively. Fig. 13 shows a Campbell diagram of NRRO on the disk in case of B.S.D. Compared with NRRO in the case of no damping in Fig. 7, visco-elastic damping material mostly reduces the low

5 JANG et al.: CHARACTERIZATION OF NRRO IN A HDD SPINDLE SYSTEM DUE TO BALL BEARING EXCITATION 819 Fig. 12. Frequency response functions of the spindle motors (a) no damping (b) bearing spacer damping. TABLE VII COMPARISON OF STATISTICAL PROPERTIES OF NRRO ON DISK Fig. 14. Frequency response functions of hard disk drives (a) no damping (b) bearing spacer damping. most frequency components of NRRO can be correlated with the bearing frequencies of the rotating components of ball bearings, i.e., ball and rotating race. Most of the frequency components of NRRO measured in the spindle motor are also measured after disks are mounted, and they are amplified through the motion of the disk. In addition, there are new frequency components on the disks, which can be explained by the change of the frequency response function due to the dynamic characteristics of the disk. This research shows that visco-elastic damping material reduces NRRO by damping out ball bearing excitation, and it improves the frequency response function in the frequency region where the loss factor of the damping material is effective. Visco-elastic damping material may contribute to the high track density of the hard disk drive. Fig. 13. Campbell diagram of NRRO on the disk with B.S.D. frequency components of NRRO below 700 Hz because its loss factor is effective on that frequency region as shown in Fig. 10. Fig. 14 shows the frequency response function of the hard disk drive with no damping and B.S.D. Visco-elastic damping material improves the frequency response function on the frequency range corresponding to the first three natural frequencies where the loss factor of the damping material is effective, and it damps out the NRRO effectively. V. CONCLUDING REMARKS This research rigorously investigates the characteristics of NRRO by analyzing NRRO of the outer-race rotating spindle and the inner-race rotating spindle, respectively. In both cases, REFERENCES [1] E. J. Klein, The asynchronous runout of spindles, in Mechanical Signature Analysis, Machinery Vibration, Flow-Induced Vibration and Acoustic Noise Analysis: ASME, 1987, vol. 7, pp [2] W. O. Richter and F. E. Talke, Nonrepeatable radial and axial runout of 5 1/4 in disk drive spindles, IEEE Trans. Magn., vol. 24, no. 6, pp , [3] G. H. Jang, D. K. Kim, and J. E. Oh, New frequency domain method of nonrepeatable runout measurement in a hard disk drive spindle motor, IEEE Trans. Magn., vol. 35, no. 2, pp , March [4] K. Ono, N. Saiki, Y. Sanada, and A. Kumano, Analysis of nonrepeatable radial vibration of magnetic disk spindles, Trans. ASME J. Vib. Acoust., vol. 133, pp , [5] T. Ohmi, Nonrepeatable runout of ball-bearing spindle-motor for 2.5" HDD, IEEE Trans. Magn., vol. 32, no. 3, pp , [6] S. Noguchi, K. Tanaka, and K. Ono, Theoretical analysis of a ball bearing used in a HDD spindle motors for reduction of NRRO, IEEE Trans. Magn., vol. 35, no. 2, pp , March [7] G. Bouchard, L. Lau, and F. E. Talke, An investigation of nonrepeatable spindle run-out, IEEE Trans. Magn., vol. 23, no. 5, pp , [8] G. H. Jang, S. J. Hong, D. K. Kim, and J. H. Han, New design of a HDD spindle motor using damping material to reduce NRRO, IEEE Trans. Magn., to be published. [9] G. H. Jang, D. K. Jung, N. Y. Park, and J. S. Park, Analysis of the dynamic characteristics of ball bearing in 3.5 HDD spindle motor, in Proc. Incremental Motion Control Syst. and Devices, 1996, pp