Fluid Dynamic Bearings Research Articles

Behavior of a micron-sized air bubble in operating FDBs using the discrete phase modeling method

This paper investigates the motion of a micron-sized air bubble in the operating fluid dynamic bearings (FDBs) of a spindle motor in a computer hard disk drive. The flow field of FDBs is calculated by solving the Navier---Stokes equation and the continuity equation. The two-phase flow in the air-oil interface is simultaneously solved by using the finite volume method and the volume of fluid (VOF) method. We then analyze the motion of a micron-sized air bubble by applying the discrete phase modeling (DPM) method to the calculated flow field of FDBs. The motion of a micron-sized air bubble determined using the DPM method is verified by comparison with the trajectory of the micron-sized air bubble determined using the VOF method. The trajectories of a micron-sized air bubble with different initial positions in the FDBs are discussed.

Effect of an hourglass-shaped sleeve on the performance of the fluid dynamic bearings of a HDD spindle motor

This research investigated the characteristics of fluid dynamic bearings (FDBs) in a HDD spindle motor with an hourglass-shaped sleeve. We demonstrated experimentally that the hourglass-shaped sleeve generated through the ball-sizing process is a major source of large repeatable runout and non-repeatable runout in a HDD spindle system. We also numerically proved the effect of hourglass-shaped sleeves on pressure, friction torque, stiffness and damping coefficients, critical mass, and shock response. Finally, we proposed a robust design for FDBs with hourglass-shaped groove depths to compensate for the decrease in the static and dynamic performance of FDBs with hourglass-shaped sleeves. The proposed hourglass-shaped groove depth improves the performance of FDBs with both straight and hourglass-shaped sleeves.

Design and characterization of miniature fluid dynamic bearing using novel multi-step elliptical grooves

This paper investigates the design and characterization of a miniature fluid dynamic bearing using novel multi-step elliptical grooves for small-form-factor data storage applications and miniature fan motors. In contrast to conventional herringbone-grooved cylindrical journal bearings (HGJBs), the proposed journal bearing contains a single set of multi-step elliptical grooves (EGJB). The performance of the proposed multi-step EGJB is characterized numerically using proprietary flow field analysis software. In addition, to reduce the number of optimal full-factorial experimental design tests (323), the Taguchi parameter design methodology is used to find out the optimal design parameters of the multi-step EGJB. Results show that compared to the conventional HGJB presented by the current group in a previous study, the proposed multi-step EGJB improves the load capacity. Consequently, the proposed motor represents another solution for both existing and emerging miniaturized spindle motor applications.

Optimal design of fluid dynamic bearings to develop a robust disk-spindle system in a hard disk drive utilizing modal analysis

This research proposes an optimal design methodology for fluid dynamic bearings (FDBs) in a hard disk drive to improve the dynamic performance of the disk-spindle system. We solved equations of motion for the rigid rotor supported by FDBs with five degrees of freedom. Five modal damping ratios were selected as multi-objective functions. The constraint equations were the friction torque of the FDBs and the stiffness and damping coefficients related to under-damped vibration modes. Ten major design variables of the FDBs were chosen for this optimization problem. The steady-state whirl radius and the shock response at half-speed whirl of the rotating rigid spindle-bearing system were evaluated as RRO and NRRO, respectively. The RRO and NRRO of the optimal design were compared with those of the conventional design. Our results show that the proposed method effectively reduces RRO and NRRO.

Analysis of the oil injection process in fluid dynamic bearings of the spindle motor of computer hard disk drives

This paper numerically and experimentally investigates the oil injection process of fluid dynamic bearings (FDBs). Compressible air, which is governed by the ideal gas law, is introduced to model a nearly perfect vacuum. The two-phase flow of compressible air and oil is determined by the Navier–Stokes equation, the continuity equation, and the volume of fluid method. The proposed numerical method of this paper is verified by the oil injection experiment. This research discusses the possible sources of the air bubbles included in the lubricant of the FDBs in the oil injection process and proposes a novel oil injection process and FDBs to reduce the air bubbles.

Experimental Verification of the Optimal FDBs in a HDD Spindle Motor to Minimize Power Loss

In this paper, we experimentally verified the optimal design of the fluid dynamic bearings (FDBs) in a HDD spindle motor to minimize power consumption while maintaining the same level of dynamic performance. HDD spindle motors with optimal FDBs were prototyped, and their power consumption and dynamic characteristics were measured and compared with those of conventional ones at various operating speed. It shows that the HDD spindle motors with optimal FDBs consume less power than those of the conventional ones as predicted in the proposed optimal design methodology.

Motions of Air Bubbles Trapped in Grooved and Plane Journal Bearings of Operating Fluid Dynamic Bearings

We investigated the motion of air bubbles trapped in grooved and plane journal bearings of operating fluid dynamic bearings (FDBs) by using the finite volume method and the volume of fluid method. The motion of a trapped air bubble in grooved journal bearing is mostly determined by surface tension and the pressure difference due to the wedge effect of the groove. Also, the motion of trapped air bubbles in plane journal bearing is mostly determined by surface tension and the Coutte flow. A novel design of FDBs was finally proposed to expel the air bubbles trapped in grooved journal and plane journal bearings out of operating FDBs.

Vibration and Noise in a HDD Spindle Motor Arising from the Axial UMF Ripple

We investigated numerically the characteristics of axial unbalanced magnetic force (UMF) due to axial magnetic design in the spindle motor of a hard disk drive (HDD). The HDD spindle motor has a magnet-overhang and a pulling plate to generate the axial magnetic force, which is applied to the fluid dynamic bearings (FDBs) as a preload in order to increase the axial stiffness of the HDD spindle system. However, the axial UMF ripple with the least common multiple (LCM) harmonics of pole and slot is generated by the pulling plate and magnet-overhang in the HDD spindle motor. We also investigated the characteristics of the axial magnetic forces generated in the pulling plate and the stator core, separately. We found that the axial magnetic forces generated in the pulling plate and the stator core have opposite phases. Furthermore, we proposed an optimal position of permanent magnet with respect to the stator core. We experimentally verified that the LCM harmonics of pole and slot in the vibration and acoustic noise mostly originate from axial UMF ripple and that the proposed design can effectively minimize the LCM harmonics in the vibration and acoustic noise of HDD systems.

Extraction of Bearing Coefficients of Fluid-Dynamic Bearing Spindle Motors Using a Proof Mass and a Hammer—A Refined Approach

In this paper, we report significant theoretical and experimental improvements over the method developed by Shen et al. (“Estimating bearing coefficients of fluid-dynamic bearing spindle motors: A theoretical treatment and feasibility study” in IEEE Trans. on Magnetics, vol. 47, no. 7, 2011) to extract bearing coefficients of a fluid-dynamic bearing (FDB) spindle motor used in hard disk drives (HDD). On the theoretical side, many assumptions made in the prior mathematical model are removed. For example, effects of radial displacements, inertia, and gyroscopic moments are now accounted for. Governing equations to extract the bearing coefficients are exact thus enhancing extraction accuracy. Besides, two new schemes are developed to extract the bearing coefficients from the governing equations. On the experimental side, we use a dual-beam differential laser Doppler vibrometer (LDV) to directly measure angular displacements, thus significantly reducing measurement uncertainties. Radial displacements are also measured to increase accuracy of extracted bearing coefficients. With these improvements, three rounds of experiments were conducted to extract in-line stiffness, cross stiffness and damping coefficients of the lower radial bearing of a 15 000 r/min spindle motor.

Characteristics of Axial and Radial Surface Tension Sealing Structures for HDD FDB Spindles

This paper presents the characteristics of the axial and radial surface tension sealing structures. Based on the 6-DOF rotor-bearing coupled dynamic analysis, the movement of the shaft relative to the sleeve during and immediately after a shock is calculated. By use of the finite volume method, volume of fluid, K-e turbulence model and dynamic mesh technique, the anti-shock capacity and anti-centrifugal force capacity of the two structures are investigated. The results are significant for optimal design of the sealing structure in hard disk drive fluid dynamic bearings spindles.

화학적 에칭을 이용한 유체 및 공기 동압 베어링용 그르브 가공

Abstract - This paper presents a chemical etching system for groove manufacturing for the fluid and aero dynamic bearings. To manufacture the grooves to thrust and journal surface of the fluid and aero dynamic bearing, it is very important for grooves' depth to be smaller tolerance. It is very difficult for the internal surface of journal bearing to make the grooves precise. If the precision of the groove is not exact, we can not get the desirable performance for the target of the dynamic bearing. To make the groove of bearing precise, we propose the method of chemical etching system. It has known that the method of chemical etching can not make the groove on the internal surface of journal bearing excepts for on the surface of thrust bearing. However, this paper has shown the solution to make the grooves on it. We obtain the condition and the parameters of the system such as time, chemical material composition and so on. In this paper, we get the experimental results to verify the precise groove manufacturing for the fluid and aero dynamic bearing.Key Words : Chemical etching, Fluid dynamic bearing, Aero dynamic bearing, Groove, Thrust surface, Journal surface†교신저자, 정회원 : 인하공업전문대학 전기정보과 교수E-mail : leeyong@inhatc.ac.kr*정 회 원 : 인하공업전문대학 전기정보과 교수 접수일자 : 2012년 11월 1일 최종완료 : 2012년 11월 25일

Reduction in Adsorption of Bearing Oil From Fluid Dynamic Bearings of Hard Disk Drive Spindle Motor by Diamagnetism

In this study, we focused on the diamagnetism of ester lubricant oil for fluid dynamic bearings (FDBs) and investigated the effect of a magnetic field on oil vapors. We vaporized the lubricant oil and compared the amount of oil adsorbed on disk surfaces through two slits-one with magnets and the other without a magnet-in a preliminary experiment. As a result, we confirmed that it is possible to reduce the amount of oil adsorbed on a disk surface with the aid of a magnetic field. Based on the preliminary experiment, the outflow of the FDB oil from the hard disk drive (HDD) spindle was compared with regard to a narrow clearance with and without the magnetic field gradient of permanent magnets. The amount of outflow was decreased by the magnetic field gradient, and the reduction ratio depended on the environmental temperature. Finally, our results suggest the possibility of a gas seal generated by the magnetic field gradient.

Robust Optimum Design of Fluid Dynamic Bearing for Hard Disk Drive Spindle Motors

This paper describes the robust optimum design considering dimensional tolerances for fluid dynamic bearings (FDBs) of 2.5 in. hard disk drives (HDDs). Recently, 2.5 in. HDDs are widely used for mobile devices such as laptops, video cameras, and car navigation systems. Therefore, in mobile devices, high durability toward external vibrations is essential for high HDD performance. On the other hand, FDBs for HDD spindle motors are generally manufactured by mass production processes which will eventually require reduction of production costs. Consequently, the FDBs are demanded to be easily manufactured and expected to have an insensitive design with low variability of bearing characteristics due to manufacturing errors. In this paper, first, the vibration model of the spindle motor is constructed, and then the vibration experiment was carried out in order to verify the appropriateness of the vibration model. Second, the bearing characteristics are calculated considering dimensional tolerance using optimum design combined with the statistical method in which the dimensional tolerance is assumed to distribute according to the Gaussian distribution. The bearing characteristics are estimated by expectation and standard deviation. Finally, the results of this robust optimum design compared with ones of optimum design neglecting tolerance, and the validity of this technique, were clarified. It was found from the results that the tolerances of radial clearance and groove depth are important factors to be considered to reduce the variability of the amplitude and friction torque. In addition, the variability of the amplitude strongly depends on the groove depth.

Improvement of slider off-track vibration through design parameter modifications of a compatible FDB spindle system

Recently, the hard disk drive (HDD) industry has tried to use a compatible spindle system regardless of the number of disks because of the resulting cost reduction and standardization of components. The center of gravity (CG) location predominantly affects the disk and slider off-track vibration, which is why the rocking mode of a spindle system is affected by the CG. Any changes to the CG affect the operational vibration of the spindle system. In a compatible fluid dynamic bearing (FDB) spindle system, changing the number of disks may alter the CG. Nevertheless, research into the compatibility of FDB designs has not been undertaken. In this study, FDB design parameters were selected to reduce the slider off-track vibration with variations in the CG considering a compatible spindle system. First, a verified finite element (FE) model of a spindle system was constructed. The amplitude and frequency of the rocking mode were compared between a one-disk spindle system and a two-disk spindle system using the FE model, considering the relationship between the CG location, which is changed by the number of disks, and the location of the upper and lower journal bearings. HDD prototypes were then manufactured using the improved design. Based on the manufactured spindle system, the variations in the rocking mode characteristics and slider off-track vibration were measured and operational vibration tests were performed to verify the effect of the number of disks on the slider off-track vibration. An improved FDB spindle design was developed with a reduced rocking mode, and a compatible spindle system was proposed.

Micro Pattern Forming of Spiral Grooves in a Fluid Dynamic Bearing Using Desktop Forming System

This research explores the micro-forming process of spiral groove pattern on Fluid Dynamic Bearing(FDB), which is utilized in precision driving part of the hard disk drive(HDD), using micro desktop forming system. While EDM and ECM process has been widely used to engrave the precision pattern which generates dynamic pressure on FDBs, micro forming process is newly proposed in this study to increase the productivity and to reduce the product costs. At first, desktop forming system is designed for spiral groove pattern forming. FE simulations are followed in order to evaluate the feasibility of micro-forming. The simulation results show that forming loads of 1,500Kgf is required to fabricate micro patterns with the depth of 15 μm. Finally the formability test is carried out with various forming loads. Deformed shapes and forming loads obtained from the test are compared with those from the analysis. The results fully demonstrate that micro pattern forming techniques are available to fabricate micro spiral groove patterns in FDB.

Behavior of fluid lubricant and air–oil interface of operating FDBs due to operating condition and seal design

This paper investigated the behavior of fluid lubricant and air–oil interface of operating fluid dynamic bearings (FDBs) by using two-phase flow analysis of air and oil to describe the oil sealing mechanism of operating FDBs. The two-phase flow of fluid lubricant and air was analyzed by using the Navier–Stokes equation and the volume of fluid method of a multi-phase flow. The proposed numerical method was verified by the numerical result of the Reynolds equation and the experimental result of the prior researcher. This research also discussed the effect on the oil leakage of the operating FDBs due to the existence of inward pumping groove, tapering angle and initial position of fluid.

증기압을 고려한 플레인 저널 베어링의 동특성 변화

Fluid dynamic bearings have several advantages that low noise, high rotational stability and low friction. So, early 2000s, the HDD spindle motors have been replaced ball bearings to fluid dynamic bearing. Most of studies apply inner boundary condition that is inside of bearing pressure larger than atmosphere pressure. Therefore, they used Half-Sommerfeld or Reynolds Boundary condition. This paper investigates the dynamic coefficients of the plain journal considered vapor pressure. As a result, it shows that the vapor pressure effect cannot ignore.

A generalized Reynolds equation and its perturbation equations for fluid dynamic bearings with curved surfaces

We derived a generalized Reynolds equation and its perturbation equations using meridian and angular coordinates to calculate the characteristics of fluid dynamic bearings (FDBs) with various shapes, namely journal, thrust, conical, and spherical bearings. They were transformed to the finite element equation to calculate the pressure, load, stiffness, and damping coefficients of FDBs. We verified our proposed method by comparing the measured flying height of the coupled conical and journal bearings in the spindle motor of a hard disk drive with the simulated height. Additionally, we investigated their characteristics according to flying height and conical angle.

A miniature pump with a fluid dynamic bearing

This paper describes the development of a miniature pump having an impeller with an exit diameter of 24 mm supported with the motor rotor by a fluid dynamic bearing. Tests verify that the miniature pump is stable and quiet for rotational speeds larger than 4000 min-1. The three-dimensional turbulent flow in the entire pump flow passage and the laminar flow in the fluid dynamic bearing were then simulated numerically. The average pump performance was well predicted by the simulations. Both the tests and the simulations show that there is no obvious Reynolds effect for the miniature pump within the tested range of rotational speeds. The numerical results also show that the bearing capacity of the fluid dynamic bearing increases with the pump rotor rotational speed and the eccentricity ratio of the journal to the bushing. This pump is very compact, so it is a promising device for surgical use.

1209 流体動圧軸受における微細周期構造パターン付与による摩擦特性の変化(GS-11・16 軸受・潤滑油)

1209 流体動圧軸受における微細周期構造パターン付与による摩擦特性の変化(GS-11・16 軸受・潤滑油)