Lubricant evaluation method for FDBs using TOF-SIMS

 With the increase in recording density and data transfer rate of hard disk drive (HDD), fluid dynamic bearing (FDB) motors have been introduced due to their silence and high rotation accuracy. Although lubricant plays a major role in the development of FDB motors, it is extremely difficult to perform thorough evaluation because the quantity of lubricant used in the motor is as small as several micro liters. This paper describes a lubricant evaluation method for FDBs using the time of flight secondary ion mass spectrometry (TOF-SIMS), which enables simultaneous analyses of elements and organic molecules at PPM levels in an extremely small quantity of sample. By using this method, we found that worn metal elements generated both from spindle and bearings have substantial influences on lubricant degradation. Received: 5 July 2001/Accepted: 1 November 2001     

Prediction of the oil injection time of FDBs in an HDD spindle motor with a tied shaft by utilizing Kirchhoff’s pressure law

We propose a method to predict the oil injection time of fluid dynamic bearings (FDBs) with a tied shaft by applying Kirchhoff’s pressure law. Since the oil is injected by capillary phenomenon, the volume flow rate can be calculated by utilizing Kirchhoff’s pressure law. Then, we calculated the oil injection time of the FDBs with a tied shaft by dividing the volume flow rate by the clearance volumes of the journal bearing, the thrust bearing, and the recirculation channel (RC), respectively. We generated simulation models of the FDBs used in a 2.5″ HDD spindle motor with a tied shaft. The total oil injection times of the FDBs with and without a RC were 0.302 and 0.335 s, respectively. Also, we verified the proposed method by measuring the oil injection time of FDBs with a RC. We applied the proposed method to predict and improve the oil injection time of the FDBs with a tied shaft due to the variation of major parameters affecting the oil injection time.     

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.     

Optimization of 3.5-in. HDD spindle motors for OP-vibration performance: theoretical prediction and experimental verification

The purpose of this paper is to optimize OP-vibration performance of 3.5-in. hard disk drive (HDD) spindle motors through theoretical prediction and experimental verification. OP-vibration performance of HDD is closely related to the first rocking vibration of spindle motors because excited frequencies of 3.5-in. HDD from the environment are mostly below 500 Hz and the first rocking vibration is the only resonance in the corresponding frequencies. Therefore, minimizing first rocking vibration leads to improve OP-vibration performance of the spindle motors. In order to minimize the first rocking vibration key parameters of FDB spindle motors were selected from a previous work done by Heo and Shen (Microsyst Technol 11:1204–1213, 2005). Then, the selected parameters have been optimized to minimize the first rocking vibration through a theoretical model developed at University of Washington. Then, experiments with ten prototype FDB spindle motors have been conducted to verify the theoretical results. Each prototype motor has different spindle parameter configurations including bearing coefficients, bearing locations, and center of gravity location, etc. Also, this paper demonstrated that radial measurements of spindle rocking vibration have better correlation with OP-vibration performance than axial measurements through PES measurements. Finally, the optimized design has been manufactured by a motor maker and has also successfully verified the theoretical prediction experimentally.     

A parametric study on rocking vibration of hard disk drive spindle motors with fluid-dynamic bearings and rotating-shaft design

This paper presents a thorough parametric study to identify critical parameters controlling rocking vibration of hard disk drive (HDD) spindle motors. The spindle motors studied are of rotating-shaft design with fluid-dynamic bearings (FDB). The rocking vibration of interest results primarily from first three pairs of (0,1) unbalanced modes (also known as rocking modes, pitch modes, or gyro modes) and half-speed whirls. The parametric study shows that the transverse mass moment of inertia of the rotating part is the most critical parameter affecting the rocking amplitude. Also, FDB in-line stiffness dominates the amplitude of the half-speed whirls. Surprisingly, FDB in-line damping coefficient can considerably affect the amplitude of the second (0,1) unbalanced mode. Finally, bearing locations are very critical parameters for rocking amplitude of FDB spindles. Unfortunately, there is not a set of optimal bearing positions that will minimize vibration for all (0,1) unbalanced modes.The work was completed when the first author was a graduate student at the University of Washington, Seattle, Washington, USA.

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Improvement of component mode synthesis model for vibration analysis of hard disk drives using attachment modes

I improved the component mode synthesis (CMS) model for free and forced-vibration analyses of hard disk drives using attachment modes. The convergence and the accuracy of the proposed CMS model was improved substantially by applying an attachment mode to a FDB shaft and a pivot shaft in the stationary part model. Different formulations were used for the FDBs and the pivot bearings because of their different damping properties. In the proposed formulation, additional general coordinates corresponding to the attachment modes of the FDB shaft are introduced into the system coordinates; on the other hand, the attachment modes of the pivot shaft moderate the stiffness and damping properties of the pivot bearings. To check the improvement of the convergence and the accuracy, I performed the free and forced-vibration analyses using the previous and proposed CMS models and a full finite element (FE) model. The convergence of the natural frequencies and the frequency response function (FRF) of the disk/spindle system were extremely improved. Moreover, the FRF of the head actuator better matched the full FE model than the previous CMS model when the same number of component modes are used.     

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.     

On investigation of vibro-acoustics of FDB spindle motors for hard disk drives

The present work investigates vibro-acoustic behaviors of the fluid dynamic bearing (FDB) spindle motors for hard disk drives (HDD) through the sound spectra and the frequency response functions (FRF) of the motor structure. The quantitative evidence on the significance of the acoustic noise originated from the electromagnetic source is deduced from the sound spectra that were measured in two distinct cases of the spinning motor: in the normal operation and at the moment immediately after the power supply was disconnected. It is found that the effect of electromagnetic noise source is more dominant than the combined effect of the mechanical and aerodynamic sources. In addition, it is identified that, within the audible range of frequency, the frequency range of 13.4–20 kHz deems important to the noise problem as it is the main contributor to the acoustic noise for the FDB spindle motors. Moreover, the structural resonances that can be identified via the FRF are found to play an important role in the noise emitted by the motors. The concurrence of resonance and excitation frequencies clearly intensifies the sound spectrum, resulting in high discrete peaks, hence higher decibel level.     

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.     

Behavior analysis of air bubbles in the oil lubricant of FDBs at low speed operating conditions

This paper investigated the behavior of the air bubbles and the air–oil interface of FDBs at low speed operating conditions by using a two-phase flow analysis of air and oil to study the expelling mechanism of air bubbles in the operation of FDBs. The two-phase flow of air and oil was analyzed using the three-dimensional Navier–Stokes equation and the volume of fluid method of multi-phase flow. The proposed numerical method was verified by the experimental results. The effects of the depth and area of the groove near the outlet on the behavior of the air bubbles and the air–oil interface were also studied. This research shows that the groove near the outlet is responsible for the concave shape of the air–oil interface as a result of surface tension, and that air bubbles are expelled into the outside air when an air bubble meets the concave air–oil interface.     

Whirling, tilting and axial motions of a HDD spindle system due to the manufacturing errors of FDBs

This paper investigates the whirling, tilting and axial motions of a hard disk drive (HDD) spindle system due to manufacturing errors of fluid dynamic bearings (FDBs). HDD spindle whirls around the sleeve with tilting angle due to the centrifugal force of unbalanced mass and the gyroscopic moment of rotating spindle in addition to axial motion. The whirling, tilting and axial motions may be increased by the manufacturing errors of FDBs such as imperfect cylindricity of sleeve bore, or imperfect perpendicularity between shaft and thrust plate. They increase the disk run-out to limit memory capacity and they may result in the instability of the HDD spindle system. This paper proposes the modified Reynolds equations for the coupled journal and thrust FDBs to include the variable film thickness due to the cylindricity of sleeve bore and the perpendicularity between shaft and thrust plate. Finite element method is used to solve the modified Reynolds equation to calculate the pressure distribution. Reaction forces and friction torque are obtained by integrating the pressure and shear stress, respectively. The whirling, tilting and axial motions of the HDD spindle system are determined by solving the equations of a motion of a HDD spindle system in six degrees of freedom with the Runge-Kutta method. It shows that the imperfect cylindricity and perpendicularity increase the whirl radius, axial runout and tilting angle of the HDD spindle system. However, the degradation of dynamic performance due to the imperfect perpendicularity between shaft and thrust plate can be improved by allowing the other manufacturing error of the cylindricity of sleeve bore in such a way to compensate the bad effect of the imperfect perpendicularity.     

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.     

Robust design of a HDD spindle system supported by fluid dynamic bearings utilizing the stability analysis of five degrees of freedom of a general rotor-bearing system

This paper proposes a method to improve the robustness of a hard disk drive (HDD) spindle supported by fluid dynamic bearings (FDBs) by utilizing the stability analysis of the five degrees of freedom of a general rotor-bearing system. The Reynolds equations and the perturbed equations of the coupled journal and thrust bearings were solved by FEM to calculate the dynamic coefficients. The paper introduces the radius of gyration to the equations of motion in order to consistently define the stability problem with respect to a single variable, i.e., the mass. The critical mass, which is the threshold between the stability and instability of the HDD spindle, is determined by solving the linear equations of motion. The proposed method was applied to improve the robustness of a HDD spindle supported by FDBs by varying the groove parameters. It shows that the optimized groove design obtained using the proposed method increases both the stability and the modal damping ratio of the half-speed whirl mode. This research also determines the motions of the rotating disk-spindle system by solving its nonlinear equations of motion with the Runge?CKutta method. It shows that the groove design optimized using the proposed method has a small whirl radius in the steady state. It also shows that it has very little displacement due to the shock excitation, and that it quickly recovers to the equilibrium state.     

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.     

Transient dynamic analysis of ferro-fluid bearing spindle motor

 With areal recording density of hard disk drives (HDD) historically growing at an average of 60% per year and fast spindle speed to continue to reduce access time, it is becoming increasingly more difficult to maintain the precise positioning required of the GMR heads to read and write data. Any unexpected vibration will cause the data written to a wrong data track. Consequently, the dynamic behaviors of HDD spindle systems and their potential influences on track misregistration are key issues in disk drive design. With rapid advances in the emerging consumer device market, the fluid bearing spindle motors, which have low NRRO, low acoustic noise and high damping, are being developed as next generation spindles. This paper is to study transient dynamic performance of HDD ferro-fluid bearing spindle systems. The FEA based component mode synthesis method is used to reduce the overall spindle system dimensions. The effect of the unbalanced magnetic pulls (UMP) due to two different types of motor configurations (balanced and unbalanced configurations) on the dynamic behaviors of spindle system was investigated. The simulated results show that the motor with balanced configuration provides better spindle dynamic performance due to absence of UMP. The UMP derived from the unbalanced configuration can result in some frequency resonance interactions and adversely affect the HDD servo-tracking system. Received: 5 July 2001/Accepted: 17 October 2001     

Estimation and prediction of motor load torque applied to electrical submersible pumps

This work presents a methodology to model a time series of electrical current using state space approach that aims at estimating and predicting the load torque of electrical submersible pump motors, in addition to classifying the oil well dynamic behavior. For this purpose, the historical values of the stator current of 9 motors from subsea well production were used. To perform the validate the modeling and prediction approach test, one motor is used to identify the model parameters. In sequence, the model is compared with others 8 real motors. The model is also compared with established MATLAB/Simulink motor model. Finally, experimental test using three equipment in operation demonstrated the viability of the method. The torque estimation reached an average absolute percentage error of 2.73% and 5 min forecast presented the absolute percentage error below than 1%. With these results, it was possible to predict variations in the load of an oil well.     

Tribilogical performances of connecting rod and by using orthogonal experiment,regression method and response surface methodology

Dynamic lubrication analysis of connecting rod is a very complex problem. Some factors have great effect on lubrication, such as clearance, oil viscosity, oil supplying hole, bearing elastic modulus, surface roughness, oil supplying pressure and engine speed and bearing width. In this paper, ten indexes are used as the input parameters to evaluate the bearing performances: minimum oil film thickness (MOFT), friction loss, the maximum oil film pressure (MOFP) and average of the oil leakages (OLK). Two orthogonal experiments are combined to identify the factors dominating the bearing behavior. The stepwise regression is used to establish the regression model without insignificant variables, and two most important variables are used as the input to carry out the surface response analysis for each model. At last, the support vector machine (SVM) is used to identify the asperity contact. Compared with SVM model, the particle swarm optimization-support vector machines (PSO–SVM) can predict the asperity contact more precise, especially to the samples near dividing line. In future work, more soft computing methods with statistical characteristic are used to the tribology analyses.     

Dynamic analysis of a HDD spindle system with FDBs due to the bearing width and asymmetric grooves of journal bearing

This paper investigates the dynamic behavior of a HDD spindle system with fluid dynamic bearings (FDBs) by solving the Reynolds equation and the equations of a motion of a HDD spindle system in five degrees of freedom. FEM is used to solve the Reynolds equation in order to calculate the pressure distribution in fluid film. Reaction forces and friction torque are obtained by integrating the pressure and shear stress along the fluid film, respectively. Dynamic behaviors of a HDD spindle system, such as the whirling and the tilting motion, are determined by solving its nonlinear equations of motion with the Runge-Kutta method. This paper also proposes two design methods to improve the dynamic characteristics of a HDD spindle system without increasing friction torque, i.e., optimization of the width of the lower and the upper journal bearings and the journal bearings with asymmetric grooves.     

A bi-directional rotating fluid bearing system

 Most fluid bearing systems with grooves on the journal/thrust bearing surfaces were designed to rotate in a specified direction and cannot be reversed. This feature of such fluid bearings limits their application range and hence, a bi-directional rotating fluid bearing system is proposed. The results of numerical simulation on the dynamic characteristics of such bearing system are presented and compared with those of one-directional rotating fluid-bearing system. It shows that for the same load capacity and stiffness requirement, the bi-directional rotating fluid bearing system has a higher power consumption than that of the one-directional counterpart. However, the bi- directional rotating fluid bearing system provides the freedom of rotating spindle motor in either direction and widens the application range of fluid bearing spindle motors. Received: 5 July 2001/Accepted: 17 October 2001     

A miniature spindle motor with fluid dynamic bearings for portable storage device applications

This paper presents the design and fabrication of a miniature axial-gap spindle motor for small-form-factor optical storage applications. The motor features a fluid dynamic bearing (FDB) and is characterized by a high mechanical rigidity, excellent dynamic characteristics, and a zero cogging torque. The performance of this FDB motor is evaluated experimentally using a laboratory-built prototype. The results show that the motor has an excellent dynamic response, a small axial repeatable runout, a small tilt angle, a high rotational speed, and a low operating current. Furthermore, with overall dimensions of just 15.5 × 3.3 mm, the FDB motor is around 80% smaller than that presented by the current group in a previous study (Liu et al., J Magn Magn Mater 304:362–364, 2006). Consequently, the proposed motor represents an ideal solution for both existing and emerging miniaturized portable storage device applications.