Shigehisa Fukui

A Database for Interpolation of Poiseuille Flow Rates for High Knudsen Number Lubrication Problems

We propose the use of a Poiseuille flow rate database for rapid calculation of a generalized lubrication equation for high Knudsen number gas films. The database is created by numerical calculations based on the linearized Boltzmann equation. The proposed interpolation method is verified to reduce calculation time to several tenths of that required to perform rigorous calculations with the same accuracy

Recent progress in microtribology

Sliders having very small mass (less than 10 mg) are beginning to be used in magnetic recording devices and very light contacting loads (less than 1 μN) are expected to be used in ultrahigh-density recording devices using the point-recording technique. These newer devices will require wear rates that are virtually zero. The wear on such sliding surfaces is primarily due to the surface interaction forces rather than the load. The ultimate goal of microtribology is to create practical zero-wear devices with very small mass and very light load. Computer simulation of molecular dynamics is used to trace the movement of each atom in sliding surfaces and of each molecule in lubricant films. New tools, such as scanning probe microscopes, are being used to evaluate sliding surfaces and lubricant films. Experimental studies of microwear processes on solid surfaces and of lubrication using very thin lubricant films have recently begun. Microtribology is an important technology for development of new microdevices, and also an important science for understanding the origin of friction and wear. Close cooperation between scientists and engineers is necessary. However, as we have very little of the knowledge needed for microtribology, we must obtain much more data.

Dynamic Characteristics of Gas-Lubricated Slider Bearings Under High Knudsen Number Conditions

This paper presents numerical analyses of the dynamic characteristics of gas-lubricated slider bearings under high Knudsen number conditions using a generalized lubrication equation based on the Boltzmann equation. The present analysis is applied to the dynamic response of flying head sliders for magnetic disk storage devices. For a small slider with ultra-thin spacing, the deviations of the slip flow approximation equations are remarkable in regard to steady flying characteristics, but insignificant in regard to dynamic characteristics

Design of negative pressure slider for magnetic recording disks

This paper reports on investigations of static and dynamic characteristics of a negative pressure slider for magnetic recording disks. The negative pressure slider is composed of two narrow pressure pads, as on a cylindrical slider, separated by a rather large reverse step region. Through a modified Reynolds equation, which considers slip flow effects, the static and dynamic pressure distributions are calculated by divergence formulation and a finite difference method. Using the calculated results, the optimum reverse step depth is clarified in terms of both dynamic characteristics and wear caused by starting and stopping in contact with the disk. With a 4.6 mm length, 3.2 mm total pad width and 4 μm reverse step depth, the negative pressure slider has six times the air film stiffness of a conventional light load slider. Experimental data concerning spacing versus reverse step depth and take-off characteristics for the negative pressure slider in submicron spacing region are in good agreement with the calculated results.

Analysis of flying characteristics of magnetic heads with ultra-thin spacings based on the Boltzmann equation

Numerical analyses of flying characteristics of magnetic heads for magnetic disk storage with ultrathin spacings are presented, applying the general lubrication equation based on the Boltzmann equation as an exact solution. The analyses are then compared with those using the slip flow approximation equations with accommodation coefficients of momentum. For conventional magnetic heads with nominal submicron spacings, differences between flying characteristics based on the generalized equation and the approximation equation are not necessarily significant (10% or less). This can explain why the experimental results agree well with numerical solutions based on a slip flow approximation. The differences for small heads with ultrathin spacings (less than 0.1 mu m) can be more than 20%, indicating that for such heads the generalized lubrication equation based on the Boltzmann equation should be used to predict the flying characteristics for slider designs. >

Estimation of gas film lubrication effects beneath sliding bushings of microrotors using a molecular gas film lubrication equation

Abstract Molecular gas film lubrication effects between rotor bushings and stators in micromotors with a spacing of less than 0.1 μm are numerically clarified using a molecular gas film lubrication equation based on the Boltzmann equation. In conventional bushings, positive pressures and negative pressures are generated nearly equally and the net sum, and hence the load-carrying capacity, is extremely small so it cannot support the rotor weight. A new bushing with a step produces a net pressure that results in a load-carrying capacity that exceeds the rotors weight. Gas film lubrication effects considering surface roughness are briefly discussed on the basis of the mixed lubrication model.

Asymptotic Analysis of Ultra-Thin Gas Squeeze Film Lubrication for Infinitely Large Squeeze Number (Extension of Pan’s Theory to the Molecular Gas Film Lubrication Equation)

Ultra-thin gas squeeze film characteristics are analyzed by extending Pans asymptotic theory for infinite squeeze number to the molecular gas film lubrication equation which was derived from the linearized Boltzmann equation and is valid for arbitrary Knudsen numbers. The generalized asymptotic method is shown to solve the boundary value equation which contains the flow rate coefficient as a function of the product of pressure P and film thickness H. Numerical results are obtained for a circular squeeze film. The PH ratio and the load carrying capacity ratio to those of continuum flow both decrease when the average film thickness is less than several microns because of molecular gas effects

Dynamic Characteristics of Flying Head Slider Considering Van Der Waals Forces: Analyses Based on the Corrected Van Der Waals Force Equation for Multilayers

A dynamic model of a simple inclined flying head slider was considered in order to analyze the dynamic characteristics of a flying head slider considering the van der Waals force the air bearing force. The corrected van der Waals force equation and the linearized molecular gas-film lubrication (MGL) equation were used for the analysis. The “spacing stiffness”, kspacing , composed of the stiffnesses due to the van der Waals force and the air bearing force was defined and the flying stability of the head slider was discussed. It was found that there are flyable limit minimum spacings, and relationship between the limit minimum spacing and the disk vibration frequency (the “boundary curve of dynamic flying stability”), which provides the boundary between the stable (kspacing > 0) and unstable (kspacing < 0) domains of the flying, was presented. The basic characteristics of the flying head slider considering the van der Waals force including the dependence of the boundary curve on the film thickness of PFPE lubricant were clarified.Copyright

DSMC/MGL comparisons of stresses on slider air bearing with nano meter spacings

Summary form only given. In a computer disk drive, the spacing between the slider and disk has been decreased and the tangential force or shear stress as well as the normal force or pressure on the slider surface becomes significant for design considerations. In this paper, we calculated these stress distributions using a direct simulation Monte Carlo (DSMC) method and molecular gas-film lubrication (MGL) analyses with Kangs formulation for shear stress, and examined in detail the influences of the surface accommodation coefficients on the stresses, which characterize the gas-surface scattering at the boundary surfaces.

Dynamic Characteristics of an In-Contact Head Slider Considering Meniscus Force: Part 3—Wear Durability and Optimization of Surface Energy of Liquid Lubricant Under Perfect Contact Condition

The authors developed a computer simulator for in-contact head slider motion where a 3-DOF model of head-suspension assembly was introduced and effects of meniscus force between the slider and the disk were considered. The contact force between a contact pad and a disk surface was taken into account and the dynamic characteristics of an in-contact head slider were discussed from the viewpoints of both bouncing vibration and wear durability by not only 3-DOF head-suspension assembly model over a sinusoidal disk surface undulation but also 1-DOF head-suspension assembly model over a sinusoidal disk surface undulation. In addition to the critical frequency of bouncing, the critical frequency of pressure was introduced. As a result of 3-DOF simulation and 1-DOF analysis, it was found that there exists an optimum surface energy at which the stability of bouncing vibration and wear durability becomes highest under perfect contact condition. Furthermore, the behavior of the optimum point for several design parameters were made clear.