Kyung-Woong Kim

X-RAY STUDIES OF SNO2 PREPARED BY CHEMICAL VAPOUR-DEPOSITION

Abstract Deposits of SnO2 were formed on Si(100) wafers by chemical vapour deposition using direct oxidation of SnCl4. X-ray studies demonstrated that SnO2 films deposited at temperatures above 400°C are polycrystalline with the tetragonal rutile structure and grow with a (301) preferred orientation. The preferred orientation characteristics of SnO2 prepared by CVD can readily be explained using the standard deviation of texture coefficients of each crystal plane from the powder condition. It is concluded that the mechanism of SnO2 CVD at temperatures below 600°C is oriented overgrowth as a result of preferred nucleation on the growing surface and at temperatures above 600°C is affected by homogeneous nucleation.

Elastohydrodynamic lubrication of a finite line contact

Abstract A numerical solution of the elastohydrodynamic lubrication (EHL) of the finite line contact problem is presented for an axially profiled cylindrical roller which is rolling over a flat plane. The problem is systematically analyzed using a finite difference method and the Newton–Raphson method. Converged solutions are obtained for moderate load and material parameters. Pressure profiles and film shapes are showed and variations of the minimum film thickness with dimensionless parameters are also presented. At the middle of the roller, pressure profile and film shape are nearly the same as those of infinitely long line contacts. Near the position where the profiling start, however, the EHL results are considerably different from those of the infinite solution and both the maximum pressure and the minimum film thickness occur at this region. These analytical results bear a close resemblance to the experimental data in the literature obtained by optical interferometry. Therefore, the present numerical scheme can be used generally in the analysis of the EHL of finite line contact problems.

The Dynamic Performance Analysis of Foil Journal Bearings Considering Coulomb Friction: Rotating Unbalance Response

The dynamic performance of air foil bearings relies on a coupling between a thin air film and the frictional motion of a foil structure. A number of successful analytical techniques to predict their performance have been developed. However, the evaluation of their dynamic characteristics is still not adequate because of the mechanical complexity of the foil structure and the strong nonlinear behavior of the friction force. This work presents a nonlinear transient analysis method to predict the dynamic performance of foil bearings considering Coulomb friction. In this method, the time-dependent Reynolds equation is solved to calculate pressure distribution and a finite element method is used to simulate bump motion. The analysis is treated with a direct implicit integration technique that can handle nonlinear problems and an algorithm to solve for stick-slip motion of bump foil is applied. Using this method, the hysteretic behavior and dissipative characteristics of bump foil resulting from the unbalance excitation is observed. In addition, a number of parametric studies are performed to evaluate the effects of design parameters on the unbalance response. The parameters are friction coefficient, bump half wavelength, amount of mass unbalance, and the number of bump foil strips. The predictions of the unbalance response show that the foil bearing is very effective on the restriction of the vibration at the resonance frequency and there exist optimum values of design parameters such as friction coefficient, bump foil stiffness, and the number of bump foil strips with regard to minimizing the amplitude at the resonance frequency.

A study of the dynamic characteristics of synchronously controlled hydrodynamic journal bearings

Abstract In this paper, synchronous control of bearing is employed through a control algorithm for an actively controlled hydrodynamic journal bearing in order to suppress whirl instability and to reduce the unbalance response of a rotor-bearing system. Furthermore, a cavitation algorithm, implementing the Jakobsson–Floberg–Olsson boundary condition, is adopted to predict cavitation regions in a fluid film more accurately than the conventional analysis, which uses the Reynolds condition. The unbalance responses and stability characteristics of the rotor-bearing system are investigated for various control gains and phase differences between the bearing and journal motion. It is shown that the unbalance responses of the system can be greatly decreased by synchronous control of the bearing. There is an optimum phase difference, which gives the minimum unbalance response of the system at given operating conditions. It is also found that the stability threshold of the system can be greatly increased by synchronous control of the journal bearing.

Acoustical properties of hydrodynamic journal bearings

Abstract Results of theoretical investigations on acoustical properties of hydrodynamic journal bearings are presented. Nonlinear analysis including rotor imbalance is performed for a rotor-bearing system in order to obtain acoustical properties of the bearing. Furthermore, a cavitation algorithm, implementing the Jakobsson–Floberg–Olsson boundary condition, is adopted to predict cavitation regions in a fluid film. Acoustical properties are investigated through frequency analysis of pressure fluctuation calculated from the nonlinear analysis. Results show that the acoustical frequency spectra of the fluid film are pure tone spectra, containing the frequency of the shaft rotation and its super-harmonics. The analysis also shows that super-harmonics are predominant at the neighborhood of the fluid film reformation and rupture regions.

Numerical analysis of grease thermal elastohydrodynamic lubrication problems using the Herschel-Bulkley model

Grease thermal elastohydrodynamic lubrication (TEHL) problems of line contacts are analysed numerically. The effects of temperature and rheological parameters on grease TEHL are investigated using the Herschel-Bulkley model as a rheological model of greases. The pressure distribution, the shape of grease film, mean film temperature and surface temperature of solid wall in line contacts are obtained. It is found that thermal effects on the minimum film thickness become remarkable at high rolling speeds. The effect of yield stress of the Herschel-Bulkley model on minimum film thickness is negligible, while the flow index and viscosity parameter have significant effects on minimum film thickness.

The Static Performance Analysis of Foil Journal Bearings Considering Three-Dimensional Shape of the Foil Structure

To obtain the foil bearing characteristics, the fluid film pressure must be coupled with the elastic deformation of the foil structure. However, all of the structural models thus far have simplified the foil structure without consideration of its three-dimensional shape. In this study, a finite element foil structural model is proposed that takes into consideration the three-dimensional foil shape. Using the proposed model, the deflections of interconnected bumps are compared to those of separated bumps, and the minimum film thickness determined from the proposed structural models is compared to those of previous models. In addition, the effects of the top foil and bump foil thickness on the foil bearing static performance are evaluated. The results of the study show that the three-dimensional shape of the foil structure should be considered for accurate predictions of foil bearing performances and that too thin top foil or bump foil thickness may lead to a significant decrease in the load capacity. In addition, the foil stiffness variation does not increase the load capacity much under a simple foil structure.

A study on nonlinear frequency response analysis of hydrodynamic journal bearings with external disturbances

The nonlinear vibration characteristics of hydrodynamic journal bearings with a circumferential groove are analyzed numerically in case that external sinusoidal disturbances are given to the rotor-bearing system. The cavitation algorithm implementing the Jakobsson-Floberg-Olsson boundary condition is adopted to predict cavitation regions in the fluid film. The comparison of the steady state journal orbits using linear dynamic coefficients with the transient motion calculated from nonlinear analysis is performed. The frequency response functions obtained by linear and nonlinear analysis are also presented. When an external sinusoidal disturbance is applied to the bearing, the range of the full film region varies periodically and it becomes smaller than that of static equilibrium state. The difference between linear and nonlinear analysis increases as the excitation amplitude or the frequency of the external disturbance increases. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in San Francisco, CA October 21–24, 2001

A Noise Analysis of Cylindrical Roller Bearings Operating Under Zero External Load

The purpose of this article is to numerically investigate one source of acoustic noise in roller bearings, that which results from the motion of the rollers in the bearing under zero external load. For the sake of simplification, it was assumed that the cylindrical roller bearings are infinitely long. Furthermore, the effects of the following on the noise of the bearing were also examined: the radial clearance of the bearing, the viscosity of the lubricant, and the number of rollers. The results of the study show that the fundamental frequency of the noise components of the cylindrical roller bearing corresponds to the multiplication of the number of rollers and the whirling frequency of the roller center. The acoustical frequency spectra of the cylindrical roller bearing are pure tone spectra, containing the fundamental frequency of the roller bearing and its superharmonics. The factors that decrease the sound pressure level of a cylindrical roller bearing include low lubricant viscosity, high radial clearance, and a reduced number of rollers. The results and discussions of the present article could aid in the design of low-noise cylindrical roller bearings.

Effects of Distance Between Pads on the Inlet Pressure Build-Up on Pad Bearings

Full Navier-Stokes equations are solved numerically for a cavity region between two consecutive pads and a parallel lubricating film. Numerical solutions are obtained for a wide range of Reynolds number and various values of a distance between pads. Numerical results show that the inlet pressure build-up is significantly affected by Reynolds number and the distance between two adjacent pads. A new formula is derived of loss coefficient with Reynolds number and a distance factor, for using it in an extended Bernoulli equation, on the basis of numerical results. Experiments are conducted to investigate the validity of the formula of loss coefficient proposed by authors.