• Journal of the Korean Society of Industry Convergence
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• v.22 no.1
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• pp.47-54
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• 2019

An experimental study was conducted on the wear and friction responses in sliding tests of a micro-textured surface on laser pattern (LP) steel as reduction gear material in electric guided vehicle. In this research, the friction characteristics of laser pattern steel under different micro texture density conditions were investigated. The friction tests were carried out at sliding speeds of 0.06 m/s to 0.34 m/s and at normal loads of 2 to 10 N. Photolithography method was used to create the dimples for surface texturing purpose. Four different specimens having different dimple densities of 10%, 12.5%, 15%, and 20% were observed respectively. In this research, friction conditions as shown in Stribeck curve were investigated. Furthermore, the microscopic surface was observed using scanning electron microscope. It was found that the dimple density had a significant role on the friction characteristics of laser pattern steel conditioned as reduction gear material in an agricultural vehicle. The duty number showed that the friction condition was hydrodynamic regime. The best performance was obtained from 12.5% dimple density with lowest friction coefficient achieved at 0.018771 under the velocity of 0.34 m/s and 10N load.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.972-975
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• 2002

An understanding of tribological behavior in CMP(Chemical Mechanical Polishing) is one of the most important things to reveal the mechanism of material removal. In CMP, the contact type is thought to be semi-direct, elastohydrodynamic contact type from the Stribeck diagram, which is a combination of solid-solid direct contact and hydrodynamic lubrication with thin liquid film. This study is focused on the decay of friction force during CMP from two points of view, one of which is change of the real contact area and the other is the decrease of the elastic modulus of the pad caused by the increase of the temperature during CMP Experiments are implemented with elastic modulus measuring system and tool dynamometer. Results show that the decay of friction force during CMP results from the decrease of the real contact pressure working on an abrasive, which is induced by the decrease of elastic modulus of pad caused by the increase of temperature. And, the phenomenon is thought to be happen specially in the case that the weight concentration of abrasive in slurry is small enough.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.4
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• pp.301-306
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• 2016

High speed polishing can kinematically increase the polishing removal rate by using the conventional Preston equation, especially for hard substrates such as sapphire or diamond. However, high speed effects should be clarified beforehand considering the lubrication state and process parameter variations. In this paper, we developed a polishing experimental method and apparatus to determine the lubrication state by measuring the real time friction coefficient using two load cells. Through experiments, we obtained a boundary lubrication state above 0.35 of the friction coefficient by using low table speed and high polishing load, indicating a synchronized stable behavior in polishing head rotation. However, larger Stribeck indexes by a high speed above 200 rpm can generate a hydrodynamic lubrication state below 0.25 of the low friction coefficient. This causes the polishing head rotation to stop. A forced and synchronized head rotation is required for high speed polishing.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.11a
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• pp.304-311
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• 2001

This research numerically analyzes the dynamic characteristics of a coupled journal and thrust hydrodynamic bearing due to its groove location which has the static load due to the weight of a rotor in the axial direction and the dynamic load due to its mass unbalance in the radial direction. The Reynolds equation is transformed to solve a plain member rotating type of journal bearing(PMRJ), a grooved member rotating type of journal bearing (GMRJ), a plain member rotating type of thrust bearing (PMRT) and a grooved member rotating type of thrust bearing (GMRT). FEM is used to solve the Reynolds equations in order to calculate the pressure distribution in a fluid film. Reaction forces and friction torque are obtained by integrating the pressure and shear stress along the fluid film, respectively. Dynamic behaviors, such as whirl radius or floating height of a rotor, are determined by solving its nonlinear equations of motion with the Runge-Kutta method. This research shows that the groove location affects the pressure distribution in the fluid film and consequently the dynamic performance of a HDD spindle system.

• Journal of Korea Water Resources Association
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• v.33 no.1
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• pp.39-50
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• 2000

Generally speaking, a hydrodynamic model needs a friction coefficient (Manning coefficient or Chezy coefficient) and eddy viscosity. For numerical solution the coefficients are usually determined by recursive calculations. The eddy viscosity in numerical model plays physical diffusion in flow and also acts as numerical viscosity. Hence its value has influence on the stability of numerical solution and for these reasons a consistent evaluation procedure is needed. By using records of stage and discharge in the downstream reach of the Han river, I-D models (HEC-2 and NETWORK) and 2-D model (SMS), estimated values of Manning coefficient and an empirical equation for eddy viscosity are presented. The computed results are verified through the recorded flow elevation data.n data.

• Journal of KSNVE
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• v.7 no.5
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• pp.861-869
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• 1997

An approximate analytical method has been developed for estimating hydrodynamic forces acting on oscillating inner cylinder in concentric annulus. When the rigid inner cylinder executes translational oscillation, fluid inertia and damping forces on the oscillating cylinder are generated by unsteady pressure and viscous skin friction. Considering the dynamic-characteristics of unsteady viscous flow and the added mass coefficient of inviscid fluid, these hydrodynamic forces including viscous effect are dramatically simplified and expressed in terms of oscillatory Reynolds number and the geometry of annular configuration. Thus, the viscous effect on the forces can be estimated very easily compared to an existing theory. The forces are calculated by two models developed for relatively high and low oscillatory Reynolds numbers. The model for low oscillatory Reynolds number is suitable for relatively high ratio of the penetration depth to annular space while the model for high oscillatory Reynolds number is applicable to the case of relatively low ratio. It is found that the transient ratio between two models is approximately 0.2~0.25 and the forcea are expressed in terms of oscillatory Reynolds number, explicity. The present results show good agreements with an existing numerical results, especially for high and low penetration ratios to annular gap.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2015.11a
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• pp.102-133
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• 2015

Nature, such as plants, insects, and marine animals, uses micro/nano-textured surfaces in their components (e.g., leaves, wings, eyes, legs, and skins) for multiple purposes, such as water-repellency, anti-adhesiveness, and self-cleanness. Such multifunctional surface properties are attributed to three-dimensional surface structures with modulated surface wettability. Especially, hydrophobic surface structures create a composite interface with liquid by retaining air between the structures, minimizing the contact area with liquid. Such non-wetting surface property, so-called superhydrophobicity, can offer numerous application potentials, such as hydrodynamic drag reduction, anti-biofouling, anti-corrosion, anti-fogging, anti-frosting, and anti-icing. Over the last couple of decades, we have witnessed a significant advancement in the understanding of surface superhydrophobicity as well as the design, fabrication, and applications of superhydrophobic coatings/surfaces/materials. In this talk, the designs, fabrications, and applications of superhydrophobic surfaces for multifunctionalities will be presented, including hydrodynamic friction reduction, anti-biofouling, anti-corrosion, and anti-icing.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.12
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• pp.1035-1042
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• 2014

Reducing the friction of engine parts is an important issue in engine design. The loss of energy in the piston assembly due to mechanical friction ranges from 40 to 55%, and there is an increase in the total energy of about 5% if the friction of the piston can be removed. In order to reduce the friction loss at the level of each engine part, it is necessary to perform a comparative analysis with other engines to determine the important factors affecting the energy loss. Several studies have been performed to analyze the lubrication based on hydrodynamic modeling, since a piston lubrication system has dimensions in the nanoscale to microscale domain. Therefore, it is necessary to determine the correlations between the molecular and continuum systems. In this study, we investigated the friction changes due to the various interactions between molecules in the wall/fluid interface, where a microscopic movement of the oil film occurs along the cylinder liner of the engine.

• Journal of Mechanical Science and Technology
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• v.17 no.3
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• pp.350-356
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• 2003

In this study, a numerical analysis for the piston secondary dynamics of small refrigeration reciprocating compressors is performed. In general, the length of cylinder in this class of compressors is shortened to diminish the frictional losses of the piston-cylinder system. So, the contacting length between piston and cylinder wall is in variable with the rotating crank angle around the BDC of the reciprocating piston. In the problem formulation of the piston dynamics, the variation in bearing length of the piston and all corresponding forces and moments are considered in order to determine the piston trajectory, velocity and acceleration at each step. A Newton-Raphson procedure was employed in solving the secondary dynamic equations of the piston. The developed computer program can be used to calculate the entire piston trajectory and the lubrication characteristics as functions of crank angle under compressor running conditions. The results explored the effects of some design parameters and operating conditions on the stability of the piston, the oil leakage, and friction tosses.

• Journal of KSNVE
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• v.8 no.5
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• pp.849-855
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• 1998

An analysis of lateral hydrodynamic forces of compressor labyrinth seals is presented. Basic equations are derived using a two-control-volume model for compressible flow. Blasius' wall friction-factor formula and jet flow theory are used for the calculaton of wall shear stresses and recirculation velocity in the cavity. Linearized zeroth-order and first-order perturbation equations are developed for a small motion about the centered position by expansion in the eccentricity ratio. Integraton of the resultant first-order pressure distribution over the seal defines the rotordynamic coefficients. As an application a rotordynamic analysis of the balance drum labyrinth seal found in an ethylene regrigeration copmressor is carried out. The rotordynamic characteristic results of the labyrinth seal are presented and compared with other types of seals, honeycomb seal and smooth seal.