• Journal of the Korean Society for Precision Engineering
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• v.24 no.6
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• pp.86-95
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• 2007

This paper presents the optimal design of an exhaust system of a vacuum-compatible air bearing using a genetic algorithm. To use the air bearings in vacuum conditions, the differential exhaust method is adopted to minimize the air leakage, which prevents air from leaking into a vacuum chamber by recovering air through several successive seal stages in advance. Therefore, the design of the differential exhaust system is very important because several design parameters such as the number of seals, diameter and length of an exhaust tube, pumping speed and ultimate pressure of a vacuum pump, seal length and gap(bearing clearance) influence on the air leakage, that is, chamber's degree of vacuum. In this paper, we used a genetic algorithm to optimize the design parameters of the exhaust system of a vacuum-compatible air bearing under the several constraint conditions. The results indicate that chamber's degree of vacuum after optimization improved dramatically compared to the initial design, and that the distribution of the spatial design parameters, such as exhaust tube diameter and seal length, was well achieved, and that technical limit of the pumping speed was well determined.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2001.04a
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• pp.429-434
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• 2001

This paper has been presented the dynamic effect by the journal speed, eccentricity and source positions in order to overcome the defects of air bearing such as low stiffness and damping coefficient. Choosing two row sources position of air bearing is different from previous investigations in the side of pressure distribution of air film by the wedge effects. An experimental study was performed to compare theoretical analysis. The dynamic stiffness was measured in actual cutting. It helps predicting of air spindle s characteristic in machining of die more precisely. The results of investigated characteristics was applied to air spindle for high speed milling.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.56 no.6
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• pp.1039-1046
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• 2007

This paper presents the analysis for power consumption, mechanical vibration and acoustic noise characteristics of the Coreless and Slotless Brushless DC motor in Digital Lightening Processor(DLP) Motor with the Air-Dynamic Bearing. The Coreless BLDC motor has not the stator yoke as well as the stator slot to remove the unbalance force by the interaction between the stator yoke and Air-Dynamic Bearing clearance. The assembling tolerance and the processing error make the air-gap difference between the magnet and the stator yoke .which occurs the unbalanced electro-magnetic force in the Slotless BLDC motor. It imposes the air-dynamic bearing on the disturbance force and makes the Air-Dynamic Bearing vibrated and noised. Also, The attractive force between the magnet and the silicon steel stator yoke increases the power consumption. In this paper, the power consumption, mechanical vibration and acoustic noise of the Coreless BLDC motor and the Slotless BLDC motor with the silicon steel stator yoke are simulated, analyzed, and tested using the manufactured proto-type motors with Air-Dynamic bearing. The simulated and tested results present that the Coreless BLDC motor without the silicon steel stator yoke has the lower mechanical vibration and noise ,and lower power consumption than the Slotless BLDC motor with the silicon steel stator yoke in Digital Lightening Processor Motor with Air-Dynamic Bearing.

• Tribology and Lubricants
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• v.21 no.3
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• pp.114-121
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• 2005

As fuel cell system is environmental friendly generator, its performance depends on its air supply system. Because, fuel cell stack generates electrical energy by electron and the electron is generated by reacting between air and hydrogen. So, more and more compressed air is supplied, more and more the energy can be obtained. In this study, turbo-expander supported by air foil bearing is introduced as the air supply system used by fuel cell systems. The turbo-expander is a turbo machine which operates at high speed, so air foil bearings suit its purpose for the bearing elements. Analysis for confirming the stability and endurance is conducted. Based on FDM and Newton-Raphson method, characteristics of air foil bearing, dynamic coefficients, pressure field and load capacity, are obtained. Using the characteristics of air foil bearing, the rotordynamic analysis is performed by finite element method. The analysis (stability analysis and critical speed map) shows that turbo-expander is stability at running speed. After the analysis, the test process and results are presented. The goals of test are running up to 90,000 RPM, flow rate of 150 $m^3/h$ and pressure ratio of 1.15. The test results show that the aerodynamic performance and stability of turbo-expander are satisfied to the primary goals.

• Tribology and Lubricants
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• v.33 no.6
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• pp.309-314
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• 2017

This paper presents a theoretical investigation of the dynamic characteristics of externally pressurized air pad bearings with closed loop grooves. These grooves are made on the surface of bearings to reduce the number of supply holes so that manufacturing costs can be reduced. The semi-implicit method is applied to calculate the time varying pressure profile on the air bearing surface owing to the advantages of numerical stability and fast time tracing characteristics. The static pressure of the groove bearings is much higher than that without grooves, so the groove bearings can provide high load carrying capacity. The equation of motion considering vertical motion and tilting motion are also solved using the Runge-Kutta 4th order method. By combining the semi-implicit method and the Runge-Kutta method, fast calculations of the dynamic behavior of the air bearing can be achieved. The variations of bearing reaction force, air film reaction moment, height, and tilting angle are investigated for the step force input, which is 20% higher than the bearing reaction, when the nominal clearance is 6 mm. The effect of the groove width and the groove depth are investigated by calculating the dynamic behavior. The possibility of the air hammering with the depth of the groove is found and discussed.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.6
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• pp.1028-1033
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• 2003

The purpose of this paper is to investigate the noise and vibration characteristics of externally pressurized air journal bearings with a circular slot restrictor. To do this, the nonlinear transient analysis including rotor imbalance was performed for a rotor-bearing system. The effects of radial clearance and the length of the bearing and mass eccentricity of the rotor on the noise and vibration characteristics of the bearing are also examined. The results show that the noise and vibration of the rotor-bearing system first increase up to critical speed of the system, and then decrease up to instability threshold speed of the system as the rotational speed of the rotor increases, and the noise of the bearing is markedly influenced by the mass eccentricity of the rotor and the radial clearance and the length of the bearing.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1998.10a
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• pp.350-357
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• 1998

Slot restrictor air journal bearing has high load capacitance and high stiffness. Stability characteristics of slot restrictor air journal bearing are studied theoretically to forecast and to prevent the whirl instability. As for the high speed rotating machinery, the instability called 'whirl' occurs when the rotor rotates at a speed more than twice the resonant speed. Once the whirl occurs, rubbing contact between the journal and the bearing occurs mostly and the bearing-rotor system is destroyed ultimately. Therefore, the forecasting and prevention of the occurence of whirl instability is a very important subject especially to develop highly efficient high speed machinery. The bearing with the slot restrictor that varies about circumferencial direction is used for the purpose of the prevention of whirl instability.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2000.11a
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• pp.177-185
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• 2000

The characteristics of multi-leaf feil bearing are investigated. The Foil bearing is preloaded and has several leaf foils modeled by curved beams. An analysis of the air foil bearing was performed, considering effects of foil deflection and compressible lubrication equation simultaneously. A parametric study shows that the number of foils significantly affect the static characteristics of air foil bearings and describes what the minimum film thickness means. The results include pressure profile, load capacity, dimensionless torque and minimum film thickness in the foil bearing.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.2
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• pp.198-205
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• 2001

Recently, almost all hard disk drives employ the rotary actuator system. The performance of an HDD depends on the accuracy and speed of tracking motion. We study the dynamics of head-suspension assembly during track seek. We develop the numerical analysis program to study the dynamic characteristics of HDD suspension system considering the air bearing effects. The track seek simulation by using the developed program helps to estimate the effect of the suspension vibration on the air bearing dynamics. We calculate the behaviour of the air bearing for the given track seek profile and calculate the positioning error during track seek process due to the lateral deflection of the suspension.

• Tribology and Lubricants
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• v.36 no.6
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• pp.332-341
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• 2020

High-speed turbomachinery implements gas foil bearings (GFBs) due to their distinctive advantages, such as high efficiency, lesser part count, and lower weight. This paper provides the test results of the static structural stiffnesses and loss factors of bump-type foil thrust bearings with increasing preload and bearing deflection. The focus of the current work is to experimentally quantify variability in structural stiffnesses and loss factors among the four test thrust bearings with identical design values and material of the bump and top foil geometries using the same (open-source) fabrication method. A simple test setup, using a rigidly mounted non-rotating shaft and thrust disk, measures the bearing bump deflections with increasing static loads on the test bearing. The inner and outer diameters of the test bearings are 41 mm and 81 mm, respectively. The loss factor, best-representing energy dissipation in the test bearings, is estimated from the area inside the local hysteresis loop of the load versus the bearing deflection curve. The measurements show that structural stiffnesses and loss factors of the test bearings significantly rely on applied preloads and bearing deflections. Local structural stiffnesses of the test bearings increase with applied preloads but decrease with bearing deflections. Changes of loss factors are less sensitive to applied preloads and bearing deflections compared to those of structural stiffnesses. Up to 35% variability in static load structural stiffnesses is found between bearings, while up to 30% variability in loss factors is found between bearings.