• The KSFM Journal of Fluid Machinery
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• v.6 no.2 s.19
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• pp.62-69
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• 2003

An oil-free turbo-compressor supported by compliant foil bearings which remove oil-contamination by elimination of a conventional ball bearing and oil lubrication systems is presented. Turbo-compressor makes two individual air compressions with two impellers at a operating speed of 39,000 rpm. In this study, the rotordynamic effects caused by aerodynamic instability were investigated with variable mass flow rates. Correlations between frequencies of pressure fluctuation in two diffusers and those of excitation forces on rotor were clearly observed in an aerodynamic unsteady region. Thus, these results show that it is beneficial to design high-speed rotating turbomachinery by considering coupling effect between aerodynamic instability and rotordynamic force.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2000.11a
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• pp.498-501
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• 2000

Air dynamic bearings are inherently unstable in dynamic behavior due to the varying angle of a force produced and the nonlinear characteristics of stiffness. In this study, such dynamic behavior is obtained and compared with experimental results. A body axis coordinate system is employed to avoid the change of a moment of inertia. FDM is used to calculate the pressure distribution on the bearing surface and then the force acting on the rotor was calculated by integrating the pressure distribution. By integrating accelerations which are calculated from the equations of motion using the 4th order Runge-Kutta method, the pose of the bearing at each time step is obtained.

• Journal of the Korean Society for Precision Engineering
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• v.20 no.9
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• pp.84-91
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• 2003

This paper concerns on one-DOF non-rotating active magnetic bearing (AMB) system subject to base motion. In such a system, it is desirable to retain the axis within the predetermined air-gap while the base motion forces the axis to deviate from the desired air-gap. Motivated from this, an optimal acceleration feedforward control is proposed to reduce the base motion response without deteriorating other feedback control performances. Experimental results demonstrate that the proposed optimal feedforward control reduces the standard deviation of the air-gap to 29％ that by feedback control alone.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1999.10a
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• pp.227-230
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• 1999

In this paper, the machining characteristics of high speed ball end milling affected by the rotational error of high speed spindle using air bearing are investigated. The error motions of a spindle have generally influenced on the surface roughness, the form accuracy, the tool life, etc. in end milling. Experiments are carried out over a wide range of rotational speeds(10,000-50,000rpm). The rotational errors of the spindle are measured by the gap sensor mounted on the spindle shaft at various cutting speeds. The relations between the surface roughness and the spindle error motion are presented. Results show that the rotational accuracy of the spindle directly affects the surface roughness of the machined surface.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.05a
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• pp.271-276
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• 2006

This paper shows the rotordynamic characteristics of a turbo-generator for a BOP of a fuel cell system. The rotor-bearing system consists of magnetic shaft and compressor-turbine shaft, and the two shafts are connected by spline coupling and supported by oil free air foil bearing. Preliminary design according to several parameter is considered in detail. Static and dynamic characteristics of the AFB are estimated by the soft elasto-hydrodynamic analysis technique and the perturbation method. The results of the natural frequencies, mode shape, and unbalance response analysis are presented.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.05a
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• pp.1165-1169
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• 2003

This paper presents an approach to optimally design the air-bearing surface (ABS) of the optical flying head for near-field recording technology (NFR). NFR is an optical recording technology using very small beam spot size by overcoming the limit of beam diffraction. One of the most Important problems in NFR is a head disk interface (HDI) issue over the recording band during the operation. A multi-criteria optimization problem is formulated to enhance the flying performances over the entire recording band during the steady state. The optimal solution of the slider, whose target flying height is 50 nm, is automatically obtained. The flying height during the steady state operation becomes closer to the target values than those fur the initial one. The pitch and roll angles are also kept within suitable ranges over the recording band. Especially, all of the air-bearing stiffness are drastically increased by the optimized geometry of the air bearing surface.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.11a
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• pp.742-747
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• 2000

The optical storage device has recently experienced significant improvements, especially for the aspects of high capacity and fast transfer rate. However, it is the fact that the optical storage device has the lower access time for the randomly scattered data compared to the hard disk drives. It is, therefore, necessary to develop a new type of optical storage system. In this study, we investigate the air bearing characteristics for the optical disk drives which have the swing arm actuator similarly to the hard disk drives. Considering the requirements of the optical disk drives, we parametrize the shape of the air bearing surface and investigate its sensitivity to the flying characteristics for further optimized design outputs.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2001.10a
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• pp.243-249
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• 2001

This paper has been presented the hydrodynamic 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 the two row source position of air bearing is different from existed investigations in the side of pressure distribution of air film because of the high speed of journal and the wedge effects by the eccentricity. These optimal chooses of the two row source positions enable us to improve the performance of the film reaction force and loading force as making the high speed spindle. In this paper, the pressure behavior in theory of air film according to the eccentricity of journal and the source positions analyzed. The theoretical analysis have been identified by experiments. The results of investigated characteristics may be applied to precision devices like ultra-precision grinding machine and ultra high speed milling.

• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.475-481
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• 2001

Foil bearings have been successfully used for small high speed rotors, such as ACM(Air Cycle Machine), turbo charger, turbo compressor, high speed motor, etc. Recently advanced researches are concentrated on the high load capacity and the extreme temperature foil bearings to extend the application boundary. Some bearings are already adopted into cryogenic machines and micro gas turbines. In this paper, a foil journal bearing designed for high load capacity, which is under development, is introduced. The bearing is for the turbo refrigerator which has a rotor of 18${\~}$25 kgf rotating at 23,000${\~}$38,000 rpm. This application is well beyond conventional spectrum of foil bearings because the rotor is relatively heavy and the rotational speed is low. Therefore, the development is challenging. The foil bearing is a bump type, the size is 60mm in diameter and 50mm in length, the operating fluid is air and rotational speed is 26,000 rpm. In-house software was developed and used for bearing design. Tested maximum load capacity is 80kgf, 0.62 in terms of load capacity coefficient, and testing is being continued.

• Journal of the Korean Society for Precision Engineering
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• v.28 no.6
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• pp.745-750
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• 2011

In a hope to better understand the flow and convective heat transfer characteristics inside a ball bearing, air flow between the rolling elements and raceways at high speed bearing rotation is numerically investigated using a simplified inner geometry of bearing and a CFD technique. Flow simulation results reveal the pressure distribution of airflow and the shear stress distribution on the ball surface, of which nonuniformity becomes significant with the increasing rotational speed. Also, the local point of maximum shear stress coincides with the stagnation flow area on the surface of rolling elements. A complex pattern of three-dimensional vortex structures is found in the air flow due to the relative motion of bearing elements and three different types of vortex pairs exist around the rotating and orbiting rolling elements.