• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.803-809
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• 2005

This paper presents the theoretical model to investigate the effect of Coulomb damping in the sub-structure of a foil bearing. Foil deflection is restricted by friction of bumps. Equivalent viscous damping of the bump foils is derived from the Coulomb friction. Dynamic equation of the bumps is constituted by stiffness and damping terms. This point give the difference from Heshmat's frictionless and simple compliance bump model. The fluid is modeled with the compressible Reynolds equation. A perturbation approach is used to determine the static and dynamic performance of the bearing from the coupled fluid-structural model. The analysis result shows that the static and dynamic performance is enhanced by bump friction. This analysis technique would be extended to development of a high performance bearing.

• The KSFM Journal of Fluid Machinery
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• v.9 no.1 s.34
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• pp.47-55
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• 2006

This paper presents the theoretical model to investigate the effect of Coulomb damping in the sub-structure of a foil bearing. Foil deflection is restricted by friction of bumps. Equivalent viscous damping of the bump foils is derived from the Coulomb friction. Dynamic equation of the bumps is constituted by stiffness and damping terms. This point give the difference from Heshmat's frictionless and simple compliance bump model. The fluid is modeled with the compressible Reynolds equation. A perturbation approach is used to determine the static and dynamic performance of the bearing from the coupled fluid-structural model. The analysis result shows that the static and dynamic performance is enhanced by bump friction. This analysis technique would be extended to development of a high performance bearing.

• Tribology and Lubricants
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• v.35 no.2
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• pp.114-122
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• 2019

High-speed rotating machinery requires low cost and reliable bearing elements with low friction, stable rotordynamic characteristics, and a simple design. This study experimentally evaluates the effects of bearing-support elements on the vibrational characteristics of a small-sized, high-speed permanent magnetic motor. A series of coast down tests from 100 krpm characterize the vibrational behaviors, rotor displacement, and housing acceleration of motors supported by ball bearings, ball bearings with a metal mesh damper, and gas foil bearings, respectively. Two eddy-current sensors installed in the horizontal and vertical directions measure the displacement of the rotor at its front nut, and a 3-axis accelerometer attached to the motor housing measures the housing acceleration. The test results reveal that synchronous (1X) vibration components most significantly affect the rotor displacement and housing acceleration, independent of the bearing-support elements. The motor supported by the deep-groove ball bearings results in the largest rotor vibrations increasing with speed; this is due to the absence of a damping mechanism. Additionally, the metal mesh damper effectively reduces the rotor displacement, housing acceleration, and sound-pressure level in the high-speed region (i.e., above 40 krpm), thus implying its substantial damping performance when installed on the outer race of the ball bearing. Lastly, the gas foil bearing supported motor yields the smallest rotor displacement, housing acceleration, and lowest sound-pressure level because of its hydrodynamic airborne operation, which does not require rolling elements that may cause mechanical friction and vibrations.

• 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.

• Resources Recycling
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• v.9 no.4
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• pp.50-55
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• 2000

Recycling technology of aluminum foil scraps into aluminum flake powder by ball milling in dry or wet conditions was studied. Aluminum foil were laminated each other, elongated through microforging by the falling balls, fragmented into small foils and then changed into flake powder during ball milling. It is also possible to recycle foil scraps with thickness less than $60\mu\textrm{m}$ into aluminum paste by wet ball milling. As initial foil thickness decreases, foil is easily milled to flake powder by wet milling in mineral spirits. the appearance and the opaque character of glass painted with aluminum paste obtained by wet milling of foils are similar to those of aluminum paste made by ball milling of gas atomized powder.

• Tribology and Lubricants
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• v.38 no.2
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• pp.53-62
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• 2022

This study presents experimental measurements of the rotordynamic performance of a motor-driven small turbocompressor supported by gas beam foil journal bearings (GBFJBs) and compares the test results with the predictions of a computational model. The experiments confirmed that the rotational synchronous frequency component dominates the behavior of the overall rotor vibrations, whereas the nonsynchronous components are insignificant, indicating the rotor-bearing system remains stable up to 100 krpm. The undamped natural frequency and imbalanced response of the rotor-bearing system are predicted when integrating the finite element model of the rotor-bearing system with the predictions of the bearing dynamic coefficients. The results are in good agreement with the experimental results. In addition, base excitation test results show that the small turbocompressor can endure large external forces and demonstrate limited rotor amplitudes. A simple single degreeof-freedom rotor model using the nonlinear stiffness of the GBFJBs can effectively predict the test results.

• International Journal of Fluid Machinery and Systems
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• v.8 no.1
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• pp.23-35
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• 2015

This paper presents the rotordynamic performance measurement of oil-free turbocharger (TC) supported on gas foil bearings (GFBs) for 2 liter class diesel vehicles and comparison to floating ring bearings (FRBs). Oil-free TC was designed and developed via the rotordynamic analyses using dynamic force coefficients from GFB analyses. The rotordynamics and performance of the oil-free TC was measured up to 85 krpm while being driven by a diesel vehicle engine, and compared to a commercial oil-lubricated TC supported on FRBs. The test results showed that the GFBs increased the rotor speed by ~ 20% at engine speeds of 1,500 rpm and 1,750 rpm, yielding the reduction of turbine input energy by more than 400 W. Incidentally, an external shock test on the oil-free TC casing was conducted at the rotor speed of 60 krpm, and showed a good capability of vibration damping due to the well-known dry friction mechanism of the GFBs.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.10
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• pp.1029-1037
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• 2015

Recently, the importance of rotordynamic stability has been increased because of the tendency to employ ultra-high speeds in rotating machinery. In particular, the dynamic characteristics of gas bearings for high-speed rotating machinery need to be identified at various excitation frequencies to predict the rotor's behavior. In this study, we perform dynamic loading tests for gas-foil bearings (GFBs) to determine the bump foil structure and an air-film combined bump-foil structure for varying excitation frequencies. We calculate the dynamic characteristics from the measured force and displacement data. The air film is generated by a pressurized air supply. Based on the results, the stiffness coefficients of the bump structure and the air-film combined bump structure increased, while the damping coefficients decreased at increasing excitation frequencies. Further, the stiffness and damping coefficients of the air-film combined structure show lower values than those of the bump structure. Consequently, we identify the frequency-dependent dynamic characteristics of the bump structure and the effect of gas film on the dynamic characteristics of GFBs. Furthermore, to reveal the effectiveness of the proposed method, we perform experiments and discuss two methods of extracting the dynamic characteristics from the measured data.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.104-108
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• 2004

Micro turbine is an electric power generating system using a gas turbine whose rated power is under 300kW, and it is featured as a small, efficient. maintenance free and environment-friendly system. Air foil bearing has several advantages over conventional bearings for micro turbine because it is oilless and non-contact. Recently, air foil bearings for high temperature over $500^{\circ}C$ has been developed for the application of 65kW micro turbine system. In this paper, the development and current status are summarized in detail.

• Tribology and Lubricants
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• v.39 no.5
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• pp.203-211
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• 2023

This study experimentally investigates the effects of angular acceleration on the friction and wear performances of a gas foil thrust bearing (GFTB) using a typical GFTB with six pads. The outer radius of the bearing is 31.5 mm, the total bearing area is 2,041 mm² , and the bump foil and incline (ramp) height are both 500 ㎛. The newly developed GFTB test rig for measuring the friction torque and coefficient measures the axial load, drag torque, lift-off speed, and touch-down speed. The experiment is conducted for angular accelerations of 78.5, 314.2, and 328.3 rad/s² at axial loads of 5, 10, and 15 N, respectively. The test shows that the start-up friction coefficient increases with increasing axial load at the same angular acceleration, and the friction coefficient decreases with increasing angular acceleration under the same axial load. As the angular acceleration increases, the lift-off speed at the motor start-up increases, and the touch-down speed at the motor stop decreases. The wear distance of the GFTB for a single on/off cycle increases with increasing axial load at the same angular acceleration and decreases nonlinearly with increasing angular acceleration under the same axial load. The test results suggest that adjusting the rotational angular acceleration helps reduce bearing friction and wear.