• 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.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.98-103
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• 2004

This paper presents modeling and simulation of the bump foil bearings with consideration of the elastic behavior of the foil and gas compressibility. Heshmat had originally introduced the simple compliance model to estimate the EHL(elasto-hydrodynamic lubrication) performance. But this approach can not consider the deflection of top foil at the edge of bearing, so model is insufficient to analyze in case that the eccentricity ratio is greater than I. So the top foil is considered as a simple beam model supported by linear spring elements, and the bump foil deflection can be simple compliance model. The EHL calculations are performed for convention rigid type, classical foil type, variable pitch type and double bump type toil bearings. This paper presents that 2nd or 3rd generation bearings have excellent performance in every speeds.

• 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.34 no.3
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• pp.107-114
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• 2018

This paper presents a computational model of a gas foil journal bearing with shim foils between the top foil and bumps, and predicts its static and dynamic performance. The analysis takes the previously developed simple elastic foundation model for the top foil-bump structure and advances it by adding foil models for the "shim foil" and "outer top foil." The outer top foil is installed between the (inner) top foil and bumps, and the shim foil is installed between the inner top foil and outer top foil. Both the inner and outer top foils have an arc length of $360^{\circ}$, but the arc length of the shim foil is shorter, which causes a ramp near its leading edge in the bearing clearance profile. The Reynolds equation for isothermal and isoviscous ideal gas solves the hydrodynamic pressure that develops within the bearing clearance with preloads due to the ramp. The centerline pressure and film thickness predictions show that the shim foil mitigates the peak pressure occurring at the loading direction, and broadens the positive pressure as well as minimum film thickness zones except for the shortest shim foil arc length of $180^{\circ}$. In general, the shim foil decreases the journal eccentricity, and increases the power loss, direct stiffness, and damping coefficients. As the shim foil arc length increases, the journal eccentricity decreases while the attitude angle, minimum film thickness, and direct stiffness/damping coefficients in the horizontal direction increase.

• Tribology and Lubricants
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• v.38 no.5
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• pp.205-212
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• 2022

This paper presents experimental measurements of the structural characteristics of a two-pad beam-type gas foil journal bearing and its rotordynamic performance for a high-speed motor-driven turbocompressor. The test bearing had two top foils and two beam foils, each with an arc length of ~180°. Each beam foil was etched to obtain 40 beams with six geometries of different lengths and widths. The insertion of beam foils into the bearing housing produces equivalent beam heights. The structural tests of the bearing with a non-rotating journal revealed a smaller bearing clearance and larger structural stiffness for the load-on-pad configuration than for the load-between-pads configuration. Rotordynamic performance measurements during driving tests up to 100 krpm demonstrated synchronous vibrations and subsynchronous vibrations with large amplitudes. The test was repeated after inserting the shim between the top foil and beam foil to reduce the bearing radial clearance. The reduced bearing clearance resulted in a reduction in the peak amplitude of the synchronous vibrations and an increase in the speed at which the peak amplitude occurred. In addition, the onset speed and amplitude of the subsynchronous vibrations were dramatically increased and diminished, respectively. The rotor coast-down tests at 100 krpm show that the reduction in the bearing clearance extends the time to rotor stop, thus implying an improvement in hydrodynamic pressure generation and a reduction in bearing frictional torque.

• Tribology and Lubricants
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• v.30 no.1
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• pp.1-8
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• 2014

In this study, a three-pad gas foil journal bearing with a diameter of 40 mm and an axial length of 35 mm was modeled to predict the static and dynamic performances with regard to an increasing mechanical preload. The Reynolds equation for an isothermal and isoviscous ideal gas was coupled with a simple elastic foundation foil model to calculate the hydrodynamic pressure solution iteratively. In the prediction results, the journal eccentricity, journal attitude angle, and minimum film thickness decreased, but the friction torque increased with the preload. A quick comparison implied a lower load capacity but higher stability for a three-pad gas foil bearing compared to a one-pad gas foil journal bearing. The direct stiffness coefficients increased with the preload, but the cross-coupled stiffness coefficients decreased. The direct damping coefficient increased in the horizontal direction but decreased in the vertical direction as the preload increased. These model predictions will be useful as a benchmark against experimental test data.

• Tribology and Lubricants
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• v.30 no.3
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• pp.131-138
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• 2014

This study investigates the effects of the slopes of the rotational axis and bearing preloads on the natural frequencies and onset speeds of the instability of a rotor supported on gas foil bearings (GFBs). The predictive model for the rotating system consists of a rigid rotor supported on two gas foil journal bearings (GFJBs) and a pair of gas foil thrust bearings (GFTBs). Each GFJB supports approximately half the rotor weight. As the slope of the rotational axis increases from $0^{\circ}$(horizontal rotor operation) to $90^{\circ}$(vertical rotor operation), the applied load on the GFJB owing to the rotor weight decreases. The predictions show that the natural frequency and onset speed of instability decrease significantly with an increase in the slope of the rotational axis. In a parametric study, the nominal radial clearance and preload for the GFJB were changed. In general, a decrease in the nominal radial clearance lead to an increase in the natural frequency and onset speed of instability. For constant assembly clearance, the decrease in the preload changed the natural frequency and onset speed of instability with insignificant improvements in the rotordynamic stability. The present predictions can be used as design guidelines for GFBs for oil-free high-speed rotating machinery with improved rotordynamic performance.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2003.11a
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• pp.267-272
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• 2003

This paper presents a performance analysis model of corrugated bump foil bearings. The analyses for not only 1st generation bump foil journal bearings but also bump foil thrust bearings are performed. Static performances such as load capacity, attitude angle, pressure distribution, foil deflection, and film thickness are accurately estimated by using soft elasto-hydrodynamic analysis technique and finite difference numerical method. Also dynamic performances such as stiffness coefficients and damping coefficients are estimated by perturbation method. The analysis technique may be appliable to rotordynamic analysis, stability analysis, and optimized bearing design.

• Tribology and Lubricants
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• v.37 no.6
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• pp.261-272
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• 2021

This study investigates the structural characteristics of oil-free, gas beam foil journal bearings (GBFJBs) for use in high speed motors. Mathematical modeling was carried out, and reaction force modeling for static load was performed to predict the structural characteristics of the GBFJB. Mathematical modeling and reaction force modeling for static load are performed to predict the structural characteristics of GBFJBs. The reaction force of the test bearing against static loads was measured during experiments and compared with the predicted results. The measured experimental data reveal the nonlinear stiffness characteristics of the GBFJB against varying displacement and agree well with the predictions. Dynamic load tests using an exciter allow to identify the vibration characteristics of the GBFJB. Test results show that the vibration displacement, dynamic force, and acceleration measured on the test bearing are most dominant at the applied dynamic load (synchronization) frequency. Futhermore, the test results show that the hysteresis area recorded during the dynamic tests increases with the excitation amplitude and frequency, and that the beam stick phenomena occurr at high excitation frequencies. The single degree of freedom (DOF) vibration model aids to identify the stiffness and damping coefficient of the GBFJB, which decrease as the excitation frequency increases.

• Tribology and Lubricants
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• v.35 no.3
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• pp.190-198
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• 2019

Gas foil bearings (GFBs) enable small- to medium-sized turbomachinery to operate at ultra-high speeds in a compact design by using ambient air or process gas as a lubricant. When using air or process gas, which have lower viscosity than lubricant oil, the turbomachinery has the advantage of reduced power loss from bearing friction drag. However, GFBs may have high Reynolds number, which causes turbulent flows due to process gas with low viscosity and high density. This paper analyzes gas foil journal bearings (GFJBs) with high Reynolds numbers and studies the effects of turbulent flows on the static and dynamic performance of bearings. For comparison purposes, air and R-134a gas lubricants are applied to the GFJBs. For the air lubricant, turbulence is dominant only at rotor speeds higher than 200 krpm. At those speeds, the journal eccentricity decreases, but the film thickness, power loss, and direct stiffness and damping coefficients increase. On the other hand, the R-134a gas lubricant, which that has much higher density than air, causes dominant turbulence at rotor speeds greater than 10 krpm. The turbulent flow model predicts decreased journal eccentricity but increased film thickness and power loss when compared with the lamina flow model predictions. The vertical direct stiffness and damping coefficients are lower at speeds below 100 krpm, but higher beyond that speeds for the turbulent model. The present results indicate that turbulent flow effects should be considered for accurate performance predictions of GFJBs with high Reynolds number.