• Tribology and Lubricants
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• v.34 no.3
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• pp.75-83
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• 2018

Microturbomachinery (< 250 kW) using gas foil bearings can function without oil lubricants, simplify rotor-bearing systems, and demonstrate excellent rotordynamic stability at high speeds. State-of-the-art technologies generally use bump foil bearings or leaf foil bearings due to the specific advantages of each of the two types. Although these two types of bearings have been studied extensively, there are very few studies on leaf-bump foil bearings, which are a combination of the two aforementioned bearings. In this work, we illustrate a simple mathematical model of the leaf-bump foil bearing with leaf foils supported on bumps, and predict its static and dynamic performances. The analysis uses the simple elastic model for bumps that was previously developed and verified using experimental data, adds a leaf foil model, and solves the Reynolds equation for isothermal, isoviscous, and ideal gas fluid flow. The model predicts that the drag torques of the leaf-bump foil bearings are not affected significantly by static load and bearing clearance. Due to the preload effect of the leaf foils, rotor spinning, even under null static load, generates significant hydrodynamic pressure with its peak near the trailing edge of each leaf foil. A parametric study reveals that, while the journal eccentricity and minimum film thickness decrease, the drag torque, direct stiffness, and direct damping increase with increasing bump stiffness. The journal attitude angle and cross-coupled stiffness remain nearly constant with increasing bump stiffness. Interestingly, they are significantly smaller compared to the corresponding values obtained for bump foil bearings, thus, implying favorable rotor stability performance.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.11a
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• pp.282-289
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• 2001

Experiments were conducted to determine the structural dynamic characteristics of bump foil bearing. The housing of the bearing on the journal was driven by two shakers which were used to simulate dynamic forces acting on the bump foil strips. Three different bump foils(Cu-coated bump, silicon bump, viscoelastic bump) are tested and the dynamic coefficients of three bump foils compared, based on the experimental measurements for a wide range of operating conditions. From the test results, the high damping coefficients of viscoelastic bump are achieved and the possibility of the super-bending-critical operation is suggested.

• 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.20 no.5
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• pp.245-251
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• 2004

This paper presents the effects of the bump foil stiffness on the static and dynamic performance of the foil journal bearings. Reynolds equation is used for the static and dynamic performance analyses. To consider the deflection of top foil the top foil is modeled as a elastic beam and the bump foil is modeled as a spring. So in the static performance analysis the load capacity is compared to the various bump foil stiffness and in the dynamic performance analysis the trajectory of journal center is compared to the various bump foil stiffness.

• The KSFM Journal of Fluid Machinery
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• v.5 no.1 s.14
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• pp.55-61
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• 2002

The feasibility of an oil-free, motor-driven, two-stage centrifugal compressor supported by air bump bearings is investigated. This centrifugal compressor is driven by a 75 kW motor at an operating speed of 39,000 RPM, and a pressure ratio of the compressor is set up to 4. The analysis is performed by using bearing equilibrium position, heaving stillness, Campbell diagram, unbalance response, and stability. It is demonstrated in this paper that air bump bearings can be adopted well to an oil-free, motor-driven, centrifugal compressor. Specially, Cu-coated bump bearings have enough damping force to reduce a synchronous unbalance for rigid modes of the two-stage compressor. Futhermore, this concept may be applied to the flexible rotor system such as high speed turbomachinery with a super critical speed.

• 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.

• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.469-474
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• 2001

The feasibility of a oil-free motor-driven two-stage centrifugal compressor supported by air bump bearings is investigated. This centrifugal compressor is driven by 75kW motor at an operating speed of 39,000RPM md a pressure ratio of the compressor is up to 4. The analysis is performed, based upon bearing equilibrium position, bearing stiffness, Campbell diagram, unbalance response and stability. It is demonstrated in this paper that air bump bearings can be adopted well to a oil-free motor-driven centrifugal compressor.

• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.129-134
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• 2001

The paper presents the optimal design of a oil-free two-stage compressor, which is driven by 75 kW motor at an operating speed of 39,000 rpm, and the pressure ratio of which is up to 4. First, an attempt is made to obtain the optimal design of a bump bearing which supports a compressor rotor. Second, bump bearings and shaft are considered simultaneously, and the weighted sum of rotor weight and frictional torque is minimized. Finally, the optimal geometry of compressor wheel is considered. The mean efficiency and the - minimum efficiency are maximized respectively. The results presented in this paper provide important design information necessary to reduce the energy loss.

• 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.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.06a
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• pp.236-242
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• 2001

In this paper the effect of bump compliance, load, and the number of pad on the lift-off speed is studied. When the load is greater and bump compliance lower, the shaft is lifted off at higher rotating speed. And when the load is applied near the center of pad, lift-off speed is lower. When the number of pad increases, the lift-off speed is higher. The lift-off characteristics can be used to lengthen the life time of the coating and design the rotating machinery supported by bump bearings.