Airfoil bearings offer many advantages over oil-lubricated bearings, but they have reliability issues during start∕stops (wear) and limited heat dissipation capability. To address these issues, a hybrid airfoil bearing (HAFB) combining hydrodynamic airfoil bearing with hydrostatic lift was introduced previously by one of the authors of this paper. Their studies show that HAFB has superior performance compared to its hydrodynamic counterpart in load capacity and cooling performance. In this article, the bearing stiffness and damping coefficients of HAFB are calculated using a linear perturbation method developed for HAFB. Simulations showed that feed parameter and supply pressure affect the dynamic characteristics of HAFB. With an increase in either the supply pressure or the feed parameter, the rotor centers itself and hence one sees a decrease in direct stiffness. Simulations showed that the cross-coupled stiffness could be reduced by increasing either the supply pressure or the feed parameter. Direct damping showed increasing trend with the supply pressure and the feed parameter. Frequency-domain analysis of the bearing coefficients was also performed. The direct damping showed marginal changes with supply pressure but showed rapid increase with increasing excitation frequencies. The damping converged to null values for all the pressures for supersynchronous excitations. The loss in damping with high stiffness values for high frequency excitation is a typical hardening effect of gas bearings. In almost all the cases, there are rapid decreases in cross-coupled stiffness and damping and the values show converging trends in supersynchronous regime.

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Air foil bearings have compliant elastic foundation to provide compliance to bearing surface and damping. The most widely used elastic structure is the corrugated bump foil. All the currently available formulas for the bump stiffness [1–3] do not consider bending moments at the bump supports and within the straight contact patch which connects the bumps. However, for continuous bump foils, bending moments at the support points where contact is made with the bearing sleeve cannot be neglected. This paper introduces a set of new formulations for stiffness of bump foils considering the bending moments at the support points. All the formulas were derived using computer algebra software. The results are compared with the result from other formula [3].

From the recent literature, it is revealed that pipe bend geometry deviates from the circular cross-section due to pipe bending process for any bend angle, and this deviation in the cross-section is defined as the initial geometric imperfection. This paper focuses on the determination of collapse moment of different angled pipe bends incorporated with initial geometric imperfection subjected to in-plane closing and opening bending moments. The three-dimensional finite element analysis is accounted for geometric as well as material nonlinearities. Python scripting is implemented for modeling the pipe bends with initial geometry imperfection. The twice-elastic-slope method is adopted to determine the collapse moments. From the results, it is observed that initial imperfection has significant impact on the collapse moment of pipe bends. It can be concluded that the effect of initial imperfection decreases with the decrease in bend angle from 150∘ to 45∘. Based on the finite element results, a simple collapse moment equation is proposed to predict the collapse moment for more accurate cross-section of the different angled pipe bends.

A few years ago, a hydrostatic air foil bearing using compression springs as elastic foundation was firstly introduced by Kim and Park [1]. This paper presents recent experimental results on load capacity of a new hydrostatic air foil bearing made of corrugated bump foils. The new hydrostatic air foil bearing was designed with higher structural stiffness than the first design by [1]. A new test rig was also designed and constructed to measure load capacity of the bearing at higher speeds. The new test results indicate the hybrid operation has similar load capacity to that of hydrodynamic operation at high speeds due to dominance of hydrodynamic pressure. However, comparative tests at low speed (10,000 rpm) showed noticeable increase of load capacity in hydrostatic air foil bearing, manifesting effective hydrostatic levitation feature at low speeds.