Young-Cheol Kim

The Dynamic Performance Analysis of Foil Journal Bearings Considering Coulomb Friction: Rotating Unbalance Response

The dynamic performance of air foil bearings relies on a coupling between a thin air film and the frictional motion of a foil structure. A number of successful analytical techniques to predict their performance have been developed. However, the evaluation of their dynamic characteristics is still not adequate because of the mechanical complexity of the foil structure and the strong nonlinear behavior of the friction force. This work presents a nonlinear transient analysis method to predict the dynamic performance of foil bearings considering Coulomb friction. In this method, the time-dependent Reynolds equation is solved to calculate pressure distribution and a finite element method is used to simulate bump motion. The analysis is treated with a direct implicit integration technique that can handle nonlinear problems and an algorithm to solve for stick-slip motion of bump foil is applied. Using this method, the hysteretic behavior and dissipative characteristics of bump foil resulting from the unbalance excitation is observed. In addition, a number of parametric studies are performed to evaluate the effects of design parameters on the unbalance response. The parameters are friction coefficient, bump half wavelength, amount of mass unbalance, and the number of bump foil strips. The predictions of the unbalance response show that the foil bearing is very effective on the restriction of the vibration at the resonance frequency and there exist optimum values of design parameters such as friction coefficient, bump foil stiffness, and the number of bump foil strips with regard to minimizing the amplitude at the resonance frequency.

The Static Performance Analysis of Foil Journal Bearings Considering Three-Dimensional Shape of the Foil Structure

To obtain the foil bearing characteristics, the fluid film pressure must be coupled with the elastic deformation of the foil structure. However, all of the structural models thus far have simplified the foil structure without consideration of its three-dimensional shape. In this study, a finite element foil structural model is proposed that takes into consideration the three-dimensional foil shape. Using the proposed model, the deflections of interconnected bumps are compared to those of separated bumps, and the minimum film thickness determined from the proposed structural models is compared to those of previous models. In addition, the effects of the top foil and bump foil thickness on the foil bearing static performance are evaluated. The results of the study show that the three-dimensional shape of the foil structure should be considered for accurate predictions of foil bearing performances and that too thin top foil or bump foil thickness may lead to a significant decrease in the load capacity. In addition, the foil stiffness variation does not increase the load capacity much under a simple foil structure.

Performance Analysis of Double-Bump Air Foil Bearings

This paper presents a theoretical model for the analysis of double-bump air foil bearings (AFBs). The stiffness and damping coefficients of the double bump vary depending on the external load and its friction coefficient. In the case of a lightly loaded condition where only the upper bump contributes to deformation, the double bump is in the single active region. In the case of a heavily loaded condition where both the upper and lower bumps contribute to deformation, the double bump is in the double active region. So the double bump can be either in the single or double active region depending on vertical deflection. The equivalent stiffness and damping coefficients of the bump system are derived from the vertical and horizontal deflection of the bump, including the friction effect. A static and dynamic performance analysis is carried out by using the finite difference method and the perturbation technique. The results of the performance analysis for a double-bump AFB are compared with those obtained for a single-bump AFB. This paper successfully proves that a double-bumped AFB has higher load capacity, stiffness, and damping than a single-bump AFB in a heavily loaded condition.

THE STATIC PERFORMANCE ANALYSIS OF AIR FOIL JOURNAL BEARINGS CONSIDERING THREE-DIMENSIONAL STRUCTURE OF BUMP FOIL

The calculation of bump foil deflection is very important to predict the performance of foil bearings more accurately, because the foil bearings consist of top foil and its elastic foundation usually called bump foil. For the purpose of this, a finite element model considering 3-dimensional structure of the bump foil is developed to calculate the deflection of inter-connected bump. The results obtained from the suggested model are compared and analyzed with those from the previous proposed deflection models. In addition, load capacity of the foil bearings is analyzed by using this model.Copyright