Determination of Dynamic Coefficients of Air-Ring Bearings

Abstract

An externally-pressurized journal air bearing (AB) with an air-ring (AR), or air-ring bearing (ARB), for a balanced, rigid and light-weight rotor is studied. An elastic structure in the form of an AR is provided between the bearing-bushing and the casing. The ARB is analyzed to determine the dynamic coefficients (DC) at various angular velocities and angular frequencies of vibration of the journal in its range of operation. These DC can then be used to predict the dynamic stability of the rotor ARB system against self-excited (SE) vibration. A numerical simulation procedure is followed to determine the DC. The ARB is modeled as a two-degrees of freedom system. During the simulation, the journal follows a prescribed harmonic motion. Self-exciting forces due to flow dynamics inside an ARB induce this motion. Three-dimensional (3-D) flow equations are solved on a moving/deformable grid using ANSYS®, to compute the pressure (p) distribution in the ARB. Unlike in previous studies, in this study the bushing displacement is determined by the instantaneous p-distribution in the ARB. DC of both AB and AR are determined simultaneously by considering the interaction between the AR and the AB regions through the feed-holes in the bushing. Time-dependent displacement, velocity, and load-carrying capacity obtained by numerical simulation are used to evaluate the DC. Incorporation of an AR around an AB can prevent SE vibration due to positive values of direct damping coefficients of AR. A 3-D flow analysis can reveal the realistic nature of flow in an ARB.