CAVITATION EFFECTS ON THE DYNAMICS OF JOURNAL BEARINGS
Jerzy T. SAWICKI
D.E. Bently & A. Muszynska Endowed Chair
Rotor-Bearing Dynamics and Diagnostics Laboratory
Fenn College of Engineering
Cleveland State University
Cleveland, Ohio 44115-2425, U.S.A.
A numerical procedure incorporating cavitation effects on the dynamics of journal bearings is presented. A two-dimensional linear stability analysis considering the fluid flow in both full film and cavitation regions for a plain cylindrical journal bearing and four multi-lobe bearings are presented. The Lund’s infinitesimal perturbation procedure is applied to Elrod’s universal equation for evaluation of unsteady pressure gradients. Based on JFO theory, the pressure distribution, film rupture, and reformation boundaries can be obtained using Elrod’s universal equation, for a given operating position of the journal. In this work, it is assumed that for infinitesimal perturbation of journal about equilibrium position, the film rupture and film reformation boundaries are same as those obtained for steady state. However, the unsteady pressure gradients in the full film region are evaluated taking into consideration the perturbed flow parameters in the cavitation region, i.e., at both rupture and reformation boundaries. The linearized stiffness and damping coefficients, whirl frequency ratio, and threshold speed for various values of eccentricity and L/D ratios are obtained for a plain cylindrical journal bearing with an axial groove along the load line. Measured data of dynamic coefficients for a 120° partial arc bearing are chosen for comparison with this work. Results show good agreement between the theoretical and experimental results. Results of stiffness and damping coefficients are presented for two-axial groove, elliptical, three-lobe and offset cylindrical bearing for various L/D and eccentricity ratios. A transient analysis of submerged journal bearing incorporating the mechanism of shear between the liquid sublayer and air cavity in the cavitation zone is also presented. Using the mass conservation principles, Elrod’s universal equation is modified to take into consideration the shear of air cavity in the cavitation zone. Results of transient response for the submerged journal bearing using the present approach are compared with the Elrod’s universal equation based on the striated flow in the cavitation region. The limit cycle journal motion using the present approach predicts higher eccentricity ratios.