Dynamic Stability Analysis of Noncircular Three Lobe Journal Bearings with Couple-stress Lubricant

Design of the bearings with different geometric structures and using additives along with lubricants are among the proposed solutions to improve the dynamic stability of the hydrodynamic journal bearings. It is inevitable to apply new analytical models for evaluating the performance of non-Newtonian lubricants compared to Navier-Stokes equations. Couple stress fluid is a sample of these models, which is employed in this study to investigate the stability of noncircular 3-lobe bearings. To do so, the modified Reynolds equation is first derived based on the couple stress theory. Then, the dynamic pressure components and equivalent stiffness and damping coefficients of the lubricant are calculated by assuming a linear model for the rotor disturbances as limit cycle oscillations (LCOs). Finally, the critical mass parameter and whirl frequency ratio are obtained to determine the bearing stability range by simultaneously examining the Reynolds and rotor motion equations. The results show the bearing stability and lubricant damping ability improve by increasing the couple stress parameter. Also, the effects of upgrading lubricant characteristics on bearing performance significantly enhance with increasing the amount of 3-lobe bearing noncircularity.