The Effect of Bearing Properties on the Eigenvalues of a Rotordynamic System

Abstract

This work examines how the natural frequencies and modal damping ratios of a shaft/rotor system are affected by changes in lubricant viscosity and bearing clearance. The analysis considers a uniform elastic shaft with a single rigid rotor mounted away from mid-span, and supported by fully cavitated, π-film, short-length, plain journal bearings. The transverse displacements and cross-sectional rotations are described by Ritz series, and standard fluid film lubrication theory is used to describe the forces generated by the bearings. The clearance and lubricant viscosity are independently adjustable at each bearing. A Campbell diagram, which depicts the imaginary part of the systems eigenvalues as a function of rotation rate, is used to identify critical speeds for the nominal system. With the rotation rate held fixed at a critical value, lubricant viscosity and bearing clearance are allowed to vary, and the fluctuations of natural frequency and modal damping ratio are evaluated.