A Lateral Vibration Analysis of a Flexible Rotor-Bearing System

Abstract

An unbalanced rotor will generate excessive force on various components of the machine adversely affecting the machines longevity. Analytic study of rotor vibration plays an important role in design, and fault detection and diagnosis of rotating machines. In this paper, vibration characteristics of a simple rotor system due to a central mass unbalance were analysed numerically under two cases. In Case I, bearings have symmetric, isotropic stiffness and damping matrices, while in Case II they have asymmetric, non-isotropic stiffness and damping matrices. It was found in Case I that critical speed occurs at 1690 rpm and the orbital trajectories of rotor mass stations are circular and co-rotational. In Case II, vibration in x-direction and y-direction maximize at 1660 rpm and 1680 rpm, respectively. Response orbit of either the bearing or the disk is elliptic and the size, shape, and orientation of the elliptical orbit changes with shaft speed, and also from the bearing to the disk. Further, both cases show a significant shaft bending contribution to the total response displacement of the disk.

Keywords: Rotor Dynamics, Response Orbit, Asymmetrically Bearing Stiffness Matrix, Mass Unbalanced Rotor