Modal analysis of rotor-shaft system under the influence of rotor-shaft material damping and fluid film forces

Abstract

The present work attempts to study the influences of internal rotor material damping and the fluid film forces (generated as a result of hydrodynamic action in journal bearings) on the modal behaviour of a flexible rotor-shaft system. This is relevant as both journal bearing and the internal material damping introduce tangential forces increasing with the rotor spin speed. Such forces considerably influence the dynamic behaviour of a rotor and tend to destabilize the rotor-shaft system as spin speed increases. Under this system of forces the modal behaviour of the rotor-shaft is studied to get better ideas about the dynamic behaviour of the system, estimated in terms of modal damping factors, stability limit speed, the frequency response functions, as well as the direction of whirl of the shaft in different modes. It is seen that correct estimation of internal friction, in general, and the journal bearing coefficients at the rotor spin-speed are essential to accurately predict the rotor dynamic behaviour. This serves as a first step to get an idea about dynamic rotor stress and, as a result, a dynamic design of rotors.