The Effect of Disk Location, Shaft Length and Imbalance on Fluid Induced Rotor Vibrations

Rotors mounted on fluid film bearings can experience fluid induced instabilities that manifest as severe vibration. The setup of the fluid film bearing greatly contributes to the dynamic characteristics of these systems. In this paper, the influence on fluid induced rotor vibrations of shaft length, disk axial separation distance from the fluid film bearing and disk imbalance are studied through experiment and simulation. The basic components of the rotor are the shaft, a disk with variable imbalance, a sleeve and a fluid film bearing. Experimental shaft vibration data was collected and analyzed by computer. The finite element method was used to model the shaft, disk and bearings. The stiffness and damping maps and Campbell diagrams for the system were produced and used to analyze stability. Results indicate that increasing the distance of the disk from the fluid film bearing increases the instability threshold, whereas increasing the shaft length decreases the instability threshold. Changes in shaft length and disk location do not result in noticeable effects on the instability amplitude. Although the mass imbalance produces vibrations in same order as the frequency of shaft rotation, it also affects the oil instability at a frequency of about half that of the shaft rotation.