Identification of vibrational modes of rotating rigid shaft-flexible support system with multiple flexible disks

Abstract

The recent trend toward high rotational speed and low vibration level for high performance and efficiency in the design of rotating machines demands the inclusion of both the disk flexibility and shaft dynamics in the analysis model for accurate vibration analysis. This is especially important in the areas such as gas turbines, industrial compressors, hard disks and CD-ROM drive systems. However, the analysis model consisting of both flexible shaft and disks is so complex that the coupling between the disk and shaft is not yet clearly understood, which necessitates a simpler model. In the case that disk flexibility influences significantly the vibrational characteristics of the whole system, it is very difficult to identify the modal parameters by the conventional modal testing method utilizing mode shapes, since the shaft and disk modes are coupled with each other and their modes tend to be closely packed in the frequency domain. In this thesis, the coupled vibrations between the shaft and disk elastic motions and the corresponding mode splits, which can be used to identify effectively the vibrational modes of rotating machines, are investigated. The analysis model assumes that the shaft is rigid, the bearings are isotropic and the disks behave according to the classical plate theory. The equations of motion are derived by using the extended Hamiltion``s principle, and discretized by expressing the disk deformation as a series of the eigenfunctions of the disk. According to the coupling mechanism of shaft and disk modes, the equations are classified as: bending coupled motion associated with the disk modes with one nodal diameter, axially coupled motion associated with the disk modes with no nodal diameter and uncoupled motion associated with the disk modes with more than one nodal diameter. The bending coupled modes are classified as the balanced and unbalanced modes. The differences in magnitude between forward and backward modal frequencies, referred to as ...