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International Journal of Rotating Machinery
Volume 2010, Article ID 385947, 12 pages
Research Article

Effects of the Hot Alignment of a Power Unit on Oil-Whip Instability Phenomena

Politecnico di Milano, Dipartimento di Meccanica, Via La Masa, 1, 20156 Milano, Italy

Received 9 April 2010; Accepted 29 July 2010

Academic Editor: Jerzy Sawicki

Copyright © 2010 A. Vania and P. Pennacchi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


This paper shows the results of the analysis of the dynamic behaviour of a power unit, whose shaft-train alignment was significantly influenced by the machine thermal state, that was affected in operating condition by high subsynchronous vibrations caused by oil-whip instability phenomena. The dynamic stiffness coefficients of the oil-film journal bearings of the generator were evaluated considering the critical average journal positions that caused the instability onsets. By including these bearing coefficients in a mathematical model of the fully assembled machine, the real part of the eigenvalue associated with the first balance resonance of the generator rotor became positive. This paper shows the successful results obtained by combining diagnostic techniques based on mathematical models of journal bearings and shaft train with detailed analyses of monitoring data aimed to investigate the effects of the hot alignment of rotating machines on the occurrence of oil-whip instability onsets.