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International Journal of Rotating Machinery
Volume 2014, Article ID 351261, 17 pages
Research Article

On the Instability Threshold of Journal Bearing Supported Rotors

Laboratory of Rotating Machinery, Department of Mechanical Design, Faculty of Mechanical Engineering, University of Campinas, 200 Mendeleiev Street, 13083-970 Campinas, SP, Brazil

Received 18 July 2013; Accepted 31 October 2013; Published 3 February 2014

Academic Editor: Hooshang Heshmat

Copyright © 2014 Ricardo Ugliara Mendes and Katia Lucchesi Cavalca. 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.


Journal bearing supported rotors present two kinds of self-excited vibrations: oil-whirl and oil-whip. The first one is commonly masked by the rotor unbalance, hence being rarely associated with instability problems. Oil-whip is a severe vibration which occurs when the oil-whirl frequency coincides with the first flexural natural frequency of the shaft. In many cases, oil-whip is the only fluid-induced instability considered during the design stage; however, experimental evidences have shown that the instability threshold may occur much sooner, demanding a better comprehension of the instability mechanism. In this context, numerical simulations were made in order to improve the identification of the instability threshold for two test rig configurations: one on which the instability occurs on the oil-whip frequency, and another which became unstable before this threshold. Therefore, the main contribution of this paper is to present an investigation of two different thresholds of fluid-induced instabilities and their detectability on design stage simulations based on rotordynamic analysis using linear speed dependent coefficients for the bearings.