Many common faults and malfunctions in rotating machines mainly cause synchronous vibrations (1X). Very high 1X vibration levels can occur, in case of severe faults. Large journal orbits inside oil-film journal bearings may generate non-linear effects in the oil-film forces, whose presence can be detected by means of the appearance of not negligible super-synchronous vibrations of the shafts.

In this paper, a model-based method has been used to study the effects of non-linear oil-film forces on the machine dynamic behavior that may occur during runups and rundowns.

In general, it is possible to suppose that the importance of the non-linear behavior of oil-film journal bearings, and then the level of the super-synchronous vibrations, increases with the amplitude of the 1X vibrations caused by the primary fault. However, the numerical results of this study and the experimental evidences found in the monitoring data of a real machine have shown that the super-synchronous harmonic components of the oil-film forces may excite resonances of the shaft-train causing unexpected amplifications of the super-synchronous vibrations. This may make difficult the recognition of the presence of non-linear effects in the machine dynamic behavior and the identification of the actual cause of abnormal vibrations.

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