This paper is concerned with instability of a rotor that arises due to fluid film forces of a journal bearing. The half frequency whirl and the resulting oil whip phenomena is explained by a consideration of flow balance in a bearing which looses the load carrying capacity. The threshold instability criterion for a rigid rotor in plain cylindrical bearings is given in the form of a chart by obtaining the solution of equations of motion with the corresponding spring and damping coefficients. Both translatory and conical whirls are considered. The analysis is then presented for a flexible rotor and a simple procedure is given to obtain the instability threshold speed based on the rigid rotor criterion. The procedure does not involve any trial and error iteration calculations and is illustrated by a case study.

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