An analysis is made of the instability in rotating systems induced by internal damping in the rotor. The stability boundary is defined in terms of the ratio of the external damping of the system to the internal damping in the shaft. It is shown that the induced-whirl mode is not necessarily the fundamental mode. For various damping conditions, a particular whirl mode is induced. It is shown that, in general, the whirl mode induced at the instability boundary occurs in the mode whose whirling speed is approximately one half the rotational speed.

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