It is a rather common opinion that one of the most important symptoms of a crack growth in rotating machinery is the change in the twice per revolution vibrational behavior. This derives from the generalization of simple models, like Jeffcott rotors, to real overhung machines and is generally true. Anyhow there exists also cases in which the combination of other facts, like the local stiffness of the rotor in correspondence of the crack position and the presence of coupling misalignment between the rotors that compose the shaft-line, make the twice per revolution vibration practically absent. In this paper, a model based diagnostic methodology is presented that allows to identify the presence of a crack. Its application is proven by the experimental case of a crack in the load coupling of a power plant gas turbine.

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