Supersynchronous Vibrations Caused by Transverse Annular Cracks in Rotating Machines

In general, the propagation of transverse cracks in horizontal shafts of rotating machines causes major changes in the synchronous vibrations and minor but significant changes in the supersynchronous vibrations. Often, abnormal changes of the twice per revolution harmonic component of the machine vibration is considered the most important symptom of a crack propagation. However, in rotating machines subjected to considerable thermal transients, like steam and gas turbines, it is not unusual that multiple crack initiations occur at the root of a large number of blades of a single wheel. This type of failure may generate annular transverse cracks. This paper shows the results of a parametric analysis performed to study the effects of the basic geometrical characteristics of annular cracks on the periodic changes of the local flexural stiffness of the shaft, caused by the machine weight, as well as on the machine dynamic behaviour. This investigation has shown that annular cracks cause only minor changes of the shaft supersynchronous vibrations. These results have been confirmed by the experimental dynamic behaviour of a steam turbine in which a very deep annular crack propagated.