On the effects of transverse annular cracks in rotating machines

In general, abnormal changes of the twice per revolution harmonic component of the shaft vibration are considered the most important symptom of a crack propagation in horizontal shafts of rotating machines. This paper shows the results of a model-based investigation performed to study the influence of some basic geometrical characteristics of full-annular cracks on the shaft dynamic behaviour. This study has shown that, contrary to common cracks, full annular cracks may cause only minor changes of the shaft super-synchronous vibrations. These results are in good accordance with the experimental dynamic behaviour experienced by a steam turbine in which a very deep annular crack propagated.