VIBRATIONAL CHARACTERISTICS OF THE CANTILEVER ROTOR WITH A CRACKED SHAFT

The vibrational characteristics of the high speed cantilever rotor with a cracked shaft are investigated to find an optimum vibration monitoring method for a turbine rotor. The model studied is established based on the structure of a real turbine rotor.The transfer matrix-direct integration method proposed by the first author in reference (2) is applied to the theoretical analysis. In consequence it is possibleto avoid making any assumption about the whirling orbit, and modification of some real parameters and whirling orbit of the disk is performed sequentially in the process of numerical integration. In experimental investigation some measures are taken to create a crack very similar to a real crack on the shaft.The results of Fourier analysis of the unbalance responses obtained from both calculations and experiments show that in signature of vibration of the cracked rotor exist very strong 2nd anc 3rd harmonics. The triple-frequency harmonic increases with the rotating speed and reaches maximum at the speed being equal to 1/3 of the critical speed.Since the second harmonic can be caused by the other factors such as misalignment,as a result of this research the authors suggest that operating the rotor in the vicinity of 1/3 of the critical speed and monitoring the triple-frequency component be the best method to detect a crack for this kind of turbine.