Condition Monitoring of Cracked Rotating Shafts by Signal Analysis

The dymanic behaviour of cracked rotating shafts has been obtained by regarding theoretical and experimental publications and by carrying out tests with various test rigs. The aim of the investigation was to detect differences in the vibrational signals. This information was used for detection of the presence of a crack. From the literature it can be seen that faults such as fatigue cracks change the characteristics of the vibrations. However, different investigators detected different vibrational behaviour and recommended different crack indicators. The majority of papers deal with theoretical considerations and simulations. This made it necessary to carry out experimental work. The conclusions of other research work was presented. It could be seen that the increase of the twice per revolution harmonic indicates the presence of a crack. However, as explained, this recommendation is not free of inconsistences. Other indicators have also been detected. The results of the experimental wo rk showed that indeed the twice per revolution harmonic increased in most of the cases when a cracked shaft was compared with an uncracked one of the same properties. Since, in particular, this harmonic is influenced by a variety of parameters it was not detected in all of the tests, even when great effort had been made to keep these parameters unchanged. Another result that was expected is the decrease of the natural frequency of the shaft when it contains a crack. This could easily be confirmed in free vibration tests and with use of the hammer impact by regarding the resulting frequency response. An effect that is not mentioned in the literature explicitly, was that the harmonics of the rotating shaft are amplified in the region of the natural frequency of the shaft. This happened very clearly for the first mode, and under certain circumstances also for the second mode. It can be assumed that this effect is caused by the high damping of the material used, polymer, under forced vib r ation, and would not be obtained for metal shafts. This information obtained can be used to design an on-line condition monitoring tool for rotating machinery