Coupled forms of non-linear vibrations as a new tool of crack detection in rotating shaft

It is widely acknowledged from literature that material or structural imperfections of the shaft crack type are capable, in some cases, of generating coupled forms of lateral-axial-torsional vibrations in the rotating machine. Such information, however, pertains to small or model objects of that type. In the paper presented are investigations into the influence of the shaft crack on the dynamic state of a large, multi-supported rotating machine. Determined have been the cases where the crack is capable of inducing pronounced forms of vobrations, which can serve as basis for more reliabe and effective diagnostics, or the cases, where the mechanism of coupling deteriorates, despite the existence of crack with significant depths. Determined has been the degree of sensitivity of nono-linear and mutually coupled system vibrations on crack propagation and conducted had been the assessment of their applicability as a diagnostic determinant of a the dynamic state in a large machine. Indicated have also been the cases, where traditional diagnostics, based on the analysis of constituent vibration spectra of 2X type in the case of lateral vibrations and 1X and 2X for axial and torional vibrations can be misleading. Conducted have also been investigations into the influence crack location and propagation on operation of rotating machine after exceeding the stability threshold and hence system operation under conditions of existence and formation of oil whirls. It turned out that the crack can sustain and reinforce existing sub-harmonic components in the vibration spectrum whereas oil whirls efficiently impair the effect of vibrations coupling. Investigations have been conducted based on the advanced models and suites of computer codes for the analysis of complex systems of the type: line of rotors-slide bearings-foundation, enabling generation of diagnostic relations defect-symptom also in cases of large displacements and strongly non-linear external interactions occuring the system. Presented have been the examples of experimental verification of applied research tools.