Date of Award

2011

Degree Type

Thesis

Degree Name

Master of Engineering

School

School of Engineering

Faculty

Computing, Health and Science

First Advisor

Dr Helen Wu

Abstract

Detection of the shaft crack in a rotating machine is one of the most challenging problems in equipment predictive maintenance. In the available literature, various crack detection methods have been applied to study the dynamic behaviour of a cracked shaft. This study sought to attempt a vibration-based method. Elastic shafts with three different types of transverse cracks, including experimentally-induced fatigue crack, welded shaft crack, and wire-cut crack, were fabricated, and used to analyse the bending stiffness and frequency response in the vertical direction. The results from the cracked shafts were compared with that of an intact shaft. Bending stiffness of different shafts was measured as a function of rotation angle of the shafts. Among the three different crack types, the bending stiffness of the fatigue crack shaft showed a typical breathing behaviour, which was consistent with the previous theoretical results. The welded shaft crack also demonstrated opening and closing characteristics, but the stiffness was found to be much lower compared with that of a fatigue cracked shaft. As for the wire-cut crack, no breathing mechanism was observed for any rotational angle, due to the big width of the gap. Therefore, it is concluded that the fatigue induced crack is the most accurate method to evaluate the vibration characteristics of cracked shafts. Our results also indicated that existing switching model and harmonic models cannot describe the periodic stiffness of a transverse shaft crack accurately. Modal analysis was carried out on the intact shaft, as well as the three types of cracked shafts. Frequency responses in the X-axis direction were obtained. The correlation between the bending stiffness and the resonant frequency was examined, and it was experimentally proved that the decrease in resonant frequency was almost proportional to the reduction in the stiffness. Also, the amplitude of vibration response was found to be amplified by the crack element. The cause and implications of these results were analysed, and they are expected to deepen our understanding of crack diagnosis using vibration method.

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