Cracked and Unbalanced Rotating Shaft Behaviour During Start Up: Analyzing the Response by Conventional Fourier Transform and Wavelets

摘要

Rotating machinery operation can be much dangerous when a fatigue and/or growing crack develops in the shaft. Longer time intervals between overhauls and maintenance cost reduction programs require effective on-line monitoring of the machine condition. Previous works show a great difficulty to detect incipient cracks merely monitoring and studying machine vibration during steady state operation; from there analyzing transient system response during frequency sweep such as start up/stopping the rotor has been highly recommended by the first author of this work as well as several other authors. In this work a cracked unbalanced Jeffcott rotor realizing a frequency sweep through a machine start up with two different time intervals are studied. Crack-imbalance interaction yields involved responses complicating the diagnosis. For the first time conventional Fourier Transform, FFT, as well as wavelet analyses are both applied to start up cracked-imbalanced rotors and their analyzing capabilities are compared. Due to the transient nature of the analysis typical FFT reduces its efficiency, while cascades and wavelet transform both capture interesting phenomena, yet the latter seems more adequate. A discussion of results and some conclusions are offered.