Crack Detection in an Overhung Rotor System Using External Harmonic Excitation

Abstract

This paper presents an experimental approach for crack identification in an overhung rotor system supported on rigid bearings. The existence of coupling phenomena between bending and lateral vibration due to the presence of crack is used for the identification of crack in an overhung rotor system. The natural frequency of the rotor system has been obtained using impact hammer analysis and compared with the finite element analysis. The crack identification procedure involves the application of external axial excitation through a thrust shaker. The overhung rotor system is excited axially through a shaker with frequency equal to its first natural frequency and subharmonic of first natural frequency obtained through impact modal analysis. The response measured in the lateral and radial direction shows that there is a difference in the amplitude of the external excitation frequency and its harmonics measured for crack and healthy rotor system. The lateral response has more amplitude peaks at excitation frequency and its harmonic for cracked rotor when excited with frequency equal to the first natural frequency and its subharmonic. The peaks are more prominent for subharmonic of first natural frequency as external excitation. A considerable difference in the amplitude of X and 2X components can be seen in radial response for cracked rotor and healthy rotor when excited with frequency equal to the first natural frequency and its subharmonic. The application of external harmonic excitation with frequency equal to first natural frequency and its subharmonic can be very well used for crack detection in overhung rotor system without stopping it.