Local resonance of crack-imbalance orientations and orbital evolution to detect mid-span rotor cracks: Part 2, experimental validation

Rotating equipment in petroleum and electrical Mexican industrial sectors is ageing therefore structural assessment by vibration monitoring is advisable. Crack detection is most feasible during frequency sweeps such as machine shut-downs. Then several resonances occurring at rational fractions of the fundamental critical speed, here named Local Resonances, can indicate structural problems. A laboratory rotor system having a configuration as the extended Jeffcott Rotor on journal bearings (see Gómez-Mancilla et-al) as used in part 1 is experimentally tested, confirming the existence of several Local Resonances. Numerical and experimental data of the monitored vibration are post-processed and analyzed using FFT and frequency sweep subtraction with emphasis on the Local Resonances. Bode plot, orbital shapes and the role of various synchronous harmonic components of test responses are analyzed validating the cracked mid-span shaft model predictions. The strong influence of the residual imbalance with the crack is confirmed and validated through experiments by varying the imbalance and crack vectors relative orientation. These facts can be used for crack detection considerably facilitating its detection during machine coast-down. Moreover, applying modal analysis conjointly with this modeling approach for the first mode the detection of cracks at other locations and/or configurations becomes feasible. A list of experimental based conclusions is offered.