CRACK BREATHING MECHANISMS IN ROTOR-BEARINGS SYSTEMS, ITS INFLUENCE ON SYSTEM RESPONSE AND CRACK DETECTION

摘要

Gravity dominated rotors supported on fluid-film bearings, flexibly coupled to other shafts having disc and bearing mass and a transversal crack at shaft mid-span exhibit a clear first bending response; then a small degree of freedom model is still powerful for qualitative analysis and can provide cracked behavior guidelines. Focusing on crack identification based on monitoring system response analysis, three crack breathing and always open crack phenomena are studied and compared. Literature reports several crack breathing dynamics, some simple, others complete and complicated; yet actual impact on system response by selection of specific breathing is controversial. Shaft stiffness variation increases nonlinearly with crack depth, and mildly diminishes with shaft length. It is found that the specific description for notched and crack breathing models can have a mild or a significant impact on rotor-bearing response and associated local resonances. In breathing models every harmonic term excites that component as well as its higher harmonics, adding local resonances with super-sub-synchronous frequencies. Breathing description, or its absence, significantly affects orbit amplitudes and shapes, modifies spectral frequency amplitudes and phase angles. Finally, contrary to crack breathing induced behavior, simulation and tests show that an open crack excites only a single local resonance, the gravity critical. Therefore, in model-based crack detection methods proper breathing selection is important to enhance damage detection performance.