Dynamic response of a cracked rotor with an unbalance influenced breathing mechanism

The vast majority of studies on cracked rotors assume that the breathing response of a fatigue crack is weight-dominant i.e. the effect of dynamic forces on the breathing response is negligible. In this study, the assumption of weight-dominance is removed and the coupling effect of unbalance angle and magnitude on the breathing behaviour of a crack is examined. The proposed breathing model is shown to be greatly influenced by unbalance eccentricity and rotor speed, whereas weight-dominant breathing models are unaffected by these factors. A significant difference in the vibration behaviour of a weight-dominant model and the proposed model was particularly seen around the critical speed of deeply cracked rotors. High unbalance eccentricity and a 180° placement of the unbalanced mass resulted in the disappearance of 2X and/or 3X harmonic components at one-half and one-third of the rotor critical speed when the vibration was predicted using the proposed model. This result suggests careful placement and size of the unbalance mass may allow for the isolation of rotor cracks from other rotor faults in the frequency domain by negating the effects of the crack breathing.