This research study aims to develop an in-line real-time diagnostic capability for turbomachinery combining rotordynamics, fracture mechanics, and tribology to predict when equipment should be taken out of service. The concept is to use system condition metrics derived from the application of experimental modal analysis to operational response data. The research will begin with data derived from an analytical model of a rotor-shaft system having moderate sized fatigue cracks, bearing wear, and lubricant deterioration and starvation in any combination. Synthetic response measurements matching current technology for rotordynamic systems will be derived from the model, and processed by a technique for experimental modal analysis to accurately indicate the condition of the system single or multiple faults and their criticality. The signal-to-noise ratios required to implement concepts as in-line diagnostics for operational systems will be identified. Viable concepts will be subjected to a preliminary experimental validation using the onsite rotordynamics laboratory.
Turbomachinery is important to many aspects of our society, especially for power generation, water supply systems, aviation and naval systems. Avoidance of catastrophic failure is presently performed primarily on a scheduled basis. Development of a technique that identifies when systems actually require maintenance, thereby avoiding unnecessary shutdowns, would have tremendous economic impact, because it would reduce the frequency with which expensive inspection procedures are performed, as well as lessen requirements for excess capacity. This study focuses on the presence of cracks in the shaft and impending bearing and lubrication failure. It would use current technology for response measurement, so it would not require major alterations to existing systems. Further development would make it implementable as an in-line diagnostic and prognostic tool, which could be employed in parallel with other in-line diagnostic techniques.
Diagnosis of transverse fatigue cracks in rotordynamic systems has proven difficult. There is a two-fold challenge for on-line diagnosis of transverse fatigue crack parameters. First, crack diagnostics require determining two important parameters: the crack's depth and location. Second, the nature of rotating machinery permits response measurement only at specific locations. This work concerns the diagnosis of gaping crack parameters; the goal is to provide metrics to diagnose a crack's depth and location. To this end, a comprehensive approach is presented for modeling an overhung cracked shaft. The equations of motion, as well as transfer matrix methods, are given for two linear gaping crack models: a notch and a gaping fatigue crack. The notch model best approximates experimentally manufactured cracks, whereas the gaping fatigue crack model is likely more suited for real fatigue cracks. In addition, improvements to an existing experimental rotordynamic test rig are performed and discussed, with specific emphasis placed on crack detection and diagnosis. Real-time frequency spectra and angular orbits are provided over a range of shaft speeds for a cracked shaft. Crack diagnosis routines are established using free and forced response characteristics. Transfer matrix techniques are used to expediently obtain the steady-state system response. The rotor's angular response is primarily employed in this work for crack detection and diagnosis. The overhung shaft induces an increased sensitivity to variations in crack depth and location by reducing the shaft stiffness. Importantly, the overhung rotor permits direct measurement of the rotor’s dynamic response. Under the influence of gravity, the steady-state response of a crack rotating shaft displays a prominent 2X harmonic, appearing at a frequency equal to twice the shaft speed. When the operational speed is one-half of the critical speed, resonance conditions are observed; hence, this shaft speed is referred to as the ½ critical speed or 2X resonance frequency. This work demonstrates that the profile of the 2X harmonic versus shaft speed is a capable diagnostic tool. Identification of the 2X resonance frequency restricts the crack parameters to certain pairs of location and depth. Following this limiting process, the magnitude of the 2X harmonic is used to identify the crack's depth and location. Orbital shapes at the rotor are discussed and compared to experimental results. Orbital modes of the shaft deflection are likewise discussed. Quantitative results and qualitative observations are provided concerning the difficulty of crack detection and diagnosis.
