Vibration analysis of nonlinear-dynamic rotor-bearing systems and defect detection

• Κύριος συγγραφέας: Χασαλεύρης, Αθανάσιος
• Άλλοι συγγραφείς: Παπαδόπουλος, Χρήστος
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2010
• Θέματα: Fault detection; Rotor bearing systems; Nonlinear systems; Breathing cracks; Cracked rotors; Worn bearings; Continuous rotors; Coupled vibrations; Ανίχνευση βλάβης; Συστήματα αξόνων εδράνων; Μη γραμμικά συστήματα; Αναπνέουσες ρωγμές; Ρηγματωμένοι άξονες; Φθαρμένα έδρανα; Συνεχείς άξονες; Συζευγμένες ταλαντώσεις; 621.82

This work focuses in two main directions of rotor dynamics field, the simulation of rotor bearing systems and the fault diagnosis. From the serious multiple faults that can appear in a rotor bearing system two of them are the target of current research: the transverse fatigue crack of a rotor and the radial extended wear in a bearing. The transverse crack is a defect able to bring a catastrophic failure of the system when the growth (depth) takes high percentage values relatively to radius of the shaft (i.e. >60%) and the symptoms of crack presence have been widely investigated during last four decades yielding efficient methods for the early crack detection. On the other hand the defect of bearing wear is much less investigated without results connected with wear diagnosis methods. Concerning previous works in those two defects the current dissertation’s persuasion is firstly to make a proposal in bearing wear detection, secondly to achieve a method definition able to detect a breathing transverse crack in a different way from those referred to literature. For the subject of crack detection, a different crack breathing model is proposed with emphasis in coupled local compliances definition and their variation during rotation while for the subject of bearing wear detection, a wear model from the literature is used with emphasis in rotor bearing system construction in a different way in relation to what up to now is available in literature. The rotor bearing system construction (simulation) is a matter widely investigated since early 60’s and some points of the current work try to differ in the way that the rotor and the fluid film bearings interact in discrete time. The concept of nonlinear fluid film forces is confronted in this work leaving out the nonlinear stiffness and damping bearing fluid film coefficients and assuming that during the journal whirling no equilibrium point must be defined in order to evaluate the future progress of vibration. Towards generality the fluid film bearings are not defined geometrically as short or long. These two specific geometric assumptions of short/long bearing appear widely in real machines and yield analytical expressions of fluid film forces but in current work the finite fluid film bearing is used demanding the well known finite difference method in order to evaluate the impedance forces, as many researches have propose. Both defects are met in a rotor bearing system parted from a continuous rotor and finite fluid film bearings. An entire chapter is dedicated in the way that Rayleigh equation of rotor motion incorporates internal damping using exclusively Real number confrontment, and in the way that fluid film forces react in rotor motion by defining boundary conditions in every discrete time moment. The definition of boundary conditions in discrete time makes them functions of the entire system response yielding a nonlinear dynamic system with the resulting time xx histories to be characterized from periodicity or quasi-periodicity sometimes depending in the defects presence. An extended analysis of time histories of the intact and the defected system is made in order to invest the symptoms of each defect in magnitudes of time and frequency domain. Timefrequency analysis is performed using continuous wavelet transform in virtually or really (the former from simulation, the latter from experiment) acquired time histories in order to extract the variable coupling phenomenon exclusively due to the breathing crack from the other two main reasons of coupling, the bearings and the shaft. Vertical response due to crack coupling is amplified when the crack coupled compliances become larger under an electromagnetic horizontal excitation in the rotor. This rapid in time variable coupling due to crack is used at last in order to detect the crack presence. The external excitation is used also in the case of wear detection since results of time-frequency analysis yield unexpected amplification of specific harmonics when the wear defect is present. Both considerations about the corresponding fault detection are tried in a real experimental system after the observation that response of the current rotor bearing simulation converges with the response of the physical system in characteristics that are judged important for the method robustness. The general speculation is that both defects have to be detected without the need of operation interruption since this cannot be feasible (high cost) in real turbo machinery plants and in an early growth that coincides with safe machine operation. The defect growths have to be at least 10% (of radius) for the crack and 20% (of radial clearance) for bearing wear so as the methods to be efficient.