Investigation on the Dynamic Characteristics of a Rotor Suffering Impact Foundation External Excitation

High speed rotating machines usually include such components as rotors, bearings, casings, foundations and are widely used in many industries. However their rotors may all face foundation external excitation problems during machine operation and service. Therefore design engineers are interested in accurate rotor response prediction when support structure is subjected to sudden impact excitation in order to set sufficient clearances and ensure machine safe operation. The problem is relevant for marine engines (gas and steam turbines) for evaluation of their reliable operation for the case when a hull of the ship is exposed to impact from giant sea waves. The paper describes methodology for creation of rotor-bearing-support system for HP steam turbine rotor of a transport marine engine whose support structure was subjected to impact excitation. The impact phenomenon was further studied on the base of developed experimental test rig with a simplified rotor structure mounted on foundation with a flexible suspension system. Two numerical models were used for verification of the experimental results: the model with a simplified rotor representation (massless shaft) on a lumped mass foundation structure and a model with beam type rotor on a lumped mass foundation. Proposed numerical models showed adequate results for rotor response prediction, what was confirmed by similarity of obtained curves for impact excitation coefficients and comparability of rotor disk orbits for experiment and simulation. Experimental testing confirmed that external foundation impact excitation may significantly influence on maximum deviations of rotor disk orbits in comparison with the case of rotor normal operation without excitation. Simulation and experiment results revealed that impact excitation coefficients within the tested range of amplitudes and rotor speeds increased almost linear and were proportional to maximum displacement amplitude measured on foundation. For subcritical and supercritical speeds impact excitation coefficients were close in values and increased faster in comparison with excitation performed for the speeds close to rotor critical speed. Proposed method for model creation and analysis could be further used for rotordynamic simulations of more complicated machines e.g. marine power engines.