Traditional experimental modal testing techniques rely on controlled and measured excitation together with measured responses in order to identify the mode shape, natural frequency, and damping factor of each mode. Applying a controlled and measured excitation to a rotor train when in operation is logistically difficult and especially challenging in the field. Operational modal analysis (OMA) identifies the modal parameters of a system from measurement of response due to some (unknown) excitation. OMA has proven successful over the past several decades on nonrotating structures but has relatively rarely been applied to rotating machinery. Case studies are presented demonstrating the use of OMA in identifying torsional modes on an electric motor driven reciprocating compressor, on a diesel engine driven fire water pump, and on a marine propulsion system. In contrast to lateral modes, torsional modes of rotor trains are typically not speed dependent. However, phenomena exist whereby the torsional modes may be different at stand still, off-load and at different loads. The case studies provide examples of such phenomena and also of significant differences between predicted and measured behavior which suggests that improvements in industrial practice would be beneficial. Such improvements should be based on reconciliation of measured and predicted behavior and OMA offers a valuable tool to facilitate this. OMA provides a significant benefit in investigating and understanding torsional behavior in operation.

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