Extensive fleet experience with the LM2500 marine gas turbine engine has identified it as an engine that exhibits wear-accelerating vibration effects. The critical speeds and associated mode shapes were not well understood by U.S. Navy engineers. To help deal with vibration-related problems, an analytical model was developed to calculate engine rotordynamic and structural response. The procedure is a multilevel, multirotor hybrid extension of the classical Myklestad-Prohl method. Presented herein are some of the model’s predictions, and correlations with actual engine vibration measurements. The model predicted in excess of 20 different critical speeds in the engine’s operating range. Because of the engine’s structural flexibility, most of the critical speeds were engine casing and structural support resonances, driven by imbalance or misalignment in one or both of the engine rotors. Rotor-bending critical speeds were found to be strongly influenced by engine casing and support structure stiffness and mass. Using the model’s predicted mode shapes, new mounting locations for accelerometers could be selected to determine vibration severity at various frequencies better. This has given the U. S. Navy new insights into fleet vibration problems, and provides a useful tool for achieving reduced engine removals.

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