An experimental evaluation of a gas turbine/compressor train yielded excellent correlation between measured and predicted torsional critical speeds. Torsional strain signals acquired at a coupling location provide basic data from which damping factors at resonance are found to vary from 1 to 6 percent, depending on mode shape. Resonant stress levels for the mode with low damping would be a factor of 2 higher than predictions based on industry accepted assumptions. Other modes are found to have considerably more damping than expected. Experimental data and a coupled torsional-lateral damped eigenvalue analysis show that this damping variation is related to pinion and gear lateral bearing motion. Conservative damping values are predicted by the coupled analysis.

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