Torsional Vibrations Caused by Geared Coupling in a Shaft Train Driven by a Steam Turbine

Rotating machines that include geared shaft-trains can be affected by a dynamic behavior that is influenced by coupling effects between lateral and torsional vibrations. These latter vibrations are rarely measured by means of permanent probes in industrial machines; therefore, these phenomena can be often pointed out only by detecting abnormal harmonic components in the frequency spectrum of the lateral vibrations measured during common monitoring activities. Then, the diagnosis of the occurrence of coupling effects between torsional and lateral vibrations can be supported by the results provided by model-based techniques. In this paper, the results of the analysis of the dynamic behavior of a small power unit that was affected by abnormal lateral vibrations caused by the excitation of torsional normal modes of the geared shaft-train are shown and discussed.