Hysteresis and torsional-lateral vibration coupling in a complex shaft line supported by hydrodyanamic journal bearings

In some rotating machinery for specific industrial applications the driving as well as resistance torques or the inertias reduced to the rotation axis may be nonstationary, thus affecting system dynamics. Under such operating conditions, in some peculiar cases torsional response and rotational motion irregularity may influence system lateral vibrations. The present paper shows how such coupling phenomena may become significant in particular conditions, where the occurrence of fluid–structure interactions causes a reduction in stability threshold of hydrodynamic journal bearings and torsional energy yields a hysteresis behaviour in system synchronous lateral response. A hypothesis based on Hopf bifurcation theory (HBT) is formulated in order to justify how and under which operating conditions such coupling phenomenon can develop. In order to validate such hypothesis, an experimental campaign is performed on a real-size shaft line including a TG rotor for heavy-duty power plants mounted on hydrodynamic bearings. The detected rotor-bearings system lateral operating response is found to become more complex in presence of a pulsating driving torque inducing significant angular speed oscillation as well as a dynamic perturbation, which causes strong coupling from torsional to lateral vibrations. Such coupling entity has been experimentally found to be dependent from excitation frequency with respect to revolution one. Particularly, localized hysteresis and jump-up phenomena are detected in trends of fundamental order contents measured during run-up and run-down tests when such torsional response is present. Consequently, a hydrodynamic bearing numerical model is built that can solve Reynolds equation in unsteady conditions in order to quantify journal lateral vibrations amplitude in presence of both an angular speed oscillation and a dynamic perturbation, both characterized by well-defined amplitude and characteristic frequency. The proposed approach, validated by means of both suitable measurements and numerical simulations, can justify the existence of such coupling phenomena. The detected anomalous response characterized by localized hysteresis is ascribable to journals unstable behaviour onset within hydrodynamic bearings due to operating angular speed irregularity related to an induced torsional vibration.