Experiments and modelling of a three-bearing flexible rotor for unbalance response and instability thresholds

Experiments were conducted on a three-bearing flexible rotor having two concentrated disks. Both rotor unbalance vibration response through critical speed and instability threshold speed were experimentally obtained over a wide range of journal bearing Sommerfeld number through accurate control of journal bearing static load. Vibration simulations of this laboratory rotor-bearing system were performed, first as a linear system and second as a non-linear system accounting for fluid-film bearing forces within a time-transient numerical time-step integration algorithm. With the time-transient nonlinear model, simulations were made to predict non-linear limit cycles, thus providing comparisons with experimentally determined instability threshold speed hysteresis loops. Comparisons between predictions and tests show the overwhelming influence of the mid-span location of the single journal bearing, the least accurate component in the model.