Using transfer functions to model flexible supports and casings of rotating machinery

Flexible bearing supports and casings may have an important influence on the dynamic behavior of rotating machinery. The contribution of these components is not always included in rotor analyses because an accurate model of the foundation, pedestals and casing may be difficult and costly to obtain. A fast and accurate method is introduced to include the effects of bearing supports and casings in rotordynamic analyses using transfer functions. These transfer functions are calculated from frequency response functions of the support structure at the bearing locations. Transfer functions can be extracted from finite element models, modal models or measured experimentally. Experimentally, the use of transfer functions only requires the measurement of frequency response functions of the support structure at the bearing locations, therefore, reducing the measurement time and effort compared to the measurement of the mode shapes.;A flexible rotor supported by fluid film bearings on flexible supports was used to verify the validity of this method. The rotor model was verified using modal analysis. The support characteristics were determined experimentally by measuring the frequency response functions of the support structure at the bearing locations. These frequency response functions were then used to calculate polynomial transfer functions that represented the bearing supports. Fifteen support configurations were used with one set of tilting pad bearings and two sets of three lobe bearings. Unbalance response and stability predictions were compared with measured experimental data. The predicted first and second critical speeds agreed with the measured critical speeds within 3.6% for all support configurations. The prediction of stability thresholds agreed with the measured stability threshold for all support configurations within a confidence bound for the logarithmic decrement of +/-0.01. Predictions using other common support representations are included to show the effectiveness of transfer function representation of flexible bearing supports and casings.