To evaluate numerically the vibration behaviour (stability and unbalance response) of rotor bearing foundation (RBF) systems such as turbomachinery which incorporate rotors running in several flexibly mounted hydrodynamic bearings, one needs input data parameters which adequately describe the bearings, the rotor, the foundation, the unbalance state and the system configuration state (the relative transverse alignment of the bearings). Adequate parameters for the last three are particularly hard to obtain for existing installations; and worldwide effort has been devoted to trying to identify these parameters, using field obtainable measurements of the instantaneous relative displacements between the rotor journals and their respective bearing housings. These measurements are then processed using Reynolds equation to determine the instantaneous bearing reaction forces acting on the rotor and foundation, which forces are then an essential ingredient of the desired parameter identification procedures [1]. The problem is in the accuracy to which one can reliably calculate these forces, even if it be assumed that Reynolds equation is otherwise a perfect model. This is felt to be particularly problematical because of the nonlinearity introduced into the RBF system by the hydrodynamic bearings.

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