Influence of Rotor Suspension Anisotropy on Oil Film Instability

A crucial problem of turbomachinery is the oil film instability on increasing the angularspeed, which is correlated with the asymmetry of the bearing stiffness matrix and resemblesthe hysteretic instability somehow. As a beneficial effect is exerted on the latter bythe anisotropy of the support stiffness, some favorable effects have been recently foundby the author also for the former, whence a systematic analysis has been undertaken. Theinstability thresholds may be detected by the usual conventional methods, but a detailedanalysis may be carried out by closed-form procedures in the hypothesis of symmetry ofthe rotor-shaft-support system, which condition approaches the real working of turbomachinesquite often. Altogether, the results point out an improvement of the rotor stabilityfor low Sommerfeld numbers by softening and locking the support stiffness in the verticaland horizontal directions, respectively. Nonetheless, the partial support release on oneplane implies lower instability thresholds for large Sommerfeld numbers, but this drawbackmay be obviated by a sort of “two-mode” stiffness management, with some verticalflexibility for heavy loads and full blocking for light loads. Otherwise, it is possible tocombine the anisotropic supports with journal bearing types that offer favorable stabilitybehavior in the range of large Sommerfeld numbers. Basing on approximate but realisticmodels, the present analysis elucidates the changes of the rotor-shaft unstable trend onvarying the external stiffness of the supports and gives tools for a rapid calculation of theexpected instability thresholds.