Preventing the oil film instability in rotor-dynamics

Horizontal rotor systems on lubricated journal bearings may incur instability risksdepending on the load and the angular speed. The instability is associated with the asymmetryof the stiffness matrix of the bearings around the equilibrium position, in like manner as theinternal hysteretic instability somehow, where some beneficial effect is indeed obtainable by ananisotropic configuration of the support stiffness. Hence, the idea of the present analysis is tocheck if similar advantages are also obtainable towards the oil film instability. The instabilitythresholds are calculated by usual methods, such as the Routh criterion or the direct search forthe system eigenvalues. The results indicate that the rotor performances may be improved inthe range of low Sommerfeld numbers by softening the support stiffness in the vertical plane,and hardening it on the horizontal one, up to the complete locking, though this advantage hasto be paid by rather lower instability thresholds for large Sommerfeld numbers. Nevertheless, a"two-mode" arrangement is conceivable, with some vertical flexibility of the supports for largejournal eccentricity, and complete locking for small eccentricity. As another alternative, thesupport anisotropy may be associated with the use of step bearings, whose particularcharacteristic is to improve the stability for small eccentricities.