A Theoretical Study of Stability of a Rigid Rotor Under the Influence of Dilute Viscoelastic Lubricants

Abstract

An attempt is made to study theoretically the stability of a symmetrical, horizontal, rigid rotor mounted on two identical short bearings. Dilute viscoelastic solutions are used as lubricants. Experimental studies reveal that at high shear rates, which occur in the journal bearing clearance space, the Weissenberg effect is degraded while the relaxation time remains unaltered. Harnoy proposed a constitutive equation which decouples the Weissenberg effect and relaxation time may be used to depict the constitutive relations for lubricants under above-mentioned conditions. The study reveals that an apparent inertia tensor, arising out of linearized elasto-viscous acceleration dependent forces, has a predominant effect on the stability of such a rotor. As the stability map is considerably modified by the use of viscoelastic lubricants, the high speed instability limits are extended providing an incentive for using this type of lubricants. However, some new instability patches show off which correspond to lower speeds of a rotor. Nature of their instabilities is still to be established. A rotor may, however, be so designed that it never falls inside such patches of instability during its operation and speeding up period.