Misalignment in Combi Bearing: A Cause of Parametric Instability in Vertical Rotor Systems

Abstract

The dynamic characteristics of the combibearing (combined thrustjournal bearing) in vertical rotor systems were analytically modeled and experimentally verified in the authors’ previous publications. An angular misalignment, which may be caused by a possible manufacturing or assembling error, is introduced in the combibearing’s rotating collar. A new model of the defective combibearing has been derived. The derived model shows that the angular misalignment in the combibearing’s rotating collar generates an asymmetry in the rotor system at the combibearing’s location. The rotor system’s stiffness in its two translational X and Y directions differ at the combibearing’s location. Constant parameters and/or coefficients in rotating asymmetric structures appear to change with time when observed in the stationary frame. These time dependent parameters (coefficients) are the source of the socalled parametric instability in rotating systems. If the collar angular misalignment is located in the XZ plane all rotor motions in this plane at the contact point between the combibearing and the rotor will be coupled. A parametric instability is observed within certain ranges of the rotor speed, depending on the magnitude of the angular misalignment.