Characteristics of oil film nonlinearity in bearings and its effects in rotor balancing

Abstract

Since unbalance is inherent to any rotating machine, and some effects like coupling misalignment and thermal bow, which are not taken into account in pre-balancing, can lead to changes in unbalance distribution, in-field rotor balancing is sometimes a necessary action. Traditionally, the influence coefficients method has been used as the main balancing procedure, and due to the low accessibility in assembled rotating machinery, generally only the journal displacements inside hydrodynamic bearings are available for the balancing procedure. However, influence coefficients method assumes linearity of rotor response and it is known that hydrodynamic bearings can sometimes present a strong nonlinear relationship between bearing forces and journal displacements, making balancing difficult or even impossible. Otherwise, the bearing housing vibrations are directly proportional to the bearing forces and are independent of the journal displacements, therefore avoiding problems related to oil film nonlinearities. This paper proposes the use of bearing housing accelerations as influence coefficients and verifies experimentally that balancing can be successful also in highly nonlinear oil film bearing conditions. First, the oil film nonlinear characteristics are introduced by means of a simple analytical model. Then, different behaviors have been experimentally investigated by means of a test rig rotor supported by three hydrodynamic bearings, designed to enhance oil film nonlinear effects. Nonlinear effects are quantified through deviations with respect to linear behavior and development of higher harmonics in the response of the oil film. It is shown that these effects can seriously affect the possibility of balancing with the use of influence coefficients. Finally, it has been verified experimentally that balancing with the use of bearing housing acceleration to calculate influence coefficients was successful, as expected.