Unbalance Identification and Balancing of Nonlinear Rotodynamic Systems

Abstract

This paper is devoted to the unbalance identification and balancing of an aircraft engine rotor running on Squeeze Film Damper (SFD) bearings that show highly nonlinear features. The high pressure (HP) rotor in a twin-spool assembly cannot be accessed under operational conditions because of the restricted space for instrumentation and temperatures that are beyond the safe operating limits of the sensors. This motivates the use of a non-invasive procedure, requiring prior knowledge of the structure. The only such method for rotating machinery with non-linear bearings reported in the literature is highly limited in its application (e.g. assumes circular centred orbits). The methodology proposed in this paper is aimed at overcoming such limitations. It uses the Receptance Harmonic Balance Method (RHBM) to generate the backward operator using vibration measurements taken from sensors installed on the engine casing. The operator is then inverted using either Least Squares Fit (LSF) or Singular Value Decomposition (SVD). The resulting solution is the equivalent unbalance distribution in prescribed unbalance planes of the HP rotor which are consequently used to balance it. This method is validated using two distinct rotordynamic systems and simulated casing vibration readings corrupted by different noise levels.