Global nonlinear dynamic characteristics of rod-fastening rotor supported by ball bearings

Abstract

Flexible rod-fastening rotor-ball bearing system (FRRBS) is usually adopted in aircraft engines. In this paper, a simplified model for the FRRBS is proposed, and the global nonlinear dynamic characteristics of FRRBS are investigated. The FRRBS has two kinds of special structural features: rods and interfaces. The circumferentially distributed rods are modelled as a constant stiffness matrix with an additional moment (when unequal pre-tightening occurs) without mass effect. The interfaces between disks are modelled as stiffness matrices with normal and tangential contact stiffness, which are determined by pre-tightening force. Then, the shaft is discretized by Timoshenko elements and supported by ball bearings with radial internal clearance. Periodic motions and bifurcation sets are obtained by shooting method. The result shows that the insufficient pre-tightening brings about abnormal vibrations through quasi-periodic instability in high speed region, and the unequal pre-tightening leads to severe vibrations in stable periodic operation state. Moreover, the mass eccentricity would be the most nonlinear source for the nonlinear instabilities. Generally, this paper presents a method for modelling a FRRBS to study the nonlinear dynamic characteristics of the FRRBS.