Multi-harmonic motions of bearing cage affected by rotor unbalance

Abstract

The motion of bearing cage in the rotor system displays very complicated behavior affected by rotor unbalance. In order to gain an insight into the complex cage motions due to rotor unbalance, a scaled rotor test rig is designed and developed that is made to be dynamically similar to the actual rotor system in aero-engines. Eddy transducers are used to measure the cage motions and monitor the rotor vibrations. Based on the test rig, a dynamic model of its rotor system with four degrees-of-freedom is developed to determine the reaction forces due to rotor unbalance, which are introduced as boundary conditions for the bearing dynamic analysis. And then, a three degrees-of-freedom dynamic model of bearing cage taking the obtained reaction forces into account is also proposed. Finally, the experiment and simulation of the cage motions are carried out and compared at different rotating speeds and rotor unbalances. The experimental results of the cage motions agree with the theoretical predictions and reveal that the cage motions generally contain the multiple harmonic components, such as the cage rotating frequency and its multi-frequency, the inner ring rotating frequency, and also some combination frequencies due to rotor unbalance. The amplitudes of the inner ring rotating frequency and combination frequencies of the cage motions increase with the increment of the rotor unbalance values. Besides, the trajectories of cage are affected by the rotor unbalances.