Modeling and vibration analysis of misaligned rotor-ball bearing systems

Abstract

In rotating machinery, misalignment may be present because of improper machine assembly and thermal distortion of the bearing housing supports, which is known to be the second most common malfunction after unbalance. It causes abnormal preload to be transmitted through bearing, shaft and coupling elements, leading to an elliptic or a banana shaped orbit and a large axial vibration. In practice, however, the perfect alignment between the connected machines cannot be attained. In spite of its importance and frequent observations, few researchers have paid attention to this important issue. A majority of the recent studies on misalignment have mainly modeled the reaction loads of a misaligned flexible coupling, and analyzed the effects of coupling misalignment on the 2x radial vibration. In the studies, they ignored, due to complexity in analysis, the change in bearing stiffness due to misalignment. However, the preloads and deformations of bearing elements due to misalignment significantly change the bearing stiffness and thus give significant influences on unbalance response and critical speed of the rotor system. The primary objective of this study lies in deriving a dynamic model for flexible coupling-rotor-ball bearing systems with the misalignment, investigating vibration characteristics related to the misalignment and verifying the theoretical development for the misaligned rotor system. In this study, the theoretical model for the flexible coupling-rotor-ball bearing system with the misalignment is derived, including the reaction loads and deformations of the bearing and coupling elements as the misalignment effects. The time response and the effective bearing stiffness for a simple rotor system are numerically calculated, using the Runge-Kutta integration scheme. In the theoretical analysis, the effective bearing stiffness is defined, using the relationships between the bearing reaction loads and deformations. Then, the effects of misalignment on the w...