Modal analysis of a rotor and investigation of the effect of bearing rigidities on the natural frequencies

Rotors are rotating systems commonly used in industry. In the case the rotor has a critical speed at rotating frequency a small unbalance can damage the rotor and the system. Therefore, it is very important to know the dynamic properties of rotors. For this purpose, besides the finite element method, experimental modal analysis is widely and effectively used. Accuracy of the results obtained from FE method depends on the assumption made. However, modeling of the contacts on the structure quite difficult. Accuracy of the vibration analysis results depends on the modeling of the rotor bearings accurately. Springs and damping elements are used to model the bearings. For a correct analysis, it is important that their values are close to reality. In this study, a rotor consisting of a shaft-disc and flexible bearings are considered and both the numerical and experimental vibration analyses are performed. The effect of bearing stiffness on the natural frequency of the system is investigated and it is shown that as the bearing rigidities increase, the natural frequencies increase. Moreover, the structural modification technique based on SMW formula which is known in matrices theory is used to determine the stiffness of the bearings. It is shown both numerically and experimentally that the method is very effective.