Vibration analysis of rotors for the identification of shaft misalignment Part 1: Theoretical analysis

Abstract

Shaft misalignment in machinery causes preload forces to be generated in couplings which are then transmitted to the different machine components, thus reducing their lifetime. Shaft misalignment is a major cause of vibration in machines. Based on tests on coupling stiffness, a new coupling finite element stiffness matrix has been deduced. This has been used in the finite element analysis of a two-rotor system connected by a flexible coupling, to calculate the mechanical vibration resulting from mixed angular and parallel shaft misalignments with residual unbalance. The calculated vibration spectra were determined for two flexible couplings: a three-pin Renold coupling and a three-jaw Lovejoy coupling. The results and analysis indicate that the vibration generated by shaft misalignment is caused by the variation in coupling stiffness on rotation, and that the forcing frequencies generated are harmonics of the shaft's speed and directly depend on the frequency of the variation in coupling stiffness. In addition, the amplitudes of the measured vibratory components were found to rely directly upon the frequency response function that is related to the coupling and measurement points.