Experimental investigations on vibration response of misaligned rotors

Abstract

Misalignment is one of the most commonly observed faults in rotating machines. However, there have been relatively limited research efforts in the past to understand its effect on overall dynamics of the rotor system. In the existing literature, there is confusing spectral information on the rotor vibration characteristics of misalignment. The present study is aimed at understanding the dynamics of misaligned rotors and reducing the ambiguity so as to improve the reliability of the misalignment fault diagnosis. Influence of misalignment and its type on the forcing characteristics of flexible coupling is investigated followed by experimental investigation of the vibration response of misaligned coupled rotors supported on rolling element bearings. Steady-state vibration response at integer fraction of the first bending natural frequency is investigated. Effects of types of misalignments, i.e. parallel and angular misalignments, are investigated. The conventional Fourier spectrum (i.e. FFT) has limitations in revealing the directional nature of the vibrations arising out of rotor faults. In addition, it has been observed that several other rotor faults generate higher harmonics in the Fourier spectrum and hence there could be a level of uncertainty in the diagnosis when other faults are also suspect. The present work through use of full spectra has shown possibility of diagnosing misalignment through unique vibration features exhibited in the full spectra (i.e. forward/backward whirl). This provides an important tool to separate faults that generate similar frequency spectra (e.g. crack and misalignment) and lead to a more reliable misalignment diagnosis. Full spectra and orbit plots are efficiently used to reveal the unique nature of misalignment fault not clearly brought out by the previous studies, and new misalignment diagnostics recommendations are proposed.