Application of Full Spectrum Analysis for Rotor Fault Diagnosis

Abstract

Machine vibration signal carries abundant information, including the machine health condition. Reliable and foolproof fault detection needs accurate knowledge of the dynamic response features of the faulty system as well as proper method to extract it. The paper presents experimental investigation of steady state vibration response of the rotor bearing system with rotor faults such as unbalance, crack, rotor-stator rub and misalignment at sub-critical rotational speeds. Test rigs are designed and fabricated for the purpose. The conventional Fourier spectrum (i.e., FFT) has limitations in exhibiting the whirl nature (i.e., forward/backward whirl) of the rotor faults. It has been observed in the past that the several other rotor faults generate higher harmonics in the Fourier spectrum. Hence there is always a level of uncertainty in the diagnosis based on FFT when other faults are also suspected. Present work through the use of full spectra has shown possibility of diagnosing these rotor faults through unique vibration features exhibited in the full spectra. The present investigation focuses on the directional nature of higher harmonics, in particular the 2X component. This provides an important tool to separate rotor faults that generate similar frequency spectra (e.g., crack and misalignment) and lead to a more reliable fault diagnosis. Crack, rub and misalignment fault identification through a full spectrum analysis is verified on a laboratory test rotor set-up.