Detection of Unbalance in Rotating Machines Using Shaft Deflection Measurement during Its Operation

Abstract

Vibrations are found almost everywhere in rotating machines. Vibrations in rotating machinery are commonly the result of mechanical faults including mass unbalance, coupling misalignment, mechanical looseness, and many other causes. Unbalance is the most cause of machine vibration, an unbalanced rotor always cause more vibration and generates excessive force in the bearing area and reduces the life of the machine. In this paper 'Deflected Shape of Shaft' (DSS) of a rotating machine was found for detecting unbalance in its rotating components. The change in deflection shape gives the presence of unbalance in the shaft. Experiment reveals that a significant change in the DSS as an early warning indicator of unbalance in the rotating components. Tests were performed on a machinery fault simulator under various conditions of unbalance. Vibration data in terms of displacement was simultaneously acquired using a FFT (Fast Fourier Transform). The rotor shaft displacements were measured at different speeds using FFT at both unbalanced and balanced condition. The experimental frequency spectra were taken for both balanced and unbalanced condition. The presence of unbalance produces a change in the DSS at the rotor running speed and this data was extracted by conducting the experiments. A comparison was then performed with the theoretical calculation and also the vibration data acquired from FFT, at different running speeds. The results of this work provide a new method for detecting machinery unbalance, and offer a simplified approach for on-line fault detection in operating machinery.