Reactec: Vibration Analysis

Vibration Analysis

Examination of Data

Figure 1 Raw data in the time-domain following a knock test

Vibration analysis begins by examining the raw vibration data in the time-domain – a graph with time and amplitude as axes (Figure 1). An estimate of the quality of the data can immediately be assessed and important features such as the maximum amplitude (for applications involving stroke length) and the decay of vibration following a bump test or jolt can give an indication of the structures natural damping (Figure 1).

Fast Fourier Transform

Figure 2. Data converted into the frequency domain using an FFT algorithm

To determine a structure or machine’s vibration characteristics the raw data is transformed into the frequency domain using a Fourier transform. This calculation is performed using a Fast Fourier Transform (FFT) algorithm which allows the data to be shown as the Root Mean Square (RMS) amplitude against frequency (Figure 2). Most vibration signals are composed of a series of superimposed waves with different frequencies – for example the signal in Figure 1 is a superposition of three signals with 30, 60 and 100 Hz. FFT diagrams are very useful in separating out these signals and determining their relative importance based on amplitude.

Structural and Forced Vibrations

Figure 3. Waterfall diagram of a wind turbine with a colour scale for amplitude (cm/s).

A vibrating system can be more thoroughly analysed by performing FFT on selected windows of raw data and plotting the resulting amplitude against time to produce a waterfall diagram (Figure 3). These are extremely useful for discerning a structural vibration from a forced vibration in rotational machinery. Forced vibrations are those related to alignment of drive shafts, gear meshing, bearings etc and their frequency will vary with rotational speed of the machine. A Structural vibration relates to the harmonics of the structure and has a constant frequency. When a waterfall diagram is plotted the structural vibration will appear as a straight line (constant with respect to time) while the forced frequencies will move to higher and low frequency as the machine’s rotation speed changes. Figure 3 shows a waterfall diagram produced by Reactec engineers of a household scale wind turbine – here the vibrations from the rotational of the drive shaft, blade pass (three times the rational speed of the shaft as there are three blades) and the generator - wiggle across the diagram as the wind speed and therefore rotational speed changes.