| Measurements on run-ups and coastdowns are measurements of non-stationary signals and require special techniques, like the use of tracking filters, three-dimensional plots and the use of external sampling with FFT analyzers. The purpose is first to identify so-called �critical frequencies�, where rotating elements, e.g. the shaft, housings or supporting structures go into resonance, and second to distinguish between machine resonances and rotational-speed related vibrations (orders). The first can be achieved using fairly simple instrumentation, while the second requires more complex instrumentation such as an FFT analyzer. Measurement of critical frequencies requires a tracking filter to track the vibration at the fundamental frequency of the machine. Amplitude and phase (referenced to a mark on the shaft) of the vibration should be recorded. Two popular representations of the measured data are the Bode’ plot, where amplitude and phase of vibration are separately displayed against speed, and a Nyquist plot where the amplitude, phase and speed are combined in one polar diagram. These measurement can be taken using either casing vibration sensors, to determine resonances in the machine or supporting structure, or non-contacting sensors to examine the behaviour of the shaft. In cases where non-contacting sensors are used, be aware of problems that may exist due to electrical and mechanical run-out. | | |
