Diagnostic Techniques-Part 3
Last Updated: 06/24/2017
Transient analysis is the study of how various
signals change during a machine's process or load change. This analysis is particularly
important during a machine's startup or shutdown when critical speeds or natural
resonances are encountered. Observing the overall amplitudes or individual spectra will
not be sufficient to document how the entire machine behaves during transient conditions.
Data collection modes fall into two general categories: observing transducer output
signals with respect to machine speed and observing transducer output signals with respect
to time. Sometimes, a combination of the two modes may be required.
The Bodé plot is essentially two displays: phase angle versus machine speed and amplitude versus machine speed. Analysis of the plot can determine at what speed(s) a critical speed exists, the amount of runout associated with a Non-Contact PickUp (NCPU), the balance condition, system damping, and the operating phase angle and amplitude at various machine speed.
The center of the plot represents zero speed and zero amplitude with subsequent amplitude and phase angle measurements plotted with their associated machine speed. Phase angle measurements are plotted around the circumference of the chart, against machine rotation direction. Polar plots are always filtered to machine running speed or some multiple of the machine speed, depending upon the fault being investigated.
Machine critical speeds and natural resonances are displayed as loops, with the critical speed situated 90 from the start of the loop. This characteristic makes identification of resonances and critical speeds easy. Other information available from the plot are: slow speed runout associated with NCPUs, system damping, balance condition, and operating phase angle and amplitude at various machine speeds.