Advanced Analysis Beyond Waveform And Spectrum

All types of problems can be determined from relative motion of machine components including the three noted in the answers.

Cross-channel phase shows amplitude and relative phase of two components with simultaneous measurement. Phase provides relative motion.

A Bode plot does take some time to set up, as it requires and accurate reference for speed and phase on each sample of vibration amplitude.

Long Time Waveform is the tool of choice to capture a short-term event. By looking over the waveform and panning and zooming until the event is noted, the spectrum of that area only can be displayed and analyzed.

It is the Bode plot that shows both vibration amplitude and phase data versus speed. An amplitude peak and a phase change of 180 degrees pinpoints a natural frequency.

Bump testing is not a calibrated force and response function, as required for modal analysis. It is simply an impact force exciting natural frequencies.

The beauty of cross-channel is that it does not require a reference input, just two simultaneous measurements of vibration.

It will take some care, but one can excite a machine structure with a bump even while it is running, and by taking a before and after spectral plot and overlay, responsive natural frequencies will be displayed.

Peak hold spectral data is typically most effective in capturing and displaying an excited natural frequency. It will show a raised noise floor and skirts on both sides of the resonance condition excited by the bump.

All three tools are effective to detect resonance and natural frequency. Coastdown can show a cascade spectral display or Bode plot. Bump test provides a peak hold of excited frequencies. Long time waveforms can be panned and zoomed for frequency content.