Advanced Analysis Beyond Waveform And Spectrum

1. Cross-channel phase measurement can detect the following problem.

Soft foot

Misalignment

Couple unbalance

All of the above

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

Explanation

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

2. What tool can be used to determine relative motion of machine components?

Coastdown

Bump Test

Cross-channel Phase

Long Time Waveform

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

Explanation

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

3. A Bode plot provides both the vibration magnitude and phase display for each speed measurement and requires a reference input.

True

False

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.

Explanation

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.

4. In trying to capture an event on a machine that is really hard to catch with one sample, one would use:

Coastdown

Cross-channel phase

Long Time Waveform

Bump Test

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.

Explanation

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.

5. What is a plot type that is commonly used with a coastdown for resonance detection?

Orbit

Time waveform

Single point spectrum

Bode

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.

Explanation

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.

6. Bump testing requires a calibrated modal hammer.

True

False

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

Explanation

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

7. Cross-channel phase measurement requires a reference signal (tachometer) input.

True

False

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

Explanation

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

8. A Bump test cannot be performed for any analysis work while the machine is running.

True

False

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.

Explanation

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.

9. Bump testing typically uses the following type of averaging on the spectral processing.

Linear

Logarithmic

Exponential

Peak Hold

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.

Explanation

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.

10. What tool can be used to determine natural frequencies on a machine?

Coastdown

Bump Test

Long Time Waveform

All of the above

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.

Explanation

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.