Check Your Skills In Predictive Maintenance

1. With an induction motor at 70% load, one can use a current clamp and spectral analysis to pinpoint cracked or broken rotor bar problems.

True

False

In fact, analysis charts exist to pinpoint the severity of the problem when looking at the first pole-pass sideband amplitude relative to the line frequency amplitude.

Explanation

In fact, analysis charts exist to pinpoint the severity of the problem when looking at the first pole-pass sideband amplitude relative to the line frequency amplitude.

2. A good way to determine impacting in a captured time waveform is Crest Factor, calculated by true peak value divided by RMS.

True

False

This is one good tool in working with time waveform data. Another way is to assess true peak-to-peak values.

Explanation

This is one good tool in working with time waveform data. Another way is to assess true peak-to-peak values.

3. In just looking at high-frequency acceleration data, one can see the early signs of rolling-element bearing problems.

True

False

A raised noise floor and “haystack” will show up in the upper end of the spectral display.

Explanation

A raised noise floor and “haystack” will show up in the upper end of the spectral display.

4. If I have a VFD and I suspect that there is a resonance condition somewhere in the operating range, I can run a Bump Test to find the problem.

True

False

This is one way to find out, but it usually requires the machine to be stopped. Another good method is to perform a Coastdown and Bode analysis.

Explanation

This is one way to find out, but it usually requires the machine to be stopped. Another good method is to perform a Coastdown and Bode analysis.

5. A sure way to find bearing faults is to use Demodulation and Enveloping techniques, and these will work well without knowing bearing details.

True

False

While the initial data might show a tell-tale sign of problems, the actual bearing fault frequencies need to be overlaid on the spectral plot to confirm the actual bearing with the problem.

Explanation

While the initial data might show a tell-tale sign of problems, the actual bearing fault frequencies need to be overlaid on the spectral plot to confirm the actual bearing with the problem.

6. If a machine is a bit unbalanced at a running speed of 1800, the condition with the vibratory forces will be twice as bad when going to 3600 RPM.

True

False

The vibratory forces for unbalance are proportional to the square of the speed change. In this case, it will be a factor of 4!

Explanation

The vibratory forces for unbalance are proportional to the square of the speed change. In this case, it will be a factor of 4!

7. I cannot find the natural resonance of a machine structure without first turning it off and using a Bump Test or shutting it down and capturing Coastdown data during its coast.

True

False

One way to capture data on a machine in service is to bump it while running and capture a long time waveform. You can then pan and zoom on the data when the structure is excited and when it is in the normal running condition and compare spectral data overlays.

Explanation

One way to capture data on a machine in service is to bump it while running and capture a long time waveform. You can then pan and zoom on the data when the structure is excited and when it is in the normal running condition and compare spectral data overlays.

8. A rotor that is considered "rigid" does not have to be balanced at running speed. In fact, it can be properly balanced all the way down to 500 RPM.

True

False

In fact, for safety reasons, it is recommended that the rigid rotor be balanced at a lower speed. Its characteristics are linear, and it will remain balanced from the lower speed to its intended operating speed.

Explanation

In fact, for safety reasons, it is recommended that the rigid rotor be balanced at a lower speed. Its characteristics are linear, and it will remain balanced from the lower speed to its intended operating speed.

9. Having a vibration reading at 1X running speed with amplitude that is 80% or greater than the overall level is a sure sign of unbalance.

True

False

While this is certainly one leading indicator, other areas need to be checked such as amplitude and phase readings in several axes and eliminating structural looseness and resonance as possible faults.

Explanation

While this is certainly one leading indicator, other areas need to be checked such as amplitude and phase readings in several axes and eliminating structural looseness and resonance as possible faults.

10. In order to do well with the analysis of gearbox issues, it is necessary to calculate the GMF and have data collected to at least 2X GMF.

True

False

As high amplitudes occur at 2X and 3X GMF along with sidebands of shaft speeds, it is necessary to go out to 3.25 GMF with Fmax.

Explanation

As high amplitudes occur at 2X and 3X GMF along with sidebands of shaft speeds, it is necessary to go out to 3.25 GMF with Fmax.