CVA Bearing Analysis & Fault Detection 2 — Questions and Answers
Question 1: What is the crest factor of a vibration signal?
- The ratio of peak amplitude to RMS amplitude (Correct answer)
- The ratio of RMS amplitude to average amplitude
- The ratio of peak-to-peak amplitude to RMS amplitude
- The ratio of velocity RMS to acceleration peak
Correct answer: The ratio of peak amplitude to RMS amplitude
Crest factor is the ratio of peak to RMS amplitude; for a pure sine wave it equals √2 (≈1.414), while impulsive bearing faults increase the peak without proportionally increasing RMS, raising the crest factor.
Question 2: What does a 'sideband' pattern around BPFI indicate in bearing diagnostics?
- Random impulsive noise from bearing contamination
- Modulation of the inner race defect frequency by shaft speed, confirming an inner race defect (Correct answer)
- Cage damage causing amplitude modulation at FTF
- Resonance of the inner race natural frequency
Correct answer: Modulation of the inner race defect frequency by shaft speed, confirming an inner race defect
Because the inner race rotates with the shaft, an inner race defect produces sidebands spaced at shaft speed around BPFI harmonics, which confirms the inner race as the defect location.
Question 3: What is the ISO 10816 standard's primary concern regarding bearing vibration?
- Defining bearing defect frequency calculation formulas
- Setting velocity-based vibration severity limits for overall machine vibration assessment (Correct answer)
- Specifying accelerometer mounting requirements for bearing measurements
- Establishing bearing life rating (L10) calculation methods
Correct answer: Setting velocity-based vibration severity limits for overall machine vibration assessment
ISO 10816 (now ISO 20816) establishes overall vibration velocity severity limits for evaluating machine condition, providing zone classifications that guide maintenance decisions based on RMS velocity.
Question 4: What is the Spike Energy (gSE) measurement, and what makes it useful for bearing analysis?
- An overall RMS measurement in the 10–1000 Hz range used for general machinery trending
- A high-frequency (5–50 kHz) energy measurement sensitive to very early-stage impacting from bearing and gear defects (Correct answer)
- A peak displacement measurement used to detect shaft bow in slow-speed machines
- A phase-referenced measurement used to locate the angular position of bearing defects
Correct answer: A high-frequency (5–50 kHz) energy measurement sensitive to very early-stage impacting from bearing and gear defects
Spike Energy measures the energy content of high-frequency stress waves (typically 5–50 kHz) generated by bearing or gear impacting, and its trend over time provides early warning of developing defects before they appear in normal velocity spectra.
Question 5: In which bearing failure stage would you expect to see harmonics of bearing defect frequencies clearly visible in the low-frequency velocity spectrum?
- Stage 1 (very early)
- Stage 2 (early)
- Stage 3 (intermediate) (Correct answer)
- Stage 4 (advanced, near failure)
Correct answer: Stage 3 (intermediate)
In Stage 3 of bearing failure, distinct harmonics of bearing defect frequencies (BPFO, BPFI, BSF) appear clearly in the normal velocity spectrum along with sideband development, indicating significant bearing damage.
Question 6: What type of bearing fault is indicated by a non-synchronous frequency at approximately 0.4–0.5× shaft speed with harmonics?
- Inner race defect (BPFI)
- Cage defect (FTF harmonics) (Correct answer)
- Rolling element defect (BSF and harmonics)
- Outer race defect (BPFO)
Correct answer: Cage defect (FTF harmonics)
FTF (cage frequency) at approximately 0.4–0.45× shaft speed with multiple harmonics indicates cage damage or defect; cage faults are often related to lubrication failure or contamination.
CVA Bearing Analysis & Fault Detection 2
What is the crest factor of a vibration signal?