CMRT Predictive Maintenance Techniques 2 — Questions and Answers
Question 1: In vibration analysis, a dominant 1× running speed (1X) frequency component most commonly indicates which condition?
- Bearing inner race defect
- Mass imbalance (Correct answer)
- Gear mesh problem
- Cavitation
Correct answer: Mass imbalance
A dominant 1× component in the frequency spectrum is the classic signature of mass imbalance, which produces a force once per shaft revolution.
Question 2: The Ball Pass Frequency Outer Race (BPFO) bearing defect frequency is determined by which set of parameters?
- Shaft speed and bearing radial clearance
- Number of rolling elements, shaft speed, and contact angle (Correct answer)
- Shaft speed and outer race diameter only
- Rolling element diameter and cage frequency
Correct answer: Number of rolling elements, shaft speed, and contact angle
BPFO = (N/2) × shaft speed × (1 − (Bd/Pd) × cos α), where N is the number of rolling elements, Bd is ball diameter, Pd is pitch diameter, and α is the contact angle.
Question 3: When performing vibration analysis, which measurement parameter is generally most sensitive to high-frequency bearing defect impacts?
- Displacement
- Velocity
- Acceleration (Correct answer)
- Jerk
Correct answer: Acceleration
Acceleration is most sensitive to high-frequency shock events such as early-stage bearing defects because it amplifies high-frequency content that displacement and velocity would attenuate.
Question 4: A 'waterfall plot' (cascade plot) in vibration analysis is used primarily to:
- Display vibration amplitude versus time at a single frequency
- Show frequency spectra stacked over time or speed to reveal trends (Correct answer)
- Compare two sensors mounted on opposite bearing housings
- Identify phase relationships between vibration signals
Correct answer: Show frequency spectra stacked over time or speed to reveal trends
A waterfall plot stacks successive FFT spectra along a time or speed axis, making it easy to identify how frequency components change over time or during run-up/coast-down.
Question 5: Resonance in a mechanical structure is most likely to occur when:
- Operating speed exceeds the critical speed by more than 20%
- A forcing frequency coincides with the structure's natural frequency (Correct answer)
- Bearing clearance exceeds manufacturer tolerances
- Lubricant viscosity drops below the recommended range
Correct answer: A forcing frequency coincides with the structure's natural frequency
Resonance is a condition where an external forcing frequency matches the natural (resonant) frequency of the structure, causing amplified vibration response.
Question 6: In a Fast Fourier Transform (FFT) vibration spectrum, sidebands spaced at exactly 1× shaft speed around a gear mesh frequency typically indicate:
- Gear tooth wear uniformly distributed around the gear
- Gear eccentricity or pitch-line runout modulating the mesh signal (Correct answer)
- Bearing outer race defect on the gear shaft
- Resonance of the gear housing
Correct answer: Gear eccentricity or pitch-line runout modulating the mesh signal
Sidebands at ±1× shaft speed around gear mesh frequency indicate amplitude or frequency modulation caused by eccentricity or runout of the gear, not uniform wear.
Question 7: Which vibration measurement transducer type is most appropriate for measuring absolute shaft displacement in sleeve (journal) bearing machines?
- Piezoelectric accelerometer
- Velocity geophone
- Eddy-current proximity probe (Correct answer)
- MEMS accelerometer
Correct answer: Eddy-current proximity probe
Eddy-current proximity probes measure relative displacement between the shaft and bearing housing directly, making them ideal for journal bearing machines where shaft orbit is the key parameter.
CMRT Predictive Maintenance Techniques 2
In vibration analysis, a dominant 1× running speed (1X) frequency component most commonly indicates which condition?