CRE Equipment and Tools Knowledge 3 — Questions and Answers
Question 1: In infrared thermography, an electric motor shows a hot spot on one phase connection that is 25°C above the other two phases. This most likely indicates:
- Normal thermal gradient during operation
- A loose or high-resistance electrical connection (Correct answer)
- Bearing overheating
- Stator core lamination fault
Correct answer: A loose or high-resistance electrical connection
A localized hot spot on one phase connection compared to balanced phases indicates increased contact resistance from a loose or corroded connection.
Question 2: Which tool measures the dynamic stiffness of a machine structure to identify resonance conditions?
- Tachometer
- Impact hammer with accelerometer (modal analysis) (Correct answer)
- Particle counter
- Clamp-on ammeter
Correct answer: Impact hammer with accelerometer (modal analysis)
Impact hammer testing excites the structure across a frequency range and, combined with an accelerometer, generates a frequency response function revealing resonances.
Question 3: An oil sample shows a ferrous particle count spike with large, chunky particles (>40 µm) on a ferrograph. This pattern is associated with:
- Normal running-in wear
- Severe sliding or fatigue wear indicating imminent failure (Correct answer)
- Water contamination
- Additive precipitation
Correct answer: Severe sliding or fatigue wear indicating imminent failure
Large ferrous particles (>40 µm) with severe morphology indicate heavy wear modes such as adhesive or fatigue spalling, signaling impending failure.
Question 4: A portable vibration balancer requires a trial weight run to determine which quantity?
- Bearing defect frequency
- The influence coefficient (sensitivity of vibration to added weight) (Correct answer)
- Oil film thickness
- Rotor critical speed
Correct answer: The influence coefficient (sensitivity of vibration to added weight)
The trial weight run measures how the vibration vector changes per unit of added mass, establishing the influence coefficient for calculating the correction weight.
Question 5: Acoustic emission (AE) testing is MOST advantageous compared to vibration analysis for detecting:
- Low-speed bearing defects (<10 RPM) (Correct answer)
- Mass imbalance
- Gear mesh harmonics
- Shaft resonance
Correct answer: Low-speed bearing defects (<10 RPM)
Acoustic emission is highly sensitive to stress wave events regardless of speed, making it superior for detecting defects in very slow-rotating bearings where vibration amplitudes are too low.
Question 6: When performing a pump performance test, cavitation is confirmed by observing:
- Increased discharge pressure and reduced current draw
- Reduced head and flow with increased noise and vibration at random frequencies (Correct answer)
- Stable performance curve with elevated temperature
- Higher suction pressure than design
Correct answer: Reduced head and flow with increased noise and vibration at random frequencies
Cavitation causes vapor bubble collapse producing broadband noise and vibration, along with head and flow deterioration due to vapor pockets in the impeller.
Question 7: A torque wrench calibration certificate shows a ±4% accuracy at full scale. When tightening a critical fastener to 100 Nm at 50% full scale, the actual torque uncertainty is approximately:
- ±4 Nm
- ±8 Nm (Correct answer)
- ±2 Nm
- ±1 Nm
Correct answer: ±8 Nm
Full-scale accuracy of ±4% applied to a 200 Nm wrench equals ±8 Nm absolute error, regardless of the actual reading; at 50% scale the uncertainty is still ±8 Nm.
CRE Equipment and Tools Knowledge 3
In infrared thermography, an electric motor shows a hot spot on one phase connection that is 25°C above the other two phases.
This most likely indicates: