CMRT Predictive Maintenance Technologies Questions and Answers 1 ā Questions and Answers
Question 1: A vibration analyst observes a predominant spectral peak at two times (2x) the running speed on a direct-coupled pump and motor. This is a classic indicator of which of the following mechanical faults?
- Component imbalance
- Angular misalignment (Correct answer)
- Advanced bearing wear
- Structural looseness
Correct answer: Angular misalignment
A strong vibration signal at twice the running speed (2x) is a primary indicator of angular misalignment between coupled shafts. Imbalance typically shows at 1x, bearing faults have their own specific non-harmonic frequencies, and looseness often presents as multiple harmonics (1x, 2x, 3x, etc.).
Question 2: While performing an infrared scan on an energized Motor Control Center (MCC) panel, a technician identifies a circuit breaker's terminal lug that is significantly hotter than adjacent ones under similar load. What is the most probable cause of this thermal anomaly?
- High electrical resistance (Correct answer)
- Excessive motor amperage draw
- Incorrectly sized circuit breaker
- High ambient air temperature
Correct answer: High electrical resistance
Heat is generated when current flows through resistance (P=IĀ²R). A loose, corroded, or improperly crimped connection creates high resistance, which manifests as a localized hot spot under electrical load. Since adjacent components under similar load are not hot, it rules out motor load or ambient temperature as the primary cause.
Question 3: A technician is using an ultrasonic sensor to perform condition-based lubrication on a motor bearing. What is the primary indicator that the correct amount of grease has been applied?
- The temperature of the bearing begins to decrease.
- A small amount of grease begins to purge from the bearing seal.
- The overall vibration amplitude of the motor decreases.
- The decibel (dB) level of the bearing drops to a normal baseline. (Correct answer)
Correct answer: The decibel (dB) level of the bearing drops to a normal baseline.
Ultrasonic instruments detect the high-frequency sounds of friction within a bearing. As the correct amount of grease is applied, it lubricates the components, reducing friction and the associated sound. The technician listens for the decibel (dB) level to drop to a pre-determined baseline for a healthy bearing. Over-greasing can cause the dB level to rise again due to churning.
Question 4: An oil analysis report for a large hydraulic system shows a high particle count, an elevated silicon (Si) level, and a noticeable increase in oil viscosity. Which of the following is the most likely root cause?
- Thermal breakdown (oxidation) of the oil
- Internal pump component fatigue
- Ingress of dirt and dust from the environment (Correct answer)
- Water contamination from a heat exchanger leak
Correct answer: Ingress of dirt and dust from the environment
Silicon (Si) is the primary elemental indicator of dirt/sand ingress, often from a faulty breather or seal. This external contamination is abrasive, which explains the high particle count from accelerated wear. Dirt particles can also cause the oil to thicken, leading to an increase in viscosity.
Question 5: Which of the following predictive technologies is most effective for detecting broken or cracked rotor bars in an AC induction motor while the motor is in operation?
- Airborne Ultrasonics
- Motor Current Signature Analysis (MCSA) (Correct answer)
- Infrared Thermography
- Strobe Light Analysis
Correct answer: Motor Current Signature Analysis (MCSA)
Motor Current Signature Analysis (MCSA), a form of Motor Circuit Analysis, analyzes the motor's current draw. Broken rotor bars create distinct sideband frequencies around the main 60 Hz (or 50 Hz) line frequency peak in the current spectrum, allowing for early and accurate detection while the motor is running under load.
Question 6: When considering the P-F Curve, which of the following predictive maintenance technologies would typically be the EARLIEST to detect the onset of a subsurface bearing fault, such as a microscopic crack?
- Acoustic Emission / Stress Wave Analysis (Correct answer)
- Vibration Analysis
- Infrared Thermography
- Human Senses (audible noise)
Correct answer: Acoustic Emission / Stress Wave Analysis
The P-F curve shows the progression from a potential failure (P) to a functional failure (F). The very first sign of a subsurface fault like a micro-crack is the release of energy in the form of high-frequency stress waves, known as acoustic emissions. This occurs long before the fault is large enough to generate significant vibration, heat, or audible noise.
CMRT Predictive Maintenance Technologies Questions and Answers 1
A vibration analyst observes a predominant spectral peak at two times (2x) the running speed on a direct-coupled pump and motor.
This is a classic indicator of which of the following mechanical faults?