CVA Data Collection & Instrumentation 2 — Questions and Answers
Question 1: What is the difference between overall vibration level and spectral analysis in a condition monitoring program?
- Overall level measures low-frequency vibration only; spectral analysis covers high frequencies
- Overall level provides a single severity value for trending; spectral analysis identifies the frequency and source of vibration components for diagnostics (Correct answer)
- Overall level is measured with proximity probes; spectral analysis requires accelerometers
- Overall level uses velocity; spectral analysis always uses acceleration
Correct answer: Overall level provides a single severity value for trending; spectral analysis identifies the frequency and source of vibration components for diagnostics
Overall vibration level (e.g., overall RMS velocity) provides a single trending metric for alarm purposes, while spectral analysis breaks the signal into frequency components that can be linked to specific machine faults for root cause identification.
Question 2: What is the ISO 20816 standard primarily used for?
- Specifying bearing defect frequency calculation methods
- Evaluating machinery vibration severity based on overall RMS velocity measured on non-rotating parts (Correct answer)
- Defining accelerometer calibration procedures and intervals
- Setting balance quality grades for rotating machine components
Correct answer: Evaluating machinery vibration severity based on overall RMS velocity measured on non-rotating parts
ISO 20816 (updated from ISO 10816) provides zone-based vibration severity limits in RMS velocity for various machine types, guiding maintenance decisions based on measured vibration on bearing housings and machine supports.
Question 3: What does 'route-based data collection' mean in a predictive maintenance program?
- A continuous online monitoring system that automatically collects data 24/7
- A systematic schedule where a technician collects vibration data at predefined measurement points and intervals using a portable analyzer (Correct answer)
- A random sampling strategy for identifying unknown machine faults
- An automated robotic system that measures vibration without human intervention
Correct answer: A systematic schedule where a technician collects vibration data at predefined measurement points and intervals using a portable analyzer
Route-based collection organizes measurement points into logical geographic routes that a technician follows on a periodic schedule (weekly, monthly) using a portable data collector, enabling consistent trending and anomaly detection.
Question 4: What is the significance of measurement point location when collecting vibration data on a bearing housing?
- Location affects only the phase of the measurement, not the amplitude or frequency content
- The measurement should be taken as close as possible to the bearing load zone for the most representative signal (Correct answer)
- Bearing housing measurements are invalid; only shaft measurements with proximity probes are acceptable
- Location has no significance as long as the sensor is within 6 inches of the bearing
Correct answer: The measurement should be taken as close as possible to the bearing load zone for the most representative signal
Measuring as close as possible to the bearing load zone minimizes the transmission path and attenuation between the fault source and the sensor, ensuring the highest signal-to-noise ratio for defect detection.
Question 5: What is the purpose of a velocity transducer (seismic pickup) in vibration measurement?
- To measure acceleration directly at high frequencies above 10 kHz
- To measure the absolute velocity of a machine casing using an internal mass-spring system with velocity coil output (Correct answer)
- To measure relative shaft displacement in fluid-film bearing machines
- To provide a once-per-revolution reference signal for phase measurement
Correct answer: To measure the absolute velocity of a machine casing using an internal mass-spring system with velocity coil output
A velocity transducer (geophone or seismic pickup) uses an internal mass-spring-damper system and electromagnetic coil to produce an electrical signal proportional to absolute casing velocity, without requiring external power.
Question 6: Which of the following best describes the function of a tachometer in vibration analysis?
- It measures the amplitude of vibration at running speed frequency
- It provides a once-per-revolution signal used to determine shaft speed and as a phase reference for synchronous measurements (Correct answer)
- It filters the vibration signal to remove components below running speed
- It measures the axial position of a thrust bearing rotor
Correct answer: It provides a once-per-revolution signal used to determine shaft speed and as a phase reference for synchronous measurements
A tachometer (optical, magnetic, or proximity type) generates a pulse once per shaft revolution, providing precise speed information and a phase reference essential for balancing, order tracking, and synchronous averaging.
CVA Data Collection & Instrumentation 2
What is the difference between overall vibration level and spectral analysis in a condition monitoring program?