CVA Bearing Analysis & Fault Detection 1 — Questions and Answers
Question 1: What are the four fundamental bearing defect frequencies for a rolling element bearing?
- BPFO, BPFI, BSF, FTF (Correct answer)
- 1×, 2×, 3×, 4×
- GMF, BSF, FTF, RPM
- NF, FF, SF, BF
Correct answer: BPFO, BPFI, BSF, FTF
The four fundamental bearing defect frequencies are BPFO (Ball Pass Frequency Outer race), BPFI (Ball Pass Frequency Inner race), BSF (Ball Spin Frequency), and FTF (Fundamental Train Frequency or cage frequency).
Question 2: Which bearing defect frequency is typically the lowest and corresponds to cage rotation?
- BPFO
- BPFI
- BSF
- FTF (Correct answer)
Correct answer: FTF
The Fundamental Train Frequency (FTF) represents the rotational speed of the cage and is typically 0.35–0.48× shaft speed, making it the lowest of the four bearing defect frequencies.
Question 3: In the early stages of rolling element bearing failure, defect-related energy typically appears first in which frequency range?
- Sub-synchronous (below 1× running speed)
- 1× to 3× running speed range
- Ultrasonic frequencies (20 kHz and above), then high-frequency resonances (Correct answer)
- At the exact bearing defect frequency in the low-frequency spectrum
Correct answer: Ultrasonic frequencies (20 kHz and above), then high-frequency resonances
Early bearing defects generate very low-energy, high-frequency stress waves that first excite ultrasonic frequencies and bearing/structural natural frequencies; bearing defect frequencies in the low-frequency spectrum appear in later fault stages.
Question 4: What is the envelope (demodulation) technique used for in bearing analysis?
- Removing electrical noise from proximity probe signals
- Detecting the repetitive impact pattern of a bearing defect by bandpass filtering and demodulating a high-frequency resonance (Correct answer)
- Integrating acceleration to velocity for low-frequency analysis
- Measuring bearing clearance by shaft orbit analysis
Correct answer: Detecting the repetitive impact pattern of a bearing defect by bandpass filtering and demodulating a high-frequency resonance
Envelope analysis bandpass filters the signal around a high-frequency resonance excited by bearing impacts, then demodulates (rectifies and low-pass filters) to extract the repetition rate of the impacts for bearing defect frequency identification.
Question 5: What does BPFO stand for, and what does it represent?
- Bearing Peak Force Output; the maximum load a bearing can sustain
- Ball Pass Frequency Outer race; the rate at which rolling elements pass a defect on the outer race (Correct answer)
- Bearing Pre-load Force Offset; the axial preload applied to angular contact bearings
- Base Point Frequency Oscillation; the resonant frequency of the bearing housing
Correct answer: Ball Pass Frequency Outer race; the rate at which rolling elements pass a defect on the outer race
BPFO (Ball Pass Frequency Outer race) is the frequency at which rolling elements strike a defect on the outer ring, calculated as (N/2) × shaft speed × (1 - Bd/Pd × cos θ), where N is the number of rolling elements.
Question 6: Which parameter is used in high-frequency bearing diagnostics to quantify the overall impulsiveness of the bearing signal?
- RMS velocity
- Peak displacement
- Kurtosis (Correct answer)
- Crest factor only
Correct answer: Kurtosis
Kurtosis measures the statistical 'peakedness' or impulsiveness of a signal; a healthy bearing has a kurtosis near 3, while damaged bearings with impulsive impacts show kurtosis values significantly above 3.
CVA Bearing Analysis & Fault Detection 1
What are the four fundamental bearing defect frequencies for a rolling element bearing?