| name | bearing-noise |
| description | Bearing noise analysis — defect frequencies (BPFO/BPFI/BSF/FTF), envelope analysis, kurtosis, bearing signal processing, lubrication noise, ISO 15242, SKF bearing diagnostics. |
| metadata | {"priority":7,"promptSignals":{"phrases":["bearing noise","bearing vibration","bearing defect frequency","BPFO BPFI","envelope analysis bearing","bearing diagnosis"],"minScore":3}} |
Bearing Noise and Vibration Diagnostics — Complete Skill
Bearing Defect Frequencies
Fundamental Bearing Frequencies (from geometry):
Ball Pass Frequency Outer Race (BPFO):
BPFO = (N_b/2) × f_r × (1 - (d_ball/D_pitch) × cos α) [Hz]
N_b = number of balls; f_r = shaft rotation frequency [Hz]; d_ball = ball diameter; D_pitch = pitch diameter; α = contact angle
Ball Pass Frequency Inner Race (BPFI):
BPFI = (N_b/2) × f_r × (1 + (d_ball/D_pitch) × cos α) [Hz]
Ball Spin Frequency (BSF):
BSF = (D_pitch / (2 × d_ball)) × f_r × (1 - (d_ball/D_pitch)² × cos²α) [Hz]
Fundamental Train Frequency (cage; FTF):
FTF = (1/2) × f_r × (1 - (d_ball/D_pitch) × cos α) [Hz; FTF ≈ 0.35–0.45 × f_r]
Approximate (for quick estimate):
BPFO ≈ 0.4 × N_b × f_r
BPFI ≈ 0.6 × N_b × f_r
Harmonics: 2×BPFO, 3×BPFO, etc. appear as defect develops
Example (6205 bearing, 1500 RPM, 8 balls, d/D = 0.33):
f_r = 1500/60 = 25 Hz
BPFO ≈ 0.4 × 8 × 25 = 80 Hz
BPFI ≈ 0.6 × 8 × 25 = 120 Hz
Bearing Noise Sources
Raceway defect: spall or pit → impulsive excitation at defect frequency; modulated by load zone
Rolling element defect: excitation at BSF (symmetric) or 2×BSF (one defect side)
Cage defect: FTF; rattle or cracks
Geometric imperfections: waviness → harmonics at Z × f_r (Z = number of balls)
Lubrication: grease insufficient → metal contact → broad-band noise; starvation → high frequency hiss
Signal Processing Methods
Time Domain — Statistical Indicators
RMS vibration: X_rms = √(Σ x_i² / N) [m/s²; overall signal level; low sensitivity to impulsive]
Peak: X_peak = max|x_i| [high sensitivity to large impacts]
Crest factor: CF = X_peak / X_rms [> 6 → early bearing fault; > 12 → advanced]
Kurtosis K₄:
K₄ = (1/N) Σ(x_i - μ)⁴ / σ⁴
Normal signal: K₄ ≈ 3.0 (Gaussian)
Impulsive bearing signal: K₄ > 4–6 → early fault; > 8 → significant fault
Limitation: kurtosis decreases when many impacts overlap (advanced damage)
Envelope (Demodulation) Analysis
Process:
- Bandpass filter raw accelerometer signal around high-frequency bearing resonance (f_res = 2–20 kHz typically)
- Rectify: |x(t)|
- Low-pass filter (or compute envelope via Hilbert transform)
- FFT of envelope signal → peaks at BPFO, BPFI, BSF, FTF
Why envelope:
Defect impacts ring structural resonance → high-frequency burst → modulation at defect frequency
Envelope analysis extracts modulation → defect frequency visible
Spectral Kurtosis (SK):
Frequency-dependent kurtosis; identifies which frequency band has most impulsive content → optimal filter selection for envelope analysis
Frequency Spectrum Analysis
Fault indicators:
| Fault | Spectrum feature |
|---|
| Outer race | BPFO harmonics; sidebands at f_r |
| Inner race | BPFI harmonics; sidebands at f_r |
| Ball | BSF (less common; variable load zone) |
| Cage | FTF; sidebands around bearing tones |
| Misalignment | 1×, 2×, 3× running speed; axial component |
| Unbalance | 1× running speed dominant |
| Looseness | 0.5×, 1.5× running speed harmonics |
ISO 15242 — Running Accuracy Tests
Vibration frequency bands:
Low (50–300 Hz); medium (300–1800 Hz); high (1800–10,000 Hz)
Measured as velocity [μm/s] or acceleration [m/s²] per band
V₁₂₅ test: test at 1800 RPM; velocity signal in each band
Bearing quality grades: 4, 5, 6 (higher = quieter)
Condition Monitoring Approach
Trending:
Measure: RMS overall vibration, BPFO amplitude, kurtosis — every 2 weeks or 1 month
Plot trend vs. time → alert when parameter crosses alarm threshold
ISO 10816 vibration severity:
Zone A: good (< 2.3 mm/s RMS); Zone B: acceptable; Zone C: alarm; Zone D: trip
Bearing remaining life estimation:
Stage 1 (healthy): K₄ ≈ 3; no peaks at defect frequencies
Stage 2 (early fault): K₄ > 4; BPFO visible in envelope spectrum
Stage 3 (developed fault): K₄ > 8; multiple harmonics; sidebands
Stage 4 (late fault): K₄ may decrease; RMS increasing; broadband noise
P-F interval: time from detectable defect to functional failure
High-speed bearing: P-F = days to weeks
Low-speed bearing: P-F = weeks to months
Lubrication-Related Noise
Metal-to-metal contact sound:
Insufficient lubrication → high-frequency broad-band noise increase (> 2 kHz)
Re-grease → noise drops → confirm lubrication effect
Grease noise:
New grease (shear thickening): temporary high noise at startup; settles in 5–30 min
Old/oxidized grease: crackling, high frequency → relubricate
Oil whirl (fluid film bearings):
Instability at ~0.46 × f_r (sub-synchronous); distinct peak just below 0.5×
If speed increases and peak tracks with 0.5× → oil whirl → oil whip (resonant) danger
Standards
| Standard | Scope |
|---|
| ISO 15242 | Bearing noise test methods |
| ISO 10816 | Vibration limits for rotating machinery |
| API 670 | Machinery protection (vibration measurement) |
| ISO 13373 | Condition monitoring — vibration |
| SKF Bearing Technical Handbook | Diagnostic guidelines |
Output
Provide: defect frequencies BPFO, BPFI, BSF, FTF [Hz] for given bearing at operating speed, measurement method (accelerometer bandpass + envelope, ISO 15242 velocity), diagnostic parameter thresholds (kurtosis alert/alarm, RMS ISO zone), spectrum interpretation for identified peaks, lubrication assessment (κ and noise correlation), remaining life stage assessment (1–4), recommended monitoring interval, and applicable standard (ISO 15242, ISO 10816, API 670).
