Bearing Failure Analysis with Vibration Signature — ISO 10816 + ISO 13373

Bearings are the most frequently failing component in rotating machinery — 50-70% of unscheduled downtime in pumps, motors, fans, and gearboxes comes from bearings. Remarkably, bearings do not fail randomly — they degrade along a predictable pattern + can be detected via vibration signature 1-6 months before catastrophe. The standards ISO 10816 + ISO 13373 are the framework that industries (oil & gas, power, manufacturing) use worldwide. This article interprets the characteristic frequency formulas + 8 failure modes + alarm thresholds

Why Bearings Need Vibration Analysis

Bearing degradation timeline is divided into 4 stages (ISO 13373-3):

1. Stage 1 (incipient defect) — sub-surface stress, detected at 250-350 kHz ultrasonic
2. Stage 2 (developing defect) — micro-spall, peak at resonance frequency (700-2,000 Hz)
3. Stage 3 (advanced defect) — visible peak at characteristic freq (BPFO/BPFI/BSF/FTF)
4. Stage 4 (catastrophic) — random broadband + heat + audible noise — fails within 1-7 days

Detection at Stage 2-3 gives 1-6 months to plan replacement. Stage 4 = emergency

Characteristic Frequencies — Formulas + Example

Geometry formulas:

• FTF (Fundamental Train Frequency) — rotation of the cage: $$\text{FTF} = \frac{1}{2} \left(1 - \frac{D_b \cos\alpha}{D_p}\right) \times \frac{n_{shaft}}{60}$$
• BPFO (Ball Pass Frequency Outer race) = n × FTF
• BPFI (Ball Pass Frequency Inner race) = n × (n_shaft/60) × ½(1 + D_b·cos α / D_p)
• BSF (Ball Spin Frequency): $$\text{BSF} = \frac{D_p}{2 D_b} \times \frac{n_{shaft}}{60} \left(1 - \left(\frac{D_b \cos\alpha}{D_p}\right)^2\right)$$

Variables:

• n = number of balls
• D_b = ball diameter
• D_p = pitch diameter (center-to-center of inner-outer raceway)
• α = contact angle (deg) — radial bearing ≈ 0°, angular contact = 15-40°
• n_shaft = shaft RPM

Example: SKF 6205-2RS deep groove ball bearing, motor 1,750 RPM

• n = 9 balls, D_b = 7.94 mm, D_p = 38.5 mm, α = 0°
• FTF = ½ × (1 - 7.94/38.5) × 1750/60 = 11.6 Hz
• BPFO = 9 × 11.6 = 104.5 Hz
• BPFI = 9 × (1750/60) × ½(1 + 7.94/38.5) = 157.6 Hz
• BSF = (38.5/15.88) × (1750/60) × (1 - (7.94/38.5)²) = 68.7 Hz

In the FFT spectrum, if a high peak appears at 104.5 Hz + harmonics → outer race defect

SKF / Timken / Schaeffler offer free online calculators + Bearing Failure Atlas — input bearing model + RPM for instant results

8 Bearing Failure Modes (ISO 13373 + SKF Atlas)

flowchart TD
  A[Vibration Signature] --> B{Where peak?}
  B -->|BPFO + harmonics| C[Outer race spalling/pitting]
  B -->|BPFI + sidebands| D[Inner race spalling]
  B -->|BSF + cage modulation| E[Ball/Roller defect]
  B -->|FTF + harmonics| F[Cage damage]
  B -->|1× + 2× + 3× shaft| G[Misalignment / overload]
  B -->|Broadband 1-10 kHz| H[Lubrication failure]
  B -->|Periodic at FTF × shaft| I[Electrical erosion fluting]
  B -->|Random broadband noise| J[Skidding / slip]
  C --> K[Pattern: peak grows
over weeks-months]
  D --> K
  E --> K
  F --> L[Critical — fail in days-week]
  G --> M[Realign + reduce load]
  H --> N[Check lubrication system
+ contamination]
  I --> O[Add shaft grounding
VFD-driven motor]
  J --> P[Reduce speed
+ check preload]

ISO 10816 / ISO 20816 — Overall Vibration Limits

ISO 10816 (renamed ISO 20816 from 2016) divides rotating machines into 4 classes by size and foundation:

Overall vibration zones (mm/s RMS):

These values are overall vibration. For bearing-specific defects, use envelope detection (ESP) at the specific frequency

Envelope Detection — Method for Bearing Defect

Standard FFT often misses early-stage bearing defects because:

• Bearing defects generate "impacts" — short pulse signals
• The pulse spreads energy across the spectrum
• The peak at the characteristic frequency is lower than the running-speed peak

Envelope Detection (ESP) / Demodulation:

1. Band-pass filter at the resonance frequency (bearing level) — 500 Hz - 10 kHz
2. Rectify the signal (full-wave)
3. Low-pass filter
4. FFT
5. Result: an envelope spectrum where the characteristic frequency stands out clearly

Modern vibration analyzers (SKF Microlog, CSI 2140, GE Bently Nevada) have envelope built-in. Threshold:

• < 0.1 g RMS — healthy
• 0.1-0.5 g — early defect
• 0.5-1.5 g — developing

• 1.5 g — advanced, plan replacement

• 3.0 g — critical, stop ASAP

Trend Analysis + Alarm Strategy

flowchart LR
  A[Periodic Measurement
weekly/monthly] --> B[Plot overall + envelope
vs baseline]
  B --> C{Trend?}
  C -->|Stable| D[Continue routine
monitoring]
  C -->|Increasing
< 6 dB above baseline| E[Increase frequency
weekly → daily]
  C -->|+6 dB Warning| F[Plan replacement
spare ready]
  C -->|+12 dB Alarm| G[Immediate plan
replace 30 days]
  C -->|+18 dB Critical| H[Stop machine
replace immediately]

Baseline reset rule:

• When a new bearing is installed → measure 24-48 hr after break-in → set as the new baseline
• Keep a separate baseline for each machine

Sensor + Equipment Selection

Accelerometer type:

• Piezoelectric IEPE (industrial standard) — 100 mV/g, frequency response 1 Hz-10 kHz
• MEMS — cheaper, smaller, integrates with IoT — frequency response 1 Hz-5 kHz
• Eddy current proximity probe — measures shaft displacement directly, for turbomachinery

Sensor installation:

• Mount: stud (best), magnetic (good), handheld (worst — used only for routine checks)
• Location: bearing housing, axial + radial + tangential 3 axes
• Cable: shielded twisted pair, ground at the sensor end only

Predictive Replacement Economics

Example: a plant with 50 pumps, 75 kW motors:

Predictive saves 60-70% vs reactive + 25-30% vs time-based

6 Procurement Guidelines

1. TOR should specify ISO 10816 / 20816 + ISO 13373 for the vibration program
2. Vibration analyzer with FFT + envelope built-in — not just an overall meter
3. Training: ISO Cat I/II/III — certified vibration analyst (certified by Mobius, BINDT, IRD)
4. Database software — store trend data + auto-alarm + reporting. Mainstream: SKF Aptitude, CSI AMS, GE System 1
5. Permanent sensors for critical machines — IEPE accelerometer + protective housing
6. Annual recalibration of sensor + analyzer per an ISO 17025 lab

Summary

Bearing failure can be detected via vibration signature 1-6 months before catastrophe. ISO 10816 / 20816 sets the overall vibration limit per machine class. Characteristic frequencies (FTF, BPFO, BPFI, BSF) are calculated from geometry — peaks at these frequencies in the FFT envelope = bearing defect. The 8 failure modes have different signatures — outer race spalling (BPFO), inner race (BPFI), cage (FTF), ball (BSF), misalignment (1×/2×/3× shaft), lube failure (broadband 1-10 kHz). A predictive strategy saves 60-70% vs reactive

Sahawatthanakit provides a vibration analysis program — initial baseline + periodic measurement + report with recommended actions for pumps, motors, fans, and gearboxes in Thai plants — consult our team to request a machine audit

Frequently Asked Questions

What does vibration analysis tell us about a bearing? A bearing has a unique vibration signature at FTF/BPFO/BPFI/BSF — peaks at these frequencies in the FFT envelope = defect. Detected 1-6 months before catastrophe, cutting unplanned downtime by 70-90%

Where does ISO 10816 measure? Non-rotating parts — bearing housing, foundation. mm/s RMS. Divided into Class I-IV by size + foundation. Class III rigid > 75 kW: Good < 1.8, Acceptable < 4.5, Alarm < 7.1, Damage > 7.1

FFT spectrum analysis? Accelerometer time signal → FFT → frequency spectrum. Bearing fault peaks at the characteristic freq (not the shaft frequency). Tool: handheld 30-120k, analyzer 250k-1.5M, permanent 1M+

FTF/BPFO/BPFI/BSF formulas? From bearing geometry: n balls, Db diameter, Dp pitch, α contact angle, n_shaft RPM. SKF/Timken/Schaeffler offer free calculators

8 failure modes? Outer race spalling (BPFO), Inner race (BPFI), Ball (BSF), Cage (FTF), Misalignment (1×/2×/3×), Lube failure (broadband 1-10 kHz), Electrical erosion (periodic FTF × shaft), Skidding (random broadband)

Where to set the alarm? ISO 10816 overall limit per class. Envelope-specific: +6 dB warning, +12 dB alarm. Reset baseline after bearing replacement