Rolling element bearings are among the most frequently failing components in rotating equipment. The good news is that bearing damage usually develops in stages rather than suddenly, and each stage leaves a different trace in vibration. To read those traces, you first need to understand bearing fault frequencies.
The four basic bearing frequencies
A bearing has four main parts: outer race, inner race, rolling elements (balls or rollers) and cage. When one of them has a defect, every contact with the defect produces an impact, and the impacts repeat at a specific frequency:
- BPFO (Ball Pass Frequency Outer): the rate at which rolling elements pass a defect on the outer race.
- BPFI (Ball Pass Frequency Inner): the same for an inner race defect.
- BSF (Ball Spin Frequency): the rotation frequency of a rolling element about its own axis.
- FTF (Fundamental Train Frequency): the rotation frequency of the cage.
Formulas
With shaft frequency fr = rpm / 60, number of rolling elements Nb, element diameter Bd, pitch diameter Pd and contact angle φ:
FTF = fr/2 · (1 − Bd/Pd · cosφ)BPFO = Nb · fr/2 · (1 − Bd/Pd · cosφ)BPFI = Nb · fr/2 · (1 + Bd/Pd · cosφ)BSF = Pd/(2·Bd) · fr · (1 − (Bd/Pd · cosφ)²)
If the geometry is unknown, BPFO ≈ 0.4 · Nb · fr and BPFI ≈ 0.6 · Nb · fr are useful rough estimates. To calculate them for your own bearing, use our Bearing Fault Frequency Calculator.
Example
For a deep groove ball bearing at 1480 rpm with 9 balls, 7.94 mm ball diameter, 39.04 mm pitch diameter and 0° contact angle:
- fr ≈ 24.67 Hz
- BPFO ≈ 88.4 Hz (≈ 3.58X)
- BPFI ≈ 133.6 Hz (≈ 5.42X)
- BSF ≈ 58.1 Hz, FTF ≈ 9.8 Hz
Notice these frequencies are not integer multiples of running speed. Peaks that do not line up with exact multiples of 1X are one of the key clues of a bearing defect.
The four stages of bearing failure
Stage 1 — Microscopic onset: the defect is tiny. There are no symptoms except in ultrasound and very high-frequency measurements (e.g. 20–60 kHz).
Stage 2 — Resonance excitation: impacts excite the natural frequencies of bearing and housing parts (a few kHz). Envelope analysis demodulates the repetition rate of these high-frequency impacts and clearly reveals BPFO/BPFI.
Stage 3 — Fault frequencies in the spectrum: BPFO, BPFI and their harmonics appear in the normal velocity spectrum. Inner race defects typically show 1X sidebands (the defect rotates with the shaft in and out of the load zone); ball defects show FTF sidebands.
Stage 4 — Advanced damage: discrete peaks turn into a broadband "haystack", overall level and temperature rise, and 1X may increase. Bearing replacement should not be postponed at this point.
Why envelope analysis matters
In the early stage, bearing impacts are very short and low in energy; in the velocity spectrum they are lost among large components such as unbalance or misalignment. Envelope analysis:
- High-pass filters the signal to remove low-frequency machine vibration,
- Extracts the envelope (amplitude modulation) of the remaining impact train,
- Takes the spectrum of that envelope, clearly revealing the repetition frequency of the impacts — the bearing frequencies such as BPFO and BPFI.
Common mistakes
- Looking only at overall level: the mm/s RMS overall level catches bearing faults only at stage 3 or 4.
- Wrong bearing data: a replacement bearing from a different manufacturer can change the geometry and therefore the frequencies.
- Mistaking lubrication problems for defects: poor lubrication raises high-frequency energy without a specific frequency; track it with ultrasound while greasing.
Conclusion
Bearing failures may be inevitable, but they do not have to be a surprise. With the right measurement point, envelope analysis and regular trending, bearing replacement can move into a planned stop. Learn about our vibration analysis service or write to us.
