What Is BPFO? — Bearing Glossary

BPFO (Ball Pass Frequency Outer — the frequency at which rolling elements pass a defect on the outer raceway) is the characteristic vibration frequency that appears in the spectrum when the outer ring of a bearing is damaged — each time a rolling element strikes the defect, it generates an impact pulse.

BPFO is the most reliable diagnostic frequency in bearing vibration analysis per ISO 13373 because the outer ring is stationary. A defect on the stationary outer ring receives an impact at exactly the same load intensity every time a rolling element passes — the signal is steady, clear, and easy to track over time. See the full vibration monitoring guide at bearing vibration monitoring and shop deep groove ball bearings with published BPFO factors. BPFO typically falls in the range of 3–8 times rotational speed (3–8× RPM/60 Hz).

Calculation and Practical Application

Formula: BPFO = (Z/2) × n × (1 − d/D × cos α)

Where: Z = number of rolling elements, n = rotational speed (Hz), d = rolling element diameter, D = pitch circle diameter, α = contact angle.

For bearing 6205 (Z=9 balls, d=7.94 mm, D=38.5 mm, α=0°):

• At 1,450 rpm (24.17 Hz): BPFO = (9/2) × 24.17 × (1 − 7.94/38.5) = 4.5 × 24.17 × 0.794 = 86.5 Hz

Each manufacturer publishes BPFO multiplier factors in their catalog (denoted BPFO_factor or Z × Fo). Quick lookup: use the factor in the catalog or in software such as SKF @ptitude or Emerson AMS. The BPFO factor for 6205 = 3.59, so BPFO at 1,450 rpm = 3.59 × (1,450/60) = 86.7 Hz — consistent with the manual calculation.

In the vibration spectrum, BPFO severity progresses as: mild fault — single peak at fundamental frequency. Moderate fault — BPFO peak plus harmonics (2×BPFO, 3×BPFO). Severe fault — BPFO peak plus sidebands spaced at shaft frequency (1× RPM).

Practical Example: Detecting Outer Ring Fault in 6308 C3

At a seafood processing plant in Ca Mau, a pump running at 2,900 rpm used a 6308 C3 bearing (d=40, D=90, B=23 mm, Z=8 balls). BPFO factor = 3.57. At 2,900 rpm: BPFO = 3.57 × (2,900/60) = 172.5 Hz. Spectrum analysis detected a peak at 172 Hz (±1% measurement tolerance) with amplitude 0.8 mm/s — exceeding the 0.5 mm/s alert threshold. The bearing was replaced at the next scheduled stop, avoiding an unplanned failure.

Reading BPFO in the Spectrum: Severity Levels

Knowing the BPFO frequency is only the first step. Interpreting what it means for remaining life requires tracking both amplitude and spectral pattern over successive measurements.