What Is BPFI? — Bearing Glossary

BPFI (Ball Pass Frequency Inner — the frequency at which rolling elements pass a defect on the inner raceway) is the characteristic vibration frequency in the spectrum when the inner ring of a bearing is damaged — each rolling element striking the defect on the rotating inner ring generates an impact pulse.

BPFI is more difficult to interpret than BPFO because the inner ring rotates. As the defect moves into and out of the load zone with each revolution per ISO 13373, it generates strong impacts when loaded and weaker impacts when unloaded. For the complete vibration monitoring framework covering BPFI and all other bearing frequencies, see bearing vibration monitoring and browse deep groove ball bearings for your rotating equipment. This variation is called amplitude modulation — BPFI is amplitude-modulated at shaft frequency (1× RPM), producing characteristic sidebands at BPFI ± k×RPM in the spectrum. These sidebands are the key signature distinguishing inner ring faults from other frequency sources.

BPFI Formula and Calculation Examples

Formula: BPFI = (Z/2) × n × (1 + d/D × cos α)

For bearing 6205 (Z=9, d=7.94 mm, D=38.5 mm, α=0°) at 1,450 rpm (24.17 Hz): BPFI = (9/2) × 24.17 × (1 + 7.94/38.5) = 4.5 × 24.17 × 1.206 = 131.2 Hz

BPFI factor for 6205 = 5.42. Check: 5.42 × 24.17 = 131.0 Hz — consistent. Comparison: BPFO for 6205 = 3.59 × 24.17 = 86.8 Hz — BPFI is always higher than BPFO for the same bearing at the same speed.

In the spectrum, an inner ring fault appears as: a peak at the BPFI frequency plus sidebands at BPFI ± 1×RPM, ± 2×RPM. Strong sidebands are characteristic of inner ring damage and are not seen with outer ring faults. Sideband amplitude reaching 30–60% of the main BPFI peak indicates moderate fault severity.

Practical Example: Detecting Inner Ring Fault in NU2216 E

At a rubber factory in Dong Nai, a roller press running at 1,460 rpm used cylindrical roller bearing NU2216 E (d=80, D=140, B=33 mm, Z=14 rollers). BPFI factor = 7.94. At 1,460 rpm: BPFI = 7.94 × (1,460/60) = 193.1 Hz. Spectrum analysis detected a peak at 193 Hz with sidebands at 168.6 Hz and 217.4 Hz — each spaced 24.3 Hz apart (= 1× RPM). Diagnosis confirmed: inner ring fault on NU2216 E. The bearing was replaced within 3 weeks at a planned stop, avoiding sudden roller fracture.

Sideband Pattern as a Severity Indicator

The number and amplitude of sidebands around the BPFI peak are a more reliable severity indicator than the peak amplitude alone. As inner ring damage progresses, additional sideband pairs appear at ±2×RPM, ±3×RPM, and their amplitudes increase.