What Is BSF? — Bearing Glossary

BSF (Ball Spin Frequency — the rotational frequency of a rolling element) is the characteristic vibration frequency when the surface of a ball or roller is damaged — as the rolling element rotates, a surface defect strikes first the inner ring then the outer ring, generating a distinctive double-pulse pattern per revolution.

BSF is typically lower than both BPFO and BPFI and is harder to detect because the signal is weak, per SKF bearing diagnostics guidelines. For practical guidance on setting up a bearing frequency monitoring program covering BSF and all fault modes, see bearing vibration monitoring and browse deep groove ball bearings with published BSF factors. When a rolling element is defective, it contacts the inner ring then the outer ring with each rotation, so pulses appear in pairs per rolling element revolution. Because the rolling element simultaneously spins and orbits around the bearing axis, its spin speed depends on both bearing geometry and shaft speed. Rolling element damage is less common than inner or outer ring damage — but more dangerous. A fractured rolling element can seize the entire bearing within seconds, with no gradual vibration warning beforehand.

BSF Formula and Calculation

BSF = (D/2d) × n × (1 − (d/D × cos α)²)

For bearing 6205 (Z=9, d=7.94 mm, D=38.5 mm, α=0°) at 1,450 rpm (24.17 Hz): BSF = (38.5 / (2 × 7.94)) × 24.17 × (1 − (7.94/38.5)²) = 2.42 × 24.17 × (1 − 0.0425) = 56.0 Hz

BSF factor for 6205 = 2.31. Check: 2.31 × 24.17 = 55.8 Hz — consistent.

Comparing all four bearing frequencies for 6205 at 1,450 rpm:

• BPFO = 3.59 × 24.17 = 86.8 Hz
• BPFI = 5.42 × 24.17 = 131.0 Hz
• BSF = 2.31 × 24.17 = 55.8 Hz (lowest)
• FTF = 0.40 × 24.17 = 9.7 Hz (cage)

BSF appears in the spectrum with harmonics — 2×BSF is the most prominent, because the rolling element contacts both rings once per revolution. A weak BSF fundamental peak with a clear 2×BSF peak is the typical early-stage pattern.

Practical Example: Rolling Element Fault in 30207

At a wood processing plant in Binh Duong, a planer running at 1,450 rpm used a tapered roller bearing 30207 (d=35, D=72, B=17 mm). BSF = 2.48 × (1,450/60) = 59.9 Hz. Spectrum analysis detected a strong peak at 119.8 Hz (2×BSF) — with no clear BPFO or BPFI peaks present. Inspection confirmed a crack on one tapered roller. Left two more weeks, the roller would have fractured causing sudden shaft seizure.

Bearing Frequency Comparison Across Types

Rolling element dimensions vary significantly between ball bearings and tapered roller bearings. A larger d/D ratio (fatter rolling elements relative to pitch diameter) reduces BSF while increasing BPFI. Always use manufacturer catalog factors rather than estimates.