What are bearing defect frequencies?

Bearing defect frequencies are characteristic vibration frequencies generated when a rolling element bearing has a localized defect on one of its components (outer race, inner race, rolling elements, or cage). Each component produces a unique frequency based on bearing geometry and shaft speed.

How to identify an outer race fault?

An outer race fault appears at the BPFO frequency and its harmonics (2×, 3× BPFO) in the vibration spectrum. Since the outer race is typically stationary, the impacts produce a clean, evenly spaced pattern. BPFO is the most common bearing defect frequency.

What is the difference between inner race and outer race faults?

Outer race faults appear at BPFO and are usually cleaner in the spectrum because the defect is in the load zone at a fixed position. Inner race faults appear at BPFI with sidebands at shaft speed, because the defect rotates in and out of the load zone.

Why do harmonics matter in bearing analysis?

Harmonics (2×, 3× etc.) of bearing defect frequencies indicate defect severity. Early-stage defects show mainly the fundamental frequency. As damage progresses, more harmonics appear with increasing amplitude. The presence of multiple harmonics suggests an advancing fault.

Where can I find bearing geometry data?

Bearing geometry data (number of rolling elements, ball diameter, pitch diameter, contact angle) can be found in manufacturer catalogs from SKF, FAG/Schaeffler, NSK, Timken, NTN, and others. Many manufacturers also provide online tools and databases. Some vibration analysis software includes built-in bearing databases.

Free Engineering Tool #035

Bearing Defect Frequency Calculator

Calculate BPFO, BPFI, BSF, and FTF from bearing geometry and shaft speed. Includes 2× and 3× harmonics for vibration diagnostics.

BPFO BPFI BSF FTF

Results

BPFO — Outer Race Defect Frequency

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BPFI — Inner Race Defect Frequency

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BSF — Ball Spin Frequency

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FTF — Cage (Train) Frequency

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Shaft Frequency

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Harmonics Table

Frequency	1× (Hz)	1× (Order)	2× (Hz)	2× (Order)	3× (Hz)	3× (Order)

Bearing Defect Frequencies

When a rolling element bearing develops a localized defect, the repetitive impact generates characteristic frequencies that depend on the bearing geometry and shaft speed. The four fundamental defect frequencies are:

Fundamental Train Frequency (FTF)

The cage rotational frequency. An elevated FTF indicates cage wear or damage.

Ball Pass Frequency — Outer Race (BPFO)

The rate at which rolling elements pass a point on the outer race. This is the most common defect frequency as outer race faults are statistically most frequent.

Ball Pass Frequency — Inner Race (BPFI)

The rate at which rolling elements pass a point on the inner race. Inner race faults typically show sidebands at shaft speed around BPFI.

Ball Spin Frequency (BSF)

The rotational frequency of a rolling element about its own axis. Rolling element defects often appear at 2× BSF because the defect contacts both races per revolution.

Note: These formulas assume no slip — actual frequencies may differ by 1–3% due to slip and elastohydrodynamic effects. In practice, use these as approximate markers when searching the vibration spectrum.

Practical Example

Example — Bearing 6205, 1500 RPM

Given: Z = 9, B_d = 7.94 mm, P_d = 38.5 mm, α = 0°, RPM = 1500

Shaft freq = 1500/60 = 25 Hz

B_d/P_d = 0.2062, cos(0°) = 1

FTF = 25/2 × (1 − 0.2062) = 9.92 Hz

BPFO = 9/2 × 25 × (1 − 0.2062) = 89.31 Hz

BPFI = 9/2 × 25 × (1 + 0.2062) = 135.69 Hz

BSF = (38.5/(2×7.94)) × 25 × (1 − 0.2062²) = 58.55 Hz

⚠️ Note: Contact angle α is 0° for deep groove ball bearings. For angular contact bearings it is typically 15–40°, and for tapered roller bearings 10–30°. Using the wrong contact angle will give incorrect frequencies.

Bearing Defect Frequency Calculator | BPFO BPFI BSF FTF

Published by admin on February 15, 2026 February 15, 2026