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 can an outer race fault appear in a spectrum?

An outer-race defect can produce components near BPFO and its harmonics. FTF or other sidebands can also occur because load, slip, transfer path and modulation affect the spectrum. A frequency match alone does not prove a defect; confirm with envelope analysis, trends and inspection context.

What is the difference between inner and outer race indicators?

Outer-race defects are investigated around BPFO; inner-race defects around BPFI. Shaft-speed modulation is common around BPFI, while BPFO can also show FTF or other sidebands. Interpret the pattern together with loading, measurement position and envelope spectrum.

Why do harmonics matter in bearing analysis?

Harmonics and sidebands can strengthen a bearing-fault hypothesis, but their count is not a universal severity scale or remaining-life calculation. Evaluate amplitude trends, envelope spectra, noise floor, operating condition and corroborating measurements.

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. Peaks near BPFO can support an outer-race fault diagnosis, but occurrence and amplitude depend on load zone, transfer path, slip and measurement position.

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 kinematic formulas assume pure rolling with no slip. Actual frequencies can shift with load, clearance, lubrication, speed and slip; use calculated values as search markers rather than exact proof of a defect.

Practical Example

Worked example — stated geometry, 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) = 89.30 Hz

BPFI = 9/2 × 25 × (1 + 0.2062) = 135.70 Hz

BSF = (38.5/(2×7.94)) × 25 × (1 − 0.2062²) = 58.03 Hz

⚠️ Note: Use the bearing manufacturer’s internal geometry and contact angle for the exact bearing variant. Do not infer contact angle or rolling-element dimensions from the designation alone.