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.
Results
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
Given: Z = 9, Bd = 7.94 mm, Pd = 38.5 mm, α = 0°, RPM = 1500
Shaft freq = 1500/60 = 25 Hz
Bd/Pd = 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.
Bearing defect frequencies are characteristic vibration frequencies generated when a rolling element bearing has a localized defect. Each bearing component — outer race, inner race, rolling elements, and cage — produces a unique frequency pattern based on bearing geometry and shaft speed.
The four key frequencies are: BPFO (Ball Pass Frequency Outer), BPFI (Ball Pass Frequency Inner), BSF (Ball Spin Frequency), and FTF (Fundamental Train/Cage Frequency).
An outer-race defect can produce components near BPFO and its harmonics. FTF or other sidebands can also be present because load, slip and modulation affect the response.
A frequency match alone is not proof. Confirm with envelope analysis, trends, operating condition and inspection context.
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 both patterns together with load, measurement position, envelope spectrum and trends.
Harmonics and sidebands can strengthen a bearing-fault hypothesis, but their count is not a universal severity scale or remaining-life calculation.
Evaluate amplitudes and trends together with envelope spectra, noise floor, operating condition and corroborating measurements.
Use geometry supplied by the bearing manufacturer or a verified bearing database for the exact designation and internal design suffix.
Bore and outside diameter do not uniquely determine rolling-element diameter, pitch diameter or contact angle. If exact geometry is unavailable, report the calculated frequencies as an estimate and validate against measured spectra; do not present inferred geometry as manufacturer data.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional vibration analysis instruments for bearing diagnostics, field balancing, and condition monitoring. Used in 50+ countries.
