AiVibeQuality, Reliability & Maintenance
Bearing Fault Frequencies (BPFO, BPFI, BSF, FTF)
Bearing fault frequencies are the characteristic frequencies at which defects on the outer race, inner race, rolling elements or cage of a rolling-element bearing generate impacts, calculated from shaft speed and bearing geometry.
When a rolling element passes over a localised defect, it produces a short impact, and because bearing kinematics are regular, these impacts repeat at predictable rates. The ball pass frequency of the outer race (BPFO) applies to outer race defects, the ball pass frequency of the inner race (BPFI) to inner race defects, the ball spin frequency (BSF) to rolling element defects, and the fundamental train frequency (FTF) to the cage. Each is calculated from shaft speed, the number of rolling elements, rolling element diameter, pitch diameter and contact angle.
Analysts compare peaks in vibration spectra, and especially in envelope spectra, with these calculated frequencies to identify which bearing component is damaged. Inner race defects often appear with sidebands spaced at shaft speed, because the defect rotates in and out of the load zone. Bearing manufacturers and condition monitoring software provide fault frequencies for catalogued bearings, often expressed as multiples of shaft speed. The frequencies are generally not integer multiples of running speed, which helps distinguish bearing faults from unbalance or misalignment.
Calculated frequencies are approximate because rolling elements slip slightly, so measured peaks may differ a little from theory. Correct bearing identification and accurate speed are essential, and variable-speed machines need speed measurement or order analysis. Early defects produce weak signals buried in other vibration, which is why demodulation techniques such as envelope analysis are used. Bearing replacement is governed by the OEM documentation and bearing manufacturer guidance.
Key points
- BPFO and BPFI relate to outer and inner race defects.
- BSF relates to rolling elements and FTF to the cage.
- Calculated from shaft speed and bearing geometry.
- Non-synchronous frequencies help separate bearing faults from other faults.
Where AiVibe comes in
AiAmbA AI Factory use cases include predictive maintenance and visual inspection on textile, automotive and electronics lines. AiAmbA IoT normalises industrial signals from protocols such as OPC UA and MQTT, with driver availability confirmed per installation.
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