Bearing Fault Frequency Calculator
Enter a rolling-element bearing’s geometry — number of balls/rollers, ball diameter, pitch diameter, contact angle — and the shaft RPM to compute the four classic defect frequencies: BPFO (outer race), BPFI (inner race), BSF (ball/roller spin) and FTF (cage), each with its 2X and 3X harmonics. These are the frequencies to look for in a vibration spectrum when you suspect a bearing fault.
ℹ These are the exact published rolling-element formulas, but every result is only as good as the geometry you type in — read the ball count, ball/pitch diameter and contact angle from the bearing manufacturer’s datasheet. This tool deliberately has no built-in part-number database: it will not invent SKF/NSK/FAG/Timken dimensions for you. Results assume the listed geometry is exact and ignore slip, so real fault peaks land near, not exactly on, these values. Matching a spectral peak to a defect frequency is a strong indicator, not a diagnosis — confirm with a calibrated accelerometer and a trained analyst.
Bearing geometry & shaft speed
Fault frequencies & harmonics
No geometry? Rule-of-thumb estimate
When you cannot get the ball/pitch diameter, these approximate shortcuts use only the ball count and shaft speed. They are rough — expect ±10–20 % error — so always prefer the geometry calculator above when a datasheet is available.
How It Works
A rolling-element bearing has four moving parts that each generate a characteristic repetition rate when a localised defect — a spall, pit or crack — is struck by a passing ball or roller. The shaft turns at fr = RPM/60 hertz (the inner race rotates while the outer race is held in the housing). From the geometry the four defect frequencies follow directly, where Nb is the ball count, Bd the ball diameter, Pd the pitch diameter and θ the contact angle:
- BPFO — ball pass frequency, outer race = (Nb/2)·fr·(1 − (Bd/Pd)·cosθ)
- BPFI — ball pass frequency, inner race = (Nb/2)·fr·(1 + (Bd/Pd)·cosθ)
- BSF — ball spin frequency = (Pd/(2·Bd))·fr·(1 − ((Bd/Pd)·cosθ)²)
- FTF — fundamental train (cage) frequency = (fr/2)·(1 − (Bd/Pd)·cosθ)
Each defect frequency comes with harmonics (2X, 3X …) because the impact is a sharp, repeating pulse rather than a pure sine wave, so it is rich in overtones. In practice you look for a peak at the calculated frequency and a train of harmonics above it. An inner-race fault (BPFI) and a ball fault (BSF) also produce sidebands spaced at the shaft speed (BPFI) or the cage rate (BSF), because the defect moves in and out of the load zone once per revolution — the sideband pattern is itself a clue to which element is damaged.
What each defect tends to sound and look like: an outer-race fault is the most stationary — a steady, periodic rumble or growl with a clean BPFO peak and strong harmonics, little sideband activity. An inner-race fault rises and falls once per shaft revolution as it rotates through the load zone, giving a BPFI peak flanked by 1X sidebands and an amplitude that “breathes.” A ball/roller fault produces BSF with FTF-spaced sidebands and can be intermittent as the element re-orients. A cage fault (FTF, always below 0.5X shaft speed) is usually a late-stage, low-frequency looseness that often shows up only as sidebands on the other peaks. These order→fault and sideband→defect associations are widely used diagnostic conventions, not guarantees — bearing slip, multiple defects, mounting and load all blur the picture.
Two honest cautions. First, the calculator assumes the geometry is exact and ideal; in reality the contact angle changes with load and rolling elements slip by a percent or two, so measured peaks land a little off the computed values — analysts search a small window around each frequency, not a single bin. Second, you must verify your inputs: get Nb, Bd, Pd and θ from the bearing datasheet (if you only know bore and OD, Pd ≈ (bore + OD)/2 is a reasonable estimate, Bd less so). We deliberately do not ship a part-number lookup table, because publishing invented SKF/NSK/FAG/Timken geometry would be worse than no data — the geometry calculator is the honest tool here.