Bearing fault frequency calculator (BPFO, BPFI, BSF, FTF)
Enter the speed and bearing geometry and see where to look in the spectrum for inner race, outer race, ball and cage faults. If you do not have the geometry, use the rule of thumb.
Take the bearing geometry (n, Bd, Pd, φ) from the manufacturer's catalogue; the internal geometry of the same bearing number can vary by manufacturer and series.
Estimate the pitch diameter
Pd ≈ (bore + outside diameter) / 2 is only a rough approximation; the element diameter Bd cannot be found this way.
Enter the values to see a result.
This calculation assumes no slip and ideal geometry; real frequencies can deviate by 1–2% because of slip, load and mounting. A peak at these frequencies is not proof of a fault on its own; this is a preliminary assessment.
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How to use it
A
Enter the speed (rpm or Hz) and choose which ring rotates.
B
Take the number of elements, element diameter, pitch diameter and contact angle from the bearing catalogue, or pick the rule of thumb.
C
Look for the frequencies, harmonics and sidebands in the table in your own spectrum; the chart shows where to look.
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What do the four fault frequencies tell you?
When a defect (crack, dent, spall) in a bearing is rolled over by each rolling element it produces a small impact; the time between impacts depends on where the defect is. BPFO is the outer race defect frequency, BPFI the inner race, BSF the ball or roller defect, and FTF the cage rotation frequency (cage problems or heavy slip).
For a rotating inner ring with a stationary outer ring, BPFO plus BPFI equals n×f; that is where the 0.4 and 0.6 rule comes from. A ball defect often shows up at 2×BSF, because the ball strikes both races, modulated at the cage frequency (FTF).
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Harmonics and sidebands
Impacts are impulses, not sine waves, so peaks also appear at multiples of the fundamental (harmonics); in an advanced fault three, four or more harmonics can be seen. The table lists the first five.
A defect passing through the load zone is amplitude-modulated. A defect on the rotating ring shows sidebands spaced at the running speed on both sides of the main peak; a ball defect shows sidebands spaced at the cage frequency. The presence of sidebands strengthens the case that a peak really belongs to a bearing defect.
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Reading the results correctly
The calculated values rest on ideal kinematics. In reality slip, load and changes in contact angle can shift the measured frequency by a few per cent, so look in a narrow band around the calculated value and check the peak is not confused with running-speed multiples (1×, 2× …).
Take the geometry from the manufacturer's catalogue. The internal geometry of the same bearing number can vary by series, so this tool does not supply a bearing list. For early-stage, high-frequency faults envelope analysis is used; this tool only gives the frequencies to look for, not a diagnosis.
FAQ
- Is the data I enter sent anywhere?
- No. The calculation runs entirely in your browser; values are not sent to any server or stored.
- What are BPFO, BPFI, BSF and FTF?
- They are the characteristic frequencies of bearing defects: BPFO for the outer race, BPFI for the inner race, BSF for a ball (roller) defect and FTF for the cage. Each depends on the bearing geometry and speed.
- Where do I find the bearing geometry?
- In the bearing manufacturer's catalogue or an online bearing database: number of elements, element diameter, pitch diameter and contact angle. Because the internal geometry of the same bearing number can differ between manufacturers, this tool does not provide example bearings.
- What can I do if I do not know the geometry?
- In rule-of-thumb mode BPFO ≈ 0.4·n·f and BPFI ≈ 0.6·n·f are calculated from the number of elements and the speed. The rule is a good starting point for bearings with 6–12 elements but is not exact; BSF cannot be calculated with it.
- I see a peak at a calculated frequency; is the bearing faulty?
- Not by itself. You also need harmonics and sidebands, growth of the peak over time, and to rule out other sources (gears, electrical, running-speed multiples). Treat the result as a preliminary assessment.
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