Condition monitoring
Bearing defect frequencies: what each ring or part produces
The four formulas and a plain table of what each defect looks like in the spectrum and how it develops.
The four frequencies
Every bearing has four characteristic frequencies that depend on its geometry and the shaft speed. Take the exact values from the bearing maker's data or from the monitoring software. The orders are fixed numbers: they do not change when the speed changes.
Formulas
| Defect | Name | Formula (n = shaft speed in Hz) |
|---|---|---|
| Outer race | BPFO | (Z ÷ 2) × n × (1 − d/D × cos α) |
| Inner race | BPFI | (Z ÷ 2) × n × (1 + d/D × cos α) |
| Cage | FTF | (n ÷ 2) × (1 − d/D × cos α) |
| Ball or roller spin | BSF | (D ÷ 2d) × n × (1 − (d/D × cos α)²) |
The symbols
Z is the number of rolling elements in one row, d is the rolling element diameter, D is the pitch diameter and α is the contact angle. For a rough check, BPFO is about 0.4 × Z × n and BPFI about 0.6 × Z × n.
What each defect looks like
| Defect | Pattern | Typical development |
|---|---|---|
| Outer race | Steady series at BPFO and multiples | Usually the first to show on a fixed load zone. Can be a fast rise under heavy load. |
| Inner race | BPFI with sidebands at shaft speed | The sidebands show the defect passing through the load zone each turn. |
| Rolling element | BSF, often at twice BSF, with cage-frequency sidebands | Harder to see. Look for sidebands at the cage frequency. |
| Cage | FTF and low-frequency modulation | Often follows other damage, or a lubrication failure. |
Four stages of a bearing fault
| Stage | What changes | Best way to see it |
|---|---|---|
| 1 | Microscopic damage below the surface | High-frequency methods such as ultrasound or acoustic emission |
| 2 | Small defects excite natural frequencies of the bearing parts | Envelope analysis and shock pulse methods |
| 3 | Defect frequencies and harmonics appear in the ordinary spectrum | Ordinary spectrum and envelope |
| 4 | The noise floor rises and the pattern turns random. The bearing is close to failure. | Overall vibration and temperature, but too late to plan |
Useful rules
- Always compare the fault in orders, not hertz, when the speed changes.
- A single peak is not enough. Look for the harmonics and the sidebands.
- Confirm every diagnosis by opening the bearing at least a few times, so the data becomes trusted.
Takeaway. Know the four orders of your bearing before you look at the spectrum. They turn a forest of peaks into a short list of suspects.
Put it to work in the tool