Tools · Diagnostics
Bearing defect frequencies
Where to look in the spectrum for outer race, inner race, rolling element and cage defects on a rolling element bearing.
Defaults are approximate figures for a 6205 deep-groove ball bearing. Get exact geometry from the manufacturer.
| Frequency | Hz | CPM | Orders (×) |
|---|
Harmonics (Hz)
| 1× | 2× | 3× | 4× |
|---|
The formulas
FTF = (fr/2) · (1 − (d/D)cosθ)
BPFO = (n·fr/2) · (1 − (d/D)cosθ)
BPFI = (n·fr/2) · (1 + (d/D)cosθ)
BSF = (D/2d) · fr · (1 − ((d/D)cosθ)²)
These assume pure rolling with no slip. In practice the measured peaks usually sit slightly off the calculated values (a percent or two), so look for a peak near the line with its harmonics and sidebands rather than an exact match.
- BPFO (outer race): usually a clean series of harmonics, since a stationary outer-race defect is always in the load zone.
- BPFI (inner race): the defect rotates in and out of the load zone, so expect sidebands at 1× shaft speed.
- BSF (rolling element): often appears at 2×BSF, because a ball defect hits both races each spin, with sidebands at FTF.
- FTF (cage): sub-synchronous, usually around 0.4× shaft speed. Rarely a peak on its own; shows up as sideband spacing.