The question a bearing calculator cannot answer
Manufacturer tools answer “I have this bearing, what are its frequencies?”. In the field the problem runs the other way: you already have a peak at 237 Hz and you need to know what could be sitting there. This sheet builds every frequency your machine is expected to produce and sorts them by frequency — the order you actually search in.
It also does something no manufacturer tool does: it flags collisions. When two expected frequencies land within a hair of each other they read as a single peak, and the diagnosis becomes a coin flip between two faults with completely different costs.
Your machine
Bearings, by measurement point
Bearing not in the list? Enter its geometry
Define it once and it becomes selectable at all four points. These four numbers are all the frequencies need, and none of them are printed in a commercial catalogue — you take them off the bearing.
Units do not matter: every formula depends on Dw/Dm, which is a ratio. Millimetres or inches give the same answer as long as both use the same one. Contact angle is 0° for deep groove ball bearings.
When two frequencies collide, how do you tell them apart?
A collision is not a dead end — it is a question with three answers, and none of them needs extra hardware.
| Test | What it separates |
|---|---|
| Change the speed | Blade pass, harmonics and every mechanical frequency track shaft speed exactly. On a direct-on-line motor, 2×FL stays fixed even if mechanical speed drifts slightly; with a VFD the electrical frequency moves with the drive output and this test loses its fixed reference. Bearing frequencies move, but not exactly — they carry slip. |
| Look for sidebands | BPFI usually arrives modulated: sidebands spaced 1× on both sides, because the defect enters and leaves the load zone once per revolution. A pure blade pass does not require those sidebands — but eccentricity, aerodynamic modulation or rotor-stator interaction can also produce them around blade pass. It is evidence, not proof. |
| Look at the high-frequency band | Elevated kurtosis and crest factor support the hypothesis of impacts, typical of certain bearing defects. They are not exclusive to bearings: looseness, rubbing and other impulsive phenomena raise them too. This test works even without a spectrum. |
The 53 bearings in the selectors carry factors verified one by one against SKF Product select, the manufacturer’s own tool. They are the same values published in the bearing fault frequency table — one source, two views. Element counts are never interpolated: they are not monotonic with size.