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Bearing defect frequencies & ISO severity

Enter a bearing designation and a shaft speed to get BPFO, BPFI, BSF and FTF. Enter a measured velocity to get its ISO 10816 / 20816 zone. Every geometry value is editable, because a 6205 is not the same bearing at every manufacturer.

Part 1 · Defect frequencies

Which frequency belongs to which fault.

Bearing & shaft

TYPE A DESIGNATION, OR ENTER GEOMETRY DIRECTLY

Designation

Shaft speed rev/min

Rolling elements count

Element diameter mm

Pitch diameter mm

Contact angle degrees

Geometry differs between SKF, FAG, NSK and others for the same designation. For anything critical, take the four values from your bearing's own datasheet.

DEFECT FREQUENCIES

Shaft rotation – Hz

BPFO — outer race

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Usually the first defect to appear. Outer race is stationary and loaded in one zone.

BPFI — inner race

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Sidebands at shaft speed are the giveaway — the fault passes through the load zone once per revolution.

BSF — ball spin

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Often appears with FTF sidebands. Frequently shows at 2×BSF, since the defect strikes both races.

FTF — cage

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Always below shaft speed. On its own it usually signals cage wear or looseness rather than a race defect.

Values in Hz. Multiply by 60 for CPM. Envelope (demodulated) analysis detects these long before they appear in a plain velocity spectrum.

Part 2 · Severity

You measured 4.2 mm/s. Is that a problem?

Enter an overall velocity reading (RMS, 10–1000 Hz) and pick the machine class. The answer is a zone, and each zone maps to a decision — not just a colour.

Machine class

Measured velocity mm/s RMS

0–––

Picking the right class

Your machine	Use
Pumps, small motors, spindle drives up to ~20 hp	Class I
Motors and machines 20–100 hp on standard mounts	Class II
Large motors, compressors, mill drives on rigid concrete foundations	Class III
Turbines, turbo-compressors, large sets on flexible or spring mounts	Class IV
Gearboxes 15–300 kW, grouted to a rigid base	Group 2, rigid
Gearboxes and drives 15–300 kW on skids or soft mounts	Group 2, flexible
Mill drives, large gearboxes, sets above 300 kW	Group 1

Rule of thumb for support: if the first natural frequency of the machine-plus-foundation is above running speed, treat it as rigid. Concrete plinth — rigid. Steel skid or anti-vibration mounts — flexible.

On the standard: ISO 10816-3 has been superseded by ISO 20816-3, which keeps the same evaluation zones. Zone boundaries above are the commonly cited values for these classes; the standards themselves are published by ISO and should be purchased for compliance work. This tool computes and interprets — it does not reproduce them.

Reading the result

A number is a snapshot. A trend is a decision.

TREND BEATS THRESHOLD

2.0 rising is worse than 4.0 steady

A machine that has sat at 4.0 mm/s for two years is telling you less than one that moved from 1.8 to 2.6 in a month. Absolute limits catch the obvious; slope catches the developing.

OVERALL HIDES DETAIL

A healthy total can hide a screaming BPFO

Overall velocity is broadband. An early outer-race defect can add very little to the total while being unmistakable in the envelope spectrum at the frequency you just calculated above.

SPEED CHANGES EVERYTHING

Variable-speed spindles need live RPM

Every frequency on this page scales with shaft speed. On a spindle that runs 800–12,000 rev/min, defect bands move constantly — which is why we read live RPM from the controller rather than assuming it.

From calculator to continuous

This is one reading. MachineWise watches these frequencies every minute.

You just did by hand what our vibration monitoring does continuously on every critical bearing — tri-axial sensing, live FFT with envelope analysis, per-machine learned baselines, and a plain-language alert naming the component and an estimated time to fault.

See vibration monitoring → Bearing life tracking