Can average bearing load be used directly in a fatigue-life calculation?

The short answer

Not generally. Bearing fatigue damage is nonlinear in load, and changing speed changes the number of loaded revolutions. A simple average can underrepresent short, heavier loading periods. Wind-generator bearing review should retain a load-duration or load-revolution spectrum under the selected calculation method.

See why the average can mislead

For an explicitly hypothetical constant-speed ball-bearing example using cubic load weighting, equal time at 1 and 3 kN gives an equivalent load of the cube root of 14, about 2.41 kN. The arithmetic average is 2 kN. This example only demonstrates nonlinear weighting. It does not include lubrication, contamination, reliability adjustment, oscillation or a real wind duty and is not a MARSTON life calculation.

Retain the operating exposure

If speed varies, equal hours need not mean equal numbers of contact cycles. Record the pairing between load and speed rather than average each independently and assume the correlation disappears. The duty definition should also identify stationary heavy-load cases requiring a separate static assessment. Schaeffler’s equivalent-operating-value treatment makes those distinctions explicit; the chosen fatigue exponent and adjustment terms must match the actual bearing and calculation basis.

Ask for the spectrum behind a life claim

Request the load/speed bins, exposure fractions, equivalent-load equation and treatment of rare or excluded events. Check that the fractions cover the stated duty and that units remain consistent. If only an average bearing load is supplied, ask what evidence justifies that simplification. Do not invent a wind-duty factor, turn the illustrative equivalent load into years or promise a bearing replacement interval without the exact equipment and application assessment. The spectrum should identify whether percentages represent time or revolutions. Those fractions coincide only under appropriate speed assumptions, and confusing them can change the calculated equivalent load without any arithmetic error in the formula.