Why is the average force not enough for a variable-duty bearing-life calculation?

LILY Bearing | 2026-10-28

Best answer

Keep each load segment instead of averaging the story away

A machine's duty record should identify load magnitude, direction, speed and duration for each significant operating segment.

In a basic ball-bearing fatigue example at constant speed, the load weighting involves the third power of equivalent load, rather than a simple force average. Higher-load segments can consequently matter much more than their share of operating time suggests. If speed changes, time shares and revolution shares are no longer the same thing. The correct duty model must therefore be selected before the segments are combined. A daily average force is insufficient evidence when it conceals short but repeated heavy-load periods.

Compare two averages using an explicitly simplified example

Assume the equivalent load is 1 kN for 90% of revolutions and 3 kN for the remaining 10%, with constant speed and a valid ball-bearing basic-life model.

The arithmetic force average is 1.2 kN. The cubic mean is [0.9 × 1³ + 0.1 × 3³]^(1/3), approximately 1.533 kN. That difference shows why the load history cannot always be replaced by an ordinary average. These numbers are an editorial calculation, not machine measurements. Keep shock checks, lubrication conditions and load-direction changes separate where they fall outside this simplified model, and have the actual duty calculation checked for the specific bearing arrangement.