Why should radial and axial bearing loads not simply be added?

The short answer

Because radial and axial forces load a bearing’s contacts differently. Their fatigue or static effect is represented by the bearing-maker’s applicable equivalent-load relationship, not usually by arithmetic addition. A wind-generator bearing calculation must use the selected bearing geometry, arrangement and load conditions.

Keep physical forces and equivalent load distinct

Radial and axial components are real directional forces in the assembly. Equivalent bearing load is a model quantity used for a stated calculation. It is not necessarily their vector resultant or their scalar sum. The coefficients and any regime boundaries depend on bearing type and manufacturer method. A formula copied from another product can produce a plausible number with no valid relationship to the selected bearing.

Check the arrangement before choosing factors

An angular-contact pair can distribute axial and radial loading differently from a single radial bearing. Internal axial forces and ring support can also affect the individual bearing loads. Begin with the actual support reactions, then apply the appropriate method to each bearing or assessed set. If the bearing catalogue requires a ratio-dependent factor, do not select that factor using forces measured at an unrelated rotor or housing boundary.

Request a reproducible calculation

Ask for the full bearing identity, radial and axial load definitions, relevant factor table and equation under the applicable catalogue issue. Distinguish the dynamic equivalent load from the static equivalent load if both are used. NSK and Schaeffler describe manufacturer-based bearing calculations; neither confirms MARSTON hardware. The immediate decision is whether the submitted calculation represents the real contact arrangement, rather than whether adding two headline force values produces an apparently conservative result. Where a calculation changes factor regime at a load ratio, retain the ratio and selected branch. That makes the calculation reviewable and prevents a copied coefficient from silently surviving a changed axial-load case.