The factor of safety (FoS) in actuator sizing serves a specific purpose: it provides margin for forces and conditions that are not fully captured by the model. The right FoS depends on how many of those forces you have actually modeled.
Simple mode — FoS 2.0 or higher:
The Simple calculator models only bearing friction and inertia. It does not account for web tension lateral forces, umbilical drag, floor grade, or bearing misalignment. An FoS of 2.0 compensates for these unmodeled terms. If you suspect unusually high secondary forces (heavy cable bundles, steep floor grade, old bearings), consider FoS 2.5 or higher.
Detailed mode — FoS 1.5 to 2.0:
The Detailed calculator models all six force terms explicitly: bearing friction, inertia, web tension lateral component, umbilical drag, floor grade, and misalignment. Because the model is more complete, a lower FoS is justified — typically 1.5 to 1.75. An FoS of 2.0 in Detailed mode is conservative and appropriate for critical applications or uncertain input values.
What the FoS covers in each case:
- Simple mode FoS — compensates for missing force terms plus measurement uncertainty
- Detailed mode FoS — compensates primarily for measurement uncertainty, bearing condition variation, and transient load spikes
A common mistake is applying an FoS to compensate for terms you should be modeling. If you know the web tension and wrap angle, model the lateral force directly in Detailed mode rather than inflating the FoS in Simple mode. The FoS should cover unknowns, not missing terms.
When in doubt, use the actuator sizing calculator in both modes and compare results. If they diverge significantly, the secondary forces are meaningful and Detailed mode gives the more accurate answer.