11.3 Choosing Bending/Pitting safety factors

An extra margin of safety can be specified by assigning SF > 1.0 and/or SH > 1.0. Since pitting fatigue is slowly progressive, and pitted gear teeth usually generate noise which warns the gearbox operator that a problem exists, pitting failures are not usually catastrophic.Bending fatigue frequently occurs without warning and the resulting damage may be catastrophic.

The safety factors should be chosen with regard to the uncertainties in the load and material data and the consequences of a failure. Small safety factors can be used where the loads and material data are known with certainty and there are small economic risks and no risk to human life. However, if the loads and material data are not known with certainty and there are large economic risks or risks to human life, larger safety factors should be used. The bending fatigue safety factor is frequently chosen greater than the pitting safety factor (SF > SH) since bending fatigue may be catastrophic. However, SF should not be too large because it leads to coarse-pitch teeth which may be noisy and prone to scoring failures.

Choosing a safety factor is a design decision that is the responsibility of the engineer. It must be carefully selected accounting for the uncertainties in:

Consider the need to conserve material, weight, space or costs. Most importantly, consider:

11.3.1 Factor for minimal normal tooth thickness at tip

This is the multiple of normal module which must exist at the tip. This factor is used to warn against pointed tip designs.