The load carrying capacity of spur gears may be calculated by ISO 6336 using influence factors. The face load factor considers the impact of the non-uniform load distribution over the face width. Even if the gears had perfect geometry, the load would not distribute uniformly along the contact lines. The face load factor depends on deformations of all parts of the containing gearbox and mainly of the teeth, gears and shafts as well as on manufacturing and assembly deviations.