Modern gear design must consider different factors which can have an impact on it other than the one included in the Lewis equation as:
- Pitch line velocity: Greater the linear velocity of the gear teeth is and greater is the impact of successive teeth as they come into contact. These impacts are caused by the impossibility to design a absolute perfect tooth profile since deflections are inevitable.
- Manufacturing accuracy: this has a great impact on loading. Moreover, it determines whether teeth share the load when pairs of teeth are theoretically in contact.
- Contact ratio: for gears with 1<G<2, the transmitted load is divided by two teeth pairs each time a contact occurs. It is important to take into consideration that there are two loading conditions:
- Part of the load at the tooth tip
- The whole load at the point of highest single tooth contact.
- Stress concentration
- Degree of shock loading
- Accuracy and rigidity of mounting
- Inertia moment of the gears and attached rotating members: Momentary angular accelerations and decelerations of rotating members are caused by small tooth errors. With large inertia, rotating members tend to resist acceleration, causing large momentary tooth loads. It is possible to reduce the harmful inertia effect using the torsional elasticity.
Moreover, the gear tooth bending fatigue issue needs an evaluation of the following points:
- Fluctuating stresses in the tooth fillet
- Fatigue strength of the material
Gear-bending strength calculations are commonly based on the assumption that the tooth-bending fatigue strength has a normal distribution with one standard deviation being about 8 percent of the nominal endurance limit.