The loading conditions assumed by the original Lewis equation are very conservative.
A modification that results in a more realistic situation was made by Dudley
(Reference 3), that takes into account multiple teeth sharing load. When the contact ratio factor is added as well, the modified Lewis equation becomes:

W_t   =  m_pSFY   ₍    600    ₎            for steel gears                              (54)
             P_d                600+V 
where the contact ratio m takes into account the fact that when the load is at the tip of the tooth, it is shared by a second pair of teeth.

The following tables are useful in determining gear load ratings:
Table 1.15                 :  Ratings for steel spur gears
Tables 1.16 & 1.17   :  Ratings for small-pitch spur gears
Table 1.18                  : Ratings for hardened steel helical gears
Tables 1.19 & 1.20    : Ratings of worms and worm gears.

13.2 Dynamic Strength
Equations 52 and 54 give adequate results for gear meshes that are in a static situation. When gears are in action, however, tooth loading is greater than the static value due to dynamic effects. In a gear system, dynamic forces arise from a combination of the masses involved, their elasticity and the forcing function representing the prescribed motion. Inaccuracies in gear-tooth profiles cause accelerations and decelerations during gear action which reflect as inertia forces, and can greatly exceed static tooth loading. The severity of dynamic forces is a function of pitch-line velocity and tooth errors.An accurate prediction of dynamic forces is very difficult. Various factors and formulas have beer, devised to increase the static tooth force to a value that safety represents the dynamic condition. A

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