Lewis Equation

Bending stress for gears is given by the Lewis equation.

Approximating the tooth as a rectangular cantilever beam, recall that

where and .

Thus, the Lewis equation is given as:

By similar triangles, we have

With this, we can get the Lewis equation as

where is a form factor. can be found from a table:

To account for dynamic effects, a dynamic factor is used:

where , with being the pitch line (linear) velocity.

, and can be found from a table:

Allowable Stress

Allowable stress can be obtained from:

  • Yield strength for moderate operating conditions
  • Fatigue strength (or endurance limit ) for infinite life bending
  • Safety factor is applied to compute :

Example