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 :