Permissible Working Stress for Gear Teeth

Introduction:
The allowable static stress is the stress at the elastic limit of the material.
Permissible Working Stress for Gear Teeth

• The permissible working stress (σw) in the Lewis equation depends upon the material for which an allowable static stress (σo) may be determined.
• The allowable static stress is the stress at the elastic limit of the material. It is also called the basic stress. In order to account for the dynamic effects which become more severe as the pitch line velocity increases, the value of σw is reduced.

According to the Barth formula, the permissible working stress,

σw = σo × Cv

where
σo = Allowable static stress, and
Cv = Velocity factor.

The values of the velocity factor (Cv) are given as follows :

The following table shows the values of allowable static stresses for the different gear materials.

Note : The allowable static stress (σo) for steel gears is approximately one-third of the ultimate tensile stregth (σu) i.e. σo = σu / 3.