Is yielding allowed in gears?

[meemouk]

The codes of practice for bending and pitting do not mention whether the permissible stress is calculated for the yield strength or the ultimate strength of the material. Do you know any resources that explain this please?

 

[tbuelna]

Common design formulas for gear tooth bending and pitting limits are based on fatigue properties for the material and heat treatment used, which is not the same as YTS or UTS. Tooth bending fatigue is usually based on a mean tensile stress at the root fillet, and pitting is usually based on the mean contact stress at the working surface of the tooth flank, for a given number of load cycles. The tensile and surface compressive stress fatigue limits for gear materials are always well below YTS, so you should never experience yielding in a properly designed gear under normal operating conditions. You can find some detailed explanations in the AGMA gear design manuals.

 

[drawoh]

meemouk,

If your gear exceeds yield stress, then it will plasticly deform, changing your tooth profile.

This is aside from anything else that might happen.

--
JHG

 

[meemouk]

Thank you tbuelna and drawoh,

Since fatigue limit is the main deign check for gears, why is it very important to include the yield and tensile strength of the material in gear design software?