Drawing of S-N curve of ISO 6336 standard gear contact strength

20mncr5 hardenability alloy steel, which is commonly used in Europe and China, is selected as the material for tractor gearbox gear design. This material has good hardenability, and can ensure the core hardness and effective hardening depth of the gear after carburizing and quenching.

In the international standard ISO / TS 6336-5, the required contact strength of the standard test gear is σ hlim = 1500N · mm-2. Under this contact strength, the number of stress cycles, that is, the life of the test gear is 3 × 107 times. It is also suitable for the standard design of 20mncr5 material. It can also get its own S-N curve data with the accumulation of experience or experiments.

In ISO / TS 6336-2 standard, the S-N curve is not given directly, but the ZnT curve of life coefficient is given. According to the curve of life coefficient, the S-N curve of contact life can be obtained, as shown in the figure.