Contact fatigue limit prediction method for heavy-duty gears considering hardness gradient characteristics

Hardening gradients have been introduced to heavy-duty gears to enhance their bearing capacity, but this has led to an increase in the complexity and difficulty of fatigue analysis. To explore ways to mitigate this, in this work, a contact fatigue model of an 18CrNiMo7-6 gear pair with a hardness gradient was established, and the Brown‒Miller‒Morrow multiaxial fatigue criterion was adopted to calculate the contact fatigue lifespan under different contact pressures. The gear contact fatigue limit was then determined by fitting the stress–number of cycles (S‒N) curve. The results of this method are verified by gear contact fatigue tests. Furthermore, the effects of case hardening depth (CHD) and surface hardness (SH) on the contact fatigue lifespan and limits were explored, and we found that the contact fatigue limit of a typical carburized gear was more affected by SH than CHD. In addition, we also examined which hardness gradients were more beneficial for the fatigue performance of gears with atypical hardening gradients.