Rolling contact fatigue analysis of the soft zone for the main bearing in a tunnel boring machine.

• The hardness of the soft zone is much lower relative to the hardened surface of the raceway. • A hierarchical finite element modeling method of the main bearing is proposed. • The principle of selecting the optimum hardened depth is given. • The effect of the soft zone on the contact characteristics and fatigue life of the raceway is investigated. • This analyzing method can quickly and efficiently clarify the contact state in the soft zone during the rolling process. A hierarchical finite element modeling method is proposed to study the problem of fatigue spalling easily due to low hardness in the soft zone for the main bearing of a tunnel boring machine. Quasi-static analyses of the main bearing are performed based on the global symmetric model and verified by relative displacement tests of the rings. The invariant contact pressure is replaced by the rolling contact of the rollers in the local sub-model. The contact load is used as an input to solve the stress field, and then the effects of soft zone and hardened depth on the contact characteristics and fatigue lifetime of the sub-model are investigated. The results show there are stress peaks at the junction of the hardened and soft zones. The effect of hardened depth on the stress distribution and fatigue lifetime is relatively small. However, the optimum hardened depth is influenced by the contact load and the permissible stress of the material. The minimum lifetime of the raceway during rolling occurs in the soft zone. This paper achieves a fast and effective rolling contact fatigue analysis of the main bearing, which provides a theoretical basis for optimizing the main bearing design. [ABSTRACT FROM AUTHOR]