To estimation of contact-fatigue durability of thermo-chemically strengthened gears

Introduction. The specification of the strength characteristics of the diffusion layer of carburized (C) and nitro-carburized (NC) involute gears is described. Regulated by GOST 21354-87, the dependence of the surface endurance limit of steel parts on hardness in the region of finite (up to 10-12 million cycles) life is overstated. The work objective is to determine the maximum allowable load level of the involute gearing under the conditions of alternative failure forms and the area of correct application of GOST 21354-87 recommendations. Materials and Methods. In the case-hardened parts, both deep and surface processes of contact fracture occur simultaneously. However, these processes have essentially different mechanisms of contact-fatigue durability with fundamentally different mechanisms of the contact-fatigue life. The high-end fracture mode is determined by a variety of factors, the key ones being the level of intensity and the properties of the near-surface layers of the material. The loading capacity of the diffusion layer is investigated on the basis of the experimental data by the strength assessment procedure of a structurally inhomogeneous material. Here, Pisarenko-Lebedev generalized criterion of limiting state is used. Research Results. Consideration of the effect of the plastic properties of the material and its variation within the diffusion layer has allowed significantly increase the forecasting validity of the loading capacity level of the carburized and nitro-carburized gears for contact strength. The occurrence of the threshold value of the working surface hardness of the wheels teeth is confirmed at the level of 60-61 Rockwell units. This level increase leads to the decrease in contact-fatigue life. The plasticity parameter impact on the capability level and the variation of this parameter value for different types of the thermo-chemical treatment (TCT) is assessed. In the finite life area, deep contact fractures are leading. And the limits of contact fatigue life upon surface pitting readings should be oriented to the allowed normal deep contact stress. Discussion and Conclusions. The obtained results are in qualitative agreement with the experimental data, but they exceed them a little in some cases. Studies on the plasticity parameter value and its variation depending on the chemical composition of the steel and the TCT type are needed. But already at this stage, the results obtained can be used in the design of heavy-loaded case-hardened gears to determine the surface hardness and the diffusion layer thickness. The application domain includes the tractor track gear transmissions, oil-producing beam engines, mining and handling facilities.