The paper deals with a methodological basis for determining the energy resource of mechanical transmission parts for mining machines. Based on the analysis of the accumulation of damage in metal gears, a method of estimating residual life of coarse-toothed wheels by periodically measuring the hardness of the surface layer of the teeth is justified. The regulari- ties in change of hardness of coarse-tooth gear, conditioned by a change in metal strength properties that take into ac- count the micro- and macromechanisms of plastic and elastic deformation, distortion of the metal crystal lattice with forma- tion and movement of vacancies and dislocations. Experimental setup was built and the results of laboratory experiments are given related to the process of destruction of non-standard samples under tension, pure alternating bending, combined bending and tensile loads. The parameters of the samples deformation and changes in the local hardness of the metal in the regions adjacent to the fracture surface are found. Comparison of dimensions and hardness values of the sample al- lows concluding that a larger deformation corresponds to a greater increase in hardness, their limit value for the material being in the fracture zone. It is established that the detected changes in the local hardness occurs in areas of increased stresses above the limit of proportionality and the work of fracture forces attributed to dislocations density adjacent to the fracture plane expressed in terms of hardness increment is constant.