The paper presents the relevance of advanced assessment of the technical condition of complex technical systems. One of which is the gearbox of a mining machine. The main reasons for early failures of bearings are shown. The main share is occupied by problems of lubrication, incorrect installation and maintenance. The analysis of types of destruction of gears of mining machines is presented. The main share is occupied by the surface coloring of the teeth and sudden fracture of the tooth as a result of dynamic load. The theoretical basis for determining the heat release of bearings is given. The calculation of heat release is given of a shaft belt conveyor gearbox as an example, which is confirmed by thermographic studies under operating conditions. Justification of criteria for evaluating the technical condition of gearboxes of mining machines in the form of temperature anomalies recorded on the surface of the body, The actual load on the output shaft is taken into account.