Abstract The article discusses the factors affecting the operation of sliding bearings. It is shown that based on the fundamental principles of hydrodynamic lubrication theory, transitioning from the formula characterizing the load-carrying capacity of the lubricant layer in sliding bearings to the parameters of oil film thickness and clearance allows for the derivation of expressions to determine functional clearances within which the bearing will operate under conditions of hydrodynamic friction. As a result of the calculations conducted for the sliding bearing of the conveyor drive reducer the limiting functional clearances were defined for the use of bushings of various lengths in accordance with GOST ISO 4379–2006. A rational length for the sliding bearing trunnion is justified based on the value of the maximum functional clearance and its influence on the deviation of the center distance in the gear transmission of the reducer. A new H8/f7 fit is proposed instead of the recommended H8/g7 and H8/e7 fits according to GOST ISO 4379–2006, as this will ensure the presence of a hydrodynamic wedge at the beginning of operation and provide the most rational material reserve for wear. The practical significance of the research lies in the applicability of the proposed approach in the design of sliding bearings operating under conditions of hydrodynamic lubrication, aimed at determining the rational length and fit of the mating elements in the bearing assembly.