In the motor design, the cooperation between the bearing inner ring and the shaft is comprehensively considered according to the nature, direction, and size of the load, the allowable operating temperature, the size, structure, and accuracy of the bearing, the machining accuracy of the armature shaft, and the structural characteristics. . For the bearing of the traction motor, it is obviously slightly larger than the critical interference between the inner ring and the shaft of the general motor bearing. Although different countries have different calculation methods for the interference fit, the basic factors considered in the calculation are roughly the same, and the calculation results are basically similar, so the actual interference fit level is also similar. However, since most of these calculation formulas are calculated by calculation coefficients extracted from experience, the range of coefficients is often very wide, which makes the calculation results often difficult to explain. To this end, we take the critical conditions proposed in the thermal resistance theoretical model as the basis for calculation, and thus obtain a new and reasonable method for selecting the amount of interference. Under the condition of the upper temperature limit allowed by the bearing, the minimum assembly interference that keeps the interference fit between the bearing inner ring and the shaft unchanged is called the critical interference. This critical interference essentially reflects that the linear expansion coefficient of the bearing steel is 11% higher than that of the armature shaft steel, and in the rated state, the two are in different positions of the bearing operating temperature field, and there is a certain temperature difference. while still maintaining its minimum critical condition in close contact. But this critical condition is only based on the thermal expansion of the inner ring to the state where the interference has just disappeared. The minimum interference of the bearing should also take into account the deformation of the inner ring, the frictional conditions that the inner ring does not move relative to the shaft, the flattening of the machining marks, and the allowable measurement deviation.