To ensure the accuracy of the size of the core file, otherwise affect the wear axis. Bearing gear size accuracy to meet the requirements of the drawings, or the impact of assembly, bearing diameter is too small to make the bearing loose, too general assembly difficulties or parts damaged or temperature rise too high. Bearing block and shaft gear are coaxial, will produce vibration and noise should not exceed the difference degree of symmetry. Otherwise it will cause the difficulty of assembling the distance between the two bearing gears (i.e., the shoulder distance) to be accurate, otherwise it will cause the rotor core to produce axial offset or bearing card dead phenomenon.