When the assembly is installed, the position of the positive case shall be changed with the rotor axis as the reference. The axial insertion depth and radial clearance of the air inlet or sealing ring of the casing and the inlet ring of the impeller shall be adjusted to the scope specified in the accompanying documents. The interface between the impeller of the centrifugal fan and the air inlet is in the form of a mouth-to-mouth and a socket. The axial clearance in the counterpart form is generally less than 0.008 to 0.012 times the diameter of the impeller, but the radial one-side clearance is not more than 0.0015-0.0040 times the diameter of the impeller. The shaft hole of the rear side plate of the casing shall be coaxial with the main shaft and shall not be contacted with each other.