PURPOSE: To facilitate assembly and disassembly of a sealing for the lower main shaft by such a construction that the conical baffle part of a draft tube is provided separate from he draft tube.

CONSTITUTION: A draft tube 5 and a conical baffle part 6 thereof are provided separately and jointed with bolts. First a conical baffle part is formed in a draft tube 5, and a main shaft 3 equipped with an upper runner 1 and a lower runner 2 is incorporated, and then a sealing 6 for the main shaft is installed at the draft tube 5. In this condition, the conical baffle part 7 is hung at the lower runner side, and thereafter inside the sealing 6 are installed a lower packing 8, packing holder 11, packing 9, holder 12, packing 10, and cover 13 one after another. Thus assembly of lower shaft sealing can be made one after another from parts situated below in the same manner as in assembling the upper sealing.