| 摘要 |
PROBLEM TO BE SOLVED: To industrially produce a drop shaft as much as possible. SOLUTION: It is possible to industrially produce the drop shaft 1 by saving manual operation as the drop shaft 1 is constituted by constituting a vertical pipe conduit of concentric inner and outer double pipes 3, 5, forming spiral grooves 7, 8 by blow-molding under a cold parison method on an inner surface of the outer pipe 3 and an outer surface of the inner pipe 5 and screwing and fitting an outer peripheral edge 6 and an inner peripheral edge of a spiral guide plate 4 in these spiral grooves 7, 8.
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