| 摘要 |
PROBLEM TO BE SOLVED: To improve rigidity and productivity and further contrive lightweight by making the outside diameter of the base end to the edge as a lower pole substantially uniform, and using a straight pipe whose base pipe part has multi- pipe structure. SOLUTION: In a lower pole 10, a base pipe part 10a making the height part L1 of about a fifth or less to the entire length of a pole body 1 is made double pipe structure in which a reinforcing inner pipe 2 is adhered by shrinkage fitting on the inside, and the other part is made single pipe structure. In other words, the reinforcing inner pipe 2 (thickness of 2.0-4.5 mm) is inserted in the base part inside of a metal pipe (outside diameter of 190 mm and thickness of 4.5 mm) being a base material in a free state, the base material is drawn down to the outside diameter of 180 mm of the reinforcing inner pipe 2 by hot working, and thereafter the reinforcing inner pipe 2 is inserted in and adhered to the base pipe part 10a by cooling. Next, the upper pole 11 of a smaller diameter than it is continued to the lower pole 10 to be made the pole body 1. Thereby durability is enhanced, in addition, a cost is reduced and further the efficiency of working can be improved.
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