| 摘要 |
PROBLEM TO BE SOLVED: To improve rigidity and productivity and further contrive lightweight by making an outside diameter from the base end to the edge as a lower pole substantially uniform, and using a straight pipe for an inside diameter molded into a small diameter as compared with the other part. SOLUTION: The steel pipe (outside diameter of 190 mm) of the wall thickness of 6.0 mm is made a base material to manufacture a taper steel pipe by spinning processing, and the lower pole 10 of about a half or less length to a entire length L is obtained. Then, the thickness of a base pipe part 10a forming the part L1 of about a fifth or less height to the entire length remains 6.0 mm, tension is increased as the remaining part 10b goes to the edge, and it is molded so as to thin the thickness up to 4.5 mm. Next, an upper pole 11 is continued to the lower pole 10 to form a pole body 10. A step is formed so that the ratio of the outside diameter D2 of the upper pole 11 to the outside diameter D1 of the lower pole 10 is 0.8 or less. Thereby durability can be enhanced, a cost can be remarkably reduced and further the efficiency of working can be improved.
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