| 摘要 |
PROBLEM TO BE SOLVED: To locally increase the strength of a boom foot part, which requires the high strength, without especially increasing the size and weight of a boom. SOLUTION: A bent part T is formed in an upper plate 22a in a position near the boom foot part of a boom body 22 so as to increase the strength of the boom foot part which serves as a part for coupling the boom 20 to a mounting bracket 21a. In the upper plate 22a with the bent part T, a surface of a part for coupling the upper plate 22a to a boss part 23 is taken as P1; a surface of a part for a shift to an intermediate bent part B is taken as P2; and a surface in a position of the bent part T is taken as P3. In this case, an angle, which a lower plate 22b forms with a line which connects P1 with P2, is taken asθ1, an angle which the lower plate 22b forms with a line which connects P1 with P3, that is, an opening angle between the upper plate 22a and the lower plate 22b on the side of the boom foot part is taken asθ2 and made greater than the angleθ1; and an angle, which the upper plate 22a forms with the lower plate 22b, is made smaller than the angleθ1 in a part from the position of the bent part T to a part provided to continue into the bent part B.
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