| 摘要 |
PROBLEM TO BE SOLVED: To improve durability of a bending portion even when a boom is lightened by forming a rib due to a recess or a projection on right and left side plates of the bending portion in the longitudinal direction of the boom. SOLUTION: Ribs 27A, 27B due to a recess or a projection are formed on right and left side plates of a bending portion 10C in the longitudinal direction of the boom to moderate stress concentrating to the bending portion 10C due to a reinforcing action by the ribs 27A, 27B. In other words, the stress concentration to the bending portion 10C is moderated by the ribs 27A, 27B even when a conventional plate material for blocking the inside of a cylinder of the bending portion is not provided, and the boom is lightened by eliminating the plate material. A cross section of the ribs 27A, 27B may be semi circular arc as well as trapezoidal, and number of the ribs 27A, 27B may be two or more. When the ribs 27A, 27B are structured by forming folding stage portions 32A, 32B crossing the longitudinal direction of the boom on the right and left side plates, a twisting rigidity of the bending portion 10C can be improved.
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