Boom of a working machine

摘要

A main front boom member (second boom member) (8) includes a pair of side plates (20) configured to sandwich a distal end of a main boom member (first boom member) (6) from both sides of the main boom member in a horizontal direction at base ends of the side plates. The pair of side plates are connected to each other in the horizontal direction by a rigid member (23), and a closed cross-section portion is defined by the rigid member and the side plates. Side-plate extending sections are formed by extending the side plates from the connection-pin insertion sections in a direction away from the boom connection pin (horizontal shaft). A cylinder attachment part, through which a cylinder supporting pin (27) extends, is connected to the side-plate extending sections. The cylinder supporting pin supports one end of the boom cylinder such that the boom cylinder (11) is rotatable. At least a portion of the cylinder supporting pin is disposed closer to the main boom member (6) than the closed cross-section portion in a linear direction connecting the boom foot pin (first-boom member base-end pin) and the horizontal-shaft insertion sections.