HYDRAULIC CONTROL CIRCUIT FOR CONSTRUCTION MACHINE

摘要

A hydraulic control circuit has a hydraulic cylinder for vertically moving a heavy object, a hydraulic pump driven by an engine as a drive source to supply hydraulic pressure to the hydraulic cylinder, a recovery oil passage which, when the heavy object is lowered, feeds oil discharged from a weight holding-side oil chamber to the oil chamber on the side opposite the weight holding side, and an engine speed setting device. The hydraulic control circuit is adapted to improve fuel consumption and to match lowering speed of the heavy object to a value set by the engine speed setting device. A recovery control valve (13), whose degree of opening is regulated according to a control command from a control device (18), is placed in a recovery oil passage (C). The control device (18) performs engine speed reduction control which, when a boom is lowered, reduces the speed of the engine (E) to or below an engine speed for reduction control, and the control device also performs recovery amount regulating control for increasing or decreasing the degree of opening of the recovery control valve (13) according to a preset target speed set by an accelerator dial (20).