| 摘要 |
<p>A construction machine comprising a lower carriage (12); a working attachment (17) attached rotatably to an upper rotating structure (14); and posture detecting means for detecting a posture of the working attachment, characterized in that the construction machine has a control circuit comprising: a rotating direction control valve (29) for controlling rotation of the upper rotating structure; an unloading valve being a cut-off valve (33) adapted for opening and closing motions to control bleed-off of said rotating direction control valve (29), wherein an outlet of a center bypassing oil path for return of a main pressure oil, said center bypassing path extending through a neutral position of said rotating direction control valve (29), and an oil tank (32) are in communication with each other through said cut-off valve (33); a controller (16) adapted to control opening and closing of said cut-off valve (33) on the basis of detection signals from said posture detecting means and rotating operation detecting means (42L,42R), said controller (16) providing an operation command signal to an electromagnetic proportional pressure controlling valve (35) in such a manner that an operation command pilot pressure is provided to said pilot port (34) of the cut-off valve (33). <IMAGE></p> |
