| 摘要 |
PURPOSE:To obtain a heavy drilling mode with a sense of powerfulness of a machine by providing means for increasing relief set pressure respectively on an LS relief valve and a main relief valve arranged on a pump discharge circuit, and operating the main relief valve when a negative load pressure to be applied to an actuator is increased. CONSTITUTION:When an operator sets a switch 26 ON, a solenoid of a solenoid valve 23 is excited and pressure oil sent from a hydraulic pump 24 is applied to an LS relief valve 21 through a pilot circuit 25. A relief set pressure of the LS relief valve 21 gets higher than that of a main relief valve 22. In case that a negative load pressure to be applied to an actuator is increased and a discharge pressure of a pump 1 exceeds a specified value, the main relief valve 22 is operated, while the LS relief valve 21 is not operated. A differential pressure between a pump discharge pressure Pp and a pilot pressure PLS, therefore, todes not exceed a set pressure of a regulator valve 17. The pump 1 is thus driven with a maximum tilting plate angle, and cut-off, is not carried out. |
