Patent ID: 7913490

Claim:
A hydraulic circuit for a construction machine, comprising: first to third hydraulic pumps; a first switching valve composed of valves installed in a flow path of the first hydraulic pump and shifted to control hydraulic fluid fed to working devices; a second switching valve composed of valves installed in a flow path of the second hydraulic pump and shifted to control hydraulic fluid fed to working devices; a third switching valve composed of valves installed in a flow path of the third hydraulic pump and shifted to control hydraulic fluid fed to working devices; a fourth switching valve composed of valves installed on upstream sides of the flow paths of the first and second hydraulic pumps, respectively, and shifted to control hydraulic fluid fed to left and right traveling devices; a confluence switching valve installed on a downstream side of the flow path of the third hydraulic pump and shifted by a signal pressure fed to a signal line to selectively supply the hydraulic fluid from the third hydraulic pump to either the working devices on the first hydraulic pump side or the working devices on the second hydraulic pump side; a signal line for traveling devices which is connected to the signal line for the confluence switching valve and in which a signal pressure is formed when the fourth switching valve is shifted; signal lines for working devices which are connected to the signal line for the confluence switching valve and in which a signal pressure is formed when any one of the first to third switching valves is shifted; a shuttle valve, installed at an intersection of a flow path that is connected to a signal line connected to the signal line for the traveling devices and the signal line for the confluence switching valve and a flow path that is branched and connected to the signal line for the confluence switching valve and the signal line for the working devices, for selecting any one of the signal pressure formed in the signal line for the traveling devices and the signal pressure formed in the signal lines for the working devices; and a signal pressure output port installed in the signal line for the traveling devices so that the signal pressure formed in the signal line for the traveling devices can be used as the signal pressure for travel boosting or travel alarming.