Patent ID: 8607557

Claim:
A hydraulic control system for an excavator including an upper swing structure, a lower driving structure, first and second hydraulic pumps which provide fluid pressure to a plurality of working device actuators including a boom cylinder installed on the upper swing structure, an arm cylinder, a bucket cylinder, and a swing motor, the hydraulic control system comprising: at least one first working device control valve having a first traveling control valve and a first boom control valve successively installed along a first center bypass line from a downstream side of the first hydraulic pump; at least one second working device control valve having a second traveling control valve and a second boom control valve successively installed along a second center bypass line from a downstream side of the second hydraulic pump; a third hydraulic pump providing fluid pressure to the swing motor that is installed on a third center bypass line; a swing control valve installed on a downstream side of the third hydraulic pump and shifted, in accordance with a valve switching signal input from the outside, to supply hydraulic fluid discharged from the third hydraulic pump to the swing motor; a confluence control valve connected to and installed on a flow path between the swing control valve installed on the third center bypass line at the downstream of the third hydraulic pump and at least one of the working device control valves connected to the first and second center bypass lines, and shifted, in accordance with the valve switching signal input from the outside when the swing control valve is in a neutral position, to supply the hydraulic fluid discharged from the third hydraulic pump to at least one of the working device control valves selectively connected; and variable confluence lines connected and installed between an output port side of the confluence control valve and the at least one of the working device control valves selectively connected to make the hydraulic fluid supplied from the third hydraulic pump join the hydraulic fluid in the first or second hydraulic pump in accordance with spool shifting of the confluence control valve, wherein the variable confluence lines are connected to and installed on a flow path between an output port of a second arm control valve and the arm cylinder.