Patent ID: 7207175

Claim:
A hydraulic control device for controlling a boom-arm combined operation in an excavator, comprising: a first hydraulic pump (P 1 ) and a second hydraulic pump (P 2 ); a boom high-speed control spool ( 5 b ) and an arm low-speed control spool ( 7 a ) provided on parallel lines ( 11 , 13 ) in tandem from upstream to downstream for receiving hydraulic flow from the first hydraulic pump (P 1 ) via the parallel lines ( 11 , 13 ); a boom low-speed control spool ( 5 a ) and an arm high-speed control spool ( 7 b ) provided on parallel lines ( 15 , 17 ) in tandem from upstream to downstream for receiving hydraulic flow from the second hydraulic pump (P 2 ) via the parallel lines ( 15 , 17 ); and a boom priority valve ( 25 ) provided on the parallel line ( 17 ) interconnecting the second pump (P 2 ) and the arm high-speed control spool ( 7 b ) for throttling the parallel line ( 17 ) to cause the hydraulic flow of the second hydraulic pump (P 2 ) to be supplied to the boom low-speed control spool ( 5 a ) prior to the arm high-speed control spool ( 7 b ), wherein the boom priority valve ( 25 ) has a pressure receiving part ( 25 a ) remaining in fluid communication with a boom pilot valve ( 20 ) through a boom priority control signal line ( 27 ) bifurcated from a boom-raising pilot line ( 21 ) and a variable orifice section (X) for reducing an opening area of the parallel line ( 17 ) in proportion to the magnitude of a boom priority control signal pressure delivered to the pressure receiving part ( 25 a ) by way of the boom priority control signal line ( 27 ).