Hydraulic drive device for hydraulic excavator

A hydraulic drive system for a hydraulic excavator that performs digging work and grading work by a combined operation of boom raising and arm crowding, said system comprising:

• a boom cylinder and arm cylinder for driving a boom and arm of a front working implement, respectively,
• a main hydraulic pump as a generation source of a hydraulic pressure to be fed to said boom cylinder and arm cylinder,
• a directional control valve for said boom to control a flow of pressure oil to be fed from said main hydraulic pump to a bottom chamber or a rod chamber of said boom cylinder and to control an extending or retracting direction of said boom cylinder and a speed thereof,
• a directional control valve for said arm to control a flow of pressure oil to be fed from said main hydraulic pump to a bottom chamber or a rod chamber of said arm cylinder, and to control an extending or retracting direction of said arm cylinder and a speed thereof,
• a reservoir line connecting said directional control valve for said arm cylinder with a working oil reservoir,
• a communication oil passage that connects the rod chamber of said arm cylinder with the bottom chamber of said boom cylinder via said directional control valve for said arm cylinder and said directional control valve for said boom cylinder,
• a restrictor that is arranged in a release-side oil passage on a switching position of said directional control valve for said arm cylinder which drives said arm cylinder in the extending direction,
• a pilot oil passage that guides pressure oil in the rod chamber of said arm cylinder as a hydraulic pilot pressure,
• a combined flow-rate control valve for said boom which is arranged on said communication oil passage to control opening and closing of said communication oil passage by a pressure value in said pilot oil passage, and
• a meter-out flow-rate control valve for said arm which is arranged on said reservoir line to control opening and closing of said reservoir line by a pressure value in said pilot oil passage, wherein
• upon performing the combined operation of boom raising and arm crowding,
• when a pressure in the rod chamber of said arm cylinder has decreased to a predetermined pressure value indicative of digging work being performed, said reservoir line that connects the rod chamber of said arm cylinder with the working oil reservoir is opened by switching said meter-out flow-rate control valve for said arm to an opening position by the hydraulic pilot pressure and said communication oil passage is closed by switching said combined flow-rate control valve for said boom to a closing position by the hydraulic pilot pressure, and
• when a pressure in the rod chamber of said arm cylinder has increased to a predetermined pressure value indicative of grading work being performed, said reservoir line that connects the rod chamber of said arm cylinder with the working oil reservoir is closed by switching said meter-out flow-rate control valve for said arm to a closing position by the hydraulic pilot pressure and said communication oil passage is opened by switching said combined flow-rate control valve for said boom to an opening position by the hydraulic pilot pressure so that the pressure oil in the rob chamber of said arm cylinder is fed to the bottom chamber of said boom cylinder.