Hydraulic drive system for hydraulic working machine

A hydraulic drive system for a hydraulic working machine equipped with a swing structure and a working implement, the hydraulic drive system comprising:

• a hydraulic pump;
• a swing motor unit driven by a hydraulic fluid delivered from the hydraulic pump, the motor unit including a hydraulic swing motor for rotating the swing structure;
• a control valve including a swing-directional control valve, the control valve controlling flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic swing motor;
• a first swinging relief valve with a shockless function, the first swinging relief valve disposed inside the swing motor unit and connected to an actuator hydraulic line that connects to the swing motor, which limits a driving or braking pressure of the hydraulic swing motor in order to prevent the driving or braking pressure from exceeding a first setting pressure (P1), starts to open at a relief starting pressure (P0) below the first setting pressure (P1), and reduces an increase in internal pressure due to an abrupt increase in the driving or braking pressure;
• a second swinging relief valve connected to an actuator hydraulic line in parallel with respect to the first swing relief valve, and which limits the driving or braking pressure of the hydraulic swing motor to prevent the driving or braking pressure from exceeding a second setting pressure (P2) that is lower than the first setting pressure (P1);
• and
• a selector for selecting either one of a first relief mode and a second relief mode, wherein in the first relief mode relief characteristics of the first swinging relief valve are left operatively intact, and in the second relief mode the first setting pressure (P1) of the first swinging relief valve is reduced to the second setting pressure (P2) of the second swinging relief valve,
• wherein the second setting pressure (P2) of the second swinging relief valve is set to a value ranging between the relief starting pressure (P0) and the first setting pressure (P1) of the first swinging relief valve, and
• wherein the selector includes a changeover valve disposed at an upstream side of the second swing relief valve between the actuator hydraulic line and the second swinging relief valve, and the selector selects the second relief mode by switching the changeover valve to a communicating position and selects the first relief mode by switching the changeover valve to an interrupting position.