Hydraulic driving unit for working machine, and method of hydraulic drive

A hydraulic drive system for a working machine comprising:

• an engine (1) having a fuel injection control unit (12, 13) capable of performing control in at least a part of a governor region based on an isochronous characteristic, a reverse drooping characteristic, or a combined one of the isochronous characteristic and the reverse drooping characteristic;
• a variable displacement hydraulic pump (2) driven by said engine;
• and
• a plurality of hydraulic actuators (3-6) driven by a hydraulic fluid delivered from said hydraulic pump,
• wherein said hydraulic drive system comprises a regulator (16) for controlling a displacement of said hydraulic pump (2);
• a pressure sensor (14) for detecting a delivery pressure of said hydraulic pump;
• pump absorption torque control means (82) for controlling said regulator (16) such that, when the delivery pressure of said hydraulic pump detected by said pressure sensor exceeds a first predetermined pressure (P1), the displacement of said hydraulic pump does not exceed a value decided in accordance with a preset pump absorption torque curve (20);
• and
• flow rate compensation control means (83, 85;
• 17A, 83A, 83B, 84A, 85) for controlling said regulator (16) such that, when the delivery pressure of said hydraulic pump is not higher than the first predetermined pressure (P1), the displacement of said hydraulic pump is increased as the delivery pressure of said hydraulic pump lowers from a second predetermined pressure (P1).

A hydraulic drive method for a working machine comprising:

• an engine (1) having a fuel injection control unit (12, 13) capable of performing control in at least a part of a governor region based on an isochronous characteristic, a reverse drooping characteristic, or a combined one of the isochronous characteristic and the reverse drooping characteristic;
• a variable displacement hydraulic pump (2) driven by said engine;
• and
• a plurality of hydraulic actuators (3-6) driven by a hydraulic fluid delivered from said hydraulic pump,
• wherein when a delivery pressure of said hydraulic pump (2) exceeds a first predetermined pressure (P1), a displacement of said hydraulic pump is controlled such that the displacement of said hydraulic pump does not exceed a value decided in accordance with a preset pump absorption torque curve (20), and
• when the delivery pressure of said hydraulic pump is not higher than the first predetermined pressure (P1), the displacement of said hydraulic pump is controlled such that the displacement of said hydraulic pump is increased as the delivery pressure of said hydraulic pump lowers from a second predetermined pressure (P1).