Work machine

A work machine comprising:

• a first hydraulic circuit in which a hydraulic motor as a first actuator for drivingly swinging a revolving upperstructure and a first pump/motor enabling the outflow/inflow of hydraulic fluid in both directions and being controllable in displacement are connected in a closed circuit style by a flow path enabling hydraulic fluid to flow and in which a first switching device is provided for selectively opening the flow path between the hydraulic motor and the first pump/motor;
• a second hydraulic circuit in which a second hydraulic actuator differing from the hydraulic motor and a second pump/motor enabling the outflow/inflow of hydraulic fluid in both directions and being controllable in displacement are connected in a closed circuit style by a flow path enabling hydraulic fluid to flow and in which a second switching device is provided for selectively opening the flow path between the second hydraulic actuator and the second pump/motor;
• a combining flow path connected between the first hydraulic circuit and the second hydraulic circuit;
• a first combining flow path switching device that selectively opens the first combining flow path;
• and
• a control device that controls the first and second pumps/motors, the first and second switching devices and the first combining flow path switching device, wherein:
• the control device includes a swing deceleration detection section that detects the state of the revolving upperstructure being decelerated, a pump operation judgment section that judges the operation state of the second pump/motor, and a control section that controls the displacements of the first and second pumps/motors and the selective openings of the first and second switching devices and the first combining flow path switching device;
• and
• when the swing deceleration detection section detects the state of the revolving upperstructure being decelerated, when the pump operation judgment section judges the state that the second pump/motor is in the state of not supplying hydraulic fluid to the second hydraulic actuator, and when inertia energy attendant on the swing operation is unable to be regenerated by the first pump/motor only, the control section outputs an open signal to the first switching device, outputs a closing signal to the second switching device, outputs an open signal to the first combining flow path switching device that merges the second hydraulic closed circuit and the first hydraulic closed circuit, and further controls the displacement of the first pump/motor and the displacement of the second pump/motor to make respective suction pressures higher than respective discharge pressures so that the first pump/motor and the second pump/motor operate as motors,
• a third hydraulic circuit in which a third hydraulic actuator differing from the first hydraulic actuator and the second hydraulic actuator and a third pump/motor enabling the outflow/inflow of hydraulic fluid in both directions and being controllable in displacement are connected in a closed circuit style by a flow path enabling hydraulic fluid to flow and in which the third switching device is provided for selectively opening the flow path between the third hydraulic actuator and the third pump/motor;
• a second combining flow path connected between the first hydraulic closed circuit and the third hydraulic closed circuit;
• and
• a second combining flow path switching device that selectively opens the second combining flow path, wherein:
• when the swing deceleration detection section detects the state of the revolving upperstructure being decelerated, when the pump operation judgment section judges that the third pump/motor is in the state of not supplying hydraulic fluid to the third hydraulic actuator and when inertia energy attendant on the swine operation is unable to be regenerated by the first and the second pumps/motors only, the control section further outputs a closing signal to the third switching device, outputs an oven signal to the second combining flow path switching device that merges the third hydraulic closed circuit and the first hydraulic closed circuit, and controls the displacement of the third pump/motor to make the suction pressure become higher than the discharge pressure so that the third pump/motor operates as a motor;
• the first hydraulic closed circuit is further provided with a variable relief valve capable of controlling a relief pressure for the hydraulic fluid in the first hydraulic closed circuit;
• when the swing deceleration detection section detects the state of the revolving upperstructure being decelerated, the control section raises the relief pressure for the variable relief valve and then controls respective displacements of a pump/motor of the second and third pumps/motors which is not supplying hydraulic fluid to the second or third hydraulic actuator, and the first pump/motor based on the pressure difference detected by the pressure detection section so that in the first and second pumps/motors, a suction pressure becomes higher than a discharge pressure.