Hydraulic closed circuit system

A hydraulic closed circuit system, comprising:

• a hydraulic cylinder device;
• a first hydraulic pump of a bidirectional delivery type connected to the hydraulic cylinder device in such a manner that a hydraulic closed circuit is made;
• a second hydraulic pump of a bidirectional delivery and bidirectional variable displacement type, connected at one of paired delivery ports thereof to a bottom side of the hydraulic cylinder device and at the other of the paired delivery ports to a tank;
• a prime mover that drives the first and second hydraulic pumps and recovers motive power from the first and second hydraulic pumps;
• and
• a pump capacity control unit configured to:
• detect a direction in which the hydraulic cylinder device operates,
• detect a pressure applied on a lower-thrust side of the hydraulic cylinder device, and
• control a capacity of the second hydraulic pump such that a flow rate of a hydraulic fluid during extension/retraction of the hydraulic cylinder device becomes balanced between the first and second hydraulic pumps and the hydraulic cylinder device,
• wherein the pump capacity control unit, during an extending operation of the hydraulic cylinder device, is further configured to:
• when the pressure on the lower-thrust side of the hydraulic cylinder device is lower than a reference pressure value, increase the capacity of the second hydraulic pump, and
• when the pressure on the lower-thrust side of the hydraulic cylinder device is higher than the reference pressure value, decrease the capacity of the second hydraulic pump, and
• wherein the pump capacity control unit, during a retracting operation of the hydraulic cylinder device, is further configured to:
• when the pressure on the lower-thrust side of the hydraulic cylinder device is higher than the reference pressure value, increase the capacity of the second hydraulic pump, and
• when the pressure on the lower-thrust side of the hydraulic cylinder device is lower than the reference pressure value, decrease the capacity of the second hydraulic pump.