In a hydraulic closed circuit system with hydraulic pumps which maintains a well-balanced flow rate by automatically controlling the flow rate, a first hydraulic pump is connected to a hydraulic cylinder device such that the hydraulic closed circuit is made, a second hydraulic pump is connected at one of paired delivery ports to a bottom side of the hydraulic cylinder device and at the other of the ports to a tank, and a prime mover drives the first and second hydraulic pumps and recovers motive power from these pumps. A pump capacity control unit detects a moving direction of the hydraulic cylinder device, and a pressure in a lower-thrust side of the device, and controls a capacity of the second hydraulic pump so that the flow rate during the extension/retraction of the hydraulic cylinder device becomes balanced between the first and second hydraulic pumps and the hydraulic cylinder device.

A hydraulic system and method of controlling such on a machine is disclosed. The hydraulic system may comprise a primary hydraulic circuit that includes a first load sense controlled pump, a secondary hydraulic circuit that includes a second load sense controlled pump, a flow sharing valve, and a load sense arrangement. The load sense arrangement may include a main resolver configured to select a higher pressure signal between the primary and secondary hydraulic circuits, and a load sense valve having an open position at which pressure signal flow between the primary hydraulic circuit and the main resolver is allowed, and a closed position at which pressure signal flow between the primary hydraulic circuit and the main resolver is blocked. In an embodiment, when the load sense valve is in the closed position, the second load sense controlled pump may be controlled only by the secondary hydraulic circuit.

A hydraulic system for a construction machine having a pressure compensation valve is disclosed, which improves operability when a combined operation of a swing device and an attachment is performed. The hydraulic system includes a hydraulic pump, attachment and swing operation devices, an attachment actuator and a swing motor connected to the hydraulic pump, a first control valve controlling hydraulic fluid being supplied to the actuator, a second control valve controlling hydraulic fluid being supplied to the swing motor, a check valve installed on a flow path for detecting load pressures of the actuator to take a maximum pressure of the load pressures, a first pressure compensation valve having an opening amount that is controlled by a difference between the maximum load sensing pressure and the pressure of a discharge flow path of the first control valve, a second pressure compensation valve having an opening amount that is controlled by a difference between the maximum load sensing pressure and the pressure of the discharge flow path of the second control valve, a maximum load pressure sensor detecting the pressure of a maximum load sensing pressure line, and a control unit controlling a discharge flow rate of the hydraulic pump, wherein the system is configured so that the swing load pressure is not connected to the first pressure compensation valve on the attachment side when a combined operation of the swing device and the attachment is performed.

A control method for controlling retraction of a boom is applied to a hydraulic system which combines retraction of the boom by self-weight with retraction of the boom by pressure. The control method includes: when the boom is retracted, controlling the self-weight retraction proportional valve to be at a maximum opening; obtaining an actual speed at which the boom is retracted; controlling a rotation speed of the pressure retraction motor to rise or an opening degree of the pressure retraction proportional valve to increase when the actual speed is less than a preset speed; and controlling the rotation speed of the pressure retraction motor to reduce or the opening degree of the pressure retraction proportional valve to decrease when the actual speed is greater than the preset speed.

A floating control system for a fixed displacement pump system is disclosed, which relates to the technical field of floating mechanisms and includes: a fixed displacement pump, floating control valves, a floating mechanism, boom function valves, an actuator and a control device, where the fixed displacement pump, the floating control valves and the floating mechanism are connected in sequence, the fixed displacement pump, the boom function valves and the actuator are connected in sequence, the floating control valves include a pressure reducing valve and a floating switching valve, and the boom function valves include a fixed differential overflow valve and a proportional directional valve; and the fixed differential overflow valve and the proportional directional valve are connected in parallel between the fixed displacement pump and an oil return tank, the floating switching valve and the fixed differential overflow valve are connected through a feedback oil path.

A hydraulic system for a working machine includes a prime mover, a boom cylinder, a control valve, a first hydraulic pump to deliver pilot fluid to switch the control valve, a second hydraulic pump to deliver hydraulic fluid to activate the boom cylinder, a hydraulic controller configured or programmed to control the second hydraulic pump to set a load-sensing (LS) differential pressure, a first pilot fluid passage, a second pilot fluid passage branching off from the first pilot fluid passage and connected to the hydraulic controller, a solenoid valve to change a pilot pressure that is a pressure of the pilot fluid applied to the hydraulic controller, and a pressure compensator to increase the LS differential pressure as a temperature of the hydraulic fluid including the pilot fluid decreases.

Problem: In a case where the hydraulic actuator A supplied with hydraulic oil only from the first hydraulic pump and the hydraulic actuator B supplied with hydraulic oil from the first and second hydraulic pumps are operated simultaneously, even if the required flow rate for the first hydraulic pump of the hydraulic actuators A and B exceeds the first hydraulic pump maximum discharge flow rate, the hydraulic actuators A and B can be driven at the operation speed corresponding to the operation amount of the operation means. Solution: A target flow rate correction means is provided for setting the bucket cylinder required flow rate as a target flow rate from the hydraulic pump to the bucket cylinder, a flow rate obtained by subtracting the bucket cylinder required flow rate from the pump maximum discharge flow rate as a target flow rate from the hydraulic pump to the boom cylinder, and setting a flow rate obtained by subtracting the target flow rate from the hydraulic pump to the boom cylinder from the bucket cylinder total required flow rate as a target flow rate from the hydraulic pump to the boom cylinder.