A hydraulic system includes a first fixed displacement pump having a first pump output and a second fixed displacement pump having a second pump output. A first actuator requires a first required flow rate to perform a first function. A second actuator requires a second required flow rate to perform a second function. A combined flow control valve is controllable between a first state connecting fluid communication between the first fixed displacement pump and the tank so that only the second pump output is directed to the second actuator, and a second state disconnecting fluid communication between the first fixed displacement pump and the tank to combine the first pump output and the second pump output such that the combined first pump output and the second pump output is directed to the first actuator.

A hydraulic power system for a downhole device, including a first motor, a first hydraulic pump, a second hydraulic pump, a first main oil circuit, a second main oil circuit, a switching control module and a first execution module. The first motor has a first output shaft which drives the first hydraulic pump and has an oil outlet connected to an input end of the first main oil circuit and a second output shaft which drives the second hydraulic pump and has an oil outlet connected to an input end of the second main oil circuit; the first execution module is connected to an output end of the first main oil circuit; displacement of the first hydraulic pump is smaller than that of the second hydraulic pump; and the switching control module is connected between the first main oil circuit and the second main oil circuit.

A work vehicle includes a valve for switching between a merging position for causing a first oil passage for driving a bucket and a second oil passage for driving an arm to communicate with each other, and a diverging position for separating the first oil passage and the second oil passage. The work vehicle is adapted to switch the valve from the merging position to the diverging position, when one of a pump pressure in the first pump and a pump pressure in the second pump comes to be equal to a predetermined value during an excavation operation. The work vehicle is adapted to control both pumps, such that the amount of the oil discharged from the first pump is larger than the amount of the oil discharged from the second pump, when the pump pressure in the first pump is equal to or more than the predetermined value.

A hydraulic-pump flow-rate calibration system includes: a variable capacitance type hydraulic pump that supplies an operating fluid to a hydraulic actuator; a regulator that changes the dispense flow rate of the hydraulic pump according to a flow rate command signal; a flow rate detection device that detects the flow rate of the operating fluid; a control device that outputs the flow rate command signal to the regulator to control the regulator; and a calibration device that calculates an actual measurement characteristic of the dispense flow rate for the flow rate command signal, and performs, on a preset reference characteristic, calibration based on the actual measurement characteristic. The actual measurement characteristic is calculated as a result of the flow rate of the operating fluid being detected by the flow rate detection device during output of a predetermined flow rate command signal from the control device to the regulator.

A digital pump axis control system having a circuit including an electric engine, powering first and second hydraulic machines connected in a rotationally locked manner to each other. At least one cylinder has a first chamber connected through a first pipeline to the first hydraulic machine and a second chamber of the cylinder is connected through a second pipeline to the second hydraulic machine. A first valve is arranged in the first pipeline; a second valve is arranged in the second pipeline; a third valve is arranged in a third pipeline, wherein the third pipeline connects a portion of the first pipeline between the first hydraulic machine and the first valve and a portion of the second pipeline between the second hydraulic machine and the second valve. An open tank provides hydraulic fluid to inlets of the first and second hydraulic machines. The first and second hydraulic machines are digital variable displacement pumps, each providing a positive and a negative displacement of hydraulic fluid.

A hydraulic system includes a first fixed displacement pump having a first pump output and a second fixed displacement pump having a second pump output. A first actuator requires a first required flow rate to perform a first function. A second actuator requires a second required flow rate to perform a second function. A combined flow control valve is controllable between a first state connecting fluid communication between the first fixed displacement pump and the tank so that only the second pump output is directed to the second actuator, and a second state disconnecting fluid communication between the first fixed displacement pump and the tank to combine the first pump output and the second pump output such that the combined first pump output and the second pump output is directed to the first actuator.

An object of the present invention is to provide a construction machine that allows easy and accurate calibration of a pressure sensor and enables accurate control of hydraulic actuators. A controller increases the delivery pressure of a hydraulic pump in a state in which first and second meter-out valves and a second meter-in valve are closed and in which a first meter-in valve is opened, and calibrates a first pressure-calculation map such that a pressure calculated on the basis of the first pressure-calculation map matches a pressure calculated on the basis of a supply-pressure-calculation map, and increases the delivery pressure of the hydraulic pump in a state in which the first and second meter-out valves and the first meter-in valve are closed and in which the second meter-in valve is opened, and calibrates a second pressure-calculation map such that a pressure calculated on the basis of the second pressure-calculation map matches the pressure calculated on the basis of the supply-pressure-calculation map.

This hydraulic drive device for a work vehicle includes a main pump (1) of a variable displacement type or a fixed displacement type discharging pressure oil, a main flow passage (F1) for supplying pressure oil of the main pump to an actuator, a sub-pump (5) of a fixed displacement type discharging pressure oil, a sub-flow passage (F2) for making pressure oil of the sub-pump merge with the main flow passage and supplying the pressure oil to the actuator (2), a merging directional valve (6A) for connecting or cutting off the main flow passage and the sub-flow passage, a controller (30) for controlling operation of the merging directional valve, and a relief valve (7A) arranged in the sub-flow passage, in which the relief valve has a pressure override characteristic having a tendency that the relief pressure increases from a cracking pressure to a set pressure as a relief flow rate increases.

A working machine includes a hydraulic pump, a first traveling device to be driven by a first traveling hydraulic actuator, a second traveling device to be driven by a second traveling hydraulic actuator, a first output tube to connect a first output port of the hydraulic pump to the first traveling hydraulic actuator, a second output tube to connect a second output port of the hydraulic pump to the second traveling hydraulic actuator, a first operation device to operate the first traveling device, a second operation device to operate the second traveling device and a correction mechanism to equalize a driving force of the first traveling hydraulic actuator and another driving force of the second traveling hydraulic actuator when the first operation device and the second operation device are operated each at same operation extents to perform a straight-traveling operation.

This construction machine includes a first hydraulic drive device that is driven by a first prime mover and a second hydraulic drive device that is driven by a second prime mover. The first hydraulic drive device has a first closed circuit that connects a first hydraulic actuator and a first closed-circuit pump and a first assist flow path that connects the first closed circuit and a first open-circuit pump and that supplies pressure oil from the first open-circuit pump to the first closed circuit. The second hydraulic drive device is provided with a second closed circuit that connects a second hydraulic actuator and a second closed-circuit pump. The present invention also includes a first emergency flow path that branches from the first assist flow path and connects to the second closed circuit and that supplies pressure oil from the first open-circuit pump to the second closed circuit.