A work vehicle includes: an operation tool that is operated by an operator; and a controller that determines a target flow rate for hydraulic oil fed to a hydraulic device on a basis of the amount of operation of the operation tool. The controller calculates a bleed-off target flow rate on a basis of the flow rate of hydraulic oil fed from a hydraulic oil pump and the target flow rate for hydraulic oil fed to the hydraulic device, calculates a bleed-off throttle differential pressure on a basis of a pressure of hydraulic oil fed from the hydraulic oil pump and a pressure of hydraulic oil in a hydraulic oil tank, calculates a bleed-off target opening area on a basis of the bleed-off target flow rate and the bleed-off throttle differential pressure, and controls a hydraulic oil control valve such that the bleed-off target opening area is achieved.

A hydraulic system for a construction machine having a protection device is disclosed, which can limit a flow rate of hydraulic fluid that is discharged from a hydraulic pump in accordance with a hydraulic fluid temperature and notify an operator of information on the hydraulic fluid temperature. The hydraulic system for a construction machine includes a hydraulic actuator operated by hydraulic fluid that is supplied from a hydraulic pump; a control valve installed in a flow path between the hydraulic pump and the hydraulic actuator to control a flow rate of the hydraulic fluid that is supplied from the hydraulic pump to the hydraulic actuator; an oil cooler installed in a return flow path from the control valve to an hydraulic fluid tank to cool the hydraulic fluid; a hydraulic fluid temperature sensor detecting in real time a hydraulic fluid temperature of the hydraulic fluid tank; a hydraulic pump regulator controlling a discharge flow rate of the hydraulic pump through adjustment of an inclination angle of a swash plate of the hydraulic pump; and a controller comparing the hydraulic fluid temperature detected by the hydraulic fluid temperature sensor with a predetermined hydraulic fluid temperature, and outputting a control signal to the hydraulic pump regulator so as to limit the discharge flow rate of the hydraulic pump to or below a predetermined flow rate if the detected hydraulic fluid temperature is equal to or lower than a predetermined lower limit value, or equal to or higher than a predetermined upper limit value.

A hydraulic control device includes: a hydraulic pump connected in parallel to steering and boom cylinders; a steering control valve that controls the direction of operating oil flowing through the steering cylinders; a boom control valve that connects the hydraulic pump to a tank when the valve is at a neutral position and controls the direction of the oil flowing through the boom cylinders when the valve is at an offset position; a meter-in pressure compensator that increases flow rate of the oil flowing through a variable restrictor of the steering control valve in accordance with pressure in front of and behind the restrictor; and a bleed-off pressure compensator that decreases flow rate of the oil flowing through the boom control valve in accordance with the increase in pressure of the oil flowing through the steering cylinders to maintain the predetermined pressure of the oil in the steering control circuit.

A hydraulic control device includes: a hydraulic pump connected in parallel to steering and boom cylinders; a steering control valve that controls the direction of operating oil flowing through the steering cylinders; a boom control valve that connects the hydraulic pump to a tank when the valve is at a neutral position and controls the direction of the oil flowing through the boom cylinders when the valve is at an offset position; a meter-in pressure compensator that increases flow rate of the oil flowing through a variable restrictor of the steering control valve in accordance with pressure in front of and behind the restrictor; and a bleed-off pressure compensator that decreases flow rate of the oil flowing through the boom control valve in accordance with the increase in pressure of the oil flowing through the steering cylinders to maintain the predetermined pressure of the oil in the steering control circuit.

A hydraulic system includes a hydraulic valve, a load-holding valve arrangement, a flow-control valve arrangement, a working pressure sensor, a differential pressure sensor, and a speed sensor. The load-holding valve arrangement includes first and second load-holding valves connected to opposite ends of the hydraulic cylinder. The flow-control valve arrangement includes a directional flow-control valve and a hydraulic pump. The differential pressure sensor measures pressure over the load-holding valve arrangement. A desired extension or retraction speed of the hydraulic cylinder is controlled by adjusting the position of the directional flow-control valve, the first load-holding valve and the second load-holding valve based on a measured working pressure in the hydraulic system and a measured speed of the hydraulic cylinder or a differential pressure over the load-holding valve arrangement.

To provide a driving device capable of improving the operability of a plurality of single rod hydraulic cylinders. The present invention takes a construction provided with a closed circuit A in which first and third hydraulic pumps 12, 14 and a boom cylinder 1 are connected through selector valves 43a, 45a in a closed-circuit fashion, a plurality of open circuits E, F provided with second and fourth hydraulic pumps 13, 15 and selector valves 44a, 46a that switch the supply destinations of hydraulic oils flowing out from the second and fourth hydraulic pumps 13, 15, and a connection passage 301 connected to sides of the selector valves 44a, 46a from which sides hydraulic oil flows out, and connected to the closed circuit A.

To provide a driving device capable of improving the operability of a plurality of single rod hydraulic cylinders. The present invention takes a construction provided with a closed circuit A in which first and third hydraulic pumps 12, 14 and a boom cylinder 1 are connected through selector valves 43a, 45a in a closed-circuit fashion, a plurality of open circuits E, F provided with second and fourth hydraulic pumps 13, 15 and selector valves 44a, 46a that switch the supply destinations of hydraulic oils flowing out from the second and fourth hydraulic pumps 13, 15, and a connection passage 301 connected to sides of the selector valves 44a, 46a from which sides hydraulic oil flows out, and connected to the closed circuit A.

A working machine is disclosed. A boom supports an arm. An upper revolving body supports the boom. A lower traveling body mounts the upper revolving body in a rotatable state. A hydraulic actuator drives a hydraulic oil while relieving the hydraulic oil. A first pump supplies the hydraulic oil to the hydraulic actuator. A pump state detector detects load of the first pump. A controller reduces a flow rate of the hydraulic oil from the first pump to the hydraulic actuator when the pump state detector detects a predetermined state.

A working machine is disclosed. A boom supports an arm. An upper revolving body supports the boom. A lower traveling body mounts the upper revolving body in a rotatable state. A hydraulic actuator drives a hydraulic oil while relieving the hydraulic oil. A first pump supplies the hydraulic oil to the hydraulic actuator. A pump state detector detects load of the first pump. A controller reduces a flow rate of the hydraulic oil from the first pump to the hydraulic actuator when the pump state detector detects a predetermined state.

In a fluid circuit for an air cylinder connected to a switching valve provided with exhaust ports, a head-side pressure chamber is connected to the switching valve by a first pipe, and a rod-side pressure chamber is connected to the switching valve by a second pipe. A first restrictor is disposed at a connection point between the first pipe and the switching valve or in the vicinity of a first output port of the switching valve, and a second restrictor is disposed at a connection point between the second pipe and the switching valve or in the vicinity of a second output port of the switching valve.