A hydraulic system for a working machine includes a traveling motor to output power to a traveling device in the working machine, a first detector to detect a physical quantity that changes in accordance with a traveling state of the working machine and that is detected at intervals of a predetermined period, and a controller to execute automatic shift-down to automatically reduce a rotational speed of the traveling motor to a first speed stage when the rotational speed of the traveling motor is in a second speed stage higher than the first speed stage. The controller is configured or programmed to determine a tendency or degree of change in the physical quantity, based on values of the physical quantity detected at intervals of the predetermined period by the first detector.

A hydraulic system for a working vehicle includes a first hydraulic pump to deliver pilot fluid to a control valve for a hydraulic actuator whose highest load pressure acts on a first fluid passage, and a second hydraulic pump to deliver hydraulic fluid whose pressure acts on a second fluid passage. A hydraulic controller is operable to control a load-sensing (LS) differential pressure between the highest load pressure and a delivery pressure of the hydraulic fluid from the second hydraulic pump. A third fluid passage to which the second hydraulic pump delivers the hydraulic fluid branches to a fourth fluid passage for flow of the pilot fluid. A solenoid valve is operable to change a pilot pressure of the pilot fluid for the hydraulic controller, and a controller is configured or programmed to control the solenoid valve to adjust the pilot pressure to change the LS differential pressure.

A circle drive system with clutch protection in motor graders is disclosed. The circle drive system includes a hydraulic motor that is configured to receive a pressurized fluid and generate a torque for a rotation of a circle and a moldboard. The circle drive system also includes a gearbox to transfer the torque generated by the hydraulic motor to the circle. Further, the circle drive system includes a clutch connected to the hydraulic motor at one end and to the gearbox at another end. In addition, the circle drive system also includes a hydraulic circuit having a pressure relief valve to relieve the pressurized fluid supplied to the hydraulic motor when a pressure of the pressurized fluid exceeds a predefined threshold. The pressure relief valve thereby, limits the torque generated by the hydraulic motor to less than or equal to a torque capacity of the clutch.

A method for controlling a work vehicle includes driving a first hydraulic pump and a second hydraulic pump by an engine to supply hydraulic fluid to a first hydraulic motor and a second hydraulic motor, respectively, to drive a first traveling device and a second traveling device, respectively. An operation state of a direction input device to operate a traveling direction of the work vehicle is detected. Based on the operation state detected, whether or not a traveling state of the work vehicle is a turning state. When it is determined that the traveling state is the turning state, a rotation command to decrease a target rotational speed of the engine from a first rotational speed to a second rotational speed is output.

A hydraulic system for a working machine, the system comprising: an electric machine connected to a first hydraulic machine and to a second hydraulic machine via a common axle, an output side of the second hydraulic machine being connected to an input side of the first hydraulic machine, wherein the first hydraulic machine is a variable displacement hydraulic machine with unidirectional flow; at least one hydraulic consumer hydraulically coupled to an output side of the first hydraulic machine via a supply line and configured to be powered by the first hydraulic machine; a first return line hydraulically coupling the hydraulic consumer to the input side of the first hydraulic machine.

A controller mounted in a work machine limits a velocity at which a work device approaches a design surface to be equal to or lower than a predetermined limiting velocity in such a manner that the work machine is located above the design surface when an operation device is operated. The controller determines whether a work phase of the work device is compaction work on the basis of a posture of a bucket with respect to the design surface in a case in which the operation device instructs the work device to approach the design surface, and sets the limiting velocity when determining that the work phase of the work device is the compaction work to be higher than the limiting velocity when determining that the work phase of the work device is other than the compaction work.

Provided is a hydraulic circuit, for a construction machine, which drives an actuator by merging pressure oil from a fixed-volume pump into a center bypass oil path from a variable-volume pump to an oil tank, wherein the flow rate of flow from the fixed-volume pump to the center bypass oil path can be controlled in accordance with a requested flow rate of the actuator. A distribution direction-switching valve, which has a first oil path from a fixed-volume pump to an oil tank and a second oil path from the fixed-volume pump to a first center bypass oil path, has a first signal reception unit which causes a spool to slide in a direction in which the first oil path is formed, and a second signal reception unit which causes a spool to slide in a direction in which the second oil path is formed, and determines a distribution ratio of pressure oil flowing to the first oil path and the second oil path in accordance with the difference in size of the signals received by the first signal reception unit and the second signal reception unit, the first signal reception unit receiving a signal based on a negative control signal.

A direction control valve group for a construction machine that controls an amount of pressure oil supplied to a hydraulic actuator from a hydraulic pump that discharges the pressure oil is provided. The direction control valve includes a cylinder port that supplies the pressure oil to the hydraulic actuator, a bridge passage that is switchably connected and disconnected to the cylinder port according to a change in position of a first spool, and an internal passage that supplies the pressure oil discharged from the hydraulic pump to the bridge passage. The first spool is provided in the internal passage.

A drive control method of a hydraulic actuator of a construction machine includes: determining whether a rotation operation lever and a working device operation lever are operated; calculating the required pressure of a hydraulic cylinder fix a working device according to the operation amount of the rotation operation lever; calculating the required flow rates of a swing motor and the hydraulic cylinder for the working device, the required flow rates corresponding to the operation amounts of the working device operation lever and the rotation operation lever; calculating the opening areas of the first and second proportional solenoid valves of an inlet side and an outlet side by using the calculated required pressure and required flow rates of the hydraulic cylinder for the working device and the swing motor; and calculating current values to be inputted into the first and second proportional solenoid valves of the inlet side and the outlet side according to preset data values or a table in comparison with the calculated opening areas of the first and second proportional solenoid valves of the inlet side and the outlet side.

A hydraulic control system includes a flow rate control valve; a variable displacement hydraulic pump which is connected to the flow rate control valve and discharges hydraulic pressure to the flow rate control valve; a pump discharge pressure detector which is installed in a section where the flow rate control valve and the variable displacement hydraulic pump are connected, and detects the discharge pressure of the hydraulic pressure discharged from the variable displacement hydraulic pump to the flow rate control valve; and a hydraulic controller including a detection unit which is connected to the pump discharge pressure detector, detects the discharge pressure of the variable displacement hydraulic pump and converts the discharge pressure to a pump discharge pressure value, a comparison unit which receives the pump discharge pressure value from the detection unit, compares the pump discharge pressure value with a pre-stored lowest recognition pressure value, and determines whether the pump discharge pressure value is greater or less than the lowest recognition pressure value, and a calculation unit which, by interworking with the comparison unit, if the pump discharge pressure value is less than the lowest recognition pressure value, calculates a control pressure by recognizing the control