A hydraulic system of a work machine includes a first hydraulic device to operate in a first operation mode while pressure of hydraulic oil supplied from a hydraulic pump via a first oil passage is equal to or higher than a first pressure threshold. The hydraulic oil in the first oil passage is to be discharged via a second oil passage. A pilot check valve is provided in the second oil passage and has a pilot port to receive a pilot pressure of the hydraulic oil. The pilot check valve is closed to stop discharging the hydraulic oil in the first oil passage through the second oil passage while the pilot pressure is lower than the fourth pressure threshold. The pilot check valve is opened while the pilot pressure is higher than or equal to the second pressure threshold. The first hydraulic device is an operation control valve.

A shovel includes a lower traveling body, an upper turning body turnably mounted on the lower traveling body, a hydraulic pump mounted on the upper turning body, a hydraulic actuator configured to be driven with hydraulic oil discharged by the hydraulic pump, a bleed valve configured to control the flow rate of a portion of the hydraulic oil discharged by the hydraulic pump, the portion flowing to a hydraulic oil tank without going through the hydraulic actuator, and a control device configured to control the opening area of the bleed valve in accordance with the magnitude of pulsation in the pressure of hydraulic oil supplied from the hydraulic pump to the hydraulic actuator.

A hydraulic system (600) and method for reducing boom dynamics of a boom (30), while providing counter-balance valve protection, includes a hydraulic cylinder (110), first and second counter-balance valves (300, 400), and first and second control valves (700, 800). A net load (90) is supported by a first chamber (116, 118) of the hydraulic cylinder, and a second chamber (118, 116) of the hydraulic cylinder may receive fluctuating hydraulic fluid flow from the second control valve to produce a vibratory response (950) that counters environmental vibrations (960) on the boom. The first control valve may apply a holding pressure and thereby hold the first counter-balance valve closed and the second counter-balance valve open.

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.

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.

A hydraulic drive system of construction machine including: turning motor; turning operation device that outputs turning operation signal corresponding to inclination angle of operating lever; turning direction switching valve including spool and driver, driver receiving command current and driving spool, turning direction switching valve increasing amount of hydraulic liquid supplied to turning motor and amount of hydraulic liquid discharged from turning motor with increase in command current; controller that feeds command current to turning direction switching valve, wherein command current increases in accordance with increase in turning operation signal; and pressure sensor that detects outflow pressure of turning motor. Where turning operation signal decreases, when outflow pressure of turning motor, which is detected by pressure sensor, is higher than threshold and is increasing, controller feeds command current to turning direction switching valve, wherein a moving speed of spool is kept to be less than or equal to limiting value.

An implement attachment device, for selectively attaching an implement to a working machine includes a hydraulic cylinder including a cylinder housing with first and second longitudinal ends, a piston in the cylinder housing, and a piston rod fixed to the piston and protruding through the first end of the cylinder housing, an attachment element being connected to a protruding portion of the piston rod, and adapted to engage with a complementary element of the implement, and a hydraulic conduit and valve arrangement connected to the hydraulic cylinder, wherein the conduit and valve arrangement is arranged to selectively allow a communication between a piston side chamber of the hydraulic cylinder and a rod side chamber of the hydraulic cylinder.

A hydraulic drive device for a cargo vehicle includes a hydraulic cylinder supplying and discharging of hydraulic oil, an operation member that operates the hydraulic cylinder, a hydraulic pump, a lowering oil path connecting the hydraulic cylinder and the hydraulic pump, an operation valve disposed in the lowering oil path, a bypass oil path that branches off from the lowering oil path, a bypass flow rate control valve disposed in the bypass oil path and that controls a bypass flow rate, and a resistance element that is disposed closer to the hydraulic cylinder than the operation valve in the lowering oil path and that increases a fluid resistance. A pilot flow path of the bypass flow rate control valve is connected to a part of the lowering oil path between the hydraulic cylinder and the resistance element.

A hydraulic actuator; a control valve that controls supply and discharge of hydraulic oil to and from the hydraulic actuator; a pilot operation valve connected to the operation valve by a pair of pilot lines; a proportional solenoid pressure-reducing valve provided on at least one of the pair of pilot lines; an operation detector that outputs an operation amount signal in accordance with an inclination angle of an operating lever of the pilot operation valve; and a controller that controls the proportional solenoid pressure-reducing valve such that, from immediately after a change amount per unit time in the operation amount signal outputted from the operation detector has decreased by a threshold value or more, a pilot port pressure of the control valve gradually decreases to zero due to communication of a secondary pressure port and a tank port of the proportional solenoid pressure-reducing valve with each other.

A hydraulic actuator; a control valve that controls supply and discharge of hydraulic oil to and from the hydraulic actuator; a pilot operation valve connected to the operation valve by a pair of pilot lines; a proportional solenoid pressure-reducing valve provided on at least one of the pair of pilot lines; an operation detector that outputs an operation amount signal in accordance with an inclination angle of an operating lever of the pilot operation valve; and a controller that controls the proportional solenoid pressure-reducing valve such that, from immediately after a change amount per unit time in the operation amount signal outputted from the operation detector has decreased by a threshold value or more, a pilot port pressure of the control valve gradually decreases to zero due to communication of a secondary pressure port and a tank port of the proportional solenoid pressure-reducing valve with each other.