A hydraulic system includes: a slewing motor; a mechanical brake; and a slewing control valve interposed between a main pump and the slewing motor. A first pilot port of the slewing control valve is connected to a first solenoid proportional valve by a pilot line. A second pilot port of the slewing control valve is connected to a second solenoid proportional valve by a second pilot line. The first solenoid proportional valve and the second solenoid proportional valve are connected to an auxiliary pump by a primary pressure line. A switching valve is interposed between the auxiliary pump and the mechanical brake. The switching valve includes a pilot port that is connected to the first pilot line by a switching pilot line. The valve switches from a closed to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a setting value.

A work machine includes a frame, a traction system supporting the frame, an implement system supported by the frame, and a hydraulic system. The hydraulic system includes a hydraulic oil tank, a control circuit, an oil cooler, and a cooler bypass valve assembly. The cooler bypass valve assembly is connected to the control circuit by a control circuit return line, and includes an unloading valve configured to allow hydraulic oil to flow from the control circuit return line to the hydraulic oil tank if a pressure of hydraulic oil in the control circuit return line exceeds a first threshold, a backpressure valve configured to allow hydraulic oil to flow from the return line to the oil cooler through an oil cooler inlet line if a pressure of hydraulic in the oil control circuit return line exceeds a second threshold, and an orifice configured to limit the flow of hydraulic oil through the backpressure valve.

A vehicle control system determines an upper non-zero limit on deceleration of a vehicle to prevent rollback of the vehicle down a grade being traveled up on by the vehicle. The upper non-zero limit on deceleration is determined by the controller based on a payload carried by the vehicle, a speed of the vehicle, and a grade of a route being traveled upon by the vehicle. The controller is configured to monitor the deceleration of the vehicle, and to automatically prevent the deceleration of the vehicle from exceeding the upper non-zero limit by controlling one or more of a brake or a motor of the vehicle. The controller also is configured to one or more of actuate the brake or supply current to the motor of the vehicle to prevent rollback of the vehicle while the vehicle is moving up the grade at a non-zero speed.

A shovel includes a lower traveling body, an upper turning body mounted on the lower traveling body, an excavation attachment attached to the upper turning body, a posture detecting device configured to detect the posture of the excavation attachment, an instability detecting device configured to detect information on the instability of the upper turning body due to an excavation load, and a processor configured to correct the posture of the excavation attachment. The processor is configured to open an arm or a bucket of the excavation attachment in response to determining, based on the outputs of the posture detecting device and the instability detecting device, that the excavation load during deep excavation is more than or equal to a predetermined value.

A coupler cylinder is driven between a locked state and an unlocked state of a bucket by being supplied with hydraulic oil. A main pump supplies the hydraulic oil to the coupler cylinder. A pressure increasing valve controls the supply of the hydraulic oil to the coupler cylinder. A controller controls drive of the pressure increasing valve. The controller instructs the pressure increasing valve to stop the supply of the hydraulic oil to the coupler cylinder based on pressure in an oil passage between the main pump and the coupler cylinder.

A shovel according to embodiments of the present invention includes: a lower traveling body; an upper swivel body that is rotatably mounted on the lower traveling body; an attachment that is configured by a boom, an arm, and an end attachment; an engine that is mounted on the upper swivel body; a motor generator that is able to assist the engine; a power storage system that is mounted on the upper swivel body; a swiveling motor that is driven by electric power from the motor generator and the power storage system; temperature detection units; and a control device. The control device switches a control mode in a case where at least one of a temperature relating to the motor generator, a temperature relating to the power storage system, and a temperature relating to the swiveling motor, detected by the temperature detection units, is higher than a predetermined temperature.

Provided is an electric work vehicle having a simple configuration, yet allowing a hydraulic cylinder to function appropriately even in a low-temperature environment. An electric work vehicle includes a first hydraulic cylinder, a second hydraulic cylinder and a third hydraulic cylinder and includes also a hydraulic system for feeding work oil to these hydraulic cylinders and a traveling motor. The hydraulic system includes an oil heating circuit for heating the work oil using heat generated from the traveling motor.

Both a control for limiting operation of a machine body and a control for raising engine speed are achieved when an obstacle is detected. To this end, a machine controller performs operation limiting control by conducting a control for reducing the speed of the engine when the machine body does not require engine speed increase control and an obstacle is sensed by obstacle sensors, and the machine controller performs supply flow rate reduction control for reducing the flow rates of the hydraulic fluid supplied to hydraulic actuators from a hydraulic pump when the machine body requires the engine speed increase control and an obstacle is sensed by the obstacle sensors.

In a hydraulic system for a working machine, a controller is configured or programmed to increase an output-port pressure of one activation valve for one hydraulic device and an output-port pressure of another activation valve for another hydraulic device to a normal pressure higher than a preloading pressure from a state where the output-port pressure of the one activation valve is equal to the preloading pressure and the output-port pressure of the other activation valve is lower than the preloading pressure, by causing the output-port pressure of the one activation valve to be lower than the preloading pressure and increasing the output-port pressure of the other activation valve to the normal pressure.

A hydraulic system for a working machine includes hydraulic actuators actuated with hydraulic fluid delivered from a hydraulic pump, and control valves each of which is shiftable among shift positions to control a flowrate of hydraulic fluid flowing to the corresponding hydraulic actuator. Each control valve includes an input port, an output port, and a flowrate reduction section. When the control valve is shifted to a reduction position, the flowrate reduction section reduces a flowrate of the hydraulic fluid entering the input port and outputs the flowrate-reduced hydraulic fluid to the output port. At least one of the control valves includes a flowrate increase section. When the control valve is shifted to an increase position, the flowrate increase section outputs the hydraulic fluid having entered the input port to the output port at a flowrate larger than that of hydraulic fluid output by the flowrate reduction section.