A landing gear lifting/lowering EHA system includes: a hydraulic actuator configured to lift and lower the leg of an aircraft; at least one electrically operated hydraulic pump; a hydraulic path; a pressure sensor attached to the hydraulic actuator or the hydraulic path and configured to output a measurement signal corresponding to hydraulic pressure; and a controller configured to output a control signal to the electrically operated hydraulic pump, wherein, when a state in which the hydraulic pressure exceeds a set pressure continues for a set time, the control unit stops the electrically operated hydraulic pump in operation and resumes the operation of the electrically operated hydraulic pump after the hydraulic pressure drops to or below a second set pressure after the electrically operated hydraulic pump is stopped.

A landing gear lifting/lowering EHA system includes: a hydraulic actuator configured to lift and lower the leg of an aircraft; at least one electrically operated hydraulic pump; a hydraulic path; a pressure sensor attached to the hydraulic actuator or the hydraulic path and configured to output a measurement signal corresponding to hydraulic pressure; and a controller configured to output a control signal to the electrically operated hydraulic pump, wherein, when a state in which the hydraulic pressure exceeds a set pressure continues for a set time, the control unit stops the electrically operated hydraulic pump in operation and resumes the operation of the electrically operated hydraulic pump after the hydraulic pressure drops to or below a second set pressure after the electrically operated hydraulic pump is stopped.

A hydraulic power unit (HPU) configured for use with a pressure relief valve (PRV) having an open port and a close port is provided. The HPU includes a pneumatic primary pump, a hydraulic fluid reservoir, an accumulator, and a two position solenoid directional valve (TPSDV). The hydraulic fluid reservoir is in fluid communication with the primary pump. The TPSDV is in communication with the primary pump, the reservoir, the accumulator. The TPSDV is configured for fluid communication with the PRV. The HPU is configurable in a PRV fail open configuration and a PRV fail close configuration.

A hydraulic fluid energy regeneration system for a work machine capable of securely maintaining a brake pressure even when a solenoid valve or the like is erroneously rendered open by an electrical failure is provided. The hydraulic fluid energy regeneration system for a work machine, including a first hydraulic actuator; a second hydraulic actuator; and a first hydraulic pump that supplies a hydraulic fluid to the first hydraulic actuator via a first hydraulic line, includes: an upper control valve that is connected to a discharge side on which the hydraulic fluid is discharged from the second hydraulic actuator when an object to be driven by the second hydraulic actuator falls under its own weight, and that can regulate a flow rate of the hydraulic fluid discharged from the second hydraulic actuator; a communication hydraulic line that connects the upper control valve to a hydraulic fluid tank; a lower control valve that is provided in the communication hydraulic line and that can regulate a flow rate of the hydraulic fluid discharged from the upper control valve to the hydraulic fluid tank; and a regeneration hydraulic line that has one end side connected to a branch section between the upper control valve and the lower control valve in the communication hydraulic line and that has an other end side connected to the first hydraulic line.

A hydraulic excavator drive system includes: a first pump connected to a head-side chamber of a boom cylinder; and a second pump that supplies hydraulic oil to one of, or both, an arm cylinder and a bucket cylinder. The first pump is driven by an electric motor. The drive system further includes a switching valve that is in a first position at a boom raising operation and in a second position at a vehicle body lifting operation. The first position is a position in which the switching valve brings a rod-side chamber of the boom cylinder into communication with a tank, and the second position is a position in which the switching valve brings the rod-side chamber into communication with the second pump.

A hydraulic power unit (HPU) configured for use with a pressure relief valve (PRV) having an open port and a close port is provided. The HPU includes a pneumatic primary pump, a hydraulic fluid reservoir, an accumulator, and a two position solenoid directional valve (TPSDV). The hydraulic fluid reservoir is in fluid communication with the primary pump. The TPSDV is in communication with the primary pump, the reservoir, the accumulator. The TPSDV is configured for fluid communication with the PRV. The HPU is configurable in a PRV fail open configuration and a PRV fail close configuration.

A hydraulic circuit changeable between a state of executing a regeneration action and a state of stopping the regeneration action. The hydraulic circuit includes hydraulic pumps; a plurality of hydraulic actuators; a plurality of control valves provided for respective hydraulic actuators; a regeneration line through which return oil returned from a specific hydraulic actuator of the hydraulic actuators to a tank is supplied to a regeneration target; a regeneration valve; a meter-out valve; and a flow-path selection device selecting a flow path for the return oil from a first flow path supplying the return oil to the regeneration line to cause the regeneration action and a second flow path supplying the return oil to the control valve for the specific hydraulic actuator to stop the regeneration action.

A hydraulic stabilizing system includes a pump having first and second flow ports, and first and second actuators, each having an extend chamber and a retract chamber. A first pilot-operated check valve is installed between the pump first flow port and the extend chamber of the first actuator. A second pilot-operated check valve is installed between the pump first flow port and the extend chamber of the second actuator. A third pilot-operated check valve is installed between the pump second flow port and the retract chambers of the first and second actuators. The pilot ports of the first and second pilot-operated check valves are coupled to the second pump flow port, and the pilot port of the third pilot-operated check valve is coupled to the first pump flow port.

A hydraulic fluid energy regeneration system for a work machine capable of securely maintaining a brake pressure even when a solenoid valve or the like is erroneously rendered open by an electrical failure is provided. The hydraulic fluid energy regeneration system for a work machine, including a first hydraulic actuator; a second hydraulic actuator; and a first hydraulic pump that supplies a hydraulic fluid to the first hydraulic actuator via a first hydraulic line, includes: an upper control valve that is connected to a discharge side on which the hydraulic fluid is discharged from the second hydraulic actuator when an object to be driven by the second hydraulic actuator falls under its own weight, and that can regulate a flow rate of the hydraulic fluid discharged from the second hydraulic actuator; a communication hydraulic line that connects the upper control valve to a hydraulic fluid tank; a lower control valve that is provided in the communication hydraulic line and that can regulate a flow rate of the hydraulic fluid discharged from the upper control valve to the hydraulic fluid tank; and a regeneration hydraulic line that has one end side connected to a branch section between the upper control valve and the lower control valve in the communication hydraulic line and that has an other end side connected to the first hydraulic line.

A hydraulic circuit for a construction machine including a center bypass passage including a group of directional control valves arranged in tandem; and a bleed-off valve arranged in the center bypass passage on a downstream side of the group, wherein each directional control valve includes a first internal passage causing the pressurized oil supplied to the directional control valve to flow out to the center bypass passage and a second internal passage supplying the pressurized oil to a cylinder port, wherein a parallel passage is formed by the center bypass passage and the first internal passage by causing the pressurized oil discharged from the hydraulic pump to flow to the center bypass passage on the downstream of the directional control valve, wherein the second internal passage supplies the pressurized oil from the center bypass passage through an opening of a spool and/or the bypass passage to the cylinder port.