A hydraulic system includes a hydraulic pump, a first hydraulic actuator, a second hydraulic actuator, a first control valve to control the first hydraulic actuator, a second control valve to control the second hydraulic actuator, a return fluid tube in which a return fluid discharged from the first hydraulic actuator flows, the return fluid tube connecting the first control valve and the first hydraulic actuator, a first inner fluid tube provided in the first control valve and connected to the return fluid tube, an outer fluid tube connected to the first inner fluid tube, the outer fluid tube connecting the first control valve and the second control valve, a return-discharge fluid tube to discharge the return fluid, the discharge fluid tube being branched from the outer fluid tube, and a throttle provided in the return-discharge fluid tube.

A hydraulic system includes first and second control valves controlling a flow rate of operation fluid to be supplied to first and second hydraulic actuators, respectively, a supply fluid tube connecting the first control valve and the second control valve and being connected to a return fluid tube, and a discharge fluid tube connected to the second control valve. The second control valve is switched between primary, secondary, and tertiary positions, the primary position blocking between the supply fluid tube and the discharge fluid tube and allowing return fluid to be supplied to the second hydraulic actuator, the secondary position communicating between the supply fluid tube and the discharge fluid tube and stopping supplying the return fluid to the second hydraulic actuator, and the tertiary position communicating between the supply fluid tube and the discharge fluid tube and allowing the return fluid to be supplied to the second hydraulic actuator.

A valve (V, V1, V2) includes a housing (VG) and a piston (VK) displaceably guided therein. A first end (VK1) of the piston (VK) can be acted upon by a force, with the aid of which the piston (VK) is displaceable against a spring (F) acting upon a second end (VK2) of the piston (VK). The valve (V, V1, V2) has four switching conditions, in which four ports (A, B, P, T) in the housing (VG) are selectively connectable to one another or blocked with respect to one another. In a first switching condition, none of the ports (A, B, P, T) are connected to one another. The spring (F) is configured in such that, in the absence of an application of force onto the first end (VK1), the piston (VK) is held in a position, which corresponds to the first switching condition of the valve (V, V1, V2).

A new means of achieving a valve float position in a closed centered hydraulic control valve. The valve has a main spool position whereby pressure compensator load sense control pressure is drained to tank through the main spool. Secondly, a pin acts on the pressure compensator to prevent the main pump flow from reaching the main section spool. Finally, the new spool position adds a connection from the consumer to tank, enabling a float position.

A control valve includes a housing formed with first and second supply ports, first and second actuator ports, and a pilot port, and a spool accommodated slidably in the housing, and the spool is configured to allow the first supply port and the pilot port to communicate with each other both at the first communication position and the second communication position.

A hydraulic system includes first and second control valves controlling a flow rate of operation fluid to be supplied to first and second hydraulic actuators, respectively, a supply fluid tube connecting the first control valve and the second control valve and being connected to a return fluid tube, and a discharge fluid tube connected to the second control valve. The second control valve is switched between primary, secondary, and tertiary positions, the primary position blocking between the supply fluid tube and the discharge fluid tube and allowing return fluid to be supplied to the second hydraulic actuator, the secondary position communicating between the supply fluid tube and the discharge fluid tube and stopping supplying the return fluid to the second hydraulic actuator, and the tertiary position communicating between the supply fluid tube and the discharge fluid tube and allowing the return fluid to be supplied to the second hydraulic actuator.

A hydraulic excavator drive system includes: first and second pumps; arm cylinder; arm first control valve connected to the cylinder by an arm crowding supply line and arm pushing supply line; arm second control valve connected to the supply lines by a first and second replenishment line; and arm operation device that outputs an operation signal corresponding to an inclination angle of an operating lever. The arm second control valve is configured so, when performing an arm crowding operation, an opening area at a meter-in side changes in accordance with the operation signal, and an opening area at a meter-out side is: kept to zero when a predetermined condition is not satisfied; and kept to zero until the operation signal becomes a setting value or greater, and when the operation signal has become the setting value or greater, increases to a maximum value when the predetermined condition is satisfied.

A new means of achieving a valve float position in a closed centered hydraulic control valve. The valve has a main spool position whereby pressure compensator load sense control pressure is drained to tank through the main spool. Secondly, a pin acts on the pressure compensator to prevent the main pump flow from reaching the main section spool. Finally, the new spool position adds a connection from the consumer to tank, enabling a float position.

A hydraulic system includes a hydraulic pump, a first hydraulic actuator, a second hydraulic actuator, a first control valve to control the first hydraulic actuator, a second control valve to control the second hydraulic actuator, a return fluid tube in which a return fluid discharged from the first hydraulic actuator flows, the return fluid tube connecting the first control valve and the first hydraulic actuator, a first inner fluid tube provided in the first control valve and connected to the return fluid tube, an outer fluid tube connected to the first inner fluid tube, the outer fluid tube connecting the first control valve and the second control valve, a return-discharge fluid tube to discharge the return fluid, the discharge fluid tube being branched from the outer fluid tube, and a throttle provided in the return-discharge fluid tube.

A hydraulic excavator drive system includes: first and second pumps; arm cylinder; arm first control valve connected to the cylinder by an arm crowding supply line and arm pushing supply line; arm second control valve connected to the supply lines by a first and second replenishment line; and arm operation device that outputs an operation signal corresponding to an inclination angle of an operating lever. The arm second control valve is configured so, when performing an arm crowding operation, an opening area at a meter-in side changes in accordance with the operation signal, and an opening area at a meter-out side is: kept to zero when a predetermined condition is not satisfied; and kept to zero until the operation signal becomes a setting value or greater, and when the operation signal has become the setting value or greater, increases to a maximum value when the predetermined condition is satisfied.