A pump 1 supplies hydraulic fluid from a reservoir 2 to hydraulically operated equipment 4 through a main flow line 3, the fluid being returned to the reservoir 2 through a return supply line 5. If the pump 1 should fail the equipment 4 may be supplied from an independent pump unit which can be connected to flow and return lines 6, 7. Because the pump 1 has failed it will be incapable of operating a dump valve 15 so the fluid will not be fed to the reservoir 2. Should there be a blockage in the equipment 4 pressure will build up in a relief valve 13 which will therefore open to allow the fluid to pass directly to the return line 5 (or 7). If the system is to be used as a source of pressurized hydraulic fluid for another unit then a change over valve 12 will be operated to de-couple the unit 4 and allow the fluid to pass to the other unit through the auxiliary supply lines 6, 7.

A hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions, a pump for supplying fluid to said actuator, pilot operated meter-in valve means to which the fluid from the pump is supplied for controlling the direction of movement of the actuator, pilot operated meter-out valve means associated with each opening of the actuator for controlling the flow out of said actuator, and means for sensing a predetermined drop in a line supplying fluid to one opening of the actuator caused by a runaway load in one direction and operating the meter-out valve means to interrupt flow out of the other opening of the actuator.

A hydraulic control apparatus has at least two meter-in and two meter-out poppet valves. In order to be compact and deal with a great flow rate, the apparatus is arranged such that a pair of meter-in poppet valves and load check valves are located in an upper horizontal plane, a pair of meter-out poppet valves and solenoid-actuated pilot valves are located in a lower horizontal plane, and a pair of a pilot valve associated with the meter-out poppet valve and a reducing valve associated with the meter-in poppet valve are located in a middle horizontal plane and are respectively disposed in opposed relationship to each other, and the load check valve is vertically located in parallel relationship with the meter-out poppet valve.

A hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving an element of the actuator in opposite directions, a pump for supplying fluid to the actuator. A meter-in valve individual to each opening is provided to which fluid from the pump is supplied and a pair of lines extending from the respective meter-in valves to the respective openings of the actuator. A controller alternately supplies pressure to the respective meter-in valves for controlling the direction of movement of the load. A second pair of lines extends from the first pair of lines to tank and a variable pressure relief valve is positioned in each line of the second pair of lines. When the controller is moved to actuate one of the meter-in valves for supplying fluid to one of the openings of the actuator, the controller also actuates the variable relief valve associated with the line of the first pair of lines extending from the other of the openings of actuator to control the flow out of the other opening. The controller thus simultaneously controls the fluid flow and the pressure to the actuator and the fluid pressure from the actuator, thereby simultaneously controlling the driving and braking functions of the system.

A hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions, a pump for supplying fluid to said actuator, a pilot operated meter-in valve to which the fluid form the pump is supplied for controlling the direction of movement of the actuator, and pilot operated meter-out valves associated with each opening of the actuator for controlling the flow out of said actuator. The pressure of fluid being supplied to the actuator by the meter-in valve is sensed and caused to produce a force opposing the movement of the meter-out valve controlling flow from the actuator.