| 摘要 |
A pilot operated directional control valve communicates fluid between a hydraulic actuator, a pump and a reservoir. A lockout valve is connected between each main control valve work port and a corresponding cylinder port, and operates to reduce fluid leakage therefrom. Each lockout valve includes a pressure responsive poppet valve member which is exposed to fluid pressure in a lockout chamber. A vent control valve is connected between each lockout valve and a corresponding one of the pilot valves. Each vent control valve includes a vent check valve which controls fluid venting from the lockout chamber and a vent piston which is biased to urge the vent check valve to a closed position thereby trapping fluid in the lockout chamber. When both pilot valves are energized, both lockout valves are vented and a float valve moves to a position wherein both cylinder ports are communicated, via the lockout valves and the float valve, with each other and with the reservoir. The float function is obtained without the requirement of additional solenoids or by compromising the available modulation range of the main control valve.
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