| 摘要 |
PROBLEM TO BE SOLVED: To prevent the sudden lowering of a load by breakage of a load retaining conduit by communicating the pressure chamber of a cylinder device with the load retaining conduit at a pilot pressure below a prescribed pressure, and discharging the hydraulic fluid in the back pressure chamber of a pilot check valve at a pilot pressure exceeding the prescribed pressure. SOLUTION: A selector valve 19 is switched to position (y) at a pilot pressure not more than a prescribed pressure to close a pilot passage 8, and a valve member 13 is seated on a valve seat 16 by the load pressure of a back pressure chamber 14 and a spring 15 to arrest the flow from the bottom-side pressure chamber 3a of a cylinder device 3. A branch passage 18 is allowed to communicate with a load retaining conduit 5 through a diaphragm 20 to discharge a hydraulic fluid to a tank according to the opening of the diaphragm 20 and a control valve 2. When the pilot pressure exceeds the prescribed pressure, the selector valve 19 is switched to position (z) to separate the valve member 13 from the valve seat 16, and most of the hydraulic fluid is passed through a pilot check valve 7 and discharged to the tank through the valve 2. Accordingly, even if the load retaining conduit 5 is broken when the selector valve 19 is in the position (y), the hydraulic fluid is never discharged at once, so that an intended matter can be prevented from suddenly lowering.
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