| 摘要 |
A releasable hold-down device is provided for use in a dockleveller wherein dock-raising springs are used to raise the dockboard. The hold-down device comprises a hydraulic cylinder assembly having the piston connected to the dockboard and the cylinder connected to the dockleveller frame. The cylinder assembly must thus expand and contract to permit the dockboard to be raised by the spring and lowered by the weight of a man standing on the dockboard. The cylinder assembly has fluid-filled stem and base chambers. A connecting conduit permits fluid movement between the chambers. A valve means in the conduit controls this fluid movement. There is provided means for mechanically opening the valve means to permit the dockboard to be raised by the spring with concomitant expansion of the cylinder assembly. When such mechanical opening means is inoperative, the valve means is operative to (1) close if there is a greater fluid pressure in the stem chamber than in the base chamber (thus locking the device and preventing the spring from raising the dockboard), and (2) open if there is a greater fluid pressure in the base chamber than in the stem chamber (thus permitting the cylinder assembly to contract when a weight acts on the raised dockboard).
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