Hold-down device for dockleveller

Claim 1

A hold-down device for use in a dockleveller wherein a dockboard is pivotally mounted at its inner end to a base frame for rotation about said pivot between cross-traffic, fully raised and declined positions and wherein mechanical dockboard raising means are provided to rotate the dockboard upwardly from the declined and cross-traffic positions to the fully raised position, said hold-down device comprising: a hydraulic cylinder assembly having a stem end and a base end and comprising a piston element, having a piston head, and a cylinder element within which the piston element reciprocates, said assembly being mountable so that one of the elements may be interconnected with the dockboard and the other element may be interconnected with the base frame, whereby the cylinder assembly may expand and contract respectively in response to the dockboard being raised and lowered; said cylinder assembly having a fluid-filled stem chamber and a fluid-filled base chamber, the volume of fluid in the base chamber when the cylinder assembly is fully expanded being greater than the volume of fluid in the stem chamber when the cylinder assembly is fully contracted, said piston head being adapted to prevent fluid movement between the chambers; said fluid in the base chamber comprising a body of compressible fluid; first and second conduit means extending from the stem and base chambers; check valve means connecting said conduit means and controlling fluid movement through the conduit means between the chambers; said check valve means comprising a valve body forming a first valve chamber, a first port connecting the valve chamber with the conduit means communicating with the stem chamber, a second port connecting the valve chamber with the conduit means communicating with the base chamber, and a valve seat at the end of the valve chamber adjacent the second port, and a valve member disposed in the valve chamber, for seating on the valve seat to prevent flow through the valve chamber; first means for mechanically unseating the valve member to permit the cylinder assembly to be expanded as the dock-raising means raises the dockboard, said first means extending through the valve body wall in sealing engagement therewith and being exposed only to the low pressure in the base chamber; said check valve means being operative to close if there is a greater fluid pressure in the stem chamber than in the base chamber and the first means is inoperative, thereby preventing expansion of the cylinder assembly; said valve means being operative to open if there is a greater fluid pressure in the base chamber than in the stem chamber, thereby permitting the cylinder assembly to contract.