Fluid die spring

Claim 1

In a die assembly of the type which includes a pair of spaced die members mounted on supports guided for relative movement toward and away from each other to form a workpiece between the die members, the combination comprising, a fluid spring extending between one of said die members and its support, said fluid spring comprising a fluid cylinder fixedly mounted at one end on said support, said cylinder having an axial bore therein in which a close-fitting piston is axially slideable, said piston having a piston rod with a free end extending axially through the other end of the cylinder, said piston rod having a diameter less than the diameter of the cylinder bore so that the piston divides the cylinder into a working chamber at one side thereof and an annular chamber surrounding the rod at the opposite side thereof, said working chamber being connected with a fixed-volume reservoir of gas under relatively high pressure for biasing the free end of the piston rod against said one of said die members, said piston having an annular seal thereon for preventing leakage of said high pressure gas from said working chamber into said annular chamber, the other end of the cylinder having a high pressure annular cup-shaped seal with an open end thereon engaging the outer periphery of the rod, the last-mentioned seal being arranged on the cylinder with the open end of the cup facing axially away from the annular chamber so that, in the event the pressure in the annular chamber becomes sub-atmospheric, the open end of the cup is radially expanded by the differential pressure across the seal into firm sealing engagement with the cylinder and piston to prevent air and liquid contaminants from the atmosphere surrounding the cylinder from leaking past the last-mentioned seal into said annular chamber.