Abstract: A device for clamping an axially movable rod comprising a housing having a cylindrical inner wall and end walls that slidably support the rod. A pair of annular rod clamp rings in the housing having inner clamping surfaces for engaging the rod and frusto-conical outer surfaces, and a pair of annular pistons in the cylinder having an inner surface engaging the outer surface on the annular rod clamp rings and an outer surface slidable in the cylindrical inner wall of the housing. The pistons have spring receiving cavities radially intermediate the inner and outer surfaces thereof and the springs disposed in the cavities for moving the pistons axially in a direction to clamp the rod. A fluid pressure chamber is provided in the cylinder for actuating the pistons in a direction to release the rod.