A linear actuator includes a ram and a piston separated by a column of hydraulic fluid in a housing. Upward movement of the ram in the housing first seals a finite quantity of hydraulic fluid in the housing cavity. Continued upward ram movement then causes the piston, and hydraulic fluid beneath it, to move upwardly into a sealed gas chamber to compress a gas and create a source of potential energy. Release of the force moving the ram upwardly permits the compressed gas above the piston to expel the hydraulic fluid from the chamber against the ram thereby converting the potential energy in the compressed gas into kinetic energy in the ram. The hydraulic fluid bears directly against the ram to provide the working stroke of the impact device without the effect of metal fatigue from direct contact between the piston and ram.