Abstract: A pump for use in high pressure liquid chromatography includes a housing with a pump head at one end and an axially-movable, high-pressure piston with a small-diameter plunger reciprocating within the pump head to move fluid from an inlet through an outlet at high pressure. An attached motor rotates a circular cam fixed off center on the motor shaft to reciprocate the piston that has an inner end in engaging contact with the cam periphery. An axially-aligned, torque-compensating piston is provided and has an inner end biased by spring force against the cam opposite the high-pressure piston. A large-diameter piston head may be fixed to the outer end of the torque-compensating piston and slidably mounted within a cylinder defined at the housing end opposite the pump head. The outer end of the piston head is open to ambient atmosphere.

Abstract: A cryogenic hydraulic reciprocating piston pump includes a casing which defines a piston chamber. The sidewall of the piston chamber includes a retraction spill port as well as a pumping spill port. At the end of a retraction stroke, a retraction spill passageway that extends through the piston becomes aligned with the retraction spill port and fluid is communicated from the pressurized side of the piston to the unpressurized side of the piston to stop the retraction stroke before the piston “bottoms out”. Similarly, at the end of a pumping stroke, a pumping spill passageway that extends through the piston becomes aligned with the pumping spill port which provides communication between the pressurized side of the piston and the unpressurized side of the piston thereby stopping movement of the piston before it “bottoms out”.