A pump apparatus includes: a pump that ejects a hydraulic fluid; and a selector valve that switches a direction of a flow of the hydraulic fluid to be supplied to one of a first chamber and a second chamber of a cylinder apparatus, which is internally partitioned by a piston into the first chamber extending during an extending stroke of the cylinder apparatus, and the second chamber extending during a shortening stroke of the cylinder apparatus, and the selective valve has, at a channel connected to the second chamber, an orifice that is narrower than a channel connected to the first chamber.

A discharge plate (16) for a rotating screw machine (10), said rotating screw machine comprising: a rotor housing (12) defining an internal cavity (18) in which a plurality of rotatable screw rotors (20, 22) are located, and an outlet casing (14) defining a discharge chamber (28), wherein: said discharge plate (16) is configured to define a plurality of pressure balancing ports (39; 56) that each provide a passageway through the plate, and said discharge plate further (16) comprises a plurality of valves (52; 58) coupled to a face of the plate (16), each said valve being moveable between a position where the valve closes one or more of the pressure balancing ports (39; 56) and a position where the valve opens one or more of the pressure balancing ports (39; 56); wherein the ports (39; 56) and valves (52; 58) are arranged so that the valves (52; 58) can open into the discharge chamber (28) to open one or more of said ports (39; 56) when the discharge plate (16) is coupled between the rotor and outlet housings (12, 14) of the screw machine (10) and thereby provide one or more passageways between the rotor housing (12) and the discharge chamber (28) that enable pressure balancing to occur between the internal cavity (18) of the rotor housing (12) and the discharge chamber (28), the discharge plate (16) being configured so that fluid can pass from the rotor housing (12) to the discharge chamber (28) only via one or more of said valves (52; 58) when compression pockets between the rotors (20, 22) align with said pressure balancing ports (39; 56).