An apparatus for converting rotational movement into linear movement, said apparatus comprising: a screw member having a threaded external surface; and a sleeve member having a threaded interior surface for receiving the threaded external surface of the screw member; wherein the threaded surfaces are configured so that rotation of the screw member relative to the sleeve member, in use, causes the screw member to move axially relative to the sleeve member; and a plurality of conduits extending through the sleeve member to the threaded interior surface for applying pressure to a region between the threaded interior surface of the sleeve member and the threaded exterior surface of the screw member, characterized in that each of said plurality of conduits extends from an opening in a radially exterior surface of the sleeve member to two openings (18, 20) in a radially interior surface of the sleeve member such that, in use, fluid passes through each of said plurality of conduits from the opening in the radially exterior surface to the two openings (18, 20) in the radially interior surface; wherein the threaded interior surface of the sleeve member forms a helical ridge on the interior surface; and wherein each of the plurality of conduits extends radially inwards through the sleeve member and radially inwards through the ridge to the two openings (18, 20) in the interior surface, wherein the two openings (18, 20) are openings in both side walls (14,16) of said ridge; and wherein each of said plurality of conduits is formed by drilling radially inwards through the sleeve member and radially inwards through the ridge so as to form the conduit that is connected to the openings (18, 20) in the interior surface on both side walls of the ridge. a screw member having a threaded external surface; and a sleeve member having a threaded interior surface for receiving the threaded external surface of the screw member; wherein the threaded surfaces are configured so that rotation of the screw member relative to the sleeve member, in use, causes the screw member to move axially relative to the sleeve member; and a plurality of conduits extending through the sleeve member to the threaded interior surface for applying pressure to a region between the threaded interior surface of the sleeve member and the threaded exterior surface of the screw member, characterized in that each of said plurality of conduits extends from an opening in a radially exterior surface of the sleeve member to two openings (18, 20) in a radially interior surface of the sleeve member such that, in use, fluid passes through each of said plurality of conduits from the opening in the radially exterior surface to the two openings (18, 20) in the radially interior surface; wherein the threaded interior surface of the sleeve member forms a helical ridge on the interior surface; and wherein each of the plurality of conduits extends radially inwards through the sleeve member and radially inwards through the ridge to the two openings (18, 20) in the interior surface, wherein the two openings (18, 20) are openings in both side walls (14,16) of said ridge; and wherein each of said plurality of conduits is formed by drilling radially inwards through the sleeve member and radially inwards through the ridge so as to form the conduit that is connected to the openings (18, 20) in the interior surface on both side walls of the ridge.