Control valve for vehicle power steering systems

A hydraulic control valve for a power assisted steering system for a vehicle, the valve comprising a sleeve member having a bore in the surface of which is a first set of longitudinally extending grooves separated by lands, a valve core fitting closely within the bore of the sleeve member and relatively rotatable therein, a second set of longitudinally extending grooves, also separated by lands, on the surface of the valve core, each groove of the first set lying opposite to a land of the second set, the width of some at least of the grooves or lands varying along their lengths, an axially extending torsion bar connecting the valve core to the sleeve member, means to conduct hydraulic fluid under pressure into and out of said grooves, relative rotation between the valve core and the sleeve member acting to vary the effective width and therefore the area of orifices defined by adjacent edges of opposing grooves and lands, and so to control the restriction to flow of hydraulic fluid within the valve and thus the power assistance provided to the system, characterised in that the sleeve member and the valve core are relatively movable axially to change the length and thus further change the area of said orifices available for the flow of hydraulic fluid and thereby alter the degree of assistance provided.