Rotary vane steering gear

The fixed vanes are cast into/keyed into the cylinder (the stationary housing) and project radially inwards; the moving (rotor) vanes are cast on to/attached to the rotating rotor (coupled to the rudder stock) and project radially outwards, interleaving with the fixed vanes to form radial divisions. High-pressure oil pumped into the space between one side of the fixed and moving vanes pushes the moving vanes (and so the rotor and rudder stock) to the low-pressure side, rotating the rudder.

Several sets of vanes (normally three or four rings) are provided. The number of sets is limited by the available axial length of the rotor/cylinder and the spacing needed for the vane travel and sealing; i.e. the limitation factor is the space available and the requirement that each set carry a share of the torque within the actuator's stroke.

The moving vanes are made strong (heavy section, high-strength material) so that when the rudder is driven to hard-over the moving vane comes solidly against the stop (the fixed structure/end cover or a machined stop face) and arrests further travel, thus acting as a mechanical rudder stop that withstands the impact of flow/hydrodynamic loads without bending or fracturing.

The rotor is axially located in the cylinder so that vertical movement (rudder drift/up-lift arising from the hydrodynamic thrust tending to lift or drop the rudder) is restrained. This is achieved by providing axial thrust faces/bearings on the rotor or by designing the vanes so axial loads are carried by a supporting surface (e.g. the base/cover and a thrust face on the rotor), while the vane sealing rings keep the divisions pressure-tight. Thus the rotary vane unit both rotates the rudder and carries the vertical rudder load without permitting the rotor to ride up or down out of the cylinder.