The steering gear system above consists of the telemotor which receives a signal from the bridge wheel. This acts on the hunting gear.

The hunting gear moves displacing a control rod, this rod acts on the pump displacement control gear to alter the delivery from the pump. The delivery from the pump causes the ram to move rotating the rudder stock and hence the rudder. The other end of the hunting gear is mounted on the rudder stock.

The rotation of the rudder stock moves the hunting gear returning the operating rod for the pump to the neutral position once the rudder has reached the correct angle.

Pumps used for supplying the working fluid to the main steering gear can be of either the variable capacity reversible delivery type or the fixed delivery non-reversible type. For large capacity outputs with high rates of change in demand, the variable capacity pumps are normally fitted. These are of two main types, the Hele-Shaw variable stroke pump having radial cylinders and the swash plate variable stroke pump having axial cylinders.

Hele-Shaw Pump

Left shows the construction and operation of this type of pump which is normally driven by a constant speed electric motor. The pistons are fitted in a row of radial cylinders and through the outer end of each piston is a gudgeon pin , which attaches the slippers to the piston. The slippers are free to oscillate on their gudgeon pins and fit into circular grooves in the circular floating ring . This ring is free to rotate being mounted on ball bearings , which are housed in guide blocks , this reduces oil churning and friction losses. The latter bear on tracks are controlled by the actuating spindles, which passes through the pump casing. The movement of the floating ring by the actuating control spindle (operated by, say, the telemotor receiver) from the central position causes pistons to reciprocate in the radial cylinders so that a pumping action takes place. The direction of the pumping depends upon whether the movement is to the left or right of the central or neutral position.

The action of the pump is shown above.

It should be noted that an advantage with this system is that in reversing the direction of the flow of fluid, the pump moves from maximum delivery in one direction across to zero delivery then through zero delivery to maximum delivery in the opposite direction. The build up in fluid pressure taking place without shock loading of pipe lines supplying fluid to the main steering unit.

The pump is usually provided with an odd number of cylinders, usually seven or nine, which produces more even hydraulic flow and a better balanced pump.

Variable Stroke Reversible Swashplate Pump

This pump is sometimes called the variable stroke gear pump (V.S.G), it runs in the flooded condition, the make up tank being above the level of the pump so that all the working parts are immersed in oil. It is driven by a constant speed electric motor the volume and direction of the oil flow being controlled by means of a stroke control lever.

The VSG pump is stated to have some advantages over the Hele-Shaw , this is due to the fact that the c of g of the Hele Shaw plungers is a relatively large distance from the centre of rotation operating relatively large centrifugal forces . The VSG plunges have a c of g close to the centre of rotation creating relatively small centrifugal forces, this means that the VSG system can be run at much higher speeds and therefore can be much smaller whilst doing the same work as the Hele-Shaw. Due to centrifugal forces acting, the wear on the V.S.G. pump pistons can be greater than that for the radial type pistons There is a small clearance between the valve and cylinder blocks when running off load. When the unit comes on stroke the hydraulic pressure forces the two faces together.

External pipes connect ports to steering gear.

VSG pumps and Hele-Shaw pumps have an odd number of cylinders since calculation shows that this gives better hydrodynamic balancing (and a better starting torque when used in a pump driving hydraulic motor).