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Pumps suitable for steering gear systems

Pumps used for supplying the working fluid to the main steering gear can be of either thevariable 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 pumpsare normally fitted. These are of two main types, the Hele-haw variable stroke pump

having radial cylinders and the swash plate variable stroke pump having axial cylinders.

Hele-Shaw Pump

!eft shows the construction and operation of this type of pump which is normally drivenby 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 circulargrooves 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 orneutral position.

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The action of the pump is shown above.

$t 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 %erodelivery then through %ero 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 "&..'#, it runs in the

flooded condition, the make up tank being above the level of the pump so that all theworking parts are immersed in oil. $t 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.

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The &' pump is stated to have some advantages over the Hele-haw , this is due to the

fact that the c of g of the Hele haw plungers is a relatively large distance from the centreof rotation operating relatively large centrifugal forces . The &' plunges have a c of g

close to the centre of rotation creating relatively small centrifugal forces, this means that

the &' system can be run at much higher speeds and therefore can be much smallerwhilst doing the same work as the Hele-haw. (ue to centrifugal forces acting, the wear

on the &..'. 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. )hen theunit comes on stroke the hydraulic pressure forces the two faces together.

*xternal pipes connect ports to steering gear.

&' pumps and Hele-haw pumps have an odd number of cylinders since calculation

shows that this gives better hydrodynamic balancing "and a better starting tor+ue when

used in a pump driving hydraulic motor#.

Auxiliary Pump

ome manufacturers supply an auxiliary pump driven from the main pump shaft, which

draws oil from the replenishment tank, delivering through non return valves to each side

of the main hydraulic system. low pressure relief valve opens to return the auxiliarypump delivery back to the replenishment tank if the main system is full, at the same time

keeping e+uivalent initial pressure on the whole system. This ensures the lubrication of

the main pumps when at no stroke and resists the ingress of air into the system. Pressurefrom this pump can also be used to power the automatic helmsman control, to operate

change over valves, or to power servo control units which in large installations may be

used to operate pump stroke mechanisms and so reduce the force re+uired from the

telemotor.

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Constant Pressure Pumps

The constant pressure delivery pump is a standard production line , cheap pump it runs

constantly delivering a set volume of li+uid whose pressure must be regulated.ecirculating v/v0s allow oil to by-pass rams when stationary, an oil cooler may have to

be incorporated.

&alves must be incorporated to divert the flow of oil to one side or the other. These

normally take the form of electrically operated solenoid valves which are sub1ect to wear,as well as damage to seats and solenoid coils.

hock loading to rams and pipework causing noise and vibration as well as damage.