Pumps suitable for steering gear systems 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). Auxiliary Pump Some 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. A low pressure relief valve opens to return the auxiliary pump delivery back to the replenishment tank if the main system is full, at the same time keeping equivalent 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. Pressure from 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 required from the telemotor. Constant Pressure Pumps The constant pressure delivery pump is a standard production line , cheap pump; it runs constantly delivering a set volume of liquid whose pressure must be regulated .Recirculating v/v's allow oil to by-pass rams when stationary, an oil cooler may have to be incorporated. Valves 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 subject to wear, as well as damage to seats and solenoid coils. Shock loading to rams and pipework causing noise and vibration as well as damage.
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