Hele-Shaw PumpLeft 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.
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
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 PumpSome 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 PumpsThe 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.