The flow in this pump can be reversed bymoving the slide block, and therefore the rotor,to the right so the relation of the centers of therotor and the cylinder block is reversed from theposition shown in views A and B of figure 4-11.View D shows this arrangement. Liquid enters thecylinder as the piston travels from position 1 toposition 2 and is discharged from the cylinder asthe piston travels from position 3 to 4.In the illustrations the rotor is shown in thecenter, the extreme right, or the extreme left inrelation to the cylinder block. The amount ofadjustment in distance between the two centersdetermines the length of the piston stroke, whichcontrols the amount of liquid flow in and out ofthe cylinder. Thus, this adjustment determines thedisplacement of the pump; that is, the volume ofliquid the pump delivers per revolution. Thisadjustment may be controlled in different ways.Manual control by a handwheel is the simplest.The pump illustrated in figure 4-11 is controlledin this way. For automatic control of deliveryto accommodate varying volume requirementsduring the operating cycle, a hydraulicallycontrolled cylinder may be used to position theslide block. A gear-motor controlled by a pushbutton or a limit switch is sometimes used for thispurpose.Figure 4-11 is shown with four pistons for thesake of simplicity. Radial pumps are actuallydesigned with an odd number of pistons (fig.4-12). This is to ensure that no more than onecylinder is completely blocked by the pintle at anyone time. If there were an even number of pistonsspaced evenly around the cylinder block (forexample, eight), there would be occasions whentwo of the cylinders would be blocked by thepintle, while at other times none would beblocked. This would cause three cylinders to dis-charge at one time and four at one time, causingpulsations in flow. With an odd number of pistonsspaced evenly around the cylinder block, only onecylinder is completely blocked by the pintle at anyone time. This reduces pulsations of flow.Figure 4-12.—Nine-piston radial piston pump.4-11
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