rods, beyond the fixed wheel, and insert them intocylinders. The rods must be connected to thepistons and to the wheel by ball and socket joints.As the assembly rotates, each piston moves backand forth in its cylinder. Suction and dischargelines can be arranged so that liquid enters thecylinders while the spaces between the pistonheads and the bases of the cylinders are increasing,and leaves the cylinders during the other half ofeach revolution when the pistons are moving inthe opposite direction.The main parts of the pump are the driveshaft, pistons, cylinder block, and valve and swashplates. There are two ports in the valve plate.These ports connect directly to openings in theface of the cylinder block. Fluid is drawn into oneport and forced out the other port by thereciprocating action of the pistons.IN-LINE VARIABLE-DISPLACEMENTAXIAL PISTON PUMP.— When the drive shaftis rotated, it rotates the pistons and the cylinderblock with it. The swash plate placed at an anglecauses the pistons to move back and forth in thecylinder block while the shaft, piston, cylinderblock, and swash plate rotate together. (The shaft,piston, cylinder block, and swash plate togetheris sometimes referred to as the rotating group orassembly.) As the pistons reciprocate in thecylinder block, fluid enters one port and is forcedout the other.Figure 4-13 shows piston A at the bottom ofits stroke. When piston A has rotated to theposition held by piston B, it will have movedupward in its cylinder, forcing fluid through theoutlet port during the entire distance. During theremainder of the rotation back to it originalposition, the piston travels downward in thecylinder. This action creates a low-pressure areain the cylinder. The difference in pressure betweenthe cylinder inlet and the reservoir causes fluidto flow into the inlet port to the cylinder. Sinceeach one of the pistons performs the sameoperation in succession, fluid is constantly beingtaken into the cylinder bores through the inlet portand discharged from the cylinder bores intothe system. This action provides a steady,nonpulsating flow of fluid.The tilt or angle of the swash plate determinesthe distance the pistons move back and forth intheir cylinders; thereby, controlling the pumpoutput.When the swash plate is at a right angle to theshaft, and the pump is rotating, the pistons donot reciprocate; therefore, no pumping actiontakes place. When the swash plate is tilted awayfrom a right angle, the pistons reciprocate andfluid is pumped.Since the displacement of this type of pumpis varied by changing the angle of the tilting box,some means must be used to control the changesof this angle. Various methods are used to controlthis movement—manual, electric, pneumatic, orhydraulic.STRATOPOWER PUMP.— Another type ofaxial piston pump, sometimes referred to as anin-line pump, is commonly referred to as aStratopower pump. This pump is availablein either the fixed-displacement type or thevariable-displacement type.Two major functions are performed by theinternal parts of the fixed-displacement Strato-power pump. These functions are mechanicaldrive and fluid displacement.The mechanical drive mechanism is shown infigure 4-16. In this type of pump, the pistons andblock do not rotate. Piston motion is caused byrotating the drive cam displacing each piston thefull height of the drive cam during each revolutionof the shaft. The ends of the pistons are attachedto a wobble plate supported by a freed center pivotand are held inconstant contact with the cam face.As the high side of the rotating drive camdepresses one side of the wobble plate, the otherside of the wobble plate is withdrawn an equalamount, moving the pistons with it. The two creepplates are provided to decrease wear on therevolving cam.A schematic diagram of the displacement offluid is shown in figure 4-17. Fluid is displacedby axial motion of the pistons. As each pistonadvances in its respective cylinder block bore,pressure opens the check valve and a quantity offluid is forced past it. Combined back pressureand check valve spring tension close the checkFigure 4-16.—Mechanical drive—Stratopower pump.4-14
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