Axial Piston Pumps
In axial piston pumps of the in-line type,where the cylinders and the drive shaft are parallel (fig. 4-13), the reciprocating motion is created by a cam plate, also known as a wobble plate, tilting plate, or swash plate. This plate lies in a plane that cuts across the center line of the drive shaft and cylinder barrel and does not rotate. In a fixed-displacement pump, the cam plate will be rigidly mounted in a position so that it intersects the center line of the cylinder barrel at an angle approximately 25 degrees from perpendicular. Variable-delivery axial piston pumps are designed so that the angle that the cam plate makes with a perpendicular to the center line of the cylinder barrel may be varied from zero to 20 or 25 degrees to one or both sides. One end of each piston rod is held in contact with the cam plate as the cylinder block and piston assembly rotates with the drive shaft. This causes the pistons to reciprocate within the cyIinders. The length of the piston stroke is proportional to the angle that the cam plate is set from perpendicular to the center line of the cylinder barrel.
A variation of axial piston pump is thebent-axis type shown in figure 4-14. This type does not have a tilting cam plate as the in-line pump does. Instead, the cylinder block axis is varied from the drive shaft axis. The ends of the
Figure 4-14.—Bent-axis axial piston pump.
connecting rods are retained in sockets on a discthat turns with the drive shaft. The cylinder block is turned with the drive shaft by a universal joint assembly at the intersection of the drive shaft and the cylinder block shaft. In order to vary the pump displacement, the cylinder block and valve plate are mounted in a yoke and the entire assembly is swung in an are around a pair of mounting pintles attached to the pump housing. The pumping action of the axial piston pump is made possible by a universal joint or link.
Figure 4-13.—In-line axial piston pump.
Figure 4-15 is a series of drawings that illustrateshow the universal joint is used in the operation of this pump.
First, a rocker arm is installed on a horizontalshaft. (See fig. 4-15, view A.) The arm is joined to the shaft by a pin so that it can be swung back and forth, as indicated in view B. Next, a ring is placed around the shaft and secured to the rocker arm so the ring can turn from left to right as shown in view C. This provides two rotary motions in different planes at the same time and in varying proportions as may be desired. The rocker arm can swing back and forth in one arc, and the ring can simultaneously move from left
Figure 4-15.–Relationship of the universal joint in operationof the axial piston pump.
to right in another arc, in a plane at right anglesto the plane in which the rocker arm turns. Next, a tilting plate is added to the assembly. The tilting plate is placed at a slant to the axis of the shaft, as depicted in figure 4-15, view D. The rocker arm is then slanted at the same angle as the tilting plate, so that it lies parallel to the tilting plate. The ring is also parallel to, and in contact with, the tilting plate. The position of the ring in relation to the rocker arm is unchanged from that shown in figure 4-15, view C. Figure 4-15, view E, shows the assembly after the shaft, still in a horizontal position, has been rotated a quarter turn. The rocker arm is still in the same position as the tilting plate and is now perpendicular to the axis of the shaft. The ring has turned on the rocker pins, so that it has changed its position in relation to the rocker arm, but it remains parallel to, and in contact with, the tilting plate.
View F of figure 4-15 shows the assembly afterthe shaft has been rotated another quarter turn. The parts are now in the same position as shown in view D, but with the ends of the rocker arm reversed. The ring still bears against the tilting plate.
As the shaft continues to rotate, the rockerarm and the ring turn about their pivots, with each changing its relation to the other and with the ring always bearing on the plate.
Figure 4-15, view G, shows a wheel added tothe assembly. The wheel is placed upright and fixed to the shaft, so that it rotates with the shaft. In addition, two rods, A and B, are loosely connected to the tilting ring and extend through two holes standing opposite each other in the fixed wheel. As the shaft is rotated, the fixed wheel turns perpendicular to the shaft at all times. The tilting ring rotates with the shaft and always remains tilted, since it remains in contact with the tilting plate. Referring to view G, the distance along rod A, from the tilting ring to the fixed wheel, is greater than the distance along rod B. As the assembly is rotated, however, the distance along rod A decreases as its point of attachment to the tilting ring moves closer to the fixed wheel, while the distance along rod B increases. These changes continue until after a half revolution, at which time the initial positions of the rods have been reversed. After another half revolution, the two rods will again be in their original positions. As the assembly rotates, the rods move in and out through the holes in the fixed wheel. This is the way the axial piston pump works. To get a pumping action, place pistons at the ends of the rods, beyond the fixed wheel, and insert them into cylinders. The rods must be connected to the pistons and to the wheel by ball and socket joints. As the assembly rotates, each piston moves back and forth in its cylinder. Suction and discharge lines can be arranged so that liquid enters the cylinders while the spaces between the piston heads and the bases of the cylinders are increasing, and leaves the cylinders during the other half of each revolution when the pistons are moving in the opposite direction.
The main parts of the pump are the driveshaft, pistons, cylinder block, and valve and swash plates. There are two ports in the valve plate. These ports connect directly to openings in the face of the cylinder block. Fluid is drawn into one port and forced out the other port by the reciprocating action of the pistons.
IN-LINE VARIABLE-DISPLACEMENT AXIAL PISTON PUMP.— When the drive shaft is rotated, it rotates the pistons and the cylinder block with it. The swash plate placed at an anglecauses the pistons to move back and forth in the cylinder block while the shaft, piston, cylinder block, and swash plate rotate together. (The shaft, piston, cylinder block, and swash plate together is sometimes referred to as the rotating group or assembly.) As the pistons reciprocate in the cylinder block, fluid enters one port and is forced out the other.
Figure 4-13 shows piston A at the bottom ofits stroke. When piston A has rotated to the position held by piston B, it will have moved upward in its cylinder, forcing fluid through the outlet port during the entire distance. During the remainder of the rotation back to it original position, the piston travels downward in the cylinder. This action creates a low-pressure area in the cylinder. The difference in pressure between the cylinder inlet and the reservoir causes fluid to flow into the inlet port to the cylinder. Since each one of the pistons performs the same operation in succession, fluid is constantly being taken into the cylinder bores through the inlet port and discharged from the cylinder bores into the 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 in their cylinders; thereby, controlling the pump output.
When the swash plate is at a right angle to theshaft, and the pump is rotating, the pistons do not reciprocate; therefore, no pumping action takes place. When the swash plate is tilted away from a right angle, the pistons reciprocate and fluid 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 changes of this angle. Various methods are used to control this movement—manual, electric, pneumatic, or hydraulic.