The flow in this pump can be reversed by
moving the slide block, and therefore the rotor,
to the right so the relation of the centers of the
rotor and the cylinder block is reversed from the
position shown in views A and B of figure 4-11.
View D shows this arrangement. Liquid enters the
cylinder as the piston travels from position 1 to
position 2 and is discharged from the cylinder as
the piston travels from position 3 to 4.
In the illustrations the rotor is shown in the
center, the extreme right, or the extreme left in
relation to the cylinder block. The amount of
adjustment in distance between the two centers
determines the length of the piston stroke, which
controls the amount of liquid flow in and out of
the cylinder. Thus, this adjustment determines the
displacement of the pump; that is, the volume of
liquid the pump delivers per revolution. This
adjustment may be controlled in different ways.
Manual control by a handwheel is the simplest.
The pump illustrated in figure 4-11 is controlled
in this way. For automatic control of delivery
to accommodate varying volume requirements
during the operating cycle, a hydraulically
controlled cylinder may be used to position the
slide block. A gear-motor controlled by a push
button or a limit switch is sometimes used for this
purpose.
Figure 4-11 is shown with four pistons for the
sake of simplicity. Radial pumps are actually
designed with an odd number of pistons (fig.
4-12). This is to ensure that no more than one
cylinder is completely blocked by the pintle at any
one time. If there were an even number of pistons
spaced evenly around the cylinder block (for
example, eight), there would be occasions when
two of the cylinders would be blocked by the
pintle, while at other times none would be
blocked. This would cause three cylinders to dis-
charge at one time and four at one time, causing
pulsations in flow. With an odd number of pistons
spaced evenly around the cylinder block, only one
cylinder is completely blocked by the pintle at any
one time. This reduces pulsations of flow.
Figure 4-12.—Nine-piston radial piston pump.
4-11
