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Hydraulic Pumps 105

The cylinder block (5) and its pistons (6), rotate with the drive shaft.
Each of the pistons is attached to a slipper pad (3). The pistons and
their slipper pads are inserted in the holes of the retaining plate (4).
Therefore, the retaining plate rotates with the pistons and the cylin-
der block. It is guided to rotate in a plane parallel to the swash plate
(2) by a fixed guide (8). The trajectory of the slipper pad is determined
by the swash plate and the retaining plate. During rotation, each pis-
ton performs a reciprocating motion. During this process, a volume
of fluid, corresponding to the piston area and stroke, is sucked or
delivered via both control openings in the port plate (7).
The cylinder block is pushed against the port plate by means of a
spring (9), which minimizes the leakage through the clearance separat-
ing them at the beginning of the pump operation. When the pressure
builds up, it acts on the cylinder block with a tightening force given by
2
2
{0.25π (d − d )P}. This force acts to the right, against the repulsion force
h
due to the pressure distribution in the clearance between the cylinder
block and the port plate. The resultant force acts to reduce this clearance
and minimize the leakage through it. The pump geometric volume is
given by the expression
π
2
V = d Dztan α (4.25)
g
4
where α = Swash plate inclination angle, rad.

4.6.3 Swash Plate Pumps with Inclined Pistons
The swash plate pump with inclined pistons, also called a semi-axial
piston pump, is produced with cylinder holes inclined to its axis (see
Fig. 4.17). In this case, the pistons reciprocate in a direction inclined
to the axis of rotation by an angle, ϕ. The swash plate also inclines by
an angle, α. This design increases the piston stroke and the pump
geometric volume. In addition, the centrifugal force acting on the
pistons has a component in the direction of the pistons’ axis, F ,
x
which assists the suction stroke. The following expressions for the
piston stroke and the pump geometric volume are systematically
deduced.

α
Dsin( ) Dsin( )α
L = 2cos(ϕ − ) and L = 2cos(ϕ + ) (4.26)
α
α
1
2
π ⎧ 1 1 ⎫
V = d Dzsin( α) ⎨ + ⎬ (4.27)
2
−
+
g 8 ⎩ cos( ϕα) cos( ϕα) ⎭