Page 456 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
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Piston
Case Drive shaft flange
drain

Outlet port
Mechanical
Inlet port input shaft

Diaplacement
control Drive shaft

Piston rod
Cylinder block
Valve plate

FIGURE 7.31: Bent-axis axial piston pump: variable displacement type. Reproduced with
permission from Eaton. Copyright 1999 Eaton. All Rights Reserved.

The only difference between them is the orientation of the rotation axis of the pistons
with respect to the input shaft. In the case of the inline piston pump, the mechanical input
shaft and piston rotation axes are inline, whereas in the case of the bent-axis type, they are
not inline. Varying the angle of the swash plate in the pump changes the displacement. The
◦
typical range of swash plate angle rotation is about 15 . The displacement of the pump is
controlled by controlling the swash plate angle between its limits, that is between 0 and
◦
15 . The oil is pushed into the pump through the intake side of the port plate, sometimes
called the kidney plate because of the shape of the ports. The cylinders are filled when they
pass the inside area and the oil is pushed out on the other side of the kidney plate. When
the cylinders travel along, the swash plate pushes the trapped oil towards the kidney plate
and therefore raises the pressure. When the swash plate is perpendicular to the rotation
axis, the displacement is zero. Then, the pump is in its stand-by position and does not
provide any flow. More accurately, at stand-by the pump provides flow only to compensate
for leakage flow. It is this capability (adjustable displacement) that accounts for the energy
saving potential of variable displacement pumps by providing adjustable flow rate based
on demand.
Axial piston pumps can be fixed displacement or variable displacement, unidirectional
or bidirectional, and may have over-center control (Figure 7.32). In a fixed displacement
pump, the swash plate angle is constant. In a variable displacement pump, the swash plate
angle can be varied by a control mechanism. A unidirectional pump is intended to be
driven by an input shaft in either a clockwise or counter-clockwise direction. If the pump
has the so called over-center control capability, which is the ability to change the swash
plate angle about its neutral position in either direction, the pump output flow direction
can be changed by the over-center control even though input shaft speed direction is same.
A bidirectional pump can be driven by an input shaft in either direction and the flow
direction changes with the input speed direction. An analogous operating principle applies
for hydraulic motors. In the case of motor, the output is the shaft rotation and input is the
fluid flow. A unidirectional motor is intended to receive flow in one direction and rotate
in one direction only. Bidirectional motors can receive flow in either direction, hence can