Service Manual

HYDRAULIC COMPONENTS (cont’d)
Pump Function
A pump is a device that transfers fluids by either
suction, pressure or both.
A pump converts
mechanical energy into hydraulic pressure and
flow. Pumps can either be fixed or variable
displacement.
A fixed (constant) displacement pump
transfers a set amount of fluid during each
revolution. To change the output, the speed at
which the pump operates must be changed.
A variable displacement pump can change its
output per revolution.
Pumps By Type
Axial Piston
design pumps use a set of pistons
rotating in a cylinder block. The cylinder block is
rotated by the input shaft. The end of the pistons
contact a swashplate. The swashplate may be a
fixed angle (constant displacement) or
adjustable to a range of angles (variable
displacement).
As the cylinder block rotates, the piston (either
pulled by the swashplate or pushed by spring
pressure) extends out of the cylinder block and
draws oil into the chamber. On the opposite
side, the swashplate forces the piston in,
displacing oil from the base of the cylinder block.
Radial-Ball Piston pumps use several balls
which travel through bores inside a rotor.
As the
rotor is turned by the input shaft, centrifugal
force throws the balls out against a cam ring. Oil
from a passage in the pintle flows into the
chamber behind the ball.
With the cam ring offset, the ball is pushed back
into the bore as it rotates past the narrow side.
This forces oil back into a second passage in
the pintle.
The cam ring offset can be either fixed to
provide a constant output, or adjustable to
provide variable output.
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HYDRAULIC
I
3.4297.402
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HIGH PRESSURE
OIL OUT
LOW PRESSURE
OIL IN
Productivity Series