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HYDRAULICS
in Industrial and Mobile Applications
CHAPTER 4
OIL HYDRAULIC PUMPS
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Piston pumps– Preliminary remarks
Considerable flow rates, optimum efficiency, the ability to
reach high pressures and, in variable displacement versions,
the availability of many and precise controllers make piston
pumps the best pumps for fluid power. Piston pumps
guarantee higher performances than the pumps described
so far, but they are more expensive, more complex and need
more maintenance.
Besides manual pumps, piston pumps can mainly be
classified as radial and axial pumps. The cylindrical pumping
parts in radial piston pumps are arranged in the shape of a
star; in some cases, they are rotary and held in a block solidly
connected to the transmission shaft, while in others they form
a fixed radial block and are moved by an eccentric shaft. Axial
piston pumps have different pistons aligned and arranged in
the shape of a circle; the cylinder block can be in line with
the axis of the shaft connected to the prime mover and it
is either rotary and solidly connected to the shaft or fixed
while the shaft rotates. The so-called inclined plate ensures
the alternation of suction/pumping. In bent axis pumps, the
plate is perpendicular to the drive shaft and the transmission
of power to the pumping block is carried out by connecting
rods, cardan joint or angle gear.
The phenomenon of pulsating flow dealt with in the paragraph
on rotary pump is even more troublesome in reciprocating
piston pumps because the design of their pistons cannot
allow a totally constant flow.
Flow oscillation depends on the number of pumping parts. If
by ‘cycle’ we mean the whole stroke of a piston (suction and
delivery), the delivery cycle in a manual pump with a single
pumping part (single-acting pump) does not occur during
the first half (suction), whereas during the second half the
flow rate soars from zero to the maximum level and then
diminishes to zero again. Flow in double-acting pumps (two
62 | JuNe-july 2018 | Global MDA Journal
pistons with opposite motions) is incessant but very pulsating
(Figure 4.50).
Figure 4.50
The more right phase timed pistons there are, the less the
flow oscillates (Figure 4.51).
Figure 4.51
The ratio between the flow variation (Qmax – Qmin) and the
average flow rate Qaverage,is the irregularity grade G.
Given a single-acting manual pump that has an average flow
of 3.18 l/min and a maximum flow of 10 l/min, the irregularity
grade G is:
If we compare the oscillating trend of three-piston pumps
whose G is 14 and four-piston pumps whose G is 32.5 or six-
piston pumps whose G is 14 and seven-piston pumps whose
G is 2.5, it is clear that the irregularity is less substantial
in odd-numbered piston pumps because flows are more
overlapping.