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Radial piston pumps - Fixed displacement
Radial piston pumps with rotary radial block and eccentric
reaction ring are made up of a series of (usually odd-
numbered) pistons, arranged in a radial manner and held in
a block solidly connected to the prime mover via a cruciform
joint. Piston ends are attached to ball socket joints fixed to
the sliding blocks resting on the reaction ring(Figure 4.52).
This ring can revolve freely on a large bearing fixed to the
pump casing; the distributor shaft, on the same axis as the
drive shaft, is solidly connected to the rotary block.
When the pump is revolving, the centrifugal force in the
suction area (and the possible pressure of the fluid if there is
a booster pump) and the pressure in the delivery area push
pistons towards the reaction ring that avoids the friction of
the sliding blocks because it rotates on the bearing.
Some clearances inside the rotary distributor shaft connect
the inlets and the outlets to their pistons, so that the fluid is
sucked into the cylinders travelling the extentstroke during
the first 180° of each revolution and then it is driven out via
the outlet and the pressure port during the second half.
In radial piston pumps equipped with a fixed radial block and
an eccentric drive shaft, the transmission shaft is eccentric
vis-à-vis the pistons held by an adequately dimensioned
spring.
In the versions with a bearing on the eccentric shaft, the lower
ends of each piston do not wear out thanks to the bearing.
The shaft revolution causes the pistons to reciprocate, thus
promoting suction and delivery (Figure 4.53). Suction involves
the cross drains of the distributor that is coaxial vis-à-vis the
drive shaft; the fluid inside the pump casing flows from the
side clearings to the pistons and, if the suction valve is open,
it fills its cylinder.
Figure 4.52
During the following phase, the piston presses on the fluid,
thus switching on the delivery valve that allows the liquid
to flow out through dedicated drains and the outlet. The
connection of the pressure ports permits to connect more
devices on the auxiliary openings.
This design demands a small number of low-bore pistons
because the double ball bearing on the eccentric shaft can
sustain only limited dynamic loads. As a result, in order to
reach higher pressures and flow rates, it is essential to replace
the ball bearing on the eccentric shaft with hydrostatic
sliding blocks. The fluid film between the eccentric shaft and
the mirror-like part of the sliding blocks serves as a bearing
absorbing the radial stress due to the pressurisation of the
connected pistons. In this version, the pump, except for the
sliding blocks, built and operated with the same characteristics
as the previous one, can sustain better dynamic loads; this
promotes a higher number and bore of pistons with the
ensuing increase in pressure and flow (Figure 4.54).
Figure 4.53
Radial piston pumps equipped with hydrostatic sliding
blocksensure flow rates up to 200 l/min, working pressures
JuNe-july 2018 | Global MDA Journal | 63