BOOK IN SERIES
The centrifugal force is not enough to set vanes into motion
below 600 revolutions per minute (a few dozens more or less
depending on the materials and the design; the adhesion
force of the vanes to the stator affects their life at a rotational
speed above 2500 rpm.
Figure 4.25
In any case, the centrifugal force alone cannot guarantee
a high efficiency, especially at rather low speeds (around
800-1000 revolutions). Vanes adhere to the eccentric stator
thanks to the pressurised oil sent to their back head through
a clearance inside the rotor that connects the outlet to the
vane seats. During the inlet the vanes cause a vacuum that
sucks the fluid from the tank, while during the delivery the
liquid is sent to the outlet.
Balancing is not problematic because the pressurised fluid
in motion fills the interstices between the screws and the
pump casing, thus generating compensating radial forces;
the translation is axial and the fluid found between the rotors
avoids friction. A very accurate design of threads prevents the
fluid from being squeezed between the parts in mesh (see
gear pumps) and it also ensures the most constant flow rate
among volumetric pumps; the slight pulsating flow virtually
results from the compressibility of the liquid inside the outlet.
FIXED AND VARIABLE DISPLACEMENT PUMPS
The hydraulic pumps described so far are only fixed
displacement pumps due to their design. Other types can
be manufactured in fixed or variable displacement versions
depending on their operational peculiarities.
Fixed displacement vane pumps
Fixed displacement vane pumps are popular among many
self-propelled machines manufacturers and widely used
in medium-power stationary applications because of their
design simplicity, good value for money, reasonable efficiency
and the ability of balanced versions to reach higher average
pressures than other rotary pumps.
The essential parts of a vane pump (Figure 4.26) are the
splined rotor solidly connected to the drive shaft, rectangular
vanes arranged in a radial manner inside the stator and free
to slide into the slotsand the stator, which is eccentric vis-à-
vis the rotor and mounted on the internal wall of the casing.
The operating principle of any vane pumps is the vane shifting
into the slot, from the seat towards the eccentric stator.
When the prime mover is started, the vanes at rest inside
the slots tend to move towards the stator wall due to the
centrifugal force. The effect of the centrifugal force should
be considered carefully, except for some old versions where
vanes were directly connected to the stator through springs
inside the splines (nowadays they are still essential in systems
that need quite few revolutions).
Figure 4.26
In order to ensure a perfect tightness with the stator, vanes
have a sharp-edged end; the vertex must be positioned
towards the direction of the revolution, as shown in Figure
4.27. If the vertex was in the opposite direction, fluid pressure
would act on the vane end pushing it towards the seat.
Figure 4.27
If we observe the revolution of a single vane in obsolete fixed
displacement versions starting from the lower part (Figure
4.28), we can see that suction covers half round angle while
the other 180° are devoted to delivery. This design promotes
an excellent translation of the fluid but the whole right part,
subjected to circuit pressure, triggers a substantial radial
thrust on the rotor with the ensuing load unbalance. As we
are going to see later on, this system is still applied to variable
displacement vane pumps that cannot be modified due to
their design, yet better-balanced systems are preferred in
fixed displacement versions.
may 2018 Global MDA Journal
63