BOOK IN SERIES
Unlike the pressure in the fluid film between the bushes and
the rotor/stator unit, the pressure in these basins offsets the
pump axial loads.
Figure 4.34
In everyday language, standard pumps with single inlet and
outlet are referred to as ‘round’ and radially balanced pumps
as ‘square’ (Figure 4.35) due to their shape.
Figure 4.37
Vanes must be balanced in order to ensure high performances.
An effective system consists in putting double vanes with
opposed sharp edge in the slots (Figure 4.36). The tightness
between vanes and the stator virtually doubles and, in
order to reduce the ensuing friction, opposite cavities are
manufactured on the sides so that the pressurised fluid flows
through them and, only in the delivery area, it offsets stator
pressure.
Figure 4.38
Figure 4.35
The pressure at the base of the vane in intravane systems
(Figure 4.37) depends on its positioning: the base of the
vane is alternately subjected to suction and delivery pressure
during the revolution. The slot between the small insert and
the vane is constantly subjected to delivery pressure.
The fluid is routed to the base of the vane through small
double clearances between its seatand the external part of
the rotor (Figure 4.38 – A e B –). The constantly pressurised
fluid flows to the cavity between the insert and the vane via
a clearance between the slot and the plane face of the rotor
(Figure 4.38 – C –), which is routed by the wear plate. The
insert, driven by the pressurised fluid to the ring, does not
move in the lower part of the slot.
Figure 4.36
In single vane pumps, the thrust fluid at the base of each
vane is always at delivery pressure. This ensures an optimum
tightness, but it also results in a strong friction on the stator
wall in suction areas, which makes it wear out quickly. This
may 2018 Global MDA Journal
65