BOOK IN SERIES
Under other respects, this hydraulic generator is more similar
to standard internal gear pumps (see next paragraph), yet
it cannot fall into this category because it does not have a
component known as ‘prefill’ and it has much lower working
pressures.
This component, referred to as ‘low-pressure internal gear
pump’ in manufacturer catalogues, is characterised by a
peculiar innovation. An insert made of a deformable material
is embedded in the external part of each tooth of the
peripheral wheel; the insert balances the tolerance between
the teeth of the ring wheel and the central wheel solidly
connected to the transmission shaft, thus ensuring a firm
tightness between suction and delivery chambers (Figure
4.20).
Figure 4.21
Figure 4.20
Low-pressure internal gear pumps are available in different
versions with displacement ranging between 32 and 200
cm3/rev, pressures between 60 and 125 bar and a speed
between 400 and 3500 rpm. Both volumetric and overall
efficiency amount to about 90% under optimum conditions,
absolute suction pressure ranges between 0.6 and 0.8 bar, it
is sensitive to fluid contamination(inserts subjected to wear).
They are really noiseless and their viscosity range is very
interesting.
Internal gear pumps
The main features of internal gear pumps are noiselessness,
limited flow pulsations, higher pressures than similar external
gear pumps, high volumetric and overall efficiency. Their
main drawbacks that affected their popularity are, above all
low flow rates and a more complex design than external gear
pumps (resulting in higher costs).
Present versions have accurate axial and radial balance, as
well as an enhanced prefill part; they can reach working
pressures above 300 bar, volumetric efficiency above 95%,
overall efficiency above 90%, rotational speed between
400 and 3500 rpm (or 300 l/min at about 1500 rpm in large
versions with 200 cm 3 ).
Figure 4.22
The clockwise revolution of the internal gear (2) (Figure 4.21),
solidly connected to the transmission shaft and equipped
with bronze bearings (6) mounted on flexible bearings (5),
meshes with the peripheral gearwheel (1), which entails
fluid suction almost in the same manner it occurs in external
gear pumps. The engagement and position in line with the
prefill part (3) are promoted by the spring over the outlet that
exerts a thrust on the balancing part (8); the prefill element,
swinging around the pivot (9), moves to the middle of the
teeth of the gears.
The task of the filling element (usually called ”moustache”)
is to separate suction areas from delivery areas: the fluid
between the gear teeth, held by the gear surfaces, cannot
flow back into the inlet and it finally flows out through the
outlet after travelling a short rotational angle between the
teeth themselves. The peripheral gear (1) allows the fluid to
flow, first inside the pump and then into the outlet because
it has clearing holes that correspond to each internal part of
the teeth (root throat). The parts (7) adhering to the wheel
(1) promote axial balancing while radial balancing is ensured
by the parts (8) moved by the spring when the pressurised
may 2018 Global MDA Journal
61