BOOK IN SERIES
Yet, these pumps have some drawbacks too, i.e. a design
focused on sustaining around 150 – 280 bar, their unsuitability
for high flow applications, rather loud noise and a rather poor
overall efficiency.
Nonetheless, the strong demand for these hydraulic
generators prompts manufacturers to carry out research
and improve their products by employing special materials,
accurate heat treatments, minimum coupling tolerance
between wheels and surface precise finish in order to reduce
the drawbacks mentioned above.
External gear pumps (Figure 4.5) are essentially made up of
two twin gearwheels geared with each other that are held
in a totally smooth stator housing so as to prevent leakages
between moving and fixed parts; the 8-shaped bearings, held
in the stator along with gears, counterbalance side hydraulic
thrusts by means of dedicated seals.
Bearings or bushings are positioned in the two coversopposite
the stator where gear spindles revolve; the transmission shaft
between the prime mover and the pilot wheel is in the hole
of the front cover provided with a static seal (see Figure 4.9
and Figure 4.10). Like in most rotary and reciprocating pumps,
the inlet has a larger bore than the outlet and both of them
are usually positioned one opposite the otheron the stator
housing.
When the motor is started (Figure 4.6), lead wheel A, firmly
connected to it, makes the driven gear B move into the
opposite direction. The rotation of the teeth that come out
of the mesh entails a vacuum whose volume is equal to the
space between the two teeth; in other words, as shown
in the Figure, the tooth x that was previously in the space
between teeth y and z rotates clockwise, thus clearing a
space (previously inexistent, hence a vacuum) equal to this
volume.
During this very short phase (rotational speed can range from
few hundreds revolutions per minute to 3000 rpm), the fluid
pushed by the atmospheric pressure on the free surface of
the tank fills the space between the teeth w and k of gear A
and it is driven clockwise to the outlet. In the following phase,
the vacuum generated by the tooth y allows teeth z and r of
gear B to drive anticlock wise the oil to the right part of the
stator. Obviously, these phases alternate so as to conclude
the revolution.
Figure 4.5
When the actuating circuit is operated, delivery pressure
should be the same throughout the whole upper chamber;
actually, the inevitable leakages cause the pressure of
the whole active part (oil transferring chambers) to be
proportionally distributedand to decrease from the outlet to
the first space after suction.
The pressurised fluid inside the pump subjects gearwheels to
considerable radial loads (Figure 4.7).
91
Another problem is the accurate positioning of gearplane
faces vis-à-vis cover surface plates. If they do not form
a perfect right angle with tolerances even lessthan 2-3
thousandths of millimetres, this would promote substantial
leakages, early wear and the ensuing seizing.
Figure 4.5
www.ghmediabusiness.com
That is why it is important to use components with more
suitable tolerances and methods to limit axial and radial
Figure 4.7