BOOK IN SERIES
fluid is sent through tiny clearances in the lower part.
Lobe pumps
Two gears, one of which is axially connected to the prime
mover, are rigidly assembled respectively to each lobe. When
the lobes revolve, they suck the fluid and drive it to the outlet
(Figure 4.23).
a Medium-high flow rates
a Absolute suction pressure equal to 0.9 bar
a Overall efficiency of around 75% under optimum working
conditions
a Compatible with most hydraulic liquids
a Long-lasting
a Medium cost
Some of these characteristics are, on the one side, a plus for
this type of pumps and, on the other, a drawback because
they limit their fields of application. For instance, high flow
rates drop dramatically when pressure goes up, even if it
does not reach high levels and it compares only with the
characteristics of gear pumps.
Figure 4.23
All in all, the operating principle is the same as for external
gear pumps (note that gearwheels in lobe pumps are
not involved in the hydraulic operation because they just
transmit the rotary motion from one lobe to the other): as
lobes revolve in opposite directions at the inlet, this causes a
vacuum that makes the fluid flow in; the fluid is then driven
alternately by the left and right lobe to the outlet.
In order to avoid leakages, lobe coupling must be very precise,
which means these pumps are employed only in circuits with
very decontaminated fluids so as to prevent lobe erosion. In
addition, these pumps cannot sustain high pressures and are
subjected to rather strong pulsations due to the large space
between lobes. This is why lobe pumps are rarely used in oil
hydraulics and cheaper external gear pumps are employed
instead.
Screw pumps are better then external gear pumps in some
respects: they are less noisy and pulsating, fewer vibrations
occurs, they are compatible with similar or more viscous
fluids, they last longer and have higher flow rates at medium
pressures. Conversely, external gear pumps are more
compact and compatible mechanically speaking, they are
manufactures by many companies in a number of versions
with different flow rates; from the economic point of view,
although their price is not exorbitant, screw pumps are more
expensive than similar gearwheel pumps.
These units are found in the versions with two, three or five
rotors. In two-rotor pumps, a rotor is axial vis-à-vis the prime
mover and it moves the other rotor via gears; In three- (Figure
4.25) or five-rotor pumps, the central leading screw transmits
the revolution directly to the others. Three-screw pumps are
usually employed in oil hydraulics.
The profile of driven screws is opposite to the profile of the
leading screw and, like in gear pumps, the volume generated
by the revolution of the screws in the inlet creates a vacuum
that helps the fluid to flow up; subsequently, the fluid is
translated to the outlet by the particular profile between the
leading and driven screws.
Screw Pumps
What follows is a list of the remarkable characteristics of
screw pumps (Figure 4.24):
a Easy manufacturing process
a Working pressure up to 120 and peak pressure up to 200 bar
a Up to 5000 rev/min
a Very noiseless
a Little vibration
a Wide range of the grade of kinematic viscosity
a Almost no pulsations
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Global MDA Journal may 2018
Figure 4.24