Gear and lobe pumps operate on the principle of using mechanical means to transfer small
elements or "packages" of fluid from the low pressure (inlet) side to the high pressure (delivery)
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side. There is a wide range of designs available for achieving this end. The general characteristics
of the pumps are similar to those of reciprocating piston pumps, but the delivery is more even
because the fluid stream is broken down into so much smaller elements. The pumps are capable
of delivering to a high pressure, and the pumping rate is approximately proportional to the speed
of the pump and is not greatly influenced by the pressure against which it is delivering. Again, it is
necessary to provide a pressure relief system to ensure that the safe operating pressure is not
exceeded.
One of the most common forms of the pump is the gear pump in which one of the gear wheels is
driven and the other turns as the teeth engage illustrated in Figures 7.19. The liquid is carried round
in the spaces between consecutive gear teeth and the outer casing of the pump, and the seal
between the high and low pressure sides of the pump is formed as the gears come into mesh and
the elements of fluid are squeezed out. Gear pumps are extensively used for both high-viscosity
Newtonian liquids and non-Newtonian fluids.
Figure 7.19: Gear pump
The lobe-pump (Figure 7.20) is similar, but the gear teeth are replaced by two or three lobes and
both axles are driven; it is therefore possible to maintain a small clearance between the lobes, and
wear is reduced.
