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The Practical Pumping Handbook ......................................

Where one gear is driven by the other, the driven gear usually runs in
sleeve type bearings. The bearings and shaft journal are located in the
pump casing and flooded by the pumped fluid. Consequently, these
bearings and gears are dependent on the lubricating qualifies of the
pumped fluid.
In other types, the gears have no metallic contact with each other and
both rotors are driven by synchronized driving gears separated from the
pump chamber. As both shafts pass through the pump casing, two sets
of seals are required.
The absence of metallic contact between the surfaces of the rotors and
the casing means that the only wear that should occur will be due to
friction with the pumped fluid.
A popular modification of the gear pump has some of the gear teeth
blended together to form a Lobe Pump that provides a lower shearing
action on the pumpage.
The Internal Gear Pump has one rotor with externally cut gears
running in the bore, and meshing with, a second internally cut gear.

9.4.4 Screw pumps
Screw pumps are a special type of rotary positive displacement pump in
which the flow through the pumping elements is truly axial. The liquid
is carried between screw threads on two or more rotors and is displaced
axially as the screws rotate and mesh.
The meshing of the threads on the rotors, and the close fit of the
surrounding housing create one or more sets of moving seals in series
between pump inlet and outlet. These sets of seals act as a labyrinth and
provide the screw pump with its positive pressure capability.

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Figure 9.19: Typical screw pump (Reproduced with permission of Roper Pump Company)

m 162