types of rotary pumps are discussed-the gear pump
and the screw pump.
The GEAR PUMP is shown in figure 6-10. This
type of pump uses two spur gears that rotate in opposite
directions and mesh together at the center of the pump.
One of the gears is coupled to the prime mover (usually
an electric motor) and is called the driving gear. The
other gear, which receives its motion by meshing with
the driving gear, is called the driven gear. Note that
liquid moves as the gear teeth rotate against the casing
of the pump, thereby trapping the liquid and pushing it
around to the discharge outlet. The meshing together
of the two gears does not in itself move or pump liquid.
The meshing of the gear teeth, in effect, forms a
constant seal between the suction and discharge sides
of the pump and thus prevents liquid from leaking back
toward the suction inlet.
Very small clearances are permitted between the
meshing gears and between the gear teeth and pump
casing to avoid unnecessary friction and to allow the
liquid being handled to act as a lubricant for the
It is clear that when excessive
clearances are allowed to develop between the gear
teeth and casing or between the gears where they mesh
that the efficiency of the pump is considerably
reduced. For this reason, rotary pumps are rarely, if
ever, used to handle corrosive or abrasive liquids.
Of the several types of SCREW PUMPS, the main
difference is the number of intermeshing screws and
the pitch of the screws. Figure 6-11 shows a positive
displacement, double-screw, and low-pitch pump.
Screw pumps are primarily used for pumping viscous
fluids, such as JP-5 and diesel oil. Hydraulic systems
use the screw-type pump as the pressure supply for the
The pump may be either motor-driven or
Figure 6-10.Gear-type rotary pump.
Figure 6-11.Positive displacement, double-screw, low-pitch
In the screw pump, liquid is trapped and forced
through the pump by the action of rotating screws. As
the rotor turns, the liquid flows in between the threads
at the outer end of each pair of screws. The threads
carry the liquid along within the housing to the center
of the pump where it is discharged.
OPERATION AND MAINTENANCE OF
ROTARY PUMPS.The rotary pump is susceptible
to hydraulicking; therefore, the discharge stop valve
must be in the OPEN position before the pump is
started. In addition, it is a good operating practice to
prime these pumps before operation when possible.
This is particularly critical when the pump has been
standing idle for a period of time. This is true in spite of
the fact that rotary pumps are self-priming. Because
the liquid handled in the pump lubricates the rotating
elements of these pumps, filling the pump end with
fluid before starting prevents unnecessary friction and
wear of the rotating elements.
Rotary pump maintenance schedules stress that
proper clearance be maintained between rotating parts. .
For this reason, there should be periodic checks for
slippage. In addition, when the pump is dismantled,
the actual clearances should be carefully measured and
compared with the manufacturers specifications.
A reciprocating pump moves water or other liquid
by means of a plunger or piston that reciprocates
(travels back and forth) inside a cylinder. Reci-
procating pumps are positive displacement pumps;