The engraving herewith, Fig. 458, exhibits the general construction
of the Quimby screw pump. The four screws that act as pistons in
propelling the water are mounted in pairs on parallel shafts, and are
so arranged that in each pair the thread of one screw projects to
the bottom of the space between the threads of the opposite screws.
The screw threads have flat faces and peculiarly undercut sides; the
width of the face and the base of the thread being one-half the pitch.
The pump cylinder fits the perimeters of the threads. Space enough
is left between the screws and the cylinder and between the faces of
the intermeshing threads to allow a close running fit without actual
contact. There is no end thrust of the screws in their bearings,
because the back pressure of the column of liquid is delivered through
the suction, S, at the middle of the cylinder, therefore the endwise
pressure upon the screws in one direction is exactly counterbalanced by
a like pressure in the opposite direction.
The suction connection opens into a chamber underneath the pump
cylinder. The suction liquid passes through this chamber to the two
ends of the cylinder and is forced from the ends toward the center by
the action of the two pairs of intermeshing threads; the discharge
being in the middle of the top of the cylinder, as shown at D. The
power to drive the pump is applied to one of the shafts, and the second
shaft is driven by means of a pair of gears, shown at G.
The pump has no internal packing, no valves and no small moving parts.
The only packing is in the stuffing-boxes where the two shafts pass
through the cylinder head.
Whether driven by a belt, an electric motor, or a steam engine,
the driving power is applied directly and without the loss due to
intermediate mechanism; as the screws are not in contact with the
cylinder or with each other, the consequent absence of wearing surfaces
gives the pump great durability.
These pumps have a high efficiency against a wide range of pressures:
The power being applied direct, the thrust due to the back pressure of
the column of liquid in the delivery pipe is balanced.
As the action of the screws on the liquid is continuous, the delivery
is free from pulsation. By thus keeping the liquid in constant and
uniform motion the efficiency of the pump is increased and the pump is
made peculiarly suitable for certain specific purposes as there is no
churning effect upon the liquids handled.
These pumps are much used in connection with hydraulic elevators by
pumping directly into the elevator cylinders, as there is no pulsation.
They are also used to pump oil into pipe lines and are driven by
electric motors as well as by belts. For circulating pumps for brine
and for fire purposes these pumps have certain peculiar advantages.