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Aviation Boatswains Mate F - Aviation theories and other practices | Next
THEORY OF OPERATION |
ROTATING ELEMENT. —The rotating element
(fig. 4-8) consists of an impeller and pump shaft, shaft
sleeves and nuts, ball bearings, mechanical seals, and
a flexible coupling.
Impeller and Pump Shaft. —The impeller is a
double-suction, closed impeller. It is keyed to, and
rotates with, the pump shaft. The impeller is centered
in the discharge compartment of the pump casing and
prevented from axial movement by two shaft sleeves
and two shaft nuts. The two shaft sleeves actually act
as long spacers between the impeller and shaft sleeve
nuts. The shaft sleeves are also keyed to, and rotate
with the pump shaft. Fuel enters the center part of
the impeller from both sides of the suction chamber
and is pumped into the discharge chamber. The
impeller blades are enclosed by side plates. The
blades are designed to curve backward in relation to
the rotation of the impeller to increase pump
efficiency, and impart velocity to the fuel in the
casing. Mechanical seals (fig. 4-9) fitted on the
pump shaft guard against fuel leakage from the
pump and prevent air from entering the casing
around the shaft. The seals are installed in the
stuffing boxes provided on each side of the pump
casing.
There are two types of mechanical seals, the
John Crane, and Durametalic. The principle parts of
the John Crane mechanical seal are the stationary
floating seat, low friction sealing washer, and spring.
It is a single piece unit.
The principle parts of the Durametalic
mechanical seal are the stationary insert, seal ring,
compression ring and collar assembly, and the shaft
packing. It is a three piece unit.
Figure 4-9.—John Crane mechanical seal.
NOTE
Some parts of mechanical seals are made of
carbon and break easily. Handle mechanical
seals carefully.
Bearing Cartridges. —Both ends of the pump
shaft extend outside the upper half of the casing.
Pump shaft ends are supported by ball bearings
encased in bearing cartridges and cradled in the
bearing brackets of the lower casing half. The ball
bearings absorb radial and axial thrust, and ensure
free rotation of the pump shaft. A single ball bearing
is housed in the inboard bearing cartridge, allowing
the inboard bearing some axial movement within the
cartridge. Duel ball bearings are housed in the
outboard bearing cartridge. The ball bearings are
slipped on and held firmly against a shoulder on the
pump shaft by a lock washer and locknut. The ends of
the bearing cartridges that lie closer to the center
Figure 4-8.—Assembled rotating element.
4-12
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