They are relatively inexpensive.
Some ships that have two sets of engines use
counterrotating PTs. For example, PTs on one
They may be easily replaced.
main propeller shaft rotate clockwise while the
PTs on the other shaft rotate counterclockwise.
They can withstand high momentary
The GG rotates in the same direction for both sets
of engines. The blade angle of the wheel and the
nozzles in the PT section determine the directional
They are simple to cool, lubricate, and
rotation of the PT. On large ships where different
length propeller shafts are permitted, the engine(s)
can be mounted to the other end of the reduction
They can accommodate both radial and
gear. This allows for counterrotation of the
propellers without changing PT rotation.
You can control the output speed of the PT
They are relatively resistant to elevated
by varying the GG speed. Since only a portion
of the energy is used to drive the compressor,
the plant can be operated very efficiently.
The main disadvantages of ball or roller
For example, on a cold day you can have
antifriction bearings are their vulnerability to
foreign matter damage and their tendency to fail
without appreciable warning.
temperature variables discussed earlier in this
Usually the bearings are positioned on the
chapter account for this situation.
compressor or turbine shaft to absorb any axial
The PT is constructed much like the GG
(thrust) loads or radial loads. The ball bearings
turbine. The main differences are (I) the absence
are designed to absorb the thrust loads, and the
of vane and blade cooling air and (2) inter-
roller bearings are used to support the radial loads
locking shroud tips on the PT blades to reduce
because they present a larger working surface.
vibration. Honeycomb shrouds in the turbine case
The elements of a typical ball or roller
mate with the blade shrouds to provide a gas seal.
bearing assembly include a bearing support
They also protect the case from the high-
housing, which must be strongly constructed and
temperature gas. Two popular methods of blade
supported to carry the radial and axial loads of
retention are the bulb and the dovetail. These
the rapidly rotating rotor. The bearing housing
methods were discussed earlier in this chapter.
usually contains oil seals to prevent the oil from
leaking from its normal path of flow. The housing
also delivers lube oil to the bearing, usually
through spray nozzles.
The main bearings have the critical function
On modern engines, the bearing is mounted
of supporting the main engine rotor. For the most
in a sump. The bearing sump has a line through
part, the number of bearings necessary for
which the lube oil is scavenged back to the sump.
proper engine support is decided by the length and
The bearing sump is also vented to prevent either
weight of the engine rotor. The length and weight
a pressure or vacuum. The vent goes either to the
atmosphere or to an air-oil separator.
used in the engine. Naturally a split-spool axial
compressor will require more support than a
simple centrifugal compressor engine. The
GAS TURBINE ENGINE
minimum number of bearings required will be
three, while some of the later models of split-spool
axial compressor engines will require six or more.
Up to this point, we have pointed out the
While some engines use sleeve bearings, the
physical features and functions of a typical GTE.
GTE rotors are usually supported by either ball
In this section we will point out the basics of the
or roller bearings. In general, ball or roller
required auxiliary systems being used on most
antifriction bearings are preferred for the
GTEs of today's Navy. The systems are not
discussed in any order of importance. We will give
you a broad overview of the systems, which
include the air systems, the fuel oil system, the
accessory drive system, the lubrication system, the
They facilitate precision alignment of
starting systems, and the spark igniter system.