all points. In service, the best indication of good
alignment is good tooth contact.
The technical manual furnished with each gear
installation describes the procedures for determining the
proper depth of mesh and parallelism of gear and pinion
shafts. The length of tooth contact across the face of the
gear teeth is the key to satisfactory alignment of
reduction gears.
Poor alignment between the line shaft and the MRG
may be detected at the reduction gear. Uneven loading
of the low-speed gear train and noisy operation in certain
speed ranges are two common results of poor line shaft
to MRG alignment.
The most favorable alignment position of the main
engine to the reduction gear is when they are concentric
at full power at the proper operating temperature. The
flexible high-speed coupling is designed to handle the
transient condition of slight misalignments as the
machinery comes up to temperature. The two most
common forms of misalignment between the prime
mover and the driven shafts are angular and parallel
offset, as shown in figure 3-5.
The object of the alignment is to locate the turbine
so the axis of the spindle will be concentric with and
parallel to the axis of the reduction gear input pinion
shaft. Attaining alignment is complicated by the fact
that the turbine, reduction gear, and foundations all
Figure 3-5.—Angular and parallel misalignment.
expand as they are heated during operation to the hot
running conition. Another factor is when operating
pinion shafts move higher in their bearings under the
influence of the hydrodynamic oil film and tooth
pressure. These changes in position have been
predetermined by the manufacturer, and you can find
the offset readings in the appropriate technical manual
for the installation.
MAIN THRUST BEARING CLEARANCE
MEASUREMENTS
As you have already learned in Gas Turbine
Systems Technician (Electrical) 3/Gas Turbine Systems
Technician (Mechanical) 3, volume 1, NAVEDTRA
10563, propeller thrust is transferred from each
propulsion shaft to the hull through a Kingsbury main
thrust bearing (fig. 3-6). The Kingsbury thrust bearing
uses the wedge-shaped oil film lubrication principle.
This principle is based on an oil film between two
sliding surfaces tends to assume a tapered depth with the
thicker film at the entering side. In a Kingsbury
assembly, eight bearing shoes are installed on each side
of the thrust collar. Therefore, eight separate
wedge-shaped oil films are installed on each thrust face.
Since the bearing shoes are free to tilt slightly, the oil
automatically assumes the taper required by shaft speed,
loading, and oil viscosity.
The main thrust bearing assembly consists of the
bearing housing, two thrust rings, and a thrust collar.
The housing, thrust rings, and thrust collar facings are
all split horizontally. Each thrust ring is made up of 8
steel thrust shoes with tin babbitt facings, 16 leveling
plates, and a retainer ring. The thrust collar has a
two-piece removable steel thrust face attached to each
side. Each thrust shoe contains a hardened shoe support
with a spherical face. The support bears on the upper
leveling plate and the spherical face allow the thrust
shoe to pivot or tilt slightly in all directions. This
arrangement allows the bearing to operate on the
free-wedge film lubrication principle. One thrust shoe
on each side is fitted with a resistance temperature
element (RTE).
Due to the spring isolation system, main thrust
bearing clearance measurements are no longer taken
with a depth micrometer. All measurements are now
taken with a dial indicator that measures the deflection
of the propulsion shaft at the main flange. There are two
methods (static and dynamic) used to create shaft
deflection. The method used depends on the ship class.
The static method must be used on CG-66 and above
and all DDG-51 class ships. The dynamic method is
3-6
