The MRGs with multiple engine inputs are all rated
REMOVAL AND REPLACEMENT OF
at approximately a 21 to 1 gear reduction ratio (where
COMPONENTS
3,600 PT speed equals 168 srpm). As the rotation of the
This system seldom requires the removal or
GTEs cannot be reversed and the MRGs are identical on
twin-shaft ships, the MRGs are installed to "mirror
replacement of components. However, there is one
image" each other to allow for opposite shaft rotations.
component that you will be required to replacethe
electrohydraulic servo valve. As previously discussed,
this valve is in constant use and its probability of failure
MAINTENANCE OF COMPONENTS
is much higher than any other component in the system.
There is only one other set of components that you
The amount of MRG component maintenance you
will need to remove frequentlythe system's filters. In
will encounter on gas turbine-powered ships will
fact, you will need to remove these components even
depend on the type of MRG installed on your ship. The
more frequently than the electrohydraulic servo valve.
variations in maintenance routines will depend on the
This is because you will need to remove the filters for
type of brake assembly installed on your particular class
periodic cleaning according to the PMS. Of course, you
of ship. For example, on a DDG-51 or FFG-7 class snip,
will also need to remove them in the event of a casualty.
an externally mounted PT brake is installed on the
interface between the output shaft of the GTE and the
Now that we have discussed the maintenance and
input shaft of the MRG. In CG-47, DD-963, and
repair of the major propulsion plant systems, let's take
DDG-993 class ships, the PT brakes are internal and part
a look at the machinery that these systems support.
of the clutch/brake assembly. The systems with the
externally mounted braking systems will normally
DRIVE TRAIN EQUIPMENT
require more maintenance than those that are internal.
Next to the propulsion turbines, the drive train
In addition to the brake assemblies, there are other
equipment is probably the most significant equipment
components in the MRG system that will require
unit in the engineering plant. In this section, we will
maintenance. As a GSM, you will likely be required to
discuss the operation and care of the equipment that
perform maintenance on the following components:
transmits the GTEs' power to the propeller.
1. Sight flow indicators
2. Shock mounts and snubbers
MAIN REDUCTION GEAR
3. Attached pump angle drives
The MRG is probably the most significant
4. Vent fog precipitator
component that we will discuss in this section. The
MRG reduces the GTEs' high rpm to a workable speed
5. Dehumidifier
for the propellers. The workable speed is that rpm range
For these components, your specific maintenance duties
below the cavitation point. The speed reduction capacity
will include cleaning, inspecting, and testing.
of an MRG must be correctly proportioned according to
the size of the ship, the desired speed range, and the size
of the propeller. Since all MRGs on gas turbine-powered
Cleaning
ships are similar in design, we will discuss the
operational and maintenance requirements in basic
Cleaning the MRG is extremely important. The
terms.
MRG is equipped with numerous removable inspection
covers and maintenance access plates. Remember, the
NORMAL OPERATIONS
cleaner you maintain the MRG, the easier it will be for
you to identify new leaks.
The MRGs on the larger gas turbine-powered ships
SIGHT FLOW INDICATORS. Keeping the sight
have multiple engine inputs. To allow for multiple
flow indicators clean is an especially important
engine inputs, each engine must be coupled to the MRG
requirement. The sight flow indicators must be clean so
through a special type of clutch assembly: There are two
bearing oil flow can be properly monitored. Even
types of clutch assemblies capable of accommodating
multiple engine inputs. Each larger gas turbine-powered
though there is not a specific maintenance procedure for
cleaning the sight flow indicator glass, this component
ship will be equipped with one of these two types of
can become covered with an oil film and dirt. By
clutch assemblies.
