On the DD and DDG-993 class ships, clutch/brake
a typical multishaft ship's port and starboard
assemblies provide the means to engage, dis-
Numerous times you will be asked to describe
propulsion shaft. The CG and FFG, and DDG-51 class ships use
the MRG. An acceptable description would be
a synchro-self-shifting (SSS) overrunning clutch
that it is a double-helical, articulated, double-
to perform the same actions as the clutch/brake
reduction, locked-train reduction gear with a
assemblies on the DD and DDG-993 class ships.
21.49-to-1 reduction ratio. Double-reduction gear
sets are used when the overall speed reduction
An attached hydraulic oil pump for the CRP
ratio exceeds 10 to 1.
propeller system is driven off the lower outboard
To better understand the rotating elements of
second-reduction pinion shaft. An attached lube
the MRG, refer to figure 8-2 as the name and
oil pump is driven off the lower inboard second-
function of each element is pointed out. The two
reduction pinion. Both pumps are directly driven
high-speed input shafts (1) each drive a first-
via bevel gears and flexible couplings and are
reduction (high-speed) pinion (2) through a clutch
connected through manually operated disconnect
or clutch/brake assembly (3). The two first-
couplings. A turning gear assembly is connected
reduction pinions each drive two first-reduction
to the upper outboard second-reduction pinion.
(intermediate) gears (4). The four first-reduction
The upper inboard second-reduction pinion drives
gears drive the four second-reduction pinions (5)
through internal quill shafts and flexible
elapsed time indicator.
couplings. All four second-reduction pinions drive
a common second-reduction (low-speed) gear,
commonly referred to as the bull gear (6). The
bull gear shaft is directly coupled to the pro-
pulsion shaft (7). Integral with the bull gear shaft
The gear casing is a fabricated steel plate
is a thrust collar, not shown on figure 8-2. The
structure that encloses and supports the rotating
thrust collar transmits the fore and aft thrust from
gear elements. A lube oil header is mounted on
the propeller via the propulsion shaft to the thrust
top of the upper casing. It supplies lube oil for
bearing (8). The thrust bearing assembly is
external to the reduction gear case. It is bolted
assemblies. An opening on the bottom allows the
directly to the propulsion bedplate aft of the gear
lube oil to drain to the external sump. Covered
inspection ports permit visual observation of the
First-reduction gear elements are referred to
lube oil impingement on the gear meshes and also
as high-speed (HS) elements and the second-
permit gear inspection. The GTMs and the MRG
reduction gear elements are referred to as low-
are mounted on a common bedplate and are
speed (LS) elements. Figure 8-3 shows how the
aligned before installation on the ship. The
HS gear and LS pinion are connected by a quill
casing is secured to the bedplate by eight shock
shaft and an articulated flexible coupling to form
and noise attenuating vertical supports attached
a subassembly we call an intermediate assembly.
to the bedplate and the base.
Two intermediate assemblies provide a dual or
GTE. This arrangement, known as locked train,
does not permit independent axial or rotational
movement of the pinions. Each locked train must
The port and starboard input shafts are iden-
be timed to be sure that the load divides equally
tical on each MRG. Each input shaft is a flanged
between the two intermediate assemblies.
shaft mounted in two journal (sleeve) bearings.
Mounted on the forward end of each MRG
One end of the shaft is coupled to the GTE. A
at the end of the bull gear shaft is the OD box.
turbine brake disc is attached to this end of the
The OD box is part of the CRP propeller system.
shaft. The opposite end of the shaft is attached
The OD box serves two purposes:
to the SSS clutch. A removable cover is provided
It sends hydraulic oil and PA through the
for access to the main section of the shaft. A
rotating bull gear shaft and propulsion
triseal shaft seal is mounted at the turbine end of
shaft to the propeller hub.
the shaft to prevent oil leakage. This seal is
designed as two halves that can be removed
It positions a valve rod within the
propulsion shafting to control pitch.
without removing the shaft or the bearings.