transmission, the operator selects neutral, reverse, or
DRIVER, and the turbine may be called the RUNNER.
o n e o f s eve r a l r a n g e s o f f o r w a r d s p e e d . T h e
Often the two members are called the FRONT TORUS
transmission automatically shifts from one forward
and the REAR TORUS. (A torus is doughnut-shaped
speed/torque ratio to another based on the rpm of the
and turns on an axis.) You should become familiar with
engine and the speed of the vehicle. The automatic
all these terms so that you can understand the
transmission does not require a manually operated
manufacturers' technical manuals. Note that in figure
clutch. All support equipment used by the Navy have
2-49, the pump and turbine are torus-shaped with fins
automatic transmissions.
extending radially from its center.
Most of the fundamentals of hydraulics are put to
The pump of the fluid coupling is connected to the
work in one form or another in the automatic
engine and is rotated by the crankshaft. Usually, the
transmission. Automatic transmissions use such
pump is bolted directly to the flywheel.
hydraulic devices as relief valves, shifter valves,
The turbine is made exactly like the pump, but it is
pressure regulators, governors, and servo pistons. In
connected to the transmission input shaft. The two
most cases, the transmissions are used with fluid
members of the coupling face each other within a
couplings or hydraulic torque converters.
housing that is filled with the driving fluid (generally
oil).
FLUID COUPLINGS
When the pump goes into motion, oil is forced
A fluid coupling, sometimes called a "fluid
outward by centrifugal force around the entire
clutch," is precisely what its name implies. When it is
circumference of the pump and hurled against the
placed between an engine at the power-input end and
blades of the turbine. A continuous flow of oil against
some other mechanism, such as a transmission at the
the turbine blades is necessary to transfer sufficient
output end, it couples the two hydraulically, and there
kinetic energy to keep a vehicle in motion.
is absolutely no mechanical connection between them.
The centrifugal force of the oil as it leaves the
A simple sort of fluid coupling can be made with two
pump gives the oil the velocity it needs. The faster the
electric fans, as shown in figure 2-48. If the fans are
pump operates, the more velocity the oil has when
placed a few inches apart, facing each other, and one
leaving the pump. The design of the coupling permits
fan is plugged in so that it runs, the current of air from
the oil to return to the pump as soon as it has delivered
the running fan will cause the blades of the other fan to
its energy to the turbine.
turn. In this case, the air takes the place of the fluid.
Since the two fans are not enclosed or closely coupled,
Where the vehicle has not started to move, the
this sort of coupling is not very efficient.
turbine is stationary. For instance, the engine may be
rotating the pump at 900 rpm. The pump is
To make a more efficient coupling, oil is used, and
consequently imparting energy to the fluid, which, in
the two members (driving member and driven
turn, imparts energy to the turbine. By the time the oil
member) are mounted very close together and
returns to the pump, the pump has moved some
enclosed in a housing. In a fluid coupling, the driving
distance, making it impossible for the oil to re-enter the
member is called the PUMP and the driven member the
pump through the same set of vanes it left. Consider
TURBINE. This terminology is not universal. Some
one drop of oil as it leaves the pump, goes through the
manufacturers call the pump the IMPELLER or
AIR IS COUPLING FLUID
TURBINE
PUMP
ASf02048
ASf02049
Figure 2-48.--Basic fluid coupling, using air in place
Figure 2-49.--Fluid coupling pump and turbine.
of fluid.
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