continuous flow of hydraulic power oil to both
Five sliding blocks fit into slots machined in
sides of the servo piston. The continuous flow
the crosshead. The hub body contains five blade
results in a balanced pressure across the piston,
ports with a center post in each port. The center
holding the blade pitch constant. In this position,
posts are integral parts of the hub body. A
hydraulic pressure on both sides of the piston is
crankpin ring fits over the center post in the blade
ports. An eccentric crankpin on the underside of
sufficient to hold the desired blade pitch regardless
of changes of blade thrust loading.
each crankpin ring fits into a hole in each sliding
The flow path for the hydraulic power oil is
block. When the servomotor piston is translated
from the OD box through two drilled passages
as a result of differential pressure across the
in the regulating valve rod, through the valve rod,
piston, axial movement of the crosshead produces
and into the hollow valve pin. The oil returns
rotary movement of the crankpin rings through
through the valve pin return ports, which are
the sliding blocks and eccentric crankpins. The
continuously indexed to slots in the valve liner.
propeller blades, which are secured to the
It then flows into the chamber formed by the out-
crankpin rings, arc thus rotated, resulting in the
side diameter of the valve rod and inside diameter
desired pitch change.
of the propeller shaft and to the OD box. A PA
Figure 8-22 shows the hub assembly regulating
tube runs through the regulating valve rod and
valve, consisting of a valve liner and a valve pin,
valve pin from the OD box to the cone end cover.
which controls the position of the servomotor
piston. The valve liner is secured in the piston rod
Blades
bore. Ports in the forward and aft ends of the liner
provide a flow path for hydraulic power oil to the
Each propeller has five blades. The blades are
forward and aft chambers of the servomotor
made of nickel bronze alloy and weigh 4,300
cylinder. The valve pin fits inside the valve liner
pounds each. Each blade is attached to a crankpin
and moves axially within the liner. The amount
ring with eight bolts. A machined air channel runs
of axial travel of the pin in the liner (1/2 inch
along the blade leading edge from the root to the
total) is limited by a machined shoulder on the
blade tip. This channel provides the flow path for
piston rod bore. The valve pin contains hydraulic
PA, which discharges through small orifices
power oil ports in the forward and aft ends and
located along the leading edge. Prairie air is
return oil ports centered between. Axial move-
delivered to the blades through the PA tube that
ment of the valve pin is controlled by the valve
runs through the regulating valve rod from the
rod connected to the valve pin at the aft end and
to the OD box at the forward end.
OD box to the hub cone. Drilled passages in the
hub cone and hub body to the blade base
When the valve rod is moved in the forward
completes the flow path. A check valve in the hub
direction, the aft hydraulic power oil ports in the
cone prevents seawater from entering the PA
valve pin align with the aft ports in the valve liner;
system when air is not flowing.
the return oil ports in the valve pin align with the
forward ports in the valve liner; and the forward
Blade Seal Base Ring
ports in the valve pin are blocked off. Thus,
hydraulic power oil at 200 to 600 psig is directed
A blade seal base ring is located under the
to the aft chamber of the servomotor cylinder.
The forward chamber of the cylinder is open
outer edge of each blade port cover. The purpose
of the seal base ring is to prevent oil from
through the return oil ports. The resultant
leaking out of the hub and seawater from leaking
pressure differential across the servomotor piston
into the hub. Each seal base ring has two O-rings.
causes the piston to move forward. This results
One provides the seal between the seal base ring
in a change of blade pitch in the ahead thrust
and the blade port cover; the other provides the
direction. During pitch translation, the piston rod,
seal between the seal base ring and the hub body.
valve, and valve rod move together. Pitch transla-
Eighteen compression springs under the seal base
tion continues until movement of the valve rod
ring provide positive loading of the ring against
stops and the valve pin is centered in the valve
the blade port cover.
liner. When the valve rod is moved in the aft
direction, the action is reversed, resulting in pitch
change in the astern direction.
Tail-shaft Spigot
When the valve pin is centered in the liner,
The tail-shaft spigot has the physical
both ports in the liner are blocked off. However,
appearance of a short shaft with a flange on one
reliefs machined in the valve pin lands permit a
8-32