for hydraulic power oil from the OD box to the
end. It is bolted to the forward end of the hub
regulating valve. The rod is assembled from
body concentric with tail shaft. The tail-shaft
fabricated sections of seamless steel tubing joined
spigot, with the shaft portion extending into the
together by oil-tight couplings. The aft end of the
crosshead chamber, encloses the forward end of
rod is mechanically connected to the regulating
the chamber. The shaft portion of the spigot is
valve pin. The forward end is mechanically
the same diameter as the piston rod. It serves as
connected to the distance tube in the OD box. The
a guide and support for the piston rod and
crosshead as they move axially, during propeller
distance tube is part of the OD box piston
assembly. This provides a positive drive for
pitch translation.
the valve rod. Relative to the propeller shaft,
the distance tube is keyed to axial slots in
The crosshead chamber is designed to contain
the OD box extension shaft to prevent rotation
a constant volume of oil to prevent pressure surges
of the rod.
caused by variable seal loadings. Variable seal
loadings are caused by piston rod travel within
Axial movement of the valve rod is actuated
the chamber during pitch translation. As the
and controlled by the OD box piston assembly.
piston rod moves into or out of the crosshead
As the valve rod translates, it moves the regulating
chamber during pitch translation, it changes the
valve pin axially in the valve liner. The movement
chamber volume. The forward end of` the piston
of the valve pin ports hydraulic power oil to either
rod covers or uncovers an equal volume of the
side of the servomotor piston. This in turn results
tail-shaft spigot, thereby cancelling the effect of
in propeller blade pitch rotation. The amount of
volume change.
axial travel of the va1ve rod is equal to the travel
of the hub servomotor piston.
Two low-pressure safety valves are located in
the tail-shaft spigot. One of these valves is shown
in figure 8-21. Overpressurization in the crosshead
chamber, resulting from expansion of the oil
OIL DISTRIBUTION BOX
caused by thermal change, opens the safety valves
The OD box has two main purposes: (1) It
and bypass oil to the return oil passage. If the
provides the actuation and control of the
return oil pressure exceeds the crosshead chamber
regulating valve rod, and (2) it directs HP
pressure, the oil can flow past the piston rod seal
hydraulic oil to and from the propeller shaft
into the crosshead chamber.
flow passages. It also provides the mounting
for the manifold block assembly, local pitch
The hub purge valve is located in the hub body
indicator, linear feedback potentiometer, and
end plate that separates the servomotor cylinder
shaped readout potentiometer of the control
from the crosshead chamber. The purpose of the
purge valve is to purge oil from the forward
valves.
chamber of the servomotor cylinder when the
The regulating valve rod is actuated and
propeller is stroked beyond its design full astern
controlled by the OD box piston assembly, shown
pitch limit. If the propeller is stroked beyond its
in figure 8-23. This assembly consists of a for-
design full astern limit, the crosshead contacts the
ward and an aft piston mechanically connected
purge valve, causing the valve to open. The
to make a double-ended piston. The piston fits
open valve permits oil to flow from the
cylinder into the crosshead chamber. when the
between the forward and aft control oil chambers
of the OD box. Not shown in figure 8-23 are
pressure in the crosshead chamber exceeds the
molythane polypak seals which are installed in the
pressure setting of the tail-shaft spigot safety
valves, the safety valves open. The oil then flows
piston heads and in the annular packing gland
separating the chambers. Each chamber has a
into the return oil passage, completing the hub
single port for normal supply and return of
purge circuit.
control oil. Each chamber also has a single
emergency port for connection of the emergency
VALVE ROD ASSEMBLY
hand pump.
Control oil pressure of 350 psig is ported to
The valve rod assembly is the mechanical link
either control oil chamber by the control valve
between the OD box and the hub servomotor
regulating valve. It also provides the flow path
manifold assembly. It responds to either an
8-34
