Figure 2-42.—Hydraulic seals.
trimmed to extremely close tolerances. In some
landing gear struts, an elliptical seal is used. The
elliptical seal is similar to the O-ring seal except for
its cross-sectional shape. As its name implies, its
cross section is elliptical in shape. Both the O-ring
and elliptical seals are shown in figure 2-42.
Advances in aircraft design have made new
O-ring composition necessary to meet changing
conditions.
Hydraulic O-rings were originally
established under AN (Air Force-Navy) specification
numbers (6227, 6230, and 6290) for use in fluid at
operating temperatures ranging from -65°F to +160°F.
When new designs raised operating temperatures to a
possible +275°F, more compounds were developed
and perfected.
Recently, newer compounds were developed
under MS (Military Standard) specifications that
offered improved low-temperature performance
without sacrificing high-temperature performance.
These superior materials were adopted in the
MS28775 O-ring, which is replacing AN6227 and
AN6230 O-rings, and the MS28778 O-ring, which is
replacing the AN6290 O-ring. These O-rings are now
standard for systems where the operating
temperatures may vary from -65°F to +275°F.
Packings used in naval aircraft hydraulic
installations are manufactured from synthetic rubber.
They are used in units that contain moving parts, such
as actuating cylinders, selector valves, etc. Although
packings are made in many forms, the O-ring type is
most widely used. The U-rings, V-rings, and other
various types are obsolete in most cases and are not
discussed in this training manual.
The O-ring packing seals effectively in both
directions. This sealing is done by distortion of its
elastic compound.
Views A and C of figure 2-43
show O-rings of the proper size and installed in
grooved seats.
Notice that the clearance for the
O-rings is less than their free outer diameter. The
cross sections of the O-rings are squeezed out of
round prior to the application of pressure. In this
manner, contact is ensured with the inner and outer
walls of the passage under static (no pressure)
conditions. Views B and D of figure 2-43 show the
action of the O-rings when pressure is applied. You
should also observe, in views C and D of figure 2-43,
that backup rings are installed. In hydraulic systems
of 1,500 psi pressure or less, AN6227B, AN6230B,
and MS28775 packings are used. In such installa-
tions, backup rings are not required, although they are
desirable. In most modern aircraft with hydraulic
system pressures up to 3,000 psi, backup rings are
used in conjunction with the MS28775 packings.
Gaskets are used in the sealing of boss fittings,
end caps of actuators, piston accumulators, and other
Figure 2-43.—Action of O-rings.
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