temperatures ranging from -65F to +160F. When new
ensured with the inner and outer walls of the passage
under static (no pressure) conditions. Views B and D of
designs raised operating temperatures to a possible
+275F, more compounds were developed and
figure 8-12 show the action of the O-rings when
pressure is applied. You should also observe, in views C
perfected.
and D of figure 8-12, that backup rings are installed. In
Recently, newer compounds were developed under
hydraulic systems of 1,500-psi pressure or less,
MS (Military Standard) specifications that offered
AN6227B, AN6230B, and MS28775 packings are
improved low-temperature performance without
used. In such installations, backup rings are not
sacrificing high-temperature performance. These
required, although they are desirable. In most modern
superior materials were adopted in the MS28775
aircraft with hydraulic system pressures up to 3,000 psi,
O-ring, which is replacing AN6227 and AN6230
backup rings are used in conjunction with the MS28775
O-rings, and the MS28778 O-ring, which is replacing
packings.
the AN6290 O-ring. These O-rings are now standard
Gaskets are used in the sealing of boss fittings, end
for systems where the operating temperatures may vary
caps of actuators, piston accumulators, and other
from -65F to +275F.
installations where moving parts do not come in contact
Packings used in naval aircraft hydraulic
with the seal. Normally, the type of gasket used is an
installations are manufactured from synthetic rubber.
O-ring. In some cases it might be the same seal that is
They are used in units that contain moving parts, such
used as a packing in other installations, or it may be one
as actuating cylinders, selector valves, etc. Although
that is manufactured only for use as a gasket.
packings are made in many forms, the O-ring type is
In hydraulic systems where the operating
most widely used. The U-rings, V-rings, and other
temperature ranges from -65F to +160F, the AN6290,
various types are obsolete in most cases.
MS28778, AN623OB-1 through -25, MS28775-013
The O-ring packing seals effectively in both
through -028, -117 through -149, and -223 through
directions. This sealing is done by distortion of its
-247 O-rings are intended for use as gaskets. In systems
elastic compound. Views A and C of figure 8-12 show
where temperature limits range from -65F to +275F,
O-rings of the proper size and installed in grooved
MS28778 and designated sizes of MS28775 O-rings
seats. Notice that the clearance for the O-rings is less
are used as gaskets. Normally, O-rings designated as
than their free outer diameter. The cross sections of the
MS28778 should be used only in connections with
O-rings are squeezed out of round prior to the
straight thread tube fittings, such as boss fittings and
application of pressure. In this manner, contact is
end caps of check valves, etc.
Figure 8-12.--Action of O-rings.
8-23
