the recommended O-ring entering sleeve (a soft,
thin wall, metallic sleeve) is not available, paper
sleeves and covers may be fabricated by using the
seal package (glossy side out) or lint-free bond
paper (see views B and C of fig. 7-10).
After you place the O-ring in the cavity
provided, gently roll the O-ring with your fingers
to remove any twist that might have occurred
during the installation. After installation, an
O-ring should seat snugly but freely in its groove.
If backup rings are installed in the groove, be
certain the backup rings are installed on the
correct side of the ring.
BACKUP RINGS
Backup rings, also referred to as retainer rings,
antiextrusion devices, and nonextrusion rings, are
washer-like devices that are installed on the
low-pressure side of packing to prevent extrusion
of the packing material. Backup rings in dynamic
seals minimize erosion of the packing materials
and subsequent failure of the seal. At lower
pressures, backup rings will prolong the normal
wear life of the packing. At higher pressures,
backup rings permit greater clearances between
the moving parts. Normally, backup rings are
required for operating pressures over 1500 psi.
Backup rings can be made of polytetra-
fluoroethylene, hard rubber, leather, and other
materials. The most common material currently
used is tetrafluoroethylene (TFE). Backup rings
are available as single-turn continuous (uncut or
solid), single-turn (bias) cut, and spiral cut. See
figure 7-11. Leather rings are always furnished in
solid ring form (unsplit). Rings of TFE are
available in all three types.
Packaging and Storing
Backup rings are not color-coded or otherwise
marked and must be identified from the packaging
labels. The dash number following the military
standard number found on the package indicates
the size, and usually relates directly to the dash
number of the O-rings for which the backup ring
is dimensionally suited. Backup rings made of
TFE do not deteriorate with age and do not have
shelf life limitations. TFE backup rings are
provided by manufacturer either in individually
sealed packages or on mandrels. If unpackaged
rings are stored for a long time without the use
of mandrels, a condition of overlap may develop.
Overlap occurs when the ID of the backup ring
becomes smaller and its ends overlap each other.
To correct this overlap condition, stack TFE rings
on a mandrel of the correct diameter, and clamp
the rings with their coils flat and parallel. Place
the rings in an oven at a maximum temperature
of 1770C (3500F) for approximately 10 minutes.
Do not overheat them because fumes from
decomposing TFE are toxic. Remove and
water-quench the rings. Store the rings at room
temperature before you use them (preferably for
48 hours).
Installation
Care must be taken in handling and installing
backup rings. Do not insert them with sharp
tools. Backup rings must be inspected prior
to using them for evidence of compression
damage, scratches, cuts, nicks, or frayed con-
ditions. If O-rings are to be replaced where
backup rings are installed in the same groove,
never replace the O-ring without replacing
the backup rings, or vice versa. Many seals
use two backup rings, one on either side of the
O-ring (fig. 7-12). Two backup rings are used
primarily in situations (such as a reciprocating
piston seal) where alternating pressure direction
can cause packing to be extruded on both sides
of the gland.
Figure 7-11.—Types of backup rings.
7-12
