fasteners) are torqued to the manufacturer's
cross section enables it to seal low fluid pressures.
specifications and the lockwire method of
The greater the compression, the greater is the
ensuring that certain types of fasteners will not
fluid pressure that can be sealed by the O-ring.
come loose. In this section of chapter 6 we will
The pressure of the O-ring against the gland walls
discuss some of the new developments in fastener
equals the pressure caused by the recovery force
technology, such as the various types of locknuts,
of the compressed O-ring plus the fluid pressure.
The fluid pressure against the walls of the
which you may not be familiar with.
gland and the stiffness of the O-ring prevent fluid
THREADED LOCKING DEVICES
from leaking past the O-ring. If the downstream
clearance is large, the O-ring is forced into this
clearance (view C of fig. 6-51). The stiffness of
An important part of fastener technology has
the O-ring material prevents the O-ring from
included the development of several methods for
being forced completely through the downstream
locking mated threads of fasteners. Many of the
clearance unless that clearance is abnormally large
latest methods include the locking device or
or the pressure is excessive.
method as an integral part of the fastener
O-rings are commonly used for sealing because
assembly and are referred to as self-locking nuts
of their simplicity, ruggedness, low cost, ease of
or bolts. Self-locking fasteners are more expensive
installation, ease of maintenance, and their
than some older methods but compare favorably
in cost with pin or wiring methods.
effectiveness over wide pressure and temperature
ranges.
Failure of an O-ring can sometimes begin with
Length of Protrusion
the removal of an old O-ring. If you incorrectly
remove an O-ring with pointed or sharp tools, you
Male threads on threaded fasteners, when
can scratch or dent critical surface finishes that
installed and tightened, will protrude the distance
of at least one thread length beyond the top of
can result in seal failure.
Before installing a new O-ring, inspect the seal-
the nut or plastic locking ring. Excessive
ing surfaces for any abrasions and wipe them free
protrusion is a hazard, particularly where
of any dust, dirt, or other contaminants. Before
necessary clearances, accessibility, and safety are
important. Where practicable, the number of
installation, inspect the O-ring for any damage.
If faulty, discard it.
threads protruding should not exceed five. In no
When you install the O-ring, lubricate it. In
case should thread protrusion exceed 10 threads
most cases it is already coated with the system
unless specifically approved by the work super-
fluid or petrolatum grease. Do not stretch the
visor. (This is the one-to-ten rule.)
Where screw threads are used for setting or
O-ring more than twice its original size during
installation, and do not roll or twist it into place.
adjusting (such as valve stem packing glands and
This may leave a permanent twist in the O-ring
travel stops) or where installed threaded fasteners
and reduce its effectiveness and shorten its life.
do not strictly follow the one-to-ten rule but have
When installing an O-ring, take extreme care
given satisfactory service, the rule does not
apply. An example of an acceptable existing
to avoid forcing it over sharp edges, corners, and
threaded sections. You should use some type of
installation would be where a male thread is flush
sleeve or cover to avoid damaging the O-ring.
with the top of a nut or where more than 10
t h r e a d s protruding is of no foreseeable
consequence.
FASTENERS
Repair of Damaged Threads
The proper use of fasteners is very important
and cannot be overemphasized. Many shipboard
You can remedy damaged external threads by
machinery casualties have resulted from fasteners
replacing the fastener. In large equipment castings
that were not properly installed. Machinery vibra-
you must repair damaged internal threads to save
tion, thermal expansion, and thermal contraction
the part. You can repair internal threads by redrill-
will loosen the fasteners. At sea, loosening effects
ing the damaged thread; clean and either install
a solid wall insert or tap for a helical coil insert.
are increased by the pitch and roll of the ship. You
are familiar with such standard fasteners as nuts,
These inserts, in effect, return the tapped hole to
its original size so it takes the original mating
bolts, washers, wingnuts, and screws. In chapter
3 we discussed how nuts and bolts (threaded
fastener.
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