When gas or liquid are under pressure, system fasteners
should be sealed to prevent leakage. This is true even for fasteners that are
not under pressure, but where hostile environments such as water, gas, dust,
lubricants, cleaning solvents, etc. are present and could penetrate past
threads and leak onto the internal mechanism/operating area creating havoc.
Fastener users have a number of sealing options to effectively block these
contaminants and to contain fluids under pressure (internal/external). Once a
fastening assessment has been made, one can easily review these options against
the following criteria:
- Will the fastening be done as part of a high-speed
automated assembly operation, or will manual fastening be the rule?
- Is the main sealing concern limited to initial
installation, with little or no concern about warranty, or are there longer
range potential maintenance considerations?
- What psi/vacuum rating (internal/external) is
considered operational, and what will be the temperature conditions?
- What chemicals (including cleaning agents), gas or
other liquids will the fastener be exposed to?
- Will there be concern about field servicing, especially
spare parts availability?
- Will there be any preference for either a liquid or dry
mechanical sealing method during production and post- field servicing?
- Will vibration be a consideration - therefore requiring
additional threadlocking? If so, which
method (liquid or dry) best addresses installation time, future maintenance and
economic considerations?
- Will there be any security consideration that might
require choosing a self-sealing fastener (screw or bolt) with a
tamper-resistant head?
- Are there any other considerations unique to the
specific application?
The liquid method
(generally best suited for large automated production applications) - typically
anaerobic adhesives/sealants and other pre-applied chemicals - are manufactured
by a number of well known suppliers. Many of these suppliers offer products
that are application specific, i.e. aerospace, electronics, transportation,
durable and consumer goods. Some are designed for two-step operations involving
prep/curing. If not properly selected, some may present difficulty in removing
during service.
The dry mechanical
method is an ideal alternative for many sealing applications where fluid is
under pressure or seepage protection against the elements are prime
considerations. Efficacy, ease of installation and field maintenance are the
driving forces for deciding which of these traditional methods are best to use
with screws, bolts, nuts and rivets. There's the O-Ring or flat-washer
(deforming) technique, mounted under the
fastener head method. Both are two step processes, using a
crush and fill approach to achieve fastener sealing. Both require that
replacement parts be handy when re-installing, as neither will survive in good
condition to be re-installed. And, because neither methods are precise, there's
some question about their sealing integrity. The "dry" with operating one-step
Embedded O-Ring Fastener (originally patented by APM) seals to 20,000
psig/vacuum with operating temperatures from -160 to 500F. Upon torquing, it
becomes fully clamped with a 360 degree (metal-to-metal) seal, and can be
easily re-installed successfully many times. The stainless-steel screw and bolt
versions can even be used for a bleeding function. The reason the Embedded
design works so well is that its' silicone O-Ring is embedded in a circular
groove, strategically located under the head and next to the shank. The
asymmetrical shaped groove controls the degree of O-Ring compression into a
countersunk-threaded area for minimum wear - while still providing maximum
sealing - enabling many service reinstallations without concern about potential
sealing failure.
When needed, this class of self-sealing fasteners
accommodates a variety of threadlocking techniques such as; adjustable polymer
(normally Nylon) pellets embedded into the lower thread section; a polymer
vertical strip, also embedded into the thread section; or a pre-applied dry
coating process which becomes fused to the thread surface. They are delivered
ready for installation without the need for curing or any other special
preparation. Several versions of the dry-coating method are available,
including one that will maintain torque values through extreme temperatures
from -70 to 500F. A self-sealing stainless-steel nut is also offered by APM
that incorporates a molded silicone rubber insert. The insert features
continuous threads to lock in its' sealing capability, an important
consideration that isn't present in other sealing nut designs that just
incorporates an O-Ring. There are self-sealing rivets with embedded O-Rings to
choose from that provide high-pressure sealing; but like all rivets, will be
damaged upon removal, and therefore they are not reusable.
A high-pressure air and water-tight, self-sealing washer
version is also available from APM. This
special washer assembly consists of a silicone disc bonded to a 300 series
stainless steel contoured washer. It can be used with standard screws, bolts or
studs in a wide variety of mechanical, electrical and electronics equipment
sealing applications. All APM self-sealing fasteners are UL Recognized, and are
IP66/68 water ingress rated.
Throughout industry, APM self-sealing are used extensively
in challenging fastener applications found on equipment in laboratory and
scientific instruments, manufacturing and process, material handling and
packaging, motion control, network and
communications, test and measurement, boat and other marine/off-shore,
construction and off-road/recreational, powder and liquid handling, medical,
military and security - and any other kind of equipment that requires
wash-downs or exposure to the elements and extreme temperatures.
About the Author:
Ken Schwinn is the vice president of engineering for APM
Corp.'s Fastener Division.
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