Quantcast Door Closure/Seal Comparisons

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2.6.2.2
Considerations for Fingerstock Replacement.  Because of the delicate
nature of fingerstock materials, the door design should provide for rapid and
easy replacement of fingerstock without the requirement for special tools and
soldering.  Pneumatic doors, which are all of the sliding type, must have
available a pocket with a removable cover so that the door can be serviced or
removed.  The use of dissimilar metals or metal finishes should be minimized
in door design because of the possible battery action and corrosion which
occurs in the presence of any moisture.
2.6.2.3
Door Closure/Seal Comparisons.  Door latching mechanisms must be
designed to provide uniform wiping and compression of the fingerstock with
enough force to provide the required shielding effectiveness.  They must also
allow quick egress to satisfy the requirements of governing National Fire
Protection Association (NFPA) rules and regulations.  The pneumatic type door,
even with automatic opening and closing, requires the longest opening time,
typically seven or more seconds.  The Army Corps of Engineers Civil
Engineering Laboratory Technical Report M-313, April, 1983, Study of EMI/RFI
Seals on Shielded Enclosure Personnel Access Doors, provides an experimental
analysis of EMI door sealing mechanisms which indicates that wedge and knife
edge type doors degrade as much as 15 dB in as little as 4 months due to
routine use and exposure. The magnetic strip type of door is new, and little
long terms data is available.  The brass shimstock material covering the
magnets tends to oxidize rapidly, and a maintenance schedule similar to knife
edge doors is projected.  A further problem projected for the magnetic strip
doors is the work hardening and eventual cracking of the brass shimstock which
holds the individual magnets in place.
2.6.3
Air Duct Penetrations.  The second largest openings in shielding are
intake and exhaust of air ducting in heating, ventilation, and air
conditioning (HVAC) of the enclosures.  In order to maintain the required
shielding effectiveness through frequencies as high as 10 GHz, a waveguide
below cutoff air filter must be placed in the duct penetrations through the
shielding.  The waveguide filter may be a standard shielding manufacturer
product, typically a brass or steel core, tin-dipped, which may be similar in
appearance to a honeycomb with the walls of each small cell continuously
welded or soldered to the next cell.  Waveguide or honeycomb assemblies are
soldered in a parent framework, which is then welded or bolted into the
shield, see Figure 2.  The attenuation characteristics for each individual
cell must exceed the shielding effectiveness requirements of the total
enclosure.  The static pressure drop that the waveguide filter causes in the
airstream must be included in the HVAC calculations, as well as being
specified and controlled in the governing specifications.  The use of radio
frequency (RF) gasketing materials in the assembly and closure of the air
filter should be expressly forbidden by the written specification.
Dissimilar, and electrochemically active combinations of metal surfaces should
also be prohibited in this location where severe corrosion can be accelerated
by the combination of mechanical vibration, air flow, possible moisture, and
changing temperatures at metal-to-metal interfaces.  Filter honeycomb inserts
required for MIL-STD-285 and NSA 65-6 shielding requirements are typically one
inch in depth, with individual cell openings of 1/4 in. (6.3 mm) or less.
Custom made units may have clusters of larger diameter metal tubes,
continuously bonded at the end of a shielding plate, with hole sizes up to 1/2
in. (12.7 mm) diameter for MIL-STD-285 and NSA 65-6 required shielding
effectiveness at 10 GHz.
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