CRYOGENIC VACUUM SEALS
Oxide free indium wire will cold weld to bond even dissimilar metal flanges with different thermal coefficients to form a vacuum tight cryogenic seal. The ideal compression should give a gasket type seal of .002" - .005" thick. An important side issue is to make certain to include a jack-screw for separation. A jack-screw post is merely a threaded hole where a screw can be inserted to apply pressure to the opposing flange to 'push' the two apart. There are three acceptable techniques for seal groove design. For minimum machining and maximum reliability I prefer a semi-circular groove in one flange formed by using a ball end mill the diameter of the chosen wire to a depth of half the diameter. The other flange can have the standard o-ring smooth surface. A second technique, also reliable, safe, and sure, is to use a step groove on one of the flanges with the reverse mating form on the other flange. The allowable cross sectional area of the gap should equal about 80% of the selected o-ring cross sectional area. A chamfer on the lower edge of the reverse mating flange will allow indium to flow into the gap forming part of the seal. This technique is expensive to machine but recommended for high pressure seals. A third technique is to use a variation of the standard o-ring groove as follows: Width equal to the diameter of the chosen wire. Depth equal to half the diameter plus .005" to .015" depending on wire size.
Quantity and pattern of the bolt circle is important. The distance between bolts should be such that the compressive force fields overlap at about the 50% point. This means bolts are close together. We offer some ballpark design considerations here:
SQUARE GROOVE DESIGN:
Width of the groove times the depth should equal about 80% or less of the cross sectional area of the wire selected. For example: For .060" wire the cross sectional area is .002826 sq.in. The groove would be .060" wide and the depth would be .038". This gives a 20% excess of wire over groove cross section.
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