All-Metal Expandable Ultra High Vacuum Seal

This invention relates to sealing rings for adjacent flange surfaces, especially seal rings utilizing plastic deformation.

Mechanically separable joints in ultra high vacuum systems are most reliably sealed by plastic deformation of metal elements or gaskets. Typical seals are:

(1) Crushed wire rings, of gold, copper, or aluminum

(2) Step seal with flat gasket

(3) Coined gasket seal

(4) Knife edge seal

(5) "Conflat" seal (Varian Associates)

(6) "Cryofit" tube fitting (Raychem Corporation)

(7) "Helicoflex" seal (Carbone-Lorraine Industries Corporation)

In all these seals the sealing force is applied normal to the seal line, because no relative motion of the seal elements can be tolerated other than plastic deformation.

Some very large vacuum systems potentially of great importance cannot be sealed with any of these closure systems because the components to be sealed together cannot approach each other along paths perpendicular to the mating surfaces. An example of such seals are those between torus sections of a 16-segment vacuum vessel of a segmented Tokamak Fusion reactor. Each wedge-shaped segment spans 221/2.degree. and the closure surfaces approach each other along a sloping path of 111/4.degree., i.e., 783/4.degree. away from the normal. During gasket compression therefore the sliding between seal surfaces would be roughly five times as great as the gasket compression. What is needed is a gasket which can be installed without being loaded, then expanded and plastically deformed after the sealing surfaces are positioned and clamped together. An inflatable O-ring seal would have the necessary installation characteristics, but the service conditions sometimes (as in the Tokamak) preclude the use of elastomers.

An all-metal bakeable ultra high vacuum seal with plastic deformation of the seal which during installation has the contour of a deflated inflatable O-ring seal, but after installation assumes the sealing characteristics of a "helicoflex" seal.

The seal assembly is installed between flange surfaces in contact with only one of the surfaces until the entire closure is assembled and restrained. Sealing is then accomplished by plastic deformation of a seal assembly wrapper against the flanges. The energy to expand and deform the wrapper is provided by restoration of the shape of a shape-memory alloy component inside the wrapper as it undergoes a martensite-austenite transformation with rising temperature.

FIG. 1 is an isometric schematic of wire shaped into a helical coil;

FIG. 2 is an isometric schematic showing the installation of the wire within a wrapper;

FIG. 3 is an isometric schematic illustrating the mechanical deformation of the seal after chilling;

FIG. 4 is an isometric schematic of an installed, chilled seal; and

FIG. 5 is an isometric schematic of an installed seal after thermal deformation (warming).

In the following, the details of the invention are described via the steps for manufacture.

Refer to FIG. 1, in which a wire 1 of a shape-memory alloy in its austenitic phase is wound into a closed helical coil 11. Coil 11 is then formed into the shape of the gasket and the ends 2, 3 are joined together. In this condition it is annealed to stabilize its shape.

As shown by FIG. 2, a wrapper 4 is then put over coil 11 with its open side 10 away from the vacuum space, and its ends are joined by welding to make a continuous ring. Wrapper 4 is the material which will be elastically deformed to close the seal, and is chosen to be a substance capable of little or no additional workhardening, so that elastic deformation will proceed uniformly.

The assembly is chilled to transform the shape-memory alloy to martensite. It is then reduced in thickness in a press with chilled die-plates, and is maintained in the chilled condition. The dotted lines 5 in FIG. 3 illustrate the unchilled configuration while the solid lines shown the chilled and deformed assembly.

The chilled seal, as in FIG. 4, is placed in a groove 7 in one chilled flange 6, the mating flange 8 is slid into place along its slightly inclined approach path, and flanges 6, 8 are clamped or bolted together to resist the anticipated sealing force. The seal assembly still is in contact with only one flange 6.

The flanges 6, 8 and seal assembly are allowed to warm up through the transition temperature range of the shape-memory alloy. Coil 11 becomes again austenitic, but is prevented from again assuming its "remembered" circular cross-section, being restrained by flanges 6, 8. The restraining force acts through wrapper 4 and plastically deforms it to seal it against flanges 6, 8. This is shown in FIG. 5. Because of similarity in construction to the "Helicoflex" seal, it is anticipated that this seal can be made to cover the same size range, i.e.:

External Diameter--4 mm to 8000 mm

Section Diameter--1.6 mm to 25 mm

Min. Radius of Curvature--Three times the Section Diameter

Max. Seal Line Length--25.13 meters (82.5 feet)

There are real lower and upper limits to the operating temperature of this seal. The service temperature should preferably be above the transition temperature of the shape-memory alloy, so that the seal can be deformed, stored, and installed with the shape-memory alloy in the martensitic phase; according to the manufacturer of the Ni-Ti alloy "Nitinol", this temperature can be predetermined by minor (proprietary) alloying additions to be wherever desired between a low cryogenic temperature to well above the boiling temperature of water. If the service temperature is below the transition temperature, the seal can be installed and deformed by heating; upon cooling to the service temperature, the shape-memory component will transform without deformation to the lower strength martensitic phase, i.e., the gasket pressure will be reduced. The upper service temperature limit for very long exposure times is about 650.degree. F., above which creep begins to cause relaxation of the force resulting from the "unresolved" strain; this temperature is consistent with Nitinol being basically a titanium alloy.

The shape-memory alloy may be "Nitinol", a reasonably well characterized Ni-Ti alloy produced commercially by Raychem Corportion and Allegheny International Corporation. Equivalent results could probably be obtained using the "Proteus" copper-base shape-memory alloys available from N. V. Baekert S. A. and Metallurgie Hoboken-Overpelt in Belgium.

This seal can be used between two coaxial surfaces, not necessarily round, by radial expansion similar to that of a static O-ring. This type of seal requires that the wrapper be in the radial gap between the annular surfaces.