METAL SEAL FOR ULTRA HIGH VACUUM SYSTEM

Abstract

The present invention introduces a metal seal flange assembly for a vacuum system. A new designed metal gasket has a crosses-section shape of irregular quadrangle with two sharp angle forms by the longer base and legs. The long base of the irregular quadrangle is the vertical inner wall of the metal gasket. A preferred cross section shape of the metal gasket is trapezoid or isosceles trapezoid. This design can reduce the normal force applied to the metal seal flange assembly and reduce the number of bolts used in a limit working space.

Description

FIELD OF THE INVENTION

The present invention relates to a metal gasket, a vacuum flange for the metal gasket, and a vacuum seal flange assembly utilized in a vacuum seal at connections for ultra high vacuum (UHV) system.

BACKGROUND OF THE INVENTION

In assembling ultra high vacuum (UHV) systems, those systems operating at pressures below approximately 10-6 torr, seals and closures have presented challenging problems. For such systems, rubbers and elastomers are not suitable as seals as they are permeable to gases to an extent such that very low base pressures cannot be attained and such seals typically cannot survive the high temperatures often necessary to bake out vacuum systems. When frequent separation or proximity of heat sensitive materials makes welding impractical, plastically deformed metal seals are satisfactory. The seals, however, require very rigid flanges and many closely spaced bolts to accomplish a sufficiently tight seal, and assembly and disassembly are time consuming. Nevertheless, plastically deformed metal seals such as gold wire and flat copper washers are the most reliable seals and are used almost universally in UHV work. Many styles have been developed, from laboratory fabricated special seals to commercially available standardized seals.

Some typical seals are illustrated in FIG. 1a to FIG. 1e.

• • (1) Crushed wire rings, of gold, copper, or aluminum—these gaskets are generally made from round wire with desire diameter cut to the length of the gasket mean circumference, then form into a circle and welded. They provide positive pressures. Since only line contact, they have high local seating stress at low bolt loads. The contact faces increase in flowing into flange faces. FIG. 1a.
  • (2) Step seal with flat gasket—the soft metal gasket is crashed and deformed in groove with two flat flange members by bolt down force. FIG. 1b.
  • (3) Coined gasket seal—rectangular shaped gasket is crashed in groove by bolt down force. FIG. 1c.
  • (4) Knife edge seal—two flanges with V-shaped ridge facing each other with soft metal gasket in between and hold together by a clamp or bolt. The V-shaped ridge is filled in with the material as the gasket is deformed. FIG. 1d.
  • (5) “Conflat” seal (Varian Association)—a soft metal gasket is captured in a rigid structure which plastically deforms the gasket. FIG. 1e. Long bake out times at high temperatures relieve internal stresses and the force on the seal, allowing the joint to be leak-tight as differential thermal expansion is limited.
  • (6) “Cryofit” tube fitting (Raychem Corporation)—utilizing Nitinol alloys, a shape-memory alloy. The Cryofit connector is essentially a sleeve of Nitinol, having internal seal ridges in series, which is bored in the austenitic phase to a diameter less then the outside diameter of pipes to be joined, then chilled and transformed to martensite, and mechanically expanded to a diameter greater than the outside diameter of pipes to be joined. The connector can be slipped over the pipes, heated and transformed to austenite, whereby a hoop stress presses the series of seal ridges into the outside surface of the pipes making an excellent pipe and hydraulic connection.
  • (7) “Helicoflex” seal (Carbone-Lorraine Industries Corporation)—utilizing shape-memory alloy to seal the cavities, U.S. Pat. No. 4.445.694.

All these sealing forces, external normal force bolt down or internal pressure, are applied to deform the metal gasket. The plastic deformed metal gasket blocks the channels that connect the confined chamber and outside environment. Thereafter the chamber can be pumped down to the desired ultrahigh vacuum (UHV) with proper equipments.

Obara et al. in U.S. Pat. No. 4,988,130 provided a formula to estimate how many bolts are needed to fix a pair of flanges of thickness t using copper gasket as seal for a pipe end of a plasma vacuum vessel. These flanges each have a ring-shaped knife edge. However, Obara's suggestion is for a large round-shaped pipe end with M8 bolts, which is not suitable for a smaller size vessel with multiple opening to be sealed with thinner M5 bolts and limit space to bolt.

The present invention provides a design of metal gasket for a limited working space that is easy to be plastic deformed with less number of bolts and still provides same ability to reach desired ultrahigh vacuum (UHV) level.

SUMMARY OF THE INVENTION

The present invention provides a new design of metal gasket that provide seal for ultra high vacuum system. The metal gasket provides similar or better sealing effect with less tighten force and sustains long time baking in high temperature with limited working space than a conventional gasket.

The shape of the metal gasket may be round, square, or in any shape. However, the cross section of the metal gasket is an irregular quadrangle with two sharp angles to reduce the applied bolt down force. The long base of the irregular is the vertical inner wall of the metal gasket. A preferred cross section shape of the metal gasket is trapezoid or isosceles trapezoid with two sharp angles and the short base to long base ratio of the trapezoid is about 1:10. A preferred altitude of the trapezoid is about one half of the long base. A preferred sharp angle formed by the long base and one leg of the trapezoid is between 30 and 60 degree.

A metal seal flange assembly for a vacuum system, comprising two metallic objects having a pair of opposite surface facing each other, at least one of the metallic objects having an opening that lead to a confined chamber, a normal force is applied to the two opposite surface and form a system for vacuum; a matching groove on the second object; a metal gasket is interposed in the groove between the two metallic objects; and the cross section of the metal gasket has a shape of irregular quadrangle with two sharp angle form by the longer bas and legs. The long base of the irregular is the vertical inner wall of the metal gasket. A preferred cross section shape of the metal gasket is trapezoid or isosceles trapezoid with two sharp angles and the short base to long base ratio about 1:10. A preferred altitude of the trapezoid is about one half of the long base. A preferred sharp angle formed by the long base and one leg of the trapezoid is between 30 and 60 degree. The normal force applied to the two opposite surface is provided by numbers of bolts. A gas pass is cut on the second flange to expose the dead space to vacuum.

The metal seal flange assembly has the following distinguishing features: 1. The seal plane of flange is flat, the other flange has a groove to interpose the metal gasket; 2. cross section of the metal gasket is a trapezoid with two shape angle formed by the long base and the two legs of the trapezoid; 3. the gasket deforms inward while normal force is applied; 4. the gasket can sustain a baking temperature over 150 C without gas leakage; 5. the metal seal flange assembly provides similar or better seal outcome with smaller tightening force and less number of bolts in a limited working space; 6. a gas path is introduced to expose the dead space to vacuum and thereafter reduce the outgas issue; 7. the metal seal flange assembly uses thinner metal gasket to achieve desired vacuum level; 8. the metal seal flange assembly is easy to be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

FIGS. 1 a-e are schematic diagrammatic representation of prior art metal seal methods, FIG. 1a crush metal ring; FIG. 1b step seal; FIG. 1c coined gasket seal; FIG. 1d knife edge seal; FIG. 1e Conflat flange seal.

FIG. 2 is a photograph indicates the position of multiple holes to be sealed in chamber surface.

FIG. 3 a is a schematic diagrammatic representation of a metal seal flange assembly with trapezoidal metal gasket of the present invention.

FIG. 3 b is a schematic diagrammatic representation of metal gasket design of the present invention.

FIG. 3 c is a schematic diagrammatic representation of a cross section view of the trapezoidal metal gasket according one embodiment of present invention.

FIG. 4 is a schematic diagrammatic representation of a cross section view of the flanges and metal gasket (a) before and (b) after bolt-down force applied according one embodiment of present invention.

FIG. 5 illustrates the gas path on the seal structure that exposes the dead space to vacuum environment.

FIG. 6 is a table that compares tightening force between three different metal gaskets according one embodiment of present invention.

FIG. 7 is a table that compares different deformation amount of a trapezoidal metal gasket and leak test outcome according one embodiment of present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to specific embodiments of the invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations are not described in detail in order not to unnecessarily obscure the present invention.

As the previous discussion, the metal gasket plastic deforms during bolt down and block the connecting channels to the environment outside the confined chamber, thereafter produce the ultrahigh vacuum within the chamber with proper pump down equipments. A conventional metal gasket is an annular or rectangular shaped soft metal with a round or rectangular shaped cross section. Many metals are used for gasketing purposes. Some of the most common range from soft varieties such as lead, copper, steel, nickel, stainless, and Inconel to the high alloyed steels. Noble metals such as platinum, silver, and gold are also have been used to a limit extent. In the present invention, we will use oxygen free copper as an example to explain and clarify issues we met.

The ultimate yield stress of copper is about 200 Mpa which is equivalent to 200N/mm. To plastic deform a copper gasket with a conventional round shape cross section; the ultimate yield stress must be reached. According the formula provided in U.S. Pat. No. 4,988,130, the sealing load to deform the copper gasket requires at least 8 M8 bolts. In present invention, as the FIG. 2 illustrates, the space left for sealing multiple holes on the chamber surface is limited. It is impossible to put 8 bolts around each hole. Beside the number of bolts reduction, the present invention also requires thinner metal gaskets to reduce total weight of chamber.

The purpose of present invention is to provide a metal seal for an ultra high vacuum system, for example a multi-axis e-beam column, with less bolt-down force in a limited working space. Beside the limited working space, unlike a conventional structure, the upper (the first) flange and the lower (the second) flange of the assembly is not symmetrical. FIG. 3a illustrates the metal seal flange assembly of the present invention and FIG. 3b illustrates the metal gasket for the metal seal flange assembly. The metal gasket sit on a groove on the lower flange and the seal plane of upper flange is flat.

One embodiment of the present invention alters the shape of the cross section of the metal gasket from conventional shape to irregular quadrangle, as the FIG. 3c illustrates. The long base of the irregular is the vertical inner wall of the metal gasket. A preferred irregular quadrangle shape is trapezoid or isosceles trapezoid. The present invention will use trapezoid structure as example to illustrate embodiments of invention. The preferred trapezoid structure is, for example, the ratio between the length of upper base to which of lower base of the trapezoid is about 1:10; the altitude of the trapezoid is about half of the length of lower base; and angle form between the leg and the lower base θ is between 30 and 60 degrees. This design makes the contact plane of the upper flange 330 and the metal gasket 320 becomes a line instead of a flat surface. Reduce the contact area implies higher normal stress per unit area can be reached with a same bolt down force. Thereafter the ultimate yield stress of the metal gasket, for example oxygen free copper gasket, can be reach with less number of bolts and thinner bolts than what U.S. Pat. No. 4,988,130 suggested.

Metal gasket usually sits in a groove and plastic deform itself when the two flanges are bolted together. The plastic deformed metal gasket blocks the channels that connect the confined chamber and outside environment. At the same time the deformed metal gasket produce dead space between the two flanges and groove. During the pump down process to produce ultrahigh vacuum, the dead space will continue outgas and retard the time to reach desired vacuum level. Sometimes, the desired vacuum level is not easy to reach if the dead space is too big. One embodiment of the present invention introduces a gas path 410 on the lower flange 310 to expose the dead space formed by metal gasket 320, flange 310, and flange 330 after plastic deformation as the FIG. 4 and FIG. 5 indicates. This design can eliminate the dead space outgassing issue during pump down process and less time to reach the desired vacuum level.

The comparison between metal gasket of trapezoidal shape cross section, hexagonal shape cross section, and conventional gasket with knife edge flange is illustrated in FIG. 6. The gasket has trapezoid cross section show no gap after tighten to the ultimate and has the smallest resistance during tighten process. This is understandable since the metal gasket of trapezoidal shape cross section has a largest normal force when a same bolt-down force is applied.

The leak test of a metal seal flange assembly when different deformation amount is applied to the trapezoidal gasket is illustrated in FIG. 7. The leak test was performed with helium leak detection equipment. The best result is reached when 0.4 mm deformation which is about 20% total reduction in height of the metal gasket was applied to the assembly. At this condition, the metal gasket requires medium sealing force to tighten the assembly and the assembly passed the leak test before and after 150 C, 7 hours baking test. The metal seal flange assembly with a trapezoidal metal gasket can reach a vacuum pump down to 7.6E-10 torr.

A metal seal flange assembly 300 for ultra high vacuum system comprise of two flanges and a metal gasket. Position the metal gasket 320 between the flange 310 and flange 330, as the FIG. 3a illustrates. The shape of metal gasket may be round, squared or in any other shape. However, the cross section of the metal gasket 320 is an irregular quadrangle. The long base of the irregular is the vertical inner wall of the metal gasket. A preferred shape of cross section is trapezoid, or isosceles trapezoid. A preferred altitude of the trapezoid is about one half of the long base. The metal gasket 320 contacts the two flanges with the sharp angle by the longer base of the trapezoid. Provide normal force with bolts 340 to tight the metal seal flange assembly. During tightening process, the metal gasket 320 is deformed and provides seal to the metal seal flange assembly with flange 310 and flange 330. A gas path 410 is cut on the flange 310 to expose the dead space produced between the metal gasket and flange 310 to vacuum environment; and thereafter resolve the outgas issue during vacuum pump down.

The metal seal flange assembly 300 has the following distinguishing features: 1. The two flanges are not symmetric, one seal plane of flange is flat, the other flange has a shallow groove to set the metal gasket; 2. cross section of the metal gasket is a trapezoid with two shape angle formed by the long base and the two legs of the trapezoid; 3. the gasket deforms inward during bolt-down; 4. the gasket can sustain a baking temperature over 150 C without gas leakage; 5. the seal structure can provide similar or better seal outcome with smaller tightening force and less number of bolts in a confined working space; 6. a gas path is introduced to expose the dead space to vacuum and thereafter reduce the outgas issue; 7. the metal seal flange assembly uses thinner metal gasket to achieve desired vacuum level; 8. the seal structure is easy to manufacture.

Claims

1. A metal gasket use for vacuum seal, comprising: An annular gasket made of soft metal material with a cross section of irregular quadrangle, wherein the irregular quadrangle has two sharp angles and the long base of the irregular quadrangle is the vertical inner wall of the annular gasket

2. The metal gasket use for vacuum seal of claim 1, wherein the irregular quadrangle cross section shape is a trapezoid or an isosceles trapezoid.

3. The metal gasket use for vacuum seal of claim 1, wherein the sharp angle is between 30 degree and 60 degree.