Ultra-high vacuum metallic seal

A metallic seal design that reduces manufacturing costs by minimizing production steps and also lowers the sealing force by incorporating a thin-wall, high height/width ratio annular column. The seal has two horizontal ribs which constrain the thin-wall column from unstable buckling to thereby reduce or eliminate the tendency of the sealing dams to become inclined to sealing flange surfaces.

FIELD OF THE INVENTION

The present invention generally relates to a seal for creating a seal between a pair of sealing surfaces and, more specifically, to a metallic seal.

BACKGROUND OF THE INVENTION

Elastomeric seals generally are not suitable for ultra high vacuum (UHV) sealing applications because of the inherent open structure of polymeric chains through which molecular gaseous species can diffuse. The lowest achievable He leak rate through elastomeric seals is typically 10−8cc/sec. Using metallic seals, He leak rates in the range of 10−9-10−11cc/sec can be easily obtained. Metallic seals, however, generally require high sealing force (Fs) also referred to as mechanical contact pressure Pmcwhere Pmc=Fs/Asand Asis the area of the sealing dam. If Pmcexceeds the yield strength of the sealing flange material, flange surfaces can be brinelled when they compress the metallic seal between them to achieve UHV. If brinelled, the flange surfaces will typically require reconditioning before installing new seals. Therefore, metallic seals have been designed to reduce the force required to compress the seal by optimizing seal cross section.

Metallic seals can also offer a longer seal life compared to elastomeric seals in applications where process chemicals would otherwise degrade an elastomeric material, for example in semiconductor processing applications. Elastomeric seals are attacked by highly reactive radicals such as NF3and O2which severely damage the polymeric chain structure thereby limiting the seal life. Metallic seals made from nickel, aluminum, tin, and/or stainless steel, for example, can be used in appropriate environments in which specific alloys are found inert. Because of extremely low leakage characteristics, metallic seals are often used to seal poisonous gases, such as PH3commonly used in semiconductor processing. Metallic seals of appropriate design can achieve leak rates even lower than welded joints. For example, He can have a higher molecular diffusion rate through weld defects than through a metallic seal due to micro cracks, grain boundaries and/or porosity of the welds.

U.S. Pat. No. 6,409,180 to Spence et al. (“Spence”) discloses a UHV metallic seal design similar to the one shown inFIG. 1. The Spence seal consists of four sections: two beams1and2, a column3, two diagonal braces4and5, and two sealing dams6and7. The diagonal brace angle is 35 to 55 degrees. An recessed surface ABC between beams1and2forms variable width column3having a minimum width at the center.

When the flanges9and10compress the seal, the seal height is reduced as the column3undergoes stable buckling maintaining the sealing dam surfaces6and7parallel to the sealing surfaces of flange9and10, as illustrated inFIG. 2.

A number of steps are typically necessary for machining the Spence seals from a hollow tube. For example, and with reference toFIG. 3, one method of making the Spence seal may include:(i) machining an annular recessed surface ABC with a tool T1having the desired profile;(ii) sectioning the individual seals,(iii) machining the first sealing dam with a second tool T2; and(iv) machining the second sealing dam.

An alternative method might include:(i) machining the annular recessed surface ABC with a first tool T1;(ii) machining another annular recess with a second tool T2to form the sealing dams; and(iii) sectioning individual seals.

There are several disadvantages of the Spence seal design. For example, the large number of machining steps can increase production cost, and the variable thickness column including the braced section requires a high sealing force (Fs) and Pmcwhich can cause brinelling of the sealing surfaces of the flanges.

SUMMARY OF THE INVENTION

The present invention includes a seal design that reduces manufacturing costs by minimizing production steps and also lowers the sealing force by incorporating a thin-wall, high height/width ratio annular column. The seal has two horizontal ribs which constrain the thin-wall column from unstable buckling to thereby reduce or eliminate the tendency of the sealing dams to become inclined to sealing flange surfaces.

Accordingly, a metallic seal for sealing axially facing surfaces comprises an annular column portion surrounding a central axis and having at opposite axial ends respective sealing dams for engaging the axially facing surfaces to be sealed. At least two ribs extend radially from the annular column portion, the ribs being axially spaced apart from each other and each pair of ribs defining therebetween an annular recess. The ribs are spaced at a pitch having a length generally corresponding to an axial length of the metallic seal divided by the number of ribs. For example, for a seal having a length L and two ribs the pitch of the ribs would be L/2.

More particularly, the recess can be generally U-shape, and the ribs can extend perpendicular to the central axis. The annular column portion can extend axially beyond respective axially outermost ribs thereby forming recesses that correspond in shape to one half of the annular recess. The annular column portion can have radially inner and outer surfaces defining a column width therebetween, the column width being substantially uniform along the height of the annular column portion, and the column portion may have a height/width ration of about seven or greater. At least one rib can have a thickness dimension in the axial direction that is about equal to or greater than a radial width of the column. The respective sealing dams can be in axial alignment with the annular column portion and may have a radial width between about 0.008 and 0.016 inches.

In accordance with another aspect, a method of making a metallic seal having an annular column portion surrounding a central longitudinal axis with respective sealing dams on ends thereof for sealing axially facing surfaces, and at least two ribs extending radially from the annular column portion is provided. The method includes forming a tubular seal blank having at least three equally axially spaced apart circumferential recesses of essentially the same shape forming therebetween respective radially extending ribs, and severing the tubular seal blank along the axial midpoint of the recesses that have disposed therebetween at least two of the radially extending ribs, to thereby separate at least one metallic seal from the seal blank. The recesses can be U-shape and the ribs can extend perpendicular to the central axis. The recesses can be formed by a machining process, for example.

In accordance with another aspect, a metallic seal for sealing axially facing surfaces comprises an annular column portion surrounding a central axis and having at opposite axial ends respective sealing dams for engaging the axially facing surfaces to be sealed, and at least two ribs extending radially from the column portion, the ribs being axially spaced apart from each other and defining an annular recess between each pair of relatively adjacent ribs, and the axial ends of the annular column portion extending axially beyond the respective axially outermost ribs and defining therewith respective recesses relative to the respective sealing dams, and each recess corresponds in shape to one half of the annular recess.

The recess can be generally U-shape, and the ribs can extend perpendicular to the central axis. The annular column portion can have radially inner and outer surfaces defining a column width therebetween, the column width being substantially uniform along the height of the annular column portion. At least one rib can have a thickness dimension in the axial direction that is about equal to or greater than a radial column width.

Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings.

DETAILED DESCRIPTION

Turning now toFIGS. 4-7, and initially toFIG. 4, an exemplary metallic seal in accordance with the invention is generally indicated by reference numeral20. The ring shape seal includes an annular column portion24surrounding a central axis A and having at opposite axial ends respective sealing dams28and32for engaging axially facing surfaces to be sealed, which as shown inFIGS. 5 and 6are sealing flanges36and40. Two ribs44extend radially from the column portion24.

Turning toFIG. 5, the ribs44are axially spaced apart from each other and define therebetween an annular recess52. The recess52in the illustrated embodiment is generally U-shape, and the ribs44generally extend perpendicular to the central axis A. The column portion extends axially beyond respective ribs44and define therewith respective recesses53that correspond in shape to one half of the annular recess52.

As will be appreciated, the ribs44are axially spaced apart at a pitch P having a length generally corresponding to an uncompressed axial length L of the metallic seal20divided by the number of ribs of the seal. Thus, in the illustrated embodiment, the pitch P is generally equal to about the axial length L divided by two. In a seal having three ribs, the pitch P would be equal to about the axial length L divided by three, and so on for seals of having additional ribs. Thus, the axial extent of a base54of the recess52in the illustrated embodiment is approximately twice the axial extent of the terminal portions of the column24extending axially beyond respective ribs44and/or the axial extent of recesses53.

A radially inner surface58and a radially outer surface60of the annular column portion24define a column width Cw therebetween, with the column width Cw being substantially uniform along the axial length L of the annular column portion24. Each metallic rib44in the illustrated embodiment has a thickness dimension t in the axial direction that is about equal to or greater than the width Cw of the annular column portion24. As will be appreciated, other dimensions of the annular column portion24, recess52, and ribs44, and/or relationships therebetween, are possible.

The sealing dams28and32are each in axial alignment with the annular column portion24, with the column width Cw being the same as the sealing dam width. Thus, the annular column portion24virtually extends from one sealing surface to the other sealing surface. This results in a seal20having sealing dams28and32with the same width as constant width column24and being in axially alignment therewith.

As will be appreciated, the height/width ratio of the column24in the illustrated embodiment is generally the axial length L (height) of the seal divided by the column width Cw (width). Seals in accordance with the invention can have a column height/width ratio of wide range, for example seven (7) or greater. By increasing the height/width ratio of the thin-wall column24the sealing force (Fs) is reduced for the same amount of compression of the seal free height.

InFIG. 6, the seal20is shown in a compressed state with sealing dams28and32parallel to surfaces36and40thereby providing a tight seal. The annular column portion24is buckled in a controlled manner generally around a midpoint of the axial extent of the recess52. The controlled buckling maintains the sealing dams28and32parallel to the sealing surfaces and/or each other to facilitate a tight seal and to reduce or eliminate relative movement between the sealing dams28and32and surfaces36and40.

Turning toFIG. 7, a method for making the seal20will be described. In general the method begins with forming a tubular seal blank70having at least three equally axially spaced apart circumferential recesses52of essentially the same shape. The recesses52form therebetween respective radially extending ribs44. As will be appreciated the recesses52can be formed by any suitable process, such as by machining, for example. As illustrated, a tool T3is shown for machining the recesses52.

Once the tubular blank70and/or recesses52are formed, the tubular blank70is then severed along the axial midpoint M of the recesses70that have disposed therebetween at least two of the radially extending ribs78, thereby separating a metallic seal20from the seal blank70.

Accordingly, the seal20can be made in as few as two process steps. First the annular recesses52are machined using a single tool, followed by sectioning in the middle of alternate grooves to produce individual seals as illustrated. The resulting seal20has an annular outer surface ABC that consists of three sections: one perfectly vertical defining about 50% of the column height, and two perfectly horizontal surfaces extending radially outward defining a part of each rib. Unlike prior art seals that may have a variable column width, seal20has a column of uniform width without any diagonal brace section. The annular column width Cw is also the same as the sealing dam width which is in alignment with the annular column24such that the column structure (e.g., annular column24and sealing dams28and32) virtually extends from one sealing surface to the other sealing surface. This increases the height/width ratio of the thin-wall column and reduces the sealing force (Fs) for the same amount of compression of the seal free height.

Turning now toFIG. 8, a seal assembly includes a seal retainer104and two metallic seals20. As will be appreciated, the seal retainer104supports and locates the metallic seals20for ease of assembly. Accordingly, the seal retainer104can generally be formed from sheet metal or the like and includes holes108through which bolts or other threaded fasteners can pass. The retainer104also has cutout portions112into which the metallic seal20are inserted before being slid into the position shown. Retaining arms116are provided for securing the metallic seal20in the position shown inFIG. 8. As will be appreciated, the retainer104not only supports and locates the metallic seals20for ease of assembly, but may also serve as a spacer to prevent overcompression of the metallic seals20during assembly.