Abstract:
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.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/021,067 filed Jan. 15, 2008, which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]    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  
       [0003]    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 −8  cc/sec. Using metallic seals, He leak rates in the range of 10 −9 -10 −11  cc/sec can be easily obtained. Metallic seals, however, generally require high sealing force (F s ) also referred to as mechanical contact pressure P mc  where P mc =F s /A s  and A s  is the area of the sealing dam. If P mc  exceeds 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. 
         [0004]    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 NF 3  and O 2  which 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 PH 3  commonly 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. 
         [0005]    U.S. Pat. No. 6,409,180 to Spence et al. (“Spence”) discloses a UHV metallic seal design similar to the one shown in  FIG. 1 . The Spence seal consists of four sections: two beams  1  and  2 , a column  3 , two diagonal braces  4  and  5 , and two sealing dams  6  and  7 . The diagonal brace angle is 35 to 55 degrees. An recessed surface ABC between beams  1  and  2  forms variable width column  3  having a minimum width at the center. 
         [0006]    When the flanges  9  and  10  compress the seal, the seal height is reduced as the column  3  undergoes stable buckling maintaining the sealing dam surfaces  6  and  7  parallel to the sealing surfaces of flange  9  and  10 , as illustrated in  FIG. 2 . 
         [0007]    A number of steps are typically necessary for machining the Spence seals from a hollow tube. For example, and with reference to  FIG. 3 , one method of making the Spence seal may include:
       (i) machining an annular recessed surface ABC with a tool T 1  having the desired profile;   (ii) sectioning the individual seals,   (iii) machining the first sealing dam with a second tool T 2 ; and   (iv) machining the second sealing dam.       
 
         [0012]    An alternative method might include:
       (i) machining the annular recessed surface ABC with a first tool T 1 ;   (ii) machining another annular recess with a second tool T 2  to form the sealing dams; and   (iii) sectioning individual seals.       
 
         [0016]    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 (F s ) and P mc  which can cause brinelling of the sealing surfaces of the flanges. 
       SUMMARY OF THE INVENTION  
       [0017]    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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0024]      FIG. 1  is a cross-sectional view of prior art metallic seal in an uncompressed state. 
           [0025]      FIG. 2  is a cross-sectional view of the prior art seal of  FIG. 1  in a compressed state. 
           [0026]      FIG. 3  is a schematic illustration of a prior art method of making the seal of  FIGS. 1 and 2 . 
           [0027]      FIG. 4  is a cutaway three-dimensional view of an exemplary metallic seal in accordance with the invention 
           [0028]      FIG. 5  is a cross-sectional view of the metallic seal of  FIG. 4  in an uncompressed state. 
           [0029]      FIG. 6  is a cross-sectional view of the metallic seal of  FIG. 4  in a compressed state. 
           [0030]      FIG. 7  is a schematic illustration of an exemplary method of making the seal of  FIGS. 4-6  in accordance with the invention. 
           [0031]      FIG. 8  is a perspective view of a seal assembly including a retainer plate and a pair of exemplary metallic seals in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0032]    Turning now to  FIGS. 4-7 , and initially to  FIG. 4 , an exemplary metallic seal in accordance with the invention is generally indicated by reference numeral  20 . The ring shape seal includes an annular column portion  24  surrounding a central axis A and having at opposite axial ends respective sealing dams  28  and  32  for engaging axially facing surfaces to be sealed, which as shown in  FIGS. 5 and 6  are sealing flanges  36  and  40 . Two ribs  44  extend radially from the column portion  24 . 
         [0033]    Turning to  FIG. 5 , the ribs  44  are axially spaced apart from each other and define therebetween an annular recess  52 . The recess  52  in the illustrated embodiment is generally U-shape, and the ribs  44  generally extend perpendicular to the central axis A. The column portion extends axially beyond respective ribs  44  and define therewith respective recesses  53  that correspond in shape to one half of the annular recess  52 . 
         [0034]    As will be appreciated, the ribs  44  are axially spaced apart at a pitch P having a length generally corresponding to an uncompressed axial length L of the metallic seal  20  divided 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 base  54  of the recess  52  in the illustrated embodiment is approximately twice the axial extent of the terminal portions of the column  24  extending axially beyond respective ribs  44  and/or the axial extent of recesses  53 . 
         [0035]    A radially inner surface  58  and a radially outer surface  60  of the annular column portion  24  define a column width Cw therebetween, with the column width Cw being substantially uniform along the axial length L of the annular column portion  24 . Each metallic rib  44  in 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 portion  24 . As will be appreciated, other dimensions of the annular column portion  24 , recess  52 , and ribs  44 , and/or relationships therebetween, are possible. 
         [0036]    The sealing dams  28  and  32  are each in axial alignment with the annular column portion  24 , with the column width Cw being the same as the sealing dam width. Thus, the annular column portion  24  virtually extends from one sealing surface to the other sealing surface. This results in a seal  20  having sealing dams  28  and  32  with the same width as constant width column  24  and being in axially alignment therewith. 
         [0037]    As will be appreciated, the height/width ratio of the column  24  in 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 column  24  the sealing force (F s ) is reduced for the same amount of compression of the seal free height. 
         [0038]    In  FIG. 6 , the seal  20  is shown in a compressed state with sealing dams  28  and  32  parallel to surfaces  36  and  40  thereby providing a tight seal. The annular column portion  24  is buckled in a controlled manner generally around a midpoint of the axial extent of the recess  52 . The controlled buckling maintains the sealing dams  28  and  32  parallel to the sealing surfaces and/or each other to facilitate a tight seal and to reduce or eliminate relative movement between the sealing dams  28  and  32  and surfaces  36  and  40 . 
         [0039]    Turning to  FIG. 7 , a method for making the seal  20  will be described. In general the method begins with forming a tubular seal blank  70  having at least three equally axially spaced apart circumferential recesses  52  of essentially the same shape. The recesses  52  form therebetween respective radially extending ribs  44 . As will be appreciated the recesses  52  can be formed by any suitable process, such as by machining, for example. As illustrated, a tool T 3  is shown for machining the recesses  52 . 
         [0040]    Once the tubular blank  70  and/or recesses  52  are formed, the tubular blank  70  is then severed along the axial midpoint M of the recesses  70  that have disposed therebetween at least two of the radially extending ribs  78 , thereby separating a metallic seal  20  from the seal blank  70 . 
         [0041]    Accordingly, the seal  20  can be made in as few as two process steps. First the annular recesses  52  are machined using a single tool, followed by sectioning in the middle of alternate grooves to produce individual seals as illustrated. The resulting seal  20  has 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, seal  20  has 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 column  24  such that the column structure (e.g., annular column  24  and sealing dams  28  and  32 ) 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 (F s ) for the same amount of compression of the seal free height. 
         [0042]    Turning now to  FIG. 8 , a seal assembly includes a seal retainer  104  and two metallic seals  20 . As will be appreciated, the seal retainer  104  supports and  locates the metallic seals  20  for ease of assembly. Accordingly, the seal retainer  104  can generally be formed from sheet metal or the like and includes holes  108  through which bolts or other threaded fasteners can pass. The retainer  104  also has cutout portions  112  into which the metallic seal  20  are inserted before being slid into the position shown. Retaining arms  116  are provided for securing the metallic seal  20  in the position shown in  FIG. 8 . As will be appreciated, the retainer  104  not only supports and locates the metallic seals  20  for ease of assembly, but may also serve as a spacer to prevent overcompression of the metallic seals  20  during assembly. 
         [0043]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.