Abstract:
A tube fitting for a tube end has a first coupling member and a second coupling member joined with the first coupling member. A tube gripping device is disposed between the first coupling member and the second coupling member such that assembly of the first coupling member with said second coupling member to a pulled-up condition causes the tube gripping device to be plastically deformed and swaged into the tube to grip and seal the tube. The first coupling member engages an internal surface of the second coupling member to provide an indication of completed pull-up.

Description:
RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/522,814, filed on Sep. 14, 2006 entitled “Tube Fitting with Separable Tube Gripping Device” which is a continuation of U.S. patent application Ser. No. 10/467,444, filed on Aug. 5, 2003, now U.S. Pat. No. 7,108,288 entitled “Tube Fitting with Separable Tube Gripping Ring” which claims priority to International Patent Application Serial No. PCT/US02/03431, filed on Feb. 6, 2002 entitled “Tube Fitting with Separable Tube Gripping Ring” which claims the benefit of U.S. Provisional patent application Ser. No. 60/266,735 filed on Feb. 6, 2001 entitled “Tube Fitting with Integral Nut and Ferrule”, and Ser. No. 60/329,943 filed on Oct. 17, 2002 entitled “Tube Fitting”, the entire disclosures of which are fully incorporated by reference. 

   TECHNICAL FIELD OF THE INVENTION 
   The subject invention is generally directed to the art of tube fittings. 
   BACKGROUND OF THE INVENTION 
   Tube fittings are used to join or connect a tube end to another member, whether that other member be another tube end such as through T-fittings and elbow fittings, for example, or a device that needs to be in fluid communication with the tube end, such as for example, a valve. As used herein the terms “tube” and “tubing” are intended to include but not be limited to pipe as well. Any tube fitting must accomplish two important functions within the pressure, temperature and vibration criteria that the tube fitting is designed to meet. First, the tube fitting must grip the tube end so as to prevent loss of seal or tube blow out. Secondly, the tube fitting must maintain a primary seal against leakage. The requirement that a tube fitting accomplish these two functions has been the driving factor in tube fitting design for decades. A multitude of factors influence the design of a tube fitting to meet a desired grip and seal performance criteria, but basic to any tube fitting design will be: 1) the characteristics of the tubing that the fitting must work with, including the material, outside diameter and wall thickness; and 2) the tube grip and seal performance level required of the tube fitting for its intended applications. The goal is to design a tube fitting that reliably achieves the desired tube grip and seal functions within whatever cost constraints are imposed on the product by competing designs in the marketplace. 
   A flareless tube fitting generally refers to a type of tube fitting in which the tube end remains substantially tubular, in contrast to a flared tube fitting in which the tube end is outwardly flared over a fitting component. Flared tube ends are commonly encountered in use with plastic tubing and plastic tube fittings. The present invention is not primarily directed to plastic tubing or tube fittings because such fittings have significantly different challenges and material properties that affect the ability of the fitting to both grip the tube and provide an adequate seal. However, some of the aspects of the invention may apply to non-metal tube fittings, particularly the separable tube gripping element aspect. 
   Tube fittings that are intended for use with stainless steel and other metal tubing, for example, are particularly challenging to design in order to achieve the desired tube grip and seal functions. This arises from the nature of stainless steel which, in terms of typical commercially available tubing material, is a very hard material, usually on the order of up to 200 Vickers. Stainless steel and other metal tubing is also used for high pressure applications in which the tubing wall thickness is substantial (referred to in the art as “heavy walled” tubing). Heavy wall tubing is difficult to grip because it is not only hard but it is also not particularly ductile. Low ductility makes it more difficult to deform the tubing plastically so as to achieve a desired tube grip. 
   Tube fittings typically include an assembly of: 1) a tube gripping device, often in the form of a ferrule or ferrules, or a gripping ring-like structure, and 2) a pull-up mechanism for causing the tube gripping device to be installed on a tube end so as to grip the tube end and provide a seal against leakage. The term “pull-up” simply refers to the operation of tightening the tube fitting assembly so as to complete the assembly of the fitting onto the tube end with the desired tube grip and seal. 
   Usually a tube fitting is first assembled in a “finger tight” condition and then a wrench or other suitable tool is used to tighten or “pull up” the fitting to its final initial and complete assembled condition. The pull up mechanism most commonly used is a threaded connection of a female threaded nut component and a male threaded body component, with the tube gripping device being acted upon by these two components as they are threaded and tightened together. The body includes a tube end receiving bore with an angled camming surface at the outer portion of that bore. The most commonly used camming surfaces are frusto-conical such that the term “camming angle” refers to the cone angle of the camming surface relative to the tube end longitudinal axis or outer surface. The tube end is axially inserted into the body bore and extends past the frusto-conical camming surface. The gripping device is slipped onto the tube end and the nut is partially threaded onto the body to the finger tight position such that the tube gripping device captured axially between the camming surface and the nut. The nut typically includes an inward shoulder that drives the tube gripping device into engagement with the angled camming surface on the body as the nut and body components are threadably tightened together. The angled camming surface imparts a radial compression to the tube gripping device, forcing the tube gripping device into a gripping engagement with the tube end. The tube gripping device typically is to form a seal against the outer surface of the tubing and also against the angled camming surface. 
   The most commonly used tube gripping devices in stainless steel tube fittings today (the most commonly used are ferrule-type tube fittings) achieve tube grip by causing a front or nose portion of the tube gripping device to bite into the tube end outer surface. As used herein, the term “bite” refers to the plastic deformation of the tube gripping device into the outer surface of the tube end so as to plastically deform and indent the tubing with an almost cutting-like action to create a generally radial shoulder or wall at the front end of the tube gripping device. This “bite” thus serves as a strong structural feature to prevent tube blow out at high pressure, particularly for larger diameter tubing such as ½″ and higher. 
   Over the years there have been numerous tube fitting designs that do not rely on a “bite” type action, but rather merely radially compress the tube gripping device against the tubing outer surface, some with the effect of indenting into the tubing without creating a bite. These designs are not suitable for high pressure stainless steel tube fittings. The most common commercially available stainless steel tube fittings especially for high pressure applications have historically been of two radically distinct designs of the tube gripping device-single ferrule tube fittings and two ferrule tube fittings. 
   A single ferrule tube fitting, as the name implies, uses a single ferrule to accomplish both the tube grip and seal functions. However, it is becoming increasingly recognized that these two functions are at odds with each other when designing a tube fitting that can meet a desired tube grip and seal performance criteria. This is because the design criteria needed to assure that the tube fitting achieves an adequate tube grip usually works against the ability of the single ferrule to also provide an effective seal. Consequently, although prior art single ferrule fittings can achieve adequate tube grip in some cases, this tube grip performance comes at the expense of having a less effective seal. One result of this situation is that some single ferrule tube fittings have been designed with additional components and techniques to achieve an adequate seal. Less than optimum seal performance is particularly noted in single ferrule fittings that attempt to seal against gas, and especially high pressure gas. Single ferrule tube fittings thus are usually more suited to lower pressure liquid applications such as hydraulics, however, even in such lower pressure applications single ferrule seal performance remains less than desired. 
   For single ferrule tube fittings, the biting action is usually associated with the single ferrule being designed to bow in a radially outward direction from the tube wall in the central region or mid-portion of the single ferrule body between the front and back ends thereof. The front end of the ferrule is driven against the angled camming surface of the body by the nut pushing against the back end of the ferrule. The bowing action helps direct the front end of the single ferrule into the tube end. The bowing action is also used to cause the back end of the ferrule to likewise engage and grip the tube end. This is accomplished usually by provided an angled drive surface on the nut shoulder that engages the back end of the single ferrule so as to radially compress the back end of the ferrule into a gripping action on the tube end. In some single ferrule designs, the back end of the ferrule apparently is intended to bite into the tube end. This back end tube grip is sometimes used with the single ferrule in order to attempt to improve the tube fitting&#39;s performance under vibration because the back end grip attempts to isolate down-tube vibration from affecting the front end tube bite. 
   The use of a back end tube grip actually works against the effort to grip the tube end at the front end of the single ferrule. Ideally, the single ferrule should be completely in three dimensional compression between the nut and the camming surface of the body. Providing a back end grip actually places a counter acting tension to the single ferrule that works against the front end compression being used to provide the tube grip. Additionally, the outward bowing action tends to work against the effort to grip the tube at the front end of the single ferrule because, in order to enable the outward bowing action, the single ferrule requires a lessened mass that is adjacent the tube gripping “bite”. The outward bowing action radially displaces ferrule mass central to the ferrule body away from the tube end. Consequently, an outwardly bowed single ferrule fitting could be more susceptible to ferrule collapse, loss of seal and possibly tube blow out at higher pressures. 
   In order to achieve an adequate tube grip on stainless steel tubing, single ferrule stainless steel tube fittings have historically used a rather shallow camming angle of between ten and twenty degrees. This range of angles is referred to herein as “shallow” only as a term of convenience in that the angle is rather small. The shallow camming angle has been used in single ferrule fittings to obtain a mechanical advantage because the shallow angle provides an axially elongated camming surface against which to slide and radially compress the single ferrule front end to bite into the tube end outer surface. Hard stainless steel tubing material necessitated this elongated sliding camming action in order to be able to get the single ferrule to create an adequate bite for tube grip. Over the years, the single ferrule has been through hardened or case hardened so as to be significantly harder than the stainless steel tubing, however, the shallow camming angle is still used today in such single ferrule fittings to obtain a mechanical advantage from the ferrule sliding along the camming surface to produce the “bite” so as to assure an adequate tube grip. An example of a commercially available single ferrule tube fitting that uses a case hardened ferrule and a shallow camming angle of about twenty degrees is the CPI fitting line available from Parker-Hannifin Corporation. Another example is the EO fitting line available from Ermeto GmbH that uses a through hardened single ferrule and a twelve degree camming angle. 
   In some single ferrule designs, a non-conical camming surface has been tried whereby an attempt is made to simply press the ferrule against the outer surface of the tube end, thereby not creating a bite. The result in such cases however is a low grip or low pressure only fitting that are not well suited to stainless steel fittings. 
   The shallow camming angle and elongated camming surface and axial movement needed to achieve an adequate tube grip with a single ferrule fitting, however, compromises the ability of the single ferrule to achieve the seal function, especially in extreme environments and for sealing gas. This is because the front end of the single ferrule attempts to make the seal against the axially elongated camming surface. The radially outward bowing action causes a larger portion of the outer surface of the front end of the single ferrule to come into contact with the camming surface against which it is being driven. The result necessarily is a larger seal surface area between the outer surface of the single ferrule and the camming surface. This enlarged seal area causes an unwanted distribution of the sealing force between the single ferrule and the camming surface, and also creates a larger area for surface imperfections to allow leaks to occur. This is particularly a metal to metal seal issue (as contrasted to non-metal to non-metal seals: for example, in a plastic fitting it is usually desirable to provide an enlarged seal contact area because the more highly ductile plastic material can better form a seal between the two surfaces.) 
   Because historically the single ferrule fitting has used a shallow camming angle to achieve adequate tube grip, the less than optimum seal function is either tolerated as a recognized limitation on the application of the fitting, or additional features have been designed into the single ferrule fitting, most notably attempts to include one or more elastomeric seals with the single ferrule or with which the single ferrule cooperates to provide a better seal with stainless steel tubing. See, for example, U.S. Pat. Nos. 6,073,976 and 5,351,998. U.S. Pat. No. 6,073,976 illustrates a typical example of a single “ferrule” (called a “cutting ring” in the patent) fitting that attempts to solve the “seal” issue with added elastomeric seal. The U.S. Pat. No. 5,351,998 describes the benefits obtained by separating the tube grip and seal functions into two separate components. 
   A commercially available and highly successful two ferrule fitting used for tubing is commercially available from Swagelok Company, Solon, Ohio and is described in U.S. Pat. Nos. 6,131,963 and 3,103,373 both of which are owned in common by the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. In this two ferrule fitting, the tube grip and seal functions also are separately achieved by the use of two ferrules. The forward or front ferrule provides an excellent seal even against gas, and the back or rear ferrule provides an excellent tube grip. 
   The front ferrule achieves an excellent seal by camming against a shallow camming surface angle such as twenty degrees. This is because the front ferrule does not need to slide excessively on the camming surface in order to achieve a tube grip function. Likewise, the front ferrule is not case hardened because the primary purpose of the front ferrule is to seal and is not to bite into the tube end. Thus the relatively “softer” front ferrule achieves an excellent seal, particularly against gas, even though the body conical camming surface presents a camming angle of about twenty degrees. 
   The back ferrule achieves the tube grip function in the above noted two ferrule tube fitting. The back ferrule is case hardened to be substantially harder than the tube end. The front end of the back ferrule cams against a frusto-conical camming surface formed in the back end of the front ferrule. The ostensible angle of this camming surface is forty-five degrees, but due to the sliding movement of the front ferrule, the effective camming angle is actually a shallow angle of about fifteen to twenty degrees. Although the effective camming angle for the back ferrule is shallow, the back ferrule is not required to provide a primary seal (although it can form secondary or backup seals). The back ferrule also does not exhibit the undesired bowing action but rather grips the tube end as a function of a radially inward hinging action. As used herein, the term “hinging” refers to a controlled deformation of the ferrule such that a central region or mid-portion of the ferrule body undergoes an inwardly radial compression, as distinctly contrasted to a bowing or radially outward displacement. Thus, the effective shallow camming angle not only does not compromise the fitting seal capability, it actually substantially enhances the overall performance of the tube fitting especially for stainless steel tubing. 
   By using separate ferrules for each to achieve primarily only one of the key tube fitting functions, the two ferrule tube fitting achieves tremendous tube grip and seal functions. This prior art two ferrule tube fitting thus has enjoyed tremendous commercial success especially in the art of stainless steel tubing in part due to its performance characteristics such as high pressure rating on the order of 15000 psi, wide temperature rating of cryogenic to 1200° F. and in many applications a significant number of remakes (a “remake” refers to the loosening and re-tightening of a fitting after an initial pull-up). 
   U.S. Pat. No. 3,248,136 illustrates use of a single locking ring as opposed to a ferrule, wherein the locking ring acts against a surface having an angle that appears to be greater than twenty degrees or more, but the ring does not appear to plastically deform into the tubing but rather remains elastic so that the ring is designed to retain its original shape after pull-up, both of which are features that are unsuitable for stainless steel tube fittings of the type considered herein. Japanese utility model publication 44-29659 illustrates a tightening ring that appears to be intended to have a bowing effect and grip the tube at the front and back ends. The fitting does not appear to be applicable to stainless steel tubing as the tube is covered with a resin cover. 
   Attempts have been made to design tube fittings with a tube gripping element that separates during pull-up to function as a single element tube gripping device. Known designs place the breakaway element on the male threaded component. Additionally, the known designs either force the tube gripping element against a shallow camming surface angle or do not attempt to create a tube gripping bite into the tube wall. Thus the prior art designs suffer from the same limitations as the prior art single ferrule tube fitting designs. 
   Many applications and uses of the above-described two ferrule SWAGELOK tube fitting do not require such high pressure, temperature and remake performance characteristics. The present invention is directed to a new fitting concept that can meet lower performance characteristics without compromising overall fitting integrity and performance. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the invention, a new tube fitting concept provides an indication of completed pull-up by engaging an internal surface of one fitting component with another fitting component. In one embodiment, a tube fitting has a first coupling member and a second coupling member joined with the first coupling member. A tube gripping device is disposed between the first coupling member and the second coupling member such that assembly of said first coupling member with said second coupling member to a pulled-up condition causes the tube gripping device to grip and seal the tube. The first coupling member engages an internal surface of the second coupling member to provide an indication of completed pull-up. 
   These and other aspects and advantages of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments in view of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangements of parts, preferred embodiments and a method of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof, and wherein: 
       FIG. 1  illustrates in half longitudinal cross-section an exemplary embodiment of a tube fitting in accordance with the invention in a finger tight position; 
       FIG. 2  illustrates the embodiment of  FIG. 1  in a partially pulled up position; 
       FIG. 3  illustrates the embodiment of  FIG. 1  in a completed initial pulled up position; and 
       FIG. 4  illustrates another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance then with one aspect of the invention, a tube fitting is provided having a tube gripping device that initially is integral with one of the coupling elements and upon pull-up separates therefrom to function as a single ferrule fitting. In the preferred embodiment, the tube gripping device or ferrule is integrally formed with a female threaded nut and is attached thereto by a frangible thin web portion that breaks as the ferrule cams initially against a camming surface of the male threaded component. As a single ferrule after separation, the ferrule acts against the steep camming angle surface of a male threaded body. The steep camming surface angle is particularly advantageous when the hardness of the tube gripping device has a ratio of at least about 3.3 times greater and preferably at least 4 times greater to the hardness of the tubing material. 
   Although a number of aspects of the invention are described herein as being incorporated into the exemplary embodiments, such description should not be construed in a limiting sense. For any particular application the various aspects of the invention may be used as required in different combinations and sub-combinations thereof. Furthermore, although the present disclosure describes and/or illustrates a number of design choices and alternative embodiments, such descriptions are not intended to be and should not be construed as an exhaustive list of such choices and alternatives. Those skilled in the art will readily appreciate and understand additional alternatives and design choices that are within the spirit and scope of the invention as set forth in the appended claims. 
   Although the various embodiments are described herein with specific reference to the fitting components being made of stainless steel, and in particular 316 stainless steel, such description is intended to be exemplary in nature and should not be construed in a limiting sense. Those skilled in the art will readily appreciate that the invention may be realized using any number of different types of metal materials for the fitting components, as well as metal tubing materials, including but not limited to 316, 316L, 304, 304L, any austenitic or ferritic stainless steel, any duplex stainless steel, any nickel alloy such as HASTALLOY, INCONEL, or MONEL, any precipitation hardened stainless steel such as 17-4PH for example, brass, copper alloys, any carbon or low alloy steel such as 1018 steel for example, and any leaded, re-phosphorized or re-sulphurized steel such as 12L14 steel for example. An important aspect of the choice of materials is that the tube gripping device preferably should be case or through hardened to a ratio of at least about 3.3 and preferably 4 or more times harder than the hardest tubing material that the fitting will be used with. Therefore, the tube gripping device need not be made of the same material as the tubing itself. For example, as will be discussed hereinbelow, the tube gripping device may be selected from the stainless steel materials noted above, or other suitable materials that can be case hardened, such as magnesium, titanium and aluminum, to name some additional examples. Furthermore, the frangible feature of the tube gripping ring and female threaded nut may also be realized in non-metal tube fittings. 
   With reference to  FIG. 1 , the present invention contemplates a tube fitting  50  in which there are only two discrete components prior to assembly, namely a female threaded nut  52  and a male threaded body  54 . The nut  52  is substantially different from the typical nut used in a prior art ferrule type tube fittings. The body  54  may be the similar in general design as a typical body used in prior fittings, however, as will be explained further herein, it is preferred but not necessary that the body  54  also be optimized for proper make-up with the new nut  52 . Additionally, the body  54  need not be a discretely separate component but may be attached to or otherwise integral to another part such as a valve body, manifold or other components for example. 
   Note that in the drawings the fittings are illustrated in longitudinal cross-section but only half of the section is illustrated, it being understood that the other half is identical and omitted for clarity and ease of illustration. In all of the illustrations herein, various gaps and dimensions are somewhat exaggerated for ease of illustration. 
   The body  54  is a generally cylindrical main body  56  that has an integral extension or end  56   a . The end extension  56   a  may be a hex body, for example, or part of another component such as for example a valve body as noted hereinabove. The main body  56  may be machined from the same stock as the end extension  56   a  or may be otherwise attached such as by welding or other suitable technique. The body  56  includes a first central longitudinal bore  58  that is appropriately sized to closely and slideably receive a tube end  13 . The first bore  58  is somewhat larger in diameter than a coaxial second bore  59  that extends through the end extension  56   a  of the body  54 . Of course, if the fitting  50  is a closed end connection, the inner bore  59  would not be a through bore. 
   The tube end  13  preferably bottoms against a counterbore  60 . The body  56  is machined or otherwise formed with external male threads  62  that threadably mate with corresponding female threads  64  formed or machined in the nut  52 . It is contemplated that in order to avoid inadvertent mixing of old and new style body and nut parts with prior art fitting components, that the thread pitch on the nut and body of the present invention may be substantially different from the thread pitch values of prior art ferrule-type tube fitting nuts and bodies. This will avoid interchange problems and also allows for a course pitch that provides high axial stroke with reduced nut rotation for complete pull-up. For example, a fitting that incorporates the present invention may use course pitch threads that provide sufficient axial displacement to achieve proper pull-up in a half turn. A typical prior art fitting by comparison is pulled-up with 1¼ to 1½ turns. Nothing however prevents the designer from making the thread pitch any value suitable to a particular application, as there are other techniques to avoid interchange issues. Therefore, the one-half turn for pull-up is just one example of a variety of design choices available. 
   The central body bore  58  is preferably although not necessarily formed with a slight radially inward taper a relative to the longitudinal axis X ( FIG. 1 ) of the tube end  13  such that the diameter of the bore  58  decreases radially in the axial direction towards the counterbore  60 . For example, this taper may be about 2° to about 4°, although the selected angle is not particularly critical. The bore  58  diameter at the counterbore shoulder is just slightly less than the outer diameter of the tube end  13 . In this manner, the tube end  13  has a slight radial interference fit of a few thousandths of an inch for example with the bore  58 . This interference between the bore  58  and the tube end  13  provides an anti-rotation action to help prevent the tube end  13  from rotating during pull-up. This also reduces residual torsion stress that may be induced into the tube end due to rotation of the tube gripping element ( 80 ) during pull-up. The tube end  13  does not necessarily have to bottom completely against the counterbore shoulder  60 . This is because the interference fit helps provide a good primary seal between the bore  58  and the tube end  13 . The interference fit also helps improve the tube grip by the tube gripping element ( 80 ) by axially holding the tube end stationary during pull-up so that the full axial displacement of the tube gripping element ( 80 ) is used for proper deformation and tube grip rather than any lost axial motion or movement of the tube end during tightening. The taper of the bore  58  may extend gradually along its entire axial length or a shorter axial portion adjacent the counterbore  60 . 
   The nut  52  includes a first central bore  70  having a first diameter D 1  relative to the longitudinal axis of the fitting  50 . The nut  52  also includes a second bore  72  having a second diameter D 2  relative to the central longitudinal axis of the fitting  50 . In this embodiment, the diameter D 2  is less than the diameter D 1 . Furthermore, the diameter D 2  is sized so that the bore  72  defines a generally cylindrical wall that receives the tube end  13  ( FIG. 2 ). The first bore  70  terminates at a location that is axially spaced from the nut back end  74  to form a trepan  75 , such that the nut  52  includes a radially inwardly extending collar  76 . The collar  76  is generally defined by the back end wall  74  of the nut  52 , the smaller diameter bore  72  and the larger diameter bore  70 . 
   In accordance with a significant aspect of the invention, the nut  52  includes a tube gripping device  80  that extends axially inwardly in a somewhat cantilevered fashion from the collar  76 . The tube gripping device in this example is in the general form of a gripping ring  80  and includes an inner bore  82  that defines a substantially cylindrical wall that closely receives the tube end  13  ( FIG. 2 ). The diameter D 3  of the bore  82  may be the same as or different from the diameter of the second nut bore  72 . The cylindrical wall that defines the gripping ring bore  82  extends axially from a tapered front or nose portion  84  of the gripping ring  80 . The nose portion  84  includes an axially tapered outer surface  86  that increases in the radial dimension towards the back end of the ring  80 . The tapered outer surface  86  extends from a generally radial front end  85  of the gripping device  80 . This generally radial front end  85  joins to the inner cylindrical bore  82  at a preferably sharp corner  87 . Alternatively, however, there may be provided a circumferential recess or step or notch or other geometry (not shown) in the front end of the ring  80  having a diameter that is somewhat larger than the diameter D 3  and axially extending from the front end  85  towards the back end  74  of the nut  52 . 
   The tapered surface  86  joins the front end  85  preferably by a radius portion  89  and at its axial opposite end by a radius  86   a  to a generally cylindrical portion  91 , which in turn joins via a radius  93  to the trepan  75 . 
   It is noted at this point that the various geometry characteristics of the tube gripping device  80  (such as, for example, the various recesses, notches, tapered portions, radius portions and so on) are selected so as to effect an appropriate radially inward hinging action as will be further explained hereinafter. Accordingly, the geometry of a tube gripping device  80  will be determined by the characteristics of the material of the tubing such as hardness and the fitting components, the dimensions of the tubing and the required tube grip and seal performance needed for a particular application. Therefore, the specific embodiments illustrated herein are intended to be exemplary in nature and not limiting as to the geometry of the tube gripping device. The above referenced patents for the two ferrule fitting also illustrate additional geometry variations to facilitate the hinging effect to obtain a desired tube grip. 
   In accordance with another aspect of the invention, the tube gripping device or ferrule  80  is attached to the female threaded nut  52  by a thin frangible web portion  95 . This web portion breaks (as illustrated in  FIG. 2 ) when the ferrule  80  cams initially against a camming surface ( 88 ) during pull-up so that the tube gripping device or ferrule becomes a separate piece and functions with the nut and body in effect as a single ferrule fitting. The separated ferrule  80  has a back end  150  that is axially driven by a radially inwardly extending wall  152  of the nut  52  that serves as a drive surface for driving the ferrule  80  forward against the camming surface for completing an initial pull-up. The frangible web portion  95  is preferably designed so that upon separation of the device  80  from the nut  52 , the surface  95 a that is exposed along the break line does not interfere with the drive surface  152  of the nut during further pull-up to complete the assembly. As used herein, the terms “tube gripping device” and “ferrule” or “single ferrule” are used interchangeably when referring to the device  80  after separation from the nut  52 . 
   The ferrule  80  is machined with the frangible web  95  portion by forming a radial groove  154  that is angled generally toward the inside of the female threaded nut  52 . This groove  154  forms the back end  150  of the ferrule  80  and also the radial wall  152  of the nut that drive the ferrule axially against the camming surface after the ferrule  80  separates from the nut  52 . Preferably but not necessarily the wall  152  and the back end  150  are machined at an angle of about 75° or so relative to the tube bore axis X, although this angle may be different depending on the particular application. These surfaces  152  and  150  may be contoured to reduce galling and torque if so required. 
   With reference to  FIGS. 1 ,  2  and  3 , the tapered nose portion  84  initially engages an axially tapered camming surface  88  that forms an opening to the tube bore  58  in the main body  56 . The tapered camming surface  88  is a surface that joins the bore  58  wall to the back end wall  90  of the body  54 . This camming surface  88  is characterized by a generally frusto-conical contour. However, the shape of the surface  88  may be selected from other shapes depending on the particular ring deformation and tube gripping characteristics required for the fitting  50  in a specific application. 
   Upon a completed pull-up, the back end  90  of the body  54  contacts the trepan  75  which serves as a positive stop against over tightening. Should remakes be desired, the back end  90  may be axially spaced from the trepan  75  upon a completed first pull-up. Proper pull-up in this case may be verified using a gap gauge or other suitable technique, as is known. 
   The tube gripping ferrule  80  is shaped to effect several important functions of the fitting  50 . The ferrule  80  must, upon proper pull-up, provide a fluid-tight primary seal against the tapered camming surface  88 . This seal may be a primary outer seal for the tube fitting  50 , or may be in effect a secondary or back-up seal to any seal formed between the tube end  13  and the body  54 , for example along the bore wall  58  and/or the counterbore  60 . The separated ferrule  80  also will form a primary seal at the location where the ferrule  80  bites into the outer surface of the tube end  13  in the area where the cylindrical bore  82  of the ferrule  80  engages the tube end outer surface. Again, this primary seal may in effect be a back-up or secondary seal to any seal formed by the tube end  13  against the body  54 . In any event, the ferrule  80  must form primary seals against the camming surface  88  and the outer surface of the tube end  13 . In addition, the ferrule  80  must adequately grip the tube end  13  so as to maintain seal integrity under pressure, temperature and vibration effects, and to prevent the tube end from separating from the fitting under such circumstances. 
   In order to achieve a fluid-tight seal and tube gripping action, the ferrule  80  is designed to be plastically deformed and swaged into the tube end upon completed pull-up, as illustrated in  FIG. 3 . This result is achieved by designing the ferrule  80  to have a hinging action whereby the tapered nose portion  84  is not only driven axially forward as the nut  52  is threaded onto the body  54 , but also is radially displaced or driven into engagement with the outer surface of the tube end  13  wall. The forward end  92  of the nose portion  84  is thus compressed and embedded into the tubing wall with a resultant stress riser or bite in the region designated  94  in  FIG. 3 . The front end bite  94  produces a generally radially extending wall or shoulder  99  formed out of the plastically deformed tube end material. The shoulder  99  engages the embedded front end  92  of the gripping ring  80  to thus form an exceptionally strong mechanical resistance to tube blow out at higher pressures. The embedded front end  92  thus provides both an excellent seal and a strong grip on the tube end  13 . The ring  80  is further designed to exhibit the aforementioned radially inward hinging action so as to swage or collet the cylindrical wall  82  against the tube end at a location axially adjacent or spaced from the stress riser bite  94  and generally designated with the numeral  96 . This swaging and collet effect substantially enhances the tube gripping function and serves to isolate the embedded nose portion and bite  94  from the effects of down tube vibration and also temperature changes. 
   Although the present invention is described herein in the various embodiments as effecting an embedded nose portion and attendant swaging action, those skilled in the art will appreciate that in some applications such rigorous design criteria may not always be required, particularly for fittings that will be exposed to moderate temperature, vibration and pressure effects. Therefore, the additional design aspects of the nut, body and gripping ring set forth herein as preferred embodiments should therefore not be construed in a limiting sense but rather as selectable enhancements of the basic concepts of the invention to be used as required for particular applications. 
   In order to achieve the desired swaging action and tube grip, the ferrule  80  is designed to exhibit the hinging action that allows the tapered nose portion  84  and the central or mid-portion (as at the region of the cylindrical bore  82  or the region designated  94 ) of the gripping ring  80  to be radially inwardly compressed as it engages with the tapered camming mouth  88  of the body  56 . This hinging action is also used to provide a significant radial displacement and compression of the cylindrical wall  82  to swage a central or mid-portion of the ferrule  80  body onto the tube end  13  axially adjacent to the stress riser  94 . In the embodiment of  FIGS. 1-3 , the hinging action is facilitated by providing a preferred although not required radial inner circumferential notch  98  that is axially positioned between the cylindrical portions  72  and  82 . The notch  98  is suitably shaped to permit the ferrule  80  to plastically deform and collapse in a controlled manner so as to radially compress the cylindrical wall  82  against the tube end with the desired collet effect. The particular geometry of the gripping ring  80  will thus be designed so that as the nut  52  is further threaded and tightened onto the body  54  after the ferrule  80  separates, the ferrule  80  hinges and plastically deforms to grip the tube end and to seal both against the tube end and the tapered camming mouth  88 . Standard design procedures such as Finite Element Analysis may be used to optimize the geometry of the ring  80  based on variable factors such as the tubing material, tubing hardness and wall thickness, and required pressure, temperature and vibration performance characteristics. 
   Proper deformation of the ferrule  80  may further be controlled by selecting an appropriate contour for the tapered surface  88 . This surface engages the tapered nose of the ferrule  80  and therefore will in part determine the timing and manner of how the ferrule  80  hinges, compresses and plastically deforms to properly embed the nose portion to bite into the tubing and also provide the desired collet or swaging action. Furthermore, the contour of the camming surface  88  may be designed to achieve the desired seal between the ferrule  80  nose portion and the tapered surface  88 . This seal is important to the overall performance of the fitting, as is the seal provided between the ferrule  80  and the tube end  13 . 
   The nut  52  with its integral tube gripping ferrule  80  may be manufactured by standard machining operations, and will typically include a trepan operation to form the outer contour of the ring  80 . The other features of the nut  52  can be realized as well with known machining operations. Preferably but not necessarily the nut  52  includes wrench flats  102  to permit the user to tighten the nut  52  onto the body  54 . Those skilled in the art will readily appreciate that use of the fitting  50  only requires relative rotation between the nut  52  and the body  54 , such that either component or both may be rotated as required during a pull-up operation. 
   We have found that it is highly desirable for the camming surface  88  to form a camming angle .theta. of about 35°-60° relative to the longitudinal axis X of the fitting  50  and tube end  13 . More preferably the angle .theta. of the camming surface  88  should be 40°-50°, and most preferred the angle .theta. should be about 45°. This range of angles for the camming surface  88  differs dramatically from commonly used metal ferrule-type tube fitting designs. Commonly used tube fittings have camming surface angles in the range of 10°-25°, which is a substantially shallower angle compared to the present invention. The shallower camming angle is necessary in prior art fittings to have the ferrule slide a greater axial distance along the camming surface. This greater sliding action permits the tube gripping device to be more gradually radially deformed into the tube end to form a gripping action or bite on the tube. This is especially the case for stainless steel tubing. Prior tube fittings that included what might appear to be a steeper camming angle actually either rely on a shallow portion of the camming surface or do not produce a bite in the tubing, thereby limiting the pressure resistance of the fitting. The shallow camming angle of the prior art, however, compromises the ability of a single ferrule to form a dependable seal. In sharp contrast, the present invention utilizes a substantially steeper camming surface angle .theta., which permits the gripping ring nose portion  84  in effect to be coined into the camming surface  88  without substantial sliding action, thereby forming an excellent seal. 
   In the exemplary embodiments herein, the nose portion  84  includes the radius portion  89  that transitions to the outer tapered surface  86 . This outer surface  86  tapers generally at an angle that is not as steep as the angle of the camming surface  88 . The tapered outer surface  86  preferably tapers axially with an increasing radial dimension towards the back end of the gripping ring  80 . This tapered outer portion  86  contacts the camming surface  88  with, in effect, a generally narrow zone or line contact upon pull-up that has high stress and material coining to allow the front end of the gripping ring  80  to coin into the camming surface  88 . Therefore, the term “generally narrow line contact” is not intended to preclude an area of contact between the outer tapered surface  86  and the camming surface  88 , but rather more generally to the concept of a localized contact zone near or at the innermost extent of the camming surface  88  of high stress and material coining between the outer tapered surface  86  and the camming surface  88 . By “coin” is simply meant that the gripping ring  80  achieves a good metal to metal seal between the radius portion  89  and the camming surface  88  by forming a generally narrow circumferential line contact of metal burnished on metal to effect a gas tight primary seal between the tapered surface  86  and the camming surface  88 . 
   It is important to note that the use of a particular camming angle is not dependent necessarily on the contour of the surface  88 . In other words, the angle of interest is the angle at which the front end of the gripping ring  80  contacts the camming surface  88  to form a seal thereat. Thus, the camming surface  88  may indeed be made with a non-frusto-conical contour, but the seal is still formed by the front end of the ferrule  80  contacting a steep angled surface  88 . Additional compound angles or contours of the camming surface  88  may be used to better facilitate the hinging action and tube bite achieved by the ferrule  80 . 
   Whether the camming surface  88  is formed as a compound angled surface with additional angled portions that are steeper or shallower to facilitate the hinging action and bite of the gripping ring  80  into the tube end  13 , in accordance with this aspect of the invention, the sealing portion of the front end of the gripping ring  80  (in the exemplary embodiments the radius portion  89 ) forms the primary seal on a steep angled portion of the camming surface  88 , preferably a steep angled portion in the range of angle .theta. of about 35°-60° relative to the longitudinal axis X of the fitting  50  and tube end  13 , more preferably the angle .theta. of the camming surface  88  should be 40°-50°, and most preferred the angle .theta. should be about 45 at the location where the primary seal is to be formed. Preferably although not necessarily this primary seal is effected by a generally narrow line contact type engagement between the front end of the gripping ring  80  and the camming surface  88 . 
   The steeper camming surface angle has the additional benefit that the nose or front portion of the tube gripping device  80  may be formed with substantially more mass as compared to if the front portion had to engage a shallower camming surface angle as in the prior art single ferrule and gripping ring designs. This added mass, along with the hinging action, tends to position a substantially greater mass of material at or near the location of the tube bite  94 . This significantly strengthens the tube gripping device in resisting pressure and also strengthens the collet effect that isolates the bite from vibration and temperature effects, as contrasted to prior art single ferrule or gripping ring designs. The hinging action also results in the back end of the tube gripping device (i.e. the end opposite the nose end  84 ) from contacting the tube end, so that the entire tube gripping device is in axial and radial compression. 
   In general, for a tube gripping device such as a ferrule to embed into, bite and grip the tube end, the tube gripping device must be harder than the tube end. This is especially so for thick wall tubing. The greater axial movement of a ferrule in a shallow angle camming mouth of the prior art allows a ferrule to embed into a tube even when the ferrule is only moderately harder than the tube. Under these circumstances if the tube gripping device  80  were only moderately harder than the tube end, the device would be unable to adequately grip the tube for a steep angle camming surface because of the substantially shorter axial movement of the tube gripping device during pull-up caused by the steeper camming angle. However, in accordance with the present invention, by making the tube gripping device significantly harder than the tubing, a steeper angle camming surface may be used and is effective to cause the tube gripping device to adequately bite into the tube end to grip the tube. 
   The steeper camming angle .theta. of the present invention also results in a much shorter distance of axial displacement of the ferrule  80  during pull-up. Consequently, the nose portion  84  will need to be radially deformed and compressed into the tube end  13  with a much shorter axial displacement or sliding movement. In order to achieve the proper tube grip then, the ferrule  80  is preferably case hardened to a hardness of at least about 3.3 times harder than the tubing material. For example, if the tubing material is stainless steel, it may exhibit a hardness of up to about 200 Vickers. Therefore, in accordance with this aspect of the invention, when the fitting  50  is used with such hard materials, the tube gripping device should be hardened to a ratio of at least about 3.3 times harder than the tubing. More preferred, the tube gripping device should be hardened to a ratio of at least 4 times harder than the tubing. Still further, the entire gripping ring  80  need not be case hardened, but rather only the nose portion  84  may be selectively case hardened. 
   In accordance with this aspect of the invention, all or part of the nut  52  and body  54  may be through hardened or case hardened to increase the tube grip of the fitting  50  when used with harder tubing materials such as stainless steel. Suitable case hardening processes are fully described in U.S. Pat. Nos. 6,165,597 and 6,093,303 and copending patent application Ser. No. 09/494,093 filed on Jan. 28, 2000 for MODIFIED LOW TEMPERATURE CASE HARDENING PROCESS, issued to the assignee of the present invention, the entire disclosures of which are fully incorporated herein by reference. These processes produce a hardness of the tube gripping device of about 800 to 1000 Vickers or higher without compromising the corrosion resistance of the fitting. Other case hardening techniques however may be used as required. Case hardening of the tube gripping ring  80  allows the ring  80  to adequately grip and seal against tubing materials such as stainless steel including duplex stainless steel. The above referenced case hardening patents have an additional benefit of providing surfaces on the ring  80  that reduce or prevent galling between the ring  80  (which rotates with the nut  52 ) and the tubing. 
   Various lubricants may also be used with the tube gripping ring  80  to reduce galling and residual torsion such as, for example, PTFE greases, and greases containing molybdenum disulphide or tungsten disulphide. 
   Case hardening techniques typically will result in the entire nut  52  and integral tube gripping ring  80  to be case hardened. When the case hardening is performed on stainless steel, for example, as in the above referenced patents or patent application, an adherent oxide skin is formed. In another embodiment of the invention, a solid lubricant may be applied to the threads of the stainless steel nuts  52  to reduce friction and the hence pull-up torque during tightening. Any solid lubricant can be used for this purpose and many such solid lubricants are well known. A few examples are graphite, molybdenum disulfide, tungsten disulfide and UHMWPE (ultra high molecular weight polyethylene). These lubricants can be used neat, i.e. not combined with another material, or mixed with another material such as a resinous carrier or the like. In addition, they can be used in essentially any solid form including powders, granules and pastes. 
   Solid lubricants of this type are well known commercial products. Examples include Dow Corning® 321 Dry Film Lubricant available from Dow Corning Corporation of Midland, Mich. and Slickote® Dry Lube 100 available from Trans Chem Coatings, of Monrovia, Calif. 
   These lubricants can be applied by any standard method such as by hand, by aerosol or air spraying or by automatic equipment. Any coating thickness can be used which will provide lubricating properties. Solid lubricant thickness exceeding standard class 2 thread clearances are usually not required. If appropriate, the lubricant can also be heated to enhance its adhesion. For example, some lubricants, especially those supplied in a resinous binder, can be heated to effect cure of the binder. For example, Slickote® Dry Lube 100 can be heated following manufacturer&#39;s instructions to 300° F. for 1 hour, for example. 
   In a particular embodiment of the invention, a dry lubricant as described above is used on stainless steel nuts  52  which have been subjected to low temperature carburization using carbon monoxide as the carbon source. Stainless steel is stainless because of the thin, coherent chromium oxide film which inherently forms when the steel is exposed to air. Low temperature carburization of stainless steel parts, such as those made from AISI 316 and 316L stainless steel, usually leaves the part surfaces coated with a layer of soot. Before use this soot is usually removed by washing. When carbon monoxide is used as the carbon source in low temperature carburization, not only does soot form but in addition a heavy oxide film also forms. This heavy oxide film is considerably different from the coherent chromium oxide film which makes stainless steel stainless in that it is thicker and not coherent. Therefore, this film is also removed before use to uncover the part&#39;s carburized surface. 
   In accordance with this particular embodiment, this heavy oxide film is not removed before application of the solid lubricant. Rather, it is left on the carburized part surfaces, or at least the portions of the carburized surfaces to be lubricated. In accordance this particular embodiment, it has been found that the naturally porous structure of this heavy oxide skin acts as an anchor for binding the lubricant to the part surfaces. As a result, the lubricant is more adherent than would otherwise be the case, and hence is able to withstand repeated fitting remakes (i.e., loosening and re-tightening of the nut) without being removed. 
     FIG. 4  illustrates another embodiment of the invention in which all elements are generally the same as the prior embodiment with one variation. In the frangible web portion  95 , a stress concentrating notch  300  is formed therein. In this embodiment the stress concentrating notch  300  is formed as a generally tight radius that creates a thinner web of material  302  to promote a rapid clean break of the ferrule  80  from the nut  52 . The break thus occurs as a result of a minimal span of rotation of the nut  52  shortly past finger tight position. The shape of the break is also less ragged. Other shapes of the notch  300  may be used as required including elliptical, triangular and so on for example. 
   The invention has been described with reference to the preferred embodiment. Clearly, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.