Tube fitting for stainless steel tubing

A tube fitting that uses only two components, namely a fitting body and a fitting nut. The nut and body are adapted to be threadably coupled together by relative rotation therebetween. The nut includes an integral tube gripping ring that cooperates with a camming surface on the body when the fitting is made-up. The camming surface preferably has a steep camming angle. The ring is radially compressed against the tube outer wall to form a seal and a tight tube grip. The ring also forms a line contact-type seal against the camming surface. The ring is designed to have a hinging action and to plastically deform during pull-up to embed into the tubing for excellent tube grip, and an axially adjacent swage or collet zone that isolates any stress riser resulting from the embedding action from vibration effects. The fitting components, and particularly the ring, may be case hardened, the fitting may include a self-gauging feature to indicate sufficient pull-up to prevent excessive pull-up. Also provided is a single ferrule tube fitting that uses a steep camming surface and is at least about 3.3 times harder than the stainless steel tube end.

TECHNICAL FIELD OF THE INVENTION

The subject invention is generally directed to the art of tube fittings for stainless steel tubing. More particularly, the invention is directed to flareless tube fittings that include a steep angle camming surface.

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 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. Operating pressures and temperatures are also typically substantially lower in the plastics art. In other words, with respect to tube grip and seal, whatever works in a plastic tube fitting provides little or no guidance for a non-plastic tube fitting.

Tube fittings that are intended for use with stainless steel 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 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 for stainless steel tubing 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 stainless steel 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. In some cases, especially for larger tube sizes, a swaging tool is used to pre-install a ferrule onto the tubing. 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'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 patent 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.

Many applications and uses of the described two ferrule 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. Moreover, the present invention is directed to a new single ferrule tube fitting and variants thereof that can exhibit significantly improved performance over prior art single ferrule tube fittings, particularly for applications that do not require the high end performance characteristics of the above described two ferrule tube fitting.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a single ferrule tube fitting for stainless steel tubing is provided that achieves a significantly improved sealing function over prior single ferrule tube fittings while also achieving an excellent tube grip function. This sealing function is achieved without the use of additional elastomeric or other non-metal seals. In one embodiment, the single ferrule tube fitting utilizes a relatively steep camming angle for the camming surface of the fitting body. In accordance with this aspect of the invention, in one embodiment the camming angle is in the range of at least about 35 degrees to about 60 degrees, and preferably in the range of about forty to about fifty degrees and most preferred to be about forty-five degrees. In accordance with another aspect of the invention, a single ferrule tube fitting is provided that provides a primary seal against a relatively steeply angled camming surface. In the main embodiment this seal is formed by an outer tapered surface of the ferrule, that is not as steeply angled as the camming surface, being plastically deformed or coined with a generally narrow line contact against a camming surface angle within the above noted range. In accordance with another aspect of the invention, a single ferrule tube fitting with improved tube grip is contemplated by utilizing a case hardened ferrule with a geometry designed to enhance the ferrule tube grip. In one embodiment, the ferrule is case hardened to be at least about 3.3 times harder than the stainless steel tubing, more preferably at least 4 times harder than the stainless steel tubing. This allows the single ferrule to adequately bite into the hard stainless steel surface to grip the tube end even with a steeper camming angle and less axial travel. In accordance with another aspect of the invention, the single ferrule is plastically deformed during pull-up with a hinging effect. This hinging action produces a radially inward compression on a central or mid-portion of the ferrule body axially from a front end of the ferrule that bites into the tube end. This hinging effect places more ferrule material at the location of the tube gripping bite and produces a collet-type or swaging action of the central or mid-portion of the ferrule that helps isolate the tube gripping bite from vibration effects. In one embodiment, the ferrule geometry includes at least one interior circumferential recess to enhance the hinging action during pull-up. The hinging action also helps to keep the ferrule outer tapered surface to have a generally narrow line contact with the camming surface.

The present invention is also directed to a new tube fitting concept that utilizes the steeper camming angle and requires user assembly of only two components, namely a fitting body and a fitting nut. The nut and body are adapted to be threadably coupled together by relative rotation therebetween. The nut includes an integral tube gripping ring that cooperates with a camming surface on the body when the fitting is made-up. The ring may be machined with the nut or separately attached thereto by any convenient process such as brazing, welding or soldering, for example. The ring is radially compressed and plastically deformed against the tube outer wall to form a seal and a tight tube grip bite. The ring also forms a primary seal against the steeper camming surface. In one embodiment, the ring includes an outer tapered surface, that is not as steep as the camming surface angle, forming a generally narrow line contact-type seal against the steeper camming surface. The ring in one embodiment is designed to have a hinging action and to plastic deform during pull-up to embed the nose portion into the tubing wall for excellent tube grip, and an axially adjacent swage or collet zone that isolates the embedded nose portion from vibration effects. The fitting components, and particularly the ring, are preferably but not necessarily case hardened. The new fitting is especially useful as a stainless steel tube fitting, although the invention is not limited to any particular class of metals. In accordance with another aspect of the invention, the fitting may include a self-gauging feature to indicate sufficient pull-up and to prevent excessive tightening of the components.

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.

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 acts against a steep camming angle surface of one of the fitting components. 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. The present disclosure utilizes these aspects of the invention in two distinct general embodiments. The first general embodiment that will be described is a tube fitting arrangement in which a tube gripping device is provided that is integral with one of the two threaded components, namely the female threaded nut component. The second general embodiment is that of a tube fitting for stainless steel tubing that uses a separate single ferrule as the tube gripping device along with the male and female threaded components. Both general embodiments may share a number of common aspects of the invention many of which are optional alternatives that can be used in a variety of combinations and sub-combinations, such as the camming surface profile, the camming surface angle, geometry options of the tube gripping device, and hardness characteristics of the tube gripping device relative to 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 particular316stainless 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.

With reference toFIG. 1, the present invention contemplates a tube fitting50in which there are only two discrete components, namely a female threaded nut52and a male threaded body54. The nut52is substantially different from the typical nut used in a prior art ferrule type tube fitting for metal tubing. The body54may be the similar in design as a typical body used in prior fittings, however, as will be explained further herein, it is preferred but not necessary that the body54also be optimized for proper make-up with the new nut52. Additionally, the body54need 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 body54is a generally cylindrical main body56that has an integral extension or end56a.The end extension56amay be a hex body, for example, or part of another component such as for example a valve body as noted hereinabove. The main body56may be machined from the same stock as the end extension56aor may be otherwise attached such as by welding or other suitable technique. The body56includes a first central longitudinal bore58that is appropriately sized to closely and slideably receive a tube end13. The first bore58is somewhat larger in diameter than a coaxial second bore59that extends through the end extension56aof the body54. Of course, if the fitting50is a closed end connection, the inner bore59would not be a through bore.

The tube end13preferably bottoms against a counterbore60. The body56is machined or otherwise formed with external male threads62that threadably mate with corresponding female threads64formed or machined in the nut52. 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 bore58is preferably although not necessarily formed with a slight radially inward taper α relative to the longitudinal axis X (FIG. 2) of the tube end13such that the diameter of the bore58decreases radially in the axial direction towards the counterbore60. For example, this taper may be about 2° to about 4°, although the selected angle is not particularly critical. The bore58diameter at the counterbore shoulder is just slightly less than the outer diameter of the tube end13. In this manner, the tube end13has a slight radial interference fit of a few thousandths of an inch for example with the bore58. This interference between the bore58and the tube end13provides an anti-rotation action to help prevent the tube end13from 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 end13does not necessarily have to bottom completely against the counterbore shoulder60. This is because the interference fit helps provide a good primary seal between the bore58and the tube end13. 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 bore58may extend gradually along its entire axial length or a shorter axial portion adjacent the counterbore60.

In the embodiment ofFIGS. 1 and 2, the nut52and body54are axially dimensioned so that upon proper pull-up, for example a one-half-turn of the nut52relative to the body54, a forward end66of the nut axially contacts an end shoulder68or other axial stop surface of the body54. This feature thus incorporates a self-gauging aspect to the fitting50to thereby indicate sufficient pull-up and to prevent over-tightening of the nut52. However, for those situations where it may be desirable to permit additional axial displacement of the nut52relative to the body54, such as for example, remakes of the fitting50, the nut52and the body54may be axially dimensioned to retain an axial gap therebetween (for example, between the respective ends66and68) upon completing an initial proper pull-up. In the latter case, a gauging tool may be used to verify proper initial pull-up, such as for example, a gap measuring device to confirm that the axial gap between the surface66and68does not exceed a predetermined value or dimension. The stop feature may also be used to implement a limited number of remakes for the fitting50.

With reference also toFIG. 3, the nut52includes a first central bore70having a first diameter D1relative to the longitudinal axis of the fitting50. The nut52also includes a second bore72having a second diameter D2relative to the central longitudinal axis of the fitting50. In this embodiment, the diameter D2is less than the diameter D1. Furthermore, the diameter D2is sized so that the bore72defines a generally cylindrical wall that receives the tube end13(FIG. 1). The first bore70terminates at a location that is axially spaced from the nut back end74to form a trepan75, such that the nut52includes a radially inwardly extending collar76. The collar76is defined by the back end wall74of the nut52, the smaller diameter bore72and the larger diameter bore70.

In accordance with a significant aspect of the invention, the nut52includes a tube gripping device80that extends axially inwardly in a somewhat cantilevered fashion from the collar76. The tube gripping device in this example is in the general form of a gripping ring80and includes an inner bore82that defines a substantially cylindrical wall that closely receives the tube end13(FIG. 1). The diameter D3of the ring bore82may be the same as or different from the diameter of the second nut bore72. The cylindrical wall that defines the gripping ring bore82extends axially from a tapered front or nose portion84of the gripping ring80. The nose portion84includes an axially tapered outer surface86that increases in the radial dimension towards the back end of the ring80. The tapered outer surface86extends from a generally radial front end85of the gripping device80. This generally radial front end85may have a small angle or taper and joins to the inner cylindrical bore82at a preferably sharp corner87. Alternatively, however, there may be provided a circumferential recess or step or notch or other geometry (not shown) in the front end of the ring80having a diameter that is somewhat larger than the diameter D3and axially extending from the front end85towards the back end74of the nut52.

The tapered surface86joins the front end85preferably by a radius portion89and at its axial opposite end by a radius86ato a generally cylindrical portion91, which in turn joins via a radius93to a tapered outer wall portion95. The tapered outer wall portion95joins along a radius to the trepan75.

It is noted at this point that the various geometry characteristics of the tube gripping device80(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 device80will 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. By way of example, but without intending to be limiting,FIGS. 9-12illustrate other geometry variations for 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.

With reference toFIGS. 1,2and4, the tapered nose portion84initially engages an axially tapered camming surface88that forms an opening to the tube bore58in the main body56. The tapered camming surface88is a surface that joins the bore58wall to the back end wall90of the body54. In the embodiment ofFIGS. 1 and 2, this camming surface88is characterized by a generally convex contour. However, the shape of the surface88may be selected for other shapes depending on the particular ring deformation and tube gripping characteristics required for the fitting50in a specific application. Thus, as illustrated inFIG. 4, the camming surface88′ may be of a frusto-conical shape for example. Note also thatFIG. 4illustrates the concept that the axially tapered tube bore58may be tapered only at an axially shorter portion adjacent the counterbore60.

The tube gripping ring80is shaped to effect several important functions of the fitting50. The gripping ring80must, upon proper pull-up, provide a fluid-tight primary seal against the tapered camming surface88. This seal may be a primary outer seal for the tube fitting50, or may be in effect a secondary or back-up seal to any seal formed between the tube end13and the body54, for example along the bore wall58and/or the counterbore60. The gripping ring80also will form a primary seal at the location where the ring80bites into the outer surface of the tube end13in the area where the cylindrical bore82of the ring80engages 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 end13against the body54. In any event, the gripping ring80must form primary seals against the camming surface88and the outer surface of the tube end13. In addition, the ring80must adequately grip the tube end13so 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 ring80is designed to be plastically deformed and swaged into the tube end upon pull-up, as illustrated inFIG. 2. This result is achieved by designing the ring80to have a hinging action whereby the tapered nose portion84is not only driven axially forward as the nut52is threaded onto the body54, but also is radially displaced or driven into engagement with the outer surface of the tube end13wall. The forward end92of the nose portion84is thus compressed and embedded into the tubing wall with a resultant stress riser or bite in the region designated94inFIG. 2. The front end bite94produces a generally radially extending wall or shoulder99formed out of the plastically deformed tube end material. The shoulder99engages the embedded front end of the gripping ring80to thus form an exceptionally strong mechanical resistance to tube blow out at higher pressures. The embedded nose portion92thus provides both an excellent seal and a strong grip on the tube end13. The ring80is further designed to exhibit the aforementioned radially inward hinging action so as to swage or collet the cylindrical wall82against the tube end at a location axially adjacent or rearward of the stress riser bite94and generally designated with the numeral96. This swaging and collet effect substantially enhances the tube gripping function and serves to isolate the embedded nose portion and bite94from 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, one of the basic concepts of the present invention is the provision of a flareless tube fitting that does not use one or more ferrules, but rather uses a tube gripping ring that is integral with one of the threaded fitting components. 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 ring80is designed to exhibit the hinging action that allows the tapered nose portion84and the central or mid-portion (as at the region of the cylindrical bore82or the region designated94) of the gripping ring80to be radially inwardly compressed as it engages with the tapered camming mouth88of the body56. This hinging action is also used to provide a significant radial displacement and compression of the cylindrical wall82to swage the ring80onto the tube end13axially adjacent to the stress riser94. In the embodiment ofFIGS. 14, the hinging action is facilitated by providing a preferred although not uniformly required radial inner circumferential notch98that is axially positioned between the cylindrical portions72and82. The notch98is suitably shaped to permit the ring80to plastically deform and collapse in a controlled manner so as to radially compress the cylindrical wall82against the tube end with the desired collet effect. This result may be enhanced by including an outer notch100in the outer wall portion of the gripping ring80. The particular geometry of the gripping ring80will thus be designed so that as the nut52is threaded onto the body54, the gripping ring hinges and plastically deforms to grip the tube end and to seal both against the tube end and the tapered camming mouth88. Standard design procedures such as Finite Element Analysis may be used to optimize the geometry of the ring80based 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 gripping ring80may further be controlled by selecting an appropriate contour for the tapered surface88. This surface engages the tapered nose of the ring80and therefore will in part determine the timing and manner of how the ring80hinges, 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 surface88may be designed to achieve the desired seal between the ring80nose portion and the tapered surface88. This seal is important to the overall performance of the fitting, as is the seal provided between the gripping ring80and the tube end13.

The nut52with its integral gripping ring80may be manufactured by standard machining operations, and will typically include a trepan operation to form the outer contour of the ring80, such as the second notch100for example. The other features of the nut52can be realized with well known machining operations. Preferably but not necessarily the nut52includes wrench flats102to permit the user to tighten the nut52onto the body54. Those skilled in the art will readily appreciate that use of the fitting50only requires relative rotation between the nut52and the body54, 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 surface88to form a camming angle θ of about 35°-60° relative to the longitudinal axis X of the fitting50and tube end13. More preferably the angle θ of the camming surface88should be 40°-50°, and most preferred the angle θ should be about 45°. This range of angles for the camming surface88differs dramatically from known metal 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 θ, which permits the gripping ring nose portion84in effect to be coined into the camming surface88without a substantial sliding action, thereby forming an excellent seal.

In the exemplary embodiments herein, the nose portion84includes the radius portion89that transitions to the outer tapered surface86. This outer surface86tapers generally at an angle that is not as steep as the angle of the camming surface88. The tapered outer surface86preferably tapers axially with an increasing radial dimension towards the back end of the gripping ring80. This tapered outer portion86contacts the camming surface88with, 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 ring80to coin into the camming surface88. Therefore, the term “generally narrow line contact” is not intended to preclude an area of contact between the outer tapered surface86and the camming surface88, but rather more generally to the concept of a localized contact zone near or at the innermost extent of the camming surface88of high stress and material coining between the outer tapered surface86and the camming surface88. By “coin” is simply meant that the gripping ring80achieves a good metal to metal seal between the radius portion89and the camming surface88by forming a generally narrow circumferential line contact of metal burnished on metal to effect a gas tight primary seal between the tapered surface86and the camming surface88.

It is important to note that the use of a particular camming angle is not dependent necessarily on the contour of the surface88. In other words, the angle of interest is the angle at which the front end of the gripping ring80contacts the camming surface88to form a seal thereat. Thus, the camming surface88may indeed be made with a non-frusto-conical contour, such as the convex shape illustrated inFIGS. 1 and 2, but the seal is still formed by the front end of the gripping ring80contacting a steep angled surface88. The additional compound angles or contours of the camming surface88may be used to better facilitate the hinging action and tube bite achieved by the gripping ring80.

Whether the camming surface88is 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 ring80into the tube end13, in accordance with this aspect of the invention, the sealing portion of the front end of the gripping ring80(in the exemplary embodiments the radius portion89) forms the primary seal on a steep angled portion of the camming surface88, preferably a steep angled portion in the range of angle θ of about 35°-60° relative to the longitudinal axis X of the fitting50and tube end13, more preferably the angle θ of the camming surface88should be 40°-50°, and most preferred the angle θ 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 line contact type engagement between the front end of the gripping ring80and the camming surface88.

The steeper camming surface angle has the additional benefit that the nose or front portion of the tube gripping device80may 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 bite94. 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 end84) 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 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 device80were 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 θ of the present invention also results in a much shorter distance of axial displacement of the ring80during pull-up. Consequently, the nose portion84will need to be radially deformed and compressed into the tube end13with a much shorter axial displacement or sliding movement. In order to achieve the proper tube grip then, the gripping ring80is 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 fitting50is 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 ring80need not be case hardened, but rather only the nose portion84may be selectively case hardened.

In accordance with this aspect of the invention, all or part of the nut52and body54may be through hardened or case hardened to increase the tube grip of the fitting50when 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 ring80allows the ring80to 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 ring80that reduce or prevent galling between the ring80(which rotates with the nut52) and the tubing.

Various lubricants may also be used with the tube gripping ring80to 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 nut52and integral tube gripping ring80to 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 nuts52to 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'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 nuts52which 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'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.

With reference toFIGS. 5 and 6, in another embodiment of the invention, the fitting50′ includes a nut52that may be the same as the nut previously described hereinabove. The body54′ has been modified as follows. In this case, the body rear end wall110has been axially extended. A transition contour112may be used to join the end wall110to the axially tapered camming mouth88. All other aspects of the fitting50′ may be the same as the fitting50as described with reference toFIGS. 1-4. The extended axial length of the end wall110causes the nut inner shoulder or trepan114to axially contact the body end wall110before the forward wall66of the nut engages the body end wall68. This is illustrated in the pulled-up position ofFIG. 6. In other words, the self-gauging feature has been located at the rearward portions of the nut and body and only requires close tolerance control in the trepan area and axial length of the body rear end wall110, whereas in the embodiment ofFIGS. 1 and 2, more dimensions and tolerances are involved in insuring accurate self-gauging due to a greater number of tolerance stack-ups.

In accordance with the invention, a single ferrule tube fitting is also contemplated. The single ferrule tube fitting will be designed with many of the same concepts and advantages achieved with the integral nut and tube gripping device design described herein (wherein now the single ferrule is the tube gripping device), particularly the features of hinging, tube bite, collet or swaging effect, and high hardness ratio compared to the tube end. Therefore, those benefits will not be repeated in detail, it being recognized that the integral nut and tube gripping ring functions much like the single ferrule embodiment herein but with the ferrule being integral with the female threaded nut.

FIGS. 7,7A and8illustrate a single ferrule tube fitting181embodiment of the invention. In this example, the ferrule180is a separate part, thus providing a three piece tube fitting181including a nut182, body184and the single ferrule180. This fitting is particularly although not exclusively well-suited for use with stainless steel tubing.

The body184includes an angled camming surface186that is preferably in the range for angle θ described hereinabove and most preferably about 45°. The single ferrule180is also preferably case hardened to about a Vickers hardness of 800 or greater, or at least about 3.3 times harder than the tube end13. The front portion188of the ferrule includes a radius transition190that contacts the steep angled camming surface186during pull-up of the fitting to form a line contact-type primary seal against the steep angled camming surface186. The front portion188of the ferrule includes a generally radial front end surface188awith a forward sharp edge192of the ferrule that bites into the outer surface of the tube end13. The ferrule is plastically deformed and produces a generally radially extending shoulder194at the location of the tube bite. The ferrule180exhibits a hinging effect as described hereinbefore with respect to the tube gripping element80so that a central part or mid-portion196of the ferrule body undergoes a radially inward compression to collet or swage at or near the tube bite194. The particular geometry of the ferrule may be selected as required to facilitate the hinging action and tube gripping bite and collet action as described hereinbefore and also as described in the above incorporated patents on the two ferrule fitting. The ferrule180may be provided with an inner circumferential notch or recess198to facilitate the hinging effect, as well as a tapered outer wall200as described hereinabove. The ferrule180includes a back tapered wall202that is driven by the nut shoulder204during pull-up. The ferrule180also includes a central longitudinal and substantially cylindrical bore portion206that is axially between the front end188aand the recess198. Preferably, the tapered nose portion208is angled so as to remain out of contact with the camming surface during pull-up to facilitate the formation of a line contact primary seal.

The cylindrical bore portion206athat is axially between the back end202of the ferrule and the recess198may be the same diameter or a different diameter as the forward cylindrical portion206. Furthermore, the hinging effect may be realized such that the back end portion206aof the inner cylindrical bore remains radially spaced from and out of contact with the tube end13after pull-up.

The single ferrule180is through or case hardened to be at least about 3.3 times harder, and more preferably at least about 4 times harder, than the tube end13. The aforementioned patents on case hardening may be referred to for suitable processes although other processes may be used as required.

The single ferrule fitting181thus provides excellent tube grip and seal functions compared to prior art single ferrule fitting designs by utilizing the steeper camming surface angle for the metal to metal seal, substantially harder ferrule compared to the tubing, and strong tube grip biting action. The substantially harder ferrule, as compared to the tubing hardness, allows excellent tube grip even with the steeper camming angle, while the steeper camming angle facilitates the metal to metal seal.