Threaded joint

There is disclosed a threaded joint for coupling together rods, tubes, pipes etc. The joint consists of helical box and pin thread segments each defined by a pressure flank and a clearance flank spaced apart from each other with alternating roots and crests. The pressure flank is defined by an S-curve extending between the root and the crest, which is defined by a first curvature c1 adjacent to the root and a second curvature c2 adjacent to the crest. c1 and c2 curve in opposing directions with an inflection point “i” between curvatures c1 and c2.

FIELD

The invention relates to threaded joints for coupling together of rods, tubes, pipes and shafts, with particular application for subsurface drilling pipes and tubes.

BACKGROUND

Subsurface drilling, particularly for mineral exploration, involves the use of a rotating drill string assembled from multiple individual tubular rods that are coupled together as the drilling progresses. Typically, the rods are threaded together using threading that is provided at opposing ends of each rod. Significant axial and rotational stresses are placed on the threading during the drill rotation and drill string retraction. Furthermore, the rods should be coupled together and decoupled without the need to apply excessive torque or force to the drill rod segments.

Within the mining industry, there is an ongoing need to provide improved threaded joints that address some of the drawbacks that are present within prior art threaded joints.

SUMMARY

We describe a threaded joint for coupling together first and second members, in which each of said members comprises a central axis between respective ends thereof. The joint consists of threaded segments located on at least one end of the members, in which members may be coupled together by threading the respective segments together. In one aspect, the invention relates to a modified “buttress” thread that is self-locking.

In one aspect, the joint comprises a pin thread segment at one end of a first of said members and a tubular box thread segment at one end of a second of said members, in which the box thread segment and pin thread segment each comprise a helical thread defined by a pressure flank and a clearance flank. The box thread segment and pin thread segment each further comprises a root and a crest extending between said pressure flank and said clearance flank, wherein the pressure flank comprises an S-curve (when viewed in cross-section along an axial section) extending between the root and the crest. The S-curve is defined by a first curvature c1extending from the root and a second curvature c2extending from the crest with an inflection point “i” between curvatures c1and c2. Curvatures c1and c2are opposed, whereby c1is concave and c2is convex. The S-shaped curvature of the pressure flank may extend from adjacent to the root to adjacent to the crest.

In one aspect, curvature c1equals c2in opposed directions. One or both c1or c2may comprise a segment of a circle having a radius R1.

Alternatively, one of both c1and c2may comprise a compound curvature comprising a segment of a first circle having a radius r1R3and a segment of a second circle having a radius R2wherein R3does not equal R2. In one aspect, R2may be greater than R3.

In another aspect, the crest and root surfaces of said box and pin thread segments may each define a frustoconical surface, in which the angle of taper may be within the range of 0.75° and 1.63° relative to said central axis.

In a further aspect, the pin thread segment and box thread segment each comprises a first and second unthreaded segments at opposing ends of the helical thread, the first unthreaded segment is at a distal end of the member and having an end face defining the distal end surface of the first member. The second unthreaded segment has a radially inwardly stepped shoulder. The shoulders may each be angled relative to the perpendicular of the central axis within the range of 5 to 15 degrees. Alternatively, the shoulders may each comprise an inner region adjacent to the unthreaded segment comprising a negative slope of 12° to 15° relative to the perpendicular of the central axis and an outer region which is perpendicular to the central axis or has a negative slope of up to 4° relative to the perpendicular. The end face may comprise a mirror-image of the compound slope of the shoulder.

According to a further aspect, the clearance flanks may have a positive slope relative to the central axis by about 45°, about 60° or between 45° and 60°.

According to a further aspect, the inflection point i has a tangent with a slope relative to the central axis that is about 45°, about 60° or between 45° and 60°.

The helical thread may comprise either an unpaired helix comprising single-start thread or a paired helix comprising a double-start thread.

In a further aspect, the ratio of R2:R3above is about 3:1 or greater.

In a further aspect, R3is within the range of 0.007 inches to 0.015 inches.

In a further aspect, R3is 0.005 inches to 0.012 inches and R2is 0.024 inches to 0.060 inches.

Dimensions herein are normally provided in imperial measurements, unless otherwise specified. Directional references herein are normally with reference to the threaded tubular members being horizontal. The terms “inner”, “inwardly” and similar terms refer to the direction that is radially inwardly towards the central axis of a given threaded member. The terms “outer”, “outwardly” and the like refer to the opposed direction which is radially outwardly from the central axis.

References herein to angular deviations are generally expressed in terms of an angle from the central axis of the elongate member or, if specified, a plane which is perpendicular to the axis. The assumption is made herein that the central axis of the threaded coupling is linear. However, the present invention is equally applicable to curved members in which the central axis is non-linear. In such case, angular deviations may be considered to be based on a short segment of the central axis which closely approximates a straight line.

Definitions

For purposes of the present specification, the following definitions shall apply unless a different meaning is expressly stated or the context clearly requires a different definition.

“Rod”: means an elongate member that is threaded at one or both ends for coupling with a similar rod. A rod may be cylindrical or tapered and may have a solid or hollow core. A rod may be fabricated from any suitable material. The term “rod” may in some cases be used interchangeably with one or more of the terms “shaft”, “tube”, or “casing”.

“Thread” or “threading”: means a projecting rib or recessed groove, usually helical in configuration, which may be coupled together by threading to a similar “mating” thread.

“Box thread”: refers to the female threaded segment.

“Pin thread”: refers to the male threaded segment.

“Pressure flank”: refers to an essentially vertical or somewhat sloping surface on a thread forming between the root and crest surfaces. Normally, a pressure flank is brought into contact with a corresponding pressure flank when opposing threads are engaged with each other. The corresponding pressure flanks bear upon each other when axially load is induced on and the rod during makeup of a connection or a tensile load is applied during retraction of a drill string.

“Axial cross section”: refers to a cross section on a plane that bisects a rod through a central axis that extends between opposed ends of the rod.

“Transverse cross section”: refers to a cross section on a plane that is transverse to the central axis of the rod.

“Clearance flank”: is the flank extending between the root and crest of the threading, opposed to the pressure flank. Normally, a given clearance flank of a threading will remain out of contact with an opposed clearance flank of the mating threading when threaded together.

“Root”: refers to a cylindrical or frustoconical surface which extends between adjacent portions of a thread. The pin root is radially inward to the crest and the box root is radially outward to the crest and is normally parallel thereto. Normally, the root is co-axial with the central axis of the threading.

“Crest”: is a frustoconical or cylindrical surface which is normally parallel to the root. A crest is the surface between the pressure flank and clearance flank of a thread. The axis of the crest is normally co-axial with the central axis of the threading.

“Negative slope”: means a slope that provides an overhang between upper and lower portions of the negatively sloping surface whereby the uppermost portion of the sloping surface overhangs the lowermost portion. A negative slope normally defines a concave space beneath the overhang. For example, inFIG. 2it will be seen that shoulder30defines a concave space when viewed in cross-section. In some cases, a surface may have multiple angles whereby a negative slope may have a positively sloping portion, even when the slope as a whole is negative.

“Interference fit”: means a configuration whereby the pin thread has a slightly larger outside diameter than the inside diameter of the contact surfaces of the box thread. The portions of the pin thread that contact the box thread when fully tightened force a slight expansion of the box thread segment, to secure the respective threaded components together.

“Proximal”: refers to a direction toward a point intermediate between opposing ends of the elongate tubular member10as described herein. “Distal” refers to an opposing direction towards one of the respective ends thereof.

“Buttress thread”: refers to a thread having a trapezoid or saw-tooth profile which is designed to handle high axial force in one direction. A buttress thread has a load-bearing face which is perpendicular to the central axis or a slight slope, such as 7° or less. The opposing face has a relatively shallow slope such as about 45°.

DETAILED DESCRIPTION

The following is a detailed description of certain embodiments of the invention. The present description is not intended to limit the scope of the invention in any respect, including limiting the scope thereof to any of the specific aspects, features, details, dimensions or configurations provided in this detailed description.

FIGS. 1-11illustrate an embodiment of a modified buttress thread having a self-locking pressure flank. In this embodiment, the thread configuration relates to the threaded joint of a drill rod string. However, the present thread configuration has a range of applications, including use with a range of materials, fabrication methods and industrial applications. For example, without limitation, the thread configuration may be applicable for use with well casings, drilling tools and other components used for mineral and hydrocarbon exploration and environmental drilling.

FIG. 1depicts three tubular members10,12and13which are essentially identical in structure and which may be coupled together in end to end fashion with threading provided at the respective ends thereof. The respective threading comprises a pin thread1(seen in more detail inFIG. 7) and a box thread2(seen in more detail inFIG. 6). An exemplary pin thread1is provided on a first end segment6of a first tubular member10. A mating box thread2is provided on a second end segment8, of a second tubular member12.FIG. 1also shows an opposing end of tubular member12comprising a pin thread1. Thus, each tubular member10and12is provided with first and second threaded end segments6and8on opposing ends thereof. Tubular members10and12each further comprise a body14and16respectively, located between segments6and8. A central longitudinal axis18extends axially between end segments6and8of each of members10and12. When coupled together, the respective tubular members10and12are normally axially aligned, with a (normally) linear axis18extending between the opposing ends of the respective tubular members. It will be seen that a non-linear configuration may be provided, for example to accommodate a curved or arcuate drill string.

The outside diameter of tubular bodies14and16may be between 1.188″ to 6.5″ with a wall thickness of 0.188″ to 0.25″. The invention is not limited to these dimensions, nor to any particular dimensions identified in this specification.

As shown inFIG. 2, pin thread1is composed of a root surface20and a crest surface22. Root and crest surfaces20and22lie on respective co-axial frustoconical (conical section) surfaces which are essentially planar when seen in cross section. The respective surfaces20and22are parallel to each other and taper inwardly by an angle of between 0.75 and 1.63 degrees relative to central axis18towards the distal end of threaded segment6. Box thread2has a similar crest surface26and root surface24each being frustoconical and having a similar angle of taper of between 0.75 to 1.63 degrees inwardly towards the proximal end of segment8. The respective threads1and2are configured to provide an interference fit between the pin crest and box root surfaces and a minimal clearance between the pin root and box crest when threaded together.

As seen more clearly inFIG. 8, body14of tubular member10is stepped radially inwardly at a shoulder30, which defines the proximal margin of pin thread segment6. Shoulder30has a negative slope of about 5-15° relative to axis18whereby body14overhangs the proximal margin of segment6. A fillet31defines the inner corner between shoulder30and segment6. Fillet31merges the surface of segment6with shoulder30. An outer corner29is opposed to fillet31and defines the border between shoulder30and the outer surface of body14. Segment6is defined at its opposed, distal end by an end face32. End surface32has a positive slope similar to shoulder30(5-15°) and an outer fillet33with a radius of curvature similar to fillet segment31. Fillet33is located at the radially outer corner of end face32

Fillet segments33and50may have a minimum radius of curvature of about 0.0156″ and fillet segments31and48may have a maximum radius of curvature of about 0.0156″. These respective dimensions provide minimal or no overlap in the respective radii of curvature so as to minimize or eliminate any interference between these segments when the members10and12are coupled together. Respective segments33and50are thus brought into abutting or adjoining relationship when the thread joint is fully made up without generating an interference fit between these respective regions. In more general terms, the radius of curvature of the concave shoulder fillet segments is greater than or equal to the radius of curvature of the corresponding abutting or adjoining convex end segment fillets.

As seen inFIGS. 2, 3 and 4, when the respective box and pin threads are coupled and the tubular members10and12are initially threaded together to a non-fully tightened position, a stand-off or gap remains between the respective end surface42of the box thread and shoulder30of the pin thread, and likewise between end surface32and box thread shoulder40. At this initial pre-torqued stage, this gap is approximately 0.04-0.09 inches (see S1inFIG. 2). When torqued to proper requirements, the end surface42of the box thread contacts shoulder30of the pin thread, while a gap or stand off exists of 0.002 to 0.004 inches between end face32of the pin thread and shoulder40of the box thread.

The pin and box threads1and2are configured to provide an interference fit, whereby the pin crest22has an outside diameter of about 0.002 inches larger than the inside diameter of box thread root24. When fully made up, the crest22of the pin and the root24of the box thread has an interference fit of approximately 0.002″ on the diameter while the pin root20and the box crest26has minor clearance to allow room for thread compound and debris. By increasing the radius in the corner of the root24and pressure flank54, it decreases the stress concentration of the part.

The rear flank of the pin and box has the same geometry but the box thread depth is shallower than the pin thread depth by 0.002 to 0.005″ to provide interference between the pin crest and the box root and clearance between the pin root20and box crest26. The box crest26extends off of a secant line and intersects to the rear flank radius.

As discussed above, the diameter of pin crest22is approximately 0.002″ larger than the diameter of box root24so when the thread joint is made up “hand tight”, the pin major diameter will contact the box major diameter and there will be approximately 0.04 to 0.09″ standoff between the pin and box. When the joint is pre-torqued to the proper requirements, it will have an interference fit of approximately 0.002″ on the diameter until the box face42and pin shoulder30fully contacts. At this point there will be a gap of 0.002 to 0.004″ between the pin face32and box shoulder40.

The pin and box segments6and8have a theoretical length relative to the central axis of 1.6 to 2.6″ with the box segment8being longer than the pin segment6by up to 0.004″ to ensure proper make up.

As seen inFIGS. 6 and 7, pin and box threads have a pitch of 2 to 4 threads per inch, when provided on a single start thread. This value is doubled on an embodiment comprising a two-start thread, which comprises a paired helical threading and which is described below. The thread length relative to the central axis may be 1.6 to 2.6 inches, with the axial distance from the end face42to shoulder40of the box segment being longer than the corresponding distance of the pin segment by about 0.004 inches to ensure proper makeup. The pin crest diameter is approximately 0.002 greater than the box root diameter, to provide an interference fit.

Referring toFIGS. 6 and 7, box thread pressure flank54and corresponding pin thread pressure flank52are shown, as well as pin thread clearance flank56and box thread clearance flank58. Pin thread pressure flank52is shown in detail inFIGS. 14 through 16. Box thread pressure flanks54have identical mating configurations.FIG. 14depicts a first embodiment of pressure flank52comprising an S-shaped cross-sectional configuration, composed of an inner concave section60and an outer convex section62. Section60and62meet at inflection point64. Surface52is thus continuously curved between root20and crest22. Concave section60has a radius of curvature of R1, and convex segment62has an identical radius of curvature R1. At inflection point64, surface52has a tangent angle66. The slope of tangent angle66relative to axis18, as well as the length of radius R1, determines the thread depth. Tangent66may comprise a negative angle of between 45 to 60 degrees relative to axis18. The absence of flat surfaces on the respective pressure flanks allows the mating thread to lock while avoiding the compression characteristic of a conventional flat-surface reverse angle pressure flank.

The corresponding clearance flanks56and58are only in contact with each other until the pin shoulder30meets the box face42. At this point, contact between the pin and box threads will shift to the respective rear pressure flanks52and54. The threaded joint is then fully engaged. The initial relatively shallow attack angles of the clearance flanks56and58make it easier to start the thread by lining it up and reduce cross-threading.

The respective pressure and clearance flanks of the pin and box threads each meet the adjoining root and crest surfaces at a curved radius or fillet rather than a sharply-defined angle, as described herein. Turning first to the box thread2as seen inFIG. 6, a first convex radius or fillet63is provided at the intersection where clearance flank58meets box crest26. A second concave fillet65is provided where box clearance flank58meets box root24. A third convex fillet69is provided where box pressure flank54meets box crest26and a fourth concave fillet67is provide where box pressure flank54meets box root24.

Turning next to pin thread1as seen inFIG. 7, pin thread1has a first convex fillet80where pin thread pressure flank52meets pin crest22. A second concave fillet82defines the junction between pressure flank52and pin root20. A third convex fillet84defines the junction between pin clearance flank56and pin crest22and a fourth concave fillet86defines the junction between pin clearance flank56and pin root20.

It will be seen that when the pin and box threads are engaged, the respective convex fillets nest within the concave fillets. The radii of the respective convex fillets63, and84are larger than the radii of the respective concave fillets65, and86to ensure proper clearance during makeup of the joint.

The S-shaped curvature of pressure flank52provides a radius of curvature between pressure flank52and root20. Increasing this radius causes a decrease of the stress concentration of the threading, as shown inFIG. 24.

In one embodiment, pin and box threads1and2form a single start thread comprising an unpaired helix. In this embodiment, seen inFIG. 12, the thread has a pitch of 2.0 to 4 threads per inch. In a second embodiment, shown inFIG. 13, the thread has a double-start configuration that consists of a paired helical threading. In this embodiment, the thread pitch may be doubled from the above. One advantage of a double start thread is to provide increased contact area on the pressure flank which also reduces the distance between the last active thread and the shoulder which makes for a stronger and stiffer thread lead-in without compromising the amount of turns required to fully make up the joint. This can also allow for a shorter thread lead-in adding additional strength to the joint.

The double start embodiment shown inFIG. 12can have a “lead-in distance” x which is shorter than the corresponding “lead-in distance” y of the single start embodiment ofFIG. 13. In the present embodiment, distance x is about 23% shorter than distance y. The “lead in distances” x and y comprise the axial spacing between the outer corner29that defines the edge of face30and fillet52that defines the margin between pressure flank54and crest22of pin thread1. For purposes of measuring the lead-in distance, this measurement is derived from the proximal margin of pin thread1, where pin thread1is closest to end face30. The corresponding single start and double start embodiments of box threads2(not shown) have similar configurations to the pin threads ofFIGS. 12 and 13.

FIG. 15shows a second embodiment wherein pressure flank70comprises a similar S-shaped curve extending between root20and crest22. However, pressure flank70differs from pressure flank52in that concave segment72is composed of a dual radius curved surface. Surface72is a compound curvature, a portion of which is comprised of a primary (major) radius of curvature R3and the remainder of which is comprises a secondary (partial) radius of curvature R2, wherein R2is greater than R3. One segment of concave portion72thus has a radius of curvature of R2and is adjacent to root20, while a second portion of segment72which is adjacent to inflection point64has a radius of curvature of R3. In a similar fashion, convex segment74has a compound curvature, composed of a first segment of radius R3and an adjoining second segment of radius R2. Convex and concave segment72and74meet at an inflection point64, having a tangent66.

FIG. 16provides additional details of the single radius curvature of the embodiment ofFIG. 14. Pressure flank52can merge with crest22in a continuously curved arc. In one option, shown in a stippled75line inFIG. 16, pressure flank52can meet crest22at a flat surface75having a slope of 15° to 30° relative to crest22. Flat surface75may be introduced in a machining process after the initial threading is cut. Flat surface75is provided to remove any irregularities that may have been introduced in the initial thread cutting step due to possible difficulties in cutting a smooth, continuously curved transition between flank52and crest22. In this version, flat surface75has little or no significant effect on the performance of the thread.

Radius R1combined with the tangent angle66effectively determines the thread depth between the respective root and crest surfaces and the quantum of interference95between respective pin and box pressure flanks (see Table 1).

FIGS. 17 to 20show a further embodiment of pin thread100. In this embodiment, shoulder102(FIG. 17) has a compound negatively sloped surface consisting of an outer portion104and an inner portion106. Outer portion104is perpendicular to axis18or has a negative slope of up to 4° relative to a perpendicular to axis18. Inner portion106has a negative slope of between 12-15°. Portions104and106meet at an inflexion region108which is approximately the mid-point of shoulder102. In one aspect, inflection region108has a minimum radius of curvature of 0.156″. Inner portion106meets segment112at a curved fillet110. End surface120, seen inFIG. 18comprises a similar compound surface, consisting of an outer portion121with the same or similar taper as surface106and an inner surface122with the same or similar taper as surface104. End surface120further comprises a rounded corner124having a similar radius as fillet110. The pin shoulder102has a similar inflection region having a minimum radius of curvature of 0.156″.

FIGS. 21 and 22provide a further embodiment.FIG. 21shows a proximal portion of a box thread200adjacent to body16. Box thread200comprises an outer surface202which is continuous with the outer surface of body16. Thread200comprises a shoulder204, which comprises a tapered (frustoconical) annular surface that is undercut relative to the adjacent inner surface206of body16. Shoulder204is perpendicular to the central thread axis or tapers outwardly (towards outer surface202) and proximally and is within an angle of between 0° and 10°, preferably in the range of 5 to 10°, relative to a transverse (radial) plane of body16.

Shoulder204merges with a tapered (frustoconical) ramp segment208which extends in a distal direction from shoulder204. Ramp segment208slopes upwardly in a distal direction to merge with crest210. As such, ramp segment208tapers outwardly in a proximal direction from crest210towards shoulder204to provide a radially enlarged segment of box thread200. Ramp segment208is perpendicular to the central thread axis or has a slope of between 0° and 10°, preferably in the range of 5 to 10°, relative to the central elongate axis of body16. The length of ramp segment208(proximal to distal ends thereof) is in the range of about 0.1500.200 inches. Ramp segment208merges with shoulder204at a curved radius212.

FIG. 22shows a distal segment of a pin thread220that provides a complementary profile to box thread200. Pin thread220is provided with a sloping (frustoconical) proximal shoulder222adjacent to outer surface224of body16. Shoulder222has configuration that matches shoulder204of box thread200, whereby shoulder222undercuts outer surface224and is provided with a taper of about 0-10° relative to a plane that transversely bisects body16. Shoulder222merges with a ramp segment226which comprises a frustoconical surface having a taper of about 0-10° relative to the central elongate axis of body16. Segment16tapers radially inwardly from adjacent to the innermost (proximal) pin crest surface230, to reach a maximum diameter adjacent to shoulder222. When seen in axial cross section as inFIG. 22, ramp segment226slopes downwardly towards proximal shoulder222.

The length of ramp segment226matches box ramp segment208, namely in the range of about 0.150-0.200 inches from the proximal to distal ends thereof. Ramp segment226merges with shoulder222at a radius232.

In operation, in the embodiment ofFIGS. 21 and 22, threads200and220are threaded together thereby bringing the respective pin and box threads into engagement. The effect of the ramp segments leads to the respective pin and box lead-in being pulled into the ramp rather than pushed out when a high level of torque or torsion is applied to the respective threads.

FIG. 23shows box thread250and pin thread252according to a further embodiment. According to this embodiment, at least one of the pressure flanks of the pin and/or box threads comprises a projecting nose continuous with the pressure flank, such that the nose is adjacent to and merges with the corresponding crest. When the pin and box threads are engaged, the crest adjacent to the projecting nose crest becomes spaced apart from the root of the corresponding box or pin thread when threaded thereto.

According to this embodiment, the box and pin threads have S-curved pressure flanks254and256respectively. A flat crest258of pin thread252(at the major diameter of pin thread252) is in contact with a flat root260of box thread250(at the major diameter of box thread250) when threaded together. Box thread pressure flank254merges with an outer radius262at the minor diameter of box thread250(i.e. the innermost diameter). Outer radius262merges with a tapered segment264that angles outwardly towards the major diameter of box thread250. As a result, radius262comprises a protruding nose when seen in an axial cross section (as inFIG. 23), which protrudes inwardly towards the central axis relative to the adjacent crest266of box thread250. Crest266is thus recessed outwardly away from the central axis, when seen in axial section. In contrast, pressure flank256of pin thread252merges with a radius268which does not provide a similar nose-like projection.

It will be seen that nose262could alternatively or in addition be provided on the pin thread segment.

When box and pin threads250and252are threaded together, as seen inFIG. 23, crest266of box thread250is spaced apart from root270of pin thread252, by a gap272. It will be seen that gap272is formed and defined by the projecting portion of radius262. Furthermore, this structure maximizes the thread flank contact between pressure flanks254and256without compromising the clearance between the adjacent crest and root segments266and272of box and pin threads250and252.

Table 1 is a chart that provides an example of thread configurations in which radius R1ranges from 0.008 inches to 0.012 inches with tangent66comprising either 45 degrees or 60 degrees relative to axis18.

FIG. 24illustrates the reduction in internal stress concentration as the radius increases of the fillet between the root and the pressure flank of the box and pin threads according to one aspect of the invention.

By increasing the radius in the corner of the root and pressure flank, it decreases the stress concentration of the part

Example 3: Measurement of Torsional Strength

An embodiment was tested for torsional strength. According to this test, three threaded pipe assemblies were provided, each assembly consisting of a pair of pipes threaded together. Each pipe had a 1 inch diameter hole for receiving a torque transfer shaft. An end plate adapter interface was provided for fastening to the torque transfer shaft, consisting of a pair of flanges with pins and holes for connecting the torque transfer shaft to a torsion device. Each assembly was attached at one end, through the flange, to a bearing block shaft and a sprocket. The opposed end of the assembly was secured against rotation by attachment to a vertical post via a pin. Torque was applied to the pipe in a counter-clockwise direction using a servo-hydraulic actuator equipped with a calibrated load cell and displacement transducer, through a chain and a clevis.

Rotation of the threaded joint was measured using a second calibrated displacement transducer installed directly on the tube at approximately 1 inch from the joint. Torque was applied to the tube using angular displacement control at a constant rate of 10°/minute until failure occurred to the threaded joint. Load and angular displacement at the tube were recorded and used to plot the torque vs. angular displacement curves.

The scope of the present invention should not be limited by the preferred embodiments set forth in the examples but should be given the broadest interpretation consistent with the description as a whole. The claims are not to be limited to the preferred or exemplified embodiments of the invention.