High-strength threaded joints, particularly for lined tubes

1. A threaded joint, comprising:

a tubular coupling possessing a threaded inner surface along at least a portion of its length;

a pair of tubes that are received within the tubular coupling and comprise an external threaded surface and a substantially thread-free end, the thread-free end comprising:

a first portion comprising a thread bevel that overlaps a selected portion of the external threaded surface and that is substantially continuous with a first rounded area having a first radius of curvature;

a second portion comprising a second rounded area adjacent the first rounded area, the second rounded area having a second radius of curvature; and

wherein the first and second radii of curvature are about 1.5 mm.

2. The threaded joint of claim 1 , wherein the first rounded area of the thread-free end of the tubes extends from the thread bevel up to a point of its curvature where a tangential line to the point is perpendicular to a longitudinal axis of the tubes.

3. The threaded joint of claim 2 , wherein the second rounded area of the thread-free end of the tubes extends from a point of its curvature, where a tangential line to the point is perpendicular to the longitudinal axis of the tubes, to an inner surface of the tube.

4. The threaded joint of claim 1 , wherein the bevel comprises an angle of about 65° with respect to a line extending perpendicular to a longitudinal axis of the tubes.

5. The threaded joint of claim 1 , wherein the first and second rounded areas meet at a common point where a tangential line is perpendicular to a longitudinal axis of the tubes.

6. The threaded joint of claim 1 , wherein the first and second rounded areas are connected to one another by a transition zone.

7. The threaded joint of claim 6 , wherein the transition zone is flat and extends between a first point of curvature of the first rounded area and a second point of curvature of the second rounded area where a tangential line to the first and second points is perpendicular to a longitudinal axis of the tubes.

8. The threaded joint of claim 1 , further comprising a central section between the thread-free ends of the pair of tubes.

9. The threaded joint of claim 8 , wherein an elastomeric ring is positioned within the central section so as to be laterally compressed by the thread-free ends of the tubes.

10. The threaded joint of claim 9 , further comprising at least one metallic insert positioned inward of the ring.

11. The threaded joint of claim 1 , wherein the joint further comprises 8 round threads per inch and a diameter interference of about 0 to 2 mm.

12. A threaded joint for lined tubes, comprising:

a coupling with a threaded inner surface along its whole length and female ends; and

a pair of tubes, each having male ends, with an external threaded surface and a thread-free end comprising a nose, said tubes defining a longitudinal axis;

said joint having 8 round threads per inch, with a diameter interference from about 0 to 2 mm;

wherein each of the noses of the pair of tubes comprises:

an upper portion formed by a thread bevel with a slope of substantially 65 degrees relative to a line perpendicular to the longitudinal axis, which transitions to a first rounded area with a first radius of curvature of 1.5 mm; and

a lower portion including a second rounded area with a second radius of curvature of 1.5 mm;

said thread bevel overlapping with said thread profile and said first rounded area extending from said thread bevel to a point of its curvature where a tangential line is perpendicular to the longitudinal axis of said tubes, said second rounded area extending from a point of its curvature where a tangential line is perpendicular to the longitudinal axis to an inner surface of said tube;

wherein the tubes are lined and the first and second radii of curvature of 1.5 mm are configured to improve adhesion of the lining to the tubes and provide either a flat transition zone between the first and second rounded areas or a common point between the first and second rounded areas that coincides with said points of said rounded areas.

13. The threaded joint according to claim 12 , wherein the first and second radiuses of curvature are located one next to the other.

14. The threaded joint according to claim 12 , wherein the first and second radiuses of curvature are connected to each other by a transition zone.

15. The threaded joint according to claim 14 , wherein the transition zone is flat and extends between said points of said rounded areas where a line tangential to said points is perpendicular to the longitudinal axis.

16. The threaded joint according to claim 12 , wherein between the noses of both male threaded members is a central section.

17. The threaded joint according to claim 16 , wherein the central section includes an elastomeric ring which is laterally compressed by said noses of said tubes.

18. The threaded joint according to claim 17 , wherein the elastomeric ring includes at least one metal insert.