THREAD CLEARANCE

Abstract

A female portion is arranged to form part of a thread joint for a percussive drilling tool including a mounting sleeve having an axial end, wherein the mounting sleeve surrounds an internal cavity having an axial inner wall at the opposing end of the mounting sleeve compared to the axial end. The mounting sleeve has at least one substantially cylindrical internally threaded section having a length L.sub.1, a thread entrance positioned towards the axial end and a thread exit positioned towards the axial inner wall. A thread clearance section is positioned between the axial inner wall and the thread exit and has a length L.sub.2 and a diameter D.sub.1. A guiding section is positioned between the thread entrance and the axial end of the sleeve and has a length, L.sub.3, wherein 0 mm<L.sub.3-L.sub.2<12 mm.

Claims

1. A female portion arranged to form part of a thread joint for a percussive drilling tool, the female portion comprising: a mounting sleeve having an axial end, wherein the mounting sleeve surrounds an internal cavity having an axial inner wall at the opposing end of the mounting sleeve compared to the axial end, wherein the mounting sleeve has at least one substantially cylindrical internally threaded section having a length L.sub.1, a thread entrance positioned towards the axial end and a thread exit positioned towards the axial inner wall; a thread clearance section positioned between the axial inner wall and the thread exit having a length L.sub.2 and a diameter D.sub.1; and a guiding section positioned between the thread entrance and the axial end of the sleeve having a length, L.sub.3, wherein 0 mm<L.sub.3−L.sub.2<12 mm.

2. The female portion according to claim 1, wherein L.sub.1 is between 25-56 mm.

3. The female portion according to claim 1, wherein L.sub.2/L.sub.1>0.01×D.sub.1.

4. The female portion according to claim 1, wherein L.sub.2/L.sub.1>26%.

5. The female portion according to claim 1, wherein L.sub.2/L.sub.1<65%.

6. The female portion according to claim 1, wherein L.sub.2/D.sub.1>30%.

7. The female portion according to claim 1, wherein L.sub.2/D.sub.1<65%.

8. The female portion according to claim 1, wherein the female portion is part of a drill bit.

9. The female portion according to claim 1, wherein the female portion is a female end of a drill string rod.

10. A drill string rod comprising a female portion according to claim 1.

11. A drill bit comprising a female portion according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0014] A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings:

[0015] FIG. 1: Perspective view of a percussive drilling tool having one male and one female end.

[0016] FIG. 2: Perspective view of a drill rod having two male ends.

[0017] FIG. 3: Cross section of the female end of a drill rod.

[0018] FIG. 4: Cross section of a drill bit.

[0019] FIG. 5: Cross section of an internal profile of the cavity of the female portion to form part of a thread joint for a percussive drilling tool.

DETAILED DESCRIPTION

[0020] FIG. 1 shows a percussive drilling tool 2 whereby a drill rod 4 is threadedly coupled to a drill bit 6 of conventional design. The percussive drilling tool 2 is especially used for top hammer drilling. Shockwaves generated by a surface piston (not shown) are translated through the mated surfaces from the drill rod 4 to the drill bit 6. The drill rod 4 comprises an axially extending main length section 8 that is terminated at one end by a male end 10 and at a second opposite end by a female end 12 having a longitudinal axis 14. The drill rod 4 is capable of being coupled end-to-end with other further drill rods to form a drill string (not shown) via another thread joint. FIG. 2 shows that alternatively the drill bit 6 may be connected to a single drill rod 4 having two male ends 10.

[0021] FIG. 3 shows a cross section of the female end 12 of the drill rod 4 having a mounting sleeve 18 and an internal cavity 20, which is a hollow space, for receiving the male end 10 of the drill rod.

[0022] FIG. 4 shows a cross section of the drill bit 6 comprising an axially forwardmost drill head 16 of convention design for example comprising a rock crushing means, most typically this is a plurality of wear resistant cutting buttons projecting axially forward from the drill head (not shown); a mounting sleeve 18 that comprises an axially extending internal cavity 20 for receiving the male end 10 of the drill rod 4.

[0023] The present invention relates to a special design for a female portion 22 that forms part of a thread joint for a percussive drilling tool 2. The female portion 22 could be either the female end 12 on the drill rod 4 or the drill bit 6.

[0024] FIG. 5 shows a cross section of an internal profile of the cavity 20 of the female portion 22, in other words FIG. 5 is an enlargement of the interior of FIG. 3 or FIG. 4. The cavity 20 has an axial inner wall 24 for abutment with a male end 10 of a drill rod 4. Further, the cavity 20 has at least one substantially cylindrical internally threaded section 26 having a thread entrance 28 at the axially opposite end compared to the axial inner wall 24 and a thread exit 30 nearer to the axial inner wall 24. The threaded section 26 has a length, L.sub.1, which is defined as the length between the thread entrance 28 and the thread exit 30. The cavity 20 of the female portion 22 also has a thread clearance section 32 positioned between the axial inner wall 24 and the threaded section 26. The thread clearance section 32 is circumferential concave recess. The thread clearance section 32 has a length, L.sub.2, which is defined as the length between the thread exit 30 and the axial inner wall 24. The thread clearance section has a diameter, D.sub.1. There is a guiding section 50 on the opposing end of the threaded section 26 compared to the thread clearance section 32 for guiding the male end 10 of the rod 4 into the correct position. The guiding section 50 has a length, L.sub.3, which is defined as the length between the thread entrance 28 and an axial end 52 of the sleeve 18. The guiding section 50 can either be a constant diameter across the whole of the guiding section 50 or it could optionally be stepped so that it has at least two different diameters.

[0025] Compared to conventional designs the ratio L.sub.2 compared to L.sub.1 and/or L.sub.3 has been increased.

[0026] L.sub.3-L.sub.2 for female portions 22 according to the present invention is <12 mm, more preferably <11.5 mm, even more preferably <11 mm. L.sub.3-L.sub.2 for female portions 22 according to the present invention is >0 mm, more preferably >5 mm, even more preferably >7 mm. In one embodiment L.sub.1 is between 25-56 mm, more preferably between 30-45 mm.

[0027] In one embodiment L.sub.2/L.sub.1>0.01×D.sub.1.

[0028] In one embodiment L.sub.2/L.sub.1>26%, more preferably >32%.

[0029] In one embodiment L.sub.2/L.sub.1<65%, more preferably <50%.

[0030] In one embodiment L.sub.2/D.sub.1>30%, more preferably >38%.

[0031] In one embodiment L.sub.2/D.sub.1<65%, more preferably <50%.

[0032] Preferably, the female portion 22 is used for forming a thread joint 34 for a percussive drilling tool 2 wherein there is a bottom contact, as opposed to a shoulder contact, between female portion 22 and the male end 10 of the adjoining rod. In other words, there is contact between the axial inner wall 24 on the female portion and the male end 10 of the adjoining rod 4.

[0033] Software simulation programming with bending force added to the female portion 22 of inventive and non-inventive female portions was performed. Table 1 shows a summary of stress measurements for prior art and inventive female portions 22, it can clearly be seen that the stress in the region of the thread clearance is lower for the inventive samples.

TABLE-US-00001 TABLE 1 Summary of stress measurements Stress (MPa) in the region of the thread clearance on the female portion of a Example D.sub.1 (mm) L.sub.3-L.sub.2 (mm) threaded joint. 1 (prior art) 33.1 12.7 305 2 (invention) 33.1 11.0 290 3 (prior art) 36.5 14.3 300 4 (invention) 36.5 10.5 275