THREAD CLEARANCE
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 L1, 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 L2 and a diameter D1. A guiding section is positioned between the thread entrance and the axial end of the sleeve and has a length, L3, wherein 0 mm<L3-L2<12 mm.
The present invention relates to a female portion to form part of a thread joint for a percussive drilling tool, especially for, although not exclusively for drill bits and drill rods.
BACKGROUNDPercussion drilling is used to create a long borehole via a plurality of elongate drill string rods coupled together end-to-end by interconnected male and female threads. Alternatively, a drill bit may be connected to a single rod. The well-established technique breaks rock by hammering impacts transferred from the rock drill bit to the rock at the bottom of the borehole. The rock drill bit is mounted at one end of the drill string via a male thread on the endmost drill string rod to a female thread on the drill bit. Typically, the energy required to break the rock is generated by a hydraulically driven piston that contacts the end of the drill string (via a shank adaptor) to create a stress (or shock) wave that propagates through the drill string to a drill bit. Conventional threaded joints are described in U.S. Pat. Nos. 4,332,502; 4,398,756; 4,687,368 and DE 2800887.
Threaded joints in the percussive drilling tool, such as those between drill string rods and between the endmost drill string rod and the drill bit are subjected to bending forces during drilling from the stress waves that propagate the drill string. These bending moments fatigue the threaded joints and lead to breakage within the threaded portion of the joint. Eventually the stress will cause the threaded joint to get worn out and eventually fail.
Therefore, it is desirable to reduce the stress in the threaded joint to improve the performance of the percussive drilling tool and reduce the risk of the failure in the threaded joint. One solution to reducing stresses in the threaded joint is to increase the diameter of the male thread, however the problem with this is that it weakens the female part of the threaded joint meaning it is more likely to break. Consequently, the problem to be solved is how to decrease stress in the threaded joint to therefore increase their lifetime.
SUMMARYIt is an objective of this invention to provide a novel and improved design for threaded joints for percussive drilling tools. The objective is achieved by providing a female portion to form part of a thread joint for a percussive drilling tool 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 L1, a thread entrance towards the axial end and a thread exit towards the axial inner wall; a thread clearance section positioned between the axial inner wall and the thread exit having a length L2 and a diameter D1; a guiding section positioned between the thread entrance and the axial end of the sleeve having a length, L3; characterized in that: 0 mm<L3-L2<12 mm. More preferably, 5 mm<L3-L2<11.5 mm, even more preferably 7 mm<L3-L2<11.1 mm.
Advantageously, this reduces the stress in the female portion of the threaded joint, meaning the risk of breakages in the female portion is decreased. Furthermore, if the stress in the female portion is increased then the diameter of the male threaded part can be increased which improves the performance of the percussive drilling tool.
Preferably, L1 is between 25-56 mm, more preferably between 30-45 mm. Preferably, the total length of the female thread (L1+L2+L3) is between 70 to 86 mm. Advantageously, this is the most optimal length of threaded section for increased performance.
Preferably, L2/L1>0.01×D1. Advantageously, by increasing the ratio of the length of the thread clearance area compared to the length of the threaded section means that the stress in thread clearance area decreases.
Preferably, L2/L1>26%, more preferably L2/L1>32%. Advantageously, increasing the ratio of the length of the thread clearance area compared to the length of the threaded section means that the stress in thread clearance area decreases.
Preferably, L2/L1<65%, more preferably <50%. Advantageously, this provides sufficient length in the threaded section to achieve a secure threaded connection.
Preferably, L2/D1>30%, more preferably L1/D1>38%. Advantageously, increasing the ratio of the length of the thread clearance area compared to the diameter of the threaded section means that the stress in thread clearance area decreases. Therefore, the diameter of the thread clearance and female thread can be increased and consequently so can the diameter of the male part, which leads to increased performance of the male part and less risk of skirt failures in the female part.
Preferably, L2/D1<65%, more preferably <50%. Advantageously, this enables a secure threaded connection.
In one embodiment, the female portion is a drill bit. In an alternative embodiment, the female portion is a female end of a drill string rod.
A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings:
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.
Compared to conventional designs the ratio L2 compared to L1 and/or L3 has been increased.
L3-L2 for female portions 22 according to the present invention is <12 mm, more preferably <11.5 mm, even more preferably <11 mm. L3-L2 for female portions 22 according to the present invention is >0 mm, more preferably >5 mm, even more preferably >7 mm. In one embodiment L1 is between 25-56 mm, more preferably between 30-45 mm.
In one embodiment L2/L1>0.01×D1.
In one embodiment L2/L1>26%, more preferably >32%.
In one embodiment L2/L1<65%, more preferably <50%.
In one embodiment L2/D1>30%, more preferably >38%.
In one embodiment L2/D1<65%, more preferably <50%.
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.
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.
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 L1, 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 L2 and a diameter D1; and
- a guiding section positioned between the thread entrance and the axial end of the sleeve having a length, L3, wherein 0 mm<L3−L2<12 mm.
2. The female portion according to claim 1, wherein L1 is between 25-56 mm.
3. The female portion according to claim 1, wherein L2/L1>0.01×D1.
4. The female portion according to claim 1, wherein L2/L1>26%.
5. The female portion according to claim 1, wherein L2/L1<65%.
6. The female portion according to claim 1, wherein L2/D1>30%.
7. The female portion according to claim 1, wherein L2/D1<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.
Type: Application
Filed: Jun 29, 2021
Publication Date: Aug 31, 2023
Inventor: John HAMMARGREN (Sandviken)
Application Number: 18/013,215