The present invention relates to a thread friction locking device, especially adapted to be mounted in a thread coupling between tubes for percussive drilling.

In EP, Bl, 0 126 740 tubes for percussive drilling of the type referred to above is described. However, a frequent problem in a drill tube system of this type is that the threads, if not tightened, have a tendency to unscrew. This happens especially when the drill string of tubes is raised and the tubes disconnected from each other when drilling is finished or the drill bit must be exchanged. In order to loosen a tightened thread coupling the drill string is subjected to one or more strikes when the drill bit is in raised position, i.e. not in contact with the bottom of the drilled hole. In such a case not only the thread coupling at the highest level but also thread couplings on lower levels will become loose and it happens frequently that a thread coupling in the drilled hole gets completely unscrewed and the drill bit and one or more tubes are lost.

The aim of the present invention is to present a device for friction locking of the thread couplings between the tubes. Said aim is realized by a device that has been given the characteristics of the appending claims. Below embodiments of the invention will be described, reference being made to the accompanying drawings, where Fig.l discloses a section through a device according to the present invention, said device being mounted in a thread coupling; Fig.2 discloses a top view of a device according to the present invention; Fig.3 discloses a section along III-III in Fig.2; Fig.4 discloses in enlarged scale a portion of Fig.2; and Fig.5 discloses a section through the device according to the present invention when the nose of the male thread enters said device, said Figure also being provided with dimensional definitions.

In Fig.l a device 10 according to the invention is shown mounted in a thread coupling between a first tube 11 and a second tube 12. The device 10 is in the shape of an annular ring and will be described more in detail further down.

The first tube 11 has a spigot 13 that is provided with a male thread 14 and the second tube 12 has a boring 15 that is provided with a female thread 16. Both tubes 11 and 12 have central flushing holes 17 that align with each other when the tubes 11, 12 are assembled by the thread coupling.

The spigot 13 is also provided with a nose 18 at its free end, said nose 18 having a smaller diameter than the rest of the spigot 13.

At its bottom the boring 15 is provided with a-thread clearance 19. The thread friction locking device 10 according to the present invention is mounted in said thread clearance 19 and when the first and second tubes 11, 12 are assembled by the thread coupling the nose 18 will enter the internal periphery of the device 10 and compress the cross- section of said device 10 in radial direction in its position in the thread clearance 19.

An embodiment of the thread friction locking device 10 according to the present invention is disclosed in Figs.2-4. As mentioned above the device 10 is in the shape of an annular ring that has a certain extension in axial direction. The outer periphery of the device 10 is smooth but the inner periphery is provided with a number of axially extending grooves 20 that will allow flushing medium, preferably water or air, to enter between the end of the nose 18 and the bottom of the boring 15 and then pass between the device 10 and the nose 18 and further on to the thread coupling, said flushing medium having a cooling effect upon the thread coupling. In the disclosed embodiment the grooves 20 are spaced apart equally around the circumference by an angle « that is 30°. The angle « can of

course also have other values within the scope of the invention.

In Fig.4 one groove 20 is shown more in detail and it can be learnt that on one side of the groove 20 there is a bevel

21 that will reduce friction between the inner periphery of the device 10 and the nose 18 when the first tube 11 is rotated in a direction to tighten the thread coupling.

As is shown in the cross-sectional view of Fig.3 the outer periphery is rounded at the lower end in Fig.3, i.e. the end that is directed towards the bottom of the boring 15 in mounted position of the device 10, while the upper end in Fig.3, i.e. the end that is directed from the bottom of the boring 15 in mounted position of the device 10, is bevelled at both its outer and inner periphery. The bevel 22 on the outer periphery makes the device 10 fit into the thread clearance 19 while the bevel 23 on the inner periphery helps the nose 18 to enter the device 10. The bevel 22 is defined by the angle β in Fig.3, β preferably being about 25°. The bevel 23 is defined by the angle T in Fig.3, T preferably being about 30°. Instead of the bevel 22 the device 10 can be provided with a radiused portion.

Although the embodiment according to Figs.2-4 discloses axial grooves 20 on the inner periphery of the device 10 the invention is in no way restricted to such an embodiment. The axial grooves can also or instead be provided on the outer periphery of the device 10. It is also possible to have axial channels through the device 10. A further possible embodiment is to have helical grooves on the inner and/or outer periphery of the device. The requirement is that flushing medium can pass axially from one end of the device 10 to the other end of the device 10.

In Fig.5 the dimensions of the device 10, the nose 18 and the thread clearance 19 are defined.

The axial length of the thread friction locking device 10 is marked by 1. The cross-sectional thickness is marked by t. The following relationship between t and 1 must be valid: 0,1 i t/1 . 0,6.

The diameter of the thread clearance 19 is marked by Di. The external diameter of the nose 18 is marked by dy. The internal diameter of the thread friction locking device 10 is marked by di. The external diameter of the thread friction locking device 10 is marked by Dy. The following relationship between Di, dy, di and Dy must be valid: 0,55 i (Dy - Di)/(dy - di) i 0,95.

The thread friction locking device 10 according to the present invention is preferably made out of urethane rubber. The material that has been used in our field tests is marketed under the trade mark TREKOLLAN, types 4020 and 4013.

The material should have a hardness within the interval 80 - 105 ^β IHR.

The tensile stress of the material at 300 % extension in room temperature should be in the interval 10 - 25 MPa.

The tensile strength of the material in room temperature should be in the interval 20 - 55 MPa.

The ultimate elongation of the material in room temperature should be in the interval 400 - 850 %.

Also after aging tests according to SIS 162205 the above listed characteristics of the material must be in the given intervals.

The function of the thread friction locking device according to the present invention is best understood when studying Figs.l and 5. In Fig.5 the nose 18 of the spigot 13 enters

the internal periphery of the device 10. Due to the dimensional relationship defined above a certain compression in radial direction of the cross-section of the device 10 will take place. The bevel 23, see Fig.3, makes the entering of the nose 18 easier and avoids a compression of the device 10 in axial direction. In Fig.l the nose 18 contacts the bottom of the boring 15 and the device 10 is in its operative position. It is at once understood that said compressed device 10 exerts friction forces against an unscrewing relative rotation between the male and the female threads 14 and 16, resp.. In this connection reference is made to the bevel 21, see Fig.4. Said design decreases the friction between the device 10 and the nose 18 when the thread coupling is tightened and increases the friction when the thread coupling is unscrewed.

The invention is in no way restricted to the embodiments described above but can be freely varied within the scope of the appending claims.