Prior art A thread joint for percussive rock drilling is known through for example WO 00/19056. The known thread joint includes cooperative conical rope threads intended to improve the strength of the joint. However, the thread will be relatively sensitive to tolerance faults such that a minor error in tolerances makes that the flanks of the thread partly will not come in contact with each other.

Furthermore, connection to conventional rock drilling components is impossible.

Objects of the invention One object of the present invention is to provide a thread joint for percussive rock drilling, which minimizes the above-captioned drawbacks.

Another object of the present invention is to provide a thread joint for percussive rock drilling, which can be connected to conventional rock drilling components.

These and the other objects are realized by means of a thread joint for percussive drilling and parts therefor according to the characteristics of the independent claims with reference to the appended drawings.

Description of the drawings Fig. 1 shows a thread joint for percussive drilling according to the present invention in cross-section.

Fig. 2 shows a rock drilling component according to the present invention in cross-section with a male thread.

Fig. 3 shows a rock drilling component according to the present invention in cross-section with a female thread.

Fig. 4 shows a thread joint for percussive drilling between a female portion according to the present invention and a first conventional rock drilling rod in cross-section.

Fig. 5 shows a thread joint for percussive drilling between a female portion according to the present invention and another conventional rock drilling rod in cross-section.

Fig. 6 shows an enlarged portion of the threaded joint according to the present invention in Fig. 1.

Fig. 7 shows an enlarged portion of an alternative thread joint according to the present invention in cross-section corresponding Fig. 6.

Fig. 8 shows an alternative thread joint for percussive drilling according to the present invention in cross-section.

Detailed description of the invention In Figs. 2 and 3 are shown ends with two rock drilling rods 11,12 according to the present invention. A first drill rod 11 includes a portion 13 with substantially cylindrical male threads 15A, 15B and a second drill rod 12 includes a portion 14 with substantially cylindrical female threads 16A, 16B. Both types of ends frequently occur at one and the same rod so as to form a drill string, not shown, together with identical drill rods. The drill string is intended to keep a rock drill bit at its front end for percussive drilling during simultaneous rotation of the drill bit.

The drill bit is frequently provided with hard material buttons. The drill rod has a center line CL.

The portion 13 with male threads 15A, 15B is provided at a spigot 17, which constitutes an integral part of the first drill rod, see Fig. 2. The drill rod 11 has an abutment surface 19 provided at the free end of the spigot 17. A central flush channel 20 that terminates in the abutment surface 19 extends through the entire drill string. The abutment surface 19 connects radially outwardly to a circumferential conical part 21, see Fig. 1. An entrance of a first male thread 15A starts in or in the proximity of the conical part 21 and extends axially upwardly in left-hand turns with a pitch P1. The first thread 15A has an outer diameter D1 in the magnitude of 26-40 mm and an axial length in the magnitude of 20-50 mm.

The spigot 13 includes a transition area 22 at which the cylindrical threads meet in a continuous manner. The diameter of the spigot increases substantially step wise with a maintained pitch angle a1, to a second male thread 15B. The transition area 22 has an axial length L2 of maximum 2 times the biggest pitch of the joint. The second male thread 15B ends in an exit in or in the vicinity of a shoulder 23, which in turn connects to the jacket surface 24 of the rod 11. Also the second male thread 15B extends axially upwardly in left-hand turns with a pitch P2 from the transition area 22. The pitches P1 and P2 are substantially identical. The second thread 15B has an outer diameter D2 in the magnitude of 29-43 mm and an axial length in the magnitude of 20-50 mm. The dimensions of the both diameters D1, D2 of the respective thread 15A, 15B differ with at least 1 mm, preferably within the interval 2-5 mm and the difference is preferably about 3 mm. In a preferred embodiment D1 is about 28 mm and D2 is about 31 mm.

The portion 14 with female threads 16A, 16B is provided in a recess 25 in a female portion or extension sleeve 18, which constitutes an integral part of the second drill rod 12, see Fig. 3. The drill rod 12 has an end surface 26 at the free end of the sleeve 18. The central flush channel 20 that via the recess 25 terminates at the end surface 26 extends through the entire drill string. The end surface 19 connects radially inwardly to a circumferential conical entrance bevel 27. An entrance of a second female thread 16B starts in or in the vicinity of the conical entrance bevel 27 and extends axially downwardly in the recess 25 in left-hand turns with a pitch P4. The second thread 16B has an outer diameter D4 in the magnitude of 29-43 mm and an axial length in the magnitude of 20-50 mm.

The recess 25 comprises a transition area 28 at which the cylindrical threads meet in a continuous manner. The diameter of the recess is changed or is reduced substantially stepwise at a maintained pitch angle a2, to a first female thread 16B. The transition area 28 has an axial length L4 of maximum 2 times the biggest pitch in the joint. The first female thread 16A ends in a thread clearance 29 in the vicinity of an abutment surface or bottom 30. Also the first female thread 16A extends axially downwardly in left-hand turns with a pitch P3 from the transition area 22. The pitches P3 and P4 are substantially identical and so are the pitch angles a1 and a2. The first thread 16A has an outer diameter D3

in the magnitude of 26-40 mm and an axial length in the magnitude of 20-50 mm.

The dimensions of both diameters D3, D4 of the respective thread 16A, 16B differ with at least 1 mm, preferably the difference is within the interval 2-5 mm and is preferably about 3 mm. In a preferred embodiment D3 is about 28 mm and D4 is about 31 mm.

When the threaded joint 10 with two drill rods 11,12 for percussive drilling shown in Fig. 1 is mounted the conical part 21 of the spigot will enter within the bevel 27 of the free end surface 26 of the female portion. The rods are brought still further together in the axial direction and the first male thread 15A slides past the second female thread 16B without creating a threaded engagement between these parts. It is only when the first male thread 15A reaches the diametrically smaller first female thread 16A that the threaded engagement occurs. About simultaneously an engagement occurs between the second male thread 15 B and the second female thread 16B. Then the rods are rotated still until the abutment surface 19 of the spigot abuts against the bottom 30 in the recess. The full cross-sectional profiles of the threads 15A, 15B and 16A, 16B are of conventional form, preferably rope or round (Fig. 6) or trapezoid (Fig. 7), such that abutments occur only between certain flank portions of the respective male and female portions when they have been tightened. Each male and female thread has a full profile that includes a radius R1, R2 that is greater than 0,1 times a pitch P1-P5 of the thread. At a trapezoidal thread, a so-called T38 thread, R2 is 2 mm and the pitch P5 is 15,63 mm, whereby the relation becomes 0,13. Standard charts define a full profile.

In a tightened joint of trapezoidal threads there can be a gap of maximum 1.5 mm between the cooperating abutment surfaces of two flanks at a cylindrical portion, which gap will be gradually worn into zero. The joint in US-A-4,968,068, which patent hereby is incorporated into the present description, shows an alternative thread joint 10"' (Fig. 8) in which the present invention can be utilized, where there exists a shoulder and/or bottom abutment between the rods 11"', 12"'. The through-going flush channel 20 of the drill rod leads a flush medium, generally air or water. The sections having the smallest diameter D1 and D3 of the female thread and male thread, respectively, each comprises at least one

thread turn with a full profile. One thread turn extends 180°. The transition area 22,28 between the cylindrical sections at the respective thread has an axial extension L2, L4 of at least the half of the diametrical difference of the smallest diameter D1, D3 and the biggest diameter D2 and D4. Said transition areas may be wholly spaced from each other in a tightened joint.

The female portion 18 or the spigot 17 according to the present invention may be used for example in a rock drill bit, whereby conventional rock drilling rods with simple cylindrical threads already in store at the user may hold the rock drill bit. As an illustration of the latter an alternative thread joint 10'is shown in Fig. 4 where a spigot of a conventional rock drilling rod 11'has been mounted to a female portion 18 according to the present invention. In this joint there is engagement between the first female thread 16A and a conventional cylindrical male thread 15'. However, the second female thread 16B is not used, i. e. there exists a radial gap between the thread tips of the male thread 15'and the female thread 16B in a tightened joint.

Fig. 5 illustrates still an alternative thread joint 10"where a spigot of a conventional rock drilling rod 11"has been mounted to a female portion 18 according to the present invention. In this joint there is engagement between the second female thread 16A and a coarser conventional cylindrical male thread 15'. The first female thread 16A however is not used, i. e. the male thread 15"and the female thread 16B must transfer all chock waves in a tightened joint. The latter joint is intended to be used at easy drilling in soft rocks. It is of course obvious to instead provide the drill bit with a portion with male threads within the idea of the present invention.

Consequently, a thread joint for percussive drilling and parts therefor according to the present invention bring about that the parts according to the present invention can be combined with conventional rock drilling components in an economically advantageous manner with a minimum of parts. Furthermore, the strength of the joint has been improved for a given outer diameter. In addition, more threads than two, maximum 4 cylindrical threads, can be provided at a portion 13,14.