THREAD COUPLING FOR A DRILL STRING FOR PERCUSSIVE ROCK DRILLING

Abstract: The thread coupling is provided with a male thread (4) and a female thread (5) cooperating therewith. The male thread is arranged on a first drill string element (3) provided with a first impact surface (6). The female thread is arranged on a second drill string element (2) which is provided with a second impact surface (7). The male thread (4) has an axially variable pitch. And the female thread (5) has a thread opening (8) which is wider than the crest (9) of the male thread.

Description: Thread coupling for a drill string for percussive rock drilling The present invention relates to a thread coupling for a drill string for percussive rock drilling. In previously known thread couplings for percussive rock drilling prestressing of the drill string elements at the contact surface between them is obtained. The increased load depends on a combination of the material deformations, which are obtained when a shock wave passes through the thread coupling, and the applied torque. The increased load increases the risk for breakage in either or both of the connected drill string elements. The present invention, which is defined in the subsequent claim, aims at achieving a thread coupling where the above mentioned load increase near the contact surface between the two drill string elements is avoided. Through this one can increase the drill rate without deteriorating the service life compared to previously known equipment. This is achieved through having a male thread with variable pitch and a female thread with a thread opening which is wider than the crest of the male thread. An embodiment of the invention is described below with reference to the accompanying drawing in which fig 1 shows a drill string with a thread coupling according to the invention. Fig 2 shows a thread joint partly in section. Fig 3 shows a part of the thread joint on a larger scale with exaggerated variation of the thread pitch. The drill string shown in the drawing comprises a drill bit 1, a drill rod 2 and a shank adapter 3. Instead of a drill rod as shown one can have several. Drill bit, drill rod and shank adapter are drill string elements. A first drill string element 3 is provided with a male thread 4 and a first impact surface 6. A second drill string element 2 is provided with a female thread 5 and a second impact surface 7. The impact surfaces 6,7 abut against each other during drilling. In the shown example the abutment is situated at the outer end of drill string element 2, so-called soulder abutment. One can of course have the abutment at the outer end of drill string element 3 instead, so-called bottom abutment. As is best shown in fig 3 the male thread 4 has an axially varying thread pitch. Furthermore the female thread 5 has a thread opening which is wider than the crest 9 of the male thread 4. The female thread has substaantially constant pitch. Through these features the thread coupling can be tightened in spite of the threads not having the same pitch. If e.g. the female thread has a pitch of 15 mm the male thread can have a pitch which is 15+(x/15)*0.2 mm where x is a running coordinate in the longitudinal direction of the thread. In the shown example the pitch of the thread is largest near the outer end 10 of the thread 4. This means that the thread coupling is prestressed near the outer end 10 so that one avoids overloading adjacent the contact surfaces 6,7. If one instead of shoulder abutment, as shown, has bottom abutment the thread 4 has the smallest pitch near the outer end 10.

Claims: Claim: 1. Thread coupling for a drill string for percussive rock drilling comprising a male thread (4) and female thread (5) cooperating therewith, said male thread (4) being arranged on a first drill string element (3) and said female thread (5) being arranged on a second drill string element (2), that said first drill string element (3) comprises a first impact surface (6), that said second drill string element (2) comprises a second impact surface (7) and that said first and second impact surfaces during drilling are intended to abut against each other, c h a r a c t e r i z e d in that said male thread (4) has a variable pitch in its axial direction and that said female thread (5) has a thread opening (8) which is wider than the crest (9) of the male thread (4).

Inventors: GUSTAFSSON CHRISTER, GUSTAFSSON, CHRISTER
Applicants: GUSTAFSSON CHRISTER, UNIROC AB, GUSTAFSSON, CHRISTER
Application year: 1999
Main classification: E21B17
IPC codes: E21B17