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.