Method to establish a thread coupling

Abstract: The present invention relates to a thread coupling including at least one male thread (10) and one female thread (17), said male thread (10) being arranged upon a spigot (11) that constitutes an integral part of a first drill string element (12), and said drill string element (12) at the inner end of said spigot (11) being provided with a shoulder (13) that has a contact surface (14) directed towards the free end of the spigot (11), said female thread (17) being a part of a coupling sleeve (18) that is connected to a second drill string element (19), and the coupling sleeve (18) being provided with an internal stop surface (20). In thread couplings between drill string elements either so-called bottom stops or so-called shoulder stops are used, said principles per se having disadvantages as to stresses, energy transfer and so on. The characteristic of the present invention is the combination of a bottom and a shoulder stop in a thread coupling.

Description: The present invention relates to a method to establish both shoulder and bottom stop in a thread coupling including at least one male thread and one female thread, said male thread being arranged upon a spigot that constitutes an integral part of a first drill string element, and said drill string element at the inner end of said spigot being provided with a shoulder that has a contact surface directed towards the free end of the spigot, said female thread being provided in a coupling sleeve that is connected to a second drill string element, and the coupling sleeve being provided with an internal stop surface, said male thread and said female thread being cylindrical. In a conventional thread coupling between two elements of a rock drilling equipment for percussive drilling a so-called bottom stop is normally used, see e.g. US-A-3,537,738 concerning coupling sleeves integral with one of the drill rods and US-A-3,645,570 concerning separate coupling sleeves. In a coupling sleeve integral with the drill rod a bottom stop means that the end of the drill rod abuts the bottom of the integral coupling sleeve. In a separate coupling sleeve a bottom stop means that the ends of the rods abut each other. Also for a coupling between a drill rod and a drill bit for percussive drilling so-called bottom stops are normally used. This is exemplified in US-A-1,959,919. It is common for the above exemplified types of thread couplings that damages often start upon the externally threaded element due to extensive bending moments during the drilling. So-called shoulder stops in thread couplings between two elements in a rock drilling equipment for percussive drilling are also known. This is exemplified in US-A-1,447,855. In this type of thread coupling a deterioration of the energy transfer can occur and also extensive local stresses can be present at the shoulder stop and/or at the bottom of the internal thread. From WORLD OIL, vol. 199, Oct. 1984, page 57, it is also known a thread coupling for rotary drilling, said thread coupling having a shoulder stop and during certain conditions a bottom stop, i.e. when the make-up torque is exceeded. However, during normal drilling conditions no bottom stop is established. The present invention has the aim to present a method to establish both shoulder and bottom stop in a thread coupling of the type mentioned above, said thread coupling providing acceptable stresses and also a god energy transfer. The aim of the present invention is realized by a method that has been given by the features of the characterizing part of claim 1. Further advantageous characterizing features of the invention will be appearent from the dependent claims. Below the invention will be described with reference to the enclosed figure that shows a schematic, partly sectioned view of a thread coupling according to the invention. The thread coupling of the figure includes a male thread 10 that is provided on a spigot 11 which constitutes an integral part of a first drill rod 12 for percussive drilling. At the inner end of the spigot 11 the drill rod 12 is provided with a shoulder 13 that has a plane contact surface 14 directed towards the free end of the spigot 11. The contact surface 14 has an extension perpendicular to a longitudinal centre axis 15 of the drill rod 12. The free end of the spigot 11 has the shape of a plane end surface 16 that has an extension perpendicular to the centre axis 15. The thread coupling also includes a female thread 17 that is provided in a sleeve 18 which constitutes an integral part of a second drill rod 19. Inside the sleeve 18 the other drill rod has a plane bottom surface 20 that has an extension perpendicular to the longitudinal centre axis 15 of the other drill rod 19, said longitudinal centre axis, in the figure, coinciding with the centre axis of the first drill rod 12. From the figure it is evident that in the thread coupling according to the present invention the contact surface 14 always contacts the free end of the sleeve 18, i.e so called shoulder stop is established. It is also evident from the figure that between the end surface 16 of the spigot 11 and the bottom surface 20 inside the sleeve 18 there is an initial play δ that nominally is chosen to be zero (0) but with a positive manufacturing tolerance in the magnitude of one (1) mm. After a certain time of use, at the most within 20% of the length of life for the thread coupling, this play will be eliminated due to wearing-in of the thread coupling and consequently the thread coupling according to the invention will have both shoulder and bottom stop. From the figure it is also evident that at the left end of the thread coupling a thread clearance 21 is arranged having the aim of evening out bending stresses from manufacturing inaccuracies of the shoulder or the threads. In addition the clearance 21 evens out torsion stresses and stresses from shock or tension waves that pass through the thread coupling. The principles of the invention can be used also in a thread coupling between a shank adapter and the first drill rod, between a drill rod and a drill bit and between two drill rods having a separate coupling sleeve.

Claims: Method to establish both shoulder and bottom stop in a thread coupling for percussive drilling including at least one male thread (10) and one female thread (17), said male thread (10) being arranged upon a spigot (11) that constitutes an integral part of a first drill string element (12), and said drill string element (12) at the inner end of said spigot (11) being provided with a shoulder (13) that has a contact surface (14) directed towards the free end of the spigot (11), said female thread (17) being a part of a coupling sleeve (18) that is connected to a second drill string element (19), the coupling sleeve (18) being provided with an internal stop surface (20), said male thread (10) and said female thread (17) being cylindrical, characterized in that when coupling the unused male thread (10) and the unused female thread (17) together there is initially established a contact between the shoulder (13) and the free end of the coupling sleeve (18), a contact between the end surface (16) of the spigot (11) and the internal stop surface (20) of the coupling sleeve (18) being established subsequently during drilling due to a wearing-in of the thread coupling, said wearing-in eliminating an initial play (δ) between the end surface (16) of the spigot (11) and the internal stop surface (20) of the coupling sleeve (18). Method according to claim 1, characterized in that the initial play nominally is chosen to be zero (0) with a positive manufacturing tolerance in the magnitude of one (1) mm. Method according to claim 1 or 2, characterized in that the wearing-in is completed after at most 20% of the length of life of the thread coupling.

Inventors: LARSSON KENNETH LARS, LARSSON, KENNETH LARS
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1989
Main classification: E21B17
IPC codes: E21B17, F16L15, F16B7