Abstract:
In percussion rock drilling a screw joint for a drill run or drill string including male and female screw threads ( 5, 9 ) on the elements ( 1, 2 ) are joined together to form a drill string. Within the screw joint the male thread ( 5 ) and the female thread ( 9 ) have a trapezoidal shape and a slight conical inclination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority and incorporates by reference PCT/SE03/00752 filed May 9, 2003 and Swedish Patent Application 0201530-3 filed May 22, 2002. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
     None. 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw joint for a drill run for percussion rock drilling. 
     2. Description of the Related Art 
     Screw joints of this nature are used to join together drill string elements, such as a drill bit, one or more drill strings and a shank adapter. The screw joints include mutually co-acting male and female screw threads where an external male thread on a component element is screwed to an internal female thread on a co-acting element, so as to join said elements together. 
     Swedish Patent Specification 515518 teaches a screw joint of the aforesaid kind in which the male and the female threads are conical and the tops and bottoms of the thread profiles have a radius of curvature that is greater than 30% of the thread pitch. Such a screw thread is referred to typically as a rope thread. 
     Although such a thread functions satisfactorily in the case of drill strings of more slender dimensions, problems occur as the dimensions of the drill strings increase, resulting in shortening of the useful life of the thread. This is mainly due to the tensile stresses acting on the internal thread, causing the screw joint to be tightened excessively, or over-tightened. In turn, this makes it difficult to loosen or disconnect the joint when drilling has been terminated or when fresh drill rods shall be connected up. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a novel screw joint with which the aforesaid problems are eliminated, so as to enable the novel screw joint to be used also with drill runs or strings of coarser dimensions. 
     This object is achieved with an inventive screw joint in which the male and female threads have a trapezoidal shape with a slight conical inclination. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will now be described in more detail with reference to a non-limiting embodiment thereof, illustrated with the aid of the figures of the accompanying drawing, in which 
         FIG. 1  is a schematic broken longitudinally sectioned view of a drill rod that has a male and a female end; 
         FIG. 2  shows part of the profile of an internal screw thread; 
         FIG. 3  shows a corresponding part of the profile of an external screw thread; 
         FIG. 4  is a schematic broken longitudinally sectioned view of a drill rod that has two male ends; and 
         FIG. 5  is a schematic broken longitudinally sectioned view of a drill rod that has two female ends. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a broken longitudinally sectioned view of a drill rod, showing in particular both ends of the rod. The male end  1  and the female end  2  are connected, suitably welded at  3 , to an intermediate element  4  so as to obtain an elongate drill rod. A drill rod of this kind may typically have a length of 6 meters. A drill string may consist of a number of such rods that have been screwed together with the aid of respective screw threads. As before mentioned, the drill string includes a drill bit at one end and a shank adapter at the other end, which are also joined to the drill rods by means of corresponding screw threads. 
     As will be seen from  FIG. 1 , the male end  1  is provided with an external screw thread or male thread  5 . It will also be seen from the figure that the male thread is conical and is inclined towards the centre axis  6  (shown in chain lines) of the drill rod. The male end  1  has an end surface  7  that can be used as a stop surface, as described in more detail below. As will also be seen from the figure, the female end  2  includes an internally extending hole  8  in which the male end  1  of another drill rod can be received. The hole  8  has an internal thread, or female thread  9 , which is intended to be screwed together with the male thread  5  of another element to be connected to the element carrying the female thread. The female thread is therefore also slightly conical and has the same angle of inclination as the centre axis  6  (shown in chain lines) of the drill rod. Located at the bottom of the hole  8  is an abutment surface against with the end surface  7  of the male part  1  can bottom when screwing two elements together. 
     As shown, the female part  2  may be provided at its free end with a generally cylindrical contact surface  11  that surrounds the mouth of the hole  8 . The male part  1  may then be provided with a corresponding contact flange at a distance from the end of the male part that is shorter than the depth of the hole  8 , so that the male and female parts will reach end positions as a result of the contact between the contact surface and the contact flange, instead of the end surface  7  of the male part coming into contact with the abutment surface  10  of the female part. 
       FIG. 2  shows part of an internal thread  9  of a female part  2 . The flanks or edges  9   a ,  9   b  of the thread  9  define angles α and β, respectively, with the line  12  tangential to the apices  9   c  of the threads. The difference between the angles, α−β, is equal to two times the angle γ between the line  12  and the centre line  6 . This means that the flanks  9   a ,  9   b  define the same angle φ in relation to the centre line  6 . The flanks are also symmetrical with a straight central line between the flanks that also is perpendicular to the centre line  6 . 
       FIG. 4  is a drill road similar to  FIG. 1  except that the male end  1  as in  FIG. 1  is shown on both ends  1  of the drill rod.  FIG. 5  shows the corresponding drill road with two female ends  2 . 
     It is not necessary that the angles of the flanks to the centre line are exactly the same, but it is preferred that the flanks define essentially the same angle to the centre line  6 . 
       FIG. 3  shows a view similar to that shown in  FIG. 2 , but in this case showing a part of the outer thread  5  of a male part  1 . Also this thread  5  defines flanks or edges  5   a  and  5   b , that define angles β and α, respectively, with the line  13  tangential to the apices  5   c  of the threads. The difference between the angles, β−α, is equal to two times the angle γ between the line  13  and the centre line  6 . This also means that the flanks  5   a ,  5   b  define the same angle in relation to the centre line  6  for the same reason as that mentioned above. 
     Dimensions for which the inventive screw joint is suited are preferably thread diameters in the order of about 38-80 mm. This corresponds to drill hole diameters in the order of 64-200 mm. However, the screw joint is not restricted to those dimensions, but can be used for both finer and coarser dimensions. 
     The thread flank angles α, β are smaller than 45°, preferably between 30 and 40°, in relation to the respective lines  12 ,  13  that are tangential to the apices of the flanks or edges of the thread. The cone angle is preferably smaller than 7°, preferably about 4°. With a cone angle of 4°, the angle γ in the figures will thus be 2°. 
     An inventive thread design reduces the risk of the drill string breaking under the bending stresses to which it is subjected in the drill hole. The conical configuration of the screw thread enables the material thickness to be increased in the inner parts of respective threads, with a corresponding reduction in the risk of a breakage or fracture occurring. 
     The screw joint according to the present invention can be readily unscrewed when the drill string or drill run is withdrawn from the drill hole, but is nevertheless sufficiently stable to ensure that the screw threads will not loosen in the hole. 
     As shown in the figures and described above, the screw joints are constructed so that the drill strings have a female thread at one end and a male thread at the other end. However, it will be understood that the inventive screw joint can also be used in conjunction with splicing sleeves, although screw joints that are fully integrated in the drill strings are preferred since this eliminates the risk of damage due to loosening splicing sleeves.