An elongate drill component for percussive drilling includes a female threaded end having an outer diameter (Dot), a male threaded end, and a central section between the ends having an outer diameter (Doc). The outer diameter of the female threaded end is larger than the outer diameter of the central section. A through-going flushing channel includes a central flushing channel having a diameter (Dic) and thread flushing channels having a diameter (Dit). The ratio of an elastic section modulus of a threaded joint measured when the female threaded end is connected to the male threaded end of an identical component to an elastic section modulus of the central section, St/Sc, is more than 1.5 and in that a ratio of the diameter of the central flushing channel to the outer diameter of the central section, Dic/Doc is between 0.45 and 0.95.

RELATED APPLICATION DATA 
     This application is a §371 National Stage Application of PCT International Application No. PCT/EP2014/068118 filed Aug. 27, 2014 claiming priority of EP Application No. 13183523.3, filed Sep. 9, 2013. 
     TECHNICAL FIELD 
     The present invention relates to percussive extension drilling and, in particular, to a drill string component for use in such drilling. 
     BACKGROUND 
     Extension drilling typically involves the use of a drill bit mounted at the end of a drill string which is both rotated and subjected to longitudinal impacts. The upper end of the drill string is connected to an above-ground drilling machine which performs the rotation and imparts the impact. Such a percussive drilling technique is commonly referred to as bench drilling. The present invention can also be used in, for example, long-hole drilling, drifting and tunneling. 
     A general description of percussive drill strings comprising rods are addressed in, for example, U.S. Pat. No. 6,164,392 and U.S. Pat. No. 6,681,875. These are examples of the most used type of drill string in percussive drilling. However, these rods tend to reduce the drilling speed and drilling accuracy, as well as increase the risk of the bit becoming stuck in the ground. Prior rods are prone to overheating and subsequent failure of the thread joints. 
     A drill string comprising tubes is addressed in European Patent Number 126740. Tubes in comparison to rods are tubular in shape, i.e. the wall thickness is small as compared to the diameter of the tube. With the known solution the flushing properties improve and the elastic section modulus is increased, compared to conventional drill rods, leading to improved drilling accuracy. However, the increased elastic section modulus of the tubes leads to higher stresses in the joints. This has the effect that the joints are difficult to break. The larger outer diameter of the tube, compared to conventional drill rods, decreases the gap between the drill string and the wall of the hole being drilled, leading to problems with transportation of cuttings from the hole. 
     The present invention combines the advantages of rods and tubes into a new component. 
     SUMMARY 
     One object of the present invention is to provide a threaded drill rod with improved stiffness. 
     Another object of the present invention is to provide a threaded drill rod with improved flushing. 
     According to an aspect, there is provided an elongate drill component for percussive drilling comprising a female threaded end having an outer diameter D ot , a male threaded end, a central section between said ends having an outer diameter D oc , wherein the outer diameter of the female threaded end is larger than the outer diameter of the central section; and a through-going flushing channel comprising a central flushing channel having a diameter D ic  and thread flushing channels having a diameter D it , wherein the ratio of an elastic section modulus of a threaded joint measured when the female threaded end is connected to the male threaded end of an identical component to an elastic section modulus of the central section, S t /S c , is more than 1.5 and wherein a ratio of the diameter of the central flushing channel to the outer diameter of the central section, D ic /D oc  is between 0.45 and 0.95. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicate similar elements and in which: 
         FIG. 1  schematically shows a prior art drilling machine for percussive top hammer drilling, in a side view. 
         FIG. 2  schematically shows a prior art conventional drill string, in a side view. 
         FIG. 3  schematically shows a longitudinal cross-section of an embodiment of the invention. 
         FIG. 4  schematically shows a longitudinal cross-section of an embodiment of the invention. 
         FIG. 5  schematically shows a longitudinal cross-section of an embodiment of the invention in a connected state. 
         FIG. 6  shows a chart illustrating ratios of elastic section modules and ratios of inner and outer diameter of the invention compared with prior art. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates how a hole is made using a conventional bench drilling machine  1 . A drill string  2  connected to a drill bit, is rotated and impacted into the rock  3 , thus drilling a hole  4 . 
       FIG. 2  illustrates a conventional drill string  2  used for percussive drilling. The drill string  2  is connected to and extending from an adapter  18  at the drilling machine (not shown), and at least one rod  10  (but usually a series of rods) connecting the adapter to the drill bit. In a rod drilling machine, each rod  10  has a male screw thread  12  at one end and a female screw thread  14  at the other end. The uppermost rod  10 ′ has its female thread  14  connected to a male screw thread  16  of the adapter  18 . The remaining rods  10  are joined together in series. The lowermost rod  10 ″ has its male thread attached to a female screw thread of a drill bit  19 . The adapter  18  and the rods  10  have respective central passages extending therethrough and aligned with one another for conducting flushing fluid (usually water and/or air) which exits through outlets formed in a front face of the drill bit to cool the inserts and flush-away cuttings. The cuttings, along with the flushing fluid, are discharged upwardly through a gap formed between the drill string and the wall of the hole being drilled. 
       FIG. 3  illustrates an embodiment of the present invention. An elongate component  20  has an end  21  and an end  22 . The end  21  has a female thread  23  and the end  22  has a male thread  24 . The female thread  23  and the male thread  24  are cylindrical and comprise helical ridges and grooves and preferably have trapezoidal or rope geometries. The smallest radius of the female thread  23  and the male thread  24 , in a cross-section along the longitudinal axis of the elongate component  20 , is preferably larger than 1.5 mm. 
     Next to the end  21  the elongate component  20  has a slim section  25  with small outer diameter compared to the rest of the elongate component. Next to the end  22  the elongate component  20  has a slim section  26  with small outer diameter compared to the rest of the elongate component. Between the slim section  25  at the end  21  and the slim section  26  at the end  22  there is a central section  27 . The outer diameter of the central section is larger than the outer diameter of the end  22  and smaller than the outer diameter of the end  21 . The central section  27  of the elongate component  20  has an outer diameter D oc . The end  21  has an outer diameter D ot . 
     There is a through-going flushing channel in the centre of the elongate component  20 . Flushing media like water and/or air are flushed in this channel in order to cool the elongate component and its joints and in order to remove cuttings from the drilled hole. The through-going flushing channel is divided into several parts, a central flushing channel  28  with a diameter D ic  and thread flushing channels  29 , at both the end  21  and the end  22 , with a diameter D it . Between the central flushing channel  28  and both thread flushing channels  29  there are steps  30  such that D it  is smaller than D ic . In order to have a smooth flow of the flushing medium the steps  30  are preferably smooth with no sharp edges. 
     The end  22 , the male thread  24 , the slim section  26  and a part of the central section  27  is preferably manufactured from a single piece of material into a male component  32 . A ratio of the length of the male component  32  to the outer diameter, D oc , of the central section  27  is between 3 and 5. The end  21 , the female thread  23 , the slim section  25  and a part of the central section  27  is preferably manufactured from a single piece of material into a female component  33 . A ratio of the length of the female component  33  to the outer diameter, D oc , of the central section  27  is between 3 and 5. The middle part of the central section  27  is preferably manufactured from a single piece of material. The male component  32  and the female component  33  may be manufactured from a different type of material than the middle part of the central section  27 . The middle part of the central section  27  is preferably friction welded to the male component  32  and the female component  33 . 
       FIG. 4  illustrates an embodiment of the present invention. The elongate component  120  has a through-going flushing channel with substantially constant diameter from the end  22  to the female thread  23 . This implies that the through-going flushing channel is continuous and stepless and the diameter, D ic , of the central flushing channel  128  is substantially equal to the diameter, D it , of the thread flushing channels  129 . 
     The central flushing channel  128 , with diameter D ic  has a cross-sectional area A c . An exit area A e  for the cuttings to be flushed away from the hole  4 , is formed between the central section  27  and the wall  5  of the hole  4 . A ratio of the area A c  of the central flushing channel  128  to the exit area A e  is preferably between 0.04 and 0.26. This relationship between A c  and A e  ensures that there is sufficient space between the elongate component  120  and the wall  5  of the hole  4  for removal of cuttings from the hole  4 . 
       FIG. 5  illustrates an embodiment of the present invention in a connected state. The male thread  24  of an elongate component  20  is screwed into the female thread  23  of another elongate component  20 ′, thus connecting elongate component  20  with elongate component  20 ′. The interconnecting area between elongate component  20  and elongate component  20 ′ forms a thread joint  31 . In a drill string, several elongate components are connected in this way. 
     What characterizes the invention is the elastic section modulus of the thread joint  31  in relation to the elastic section modulus of the central section  27  in combination with the ratio between the diameter of the central flushing channel  28 ,  128  and the outer diameter of the central section  27 . The elastic section modulus is a property that correlates to the bending resistance of a cross section perpendicular to the longitudinal axis of the elongate component  20 . 
     The elastic section modulus, S t , of the thread joint  31  is calculated using the following formula: 
     
       
         
           
             
               S 
               t 
             
             = 
             
               
                 π 
                 * 
                 
                   ( 
                   
                     
                       D 
                       ot 
                       4 
                     
                     - 
                     
                       D 
                       it 
                       4 
                     
                   
                   ) 
                 
               
               
                 32 
                 * 
                 
                   D 
                   ot 
                 
               
             
           
         
       
     
     The elastic section modulus, S c , of the central section  27  is calculated using the following formula: 
     
       
         
           
             
               S 
               c 
             
             = 
             
               
                 π 
                 * 
                 
                   ( 
                   
                     
                       D 
                       oc 
                       4 
                     
                     - 
                     
                       D 
                       ic 
                       4 
                     
                   
                   ) 
                 
               
               
                 32 
                 * 
                 
                   D 
                   oc 
                 
               
             
           
         
       
     
     The ratio between the elastic section modulus's, S t /S c , is for the elongate component more than 1.5, preferably between 1.8 and 5.5 and more preferably between 2.0 and 3.5. This ratio is for known drill rods between 1.8 and 3.2 and for known drill tubes between 1.0 and 1.4. 
     The ratio between the diameter of the central flushing channel  28 , 128  and the outer diameter of the central section  27 , D ic /D oc , is for the elongate component between 0.45 and 0.95, preferably between 0.48 and 0.75 and more preferably between 0.5 and 0.65. This ratio is for known drill rods between 0.25 and 0.42 and for known drill tubes between 0.55 and 0.75. 
       FIG. 6  illustrates a graph of S t /S c  versus D ic /D oc . Known drill rods are located in area A, known drill tubes are located in area B and elongate components, according to the present invention, are located in area C. Note that area C extends to infinity in the S t /S c  direction. 
     Utilizing this specific combination of S t /S c  and D ic /D oc  gives certain advantages to the drill string. The large diameter of the central flushing channel  28 , 128 , in relation to the outer diameter of the central section  27 , gives good flushing properties which improves the removal of cuttings and cools the threads in the elongate component  20 . Cooling of the threads increases the lifetime of the threads and the drill rate can be increased. The space between the wall  5  of the hole  4  and the outer diameter of the central section  27  is large enough to enable sufficient flushing of the cuttings from the drilled hole  4 . The high elastic section modulus of the thread joints, in relation to the elastic section modulus of the central section  27 , implies that the drill string may bend without causing too high bending stresses in the thread joints. This improves the lifetime of the thread joints and makes it easier to break the threads between the elongate components. 
     Example Embodiments and Test Results 
     A surface drill rig was used to compare conventional so called T51 drill rods with an embodiment of the invention. Both the conventional T51 rods and the embodiment of the invention were manufactured from case hardened steel. The same type of threads and the same type of heat treatment were used for the T51 rods and the embodiment of the invention. The pressure drop along the length of the drill string was measured as a measure of the flushing efficiency. The following results were obtained. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Pressure 
               
               
                   
                 D oc  (mm) 
                 D ic  (mm) 
                 D ic /D oc   
                 S t /S c   
                 drop (Bar) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Conventional 
                 52.0 
                 21.5 
                 0.41 
                 2.6 
                 4.2 
               
               
                 T51 rod 
               
               
                 Embodiment 
                 56.4 
                 31.0 
                 0.55 
                 2.1 
                 2.3 
               
               
                 of invention 
               
               
                   
               
             
          
         
       
     
     It is evident that the pressure drop is much smaller for the invention compared to the conventional rod. 
     The lifetime of drill strings were compared while drilling in an underground mine. Conventional so called T45 rods were compared with an embodiment of the invention. Both the conventional T45 rods and the embodiment of the invention were manufactured from case hardened steel. The same type of threads and the same type of heat treatment were used for the T45 rods and the embodiment of the invention. The following results were obtained. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                   
                   
                 Lifetime (drilled 
               
               
                   
                 D oc  (mm) 
                 D ic  (mm) 
                 D ic /D oc   
                 S t /S c   
                 meters) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Conventional 
                 45.8 
                 17.0 
                 0.37 
                 2.6 
                 1800 
               
               
                 T45 rod 
               
               
                 Embodiment 
                 48.3 
                 28.3 
                 0.59 
                 2.4 
                 3800 
               
               
                 of invention 
               
               
                   
               
             
          
         
       
     
     It is evident that the lifetime of the invention is at least 100% longer than for the conventional rod. 
     The present invention is not limited to the above described embodiments. Different alternatives, modifications and equivalents might be used. The above mentioned embodiments should therefore, not be considered limiting to the scope of the invention, which is defined by the patent claims.