Abstract:
A device in a drill string component for percussive rock drilling including a thread for threading together with another drill string component including a complementary thread. The thread includes a thread groove formed by two thread flanks and an intermediate thread bottom. In operation one of the flanks forms a pressure flank. The thread groove has an essentially equally shaped sectional form along its axial extension. The thread bottom exhibits at least three surface portions with part-circular shape, as seen in an axial section. The surface portions with part-circular shape have increasing radiuses, as seen from each thread flank to an intermediate surface portion of the thread bottom. Also a thread joint and a drill string component.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Swedish patent application 1100398-5 filed 20 May 2011 and is the national phase of PCT/SE2012/050523 filed 15 May 2012. 
     FIELD OF THE INVENTION 
     The invention concerns a device in an drill string component for percussive rock drilling including a thread for threading together with another drill string component being provided with a complementary thread, wherein the thread includes a thread groove formed by two thread flanks and an intermediate thread bottom, and wherein the thread groove has an essentially equally shaped sectional form along its axial extension. The invention also concerns a thread joint and a drill string component. 
     BACKGROUND OF THE INVENTION 
     In order to joint drill string components for percussive rock drilling it is well known to use so called trapezoidal threads, wherein one end of a drill string component comprising a male thread is threaded together with a female thread at an end of the next drill string component. Alternatively, both ends of drill rods to be joined are threaded together over sleeves being provided with female threads at both ends. 
     Thread joints for drill string components or percussive rock drilling are subjected to high instantaneous loads and hostile environments. The threaded joints have to be drawn to moment levels that prevent loosening during operation, which means that large forces influence the respective thread wall of the male as well as the female thread. The working life length of the drill string components is related to the ability of the thread joints to resists the loads they are subjected to during operation. For that reason it is desired to provide threaded joints having the ability to better resists these loads and thereby give the components prolonged working life. 
     Another important aspect is producibility at low cost. 
     From U.S. Pat. No. 4,040,756 is previously known a threaded joint with inclined thread tops. When it concerns thread bottoms of the female as well as of the male thread, they are, however, provided with continuously curved configuration. The thread bottoms are going evenly, tangentially, over into adjacent thread flanks. 
     As further examples of the background art can be mentioned U.S. Pat. Nos. 6,196,598 and 4,687,368, whereof the latter concerns a more traditional trapezoidal thread. 
     THE AIM AND MOST IMPORTANT FEATURES OF THE INVENTION 
     It is an aim of the present invention to provide a further development of the devices according to the background art and to provide a device in a drill string component including a male thread as well as a drill string component and a method, wherein a resulting thread joint can be given greater resistibility to fatigue and thereby drilling can be performed with better economy. 
     These aims are obtained in respect of a device and a drill string component according to the above in that said thread bottom exhibits at least three surface portions with part-circular shape, as seen in an axial section, and that said surface portions with part-circular shape have increasing radiuses, as seen from each thread flank to an intermediate surface portion of the thread bottom. 
     Hereby is achieved that tension concentrations occurring in a most sensitive region of the thread, namely in connection to the root area of the pressure flank, will be widened and be levelled out as compared with the case of a conventional thread, irrespective if it concerns a trapezoidal type thread with partly linearly extending thread bottom or if it concerns a thread with evenly curved thread bottom. 
     According to the invention this is achieved in a thread that is economically advantages producible when the thread groove has an essentially equal shaped sectional shape along its extension and thereby can be produced with conventional production methods. Use of circular shapes of the surface portions results in simple and economically advantageous production. Determining the extension of the surface portions having part-circular shape in order to obtain the desired tension reducing effect can be made using conventional calculation methods. 
     It should be noted that a reduced tension concentration in this area of the thread bottom is very advantageous for the working life of the threaded joint, since already small reductions of the tension level in this area result in better resistibility to fatigue fractures and thereby thread failure. 
     Through the features of the invention it is achieved that the forces affecting the pressure flank, and to be received in the form of inner tensions in the material of the thread, will be received in a more advantageous way by the shape of the thread bottom being adapted for the reduction of forms that could increase tension concentrations. In particular it has unexpectedly been shown that the inventive construction results in extending tension distribution to at larger superficial portion of the thread bottom area and thereby reduced resulting maximal tension. In respect of the inventive construction, the surface portion having part-circular shape closest to the pressure flank is followed by a surface portion with greater radius or even with a plurality of surface portions having successively increasing radiuses, as seen in a direction towards an intermediate surface portion of the thread bottom. 
     A further advantage of the inventive construction compared to a conventional trapezoidal thread is that in a thread profile of a male thread according to the invention it will become easier to induce compressive stresses into the material through shot peening or through any other method that will plasticise material on a micro level, compared to the case in conventional threads. The reason for this is that accessibility for ejected shots will become better which leads to more even and more secure treatment of the thread. 
     Suitably said intermediate surface portion of the thread bottom is a central surface portion of the thread bottom. 
     It is preferred that said surface portions having part-circular shape are evenly passing over to one another and to an adjacent thread flank respectively. Hereby is intended that it exists tangential passages without tension inducing angles. It is preferred that they also pass directly over into each other without the intermediate or for example linear portions, since such passages do not contribute to tension level reduction. 
     Suitably a relation between a radius of a respective surface portion closest to a thread flank and a radius of a central surface portion of the thread bottom amounts to about 0.05-0.7, preferably to around 0.3-0.6, and most preferred to 0.35-0.55. 
     In one aspect of the invention, the thread groove, and in particular the thread bottom is asymmetrical as seen in said axial section. Asymmetric here means that the thread bottom as seen in the axial section lacks symmetry in respect of an imagined radial line (as indicated with interrupted line at L in  FIG. 2 ) between two adjacent thread ridges. 
     Hereby the thread bottom suitably comprises such shape that a surface portion having part-circular shape adjoining to a thread flank, which is opposite to the pressure flank, extend more inwardly towards a symmetry axis of the drill string component than a surface portion with part-circular shape joining to a thread flank, which comprises the pressure flank. Preferably a thread flank being opposite to the pressure flank extends longer towards the symmetry axis of the drill string component than a thread flank forming the pressure flank does. A ratio between a radius of the surface portion having part-circular shape adjoining to the thread flank forming pressure flank and a radius of the surface portion having part-circular form adjoining to the thread flank being opposite to the pressure flank is thus in this embodiment greater than 1 and suitably between 1 and 10. 
     It is preferred that the thread flanks have linear extension as seen in an axial section of the drill string component. 
     In an embodiment of the invention, the thread is a conical thread, wherein a suitable cone angle of the conical thread is a cone angle of 2°-8°. 
     Preferably the thread bottom, at least in a surface area adjoining to a thread flank not being the pressure flank is constructed with such depth in respect of a height of a thread top of the thread that, in case of an angular deviation of a connected thread joint, contact is established between an adjacent thread top of the male thread and a thread bottom of the female thread whereby contact in said surface area adjoining to the second thread flank with a thread top of said female thread is avoided. 
     Hereby is achieved that the properties of the thread joint for drilling with obliquely deviating joint are enhanced. 
     In a non insignificant obliqueness resulting from relatively strongly curved drill holes, the risk of fatigue fracture of the threaded joint with loss of the drill string component into the drill hole as the result would otherwise increase. 
     Through this aspect is achieved that also in case of obliquely deviating thread joints, contact is avoided between the thread top of the female thread and a thread root portion of the male thread, whereby contact thus is avoided where the greatest tension concentration would have resulted. Such contact would otherwise lead to heating in the contact area, wherein the material would be unhardened and easily damaged which tends to bring about drawbacks with fatigue fractures of the male thread, since potential tension concentrations would occur in a highly unwanted position. 
     Through the invention is achieved that tension reduction and avoiding contacts in the bottom of the male thread according to the above in an advantageous manner can be combined, and in particular in respect of asymmetrically shaped thread grooves, the male thread can be shaped to satisfy principally different aims closest to the respective thread flank. Hereby the thread can be shaped such that a smallest cross section of the thread can be made greater compared to what would be the case according to the background art. 
     In a particularly preferred aspect, the thread is a male thread for threading together with a complementary female thread arranged on another drill string component. The thread is however, also advantageously a female thread for threading together with a complementary male thread arranged on another drill string component. 
     The invention also relates to a thread joint including a male thread and a female thread, wherein the thread joint is constructed with a device according to the above. 
     The invention further concerns a drill string component from the group: a drill bit, a drill rod, a joining sleeve, a shank adapter, which includes at least one device according to the above. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The drawings show: 
         FIG. 1  diagrammatically different drill string components equipped with inventive devices, 
         FIG. 2  a part of an axel section through a thread joint, 
         FIG. 3  shows a detail of a thread joint according to  FIG. 2  in an oblique condition, 
         FIG. 4 a    is a computer simulation of a load situation and the figure shows a partial section through the inventive thread profile with indication of obtained tension distribution over the area of and below the thread bottom, 
         FIG. 4 b    is also a computer simulation of a load situation and the figure shows a partial section through a conventional thread profile with indication of obtained tension distribution over the area of and under the tread bottom, 
         FIG. 5  shows in an axial section a conical thread joint according to the invention, and 
         FIGS. 6 a - c  and 7 a - b    show details of a thread according to the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The group “drill string components”, being intended with this invention, includes drill bits, drill rods, shank adapters, joining sleeves and transfer adapters. 
     In  FIG. 1  are shown different exemplary drill string components and parts thereof with devices according to the invention, namely: 
     a) A drill rod with a male thread as well as with a female thread. 
     b) A shank adapter with a male thread. 
     c) A part of a drill rod with a conical thread. 
     d) Parts of drill rods with a male thread and a female thread, respectively. 
     e) A drill bit with a female thread. 
     f) A joining sleeve with two female threads. 
     The thread joints are for threading together drill string components for percussive drilling. Inside the drill string components extends axially continuously a flushing channel for transfer of flushing flow to the drill bit. 
     In  FIG. 2  is shown a detail of an inventive thread joint, wherein a first thread ridge  7  and a second thread ridge  7 ′ are shown belonging to the male thread. A thread ridge of the female thread is indicated with  8 . Between the thread ridges  7  and  7 ′ is positioned a thread groove  9 , that receives the thread ridge  8  in turn having an essentially equal sectional shape along its extension. 
     The thread groove  9  has a thread bottom  12 , exhibiting a plurality of surface portions having part-circular shape, which pass over into each other, namely a first surface portion Y 1  having a radius RA closest to a first thread flank  10  comprising pressure flank, a second surface portion Y 2  having a radius RC closest to a second thread flank  11  and an intermediate, here central, surface portion YC having a radius RB. The surface portions pass evenly, that is tangentially, over into each other. This is preferred, even if it can exist linear passages and also, which is not recommended uneven passages having angular steps between the surface portions. The treaded flanks form the same angle to a symmetry axis of the thread. Normally, this angle is 35°. In certain cases other angles can exist for example 45°. 
     The radius RB of the central surface portion YC is greater than both radiuses closest to the two thread flanks  10  and  11 . Suitably there is a ratio between RA respectively RC and RB of about 0.05-0.70, preferably of about 0.30-0.60, and most preferred of 0.35-0.55. Hereby the advantages are obtained relating to reduced tension levels that are presented in the above introductory part of the description. 
     In one aspect of the invention, the thread bottom  12 , in a surface area adjoining to the second thread flank  11  exhibits such a depth in respect of a height of the thread ridge  7 ′, that in the event of an oblique position of the threaded joint, contact is established first between the thread top of the thread ridge  7 ′ of the male thread with a thread bottom  13  of the female thread. 
     An imagined radial line between two adjacent thread walls is indicated with L. A smallest cross sectional radius with B.  6  indicates a flushing channel. 
     This is more evident in detail from  FIG. 3 , wherein an oblique positioning has occurred between the female thread  3  and the male thread  2 . A central axis of the female thread is indicated with double-pointed line whereas a central axis of the male thread is indicated with a single-pointed line. The oblique position is in  FIG. 3  exaggerated for clarity and is indicated with the angle α. 
     As is shown from  FIG. 3 , contact has thus been established in the area  13 - 14 , which corresponds to contact having been reached by the thread top  14  of the male thread having come to contact with the thread bottom at  13  of the female thread. In the surface area indicated with  15  being the root area of the thread ridge of the male thread, at the second thread flank  7 ′, there is no contact between the thread top of the female thread and the thread bottom of the male thread, which results in that burning or pitting in this area is avoided, whereby thus is avoided the otherwise resulting above discussed heating of the area  15 , unhardening of the same and risk of fatigue damages to the male thread. 
       FIG. 4 a    shows obtained tension distribution of the area of and below a thread bottom of a male thread  2 , which is screwed together with a not shown female thread. The figure shows a number of lines indicating the same tension, wherein the numerals (60%-85%) concerns percentage of the maximal tension which will occur in a comparative thread being a conventional trapezoidal thread according to  FIG. 4 b   . It should be noted that load distribution is the same for the two comparative objects in the  FIGS. 4 a    and  4   b.    
     As is evident from  FIG. 4 a   , the greatest tension concentration is localized to a superficial area relatively close to the pressure flank  10 . The maximal registered tension is somewhat over 85° of the maximal tension occurring in respect of the comparative thread. From the figure it is also evident that the tension is distributed over a great area and that the tension extends without jumps and unevennesses. 
     As is clear from  FIG. 4 b   , the greatest tension concentration is also here localized to a superficial area being relatively small and being relatively close to the pressure flank  10 . The maximal registered tension in respect of the comparative thread is of course by definition 100%. From the figure it is evident also that the tension is distributed in a smaller area. 
     A comparison between the results in  FIGS. 4 a  and 4 b    reveals that the maximal tension in the root area of a thread according to the invention has been reduced with up to 12-15%. Further, it is evident that the tension distribution in respect of the conventional thread, in spite of the higher tension level, is more concentrated, which results in higher effect on the material during load. 
     This good result for a thread according to the invention is surprising and indicates at the considerable enhancement can be expected as concerns resistance to fatigue of the device according to the invention in comparison to the conventional thread. 
     In  FIG. 5  is shown a conical thread according to the invention with a male thread  2  and a female thread  3 . Also in this embodiment the respective thread groove has an essentially equal sectional shape along its extension. 
     In  FIG. 6 a    is shown a detail of a thread groove with two surface portions having part-circular shape (with the respective radius RA and RB) passing evenly and directly over into each other. In  FIG. 6 b    is shown a detail of a thread groove having two surface portions with part-circular shape (with respective radius RA and RB) passing directly over into each other over a linear portion indicated with x. In  FIG. 6 c    is shown a detail of a thread groove having two surface portions with part-circular shape (with the respective radius RA and RC) passing unevenly over into each other, there is no tangential passage at the unfilled arrow. 
     In  FIG. 7 a    is shown a detail of a thread groove having a thread flank  10  and a surface portion with part-circular shape (with radius RA) passing evenly and directly over into each other. In  FIG. 7 b    is shown a detail of a thread groove having a thread flank  10  and a surface portion with part-circular shape (with radius RA) passing unevenly over into each other, that is no tangential passage at the unfilled arrow. 
     The present invention relates to drill string components for percussive rock drilling with contact surfaces between a male thread and a female thread, wherein particular and hard requirements on the behaviour of the thread is crucial for reliable function. Generally seen is intended that the present thread provides flank angles of 20°-50° and more preferred about 22.5°-47.5°. As is indicated above a usual value of the flank angle is 35° but other angle values are preferred in certain cases, for example 45°. 
     Because of the demanding conditions for devices according to the invention, drill string components for percussive rock drilling, the thread in question is relatively shallow having a relationship between (thread) profile height and (thread) profile width of 0.10-0.30, more preferred 0.20-0.30 and most preferred 0.23-0.25. With profile height is here intended the distance from the bottom of the thread (the thread groove) to the thread top, and with profile width is here intended the distance between the intersections of two straight lines extending in a central axial section along two adjacent thread flanks in a thread groove with a thread top line. 
     A ratio between profile height (see above) and pitch of the thread in an inventive device is generally 0.05-0.25 and more preferred 0.13-0.17. 
     The invention is adaptable in different types of drill string components and is particularly advantageous in respect of male threads, since in respect of these, tension concentrations are of particularly great importance for the working life, simple because of the construction of the male threads and their relative sensitivity to load. Female threads in sleeve forms can be shaped with greater inherent resistance to load.