EP0979922B1 - Tube rod - Google Patents

Abstract

A tube rod for rock drilling has a female threaded end (12) into which a male thread (either on a male connector or on the end of another tube rod) can be screwed to form part of a drill string. The female threaded end (12) is (a) of larger external diameter than the central section (10) of the rod and (b) has an annular groove (42%) with an asymmetrical profile, the flank of this groove nearest to the threaded end being steeper than the other flank. <IMAGE>

Description

• This invention relates to a tube rod for rotary/percussiverock drilling, and in particular to the way twotube rods are connected one to the other by threads. Tuberods are conventionally connected end to end by threadswhich allow rods to be screwed together (and unscrewed) toform a drill string which may consist of a large number ofconnected rods.
• Tube rods, and drill strings made up from tube rods,operate in a harsh environment. Considerable stresses arebuilt up in the rods, and these usually manifestthemselves in cracking of the rods in the area of thethreaded joints. This cracking usually leads to failureof the respective rod. It is therefore desirable todesign the threaded ends of the rods in a way which willminimise stress build up at any particular point, inparticular to minimise stress build up at the bottom ofthe female threads.
• Document EP 0 220 147 relates to a thread coupling forpercussion drilling. The drill rods comprise a male threadand a co-operating female thread. Document GB 2 285 645relates to a stress relief groove for a drill pipe. Thegrove is located in the weld neck.
• According to a first aspect of the invention, there isprovided a tube rod for percussion rock drilling havingthreaded ends, the tube rod having a hollow bore and atleast one of the ends having a female thread, the majorpart of the tube rod length between the threaded endshaving an outside diameter (the first diameter) and atleast part of the or each female threaded end having anoutside diameter (the second diameter) which is largerthan the first diameter, and an annular groove between the female threaded end and the major part of the tube rod length, the annular groovehaving a root diameter (the third diameter) which is lessthan both the first and the second diameter, the hollowbore of the rod having a region between the femalethreaded end and the major part of the tube rod lengthwhere the internal diameter is less than at either side ofthis region.
• The groove preferably has an asymmetrical profile whenseen in cross section parallel to the tube axis, and theflank nearest the threaded end is preferably steeper thanthe opposite flank.
• The adoption of these features has been shown to result ina reduction in stress build up at the bottom of the femalethreaded end.
• It is preferred for the female end to be of the seconddiameter from its end up to the annular groove. Howeverit is within the scope of the invention for the female endto be of the second diameter only in a region surroundingthe bottom of the threaded end which is where the majorstress concentration occurs, or for the female end to beof the second diameter on both sides of the annulargroove.
• In most cases, the rod will have one male threaded end andone female threaded end. However rods with two femalethreaded ends are conceivable, to be joined end to end bya tubular connector with two male threaded ends.
• The hollow bore of the rod may be reduced in internaldiameter at the inner end of the female threaded socket toform a neck, and the bore diameter tapers outwards fromthe neck to the full bore diameter of the major part ofthe tube rod length.
• The internal diameter of the rod has a region between thefemale threaded end and the major part of the tube rodlength where the internal diameter is less than at eitherside of this region. The internal diameter of the tube rodcan taper from the major part of the tube rod length downto this region of lesser internal diameter. The internaldiameter of the tube rod can also taper from the femalethreaded end down to this region of lesser internaldiameter.
• The female threaded end can have an internal shoulder atright angles to the tube axis, with the internal diameterof the tube rod tapering down from the shoulder to theregion of lesser internal diameter. The taper of theinternal diameter can be a conical taper but is preferablya radiused taper.
• The neck or region of lesser diameter of the internal borepreferably lies on substantially the same cross-section ofthe rod as the third (external) diameter of the rod.Preferably the third (external) diameter of the rod lieson a cross-section of the rod which is offset from thecentre of the neck or region of lesser diameter in thedirection towards the major part of the tube rod length.
• The threaded ends can be initially separate from the rodand can be joined to a length of constant cross sectiontube by friction welded joints, the length of constantcross section forming the major part of the tube rodlength. This is a particularly expedient method ofmanufacturing which allows standard tube stock to be usedfor the major part of the rod, with only the endsrequiring machining.
• The threaded region of the male end can extend from an annular shoulder, the shoulder being adapted to abutagainst the end of the female threaded end of another rodwhen the two rods are screwed together. The male threadedregion is preferably shorter than the female threadedrecess so that when the male threaded end is screwed intothe female threaded end of another rod until the shoulderabuts the end of the other rod, the male end does notreach the bottom of the female recess. This helps toavoid tensile pre-loading of the joint.
• The annular shoulder can be stepped and the mating end ofa female threaded end section of another rod can have ashoulder such that when the rods are screwed together, theshoulders interengage to prevent substantial radialmovement of one shoulder relative to the other. Thishelps to prevent relative radial movement at the shoulderwhich in turn helps to reduce bending of the assembledstring of rods.
• This feature can represent an independent invention, andthus according to a second aspect of the invention, thereis provided a tube rod assembly for percussion rockdrilling, the assembly including a plurality of rodsconnected end to end by joints comprising interengagingmale and female threads wherein when the joints areassembled, an end of a female threaded component is incontact with an external shoulder on the adjacent malethreaded component, the external shoulder having acircumferentially extending stepped land, with part of theend of the female threaded component fitting around theland to hinder relative radial movement between the twoends at the position of the shoulder.
• All the features set out above, either alone or incombination, increase the resistance of the tube rod tocracking, especially cracking at the last thread root atthe female end. The addition of the tapered internalbore, the increased outside diameter at the thread portionand the external groove all contribute to dispersing thestresses arising at the thread and internal profile of thefemale threaded part so that the stresses at thesecritical region are reduced.
• According to a second aspect of the invention, there isprovided a tube rod having threaded ends, at least one of the ends having a female thread, the major part of thetube rod length between the threaded ends having anoutside diameter (the first diameter) and the or eachfemale threaded end having an outside diameter (the seconddiameter) which is larger than the first diameter, whereinbetween the female threaded end and the major part of thetube rod length there is an annular groove having a rootdiameter (the third diameter) which is less than both thefirst and the second diameter, and the internal diameterof the rod having a reduced diameter neck, the narrowestpart of which coincides with the location of the thirddiameter.
• According to a third aspect of the invention, there isprovided a tube rod having threaded ends, at least one ofthe ends having a female thread, the major part of thetube rod length between the threaded ends having anoutside diameter (the first diameter) and the or eachfemale threaded end having an outside diameter (the seconddiameter) which is larger than the first diameter, whereinbetween the female threaded end and the major part of thetube rod length there is an annular groove having a rootdiameter (the third diameter) which is less than both thefirst and the second diameter, and the internal diameterof the rod having a reduced diameter neck, the internaldiameter of the rod tapering into the neck from both sidesof the neck.
• The invention also extends to a female threaded tube rodend for joining to a length of tube to contribute to theformation of a tube rod as set forth above.
• The invention will now be further described, by way ofexample, with reference to the accompanying drawings, inwhich:

Figure 1

shows a tube rod in accordance with theinvention, with the central sectionforeshortened;

Figure 2

is a cross section through the femalethreaded end;

Figure 3

is a cross section through the malethreaded end;

Figure 4

is a cross-section through an alternativemale threaded end:

Figures 5 and 6

are cross-sections through twoalternative female threaded ends: and

Figure 7

shows a second embodiment of tube rod inaccordance with the invention.

• Figure 1 shows a rod which has acentral section 10, afemale threadedend section 12 and a male threadedendsection 14. Thecentral section 10 will typically be 1.5to 2 metres in length (but can be up to 6 metres long),whilst the male and female threaded end sections willtypically each have a length of about 250 millimetres.
• Thecentral section 10 is a tube having a constant crosssection over its length. Typically the outside diameterwill be about 80 to 90 millimetres, and the insidediameter about 65 to 75 millimetres, leaving a wallthickness of about 15 millimetres.
• Theend sections 12 and 14, both of which have a throughbore, will be welded to thecentre section 10 at 16 and18. This welding can appropriately be achieved through afriction welding process.
• The female threadedend section 12 shown in Figure 2 willnormally be machined from solid material. Suitable steelsare a 3% NiCrMo grade steel or a 3% CrMo grade steel. Thesection 12 has an internally threadedend 20, and atubeend 22 which is to be welded to thecentre section 10 at18. The external and internal diameters of thesection 12at thetube end 22 will be substantially the same as thoseof thecentre section 10.
• Theend 20 of the section has anabutment shoulder 24, amouth 26 into which the male threaded end of another rodwill be introduced, a threadedregion 28 and arelievedregion 30 at the inner end of the thread section.
• Figure 2 illustrates the thread which will be formed onthe inner walls of theregion 28. This thread will be adouble start trapezoidal thread, of the type well known inconnecting tube rods for use in rotary/percussion rockdrilling. The root diameter of this thread will besubstantially the same as that of therelieved portion 30.
• Beyond therelieved portion 30, the wall thickness of thesection 12 becomes thicker and as a result the internalbore diameter becomes less. The bore has a narrow neck at32, and a tapered transition at 34 from therelievedportion 30 to the neck, and a further conically taperedportion 36 from the neck to thetube end 22. The taperedtransition 34 is shown in the form of a conical taper, butmay alternatively be a radiused taper (see Figure 6).
• The external surface of the female portion has a diameterin theregion 38 which can be the same as that of thecentral portion 10 or can be larger. The diameter at theend portion 40 is slightly larger than that of the central portion, for example 2% - 6% larger, and when the diameterat theregion 38 is larger than the central portion, itcan also be 2 - 6% larger.
• Between the first diameter and the second diameter arearegions, there is anannular groove 42 which has a rootdiameter substantially less than either the first or thesecond diameters. This groove is bounded by a smoothcurve, one flank of which closest to theend 24 is steeperthan the other flank. Although not shown in this figure,the transition between the steep flank and thesurface 24can be radiused, rather than the sharp edge shown. Itwill seen from Figure 2 that the base of the groove liesin a cross-sectional plane which incorporates theneck 32.The radius of the base part of thisgroove 42 can, forexample, be 12.5 millimetres. Between thegroove 42 andtheneck 32 the female threadedsection 12 has a greaterwall thickness than at any other region of its length.
• The male threaded end section 14 (see Figure 3) has athreadedboss 44 extending from an unthreadedcollar 46.The diameter of thecollar 46 is the same as that of thefemale end (the second diameter) so that when the male endis screwed into a female end, there is no step in theexternal diameter at the joint. However the male endcould be of the first diameter and/or could have adiameter ramp at the end to blend into the externaldiameter of the female end without a step.
• Thecollar 46 has externally a flat at 48 on which serialnumbers or other information can be stamped.
• The right hand end of the portion 14 (as seen in Figure 3)has atube end 50 which will match thetube end 22 of thefemale threaded portion and will match the cross sectionof thecentral section 10.
• The section has a throughbore 52. The threadedportion44 will have external threads to match theinternalthreads 28.
• In use, themale end 14 of one rod is screwed into thefemale end 12 of another rod. The two are screwedtogether until theshoulder 54 of one male end comes intocontact with theshoulder 24 of the female end. There isno contact between theextreme end 56 of the male end andtheinternal shoulder 58 of the female end.
• In Figures 4 to 7, the same reference numerals are usedwhere they denote parts which are directly equivalent toparts described with reference to Figures 1 to 3.
• Figure 4 shows a male threadedend section 114 whichdiffers from the section shown in Figure 3 by theintroduction of aland 155 which engages, when the rodsare screwed together, inside themouth 26 of the adjacentfemale threaded end section, and forms a close fit in thatmouth so as to prevent any significant radial movementbetween theshoulders 24 and 54. This minimises anytendency to bending in the assembled drill rod string.
• Figure 5 shows an embodiment where both theneck 132 andthegroove 142 are elongated as compared with theembodiment shown in Figure 2. This assists in reducing thestress level at the inner end of the threadedregion 28,particularly in the part of the threaded region closest totheneck 132.
• Figure 6 shows an embodiment similar to that of Figure 2,but where the taperedtransition 34 is replaced by aradiused transition 234.
• The relationship between the various parameters can be asfollows, where:

  A	represents the external diameter of the central section 10
  B	represents the maximum external diameter of the female threaded end section 14
  C	represents minimum external diameter of the female threaded end section 14 (at the groove 42)
  D	represents the smallest internal diameter of the threaded section 14 (at the throat 32)
  E	represents the core diameter of thefemale thread 28.

• The following relationships hold good for rods inaccordance with the invention:
• A is less than B
• B is up to 150% of E
• C is 100 - 130% of E
• D is 45 - 60% of E.
• In addition to the features described above which reducethe susceptibility of the rod to stress cracking, theexternal surface of the rod can be provided with externalhelically flutedportions 60, 62 (Figure 7). These flutedportions help to enhance the straightness of holes beingdrilled, by maintaining some contact with the walls of thehole while allowing flow of drilling fluid and drillingdebris past the rod up the hole. The rod is thus heldstraight in the hole, so that the drill bit is alwayspresented to the rock ahead in the correct orientation.
• There may be one (60) or two (60,62) of these flutedportions on each rod.
• It has surprisingly been found that the formation of theannular groove 42 helps to reduce stress concentrations inthe female threaded end, these concentrations otherwisearising in the area of the last thread root and internalrecess of the threadedregion 28.
• This resistance to failure is enhanced by the increaseddiameter of theregion 40 and by the internal taperedsurfaces at 34 and 36.
• These measures cause a dispersion of the overall stress sothat the stress of the critical point is reduced.

Claims (20)

1. A tube rod for percussion rock drilling havingthreaded ends (12,14), the tube rod having a hollow boreand at least one of the ends (12) having a female thread,the major part (10) of the tube rod length between thethreaded ends (12,14) having an outside diameter (thefirst diameter) and at least part of the or each femalethreaded end having an outside diameter (the seconddiameter) which is larger than the first diameter,characterised in that the tube rod includes an annulargroove (42) between the female threaded end and the majorpart (10) of the tube rod length, the annular groove (42)having a root diameter (the third diameter) which is lessthan both the first and the second diameter, the hollowbore of the rod having a region (32,24,26) between thefemale threaded end and the major part (10) of the tuberod length where the internal diameter is less than ateither side of this region.
2. A tube rod as claimed in Claim 1, wherein the seconddiameter is larger than the first diameter by between 2%to 6%
3. A tube rod as claimed in Claim 1 or Claim 2, whereinthe groove (42) has an asymmetrical profile when seen incross section parallel to the tube axis.
4. A tube rod as claimed in Claim 3, wherein the flankof the groove (42) nearest to the threaded end is steeperthan the opposite flank.
5. A tube rod as claimed in any preceding claim, whereinone of the ends (12,14) has a female thread (28) and theother end has a male thread (44).
6. A tube rod as claimed in any preceding claim, whereinthe hollow bore of the rod is reduced in internal diameterat the inner end of the female thread (28) to form a neck(32), and the bore diameter tapers outwards from the neck(32) to the full bore diameter of the major part of thetube rod length.
7. A tube rod as claimed in any preceding claim, whereinthe internal diameter of the tube rod tapers from themajor part (10) of the tube rod length down to the regionof lesser internal diameter.
8. A tube rod as claimed in any preceding claim, whereinthe internal diameter of the tube rod tapers from thefemale threaded end (28) down to the region of lesserinternal diameter.
9. A tube rod as claimed in any preceding claim, whereinthe female threaded end (28) has an internal shoulder (58)at right angles to the tube axis, and the internal tubebore tapers from the shoulder (58) down to the region oflesser internal diameter.
10. A tube rod as claimed in any one of Claims 7 to 9,wherein the or each taper of the internal diameter is aconical taper.
11. A tube rod as claimed in any one of Claims 7 to 9,wherein the or each taper of the internal diameter is aradiused taper.
12. A tube rod as claimed in any preceding claim, whereinthe neck (32) or region of lesser diameter of the internalbore lies on substantially the same cross-section of therod as the third (external) diameter of the rod.
13. A tube rod as claimed in Claim 12, wherein the third(external) diameter of the rod lies on a cross-section ofthe rod which is offset from the centre of the neck (32)or region of lesser diameter in the direction towards themajor part (10) of the tube rod length.
14. A tube rod as claimed in any preceding claim, whereinthe threaded ends (12,14) are initially separate from therod and are joined to a length of constant cross sectiontube by friction welded joints, the length of constantcross section forming the major part of the tube rodlength.
15. A tube rod as claimed in claim 5, wherein the malethreaded end (14) has a threaded region (44) extendingfrom an annular shoulder (54), the shoulder (54) beingadapted to abut against the end (24) of the femalethreaded end (12) of another rod when the two rods arescrewed together, the male threaded region (44) beingshorter than the female threaded recess so that when themale threaded end (14) is screwed into the female threadedend (12) of another rod until the shoulder abuts the endof the other rod, the male end does not reach the bottomof the female recess.
16. A tube rod as claimed in Claim 15, wherein theannular shoulder on the male threaded end section (14) isstepped and the mating end of a female threaded endsection (12) of another rod has a shoulder such that whenthe rods are screwed together, the shoulders interengageto prevent substantial radial movement of one shoulderrelative to the other.
17. A tube rod as claimed in any preceding claim, whereinthe rod has at least one externally fluted section, theexternal diameter across the crests of the fluted section being equal to or larger than the second diameter.
18. A tube rod as claimed in Claim 17, wherein theexternally fluted section forms the outside diameter ofthe female threaded end which is of the second diameter.
19. A tube rod assembly for percussion rock drilling, theassembly including a plurality of tube rods as claimed inclaim 1 connected end to end by joints comprisinginterengaging male and female threads (44,28) wherein whenthe joints are assembled, an end (24) of a female threadedcomponent is in contact with an external shoulder (54) onthe adjacent male threaded component, the externalshoulder having a circumferentially extending steppedland, with part of the end of the female threadedcomponent fitting around the land to hinder relativeradial movement between the two ends at the position ofthe shoulder.
20. A tube rod as claimed in any preceding claim, whereinArepresents the first diameterBrepresents the second diameterCrepresents the third diameterDrepresents the smallest internal diameter of the female threaded sectionErepresents the core diameter of the female thread in the female threaded end,and wherein

    A is less than B

    B is up to 150% of E

    C is 100 - 130% of E

    D is 45 - 60% of E.

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