GB2309276A - Threaded joint - Google Patents

Description

THREADED JOINT This invention relates to a threaded joint for coupling together two drill rods as part of the drill string of a rock drill.

In such a joint, there should be a large area of contact between the two coupled drill rods, for transmission of the impact forces generated by a top hammer mechanism which strikes the top end of the string. At the same time, the joint should be easy to uncouple after use, and should have a long service life.

US Patent Specification 4 687 368 discloses a typical male/female drill rod joint. Here the female part of the joint is a socket with a shoulder at the bottom, and the male part of the joint ends in an end face at right angles to the rod axis.

According to the invention there is provided a threaded joint for joining two drill tubes, the joint comprising a female threaded part at one end of a first drill tube and a male threaded part at one end of a second drill tube, wherein the female threads are formed in a bore of the first tube which has a tapered transition leading from the threaded part into the tube internal bore, the taper angle (as measured between the tapered surface and the drill rod axis) being 450 or less, the end of the first drill tube having an annular surface normal to the rod axis, and wherein the male threaded part has a shoulder between the male threads and the remainder of the second rod, the shoulder being adapted to abut the annular surface on the other rod, when the rods are coupled together.

The female threaded part (and possibly the male threaded part) can be produced separately from a rod which forms the major part of each drill tube. Either threaded part may be machined from either a solid blank or a tubular blank and subsequently joined to the rod, for example by friction welding. The threaded parts can alternatively be machined directly from the material of a rod.

The skilled man will understand that the threaded joint set forth above could equally well be used between a rod and a drill bit, or between a rod and a shank adaptor or other component for connecting a drill string to a drill. The term 'drill tube' is thus to be understood as including tubular parts of drill string components which are not drill rods as such.

Construction of the threaded joint in this way allows the strength of the rods to be maintained, ie most of the rod material is retained even where the threads are formed in the rod walls, whilst ensuring good force transmission, and an avoidance of stress-raising sharp corners.

The joint is preferably a joint for tube drilling rods, and in this case the external diameters of the first and second rods (and indeed of any other rods coupled into the string) will be constant over their length, with the pitch diameter of the threaded parts being less than the external diameter of the rods.

The taper angle of the tapered transition in the female threaded part is preferably less than 30 , and in a particularly preferred embodiment is about 100.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded view of part of a tube drilling string, with threaded joints shown uncoupled; Figure 2 shows, in cross-section, the flushing flows occurring during tube drilling; Figure 3 shows a tube drill at the bottom of a drilled hole; Figure 4 is a section through the end of a first rod, with a female thread; Figure 5 is a side view of the end of a second rod, showing a male threaded end; Figure 6 is a view showing the ends of the first and second rods coupled together; and Figure 7 is a cross-section through the coupled first and second rods.

Figure 1 shows two tube drilling rods 10 together with a button bit 12. In use, these components will be coupled together by threaded couplings 14 to form a drill string which will be driven into a rock face by a conventional top hammer mechanism. As the depth of the hole increases, further tube rods 10 will be added at the top end of the string.

It will be evident from Figure 1 that the pitch circle of the threaded couplings 14 is of smaller diameter than the external diameter of the rods themselves. As a result, the coupled drill string has a smooth external surface, as can be seen in Figures 2 and 3. This allows a minimising of the annular gap 16 between the rod 10 and the hole wall 18, and a corresponding maximising of the up-hole flushing fluid velocity, as illustrated in Figure 2. This results in very efficient flushing to remove rock chips from the hole.

Figure 4 shows one end 20 of a tube rod with an internal, female thread form generally designated 22. The tube rod has a bore 24 extending the entire length of the rod and open at both ends. The diameter of this bore at the male threaded end will be slightly smaller than the bore diameter at the female threaded end. The end 20 has an entry taper 26, a parallel walled portion 30 where the thread 22 is formed, and a tapered transition region 28 where the thread 22 stops and the bore 24 meets the end portion. It will be seen that the tube wall 32 is slightly thicker in the middle region of the tube, where the wall bounds the bore 24, than it is in the end region where the thread 22 is formed.

Figure 5 shows one end 34 of a tube rod with a male thread 36 formed on a projection 37 at the tube end. This rod also has an internal bore, indicated by dotted lines at 38. The projection 37 has a tapered leading portion 40 and ends in a shoulder 42.

It will be appreciated that each rod 10 will have one end 20 with a female thread and one end 34 with a male thread. The configuration of the threads can be as set out in our copending application 9520008.5 filed on 30 September 1995.

The external diameter of the rod 20 and of the rod 34 will be the same.

The rod ends can be coupled together to form a joint as shown in Figures 6 and 7. The open end of the rod 20 will butt up against the shoulder 42 of the rod 34, and when drilling takes place, force will be transmitted from one rod to the other across these surfaces. By making the pitch circle of the thread (indicated at 44) small, the contact area between the two abutting surfaces (the struck faces) can be maximised, to improve force transmission, to improve rigidity of the joint and to enhance the service life of the rods. We calculate the pitch circle diameter (of the joint) as the diameter which is half way between the root diameter of the female thread and the root diameter of the male thread. The pitch circle diameter of the joint will therefore be slightly larger than the pitch circle diameter of the male thread form and slightly smaller than the pitch circle diameter of the female thread form.

When wear takes place between the mating threads, the relative angular positions of the coupled rods will change to take up the wear. This should not however cause any jamming of the thread which should still be easy to uncouple.

It is suitable, for achievement of the benefits of increasing the size of the struck faces 41,42 to have the pitch circle diameter between 74% and 75% (preferably between 74.1% and 74.8%) of the external tube diameter. The angle of taper in the transition region 28 can be between 150 and 400 (inclusive angle) and the taper in the female lead in region 26 can also be between 150 and 400.

Claims (11)

Claims

1. A threaded joint for joining two drill tubes, the joint comprising a female threaded part at one end of a first drill tube and a male threaded part at one end of a second drill tube, wherein the female threads are formed in a bore of the first tube which has a tapered transition leading from the threaded part into the tube internal bore, the taper angle (as measured between the tapered surface and the drill rod axis) being 450 or less, the end of the first drill tube having an annular surface normal to the rod axis, and wherein the male threaded part has a shoulder between the male threads and the remainder of the second rod, the shoulder being adapted to abut the annular surface on the other rod, when the rods are coupled together.

2. A threaded joint as claimed in Claim 1, wherein the female threaded part has been produced separately from a rod which forms the major part of each drill tube.

3. A threaded joint as claimed in Claim 1, wherein the male threaded part has been produced separately from a rod which forms the major part of each drill tube.

4. A threaded joint as claimed in any preceding claim, wherein at least one of the threaded parts has been machined from a blank and subsequently joined to the rod.

5. A threaded joint as claimed in Claim 4, wherein the or each subsequently joined part has been joined to the rod by friction welding.

6. A threaded joint as claimed in any one of Claims 1 to 3, wherein the threaded parts have been machined:directly from the material of a rod.

7. A threaded joint as claimed in any preceding claim, which is a joint for tube drilling rods.

8. A threaded joint as claimed in Claim 7, wherein the external diameters of the first and second rods are constant over their length, with the pitch diameter of the threaded parts being less than the external diameter of the rods.

9. A threaded joint as claimed in any preceding claim, wherein the taper angle of the tapered transition in the female threaded part is less than 300.

10. A threaded joint as claimed in any preceding claim, wherein the taper angle of the tapered transition in the female threaded part is about 100.

11. A threaded joint for joining two drill tubes, substantially as herein described with reference to the accompanying drawings.