Thread structure for percussion rock drilling

A thread structure for interconnecting two elements in a percussion drill string comprising an interior cylindrical thread and an exterior cylindrical thread. The threads have abutting flanks which are straight along their abutting contact portions. The clearance (a) between the bottom portion of the exterior thread and the crest portion of the interior thread is large enough to ensure that contact does not arise at said clearance when the threads are worn.

BACKGROUND OF THE INVENTION 
This invention relates to a thread structure for interconnecting two 
elements in a percussion drill string comprising an interior cylindrical 
thread and an exterior cylindrical thread, said threads having abutting 
flanks which are straight along their abutting contact portions. 
A thread structure of this type is described in U.S. Pat. No. 3,645,570. In 
such a thread structure friction martensite is formed at the contact 
surfaces resulting in so-called pittings. This is particularly the case 
when compressed air is used as flushing medium which leads to a 
comparatively poor cooling of the contact surfaces, and when the contact 
surfaces are carburized. When these so-called pittings do arise on the 
flanks of the threads they are comparatively harmless except in the bottom 
portion of the exterior thread, however, where the stress concentration is 
high, and a surface defect does rapidly lead to fatigue failure of the 
drill element. 
The object of the present invention is to provide a thread structure of the 
above-described type in which it is ensured that contact will not arise 
between the crest portion of the interior thread and the bottom portion of 
the exterior thread when the threads are worn. 
Specifically, this invention seeks to provide an improved coupling thread 
on a drill rod which is exteriorly threaded at its one end and interiorly 
threaded at its other end, where the interior thread is provided in a 
sleeve-shaped portion of the drill rod. A drill rod of this type is 
described in U.S. Pat. No. 3,537,738. 
Other objects of the invention are to provide a drill rod of the type 
described in U.S. Pat. No. 3,537,738 having the same bending resistance 
throughout its length and to ensure that flushing medium can reach the 
threads to flow therebetween for purposes of decreasing the wear of the 
threads.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
The drill rod 10 shown in FIGS. 1 and 2 is at its one end provided with a 
sleeve-shaped portion 11, in which is formed an interior thread 12. At its 
other end the drill rod 10 is provided with an exterior thread 13. 
Further, the drill rod 10 has a flushing channel 14 traversing 
longitudinally therethrough, through which flushing medium, such as 
compressed air or water, is conducted. The bottom of the sleeve-shaped 
portion 11 forms an annular abutting surface 15 which is adapted to rest 
against a corresponding annular abutting surface 16 at the exteriorly 
threaded end of another drill rod when two drill rods 10 are screwed to 
each other. 
In the illustrated embodiment the drill rod 10 is circular in cross section 
and is provided with wrench grips 17, 18 at it ends to make possible 
holding of the drill rods during connection and disconnection thereof. 
The interior thread 12 has in longitudinal section a straight flank portion 
19 and the exterior thread 13 has a corresponding straight flank portion 
20. The threads 12, 13 have, in profile, straight crest and bottom 
portions denoted by 21 and 22, respectively, for the exterior thread and 
by 23 and 24, respectively, for the interior thread. The straight crest 
and bottom portions turn into the straight flank portions 19, 20 via 
curved portions 25, 26 on the exterior thread and curved portions 27, 28 
on the interior thread. 
In conventional manner the threads 12, 13 are designed in such a way that 
contact does arise solely between the straight flank portions 19, 20 when 
the threads are screwed together and thus not between the curved portions 
or between crest and bottom portions. Thus, abutment is avoided between 
portions which are only slightly inclined relative to the drill axis; said 
abutment might cause the threads to become wedgingly interconnected. 
As above-mentioned the purpose of the invention is to ensure that contact 
does not arise between the crest portion 24 of the interior thread 12 and 
the bottom portion 22 of the exterior thread when the threads are worn. 
Due to the asymmetrical wear of the thread profile upon wear of the 
threads the clearance a between the crest portion 24 and the bottom 
portion 22 tends to become smaller and smaller at the one side; more 
precisely at the side nearest to the abutting flank portions 19, 20. 
In a thread structure of this type it is desired to have the interspace 
between the threads as small as possible in order to counteract 
penetration of drill dust into the thread coupling. Controversially 
thereto it has been found that the life of the drill element is 
considerably lengthened if the clearance a is highly enlarged. By this it 
is ensured that contact will not even arise at the portions 22, 24 when 
the threads are highly worn. In this case, of course, contact does arise 
at the portions 21, 23 since the clearance b between the bottom portion 23 
of the interior thread and the crest portion 21 of the exterior thread is 
of conventional size. Such contact resulting in so-called pittings, 
however, does very seldom cause fatigue failure. The reason therefore is 
that the portion having the interior thread 12 has considerably higher 
bending resistance than the portion having the exterior thread 13, which 
leads to lower bending stresses in the first-mentioned portion. It has 
been found that the clearance a should be larger than 1% of the outer 
diameter Dy of the exterior thread 13 and also that the clearance a is at 
least four times the clearance b in order to ensure that contact does not 
arise between the portions 22, 24. In the illustrated embodiment the 
interior thread 12 has a height h smaller than 5% of the inner diameter Di 
of the interior thread 12. In a preferred embodiment the thread 12 has a 
height h smaller than 4.5% of the inner diameter Di for inner diameters 
smaller than 55 mm. 
In a manner known per se the threads have one entrance and the pitch angle 
.alpha. of the threads is between 5.5.degree. and 8.degree.. The pitch 
angle .alpha. is calculated from the formula 
tan .alpha.=s/D.sub.m..pi., 
where 
s=the pitch, and 
D.sub.m =the effective or mean diameter. 
The somewhat decreased pitch angle, when compared to the thread structure 
in U.S. Pat. No. 3,645,570, is due to the fact that the pitch angle in the 
present invention is adapted to hydraulically operated rock drilling 
machines, whereas the pitch angle in the thread structure in U.S. Pat. No. 
3,645,570 is adapted to pneumatically operated rock drilling machines. Due 
to the nature of the shock wave pneumatic rock drilling machines do 
tighten the thread connection harder than hydraulic rock drilling 
machines. Since the uncoupling torque increases if the pitch angle is 
decreased a comparatively small pitch angle is chosen in order to ensure 
that the thread connection is tightened hard enough during drilling by 
hydraulic rock drilling machines. In a preferred embodiment it has been 
found that the pitch angle .alpha. should decrease 0.4.degree.-0.8.degree. 
when the diameter of the threads 12, 13 is increased 10 mm, in which case 
the pitch angle .alpha. is in the order of 7.2.degree. for an interior 
thread 12 having an outer diameter of 38 mm. 
In a manner known per se the inclination angle .beta. of the flank portions 
19, 20 relative to the longitudinal axis of the percussion drill string is 
25.degree.-40.degree., with preference for values in the order of 
35.degree.. As above-mentioned abutment does arise solely at the flank 
portions 19, 20. Therefore, the flanks opposed to the flank portions 19, 
20 as well as the adjoining curved portions and the crest and bottom 
portions can be designed arbitrarily within rather broad limits. However, 
it has been found suitable to have the two opposed flanks of the threads 
substantially symmetrical with respect to the inclination of the flanks. 
Further, it has been found suitable to have the crest ant bottom portions 
of the threads straight and substantially parallel to the longitudinal 
axis of the drill string. However, the crest and bottom portions can be 
designed in other ways, for instance concave, in which case the clearance 
a is measured at the point where the distance is smallest between the 
crest and bottom portions and the height h is the largest height of the 
thread. 
Below are in tabular form values which have been found suitable for the 
clearance a, the height h of the interior thread and the pitch angle 
.alpha. for different values on the inner diameter Di of the interior 
thread and the outer diameter Dy of the exterior thread. 
______________________________________ 
Di (mm) Dy (mm) a (mm) h (mm) 
.alpha. 
______________________________________ 
35.20 38.20 0.45 1.60 7.17.degree. 
40.60 44.20 0.55 1.90 6.72.degree. 
46.60 50.80 0.65 2.20 6.31.degree. 
54.70 57.80 0.70 2.65 5.97.degree. 
______________________________________ 
Since the clearance b has a constant value, 0.1 mm, the ratio a:b is in the 
range of 4.5 to 7 for the examplified threads. 
As apparent from FIG. 2 a groove 29 is provided in the abutting surface 16. 
The groove 29 extends entirely across the abutting surface 16 and 
communicates with the flushing channel 14, thereby ensuring that flushing 
medium is supplied to the threads 12, 13 for flushing and cooling thereof. 
Alternatively, the groove 29 can be made in the abutting surface 15. 
As above-said the invention also seeks to design the drill rod 10 in such a 
way that it has the same bending resistance throughout its length so that 
the percentage of rod breakage as the final fault is decreased. For that 
purpose the reduced-diameter portion 30 of the drill rod 10 immediately 
adjacent to the exterior thread 13 has the same diameter as the remainder 
of the drill rod. 
Although being described in connection to a drill rod being externally 
threaded at its one end and interiorly threaded at its other end it is to 
be understood that the thread structure can be used in any type of drill 
elements in a percussion drill string, such as a drill rod externally 
threaded at both ends, a coupling sleeve for inter-connecting two such 
drill rods and a drill bit adapted to be connected to such a drill rod.