Note: Descriptions are shown in the official language in which they were submitted.
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sackground - Summary of the Inventlon
The present invention relates to the art of thread forms,
and more particularly to a thread structure for use on percussion
drilling parts, such as extension rods, and coupling sleeves for
connecting extension rods.
Percussion drilling operations ordinarily employ a number of
longitudinal extension drill rods joined at their ends by coupling
sleeves. These extension drill rods characteristically have ex-
ternal thread structures which cooperate with the internal thread
structures of the coupling sleeves to provide a connection there-
between. These thread forms will be referred to herein as having
top crest portions and flank portions, the lower ends of which
flanks are interconnected by root portions.
One problem arising with known thread forms used for percus-
sion drilling parts is that the threads thereof are tightened due
to the percussive rotary forces being transmitted during the dril-
ling operation. This, of course, results in a corresponding increase
in the torque necessary to uncouple the parts. It has been found
that this increase in required uncoupling torque is due to wedging
2Q between the cooperating thread structures. In a relatively new or
unworn thread, the wedging problem is attributable to the binding
between the flank portions of one thread structure and the comple-
mentary portions of the cooperating thread structure. In a used
or worn thread, the wedging problem is due to binding between the
flank and crest portions of the thread structure and the comple-
mentary structures.
U.S. Patent No. 3,645,570 to Johansson et al exemplifies
prior art attempts to resolve this problem. The thread structure
disclosed therein is profiled in such a way that, upon proper
3Q engagement of the cooperating thread structures, the crest and
root portions of the respective thread structures do not contact
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one another. More specifically, Johansson discloses a thread
design having abutting flanks which are inclined at 25 to 40
and preferably 35 to the drill rod axis. The crest and root
portions are substantially straight and parallel to the drill
rod axis and are radially spaced from the complementary portions
of the cooperating thread structure. So designed, the possibility
of wedging between portions of one thread structure with the
root portions of the cooperating thread structure is reduced.
Such thread designs do not, however, cope with the effect
of wedging after substantial wear of the thread structure has
occurred. In percussion drilling, the threads are subject to
both rotary and percussive forces. These percussive forces,
which are transmitted by the abutting flanks, cause substantial
wear thereof. As the abutting flanks are eroded, the thread
structure remaining has a portion which is in effect an extension
of the root portion and is substantially parallel to the drill
rod axis. Since the crest portion of the cooperating thread
abuts this worn extension of the root portion of the eroded
flank, wedging develops.
2Q Another common problem occurring during the use of percus-
sion drill rods is that the threads tend to break due to fatigue
stresses. It has been found that these stresses generally occur
where there is a sharp change in diameter of the rod.
The present invention provides a thread structure featur-
ing low uncoupling torque characteristics which are maintained
during the life of the thread. Further, a thread structure in
accordance with the present invention minimizes fatigue stresses
during use.
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~ The present invention more specifically provides apparatus
;~ for use in percussion drilling or transmitting rotary and downwardly
directed percussive forces to a drill bit, said apparatus comprising a
first part and second part, said first part having a cylindrical internal
thread structure which cooperates with a cylindrical external thread
structure on said second part, said thread structures on said first part
and second part being formed about a thread axis and constructed to
minimize unlocking tor4ue. The threads on said parts each have first
and second axially spaced flanks, said first flanks each having a linear
portion, said threads on each of said parts also having a crest portion
extending between the outer edges of said first and second flanks and
a root portion for interconnecting adjacent threads, said root portions
of at least said second part being continuously curved and tangentially
joined to the flank portions adjacent therewith such that local stress
concentration is reduced. The crest portions of said threads are radially
spaced from the root portion of the cooperating threads. The linear portions
on said first flanks on one of said parts abut said linear portions on
said first flanks on the other of said parts. The first flanks are axially
spaced from said second flanks to provide a thread of a thickness to
; 20 accommodate substantial erosion of said first flanks by rotary and percussive
forces during percussion drilling. The second flanks on said one part are
spaced from said second flanks on the other of said parts. The first flanks
are inclined at an angle of between 25 and 40 to the thread axis, and
the crest portion of the threads of both said first and second parts are
angled away from the abutting first flank portion of that thread toward
the root portion of that thread to form a peak portion of the crest adjacent
the abutting first flank portion such that, after substantial wear of the
abutting flank, the crest portion of that thread remains out of substantial
contact with the cooperating thread.
In accordance with another aspect of the present invention,
a thread structure is provided which reduces fatigue stress concentration
by providing continuously curved root portions. Specifically, the
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curvature of the root portions meets each of the flank portions on opposite
: sides of the root portion tangentially.
Brief Description of the Drawings
: The foregoing and other aspects and advantages of the present
` invention will become more readily apparent to those skilled in the art
. from the following detailed description as taken in conjunction with the
accompanying drawings which are a part hereof and wherein:
Figure 1 is an illustration of a drill rod coupling partly in
elevation and partly in section;
Figure 2 is a longitudinal sectional view of two cooperating
` thread structures according to this invention prior to erosion of the
: abutting flanks; and ~-
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.:: Figure 3 is a longitudinal sectional view of two cooperating
thread structures according to the invention after substantial
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erosion of the abutting flanks has occurred.
Detailed Description of A Preferred Embodiment
Referring first to Fig. l, percussion drill rod extensions
12 and 14 are interconnected by a coupling sleeve 16. The drill
rod extensions 12 and 14 have external threads 20 which cooperate
with internal threads 22 formed on sleeve 16 to provide a rigid
connection therebetween.
Figs. 2 and 3 illustrate the cooperating thread struc-
tures in enlarged longitudinal profile. Rod thread 20 has a
crest portion 24 embraced on either side by flank portions 26
and 28. Root portion 30 serves to interconnect the lower flank
portions of adjacent threads. In a similar manner, sleeve thread
-- 22 has crest portions 32 embraced on either side by flanks 34
and 36, adjacent threads being connected by root portion 38.
Contact between the two thread structures occurs at
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flanks 26 and 36, referred to hereinafter as the abutting flanks.
Flanks 34 and 28 are not abutting but rather spaced apart.
Since the abutting flanks are load-bearing surfaces, they are
preferably mated to provide the largest possible area of contact.
This can be conveniently accomplished by structuring at least a
portion of flanks 26, 36 rectilinearly. Also, in accordance
- with known practice, the abutting flanks are inclined at an -
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; angle of 25 to 40, and most preferably 35 to the drill rod
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extension axis. The nonabutting flanks are inclined at an angle
of 25 to 40 and preferably 35 to the axis of coupling sleeve ~ -
16.
Also, crest portion 24 of rod thread 20 is radially spaced
from the complementary root portion 38 of the cooperating sleeve
thread 22 so as to substantially eliminate contact therebetween.
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Also, crest portion 32 is radially spaced from root portion 30.
According to the invention, crest portion 24 is substan-
tially flat and is inclined at a slight angle with respect to
the drill rod axis. This inclination is directed away from flank
portion 26 which abuts flank 36 and inwardly toward the axis of
the extension rod, so as to provide a peak crest portion 40 im-
mediately adjacent to abutting flank protion 26. In a similar
manner, crest portion 32 of sleeve thread 22 is inclined inward-
; ly toward the axis of the coupling sleeve 16 as it extends
away from flank 36 to provide a peak crest portion 42 immediatelyadjacent abutting flank portion 36.
; Referring now to Fig. 3, the thread structure of Fig. 2
is there shown after substantial wear has occurred. Abutting
flank portions 26 and 36 have significantly eroded to where
flank surfaces 26A and 36A are in contact. Further, surfaces
48 and 50, which may be termed root extensions, have been formed
due to the wear. It will be understood that separation between
the newly exposed root extension 50 and the remaining crest por-
tion 24A is provided due to the inclination of crest portion 24
with respect to the drill rod axis. If crest portion 24 were
substantially parallel to a drill rod axis, root extension 50
would be similarly parallel to the drill rod axis, thus causing
wedging to develop between the surface 50 and crest portion 24.
On the other hand, with the crest portion inclined as set forth
hereinbefore, it is apparent that root extension 50 and crest
portion 24A do not have substantial contact, since portion 24A
is inclined at a slight angle to the drill rod extension axis
and, in fact, portion 24A and surface 50 diverge from one another
- as they extend from the abutting flank portions 26A and 36A.
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Due to this divergence, wedging is substantially precluded and
uncoupling torque is minimized. Obviously, this is also pre-
cisely what occurs due to the incline of crest portion 32 and
thus root extension 48 and crest portion 32A do not have sub-
stantial contact, even though wear occurs.
It will be understood that it is the existence and
direction of inclination rather than the extent thereof which
secure the advantages of the present invention. Thus, the
angle of inclination is not critical so long as it is adequate
to prevent binding between the newly exposed root extensions and
the remainder of the crest portions. It will be noted, however,
that the greater the angle of inclination, the more rapidly the
abutting flank portions 26 and 36 may wear. Thus, the angle of
inclination is preferably quite small to provide the most ad-
vantageous wear characteristics. Therefore, in accordance with
the preferred practice of the present invention, the angle of
inclination is approximately 3. Additionally, it will be under-
stood that the crest portion need not be linear to practice the
present invention; the crest portion could be any curvilinear
shape having a generally decreasing radial dimension with in-
creasing distance from the abutting flanks.
Referring once again to Fig. 2, in the present thread form
local stress concentrations are reduced and fatigue life im-
proved by providing continuouslv curved root portions. Thus, ~;
root portion 30 of rod thread 20 has a continuously curved shape ~`
and meets flank portions 26 and 28 tangentially. Similarly,
root portion 38 of sleeve thread 22 meets flank portions 34
and 36 tangentially. To further reduce local stress concentra-
tions, crestportions 24 and 32 can be structured so as to meet
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their respective flank portions along smoothly curved interme-
diate sections. Finally, since nonabutting flanks 28, 34 are
generally not load-bearing surfaces, they may also be continu-
ously cur~ed to provide even distribution of stress. .
. From the above description, it should be clearly under-
stood that this invention provides a percussion drill thread
structure having crest portions inclined at a slight angle to
the drill rod axis to prevent wedging after substantial wear,
and also provides continuously curved root portions to thereby
increase fatigue life.
Although the invention has been described with reference
to the connection of two drill extensions, connection with drill
bits and other drill rod members could be made as well. Also,
although the invention has been described and illustrated with
: respect to a preferred embodiment, it will be understood that
various modifications and alterations may be made without depart-
ing from the spirit and scope of the invention as defined in the
following claims.
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