CA1069881A - Thread for high torque joint - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A threaded connection for the stem of raise bore mining drills has substantially extended fatigue life as a result of relieving the radius of the bottom of the thread roots and cold working the relieved radius. The endurance life of the threaded connection is greatly increased by reducing the stress concentration in the thread root and placing the thread root in compression.

Description

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: THREAD FOR HIGH TORQUE JOINT' BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION
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This invention relates to raise bore drilling in mines.
It is common practice to drill raises from a lower level along a pilot hole with drilling bits having roller cutters arranged to cut the formation. The raise bore cutter is of substantial diameter, viz. several feet, and is subjected to high longitudinal, rotary, and bending stresses. The connection between the raise 10 bore drill and the drill pipe used to rotate the drill is subject `~
to very high stresses which have resulted in relatively early fatigue ~ailure. ~

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DFSCRIPTION OF THE PRIO~ A~T
Justman U~S. Pa-tent 3,638,7~0 discloses a rotary drill for drilling an enlarged bore alony a pilot hole. The raise bore drill has a stem which is connected to a rotary drill pipe which is subjected at the threaded joint to very high rotary, longitudinal, and bending stresses.
Kloesel U.S. Patent 3,355,192 discloses an improved threaded connection for raise bore drills which handles high loads readily and has a relatively low break-out torque.
Robbins U.S. Patent 3,129,963 discloses a threaded con-nection in which the mating pin and box have complimentary cooperating tapered sur~aces on the flanks of the threads which con-tact so that both flanks of each thread are loaded after makeup.
Lorenz U.S. Patent 3,586,353 discloses a thread arrangement for connecting solid or hollow shafts wherein the thread arrange-ment is defined in terms of the geometry, including the diameter, or diameters, of the shafts upon which the thread is employed.
Other patents showing typical threaded connections for tubular joints and the like include Duret U.S. Patent 3,508,771, Franz U.-S. Patent 3,109,672, Johansson U.S. Patent 3,645,570, and H~alsten U.S. Patent 3,388,935.
SU~MARY OF THE INVENTION
This invention relates to new and useful improvements in threaded connections and more particularly to threaded connections for raise bore drilling. The threaded connection of this inven-tion is particularly useful as a connection between the drill stem of a raise bore drill and a drill pipe connected thereto for turning and raising the drill. The threaded connection is characterized by having a thread cons-truction capable of taking very high loads and having a relieved radius at the root of the thread and work-hardened in the relieved radius area.

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Thus, accordincJ to the present invention -tllere is provided a drill pipe for connection to khe stem of a rotary drill for producing a raise bore along a pilo-t hole in an earth formation having a stem with a pla~e member thereon in a planc normal thereto supporting a plurality o roller cutters arranged to cut an enlarged bore in the earth on rotation and raising of the drill pipe. The improvement in ~he drill pipe,comprises a - - female thread for connection to a mating thread on the drill stem, with the female thread being a tapered helical buttress thread having curved, relieved~ work-hardened~ helical roots with a shallow curvature having a radius of substantial ].ength in relation to the depth of the threads and terminating in curved edges o short radius tangent to the Eac~s of the threads.

BRIEF DESCRIPTION OF TIIE DKAWIWGS

FIGURE 1 is a schematlc view of a raise bore drill showing the location of the drill in operation.
FIGURE 2 is a detail view, enlarged relatively to FIG. 1, of the raise bore drill showing the drill stem which is to be connected by the threaded connection of this invention.
FIGURE 3 is a section view of the threaded connection connecting the drill stem to the drill pipe, with Fig. 1.
EIGURE ~ is an enlarged detail sectional view of the threaded connection shown in FIG. 3. `~

DESCRIPTION OE T~IE PREFERRED ~5BODIMENTS
Mine raises are larye diameter inclined bores connecting tunnels at different levels ~nd used to transfer ore and earth by gravity to the lowermost tunnel from the tunnels above where-upon the ore and earth may be removed from the lowest tunnel by , ., ' - ~ 0~ 8 a suitable ~onveyor. Mine raise bores may range from four fee~
in diameter up to large bores which are substantially larger in size. In order to put enough axial load on drill pipe -to create enough pressure on a four foot bit to bore through rock or other hard earth formations it is necessary to bore with the drill p;pe in substantial tension. In order to bore a hole with the drill pipe in tension it is necessary to first bore a pilot hole of relatively small diameter through which the drill pipe is extended. The raise bore drill ;s then fastened to the end of the pipe and pulled upward under tension and rotated.
Due to the large size bit, e.g. four feet or larger, and the heavy axial loading of the bit the torque required to rotate the drill pipe is a very high resulting in extremely tight makeup. The drill pipe may be assembled initially hand tight and then makes up to a very tight connection as the drill is rotated. It is important that threaded connections make up sufficiently tight to carry the load adequately but 1t is also important that the makeup not be excessive which may tend to burst the box, collapse the pin, and strip and gall the threads. -An excessive makeup of a threaded joint also results in a high breakout torque which makes it very difficult to disassemble connections.
Even in drilling in tension, as in raise bore drilling, there are unusual bending moments imposed on a drill pipe carrying a large raise bit. The relatively slim pilot hole will limit lateral displacement of the drill pipe but repetitive bending stress will still be imposed on the threaded drill pipe `; connections. This tends to cause fatigue fa11ure. The failure in fatigue is a particularly severe problem at the connection from the raise bore drill stem to the bottom of the drlll pipe.

;. - '' , ,, ~, ~Oti 98~1 Refer~ing to F;gure 1, ther~ ;s shown a section throu~h the earth generally designated 1 having a horizontal tunnel 2 and a vertically extending pilot hole 3. A drill pipe ~ extends through pilot hole 3 and consists of a plurality of short sections of pipe connected by threaded joints 5 and 6. At the bottom end of the drill pipe 3 there is secured a raise bore drill 7 connected thereon by a threaded connection. As the pipe 4 is turned, the ra;se bore drill 7 is rotated and pulled upward boring out a large raise bore ~. The material dislodged from the drilling of the raise bore 8 either drops to the bottom of tunnel 2 or into a cart or other conveyor 9.
In Figure 2, the relation of the raise bore drill to the pilot hole and raise bore is shown in more detail. The raise bore drill 7 comprises a raise bore stem 10 having a threaded upper end 11 for connection to the drill pipe ~. Drill stem 10 is proYided with flats 12 cut in the upper end portion for -; receiving a wrench for makeup or breakout. Stelll 10 may also be provided with a plurality of wear pads 43 preferably of a hard wear resistant ma-terial such as tungsten carbide. At the lower end of stem 10 there are provided a pair of space plates ` 14 and 15 welded thereon and connected together to provide a base for the raise bore cutters. The cutters 16 are of a generally conical shape and carry on saddles or mounts 17.
The full details of construction of the raise bore cutter are not given inasmuch as this is a commercially available device of well-known construction. The novel features of this invention are concerned in particular with the threaded - connection 11 for connecting drill stem 10 to drill pipe 4.
In Figure 3, the threaded connection from drill stem 10 to drill pipe 4 is shown in more detail. Drill stcm 10 has C

~~ l Q~ 8 a pin 13 ~ormed on the end portion thereof extendin(J from ~
peripheral shoulder 1~. On the outer per;pheral surface of pin 13 are preferably formed two helical threads 15 an~ 16, although under some conditions a sin~le thread would suffice.
Pin 13 is externally tapered in the range of 1-1/4 to 1-3/4 inches diameter per foot, preferably 1-1/2 inch per foot as shown~ and has a maximum outer diameter considerably less than the outer diameter of stem 10. Between shoulder 14 and the adjacent ends of threads 15 and 16 there may be a tapered stress relief undercut area 17 which is tapered about the same as the threaded part of pin 13 and is preferably undercut about 1/32 inch in diameter below the threaded part of pin 13. ~ -The lower end of drill pipe 4 comprises box portion 1~
tapered the same as pin 13. The inner periphery of the box is unthreaded over an area 19 opposite the unthreaded undercut area 17 at the root of the pin 13. This unthreaded portion of the box forms the mouth 20 of the box. The remainder of the box is provided with two helical threads 21 and 22 extending from the mouth of the box to the juncture of the tapered bore ot the box with the cylindrical inner surface of drill pipe 4.
The crests of the first two turns of the box threads adjacent the mouth of the box are tapered but thereafter they vanish into the cylindrical surface defined by the inner peripheral surface of drill pipe 4.
The preferred dimensions of the pin threads are marked on Figure 4, from which it is apparent that the width of the thread crests is less than the width of the space on the pin between adjacent thread roots. In other words, the threads are narrower than the grooves. The box threads are of a similar construction whereby there are large helical spaces 23, 24 -1~5~8~1 ~
between the unloaded thread Elanks. The root of -the ~hrea~s, both of the pin ~nd o~ the box, are reli~ved as in~icated at 25 and 26 along a circ~lar curvature. The face of the indivi~ual threads is relatively flat and roun~ed along both the leading and flank edges. The relieved root portions 25 and 26 of the threads are relieved along a shallow circular curve which mer~es into the main body of the threads along a tangent to the thread surface. In addition, the relieved portions 25 and 26 are work-hardened by shot peening orthe like. The combination of a circular or curved relieved portion 25 and 26 at the root of ;~
the threads with the surface hardening provides an improvement in operation which will be described in more detail. ~ ~ -The length of the pin root and the box mouth are so related to the thread crest cones that when the pin shoulder 1~ and end face 27 of the box are engaged, even after full makeup, there are helical spaces 23 and 24 between the crests of the box threads and adjacent pin grooves. Specifically, to achieve this spacing, the box end face 27 is farther from the apex of the box thread crest cone than the pin shoulder 14 is from the apex of the pin roo-t cone. Also, the box end face 27 is farther from the apex of the box thread root cone than from the pin shoulder is from the apex of the pin thread crest cone.
The spaces 23, 24 become larger near the end of the pin due to the threads on the box vanishing into the cylindrical inner periphery of drill pipe 4~ The spaces are in communication with each other forming one continuous helical space between the threads. It will be seen that it is only the engagement of the loaded flanks of the threads that prevents the box from wobbling on the pin. The flanks of the threads are preferably at an angle of 15 degrees which is small enough to keep hoop ~ O~ 9 ~ ~

stress within reasonable limits and yet large enough to prevent wobble. The flank angle may be varied within a range of about 10 - 20 degrees.
The buttress form of the threads, that is, the 45 unengaged flanks, provides adequate strength to the threads to take the axial loads imposed not only by the tension of the pipe but also due to the bending moment tending to cause the box thread to flex or slide laterally relatively to the pin thread. The unengaged thread flanks may vary from 30 to 60 in angle.
Preferably the thread cross-section is such that the thread height is of the same order of magnitude as the average width.
Shoulder 14 and end face 27 may be slightly beveled at the ends. The bevels tend to reduce somewhat the likelihood of `~
galling.
The thread lead of one-inch per foot is not critical but rather preferable and may be varied within reasonable limits as from 3/4 inch per foot to 1-1/4 inch per foot. It is to be noted that the pin root and box mouth preferably have a length of one-inch from the shoulders 14, 27 to the commencement of the box and pin bevels leading up to the thread crest cones. The axial extent of the bevels is about 1/4 inch. The pin root and box mouth therefore have a length which is of the same order of magnitude as the lead of the thread. This length is sufficient to allow enough pin root stretch and box mouth compression after initial engagement of shoulder and end face to keep the connection tight without undue thread deformation.
It has been shown that the clearance prov;ded between the thread crests and inter-root spaces, whlch is 0.005 lnch when the pin shoulder and box end face are made up hand tight, as shown in Fig. 1, is sufficient with the taper of 3/4 inch per foot of length to allow .08 inch axial travel without the crests ~L~698~

engaging with the inter-roo-t spaces. Actually, a llttle more travel can occur without such engagement due to hoop stress in the pin and box. Axial makeup of .08 inch with a one-inch lead can be accomplished with about 1/12 revolution.
The subject connection may be used with pipes of a wide ;
range of diameter, e.g. 4 - 12 inches. The normal diameter of the pin may be varied according to the diameter of the raise bore stem in order to achieve a de~sired balance of section modulus while maintaining a satisfactory pin d;ameter. For example, a 4-3/4 inch nominal pin diameter can be used with a 6-1/~ inch OD raise bore stem; a 5-3/4 inch nominal pin diameter can be used with a 6-3/~ inch or 7 inch OD raise bore stem;
a 6-3/4 inch nominal pin diameter can be used with an 8 inch raise bore stem. In all sizes of raise bore stem and drill pipe, however, the same pin and box taper may be used, the lead may remain the same, and the thread cross-sectional dimension may remain the same.
The basic thread design described is found in U.S. Patent

3,355,192 as to many of the features thereof. The primary difference from the design found in that patent lies in the relieved root portion of the threads and the surface hardened relieved portions. The surface hardened relieved root portions have been found to improve the fatigue or endurance limit of the threaded connection substantially in raise bore drilling operations. The surface hardened relieved root portion reduces the stress concentration in the thread root and places the surface of the thread root in compression. The improvement in fatigue life or endurance has been found in field tests to be of the order of 30 - 50 percent relative to threaded connections of the same type which are not provlded with the surface hardened g ~9~81 and relieved root portions. In fact, previous experience with the basic thread design without the surface hardened relieved portions has shown that when failure has occurred it has almost always been fatigue failure at the threaded joint. Since this improved threaded connection with the surface hardened relieved thread roots has been extensively tested over a period of many months there have been no failures from fatigue.
The improved threaded connection shown and described herein has been applied as an improvement to the basic thread design of U.S. Patent 3,355,192 inasmuch as that thread is used largely in drill pipe connections and in raise bore threaded connections where high tensile, twisting, and bending forces are encountered.
The curved relieved and surface hardened root structure of the threaded connection is applicable to other threads which might be used in connecting raise bore stems to drill pipe. In most cases, however, such threads would be of a buttress thread construction because of the high stresses encountered.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a rotary drill for producing a raise bore along a pilot hole in an earth formation having a stem member for connection to and rotation by a string of drill pipe, said stem member having a plate member thereon in a plane normal thereto supporting a plurality of roller cutters arranged to cut an enlarged bore in the earth on rotation and raising of said drill pipe; the improvement which comprises a threaded joint between said stem member and pipe consisting of a threaded pin and box having tapered mated threads, said threads comprising helical buttress threads having curved, work-hardened, relieved helical roots with a shallow curvature having a radius of substantial length in relation to the depth of the threads and terminating in curved edges of short radius tangent to the faces of said threads.

2. A drill according to Claim 1 in which said threads are double buttress threads.

3. A drill according to Claim 1 in which the thread roots are hardened by peening.

4. A drill stem for use in a rotary drill for producing raise bore along a pilot hole in an earth formation adapted to be connected to a string of drill pipe, said stem being connected to a plate member normal thereto and supporting a plurality of roller cutters arranged to cut an enlarged bore in the earth on rotation and raising of said drill pipe; the improvement in said stem comprising a male tapered thread for connection to a female thread on said drill pipe, said thread comprising helical buttress threads having curved, relieved, work-hardened, helical roots with a shallow curvature having a radius of substantial length in relation to the depth of the threads and terminating in curved edges of short radius tangent to the faces of said threads.

5. A drill pipe for connection to the stem of a rotary drill for producing a raise bore along a pilot hole in an earth formation having a stem with a plate member thereon in a plane normal thereto supporting a plurality of roller cutters arranged to cut an enlarged bore in the earth on rotation and raising of said drill pipe; the improvement in said drill pipe which com-prises a female thread for connection to a mating thread on said drill stem, said female thread being a tapered helical buttress thread having curved, relieved, work-hardened, helical roots with a shallow curvature having a radius of substantial length in relation to the depth of the threads and terminating in curved edges of short radius tangent to the faces of said threads.