Screw coupling joint

A screw coupling joint having a sealing portion between an outer circumference formed in convex in an axial direction of the coupling joint at the end of the male screw portion and an inner circumference formed in taper at an inner part of the female screw portion. The female screw portion has a stopper end positioned to be contacted, when the coupling joint is assembled, by the inner end of the male screw portion. The taper is between 1.degree. and 3.degree. and the inner diameter D.sub.1 of the female screw and the outer diameter D.sub.2 of the male screw have the relationship D.sub.2 >D.sub.1 at a tangent point therebetween. The end of the male screw portion and the end of the stopper are vertical or perpendicular to the axis of the coupling joint.

A known screw coupling joint for use in oil wells is provided with a metal 
sealing portion between an outer circumference formed in the convex at an 
end of a male screw and an inner circumference formed in taper in the 
inner part of a female screw. However, because such coupling, when made up 
into a joint generally has a clearance between the end of the male screw 
and the terminal of the female screw, eddy currents are caused in the 
clearance and corrosion is accelerated. In such structure having such 
clearance, the male screw is easily driven too much and hoop tension is 
exceeded, subjecting the female screw to over tensioning and inviting 
stress corrosion at the clearance. In addition to the above mentioned 
sealing structure, there has been proposed a coupling joint which is 
provided with a stopper at the inner end of the female screw for 
contacting the end portion of the male screw, and the end portions of the 
male screw and stopper are formed in obliquity around 10.degree. to 
30.degree., respectively. Depending upon such structure, the clearance is 
not formed. The pushing-up force act on the male portion, due to the 
obliquity, so that the outer circumference of the male screw, forming the 
seal, is strongly urged to the inner circumference of the female screw and 
the sealing property is advantageously improved. However, for actuating 
the pushing-up force, it is necessary to urge the end portion of the male 
screw to the end portion of the stopper and, therefore, the bending effect 
or tearing-off force by the wedging effect act on this part and it is 
easily affected with stress corrosion. 
The purpose of this invention is to improve coupling structure and remove 
the disadvantages of the prior art. 
It is an object of the invention to provide a coupling structure with high 
sealing properties and prevent tearing-off force at the root of the 
stopper. 
It is another object of the invention to provide a coupling structure which 
avoids stress corrosion at the female screw member. 
It is a further object of the invention to provide a coupling structure 
which may easily control, in design, contacting pressure between the male 
and female screw portions. 
BRIEF DESCRIPTION OF THE INVENTION 
For accomplishing the above mentioned objects in the instant invention, a 
screw coupling joint is provided having a threaded male screw portion and 
a threaded female screw portion with a sealing portion between an outer 
convex circumference formed in an axial direction at an end of the male 
screw member and an inner tapered circumference formed at an inner part of 
the female screw member, and is structured by contacting the end of the 
male screw member to a stopper at the inner part of the female screw 
member, wherein the taper is 1.degree. to 3.degree., and the inner 
diameter D.sub.1 of the female screw member and the outer diameter D.sub.2 
of the male screw member have the relation D.sub.2 &gt; D.sub.1 at a tangent 
point therebetween, and the end of the male screw member and the stopper 
end of the female screw member are formed perpendicular to the axis 
thereof.

FIGS. 1 and 2 show an embodiment of the present invention in a coupling of 
directly connected end to end pipes. In the drawings, A.sub.1 and A.sub.2 
are pipes connected by a coupling of one embodiment of the invention. The 
portion 1 of pipe member A.sub.1 is a male screw, while the portion 2 of 
pipe member A.sub.2 is a female screw. The outer circumference 11, at the 
end portion of the male screw portion 1, is a smooth face, without screw 
thread and is formed in convex in the axial direction. The inner 
circumference 21 at the inner part of the female screw portion 2 is a 
smooth face, without screw threads and is formed in taper in the axial 
direction. A sealing portion S is formed between the smooth convex face of 
male screw portion 1 and the smooth tapered face of female screw portion 
2, forming a seal between outer circumference 11 of male portion 1 and 
inner circumference 21 of female screw portion 2 when the coupling is 
assembled, as hereinafter described. 
Inner circumference 21 of female screw portion 2, at the inner end, 
terminates in stopper 3 having a terminal face 31 contacted by end face 12 
of male screw portion 1 when the coupling is assembled, as hereinafter 
described. 
In the above structure, the terminal end face 31 of stopper 3 and end face 
12 of male screw portion 1 contacting terminal end face 31 are formed 
vertically, that is, are 90.degree. with respect to the pipe axis, in 
order not to apply a pushing-up force to the male screw portion 1 on the 
assembling of the coupling and to maintain the sealing property of the 
sealing portion S through regulation of the size of the outer 
circumference of female screw portion 2 and the size of the inner 
circumference of male screw portion 1. 
In the conventional coupling structure, there is instability in sealing 
resulting from restraining the terminal of the male screw portion by the 
stopper which is cancelled by the pushing force due to the obliquity of 
the terminal of the male screw portion and the end portion of the stopper. 
On the other hand, in the arrangement of the present invention, the taper 
of the inner circumference 21 of the female screw is 1.degree. to 
3.degree. and, further, the inner diameter D.sub.1 of the female screw 
portion and the outer diameter D.sub.2 of the male screw portion has a 
relationship of D.sub.2 &gt; D.sub.1 to thereby maintain the sealing property 
of sealing portion S. The taper angle of the inner circumference of the 
female screw portion 2 is relatively small. Thus, the tangent point is 
securely provided when the end portion 12 of male screw portion 1 contacts 
stopper 3 at terminal 31. Sealing between male screw portion 1 and female 
screw portion 2 is stabilized by the foregoing relationship. For reasons 
which are obvious, the lead thread of male screw portion has a smaller 
diameter than the lead thread of female portion 2 to avoid interference 
between the respective lead threads when the coupling is being assembled. 
In the one embodiment of FIGS. 1 and 2 described above, a stopper 4 is also 
provided about male screw portion 1 of pipe member A.sub.1 and the end 
portion 22 of female screw portion 2 of female pipe member A.sub.2. Thus, 
as the pipes are assembled to form the sealed coupling, end portion 22 of 
female screw portion 2 contacts surface 41 of stopper 4, to form a third 
metal touching seal, that is, the metal touching seals at the seal S, at 
3, between surfaces 12, 31 and at 4, between surfaces 22, 41. 
FIG. 3 shows a further embodiment of the invention in which pipes to be 
coupled end to end are coupled with a sleeve. As shown in FIG. 3., sleeve 
B has a female screw portion 2 and an inwardly projecting portion 5 at the 
axial center of the sleeve, the opposite ends of the projection 4 being 
provided with stoppers 3, 3, respectively. The ends of the pipes, FIG. 3, 
are provided with tapered, threaded male ends 1 with outer convex 
circumferences in an axial direction at the end of the male screws. The 
coupling sleeve B has, at its opposite ends, female screw portions 2, only 
one of which is fully shown in FIG. 3 but which, except for facing in 
opposite directions, are identical to the screw portions, tapered portion 
and convex portion, shown and described with respect to FIGS. 1 and 2. 
Thus, in FIGS. 1, 2 and 3, where identical reference numerals are used, 
the referenced elements and there relative relationships and angularity, 
where described, are to be taken as the same. 
In the above screw coupling joint of the present invention, the outer 
diameter D.sub.2 of the male screw portion 1, slightly larger in diameter 
than the inner diameter D.sub.1 of the female portion 2 when such 
diameters are brought into contact during assembly, is reduced in diameter 
at the tangent point as the convex and tapered circumferences are brought 
into contact and as male screw portion vertical end face 12 is brought 
into contact with terminal face 31 of stopper 3. High contacting pressure 
and secure contact conditions are provided by the taper angle of the inner 
circumference of the female screw portion. Further, because the end face 
12 of the male screw portion and the terminal 31 of stopper 3 are 
vertical, the pushing-up force, encountered in the prior art, does not 
occur on the end face of the male screw portion. Thus, tearing off forces 
about the root of the stopper are avoided and stress corrosion is 
prevented. Further, because the pushing-up force does not occur, the 
contacting pressure may be easily controlled, in design, at the sealing 
parts. 
In the assembly of the pipe coupling of the instant application torque is 
applied between the male and female pipe members of FIGS. 1 and 2 or 
between the male pipe member and female coupling member in FIG. 3. The 
applied torques to first contact and initiate deflection of the tapered 
male screw portion S, FIGS. 1, 2 and 3, is designated T.sub.2 in FIG. 2. 
The torque to engage the male and female threads is designated T.sub.4, 
FIG. 3, while the final torque to engage end face 12 of male tread portion 
1 with terminal 31 of stopper 3 is designated T.sub.1, FIG. 2. As already 
noted, relatively, the torques in both embodiments are the same. 
The relationships between the torques T.sub.2, T.sub.4 and T.sub.1 are 
shown in diagram in FIG. 4. If T.sub.1 exceeds the yield strength of the 
pipe material, rupture or breakage of the pipe at the coupling can occur. 
If, on the other hand, T.sub.1 is less than the yield strength of the pipe 
material, an incomplete coupling is formed and leakage might result. 
Therefore, T.sub.1 should be within the range between the maximum and the 
minimum of the yield strength, preferably between 30% and 80% of the yield 
point. For example, in a pipe having an outer diameter of 2 7/8 in., 
nominal wall of 6.50 lb./ft. and yield point of 55 ksi, for applying a 
compressive stress corresponding to 30% of the yield point of the material 
to the sealing parts 12 and 31 of the internal shoulder, a T.sub.1 
tightening torque of 60 kgm is required. For applying a compressive stress 
corresponding to 80% of the yield point of the material to the sealing 
parts 12 and 31, a tightening torque of 131 kgm is required. 
For purposes of design of couplings in accordance with the instant 
invention, the values or T.sub.2, T.sub.4 and T.sub.1 may be calculated, 
as follows: 
##EQU1## 
Wherein: 
.mu.:Friction coefficient 
E: Young's Modulus 
S: Diameter of tangent point of male screw 
G: Diameter of tangent point of female screw 
YP: Yield point of the material 
dso: Outer diameter of inner shoulder 
dsi: Inner diameter of inner shoulder 
dTG; djM: Outer diameter of main seal part 
P: Surface pressure of main seal part 
WTG: Contacting width of main seal part 
djo: Outer diameter of coupling 
dji: Inner diameter of pipe 
dT: Average diameter of screw 
WT: Length of screw 
Applying the foregoing to a pipe having a diameter of 2 7/8 inches and 
nominal walls, as listed, the preferred minimum and maximum values of 
T.sub.1 are calculated, as follows: 
______________________________________ 
Outer Nominal 
Diameter 
Wall 55 ksi 
(inch) (lbs/ft) 0.3 YP 0.8 YP 
______________________________________ 
27/8 6.50 60 kgm (435 lb/ft) 
131 kgm (949 lb/ft) 
7.90 80 kgm (580 lb/ft) 
176 kgm (1275 lb/ft) 
8.79 98 kgm (645 lb/ft) 
200 kgm (1449 lb/ft) 
9.50 95 kgm (688 lb/ft) 
219 kgm (1587 lb/ft) 
10.70 109 kgm (790 lb/ft) 
254 kgm (1840 lb/ft) 
______________________________________ 
The terms and expressions which have been employed in the foregoing 
description are used as terms of description and not of limitation, and 
there is no intention, in the use of such terms and expressions, of 
excluding any equivalents of the features shown and described, or portions 
thereof, but it is recognized that various modifications are possible 
within the scope of the invention claimed.