Formation of threaded elements having a self-locking action

A fatigue resistant self-locking fastener is formed by screwing two threaded elements into engagement with a part having a thread which is slightly off-size with respect to the threads of those elements, locating the two elements just in contact with one another while in engagement with said part but without tightening them against one another sufficiently to develop any substantial stress in the threads, and then securing the elements together in that relative position, preferably by welding. Because of the off-size relationship of the locating part, the threads of the two elements when so located by the part are aligned axially with one another but are turned relative to one another about their common axis to positions in which the thread of one part is not precisely a true helical continuation of the thread of the other part, so that the two threads when subsequently screwed into engagement with a single coacting threaded element will have an interfering fit therewith attaining a self-locking action and improving resistance to fatigue and tensile failures.

BACKGROUND OF THE INVENTION 
This invention relates to an improved method for manufacturing threaded 
fasteners. 
In my prior application Ser. No. 111,774 filed Jan. 14, 1980 on "Fatigue 
Resistant Self-Locking Nuts and their Manufacture," now abandoned, I have 
disclosed a unique threaded fastener which in a single structure 
accomplishes the dual purposes of attaining a highly effective 
self-locking action resisting unscrewing retation of the fastener from a 
coacting threaded element, and also achieving a vastly improved 
distribution of load forces as compared with conventional fasteners in a 
manner enabling the overall fastener to withstand higher load forces 
without failure from either tensile or fatigue effects. Instead of the 
load forces being taken predominantly by the threads at the lower portion 
of the nut, that is, the turns which are closest to the load bearing face 
of the nut, the structure of the fastener is such as to shift a large part 
of the forces which would normally be localized in the bottom of the nut 
to an upper portion of the nut, with resultant distribution of those 
forces over essentially the entire axial extent of the nut. At the same 
time, the locking action produced by the fastener is more effective than 
that found in most prior self-locking nuts, and is of a character to 
retain the self-locking action through many cycles of connection and 
detachment of the threaded parts. Further, the fastener besides 
distributing the primary load forces more uniformly than in prior 
fasteners also distributes the self-locking forces more uniformly between 
the different turns of the threads. 
The fastener of that prior invention includes a first body having a first 
preferably internal thread and a second body having a second preferably 
internal thread, with the two bodies and their carried threads being 
attached together in fixed axially aligned relative positions enabling the 
composite fastener assembly and both of its threads to be screwed into 
engagement with a single coacting threaded member. The two bodies are 
desirably secured together by fusion bonding, i.e. welding, brazing or 
soldering, electron beam welding being preferred in most instances where 
maximum strength is required. In the composite assembly, the two threads 
are turned slightly relative to one another about their common axis to 
positons in which neither is a true helical continuation of the other, so 
that they engage a coacting threaded member slightly differently and have 
an interfering fit therewith causing a self-locking action resisting 
unscrewing rotation of the composite fastener from the coacting member, 
and at the same time distributing axial load forces more uniformly among 
the different turns of the engaged threads. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide an improved method for 
forming a composite nut or other threaded fastener of the above discussed 
type, and particularly for relatively positioning the two nut sections for 
the welding or other attaching operation by screwing the sections into 
engagement with a locating part, preferably an externally threaded 
mandrel, but in a manner avoiding the development of any excessive 
stresses in the locating part or fastener sections which might resist 
unscrewing of the composite fastener from the locating part after welding. 
To achieve this result, I employ a locating part whose thread will screw 
into engagement with and mesh with the threads of the two sections, but 
which is dimensioned to allow slight axial movement of at least one and 
preferably both of the elements relative to the locating part. For 
example, a mandrel may have an external thread onto which two nut sections 
can be screwed, but with the mandrel threads being slightly undersize with 
respect to the nut threads to allow some axial play. When the two nuts or 
fastener sections are then screwed just into engagement with one another, 
the looseness in the threads will result in automatic precise locating of 
the thread of one section in a position turned slightly from a position of 
true helical alignment with the other thread. After welding of the 
sections together in this condition, the assembly can be easily unscrewed 
from the locating part, but when subsequently screwed into engagement with 
a normal thread of another part, will have the desired self-locking and 
load distributing characteristics with respect thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, there is represented at 10 a fatigue resistant self-locking nut 
of a type disclosed in my prior application Ser. No. 111,774, now 
abandoned, and which may be formed by the method of the present invention. 
The nut is typically illustrated as connected to a coacting screw or bolt 
11, with plates 12 clamped between the enlarged diameter head 13 of the 
screw and an annular load-bearing face 14 formed at the inner or lower end 
of the nut and disposed transversely of axis 15 of the screw and nut. The 
shank 16 of the screw has external threads 17 of uniform diameter along 
the entire axial extent of the shank and typically illustrated as of 
standard 60.degree. configuration as viewed in axial section. It will of 
course be understood that the particular overall assembly of parts shown 
in FIG. 1, including the plates 12 clamped between screw 11 and nut 10, is 
merely illustrative of one of the many environments in which a nut 
embodying the invention may be employed. 
The composite nut 10 includes two initially separately formed nut bodies 18 
and 19 having internal threads 20 and 21 respectively which may be 
identical with one another and are of uniform diameter, and have a thread 
profile and diameter corresponding to and adapted to engage the external 
threads 17 on screw 11. Bodies 18 and 19 are essentially rigid, and in 
most instances are formed of metal, such as steel, but are adapted to be 
deformed very slightly when tightened against a work piece such as plates 
12, and to resiliently resist such deformation in a manner developing 
internal stresses within the material of the nut bodies which act to 
return those bodies by the resilience of the material of which they are 
formed to their initial condition when the load forces are released. The 
two bodies 18 and 19 are initially formed with transverse end faces 22 and 
23 disposed perpendicular to axis 15 and adapted to abut annularly against 
one another to transmit axial forces between bodies 18 and 19. The bodies 
are rigidly secured in fixed relative positions, desirably by fushion 
bonding them together annularly at 24, preferably entirely about axis 15 
and at the locations of the two abutting surfaces 22 and 23. In some 
instances, it is contemplated that other means of attachment of the two 
bodies rigidly together may be utilized. The preferred method of 
attachment is by electron beam welding, as represented diagrammatically in 
FIG. 4, in which 25 represents an electron beam gun disposed perpendicular 
to the axis 15 of nut 10 and aimed radially inwardly at the location of 
abutting surfaces 22 and 23, to form the annular weld 24 between bodies 18 
and 19 as those bodies are turned about axis 15 relative to the gun 25. 
This relative rotation may be effected either by maintaining the electron 
beam gun 25 stationary and rotating the nuts, or vice versa by maintaining 
the nut bodies stationary and moving the electron beam gun 25 circularly 
about axis 15. The result is to fuse the material of bodies 18 and 19 
together, annularly about axis 15, and radially inwardly from their 
peripheries at the locations of surfaces 22 and 23 far enough to attain 
the desired rigid connection between the two nut bodies. 
Externally, the upper or outer nut body 18 may be noncircular to be engaged 
and turned by a wrench, while the lower body 19 may be externally circular 
and desirably larger in diameter than body 18 for most effective 
transmission of axial load forces to plates 12. The outer surface of body 
19 may flare frustoconically at 119 to an enlarged diameter, and then 
extend cylindrically at 219. In most instances, body 18 should be 
externally either hexagonal as illustrated in FIG. 2, or of 12 point 
wrench-engaging configuration. 
In the arrangement of FIGS. 1 to 3, the internal threads 20 and 21 in nut 
bodies 18 and 19 are identical and are formed integrally with those 
bodies, as by initially forming bodies 18 and 19 in unthreaded form and 
then tapping or otherwise machining threads in their interior. It is 
contemplated broadly that the threads 20 and 21 and the engaging external 
threads 17 on screw 11 may be of any desired thread profile, but in most 
instances standard 60.degree. threads are utilized. The drawings typically 
illustrate such standard threads. The two nut bodies 18 and 19 are 
preferably of identical axial extent, with that axial extent desirably 
being such that each of the internal threads 20 and 21 has several turns 
between its upper and lower ends (say three or four turns). The standard 
threads, as viewed in axial section, have opposite side faces 28 (see FIG. 
5) disposed at a 60.degree. angle to one another and have directly axially 
extending minor diameter peak portions 29 and directly axially extending 
major diameter surfaces 30. The pitch of the threads in each of the nuts 
18 and 19 is uniform for the entire axial extent of that body. More 
specifically, considering the upper nut body 18, the pitch distance a 
between the centers of successive turns of thread 20 in that body is 
uniform from the location of bottom surface 22 of the nut body to the 
upper surface 122 of that body. Similarly, the pitch distance axially 
between the centers of successive turns of lower nut 19, as viewed in 
axial section, is uniform from the bottom face 14 of nut 19 to its upper 
end surface 23, and is equal to the pitch distance a of upper body 18. The 
external thread 17 of screw 11 has an axial section essentially the same 
as that of the nut threads, with the opposite side faces 31 (see FIG. 1) 
of the screw threads being disposed at a 60.degree. angle to engage and 
apply force axially to faces 28 of the nut threads 20 and 21. 
The two nut bodies 18 and 19 and their internal threads 20 and 21 are 
aligned with one another axially, both being centered about the common 
axis 15. In order to introduce a self-locking and load distributing effect 
into the composite nut 10, the two nut bodies 18 and 19 are secured 
together by weld 24 in relative positions in which internal threads 20 and 
21 have an interfering fit with thread 17 of screw 11. Stated differently, 
it may be considered that upper body 18 before attachment to lower body 19 
is turned relative to lower body 19 about axis 15 through a slight angle, 
preferably between about 10 and 60 degrees, so that the upper thread 20 is 
then not a true helical continuation of lower thread 21, and therefore 
does not engage the thread of screw 11 in exactly the same manner as does 
thread 21. Referring to FIG. 5, which is an axial section through portions 
of the two threads 20 and 21, in a plane containing central vertical axis 
15 of the composite nut, and with threads 20 and 21 in the relative 
positions in which they are welded together, it is noted that as viewed in 
axial section, each of the individual turns 32 of thread 20 is in effect 
shifted upwardly a short distance b from the position which it would 
assume (position represented in broken lines at 34) if body 18 were in a 
rotary position in which its thread was located to be a true helical 
continuation of the internal thread of body 19. The result is that, still 
considering the two connected nut bodies as viewed in axial section (FIG. 
5), the effective pitch distance c between the uppermost turn of lower nut 
body 19 and the lowermost turn of upper nut body 18 is slightly greater 
than the pitch distance a between successive turns of nut 18 and between 
successive turns of nut 19, the difference between the pitch distances a 
and c of FIG. 5 being the previously mentioned dimension b through which 
the thread of nut 18 as viewed in axial section appears to be shifted 
upwardly relative to the thread of nut 19 as a result of the relative 
rotation of the two parts before securing them together. This axial 
offsetting of the two threads, which may be somewhat exagerated in FIG. 5 
for illustrative purposes, is just sufficient to attain a predetermined 
self-locking torque and load distributing effect in the overall composite 
nut 10. 
The present invention is concerned with an improved method of manufacturing 
the composite nut 19 thus far described, and particularly with the manner 
in which the two nut bodies are precisely located relative to one another 
for the welding or other connecting operation in a relation assuring 
exactly a predetermined degree of interference between the nut and screw 
threads. In accordance with that method, the two nut bodies 18 and 19 are 
first separately preformed with internal threads as discussed, following 
which they are both screwed onto a single mandrel 35 having an external 
thread 36. The two nuts are turned relative to one another on the mandrel 
to positions in which they just touch one another and may have slight 
frictional contact but preferably do not develop any substantial axial 
stresses in the mandrel or nut bodies (see broken line position of nut 18 
relative to nut 19 in FIG. 3). The mandrel is so formed that when the nut 
bodies are thus just in contact with one another, the nuts are 
automatically located in the relative rotary positions of FIGS. 1 to 3, in 
which their threads are not precisely true helical continuations of one 
another but are turned through a small angle, preferably between about 10 
and 60 degrees, from positions in which one would be a true helical 
continuation of the other. The nut bodies are then annularly welded 
together in that relative position as represented in FIG. 4, either by 
rotating the electron beam gun 25 about mandrel 35, and the carried nut 
bodies and their axis 15, or by rotating the mandrel and nuts while 
maintaining the electron beam gun stationary. The mandrel is preferably 
formed of copper to effectively conduct heat away from the nut threads 
during the welding operation and prevent any adverse effect on those 
threads by the heating. After the welding has been completed, the 
composite nut assembly can be easily unscrewed from the mandrel since the 
nut bodies have not been tightened against one another to an extent 
developing any substantial interference with respect to the specially 
formed mandrel thread. 
The mandrel thread 36 is given its capacity for properly locating the two 
nut bodies without tightening them together to an extent developing as 
much interference with the mandrel as is ultimately desired between the 
composite nut and screw 16, by forming the thread 36 of the mandrel to be 
slightly undersize as compared with the nut threads 20 and 21. As seen in 
FIG. 5, thread 36 may have an axial sectional profile essentially the same 
as that of internal threads 20 and 21 of the nut bodies, with the opposite 
side faces 37 of each turn of thread 36 being disposed at a 60 degree 
angle to one another, but with those faces and the major and minor 
diameter portions 38 and 39 of thread 36 all being at diameters slightly 
less than the corresponding side faces 28 and major and minor diameter 
surfaces 30 and 29 of the nut threads. As a result, the thread of each nut 
body does not interfit closely with the mandrel thread, and permits slight 
axial movement of the nut relative to the mandrel when the nuts are not 
engaged with one another. When the nuts are turned to positions of contact 
with one another, however, as in FIGS. 4 and 5, and the play between the 
mandrel and nuts is taken up and the mandrel thread engages against the 
thread of nut 18 in one axial direction and against the thread of nut 19 
in the opposite axial direction in a manner urging the nuts toward one 
another and holding their faces 22 and 23 in contact. More specifically, 
the mandrel thread engages thread 20 of nut body 18 at 40 and is spaced 
therefrom at the opposite side 41, to exert downward force against nut 18 
while the mandrel thread engages thread 21 of nut 19 at 42 and is spaced 
therefrom at 43 to exert upward force against nut 19. In this condition, 
the upper thread is not exactly a true helical continuation of the lower 
thread but is turned through the desired angle from such a position of 
true helical alignment, so that when the nut bodies are welded together 
their threads will be in the relative position of FIG. 5. 
After the composite nut 10 has been removed from the mandrel, the nut can 
be screwed onto a bolt or stud as represented at 11, and will have an 
enhanced self-locking and load distributing action with respect thereto. 
As the lower nut 19 is first advanced onto screw 11, the various turns of 
thread 21 of course mate exactly with the external thread 17 of the screw, 
and can be turned onto the screw with little or not frictional resistance. 
When the lower turn of thread 20 reaches a point of contact with thread 17 
of the screw, however, that thread 20 does not move as easily into 
engagement with the thread of the screw, but rather has an interfering fit 
therewith requiring slight deformation of the nut and/or screw threads in 
order to advance the nut farther onto the screw. Considering again FIG. 5, 
as the portion of the lower turn of thread 20 in body 18 which is 
illustrated in that figure reaches a point of contact with the external 
thread of the screw, that thread of the screw exerts force downwardly 
against the lower turn of the thread in body 18 to pull that turn 
downwardly closer to a position of true helical alignment with the thread 
of lower body 19. At the same time, upward forces are exerted on the 
thread of lower body 19, thus slightly resiliently deforming the threads 
of both bodies, and opposing forces are exerted on the thread of the screw 
to slightly resiliently lengthen the screw axially. As the rest of the 
turns of upper body 18 advance onto the screw, similar forces are exerted 
against those turns, with the result that the internal stresses developed 
in the nut and screw as they are slightly resiliently deformed maintain 
the engaged threads of the nut and screw in tight frictional engagement 
resisting unscrewing rotation of the nut from the screw and thereby 
attaining a self-locking action having a torque of predeterminable value 
dependent upon the extent to which bodies 18 and 19 were turned relative 
to one another before they were welded together. This self-locking action 
is not destroyed by application of the nut to a screw, but rather will 
repeat reliably for many uses of the nut. 
When the composite nut 10 of FIG. 1 is tightened downwardly against work 
pieces 12, the above discussed relative orientation of the threads of nut 
bodies 18 and 19 causes the axial load forces, and the self-locking 
forces, to be distributed more effectively over the different turns of the 
nut thread than if a nut of uniform pitch were utilized. Whereas in a 
conventional nut most of the load forces are taken by the lowermost turns, 
and failure therefore occurs at that location, the relative rotary 
orientation of threads 20 and 21 in FIG. 1 has the effect of shifting much 
of the load which would normally be taken by the lower turns of the nut to 
the upper thread 20 in nut body 18. This enables both the nut and screw 
threads to take a greater overall tensile load without damage to either, 
and also greatly increases their resistance to fatigue load failure under 
repetitive or fluctuating stress and strain conditions. 
While a certain specific embodiment of the present invention has been 
disclosed as typical, the invention is of course not limited to this 
particular form, but rather is applicable broadly to all such variations 
as fall within the scope of the appended claims. For example, though the 
invention has been described as applied to composite nuts having two 
sections welded together, it will be understood that the method of the 
invention may also be used for welding or otherwise securing together 
three or more nut bodies to form a multi-section nut. In addition, it is 
contemplated that one or more of the individual nuts may be of a type 
having a thread which is initially formed separately from the body of the 
nut, as a coiled element inserted into and welded within the body, as for 
instance in accordance with the teachings of my prior U.S. Pat. Nos. 
4,040,462 and 3,938,209.