Note: Descriptions are shown in the official language in which they were submitted.
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.. ~ BACKGROUND OF THE INVENTION
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A search has disclosed various types of locking thread -
`~ designs wherein a binding pressure is provided between the threads
of male and female elements to produce a positive lock. Such prior
art de6igns, for example, are shown in U.S. patentB Nos. 1,657,244;
1,697,118; 1,798,604; 1,817,295 and 1,828,856 and in French Patent
¦ No. 40,199 of 1932. These prior art locking thread designs are
believed to operate satisfacto`rily in theory, but from a practical
and commerical standpoint, are unacceptable due to the tolerance
limitations under which modern thread forming equipment must
operate. The locking thread design of the present invention on
the other hand, may be readily formed with conveniently available
equipment and ln accordance with current tolerance limitations.
`~ More importantly, the thread forms of the present invention are not
i intended to lock by binding action, as is the case with the thread
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forms of the aforementioned prior art patents but instead, locking
- action is achieved by preventing relative motion between the mating
parts and results in a locking action between the threaded elements
that appears to be at least as good as, if not superior to the
locking performance of the aforesaid prior art locking thread
deRigns .
SUMMARY OF THE INVENTION
The locking thread form of the present invention is
illustrated, by way of example, as embodied on a nut and bolt
which are employed to clamp a pair of elements in tight secured
engagement with each other. The subject thread form may be opera-
tively associated with both Standard and buttress-type threads and
utilizes a flat area or ramp at the thread root of either or both
of the bolt and nut threads. In the case where the locking thread
form is used on both elements having buttress-type threads, the
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t '. ramp at the thread root on the bolt is disposed at an angle of
approximately 30 from the bolt's axis, while the ramp at the
~ thread root of the nut is disposed at an angle of approximately
i` 22-1/2 to the nut axis. The purpose of the angles at the thread
roots i8 to allow the threads to be manufactured to normal commer-
c~al tolerances and still always make contact with the crown of the
` matlng thread and thereby prevent lateral movement between the
threaded members and thus prevent loosening under vibration or
'~ other adverse conditions. In the case where the ramp i8 utilized
on one of a pair of elements provided with Standard-type threads,
thc anglc of thc ramp may be varied within limits in accordance
with the class fit, size threaded elements and acceptable tolerance
ranges to provide for optimum locking ability with commerically
available equipment and technology. It is to be noted
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that the aforesaid angles for buttress-type threads are given by
way of example and apply particularly when the nut tfemale element)
is fabricated of a relatively softer material than the bolt or male
element, and that the angles may be the same or relatively larger
or smaller on the male and female elements depending upon the
relative degree of hardness of the materials from which these
elements are formed.
The thread design of the present invention is free running
ùnder non-loaded or lightly loaded conditions; however, at such
time as the degree of loading reaches a predetermined magnitude,,~
the thread crown or crowns move into contact with the ramp or ramps
of the opposing threaded element. Under such locking conditions,
' the crown of ~he softer thread (usually of the nut) may, depending
on the relative sizes of the mating parts, become relatively de-
formed, with an increase in magnitude of the clamping pressure
applied between the bolt and nut on the two assembled elements
causing a corresponding increase in the amount of deformation of
the threads and hence a greater degree of contact between the
threaded elements. Complete tightening of the nut on the bolt
re8ults in full engagement of the thread faces which occurs, for
example, in the case of a one-half inch diameter bolt having
"` buttress-type threads when an approximately 90 foot-pount force is
applied.
In summary of the above, therefore, the present invention
may be broatly defined as providing a fastener combination
comprising a threaded male element and a threaded female element
adapted to receive the male element in threaded engagement there-
with; each of the male and female threaded elements having a thread
form thereon defined by leading and trailing helical flanks which
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intersect to define a crown, each flank lying at an angle relative
to the common longitudinal thread axis of the fastener combination;
one of the elements including a truncated root defining a helical
flat extending between the trailing flank and the adjacent leading
' flank which faces the trailing flank, and the flat, throughout its
. axial extent, lying at an angle between about 15 and 30 relative
to the thread axis whichis substantially less than the angle of
~ the other element flank which tends to converge on the flat under
ax$al tension load, the flat angled radially outward from the trail-
;~ 10 ing to the adjacent leading flank; whereby axial tension loading
of the combination tends to displace the crown of the other thread into
contact with the flat to prevent movement between the elements,
~ transverse to the axis, and lock the elements together.
¦ BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a view in elevation of a bolt clamping two
elements together on a thread embodying features of the present
invention;
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Figure 2 is an enlarged broken view of the bolt thread
illustrated in Figure l;
. Figure 3 is an enlarged broken sectional view of the
thread of the nut and bolt when in free-running relation to each
other;
c Figure 4 is a view of the structure illustrated in
Figure 3 after the bolt has clamped the two elements illustrated
in Figure 1, with a minimum of holding force;
Figure 5 is a view of the structure illustrated ln
Figure 4, when a substantial pressure has been applied on the nut
to cause the thread of the nut to advance to the right until thread .
engagement occurs;
Figure 6 is a view of structure, similar to that
illustrated in Figure 3, with only the bolt having the root of the
thread provided with a sloping surface;
Figure 7 is a view of structure, similar to that
illustrated in Figure 6, with only the nut having the root of its
thread provided with a flat surface;
Figure 8 i8 a view of a chart showing the substantial
holdlng force provided by the thread structure herein illustrated
and described;
Figure 9 is an enlarged fragmentary cross-sectional
view of an alternate conetruction of the locking thread form of
the present invention, as shown in a free-running condition;
Figure 10 is an enlarged fragmenSary cross-sectional
view of the thread form shown in Figure 9 after a partial loading
has been applied thereto;
Figure 11 is an enlarged fragmentary cross-sectional
view of the thread form shown in Figures 9 and 10 after sub-
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r stantially complete loading has been applied thereto;
Figure 12 is an enlarged fragmentary cross-sectional
view, similar to Figure 9 of still another alternate embodiment
of the present invention; and
; Figure 13 is a graphic representation similar to Figure
8 and illustrates the comparative locking characteristics of the
locking thread form shown in Pigures 9-11.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figureæ 1-8 illustrate embodiments of the present
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invention wherein the locking thread form is associated with
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buttress-type threads. In particular, a bolt or male threaded
element 11 is shown as comprising a shank portion at one end thereof
which is formed with a thread 12, and with an enlarged head 13
being provided at the opposite end from the thread 12. The bolt,
as illustrated in Figure 1, extends through a pair of elements 14
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~' ~ which are to be clampe!d together by a nut or female threaded
element 15 having an internal thread 10 which is normally freely
running on the thread 12 of the bolt. The bolt 11 is preferably,
although not necessarily, constructed from a hardenable steel, such
as 1335, 1441 or 1340, while the nut 15 is constructed from a
relatively softer or more deformable steel than the bolt 11, such
as 1008 or 1010. It is to be understood that the bolt could also
; be constructed from a soft steel but iB preferably constructed of
a hardenable steel. As is conventional with threads of the
buttress type, threads 10, 12 have one flank thereof at a sub-
stantial angle while the opposing flanks thereof have very small
angle relative to the axis of the bolt or nut. As illustrated in
`~ Figures 2-5 inclusive, the thread of both the bolt ll and nut 15
~r has the root flattened in a manner to provide a flat surface or
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ramp which slopes relative to the thread axis; the flat 16 at the
root of the nut 15 is disposed at an angle of approximately 22-1/2
relative to the thread axis, while the flat 17 at the root of the
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bolt thread 12 is di6posed at an angle of approximately 30~ from
the axis of the thread.
It iB to be noted that the aforesaid angles will vary
with the degree of hardness of the material from which the bolt 11
and nut 15 are fabricated, and that when these elements are
fabrication so as to be of similar to identical hardness, the
angles of the flats or ramps 16 and 17 are preferably equal or
approximately equal.
When the nut 15 is in the position illustrated in Figure
3 with the crown 19 of its thread 10 disposed adjacent to the
corner 21 of the bolt 11 between the fiat surface 17 and the sloping
face 22, the nut'l5 is free running on the thread 12 and is freely
rotatable toward the left un'til the nut 15 strikes the adjacent '
element 14, whereupon the continued rotation of the nut 15 will
cause the crowns 19 and flat 16 to move to the right engag-ing the
flat sloping surface 17 and crown 23, respectively, and causing
the softer metal of the crown 19 to deform. In Figure 4, the crown
19 is illustrated as contacting the sloping flat 16 of the nut 15, -
a~ would occur when an approximately 50 foot-pound force is exerted
on a one-half inch bolt, thereby providing a substantial degree
of contact between the threads which locks the nut 15 in position
along the bolt shank. When an increased torsional force is applied
to the nut 15, an even greater degree of contact will occur between
the ramps 16, 17 and the crowns 19, 23 until the faces of the nut
'thread engages the faces of the bolt thread, as shown in Figure 5,'
which woult occur, for example, when 8 one-half inch bolt is
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sub~ected to a 90 foot-pound load. This provldes an even greater
degree of contact between the crowns 19 and 23 and the flat sloping
surfaces 17 and 16, and under these conditions, any relative
lateral movement between the nut 15 and bolt 11 is positively
prevented 80 as to assure against loosening thereof, which locked
r condition will exist until such time as a positive force is applied
' to unscrew the nut thread 10 from the thread 12 of the bolt 11 and
thereby cause the respective threads to reach the posltion illus-
h trated in Figure 3, whereupon the nut 15 is again free running
~; 10 toward the end of the bolt thread 12.
Referring to Figure 6, the bolt 11 is the same as the
bolt above described with regard to Figures 1-5 while the nut 25
has no flat areas 16 in the thread root thereof. As a result, the
crown 19 of the nut will contact th-e flat sloping surface 17 at the
root of the bolt to provide the locking of the nut on the bolt
when sufficient pressure has been applied to the nut after the
elements 14 have been clamped together.
A similar result is obtained when the relationqhip of
the threads is that illustrated in Figure 7. In this arrangement,
the thread 10 o the nut 15 has the flat 16 thereon while bolt 26
has no flat in the root area. When the nut 15 is subjected to a
predetermined degree of axial loading, such as when it is being
tightened against the element 14, further rotation of the nut 15
will cause the crown 23 of the bolt thread to contsct flat surface
16 of the nut thread 10, thereby producing distortion or penetra-
tion of the ramp 16 of the soft nut metal to a degree commensurate
; wlth the amount of pressure (torque) which is applied to clamp the
element 14. When a substantial pressure is applied, the faces
of the nut thread will advance into engagement with the fscing
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surfaces of the thread of the bolt 26, as deæcribed above. In any ~-
of the examples herein illustrated, it will be noted that the
engagement of the crowns of one or both threads with the sloping
ramp or ramps at the roots of the opposing threads will produce
the deformation or distortion of the soft metal of the nut or both
the nut and bolt, with the result that no lateral movement between
the nut and bolt will occur, which in turn assures against
loosening of the bolt and nut after assembly thereof.
The chart or graph of Figure 8 shows the result of the
same test made on four different nut and bolt combinations on a
Junkers testing machine which vibrates the tightened nut and bolt
supported thereon. The first graph line 27 represents a test
performed on a Standard-type of bolt and nut having a 7/16 inch
diameter after it was drawn up to approximately 7,000 pounds axial
force. The shaking of this bolt and nut rapidly loosened the nut
and the holding force quickly decreased to substantially below
1,000 pounds. Each square represents approximately two seconds
of time 80 that this test occurred in practically ten seconds.
The graph line 28 represents a test run on a 7/16 inch nut and bolt
of the pre~entinvention in which it will be noted that under a
clamping force of 6,500 pounds the vibration caused an initial
Brinelling of the nut and seating of the thread form (although no
rotation of the nut), but which thereafter maintained a clamping
force of between approximately three and four thousand pounds. The
third graph line 29 was the result of a test on a 7/16 inch prior
art-type lock nut and associated bolt, the lock nut being tria-agular
in this particular test, and as illustrated, a rapid drop from the
6,500 pounds applied force occurred on to the bottom of
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the chart. Grapll linc 30 discloscs 3 su~scqucnt or sccond
test of the idcntical nut and bolt testcd in conncction with
the graph line 28 and illustrates the fact that the superior
locking characteristics of the prcsent invention arc not
lessened even during reuse of the nut and bolt. The reason
that the bolt and nut lcssened to a greater degrce during the
ori*inal test depicted by line 28, as compared to the "reuse"
test depicted by line 30, is believed to be attributed to a
certain amount of thread seating and a protective coating on
the nut and bolt elements which caused an artificially low
reading of the original test data.
Figures 9-12 illustrate alternate embodiments of
the locking thread form of the present invention wherein the
thread form is operatively associated with Standard, i.e.,
American or Unified, type threads, and wherein the thread
form may be provided on either of two threaded elements and
operstively associated with the other threaded element ~hich
may be of conventional construction. In particular, Figures
9-11 illustrate a locking thread form 100 shown in association
with a pair of threaded elements 102 and 104. By way of
exsmple, the threadet element 102 may consist of a nut, while
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the threaded element 104 may consist of a bolt. ~he threaded
element 102 is formed with a Standard thread form 106, each
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thread of which comprises converging flanks 108, 110 defining
a crest 112 and having a root arca 114. Similarly, the
threaded element 104 is formed with a Standard thread form
116, each thread of which comprises flanks 118, 120, a crcst
' 122 and a root area 124.
In sccordancc with thc principlcs of the prcscnt
invcntion, each of thc root arcas on the thrcaded mcmber 104
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is formcd with an inclincd ramp 126 which is an310~0us to
the aforcdescribcd ramps 16. The ramps 126 ar~ oricntcd
at an angle a with respect to the axis of the threaded
elemen~s 102, 104, which an~le is selected so as to assure
positive enga8ement of the crest 112 thcrewith upon appli- -
cation of loading to the elements 102, 104 and consistcnt
with modern manufacturing tolerances.
More particularly> it has been found that optimum
locking ability of the elements 102, 104 can be achieved without
sacrificing any strip strength thereof by making the axial
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length of the ramps 126 equal to approximately .020 inches
ant designing the angle thereof relative to the axis of the
elements 102, 104 suth that approximately one-half the maximum
tolerance between the elements 102 and 104, plus a safety
factor of approximately .002, is taken up in the aforesaid
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sxial distance of .020 inches.
By way of example, for a Standard 3~8 inch nut
and bolt having 16 threads per inch, the maximum acceptable
dismeter limit is .3750 inches and the minimum acceptable
; diameter limit is .3595 inches. Taking the difference be~ween`~ those limits, i.e., .3750-.3595 results in .OlSS which, when
~ added to the aforementioned safety factor of .002 (which
accommodates for tool wear, etc.) equals .0175, and one-halfof .0175 equals .0087. The angle a whose tangent equals
.0087 s .020 is 23.5. Therefore, the optimum angle at which
~ the ramps 126 should be located relative to 8xis of the elements
`~ 102, 104 where it is dcsired to take up approximately one-half
the total maximum tolerance between the aforcmentionca size
and class thrcaded clcmonts in a distance of .020 is 23.5.
It will bc apprcciatcd, of course, that such an~le a will vary
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. in accordancc with chan~cs in cithcr thc sa~cty f~ctor,
: class thrcads, number Or threads pcr inch, or diamotcr of
- thrcaded clements. The followin~ chart sets forth acccptablc
angles of the ramps 126 for the respectivc size thrcaded ele-
ments, thread class and number of threads per inch, as calcu-
~ lated in accordance with the above ex~mple: .
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STANDARD SERIES SCREW THREADS
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: . Size CLASS 1 CLASS 2 CLASS 3
:............. - Thread~d Threads Angle Angle Angle
. Elcments Per Inch Required Required Required
: . - 3/8 16 23.5 .5 17.6l.5 15.9~.5
,.- ~ 7~16 14 25.2+.5 18.9+.5 17.0~.5
, . 1/2 13 26.3~.5 19.7~.5 17.8+ 5
.. 9/16 12 27.4+;5 20.5l.5 18.5l.5
:.~ 5/8 11 28.S~.5 21.4~.5 19.4~.5
12 ~ -- 20.5 .5 18.5~.5
~, 3/4 10 30;1 .5 22.6'.5 20.4 .5
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By selecting the angles of the ramps 126 in accordance with
the above, lateral movement between the elements 102, 104 upon
. applying preselected loading thereto is effecti~ely precluded,
thereby assuring that the elements 102, 104 will remain in
'~. their respective locked positions once they are moved from
~` their free-running relation sho~Yn in Figure 9 to either the
. partiaily loaded position shown in Figure 10 or the completely
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loaded position shown in Figure 11.
; Figure 12 illustrates a slightly modified embodi-
~ent of the thrcad form shown in Figures 9-11 wherein the
~ inclined ramps are located on the f~male element tnut) instead
'.~ . of the malc element, as is the case with the thread form shown
in Fi~urcs 9-11. In particular, the lockin~ thread form shown
in Fi~urc 13 is ~enerally design~ted by thc numerai 150 and is
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shown in operative association with tllrcadcd elcmcnts 152 and
154 which may consist of a nut and bolt, rcspcctivcly. The
elements 152, 154 arc providcd with Standard threads 156 and
158, respectively, with the root area of the thread 156 bcing
formed with inclined ramps 160, as hereinabove described. The
angles at which the ramps 160 arc inclined relative to the axis
of the elements 152, 154 are selected in accordancc with the
above example of the angle of the ramp 126 so as to minimize
relative lateral movement between the elements 152, 154 and
hence provide for optimum locking ability *ithin the range of
acceptable commercial tolerances. Shus, it will be seen that
the principles of the present invention are applicable when
either the male or female member is provided with the locking
thread form, and that the other member need not have any special
form other than the Standard form, thereby ~roviding for uni-
versality of application. The embodiments sho-.rn in Pigures 3-7
may be considered to be preferable to the embodiments shown in
Figures 9-12 insofar as strip strength is concerned, although
the strip strength of Standard thread forms can be improved
by slightly increasing the length of the threads.
Figure 13 is a graphic illustration similar to
Figure 8 and depicts the performance of the locking thread
form shown in Figure 12 as compared to conventional thread
designs. The illustration shown in Pigure 13 represents tests
on a 7/16 inch grade five bolt having a Standard thread form
~ith three different nuts, as performed on the Jun~ers testing
machine hereinabove described. She curvc 162 represents a test
performed on a Standard nut and bolt that had no type of locking
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means, and it will be notcd that the effectivc loc~ing ability
drops to ncarly zero in lcss than 14 seconds. She curvc rcpre-
scntcd by thc numcral 164 rcprcscnts thc resul~s of a tcst of a
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Standard t~lrcad form on a bolt and associated wlth a conventional
loc~ nut of the crimp or deformed type. This particular locking
arrangement, while superior to the arrangement shown by the
curve 162, also loses its locking ability rapidly, with the
result that the locking ability thereof is effectively non-
existent after approximately 26 seconds. Graph line 166 ~epre-
sents a test run with ~he thread form illustrated in ~igure 12
wherein the bolt has a Standard thread form and the nut is
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provided with the aforementioned ramps 126. It will be readily
apparent that the locking performance of this thread form is
highly superior to those associated with the curves 162 and 164
; and that the locking ability of the thread form of the present
invention remains at a highly acceptable level for the duration
of the testing Frocedure.
While it will be apparen~ that the embodiments
. illustrated herein are well calculated to fulfill the ob~ects
~3 above stated, it will be appreciated that the present inven-
.~ tion is susceptible to modification, variation and change
~, without departing from the proper scope or fair meaning of
the subjoined claims.
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