CA1051329A

CA1051329A - Locking thread construction

Info

Publication number: CA1051329A
Application number: CA262,783A
Authority: CA
Inventors: Horace D. Holmes
Original assignee: Individual
Current assignee: Individual
Priority date: 1975-10-08
Filing date: 1976-10-05
Publication date: 1979-03-27
Also published as: DE2645519C2; JPS553566B2; DE2645519A1; FR2327438A1; GB1528696A; SE426978B; SE7611060L; JPS5252061A; IT1072601B; FR2327438B1

Abstract

LOCKING THREAD CONSTRUCTION
ABSTRACT OF THE DISCLOSURE
A locking thread design which may be incorporated in various types of male and female threaded elements, for example, a bolt and nut, or a bolt and casting, forging or similar member having a threaded bore therein. The thread design may be of the Standard, i.e., American or Unified Standard, or buttress type and is free running until a pre-determined magnitude of loading is applied thereto, at which time the locking action of the thread occurs so as to prevent relative lateral movement between the nut and bolt and hence positively resist loosening thereof under vibration and similar adverse operating conditions. The locking thread may be embodied on either one or both of the threaded elements, and will operate effectively when the locking thread is oper-atively associated with threaded members having conventional threads thereon.

Description

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.. ~ BACKGROUND OF THE INVENTION
i 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 æ~
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^` 10513Z9 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 i ' 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 ~ dap/ ~ ~

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, 10513'~9 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 _ 3 _ AS

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, ~0513'~9 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-dap/ j ...

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- : , ; -` 10513'Z9 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 ;.
invention wherein the locking thread form is associated with . .
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 ,., ~
~' ~ 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 dap/ ~
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10513'Z9 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 .
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 dap/ ~
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-- 10513'~9 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 dap/

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`~ 10513Z9 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 dap/

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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 ~ .
the threaded element 104 may consist of a bolt. ~he threaded element 102 is formed with a Standard thread form 106, each . . .
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 ., '9' .. , . ..................... . . ., - , .. :................. - . - :. . , - , , ~: 105~3Z9 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 ~.;. .
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 .. . . .
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 ~ 1 0 ~

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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
. - - . . .... _ _ : . 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 : ~ .
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 ~, . .
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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10~3,'Z9 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 ~j .
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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, ~0513Z9 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 ~- .
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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fastener combination comprising a threaded male element and a threaded female element adapted to receive the male element in threaded engagement therewith;
each of the male and female threaded elements having a thread form thereon defined by leading and trailing helical flanks which intersect to define a crown, each flank lying at an angle relative to the common longitudinal thread axis of the fastener combination;
the female element including a truncated root adjacent the major diameter thereof, said truncated root defining a helical flat extending between the trailing flank and the adjacent leading flank which faces said trailing flank, and said flat, throughout its axial extent lying at an angle between about 15° and 30° relative to the thread axis which is substantially less than the angle of the male element flank which tends to converge on said flat under axial tension load, said flat angled radially outward from said trailing to said adjacent leading flank;
whereby axial tension loading of the combination tends to displace the crown of the male thread into contact with the flat to prevent movement between the elements transverse to said axis and lock said elements together.

2. A fastener combination as defined in claim 1 wherein the angles of the respective flanks are unequal such that the thread forms are asymmetric.

3. A fastener combination as defined in claim 1 wherein the male element is a bolt having a gripping head, and the female element is a nut having wrenching surfaces.

4. A fastener combination comprising a threaded male element and a threaded female element adapted to receive the male element in threaded engagement therewith;
each of the male and female threaded elements having a thread form thereon defined by leading and trailing helical flanks which 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 said trailing flank, and said flat, through-out its axial extent, lying at an angle between about 15° and 30° rela-tive to the thread axis which is substantially less than the angle of the other element flank which tends to converge on said flat under axial tension load, said flat angled radially outward from said trail-ing to said adjacent leading flank;
whereby axial tension loading of the combination tends to displace the crown of said other thread into contact with the flat to prevent movement between the elements, transverse to said axis, and lock said elements together.

5. A fastener combination as defined in claim 4 wherein the angles of the respective flanks are unequal such that the thread forms are asymmetric.

6. A fastener combination as defined in claim 4 wherein the male element is a bolt having a gripping head, and the female element is a nut having wrenching surfaces.

7. For use in a fastener combination comprising a threaded male element having a thread form thereon defined by leading and trail-ing helical flanks which intersect to define a crown, each flank lying at an angle relative to the common longitudinal thread axis of the fastener combination;
a female threaded element having a thread form defined by leading and trailing helical, intersecting flanks each of which lies at an angle relative to the thread axis thereof;
the female element including a truncated root adjacent the major diameter thereof, said truncated root defining a helical flat extending between the trailing flank and the adjacent leading flank which faces said trailing flank, and said flat, throughout its axial extent, lying at an angle between about 15° and 30° relative to the thread axis which is substantially less than the angle of the male element flank which tends to converge on said flat under axial tension load, said flat angled radially outward from said trailing to said adjacent leading flank;
whereby, when in the combination, axial tension loading tends to displace the crown of a male thread into contact with the flat to prevent movement between the elements transverse to said axis and lock said elements together.

8. A fastener combination as defined in claim 7 wherein the angles of the respective flanks are unequal such that the thread forms are asymmetric.

9. A fastener combination as defined in claim 7 wherein the male element is a bolt having a gripping head, and the female element is a nut having wrenching surfaces.