US 2788046 A
Description (OCR text may contain errors)
April 9, 1957 lSCREW THREAD CO TRUNCATED THREAD l Filed Dec. 15, 1952 A u i v f L u l a) N VENTIO ING 2 Shea SAN oN coMPRIsING con WITH INTEGRAL Locx SED THEREBETWEEN J. R NSTRUCTI THREADS INTERFO \mN Q Y ..Y A 4v, a mm 1 A.. @u A www uw N\ GLW N\ l f NAE N\ IRB y /Q SGE f 3 mmh v f PNE Mcmw Y OWO N RI P NN MMM v QN QN NN, `N WN NN NN .QN NT A NN l .l N t O D m R.
i y. N .f mu I, Tm 2 rum. m m. Y ...lumi 5 9 1 n l I w 9 D ..l.
STCREW-- THREAD CONSTRUCTION COMPRISING CONV ENTION AL TRUNCATED THREADS WITH INTEGRAL LOCKING THREAD INTERPOSED THEREBETWEEN Joseph Rosn,` Newportleach, Calif. Applicattonnecember 1s, 1952, seriarNo. 325,960`
s Claims; (Cl.151-22) My-'finventionv relates? to screw' threads and is-particu' lar'lyf.- directed' to improvements in mating thread constructionsto provide a controlledvdegree of interference when'assembled, inforder to resist separation of the mating parts.'
This applicationl is-related to my :zo-pending applica-v tion'si` Serial No. 309,960, ledSeptember 17, 1952, and SerialvNo. 313,280, tiled October 6, 1952. The rst of these .cases is concerned with an integral lockingvthread formed at the root of-a primary thread and the second is directed to a separately formed locking thread similarly located at the'root of a primary thread. The present invent-ionis ldirected tolan integral locking thread of the general'type disclosed'in Serial No. 309,960 but-which embodies certain features disclosed in connection with the separately formed locking'thread in Serial No. 313,280.
In`rnycolpendingapplications identified above, it is pointedoutfthat -proposediforms Yof screw thread devices having'vinterferinglparts at the crests'orvroots of the thread have'anotmet with 'universal acceptance. Thus, screw thread constructions such as that shown in the'Evans Patent No. 2,437,638 have not been widely adopted.l The metaladjacentlthe roots lof the threads on one member is-Yrequiredl to flow" orfotherwise be distorted laterally in af'dire'ction at right anglesto the direction of the applied squeezingvforces toproduce interference along theflanks ofthe'fthreads; The high unit stresses necessary to pro# duce this effect causegalling'along the anks.
The locking thread arrangement shown in the `Meerst'einervv PatentA No. 2,109 ,'7 7 8 employs a separate helical cutting element located adjacent the roots of primary threads1on abolt. The opposed ysharp corners of the helicalelement are'positioned to becomeembedded in the LmetalV ofthe bolt and the nut when they two are threaded together. The sharp corne-rs are relied upon to re'strainrelative rotation of the helical element andthe boltbut these corners produce highly objectionable localiz'ed 'stresses at the roots of the-primary thread of the bolt" Thisfis particularly undesirable in fastenings 'sub1 ject' to fatigue action under stress reversals or rapid changesl in magnitude of stress'.- The stress-raiser effect of the sharp corner digging into the metal/of 'thebolt atf'theroots of the -thread leads to fatigue failure and in consequence seriously reduces ,th'epermissible working stress '5to' becarried on the bolt.v
The/'principleobjectl of the presentinvention is to pro-v videa locking thread design whichfre'tains the advantages ofithel'integral` construction shown in said application SerialNo; 309,960, thereby eliminating the objectionable stress-'raiserleifects ofy Meersteiner, and at the same time embodying certain of the advantageous features of said application :Serial No.A 313,280. These advantageous featuresfre'late to the provision of a locking thread which islgenerally helical 'inform :but which has a varying non# constnt'ptch-with respect-'to the primary thread. The `gene'ra'llylfheh'ealelement is Vtherefdr'e shifted in its posii tion--relative 'f-to w the@ primary 'Y 'threads' in an undulating fmshlztithrougltauty its ulength? Thel undulation'* may be Patented Apr. 9, 1957 axial or in a radial direction, or both axial? and'^r`cli l The purpose of this construction is to enhance the'l'ck"- ing eiect achieved by interference with the crests: ofthe threads engaged by the locking element.
While my invention in its broader aspects` may' beferii' p'loyed with any one of a large number of 'thread forms; l have chosen to describe it in connection with American National threads of well-known conventional formi Threads under the American Nationaldesignation are V shaped with a 60 included angle' between the an'lis of adjacent threads, and in class 3 threads in this system the dimensional.tolerances are such that the flanks the mating threads may' haveclearance, but: not inter*- ference. The crests of the maleand femalethreadsfafre truncated. The secondary locking thread ofrny'n'vention engages these conventional truncated surfaces lin interferiing relationship. While my secondarylocking 'threa'dmar be located at the root of the male thread'orthe root of A"the female thread as desired, l prefer to locate the secondaryV locking thread at the root of the male thread wherever the strength of the metal of the male thread exceeds 'thato'f the female thread. member may be formed of steel while the female or" socket member may be formed of non-ferrous material,
for example, aluminum alloy. lf-the malev and female" members are `formed of the same material the secondary -iocliing thread may be placed on either member. @nev member carries the locking thread'and the other member is initially of standard conventional form. With these and other. objects in View aswill appear hereinafter reference is directed to the accompanyingl drawings in which:
Figure 1 is a sectional view partly broken away-showing a preferred embodiment of my invention. shown partially assembled on the bolt.
Figure 2 is a sectional View similar to'Figurelshow ing'the nut in engagement with the shoulder.
Figures 3, 4, and 5 are diagrammatic.v transverselsec tional views illustrating the mannerin which the position of the locking'thread element may be varied relative to the root of the primary thread on different locations along its'length to produce an undulatingaxial shiftin' thel locking thread constituting a departure from a true helix! Specically Figure'3 shows the locking Vthready shifted 'ito" the left with respect to the center of the root of the prii mary thread. Figure 4 shows a locking thread in a-tr'ue central position and, Figure 5 shows it shifted tothe right of the central position. shifting and the amount of the flank clearance are exag gerated for clarity 'of illustration.
Figure 6 illustrates another form ofthe invention and constitutes a transverse sectional view taken substantially on the lines 6 6 as shown in Figure l.
Figure 7 is a sectional view similar to Figure 1 showing a further modified construction in which thesecondary locking thread is located in the nut.
Referring to the drawings:
The nut it? may be provided with any suitable ordesi" able internal thread 11 and as shown in the drawings this internal thread may comprise an'Arnerican National, classi 3 thread of conventional form. The thread may be formed by any suitable or convenient means. The ilanks 12 and 13 of the internal thread 11 have a 60` included 'angle and the crest is truncated to form a cylindrical surface 14.H
The not itl and the internal thread llas thus described may be of conventional form.
A bolt 1.5 extends through an openingll in a istationaryf,y
Except'for the region adjacent the roots oflthe external"` thread 19, this thread `s conventionalandisproportioned In a typical installation the" male' Thel nut is' The extent Vofthe 'axial to mate with the internal thread 11 in the nut 10. The thread 19 is hereafter referred to as the primary thread.
In accordance with my invention l provide a secondary or locking thread generally designated 20 which is positioned between the convolutions of the larger thread 19 and adjacent the roots thereof. This secondary or locking thread is formed integrally with the bolt 1S and is generally (but not precisely) helical in shape. This secondary or locking thread is substantially symmetrical in crosssection and is rounded at its crest 21. It may be similar in form to the primary thread 19 except for the pronounced rounding of the crest 21 and fillets 22. These llets are positioned at the juncture of the root of the primary and secondary threads. The portions of the anks 23 and 24 of the primary thread 19 adjacent thc iillets 22 are preferably relieved as shown at 2S to provide necessary space for thread rolling tools and to allow fillets 22 of maximum size. The depth of the primary screw thread 19 may be from 31/2 to 7 times the depth of the secondary screw thread 2t) with good results.
Figure l shows how the truncated crests of the internal thread 11 are distorted when the nut and bolt are initially assembled. The crest surface designated 14 illustrates the conventional form of the thread crest before distortion occurs. When the threads 11 and i9 are screwed together the secondary or locking thread 2d presses into the crest 14 and deforme it laterally in both directions and also radially inwardly as shown by the enlargements at 26 and 26', while grooving and dividing it centrally. The deformed portions are spread laterally to increase the width of the crest and inwardly to increase the height of the thread and form an interference tit with the adjacent ilanks 23 and 24 of the primary thread 19. This crest dividing and distorting action is substantially symmetrical about the secondary thread 2d. The anlzs .i2 and 23 normally ride in engagement while the nut is being threaded on the bolt 15, although a very slight clearance 28' has been shown in Figure i, in contrast with a relatively greater clearance 28 between the flanks 13 and 24. The clearances are exaggerated in the drawings for purposes of illustration.
When the nut 10 is tightened against the shoulder 18, as shown in Figure 2, the iianks 23 on the primary thread 19 meet in substantially full surface engagement with the flanks 12 on the internal thread 1l, thereby eliminating any clearance 28 that may have been present. The clearance 28 between the flanks 24 and the iianlts 13 will then increase slightly. The iianks 12 and 23 are therefore placed in interfering engagement adjacent the roots of the thread 19 and the crest of the thread 11. it will be understood from this description that the axial load transmitted between the bolt and the nut 3!) is carried on the ilanks of the primary threads l1 and tif while the locking action is developed by the interfering engagement between the locking thread 2i! and the crests of the internal thread 11, supplemented by the interfering engagement of the anks 12 having the enlargements 26 with the anks 23, and the interfering engagement of the enlargements 26 with the flanks 24 adjacent the root. Thus, as the threads are tightened, the enlargements 26 and 25 tend to deform toward the right, as viewed in Figure 2. The degree of interference can be readily controlled in various ways, for example, by controlling the internal diameter of the crest 14. This diameter may obviously be changed by varying the size of the drill used prior to tapping of the threads 11 in the nut 1G. Varying the tap drill size is thus one effective means of varying the degree of interference.
Figures l and 2 illustrate the type of distortion which occurs between the locking thread 2G and the crests of the thread 11 on the other member whensuch other member is formed of softer material than the bolt member and locking thread. Thus, Figures l and 2 show the type of distortion which occurs when an all-steel bolt embodying my invention is threaded into a female thread provided on an aluminum alloy part. Most of the distortion occurs in the aluminum alloy part and the distortion will vary in accordance with the character of materials employed. The interfering engagement is particularly effective to prevent unscrewing movement of the parts under varying loads or vibration with or without accompanying temperature changes, and furthermore the parts may be assembled and disassembled a great many times without losing the locking effect.
As a speciic example, a diameter standard nut and bolt has 16 threads to the inch, and a clearance of about .G0075 of an inch between the confronting anks of the mating threads, or a total clearance of .0015 of an inch for each thread. In order to provide a positive lock, the crown of the thread 11 is distorted or expanded to an amount equal to about twice the total normal clearance, or about .0")3 of an inch. This provides a total excess of .G0l5 of an inch of metal on the deformed crown of the thread 11 to bind or be displaced back toward normal in effecting the lock. The same principle of distorting the thread to twice the total normal clearance applies also to the root clearance, in instances where locking engagement between the periphery of the crown of the thread lll and the root of the mating thread is desired. The locking element 2t) may be of any suitable height, as previously pointed out, and a height of .007 of an inch is satisfactory or the particular thread described.
In accordance with the present invention I prefer to preform a locking thread 20 with a non-uniform pitch so that it undulates with respect to the root of the primary thread 19. As shown in Figures 3, 4, and 5, the position of the center line X of the locking thread 20 varies along the helical length of the locking thread with respect to the center line Y of the root of the primary thread 19. More specifically, in Figure 3 the center line X is shown to the left of the center line Y. In Figure 4 the two center lines X and Y coincide. In Figure 5 the center line X is shown to the right of the center line Y. This waviness, undulation or irregularity in the pitch of the thread 20 is exaggerated in the drawings for purposes of illustration. At least a part of this variation is absorbed by elastic deformation of the material and the result is that the torque required to unscrew the threaded connection is considerably increased and the locking effect is thereby enhanced.
In the modied form of my invention shown in Figure 6 the locking thread 20a has a crest 21a which varies in diameter along the length of the thread 20a. Hills and valleys are thus produced in the thread 20, and these may be formed by a rolling tool (not shown). This form of undulation in radial dimensions of the locking thread may supplement the axial undulations in pitch, previously described, or may be used on a locking thread which is formed as a true helix. The effect of the non constant radius of lthe crest 21a of the locking thread 20a is the increase in the torque required to unscrew the thread connection, and the locking efect is thereby enhanced.
In the modied form of my invention shown in Figure 7, the locking thread 20b is provided on' the nut 10b while the bolt 15b is initially formed with a conventional thread 19h. When the nut 10b is screwed on the bolt 15b, the locking thread 20b develops an interfering engagement with the crests of the thread 19b. When the bolt 10b is screwed home against the shoulder 18b, axial distortion of the threads occurs to reduce clearances 31 on one side of the threads and to increase the clearances 32 on the other side. The locking thread 20b may be embodied in the nut 10b when the strength of the material of the nut is substantially the same as that of the bolt. In this case the locking thread 20b may be employed on either member as desired. 'Ihe other member may be of standard construction. On small parts, relief clearances similar to those shown at4 25 on Figures 1-3 may be employed if desired, to assist in thread-rolling operations. The locking thread 20b may have either axial distortion of pitch as shown in Figures 3, 4, and 5, or a change in radial dimensions as shown in Figure 6, or both.
It will be understood that the terms bolt and nut are used in a descriptive sense only and that the nut may constitute any member having an internal thread while the bolt may constitute any member having a mating external thread.
While the particular form of thread chosen to illustrate the invention has opposed flanks of equal angles, it will be understood that other forms of threads having opposing ilanlcs of unequal angles such as, for example, buttress threads, may be used with a secondary locking thread with equally good results.
Having fully described my invention, it is to be understood that I do not wish to be limited to the details herein set forth, but my invention is of the full scope of the appended claims.
1. A member having a conventional truncated substantially 60 primary screw thread for carrying axial loads and a single integral, relatively small locking thread located at the root and interposed between the convolutions of the primary screw thread, the locking thread having a rounded convex crest and said member having a rounded concave tillet between the primary thread and locking thread on either side of said locking thread, the crest diameter of the locking thread being substantially greater than the normal conventional root diameter of its associated primary thread, the cross-sectional area of said c-king thread radially outwardly of said root diameter being not greater than the combined cross-sectional areas of the spaces formed by the fillets on either side of said locking thread and lying radially inwardly of said root diameter, the crest of the locking thread varying in position relative to the primary thread along the length of said primary thread.
2. A member as defined in claim 1 in which the crest of the locking thread is undulated radially along the length thereof.
3. A member as defined in claim 1 in which the crest of the locking thread is undulated axially along the length thereof.
4. A member as defined in claim 1 in which the crest of the locking thread is` undulated radially and axially along the length thereof.
5. A member as dened in claim 1, in which the depth of the primary screw thread is 3%. to 7 times the depth of the locking thread.
6. A member having a conventional, substantially primary screw thread for carrying axial loads and a single, integral, relatively small locking thread located at the root and interposed between the convolutions of the primary screw thread, the locking thread having a rounded convex crest and said member having a rounded concave fillet between the primary thread and locking thread on either side of said locking thread, the crest diameter of the locking thread being substantially greater than the normal conventional root diameter of its associated primary thread, the cross-sectional area of said locking thread radially outwardly of said root diameter being not greater than the combined cross-sectional areas of the spaces formed by the fillets on either side of said locking thread and lying radially inwardly of said root diameter, whereby when said member is threadedly engaged with a corresponding truncated but conventional thread on another member, the crest ofthe locking thread will press into the truncated crest of said corresponding conventional thread to distort and spread it laterally to form an interference fit therewith.
7. A member, as defined in claim 6, in which the primary Screw thread has anks that include portions adjacent the root thereof disposed substantially perpendicular to the axis of the primary thread and which merge with said fillets.
8. A member, as detined in claim 6, in which a second integral locking thread is formed on the truncated crest of the primary screw thread.
References Cited in the file of this patent UNlTED STATES PATENTS 720,580 Greenfield Feb. 17, 1903 1,300,801 Woodward Apr. 15, 1919 1,807,494 Proctor May 26, 1931 1,957,095 Cole May 1, 1934 2,109,778 Meersteiner Mar. l, 1938 2,177,003 Purtell Oct. 24, 1939 2,177,100 Frame Oct. 24, 1939 2,367,213 Harding Jan. 16, 1945 2,484,644 Poupitch Oct. 11, 1949 FOREIGN PATENTS 944,369 France Nov. 2, 1948 818,140 Germany Oct. 22, 1951