US2267923A - Shear-reducing, dual verge thread for tool joints, etc. - Google Patents

Description

Dec. 30, 1941;. A. E. JQHNsoN SHEAR-'REDUCIN-G, DUAL VERGE THREAD FOR TOOL JOINTS,

Filed sept. 16,{71940 O IVO Patented Dec. 30, 1941 I ,susan-REDUCINQDUAL vEaGEfrnnEAn u j Foa'roor. Jom'rsm'rc.

Arthur E. Johnson, Taft; cam. Application September l16, 1940, Serial No. 357,005

` f 12 Claims. (Cl. 285-146) This invention is athread or tooth form for screws (male or-female), dies and othermechanical apparatus. e l .The invention resides in a thread. ofhthis class and distinguishes fromV all other formsof tooth shapes for screws and dies, .regardlessot the style of thread cross-section, in that a threadi of this invention is of constantly changing crosssection from: the end ofgreatest depth o f tooth to that of least depth `of tooth, or to the last scratc of the tooth (beit threaded'bodyorfa cutting die). This constant change of the thread section is irrespective of the general cross-section; that is, whether it is of parallel wall sides or anks or kthat such sides may be iconverged toward the lthread crest, as in V-threads.

Inza section longitudinally of a body and whichy section cuts a number of thread helices, and shows a row of teeth along the sectioma line which passes as.` a mean between theroots and crests of the thread `sections of av number of any bending; moment generatedat the l`joint one 'side ofthe pin,'or malel pipe end, yis com? pressed and the diametrically opposite side is conversely;stretched. v Ifk the flank at the top side of a `(vertical axis) {pinthread ,is for :its full length engaged byits'box thread the total engagement area is so great that stretchdue to bending moment is wholly negativedy as to. the

- vfull threaded length ofthepinand the zstress or stretch becomes` localized at the thread root nearest the. base of thefconical surface of the pinor pipe end since one full .turnof the root at `the pinbase is onthe cross-section'of the pin that is'not positively supported against oscillatory vibrations at-the top'. ofthe box or pipe into which the malev part threads,particularly so in shoulderless types oi' make-up. Thusin agood helices may be considered the pitch line ofthe series of vthe teeth and .here will be. called the mean line of thethreadisections) so as to avoid confusion as might arise. when this new thread is out on a part such as a tapered pin orpipe, and its complementaryL female yelement,. box or collar. Taper pin tool joints are a highly important device in the deep well lndustry` and it is desired to avoid confusion with,`the terms taper and pitch of the frusto-conoidall surfaces quality of material-the; fracture of a screw pin comes on'a ,plane following theifirstroot -of one thread pitch (here the .wordvpitchmeans the thread advance axially). ,f l

By the thread of this .invention the base of the conicalpin has no deep screw root'orgroove, andin fact the first; root `is preferably the last scratchf and from this the groove is gradually increased to the axially farfendV of the' thread.

` `I-Iowever the depth of `the threadmay be out so of such joints, as well as in pipejointsusing l threaded conical surfaces. .Y

It is one ofthe objects of this invention Ato providea'new thread Vfor use'inr such tool and pipe joints (and theirdies) whereby'at one and the same time substantially eliminate common. f`

shearing of the threads particularly at the maximumk end of the conic surface; to reduce'the hazardof transverse fracture of the pin` at the base of its conlc part, `and to utilize Vlatent'capacity of stretch along'ithe pin body at the root line of the threads. y

This fracture almost "invariably occursin ka cross-plane of the pin baseat `about the helical lin`eof theflrst fullrootofthethreads. Care` ful search and analysis, showsrthat this fractureiscaused `bythe fact that in'conventional tool string joints the threads "are 'of vfull depth throughout `the length of the thread. It `will therefore be seen at once that if the joint parts are well machined each and every thread pre.- sents a full depth side or `iiank area to its opposed thread onv the companion element offa pair. whether tool joint orpipe joint. Now if 55 asl to taper from a full depth at one end to any suitable fraction .of the full depth ,and not be run oi to afine last scratch. y

This run oil of the root is not to bek confused with the conventional. angular cutv al',l the bottom ofa last fewturnsof the screw thread at the butt of a threaded pin and where 'all of the remaining lthreads are of fullthread depth from rootlinetocrestline.

It can be stated at o nce that the thread; of this invention is characterized inthat in any selected body-type of. thread andtaking a mean. lineof threadk depth, and which `line -is arbitrarily oblique tothe axis of the screwthreaded part, the root line of thethread turns preferably starts from a. scratchpoint-on the said part and verges away from` the said mean of the threadsection, and likewise the crestor ridge of they thread verges inthe opposite direction. fronrthe point of start of the` thread, as to thesaid mean.'

Therefore, this; newmachine thread isja dual vergethread: its kcrestand itsroot each [verge from a minimum depth or start at zero on Athe mean-line,the scratch point, tol a full depthsection of thread at the very last turnof `the thread body about the axis of thethreaded part.` The said mean line is tov-be `understoodas the angle 0f taper ofthe taper-threaded end of aplpe,`,or

line of the angle of taper of a die, or of a tool joint part, as it is the pitch taper of the thread mean as well as the mean pitch of thread coupled parts.

The result of such a dual-verge thread-body is that in any type of thread section the two flanks of the thread gradually increase in depth from the minimum or zero point to a substantially full given depth of the thread at the end of minimum diameter of the conical surface which is threaded. n

It is interesting to note that in this new thread throughout its full length helically at no crosssection is the thread perfect to a conventional section for the reason that the thread begins at a given point, on an element, at or near the mean line of the thread and then verges each way from said line until the deepest thread body is produced at the remote end of the thread-and this remote end has its crest and root surfaces not parallel to the mean but each verged as to the mean. And another feature, every crosssection of the thread body has an absolutely uniform width at the mean of its depth.

It has been stated here that the mean line conforms to theangle line of a given taper of two tapered surfaces which are to be complementally threaded. That is one way of defining the structure of the thread but in any given case of taper it will be understood that the male part must have a rough machined conical taper of such an angle that its intended thread will have its full desired depth as a result of the fact that the crest of a pin thread verges out from the mean lineof the thread.

A further feature of the invention is that, in any case, the radial depth of the thread body, its depth oblique to the mean line, is always on a line normal to the axis of the part threaded, and is not normal to the verge line or the crest ncr of the root.

Conspicuous results are obtained here. First. the maximum depth of the thread is at maximum distance from the greatest diameter of the conical surface to be threaded, or is threaded, and the minimum depth of the thread is at the widest part of the conical surface. 'I'he root surface of the thread gradually verges in from the last scratch to the smallest diameter of the threaded part or element and therefore when the made-up joint flexes at one side under a moment of force lateral to the joint axis the incidental stretch at the convex side of the flex is distributed for a length of the element equal to the length axially of the element that is threaded. In fact' the latent stretch zone is equal to the length of the mean extending from rst to last screw helix. This mean length is many times the conventional helical-pitch distance between two or three threads of the butt of a pin or of a pipe joint. And this latent stretch is available for the threaded length of the element because when the joint is made up the single full depth thread helix at the small end of the conical part has the maximum side face interlock while all other turns of the thread to the large end of the conic surface decrease in depth with the result that the very shallow threads at the butt of the cone have capacity for some inherent stretch and in order that this stretch from helix to helix may be used it is a feature of the invention that the mean line thickness of the thread be slightly smaller in amount than the width of the slot or groove between thread coils. This will allow the threads of the pin to creep a little in the opposite groove of the female jointpart. The interlock of the threads at the last scratch is practically nil, and there is no material objection to this nil effect for the very reason that in any case of conventional full depth screw thread from end to end-of helix it is never known which` of the threads in the length of the helix may be actually in holding effect and all of the rest without load. Workmanship and temperature are disturbing factors making unreliability of fit whereas in the present invention the locking load will inevitably come, by a little wear-off of the shallower flanks, onto the deepest thread turns.

This leads to consideration of a feature of the invention wherein it eliminates a high accuracy of thread finish, as in other threads of a type with full depth throughout the thread length in contrast to the thread of this invention because in this thread it is intended to definitely localize the thread interlock on a few full depth threads at the small end of the conic surface (male or female) enough to take the load of tensile strain of the coupled elements and enable the inherent elasticity of the threaded length oi' the flexed element to absorb the bending tension instead of throwing the whole bending moment onto just the first one or two threadturns at the butt of the pin where thread shear usually happens, and instead of placing the butt root of the first thread under such strain as to result in transverse fracture of the pin at the butt.

The invention consists in certain advancements in this art as set forth in the ensuing disclosure and having, with the above, vadditional objects and advantages as hereinafter developed, and whose constructions, combinations, and details of means, and the manner of operation will be made manifest in the description of the herewith illustrative embodiments; it being understood that modifications, variations and adaptations may be resorted to within the. scope, spirit and principle of the invention as it is more particularly claimed presently.

Figure 1 is a diagram to show the dualverge of root and crest lines, of a V-type thread for a mechanical element, from the mean between crest and root of the thread; all lines emanating from a zero or last scratch point.

Figure 2 is an elevation of a conventional tool -joint pin showing by a d-otted line the usual cross-plane of fracture.

Figure 3 is an axial section of a fragmento! a tool joint make-up showing a dualverge thread of parallel-flank type; the flanks being perpendicular to the axis of the pin.

Figure 4 is anfaxial section of a fragmentY of a pin with dual-verge V-type thread body,\with the flanks symmetrical in angle to the pin axis.

A common pin 2 of a tool joint is shown in Fig. 2 as provided with a full cut V-thread 3. In a joint of a full cut thread, of 'any type where the thread is of the same height throughout,'tho

transverse breaking strain of a bending moment,

occurs at the plane whose circumference has the least reinforcement by the female surface at the joint and line f-f shows where fracture develops inthreaded joints of pipe, casing and tool strings in the well drilling and operating industry when for any reason Joined sections flex out of line. The bending moment exerts its full effortl at the cross-plane immediately above the first solid cinch between the mutually threaded elements; that is at the first cinched helix of the thread on the pin because the latent elasticity of the threaded surface can only be utilized at that circumferential zone which is not cinched in con-- tact to such a degree that thesaid zone can stretch (axially ofA the pin) at the tensioned side of the pin subjectedv to a moment of force transverse to the pin.

The present invention positively and effectively distributes vthe tension of bending strain for a I very considerable distance along the tensioned side of the flexed pin, or pipe end. This is accomplished by the use of a helical thread body 5, Fig. 3, having at one end a substantially full depth, full size (arbitrary type) helix end a andthe mean depth line M of which intersectsl The crest line vC of the thread extends to andA intersects with the vmean line M at-the zero point. The thread is distinguished in that itsfsides or flanks 6 are either normal in angle tothe axis line X of the cylindric element 2 as in Fig. 3f

showing parallel flanks normal to the axis X, or as in Fig. 4 showing a V-section thread body 5v whose sides or flanks are symmetrically inclined to the axis X. It is to be understood that the intent is to have the body depth of a thread normal to the axis X of the part 2 and not normal to the mean M.

In these several thread types it will be seen that the mean width'of the body of the helical thread is constant at any cross-section of the thread. It 'will be seen that the thread gradually diminishes in depth from the deepest end 5a to minimum at the section lying next to the last scratch S, Fig. 4, ending the thread groove. It will be seen that because of the dual verge of the crest line C and the root line R as to the mean line M there is no conventionally perfect" or full or uniform depth of thread in cross-section at any point of the whole length ef the threadroot and the pitch circle al1 converge toward a circumferential surface zone Where the thread because the depth constantly decreases .to

point O. v

Itwill be understood that this thread may be cut on a terminal of a right-cylinder in which case the crest line `C will be the periphery ofy the cylinder, or the thread may be cut on a male,

conical element in which case the crest line C will be the periphery of the cone and the mean M will verge toward the point O, as, also, will the root line R.

It will be understood that instead of running the groove tothe last scratch S, and making a very thin last thread helix, the cut can` be stopped nearer the full end 5a and thus obtain any desired substantial thread depth at the base or butt end of the pin.

The capacity of axial stretch of the tensioned side of the pin or pipe is made available because the maximum degree of interlock of a threaded pin 2 to its box l0 is carried tothe distal,` smal] end of the pin as to the top edge of the box and when a bending moment is exerted the stress is not limited to a zone at the base or butt of the pin but is dissipated along and far down the convexed side of the bending pin radially distal from its axis thereby preventing the fullv mo ment from being exerted at one of two thread helices at the top of the pin in its box, or a pipe end threaded ina pipe bore, or in a casing joint.

The root and crest surfaces of this thread are made to fit snugly the companion elementand are not disturbed by flexing action. Shearing effect is reduced around the base part of the closed. In all cases longitudinal lines v(as to the axis of the threaded member) of the crest, the

body comes to least depth or the scratch line.

vIn other words, beginning with the pitch circle.

of the deepest thread helix the longitudinal v-line from its crest and its root both converge to meet at the zone of the pitch circle of the shallowest thread cross-section. fIn the claims the word.

taper'is usedwth reference to thetaper Vof thread depth from the deepest thread to the vanishing or least depth thread; lunless-otherwise defined. Crest line, pitch line and root lineA here mean elements extending longitudinally and passing through crest, or pitch, orroot circles at any transverse plane. outermost circle of avfthread turn, and root means the circle at the valley bottom; the pitch circle is the depth mean. g

What isclaimed is: v v 1..-A coupling joint including thread Whose body cross-section at every trans-v verse plane constantly decreases from a maximum at one end of the thread to its opposite end and whose root surface and crest surface are like elongate zones and the pitch width of the thread being the same for the length of the thread and the crest and the root lines of the thread equally diverging along the thread from the vanishing zone of the shallowest thread to the maximum depth thread.

3. A coupling or joint member having an an-A nular surface provided with a screw thread whose body depth constantly decreases for its full length and whose pitch width is the same throughout, and the crest line and the root line of which both taper toward and converge with the pitch line of the shallowesthelix part of the thread, and the crest and the root surfaces being, lengthwise of the thread, similarly shaped and sized.

4. A coupling ory joint member having an annular surface provided with a screw thread whose sides or flanks each constantly change in depth throughout their length and whose crest and root zones are ysimilar in shape; the pitch width being the same for the length of the thread and the root line and the crest line of-the thread uniformly Itapering toward the pitch line at the vanishing zone of the shallowest thread end,

minishes as the diameter of said surface increases; the pitch line, the root line and the crest thread is here shownI Crest means thea body member with a screw thread characterized by a uniform une or .the thread au manning in one generan di;

rection away from the axis of the conicali'suri. faces and severally converging at the vanishing a minimum depth at the other end, and the crest and root surfaces of the thread being .of like shape for their length along the thread and both inclined axially toward and converge with the pitch line of 'the said thread body from the thread convolution of maximum height to the thread convolution of minimum height.

7. 'Ihe device of claim 6; in which .the flanks are symmetrical to the axis of said member and which axis is oblique to the pitch line of the thread body.

8. A coupling or jointk member having an annular surface provided with a thread body and which body has a crest zone and a root zone of similar size and shape, and the body width along the pitch line being the same for the length of the body, and said zones each converging toward the vanishing zone at one end of kthe thread.

9. A pair of joint making elements to be connected by a thread and which elements have a common axis and complementary male and female surfaces -thereabou a thread body on said surface of each element and the common pitch line of each thread body diverging from the said axis and the crest line and the root line of the thread converging with the pitch line on the same longitudinal axial plane at the end of the thread where the heightof the thread becomes zero; the width of theA -thread body along its pitch line being thepsame throughout the lengthl of the body, the said divergence of the pitch line from the,V axis being in the direction from 'the deepest threadk convolution toward the shallow-` est end of the thread. 4

l0. The thread device of claim 9; the `crest and the root of the thread being similar in shape and size. Y

1l. 'Ihe device of claim 9i` the elements hav-` ing. conic, fitting faces and the deepest thread cross-section, on each, being at the smaller end of the conic faces; `whereby to obtain greatest shear capacity of thread at thesmaller end of one o! said elements and to provide'shallow threads at its large end to afford greater lonl1 tudinal flexibility thereat.

112. The device of claim 9; the elements hav. ing conic. fitting faces and the deepest thread cross-section, on each, being at the smaller end of the conic faces and the thread being generally of v-cross-section and the crest and the root of the thread being widest at the larger end .of the said faces whereby to eect more directly radial.

reaction between the meeting roots and crests of the interengaged threads at thelarger end of the conic faces of the said elements.

ARTHUR E. JOHNSON.

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