US3789644A

US3789644A - Self-locking dies for making threads

Info

Publication number: US3789644A
Application number: US00200933A
Authority: US
Inventors: R Orlomoski
Original assignee: Litton Industrial Products Inc
Current assignee: Western Atlas Inc
Priority date: 1971-11-22
Filing date: 1971-11-22
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: SU571201A3

Abstract

This invention relates to self-locking screws and thread rolling dies for making such screws. The self-locking result is achieved by a rib which may be continuous or discontinuous in the flank or flanks of one or more turns of the threads over a selective length of the screw. The outermost face of the rib is generally parallel to the mating threads so that with standard threads it is disposed at approximately 60* with respect to the oppositely disposed flank on the same thread to provide initial compression and subsequent resilient bending of the rib without scoring the mating thread despite variations in fit. The screw threads and ribs therein may be made by the use of conventional flat or circular thread rolling dies in which certain selective threads in at least one of the dies, preferably the movable die, have been reformed over a suitable length in a corresponding manner.

Description

United States Patent [151 11 3,79,644 Qrlomoski Feb. 5, 1974 SELF-LOCKING DIES FOR MAKING Primary ExaminerMilton S. Mehr THREADS ABSTRACT This invention relates to self-locking screws and thread rolling dies for making such screws. The selflocking result is achieved by a rib which may be continuous or discontinuous in the flank or flanks of one or more turns of the threads over a selective length of the screw. The outermost face of the rib is generally parallel to the mating threads so that with standard threads it is disposed at approximately 60 with respect to the oppositely disposed flank on the same thread to provide initial compression and subsequent resilient bending of the rib without scoring the mating thread despite variations in fit. The screw threads and ribs therein may be made by the use of conventional flat or circular thread rolling dies in which certain selective threads in at least one of the dies, preferably the movable die, have been reformed over a suitable length in a corresponding manner.

5 Claims, 10 Drawing Figures PATENTED FEB 5 74 saw 2 or 3 Fig-6 SELF-LOCKING DIES FOR MAKING THREADS I BACKGROUND OF THE INVENTION Self-locking screws are widely used with correspondingly threaded nuts and apertures. The Locking Fastener Section of the Industrial Fasteners Institute has established specifications for the locking characteristics of such screws. The standards generally contemplate that a self-locking screw of a given size when screwed into a nut of corresponding size would require no more that a predetermined torque to make the initial installation. There is, also, a requirement for a minimum break-away torque for the same screw after it has been removed, reinserted and removed a fixed number of times, and it is desirable that the ratio of the initial drive torqueto the final removal torque be relatively low. It should be understood that while the prior art primarily includes self-locking screws, the invention disclosed herein pertains to both internal and external locking threads.

Commercially available self-locking threaded fasteners embody in or between the threads some kind of yieldable, nonmetallic material that has been secured to the threaded structure by a secondary operation. Other types utilized deformed ormismatched threads which create increased friction between the mating threads. A self-locking thread is described in U.S. Pat. No. 3,517,717 issued June 30, 1970. Application Ser. No. 26,520 filed Apr. 3, 1970, teaches the dies and the method for making that thread described in U.S. Pat. No. 3,517,717 and application Ser. No. 26,529 filed Apr. 3, 1970, discloses still another form of self-locking thread. Each of these applications and the patent has the same inventor and assignee as this application. The self-locking threads described in the patent and application have received wide acceptance in the fastener market. The present invention 'is an improvement thereover having a generally triangular, cross-sectional rib disposed on the flank of a thread and having a surface thereof cut away topresent a surface which is generally parallel to or aligned with surfaces of the mating thread.

SUMMARY OF THE INVENTION The present invention contemplates a body having self-locking threads disposed thereon wherein at least one of the threads has a quadrilateral rib on at least one flank. The rib has a bottom wall commencing at the flank intermediate the pitch line and root of thethread, an inner wall commencing nearer to the intersection of the pitch line and transverse mid-section of said thread than to the conventional line of the flank. The outer wall intersects the inner wall along an arcuate ridge line which line is generally midway in an axial direction between the root of the one flank and the crest of said one thread, said outer face of said rib having a surface disposed generally parallel to the original flank or at an included angle of approximately sixty degrees with respect to the thread face on the distal side of the thread. In one form the crest of the rib is outside the pitch line of said thread, and the included angle between the faces of the rib adjacent the crest thereof form approximately a right angle before assembly to a mating thread. The inner wall may be slightly convex and the flank upon which the'rib is-disposedmay be slightly concave. In' some forms the inner wall at some place in its width intersects the transverse plane that defines the mid-section of the thread, and the intersection of the inner wall and the third wall defines a cavity having an included angle between the walls of about 77, and a volume not less than one quarter of the volume of the thread including the rib that is radially outward of the pitch line.

In still another form the member having self-locking threads disposed thereon has at least one of the threads provided with a rib on at least one flank. The rib has a generally quadrilateral form before assembly with a meshing thread and is disposed with a base on the flank, inner and bottom walls intersecting the base, and an outer wall joining the inner and bottom walls. The outer wall forms an included angle of approximately sixty degrees with respect to the oppositely disposed flank of the thread. Preferably an obtuse angle is formed between the bottom wall and the outer wall. The crest of the rib defined by the intersection of the inner wall and the outer wall is normally outside the pitch line of the thread and normally a plurality of ribs will be disposed on successive threads.

The invention further contemplates a thread rolling die for rolling the self-locking threads of the invention on a cylindrical surface such as a screw blank. The die has at least one face having a plurality of generally parallel ridges adapted to form corresponding threads on a cylindrical workpiece and having at least one interval thereof in which a portion of the flank of at least one of the ridges is cut and formed significantly along a path parallel to the crest away from its normal flank configuration to form a rib extending outwardly beyond said flank which has a surface disposed at an angle of approximately with the opposite flank of the ridge and a cavity in the flank coextensive with said rib. The rib is disposed adjacent to a cavity in the flank which is equal in volume to the rib and the rib parallels throughout a major portion of its length, the crest of a die ridge and extends upwardly from the root adjacent to the die ridge.

Accordingly it is an object of the present invention to reduce the initial drive torque to removal torque ratio between an element having the locking thread and a conventionally threaded element.

It is another object to provide a locking thread which does not damage the mating threads particularly where those threads are manufactured of relatively low yield strength material.

It is still another object of the invention to provide a locking thread which is inexpensive to manufacture and which may be produced rapidly by means of thread rolling apparatus.

It is an object of the invention to provide dies capable of long-lived operation which will produce locking externalthreads of the invention and which will function for a large number of rolling operations.

BRIEF DESCRIPTION OF THE DRAWINGS ous rib form;

FIG. 3 is a. longitudinal section to a greatly enlarged scale of a threaded surface showing an illustrative locking rib of the present invention disposed on two threads thereof;

FIG. 4 is an axial sectional view to a greatly enlarged scale of the locking thread engaging a conventionally threaded cooperatively dimensioned internal thread;

FIG. 5 is a transverse sectional broken-away view of a die in aligned relationship to the corresponding threaded surface in FIG. 3 and taken at line 5-5 of FIG. 7;

FIG. 6 is an end view of a tool for forming the die of FIG. 5 which has been modified with the tool so as to produce a die in accordance with the invention;

FIG. 7 is a view of the upper movable die from the bottom when the die is normally positioned for use and showing the location on the surfaces thereof which have been altered by means of the forming tool shown in FIG. 6;

FIG. 8 is a top plan view of a conventional fixed die suitable for use with the movable die shown in FIG. 7;

FIG. 9 is a partially broken-away isometric view to a greatly enlarged scale of the upper surface of the movable die shown in FIG. 7 and illustrating the position of the rib on the die; and

FIG. 10 is a side elevational view of the forming tool shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2 there are shown screws 2 and 4 having a multiplicity of conventional threads 6. Selected threads 8 on each have been modified from the conventional form to include a generally quadrilateral rib 14 which may have a discontinuous or intermittent form as on screw 2 as well as continuous forms as shown on screw 4. Other forms of the invention include intermittent ribs on two flanks, intermittent ribs staggered from one flank to the other and continuous ribs on two flanks. The rib may be positioned on either or both the pressure or nonpressure flanks although it is most preferable to provide either a discontinuous or continuous quadrilateral rib on the nonpressure flank of selected threads.

According to the present invention certain of the threads are modified in shape in the manner generally indicated in FIG. 3. FIG. 4 shows the meshing relationship of the locking thread with a conventional internal thread. The rib 14 is generally quadrilateral although it should be understood that as used herein the sides may be somewhat curved without departing from the invention. Where a curved face is used a straight line approximating the mean position of the curved line shall be indicative of the angular limitations recited herein. The locking rib 14 as shown in FIG. 3 is polygonal in shape, and has an overall triangular form as defined by the dashed line 18 and inner face or wall 26 and bottom face or wall 28. However, the surface of outer wall 24 of the rib 14 is cut away to match the mating or cooperating threads so that a bearing surface which is generally parallel to the mating thread is presented. This fourth face 24 provides the quadrilateral shape referenced hereinabove. This smooth and relatively broad surface 24 has the advantage of avoiding scoring or cutting of the cooperating threads as they are torqued into engagement. The base 18 is coextensive with the flank 20 of an individual thread 8 having an opposite flank 22. The outer face or wall 24 of the quadrilateral rib 14 is most preferably disposed at a 60 included angle with respect to flank 22. The vertical centerline 29 of the thread, sometimes called the transverse midsection line will, therefore, normally form an angle of 30 with respect to flank 22 and outer face 24 of quadrilateral rib 14 in that preferred form. Some variation is possible but this 60 inclination is most preferable because conventional threads have that angle between their flanks and standard physical gaging means utilize the 60 angle as a'reference to check other thread dimensions.

A face 24 which is generally parallel to the original flank and to the mating threaded element is desirable to avoid stress concentration in the mating thread although arcuate and other curvilinear forms are satisfactory as long as the variations from' the parallel form are not so great as to cause stress concentrations. There may be slight curvature of the outer face 24 of the rib such that the center portion thereof bows away from the flank upon which it is disposed is utilized in the form of the invention illustrated in FIG. 4. The inner face 26 of the rib 14 in the most preferred form of the invention intersects face 24 at a right angle.

In the most preferred form the inner face 26 intersects the adjoining flank 20 of the thread 8 above the pitch line 27 and nearer the intersection with the transverse mid-section line 29 of a conventional thread than to the line of a conventional thread flank. The angle between the inner face 26 and the adjacent flank 20 is most preferably in the order of 77 and will generally be between 50 and 80 in various embodiments of the invention. This angle determines the amount of resilient deflection of the rib which may occur. Inner face 26 and flank 20 may be somewhat curvilinear although the angular spacing between the faces is still important to provide the necessary resiliency. The intersection of inner face 26 and outer face 24 is ordinarily outside the pitch line 27 of the thread 8. In the cross-sectional view of FIG. 3, the bottom face 28 of the quadrilateral rib 14 is shown as a straight line although the contour may vary widely without departing from the invention. Most importantly, the included angle between outer face 24 and bottom face 28 is obtuse so that upon deflection of rib 14 no sharply pointed area which would produce stress concentration is presented to the mating flank 30. In some forms of the invention, the included angle may even be degrees. The position of a conventional thread 31 in FIG. 3 is shown in phantom superimposed on a deformed thread 8 to show the relationship therebetween.

As is apparent from FIGS. 1 and 4, the assembly of threads 8 provided with ribs 14 with mating conventional threads produces deformation of rib 14 as it bears against the flank 30 of the mating conventional thread. The elastic deformation of that rib produces a locking action which prevents undesired loosening of the mating threads. It should be noted that the selflocking feature of the screw as herein disclosed, is effective with mating threads having varying degrees of tolerances. It is effective with loose as well as snug fits although it is essential that there be interference between the rib 14 and the mating thread whereby resilient deformation of the rib occurs. Rib 14 extends a sufficient distance from flank 20 so that industry torque requirements are satisfied. g

It should be understood that while the locking thread may be most inexpensively and rapidly produced by thread rolling on an external cylindrical surface, the thread may be manufactured by other means on internal surfaces as well as on external surfaces. Most preferably, the mating thread 31 of the nonlocking type is manufactured of a material at least as hard as the material from which the locking thread 8 is manufactured. Use of a softer material may result in deformation of the nonlocking thread 31 rather than deflection of the rib 14 on the locking thread 8. Ordinarily the deformation of the rib in the assembled state will not be permanent although because the stress varies over the cantilevered rib 14, it is possible that the elastic limit of rib 14 may be exceeded in certain areas so that the rib form will vary slightly after initial assembly. It will be observed in FIG. 4 that the crest 32 of the rib 14 is still disposed at a lower point than the crest 34 of thread 8 despite the deflection of rib 14. The root 70 of the locking threads 8 and conventional thread 6 is normally identical.

It is to be noted that although there is some permanent deformation which may occur after the initial connection of a locking thread with a conventional mating thread that deformation is not so great that the screw will lose its effectiveness as a locking threaded connector. Stated in other terms, the resilient nature of the ribs allows repeated insertions and removals from the mating thread while retaining the locking characteristics. The use of the continuous form of ribs 14 as shown in FIG. 2 results in the maximum locking forces for a given number of altered threads. As will be described hereinafter, the use of intermittent or discontinuous ribs 14 increases the die life where the threads are manufactured by thread rolling techniques. Ordinarily, it will only be necessary to position a rib on approximately six flanks corresponding to the threads in an axial distance equal to six times the pitch. Depending upon the torque requirements, however, a smaller or larger number'of ribs may be utilized. It is, of course, not necessary that successive flanks be deformed.

The preferred method of manufacturing the selflocking thread on external surfaces is by means of thread rolling because of the great speed and accuracy which are possible with minimum unit cost. For internal threads and certain external threads it may be desirable to manufacture the self-locking thread of the invention by means of milling, electrical discharge machining or other means. Thermoplastics may be readily formed by heating a suitably sized blank and torquing the blank into engagement with a mating thread. Planetary dies may, also, be utilized to manufacture the locking thread of the invention although flat dies are somewhat more preferable because of their reduced cost. Such dies are shown in FIGS. 7 and 8 and are indicated generally by the

numerals

52 and 54. The construction of these dies and themethod of using them is well understood in the art and details need not be shown herein. The general description of the dies, self-locking thread and method for making each described in commonly owned U.S. Pat. No. 3,517,717 is hereby incorporated by reference. It is sufficient to say that the shorter die 54 is held stationary in a thread rolling machine, the longer die 52 is moved back and forth parallel to die 54 by the thread rolling machine and a cylindrical workpiece is introduced between the properly spaced dies, and as it rolls therebetween, the metal of the workpiece is progressively formed by the teeth of a the dies 52, 54. When the movable dies 52 has carried the rotating workpiece to the end of the short die 54,

the threads of the screw will be fully formed and the completed screw will fall away.

According to the present invention, it is proposed that certain of the threads of the die 52 at a suitable location, as for example at the area 56, be deformed in the manner generally indicated in FIG. 5. The transverse cross-sectional contours of the die 52 taken through the section 56 will be seen to be complementary to the contours of the locking thread shown in FIG. 3. The longitudinal positioning of area 56 on die 52 has been selected such that when the workpiece has reached a point between the dies where it is about to roll through area 56, the screw threads of conventional nature have been almost fully formed and hence are ready to take the deformation provided by the reformed walls of the threads of the die in area 56. In addition, the longitudinal position is selected so that the ribs 14 that are formed by die 52 will not be damaged by die 54. In themost preferred form, the short die 54 has similarly reformed portions which assist in forming the rib 14. Where short die 54 has not been reformed, it is desirable to cut away a portion 57 thereof to avoid damage to the deformed thread 8. The transverse positioning of area 56 has been selected such that the threads of the workpiece will be those that are engaged when the locking thread is coupled to a mating conventionalv thread. Most preferably, they will not be the starting threads so that initial assembly is facilitated. The number of threads on the die 52 which are reformedwill vary as a function of the torque requirements of the screw as described hereinbefore.

Turning now to FIGS. 5, 6, and 9 a portion of the die 56 and the forming tool 58 for manufacturing the die are shown. FIG. 5 is a greatly enlarged cross-section of the movable die reformed area 56 and FIG. 9 is an isometric view of the same area. The forming tool 60 appears in FIG. 6. When the thread creating ridges or

threads

62, 66 were originally machined, the flanks thereof are undistorted in the manner of normal threads. Thereafter, in selected areas, and at selected distances along the flanks, the tool 60 of a length to produce the required length of rib is driven downwardly against the unhardened die flank forcing part of the metal outwardly to create the complement of the entire thread 8 including rib 14. This procedure creates a cavity complementary to the forming tool edge 69 as well as surface 88 which is relatively straight although not positivelyconstrained by any corresponding surface. The complement includes a major peak 66 and a minor peak 68 corresponding respectively to root 70 and point 72 defined by the intersection of inner face 26 and flank 20. Roots 74 of the die 52 correspond to rib peaks 76. Roots 78 of the die 52 correspond to peak 34 of the locking thread peak 34. Since this procedure is accomplished before hardening, no heating of the die 52 is necessary. If only one die is reformed, the length of the forming edge 66 corresponds to the length of the unwrapped length of the minor peak 68 and root 74 and rib 14 unless that length exceeds the unwrapped circumferential length measured at the pitch line. If both dies are reformed, the reformed areas on each may be longitudinally positioned to phase the rib formed by one die to follow the rib formed by the other so that the sum of the length of the ribs on the two dies is at least as great as the unwrapped length of the external thread produced thereby. It will be apprent that the face 82 of the die corresponds to face 24 of the locking thread and is disposed at an angle of 60 with respect to flank 84. The minor peak 68 is positioned on a rib defined by

faces

86, 88 which intersect at an acute angle.

Movable die 52 and short die 54 are hardened after all of the ribs of the selected character and length have been formed. The dies 52, 54 when used will first create normal screw threads as the workpiece is initially passed therebetween and then form the locking threads of the invention with further relative movement.

Various modifications have been conceived to maximize die life. The modifications to increase die life include increasing the radius of the leading edge of the forming tool which is depressed into the die. This affects the external thread that is rolled with the dies, so that they will have a correspondingly larger radius formed at rib crest 76. The use of a larger radius decreases stress concentration particularly in the dies which are much more vulnerable to premature failure than the screw or other externally threaded objects which may be formed with the dies since the'number of cycles ordinarily contemplated for a die exceed 100,000 as opposed to a relatively small number of cycles for a fastener.

The use of intermittent ribs 14 as shown in FIG. 1 also increases die life because the rib forming portion tends to increase stress levels. The intermittent ribs 14 are most rapidly manufactured from dies that have been modified with a forming tool having a plurality of indentations 80 in the forming edge 69 whereby a discontinuous series of depressions are made in the die 52 as shown in FIG. 10.

Still another means for increasing die life is to taper the extremities 83 of the forming edge 69 of the forming tool that is used to press into the die. More specifically as shown in FIG. the relief of this surface in the forming tool produces an indentation having end extremities of gradually decreasing depth. This means that externally threaded surfaces which are produced with the dies have ribs that are faired into the flanks upon which they are carried.

Various heat treating processes are utilized to maximize die life for particular applications. The method of manufacturing the self-locking threads with the dies will be more apparent by referring to US. application Ser. No. 26,520 filed Apr. 3, 1970, having the same inventor and assignee.

For different sizes of threaded surfaces, it is necessary to vary the angles and position of the rib as will be apparent to those skilled in the art.

In considering the present invention relative to prior self-locking threaded fasteners, reference is again made to applicants US. Pat. No. 3,517,717 showing a very thin rib; and to applicants prior development of fasteners with a massive or thick rib, as shown in US. Pat. application Ser. No. 26,529 cited above and in an article entitled New Thread Form which appeared at pages 57 and 58 of the Jan. 4, 1971, issue of Product Engineering. Reference is also made to E. M. Ilsemann, US. Pat. No. 2,301,181 granted Nov. 10, 1942. As noted above, certain problems posed in the self-locking threaded fastener field involve (1) the provision of an inexpensive threaded fastener which will have (2) a relatively low ratio of initial torque to final removal torque after several operations, while (3) accommodating the various mating threaded fasteners of different sizes as permitted by production tolerance standards for mass produced nuts and screws. It is also desirable to have a locking thread configuration which will (4) permit use of relatively hard self-locking threads with softer mating threads, so that steel locking threads could be properly used and reused in aluminum or zinc die castings, for example, without objectionable galling, extruding, or otherwise altering the mating threads.

The E. M. Ilsemann US. Pat. No. 2,301,181 is of interest in showing a thread which is provided with protrusions and recesses on the flank of the thread for locking purposes. However, only compressive forces come into play in the Ilsemann screw, and accordingly the bending or deflection action forming an important part of the present invention to provide the high resiliency and resultant low torque ratio is not present.

As compared to the prior rib structures as shown in the US. Pat. No. 3,517,717, US. Pat. application Ser. No. 26,529, and the Product Engineering article, the provision of a bearing surface represents a further advance which is particularly useful in applications involving softer metals and in connection with the accommodation of substantial variations in the size of mating threaded fasteners. It is particularly to be noted that the present invention contemplates the provision of a bearing surface as shown at 24 in FIG. 3 which extends generally parallel to the original flank of the screw thread so that it makes initial surface-to-surface area contact with the mating threaded fastener. The use of the bearing surface area distinguishes from the initial line contact of the prior locking ribs, and produces initial compressive deformation of the rib without galling or removal of metal even when relatively softer internal threads are engaged by a harder threaded screw having the self-locking rib. This permits either substantial compressive deformation of the rib followed by resilient deflection of the rib in the case of tightly fitting threads, or in the case of loosely fitting threads, a lesser degree of permanent deformation resulting from compression but still accompanied or followed by the necessary resilient deflection or bending to achieve the desired low ratio of initial to final torque after several on-off cycles.

Those skilled in the art will recognize that the invention provides a locking screw requiring a low initial drive torque while avoiding damage to the mating threads and which is inexpensive to manufacture. Similarly experience with the dies will show they are capable of long-lived operation while accurately producing threads in accordance with the invention.

1 claim:

1. A thread rolling die for rolling self-locking threads on a cylindrical surface, said die having at least one face having a plurality of generally parallel and generally uniformly spaced ridges adapted to form corresponding threads having a generally uniform pitch on a cylindrical workpiece and having at least one interval thereof in which a portion of the flank of one of said ridges is cut and formed significantly along a path parallel to the crest thereof away from its normal flank configuration to form a rib extending outwardly beyond said flank having a surface disposed at an angle of approximately sixty degrees with the opposite flank of said ridge and a cavity in said flank coextensive with said rib.

2. A thread rolling die as set forth in claim 1 wherein a given length of said rib is equal in volume to the adjacent length of said cavity.

3. A thread rolling die for rolling self-locking threads on an external surface, said die having a plurality of generally wedge-shaped parallel ridges each being spaced from an adjacent ridge a substantially equal amount and said ridges having crests and roots, at least one ridge of said die having at least one straight rib paralleling throughout a major portion of its length the crest of said one ridge and extending upwardly from the root adjacent said one die ridge and having inner and outer walls intersecting at an acute angle and disposed 5. The die as set forth in claim 3 wherein the intersection of said rib and said flank adjacent said cavity has a substantial radius therebetween to avoid stress concentration.

Claims

3. A thread rolling die for rolling self-locking threads on an external surface, said die having a plurality of generally wedge-shaped parallel ridges each being spaced from an adjacent ridge a substantially equal amount and said ridges having crests and roots, at least one ridge of said die having at least one straight rib paralleling throughout a major portion of its length the crest of said one ridge and extending upwardly from the root adjacent said one die ridge and having inner and outer walls intersecting at an acute angle and disposed to form a rib intermediate roots and crests on a workpiece, said rib on the workpiece having a surface disposed at an angle of approximately 60* with the opposite flank of said ridge on the workpiece.

4. The die as set forth in claim 3 wherein said rib is faired back into said ridge at its extreme end portions.

5. The die as set forth in claim 3 wherein the intersection of said rib and said flank adjacent said cavity has a substantial radius therebetween to avoid stress concentration.