TITLE: IMPROVED SELF-LOCKING THREADED FASTENER BACKGROUND OF THE INVENTION This invention generally relates to improvements in self-locking threaded fasteners in which locking action is obtained from a pellet or strip of resilient or thermoplastic material inserted therein. More particularly, the present invention relates to linear aromatic semicrystalline polymer (specifically poly (aryletherketone)) pellets or strips inserted into a drilled hole or milled slot respectively on a threaded fastener, in order to make the fastener self- locking and self-sealing. Fasteners with such pellets or strips exhibit a significantly higher level of prevailing torque when compared with pellets or strips of the same size composed of known materials. In addition, such pellets or strips retain their improved self-locking ability at higher temperatures and with greater consistency than was previously possible with prior art materials.

Threaded fasteners in which a locking action against loosening is obtained by a strip or pellet of nylon or similar materials are well known. These fasteners are frequently referred to as self-locking fasteners. In one form of such fasteners a hole is drilled into a portion of the threads of the fastener and a pellet is inserted into the drilled hole. The finished fastener is self-locking, self-sealing and is adjustable. Similarly, self-locking fasteners in which a locking action against loosening is obtained by a strip of nylon or similar material are likewise well known in the industry. Such self-locking fasteners are formed by milling a slot into a part of the threaded portion of the fastener and inserting and anchoring an elongated strip of nylon or similar material into the slot, which makes it self-locking and self-sealing and likewise permits adjustability thereof.

When a self-locking threaded fastener of the type described is threaded onto a mating part, the pellet or strip is compressed. The compressed locking element exerts a spring like wedging pressure, which creates strong metal to metal contact on the opposite side of the fastener. When not fully seated, this pressure forms a positive, yet adjustable, lock. When fully seated, the locking element forms a lock that resists loosening, even under extreme vibration.

Positive seal is insured by the jam-like action of the locking element which prevents leakage.

Pellet and strip locking elements do not require the fastener to be heated or the locking material to be melted in order to secure them to the fasteners. This is in contrast to so-called patch type locking elements which are generally applied by heating a fastener by high frequency induction to a temperature at which the fastener will retain sufficient sensible heat to soften and fuse a powdered thermoplastic resin. Thereafter, the defined powdered thermoplastic resin, which is most often a nylon, is entrained in a stream of air and sprayed directly on to a portion of the threads of the heated fastener forming a locking patch of resin covering part of the threads. In order for thermoplastic resin to adhere to the threads of the fastener,--------.

Users of self-locking pellet and strip type fasteners frequently require these items to meet or exceed certain test procedures or specifications. Some of these specifications, such as military specifications Mil-DTL-18240F and Mil-F-18240E, require that the fastener exhibit a specific minimum prevailing torque over up to five seated or fifteen unseated cycles or reuses of the fastener. Other standards or customer specifications further require the self-locking fastener to be able to withstand elevated temperatures (e. g. temperatures above 250°F to 275°F), yet also meet reuse specifications.

Fasteners having nylon strip or pellet self-locking elements generally exhibit acceptable locking action and permit reusability. However, such fasetners have had difficulty in consistently meeting the minimum requirements when an extended number of cycles (e. g. 15) is required by a specification. Additionally, nylon retains its self-locking properties only up to temperatures of about 275°F. Self-locking fasteners using conventional nylon locking elements therefore lose their ability to lock and can no longer pass the test procedures required under certain qualifying specifications such as military specification Mil-F-18240E, after exposure to elevated temperatures. (This military specification specifies a temperature requirement of 250°F, but its test procedures also are commonly used in evaluating the performance of locking fasteners that have been exposed to higher temperatures as well.

As a result, strip and pellet locking elements of additional materials have been introduced that exhibit a greater resistance to increased temperatures (e. g. above 275°F).. For example, strip or pellet locking elements made of a thermoplastic Kel-f have been advertised to provide effective locking action up to 340°F. Although the Kel-F self-locking fasteners exhibit a significantly better locking performance at a high temperature than nylon, the ability of such fasteners to maintain acceptable torque performance beyond two or three reuses of the fastener preclude it from meeting the requirements of many specifications such as the military specifications previously referred to. Another known material that has been used to form strips or pellets of high temperature locking material in fasteners is the polymer Vespel sold by DuPont Chemical. This material likewise, however, presented several drawbacks. To begin with, the cost of the material is very high when compared to that of other locking elements. Additionally, it is at times difficult to achieve the minimum torque requirement for fifteen cycle reuse tests, such as in the above-described military specifications. Furthermore, due to the physical properties of this material compared to other known types of locking elements, it often necessitated use of an adhesive, such as a cyanoacrylate, to glue the locking element in place into the hole or slot in the fastener. This results in both increased cost and time to produce such fasteners. Furthermore, since the melting point of cyanoacrylate adhesives generally does not exceed 252°F, such materials tend to lose their bonding ability at temperatures above that region.

Self-locking fasteners having different types of locking elements known as patch type locking elements have also been used to attempt to provide the self-locking properties at elevated temperatures. In contrast to pellet and strip locking elements, patch type locking elements usually require that the entire fastener be heated above the melting point of the locking material and/or that it be quench cooled in order to insure proper adhesion of the material. This heating requirement can be expensive, time consuming and has frequently introduced charring or degradation of certain fasteners, coatings or platings thereon. Likewise, the step of quench cooling further introduces complications and additional steps into the manufacturing process.

Moreover, the application of patch type locking elements often requires a primer or adhesive to be applied before or along with the locking materials. None of these known locking materials have claimed locking effectiveness above 450°F. For example, polyester patches made by the Long-Lok Fastener Corporation of Cincinnati, Ohio, and sold under the trade name"Poly-Lok" are advertised as maintaining effectiveness only up to 400°F. Additionally, patch type self- locking fasteners made of polypthalamide made by Nylok Fastener Corporation of Macomb, Michigan under the trade name"NYTEMPOO"are advertised as maintaining their locking effectiveness temperatures only up to 450°F.

It is therefore apparent that there exists a need in the art for an improved self-locking fastener that can exhibit improved torque results, both at room temperature and elevated temperatures, including those over 450°F.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide self-locking threaded fasteners of standard dimensions that rely upon a thermoplastic pellet or strip to provide self-locking capability and maintain the fasteners locking capability and meet certain specifications outlining parameters for acceptable self-locking performance, better and more consistently than known locking elements.

It is another object of the present invention to provide self-locking threaded fasteners of standard dimensions that can meet certain specifications outlining parameters for acceptable self- locking performance with a smaller size locking element than is now required to achieve the same result using known materials at a lower cost than known materials.

It is yet another object of the present invention to provide self-locking threaded fasteners of standard dimensions that have thermoplastic pellets or strips that provide self-locking capability and can withstand appreciably higher temperatures while still maintaining the fastener's self-locking capabilities and meeting cetain specifications outlining parameters for acceptable self-locking performance.

It is yet another object of the present invention to provide the foregoing benefits without requiring the use of heat, adhesives, primers, or quenching to apply the locking element to the fastener.

To achieve these objectives, a locking pellet or strip composed of a high temperature thermoplastic is applied to a threaded fastener by milling a slot or a hole into a portion of the threads of the fastener and inserting the locking element into that opening. This application is accomplished without any heating or quenching of the fastener and does not require any adhesive or primer to retain the element in the opening. Application can be accomplished by any one of several processes and apparatus known to those skilled in the art, such as the methods and apparatus disclosed in U. S. Patent Nos. 3,324,919,3,203,041 and 3,254,691. We have found that the result is a threaded fastener with a polymer strip or pellet locking element that exhibits greater (i) self-locking performance compared to known locking elements (ii) does not require heating or quenching of the fastener or application of adhesive to adhere the locking element to the fastener (iii) can meet certain specifications outlining parameters for acceptable self-locking performance with a smaller diameter pellet than other known locking materials, and (iv) further believe withstand exposure to a temperature of more than 450°F, for times in excess of one hour, while retaining its self-locking capabilities compared to the 275°F-450°F limitations advertised for the best prior art materials.

A variety of high temperature polymer materials can be used to obtain the desired results of the present invention. It has been found that such materials include, but are not limited to, linear aliphatic semi-crystalline polymers. More specifically, it has been found that such materials preferably include poly (aryletherketone) extended into the form of a locking pellet or strip. The foregoing polymer materials have been found to produce satisfactory pellet or strip locking elements exhibiting the above-listed characteristics. Commercially available polymers in this group include Victrexfs) PEEK, which is available from Victrex Limited of Westchester, Pennsylvania. The locking element in accordance with the present invention is applied into a milled slot or hole in the fastener in the form of a strip or pellet and preferably has a compression modulus of at least about 550,000/psi (under ASTM method D695) and a flexural modulus of at least about 594,500/psi at 23°C, (in accordance with ASTM 790).

The particular improvements contained in the present invention relate to both an improved self-locking fastener and improved high temperature self-locking fastener. The objects listed above, as well as others, will become apparent to one of ordinary skill in the art, upon review of the detailed description of the invention in combination with the following drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a side view of a male threaded fastener having a single locking pellet.

FIGURE 2 is a cross sectional view of the fastener along the line 2-2 of Figure 1 illustrating the locking pellet inserted into a hole drilled into the threaded portion of the fastener.

FIGURE 3 is a side view of a male threaded fastener having a strip of locking material inserted therein.

FIGURE 4 is a cross sectional view of the fastener along the line 4-4 of Figure 3 showing the locking strip positioned in a milled slot of the fastener.

FIGURE 5 illustrates the chemical structure of the repeat unit of a preferred locking material which may be used in the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to Figures 1 and 2, a typical screw 10 having a head portion 11 and a threaded shank 14 in which is formed a cavity 16 where some of the material of the screw 10 has been removed. Mounted in the cavity 16 is a polymer pellet 12 in accordance with the present invention. It should be understood that, although the pellet 12 is illustrated as generally circular in cross section, it could also take on a variety of other shapes, such as rectangular or hexagonal.

As illustrated, the outer surface of the pellet 12 may be disposed between the crest line 19 and the root line 13 of the thread, or may project slightly beyond the crest line 19. The pellet 12 is inserted into the cavity 16 by any variety of known techniques, such as those illustrated in U. S.

Patent Nos. 3,254,691 and 3,324,919. The pellet 12 is press fit into the cavity 16 preferably in contact with the base 17 of the cavity 16 forced against the sidewalls 18. Since the pellet 12 is press fit into the cavity 16, no heating or cooling of the fastener 10 is required to insert and retain the pellet 12 within the cavity 16.

In order to assist in retaining the pellet 12 within the cavity 16, the sidewalls 18 of the slot can optionally be beveled, chamfered, or have a groove provided therein. As a result of the physical properties of the preferred polymer for the pellet 12 of the present invention, no adhesive such as a cyanoacrylate is necessary in order to assure that the pellet 12 will be retained within the cavity 16. This use of an adhesive is often necessitated in order to insure retention of certain prior art polymer pellets such as Vespel@. Although illustrated in Figures 1 and 2, the self-locking fastener made according to the present invention is illustrated in the form of a screw, it should be understood that this invention can be used with other types of threaded fasteners, such as bolts, threaded inserts and nuts and the like and that, although not required, multiple pellets could be provided along the threaded surfaces thereof.

Turning now to Figures 3 and 4, another screw 20 is illustrated having a different embodiment of a self-locking element according to the present invention. The screw 20 has a longitudinal slot 23 formed therein, which intersects a plurality of the threads of the threaded shank 24 and is open at the end of the screw shank. Mounted within the slot 23 is a strip 26 of resilient locking polymer in accordance with the present invention. The strip 26 may be circular, rectangular or of another known shape in cross section. It is preferably press fit into the slot 23 so that the strip 26 is against the base 29 of the slot 23 and is also forced against the sidewalls 31 of the slot 23. The force exerted against the strip 26 in order to insert it into the slot 23 can cause it to be compressed into the slot so that its cross section may be altered slightly once finally inserted. Expansion of the strip 26 causes it to be firmly seated in and gripped by the walls of 31 of the slot 23 and may also at times cause it to bulge out slightly as illustrated in Figure 4. As illustrated, the outer surface of the strip 26 extends about to the crest line 30 of the threads.

Alternatively, it may be disposed between the crest line 30 and the root line 28 of the thread, or may project slightly beyond the crest line 30.

As previously described with respect to pellet type locking elements, the sidewalls 31 of the slot can optionally be provided with beveled, chamfered faces or a ridge in order to assist in retaining the strip 26 therein. As in the previously described embodiment of a pellet type locking element, the strip 26 of polymer material in accordance with the present invention is inserted into the slot 23 without having to apply external heat or cooling to the fastener and, further, does not require the use of any adhesive or primer to retain the strip 26 in the slot 23. A wide variety of known methods and apparatus can be used to insert strip elements into the fasteners, including those such as described in U. S. Patent Nos. 3,324,919 and 3,203,041. In addition, strips could be manually inserted.

The term"locking element"is used herein as a collective term to refer to a polymer strip or pellet. In the preferred embodiment of the present invention, the high temperature polymer locking element is a linear aromatic semicrystalline polymer. In particular, it is a poly (aryletherketone). The preferred structure of the preferred locking and high temperature resistant polymer of the present inventionis based upon the repeat unit shown in Figure 5.

Poly (aryletherketone) is believed to exhibit improved locking abilities at temperatures up to at least about 300°F and has a flexural modulis of at least about 594,500 psi at 23°C; 580,000 psi at 120°C; and 43,500 psi at 250°C, and a flexural strength of at least about 24,650 psi at 23°C; 14,500 psi at 120°C; and 1,813 psi at 250°C (under ASTM test method D790). In addition, the preferred high temperature polymer has a compression modulis of 550,000 psi, a compressive strength at 23°C with flow of 17,110 psi compressive strength at 23°C and across flow of 17,255 psi (under ASTM test method D695), and a sheet modulus at 23°C of 188,500 psi. In addition, the polymer has a melting point of 644°F and a level of crystallinity of about 35%. A polymer suitable for praciticing the embodiments described above include Victrex@ PEEK 450G, which is available from Victrex Limited of Westchester, Pennsylvania.

In a second embodiment where preferred locking characteristics are desired not only at room temperature, but also at temperatures above 450°-460 °F, an optional poly (aryletherketone) having a flexural modulus of about 1,421,100 psi at 23 °C, 1,160,00 psi at 120 °C, and 435,000 psi at 250 °C, a flexural strength of 30,450 psi at 23 °C, 5,220 psi at 250 °C (under ASTM method D790), a compressive strength at 23 °C (with flow) of 21,750 psi, a melting point of 644 °F and a heat distortion temperature of greater than 560 °F. A high temperature resistant polymer suitable for practicing the embodiments described above include Victrex (R) PEEK 450FC30. It is believed that strip and pellet locking elements made of the material described in this optional embodiment exhibit superior locking characteristics to known materials, both at room temperatures and at temperatures exceeding 450°-460°F and up to about 550°F.

In other optional embodiments where the preferred locking characteristics of the present invention are desired at room temperature and also at temperatures above 450°F, glass or carbon fiber can be used at amounts up to about 30% in combination with poly (aryletherketone) to form fiber reinforced locking elements. A preferred structure of these high temperature resistant polymers is likewise based on the repeat unit as illustrated in Figure 5. Examples of such preferred polymers would have either about 30% carbon or glass fiber reinforced poly (aryletherketone) and have a flexural modulus of between 1,450,000 to 2,929,00 psi at 23°C, 1,334,000 to 2,697,000 psi at 120°C, and 435,000 to 739,500 psi at 250°C, a flexural strength of 33,785 to 51,475 psi at 23°C, 25,375 to 37,700 psi at 120°C, and 10,150 to 15,225 psi at 250°C (under ASTM method D790). In addition, these optional reinforced high temperature polymers have a compressive strength at 23°C (with flow) of between 31,175 psi and 34,800 psi, and a compressive strength at 23°C across flow of 21,605 psi to 22,185 psi (under ASTM test method D695), a sheer modulus at 23°C of 348,000 psi (glass fiber) are about 35% crystalline and have a melting point of 644°F and a heat distortion temperature. High temperature polymers suitable for practicing these optional embodiments described above include Victrex (R) PEEK 450GL30 and 450CA30. It is believed that strip and pellet locking elements made of the material described in this optional embodiment exhibit superior locking characteristics to known materials and obtain their locking characteristics, both at room temperatures and at temperatures exceeding 550° and up to about 600°F.

EXAMPLES The following examples illustrate the practice of the present invention. It is to be understood that the invention is not limited to the specific conditions described therein, without limiting the scope and content of the present invention, a linear aromatic semicrystalline polymer [specifically poly (aryletherketone)] will be used as an example of the improved locking polymer to be applied in a cavity in the threads of a threaded fastener.

Example I Comparison test were performed with 10-32x 1 inch stainless steel socket head cap screws having locking elements (pellets) of conventional Kel-F, Vespel and nylon 66 material and with pellets made of poly (aryletherketone) (Victrex (g) PEEK 4506) used in the certain embodiments of this invention. The mating nuts were DOE stainless steel test nuts. The procedures of the military specification Mil-F-18240E, paragraphs 4.5.2 through 4.5.4, were used. The pellets all had. 093 inch diameter locking elements. The height of the pellets over the major diameter over each of the fasteners was. 003 inches. The parts were tested per Mil-F- 18240E, using no lubricant.

The following results were obtained: TABLE I TOROUE AT 15TH REMOVAL (Room Temnerature Minimum Breakawav Torque (In. Ibs.) Sample Nylon: Kel-F: Vespel : Present Invention: 1 Failed to meet min. Failed to meet min. 2.9 4.2 @ 6th removal @1 st removal 2 Failed to meet min. Failed to meet min. Failed to meet min. 3.5 @6th removal @2nd removal @7th removal 3 Failed to meet min. Failed to meet min. Failed to meet min. 3.2 @14th removal @2nd removal @7th removal Mil-F-18240E 2.0 2.0 2.0 2.0 Specification The pellets made of conventional Kel-F and nylon consistently failed to meet the minimum torque requirement for fifteen cycles. The pellets made of conventional Vespel met the torque requirement in only one of three samples. The pellets made of poly (aryletherketone) exceeded the for the minimum breakaway torque in the Mil-F-18240E specification for the fifteenth removal by at least 1.2" lbs. of torque, or 60%. These fasteners with pellets in accordance with the present invention further exhibited a minimum breakaway torque value grater than those with Vespel pellets for every cycle in each of the three samples.

Example II Further comparison tests were performed with 10-32x 1/2 inch stainless steel socket head cap screws having pellets of conventional Vespel material and with pellets made of poly (aryletherketone) (Victrex (g) PEEK 450G) used in certain preferred embodiments of this invention. Many nuts were DOE stainless steel test nuts. The pellets all had. 093 inch diameter locking elements and the height of the pellets over the major diameter for each of the fasteners was. 003 inches. The tests were conducted at room temperature. The parts were tested using the test procedure of Mil-F-18240E. Then it was modified to test for twenty cycles, rather than the fifteenth called for in the standard. This procedure will be referred to as the modified Mil-F- 18240E standard.

The following results were obtained: <BR> <BR> <BR> <BR> TABLE II<BR> <BR> <BR> <BR> <BR> Breakawav Torque (Inch Lbs.) Vespel Sample #1 Vespel Sample #2 Invention Sample #1 Invention Sample #2 Cycle Max Min Max Min Max Min Max Min 1 11.0 6.7 13.0 8.0 13.5 9.0 18.0 10.0 5 4.125 3.375 5.0 4.55 6.3 5.5 6.5 5.5 10 2.5 2.5 4.0 3.5 5.4 4.8 3.875 3.9 15 2.5 2.6 3.25 3.0 3.5 3.5 3.375 3.6 20 2.5 2.5 3.0 3.25 3.5 3.4 3.0 3.25 The results for the two samples using the locking material in accordance with the present invention met and exceeded the standards of the Mil-DTL-F-18240 and also consistently exhibited superior locking performance to the Vespel samples for each of the twenty cycles.

Example III One aspect of the practice of the present invention involves the application of high termperature resistant pellets or strips in such a way as to conform to Mil-F-18240E modified to provide the following test protocol and referred to here as"the modified high temperature Mil-F- 18240E standards", thereby providing a control frictional engagement between the threaded fastener and the mating fastener: 1. Assemble fastener 2. Expose the fastener to elevated temperatures for three hours 3. Cool fastener to room temperature 4. Disengage and reassemble fastener fifteen times recording torque required 5. Heat reassembled fastener to the temperature set forth in step 2 and hold it at that temperature for one hour 6. Disassemble the fastener at the temperature set forth in step 2 and measure the torque required for the sixteenth disassembly It is believed that disassembly torques for self-locking fasteners made in accordance with the present invention as previously described would meet or exceed Mil-F-18240E standards for all removals, including the sixteenth at the elevated temperatures as follows: Material Type : Elevated Temperature: Preferred poly (aryletherketone) 285°F to about 300°F<BR> Optional embodiment poly (aryletherketone) 450°F to about 550°F<BR> Optional embodiment reinforced poly (aryletherketone) 460°F to about 600°F