I. Field of the Invention
This invention relates to fasteners and in particular to self-locking metal screw threaded fasteners for locking cooperation with a complementary screw thread of a complementary member, a locking element comprising thermoplastic polymer being provided on the screw thread of the fasteners.
II. Description of the Prior Art
Villo U.S. Pat. No. 3,093,177 teaches that a patch for self-locking action may be formed on a bolt by degreasing the bolt in a hot alkali bath; locking the bolt in a special jig and heating the assembly to 475.degree. F.; applying a shaped pellet of polyamide resin (nylon) to the heated bolt under a pressure of 100 psi; cooling the bolt to 350.degree. F. while the plastic is under pressure; removing the pressure and then permitting the bolt to cool to room temperature.
Preziosi U.S. Pat. No. 3,294,139 also teaches that a screw fastener may be provided with a patch made of thermoplastic, such as nylon. The thermoplastic resin is applied to the fastener in powder form and then the fastener is heated sufficiently to melt the resin.
One disadvantage of the Preziosi patch-type threaded fasteners lies in the difficulty of obtaining close control over the thickness of the patch and, therefore, in a given production run of the fasteners, the patches vary widely in thickness. This fact may be verified by selecting a number of prior art patch bolts, the external screw threads of which are identical, within a narrow tolerance range of dimensions and screwing the bolts into nuts having internal screw threads that also lie in a narrow tolerance range of dimensions. With variations in the dimensions of the mating screw threads substantially climinated in this manner, variations in the breakaway torques through repeated cycles must be caused by variation in the thickness of the plastic patches, and it is found that the breakaway torques do vary over a wide range.
Both of the two prior art procedures for forming patches on fasteners require careful preparation of the metal surfaces of the fasteners to insure effective bonding of the plastic and, of course, such preparation steps increase the cost of production. In the Preziosi disclosure the metal is first cleaned by dipping in an acid solution and subsequently the fastener is preheated to drive off moisture. In the Villo disclosure, a bolt is given a preliminary alkali wash and if the bolt is to be cadmium plated after the patch is applied, the bolt must not only be treated with hot alkali but also must be pickled, rinsed, electro cleaned, rinsed again and finally dried before the patch is applied. Obviously these additional operations greatly increase the cost of the patch bolts, but the disclosure stress their importance in obtaining the necessary bond between the metal and the polymer patch.
Since both of the prior art disclosures require that the fastener be cleaned before the plastic material is applied, it would seem to be economical to degrease and/or otherwise clean large masses of the fastener in advance. Unfortunately, however, pre-cleaned fasteners are highly vulnerable to corrosion so that storing and handling the pre-cleaned fasteners in advance of applying the plastic patches results in deterioration of the metal surfaces to which the plastic is to be bonded.
Both of the above prior art patents disclose the patch material as extending from the root of the screw thread substantially beyond the crest, the crest being completely submerged. Thus the locking action is achieved by causing a mating complementary screw thread to penetrate relatively deeply into the patch to develop an extensive area of metal-to-plastic pressure contact to achieve adequate frictional resistance to loosening rotation between the two screw threads. Such drastic deformation of the plastic patch reduces the capability of the fastener for repeated use without excessive drop in the breakaway torque. The protruding patch can also interfere with the insertion of the bolt into a close clearance hole.
Duffy U.S. Pat. No. 3,498,352 discloses the application of a polyamide or polyester patch on a threaded fastener by first applying an adhesive to the area of the fastener over which the patch is to be formed. Thereafter, the fastener is heated and powdered plastic is spaced over the adhesive to form the desired patch. The primary disadvantage of this method is that is requires the use of an intermediate adhesive layer with its attendant higher cost and difficulty of application.
The preferred thermoplastics in the teachings of prior art patents are varieties of polyamide resins, with emphasis on Nylon 11 and Nylon 33, and in commercial practice these two resins are used almost exclusively.
Even those thermoplastic materials which have been utilized by the prior art in preparing patch-type fasteners tend to undergo some degree of crystallization during normal cooling from a molten state to room temperature and sustain a crystallization shrinkage of as high as 3 to 4 percent by volume. This crystallization shrinkage weakens the bond between the metal fastener and the patch material at the metal-to-plastic interface. This results in weakening of the bond between the plastic patch and the metal fastener which can result in premature failure of the fastener.
It would be desirable to improve the properties of patch-type fasteners of the prior art by providing a means to improve the bond between the plastic patch material and the metallic fastener element. Further, it would be desirable to provide a fastener in which the bond between the plastic patch material and the metallic fastener element and the properties of the patch material are improved by controlling the degree of crystallization that occurs in the patch material.
In addition, it would be desirable to provide both patch-type fasteners and a method for their manufacture in which the patch material is a crystallizable thermoplastic polymer having a higher shrinkage due to crystallization on cooling from a molten state to room temperature. Many of these thermoplastic polymeric materials have physical properties which are superior to those of thermoplastic materials used by the prior art in forming self-locking patch-type fasteners. However, they have been unsuitable for patch-type fasteners, prior to the present time, because of their crystallization and shrinkage on normal cooling.