1. Field of the Invention
This invention relates to a thread locking/prevailing torque fastener assembly and especially to a fastener with an improved external thread profile geometry that optimizes assembly joint integrity.
2. Background Information
In general, it is accepted that the object of having a specially designed external thread on a fastener of the thread-locking type is to create a condition of mating thread interference when the fastener is screwed into a nut anchor that has been pre-tapped in accordance with a known and accepted thread standard such as the I.S.O. standard 965/1. The term “nut anchor” as used herein is meant to include any separate nut or any anchor or support into which the fastener may be threaded.
Such mating thread interference may occur at the thread major or outer diameter as shown in FIG. 1A. That figure shows a fragmentary cross-sectional view of an external screw thread 2 assembled into a pre-tapped internal thread 3 of a nut anchor where thread interference 4 is seen at the outer diameter of the mating threads. The interference is created by having a distance from the pitch diameter 5 of the external thread 2 exceed that of the distance from the pitch diameter 6 of the internal thread to the outside or major diameter of that thread. The magnitude of the interference 4 is that which establishes the thread locking capability of the threaded fastener/nut anchor assembly.
Mating thread interference may also occur at the thread minor or inner diameter shown in FIG. 1B. There, the external thread 7 of a fastener is assembled into a nut anchor with an internal thread 8. An interference seen at 9 at the thread minor diameter creates the thread locking characteristics of the assembly.
A third type of mating thread interference is depicted in FIG. 1C. There, a fastener with an external thread 10 is shown assembled into a nut anchor having an internal thread 11. In this assembly, an interference 12 is created along one of the thread flanks.
In FIGS. 1A to 1C, the head of the fastener with the external thread 2, 7 or 10 is located in the direction of the arrow H.
It is apparent that all of the above known assemblies rely only on mating thread interference to achieve the thread locking effect. It is generally accepted in the industry that the magnitude of the mating thread interference is the governing factor behind the degree of thread locking that can be obtained by any one of the above assemblies. In all of these examples, continued re-use of the threaded fastener causes wear of the mating threads, resulting in a lesser degree of thread locking over time.
While each of the prior thread interference assemblies just described can be effective in particular applications, there are some applications where they do not achieve the desired thread locking effect, with the result that assembly joint integrity may suffer. In other words, the threaded fastener may become loosened when the jointed assembly is subjected to externally applied forces. By the term “jointed assembly” is meant an assembly wherein the threaded fastener has been subjected to a tensile force that is induced in the fastener from the applied torque used to produce an acceptable assembly.