Patent Application
20210372456
Not Applicable.
Threaded fasteners have been used for centuries to secure two or more components of an assembly together by passing through one or more through-holes and then into a concentric threaded hole (whether that be a threaded nut, a threaded blind or through-hole in another component of the clamped assembly, or any other arrangement). Threaded fasteners originally relied on the circumferential friction force created at the interface between the threads of the fastener and the threads of the nut or other threaded hole due to the axial clamping force to resist threading backward out of their engaged, functioning arrangement (i.e., “backing out”) due to loads applied to the assembly. It has long been known, however, that cyclic variations in the loading applied to the threaded fastener, whether through cyclic temperature changes, vibration or other cyclic forces applied to the clamped assembly, or other sources, can sometimes overcome this naïve approach to fastener retention, with results that range from inconvenient to costly to tragic.
Prior art contains a wide variety of attempts to solve this problem, however, even those which have been partially successful suffer from significant limitations, and some—even some of those which are widely used—may in fact harm, rather than improve fastener retention (Izumi et al. 2009).
The use of “lock wire” or “safety wire” (e.g., U.S. Pat. No. 2,890,734) is less applicable to the maintenance of clamping force in a threaded fastener than to preventing the fastener from falling out completely: that is, even very small rotations allowed by lock wire can result in a significant loss of clamping force, even though the fastener is retained by the wire after this rotation increases tension on the lock wire, balancing the peak cyclic forces driving the fastener to loosen. Moreover, the application of lock wire requires either that the fasteners thus secured either occur in groups of at least two, or that some other external anchoring point be provided to secure the lock wire, creating a limitation on the design of the assembly to be clamped, and limiting the application of this method as a retrofit to assemblies not originally designed with lock wire in mind.
A wide variety of locking washers has been developed, including simple split-ring lock washers, Belleville spring washers, curved or wave spring washers, toothed washers, multi-turn helical spring washers, and others, as well as various elaborations of these basic designs. These too suffer from disadvantages. Firstly, being a separate element from the threaded fastener itself, all lock washers necessarily increase the number of parts in an assembly, and hence the complexity, cost (both due to their own cost and by virtue of increased logistical burden), and assembly time of the assembly. Additionally, they suffer from functional deficiencies: the split-ring washer (as exemplified, e.g., in NASM35338), for instance, though commonly used, has been shown in both research (Izumi et al. 2009) and practice (Barret 1990) to often create retention characteristics which are actually worse than the same threaded fastener with no lock washer or other special retention device at all. Other washer designs which rely on the same basic principle may suffer from similar poor or negative performance, and doubt and debate exist within the professional community as to their performance, applicability, and safety. Lock washers which are intended to function by indenting or interference into the fastener or the clamped assembly (such as toothed washers) are limited in their application due to the damage which they do to the surface of the fastener or assembly (including the important consequence that whatever mechanism of corrosion protection has been applied to those surfaces can be compromised), and because they must have a greater hardness than the surface which they are intended to indent, which limits and complicates design of the assembly and selection of appropriate fasteners and washers, since high hardness is often a requirement for the performance of threaded fasteners.
Thread-locking compounds or adhesives, like lock washers, increase the complexity of the threaded fastener assembly, undesirably increasing cost and assembly time. Additionally, they tend to make disassembly more difficult, undermining one of the key advantages of threaded fasteners, namely their ease of reversibility and replacement (as compared to rivets, welds, etc.). Further, such compounds or adhesives may be significantly more susceptible to extreme high or low temperatures, extreme chemical environments, or other extremes of the potential use environment than the clamped assembly or the threaded fastener themselves are, thus limiting their potential applications.
Finally, there are a wide variety of mechanisms to improve threaded fastener retention which rely on the fastener being installed in a through-hole (such as various designs of locking nuts or the use of either identical or dissimilar paired nuts in a “jam nut” arrangement). Regardless of their other advantages or disadvantages, needless to say that these methods are limited in their application, in that they cannot be applied to threaded fasteners which are threaded into a blind hole.
One or more embodiments feature a threaded fastener the threaded body of which is divided into two segments with some mobility relative to one another, and a means to increase tension between these two segments after the fastener is threaded into a blind hole, whereby a distribution of mechanical stresses is created in the threaded body which improves fastener retention.
Advantages of one or more embodiments include applicability to uses in which the threaded fastener is threaded into a blind hole; an absence of particular vulnerabilities to environmental factors such as high temperature or hostile chemical environment; simplicity and the reduction of costs and assembly time due to a retention mechanism which is integral to the threaded fastener rather than a separate component or device; and others, as will be made apparent below.
The first embodiment is shown in isometric view in
This first embodiment is used in the following manner: The fastener is passed through one or more workpieces which are to be clamped, and threaded into a threaded blind hole, such that the threads of the threaded body 101 engage with the threads of the threaded hole, with the entire fastener turned by application of torque to the outer surface of the head 102a, as with a wrench or equivalent tool. Once the fastener has been seated into the hole with a predetermined torque, the exposed toe 301a of the blind rivet 301 is grasped with a tool and tension is applied, drawing the rivet 301 (which results in the expansion of the rivet head 302a and the snapping off of the exposed mandrel toe 301a. This creates a tension force between the lower threaded body 101a and the upper threaded body 101b of a predetermined magnitude, greater than the axial tension created in the threaded body by the initial predetermined torque; thus the loading on the threads of the upper threaded body 101b is reversed: it is now the lower faces of the threads of the upper threaded body 101b which are in contact with the upper faces of the threads of the threaded hole, rather than vice-versa. After this step, the fastener is fully installed.
The second embodiment is shown in section view in
This second embodiment is used in the following manner: The fastener is passed through one or more workpieces which are to be clamped, and threaded into a threaded blind hole, such that the threads of the threaded body 601 engage with the threads of the threaded hole, with the entire fastener turned by application of torque to the outer surface of the head 602a, as with a wrench or equivalent tool. Once the fastener has been seated into the hole with a predetermined torque, the inner rod 604 is rotated by the application of torque to the recess 604b of the inner head 604a using a corresponding key wrench, or other tool, while the threaded body 601 is prevented from rotating by being held by another wrench or other tool on the outer surface 602a of its head 602. This creates a tension force between the lower threaded body 601a and the upper threaded body 601b of a predetermined magnitude, greater than the axial tension created in the threaded body by the initial predetermined torque; thus the loading on the threads of the upper threaded body 601b is reversed: it is now the lower faces of the threads of the upper threaded body 601b which are in contact with the upper faces of the threads of the threaded hole, rather than vice-versa. Thereafter, one of the keys 604c is bent down into one of the corresponding slots 602c, preventing the inner rod 604 from loosening. After this step, the fastener is fully installed.
Thus at least one embodiment of the fastener described in this application answers a need left insufficiently addressed by prior art. While the descriptions of the two embodiments above are necessarily specific in many details, these should of course not be construed as limitations of scope, since many other embodiments—for example and without limitation, those which effect the relative movement between two portions through other methods, such as ratcheting mechanisms, thermal expansion or contraction, piezoelectric effect, hydraulic or pneumatic actuation, electromechanical actuation, or any other method—are possible within the scope of the claims contained in the “Claims” section of this application.
