Patent Application
20070025827
1. Field of the Invention
The present invention relates to fasteners, and in particular to fasteners having load capacities similar to bolts but which require no pre-drilling.
2. Description of the Related Art
Bolts are commonly used for fastening of high load structural members in light framed constructions. A bolted connection typically includes a threaded bolt fit within a pre-drilled bore hole formed through the members to be connected, and a threaded nut tightened on the protruding end of the bolt to fasten the structural members together. Often, high load bolted connections will comprise several bolts at the interface between adjoined structural members.
The predrilled bore hole diameter is preferably provided 1/32nd to, at most, 1/16th inch larger than the nominal bolt diameter. However, it frequently happens during construction that the predrilled bore holes are made larger than this range to facilitate ease of construction. This relatively larger bore hole diameter often goes unnoticed during inspection due to the bolt head and nut entirely covering the predrilled bore hole.
While relatively larger predrilled bore holes may make it easier to construct a bolted connection, bolted connections with such relatively larger bore holes present significant disadvantages in the finished structure, particularly when resisting shear loads typically applied to such connections.
First, a difference in diameter between the bore hole and bolt allows for acceleration of the bolt within the bore hole under shear loads. This acceleration transfers greater stresses to the adjoining structural members. This acceleration also creates impact forces in the bore holes, thereby increasing the chance that the structural members will split or fracture at the bolted connection. Such impact forces within bolted connections can be particularly devastating when the shear loads are cycling loads as they are in earthquakes and hurricanes. In such situations, each reversal of the shear loading tends to widen the bore hole resulting in even greater accelerations and eventual failure of the bolted connection. It is noteworthy that even properly sized bore holes allow for some play between the bore hole and bolt, thus allowing the disadvantageous acceleration and impact loads described above.
A further disadvantage to conventional bolted connections is that, in bolted connections comprising a plurality of bolts, non-uniform clearances within the various bolts and bore holes can result in non-uniform loading of the various bolts. Thus, some bolts wind up bearing more of the load than others and are subject to fatigue and failure at a greater rate than other bolts in the structural connection.
The problem of acceleration, impact loads and uneven load distribution is largely alleviated in the use of self-drilling screws, such as for example Simpson Strong-Drive®) screws described in U.S. Pat. No. 6,109,850, assigned to Simpson Strong-Tie Company, Inc. of Pleasanton, Calif., which patent is incorporated herein in its entirety. The self-drilling nature of the screw into its receiving member ensures a tight fit and prevents any play between the screw and bore formed thereby. However, conventional screws are not able to support the same loads as bolted connections and are conventionally not suitable for fastening high load structural members.
Embodiments of the present invention relate to a fastener for fastening together two or more structural members in a construction. The fastener includes helical threads formed along a portion of an outer surface of the fastener for pulling the distal end of the fastener into at least one of the structural members. The fastener further includes a cutting surface provided in an annular configuration at the distal end of the fastener for cutting into the at least one structural member as the helical threads pull the distal end into the at least one structural member. A cylindrical bore is further formed in the distal end of the fastener and through at least a portion of the fastener, the bore capable of receiving a plug from the structural member. The plug is cut by the cutting surface as the fastener is pulled into the at least one structural member.
The cutting surface may be formed by a plurality of cutting teeth. In embodiments of the invention, the teeth and helical threads are formed in such a way that the helical threads pull the fastener in at a rate at which the cutting teeth may smoothly cut into one or both structural members. The plug formed from the structural member and received within the cylindrical bore adds to the strength and performance of the fastener under shear loads.
The present invention will now be described with reference to the drawings in which:
Embodiments of the present invention will now be described with reference to
Referring now to
In embodiments of the invention, the head 102 may have a diameter of 1.5 inches and may be 0.5 inches in thickness. These dimensions may vary in alternative embodiments. To accommodate a manual or power driven tool, the head 102 may be hexagonal in shape, though other known shapes are contemplated.
In embodiments of the invention, the overall length of the fastener from proximal end 104 to distal end 110 may be 5 inches. The unthreaded shank portion 116 may be about 0.25 inches and the threaded portion 112 may be about 4 inches. The diameter of the unthreaded shank portion 116 and the outer diameter of the threaded portion 112 may be ⅞ inches. It is understood that these dimensions are by way of example only, and each of these dimensions may vary in alternative embodiments. The fastener 100 may be manufactured from 1022 steel (SAE Grade 5) with a finish coat of zinc and dichromate. Other materials are contemplated, including for example other metals such as bronze and aluminum castings, and hard plastics/composites.
Moreover, while
In operation, the distal end 110 of the fastener 100 is positioned at a point of entry into the structural member 120. In some embodiments, for example where the structural members are both formed of wood, the fastener may self-drill through both members 120, 122 to affix those members together. In other embodiments, for example where the structural member 120 is a steel plate, the first member 120 may have a preformed hole for receiving the fastener. In this instance, the fastener 100 is placed through the preformed hole in structural member 120 and against a point of entry in the second structural member 122. In either case, to insert the fastener 100, force is applied in the direction of arrow A on the head 102 of the fastener 100 while rotating the fastener 100 in a first direction so that cutting teeth 108 begin to cut into the structural member(s) 120, 122 as shown in
As the fastener is driven into the structural members 120, 122, the cutting teeth 108 make an annular cut into member(s) 120 and/or 122 so that a cylindrical portion of the structural member(s), referred to herein as plug 124, is left intact in the structural member(s), which plug fills cylindrical bore 118 of fastener 100 as the fastener is driven into the members 120, 122. In embodiments of the invention, the provision of the plug 124 with cylindrical bore 118 increases the strength and performance of the fastener 100 under shear loads. After being cut by teeth 108, the plug 124 may be a single cylindrical piece, or it may chip, splinter, break or otherwise be formed of more than one unitary piece, and still improve the strength and performance of the fastener 100 under shear loads.
In embodiments of the present invention, the configuration of the cutting teeth 108 is related to the helical threads in that the teeth configuration and thread pitch are provided to allow the teeth to cut smoothly into the members as the fastener is pulled into the members by the helical threads. For example, if the thread pitch (i.e., the number of helical threads per inch along the shank) is low so that the helical threads pull the fastener into the members relatively quickly, then if the teeth are not able to cut quickly enough, large compressive forces may be generated at the distal end of the fastener as it is pulled into the members. These forces may make it difficult to drive the fastener into the members, and, if sufficiently large, may result in “blow out,” where the structural member splits near the distal surface of one or both structural members.
In an embodiment of the present invention, the fastener may be a ⅞ inch fastener having an outer diameter of the shank 106 of ⅞ inches. In such an embodiment, the pitch of helical threads 114 may be between 8 and 15 threads per inch, or more specifically between 8 and 12 threads per inch, or more specifically 10 threads per inch. It is understood that the outer diameter of shank 106 may be larger or smaller than ⅞ inches, and that the pitch may vary outside of 8 to 15 threads per inch. In a further example, the diameter of the shank may range between ⅜ inches and 1 inch. Moreover, it is understood that where the diameter varies from that set forth above, the thread pitch may vary proportionately, disproportionately or not at all relative to the diameter.
Referring to
In embodiments of the invention, as indicated above, the size and configuration of the teeth 108, in conjunction with the threads 114, are selected to allow a smooth cutting action of the teeth into the structural member as the fastener is pulled in. However, as indicated above, the plug 124 may chip, splinter or otherwise break into multiple pieces while still adding to the structural stiffness of the fastener 100.
For a ⅞ inch fastener, there may be 6 cutting teeth 108 formed in an annular ring at the distal end of the fastener 100, though there may be more or less than that in alternative embodiments. The outer diameter of the annular ring may be slightly more than or equal to the inner diameter of the helical thread portion 112 (i.e., the diameter at the spaces between helical threads 114).
Referring to
It is understood that teeth 108 may be provided in other configurations, some with very sharp cutting teeth. As indicated above, in embodiments of the invention, there may be a relation between the cutting teeth and pitch of the threads 114. For example, if particularly sharp cutting teeth are provided which are adept at cutting quickly through the structural material, this may allow for coarser threads which pull the fastener more quickly. Those of skill in the art would understand that configuring the teeth 108 in configurations other than those set forth above may allow for appropriate thread pitches above and/or below the ranges set forth above. It is also contemplated that even though fastener 100 is provided with teeth that are especially adept at cutting through the structural members, a high pitch count for threads 114 may still be provided.
The cylindrical bore 118 may be formed by drilling through the distal end 110 of the fastener through at least a portion of the shank 106. The fastener could alternatively be formed by casting where the bore 118 is formed during the casting process. Referring to
The cutting teeth 108, threads 114 and cylindrical bore 118 allow the fastener 100 according to the present invention to be self-drilling without lead holes despite the relatively large diameter of the fastener. In alternative embodiments, a countersink or partial lead hole may be provided so that the fastener is inserted part way and self-drills the remainder of its length. The self-drilling fastener 100 according to the present invention bores its own hole in the structural members and there is no play between the fastener and the hole it forms. Thus, the initial stiffness of the connection using the present invention is higher than in predrilled conventional bolt connections, and the problems of acceleration and impact forces found with conventional bolt connections is alleviated. Additionally, where a connection includes multiple fasteners 100 according to the present invention, each of the fasteners in the connection begins resisting an applied load simultaneously, instead of at different times as found in conventional bolt connections.
As indicated above, in embodiments of the invention, the fastener 100 may be used to affix a steel plate to wood. In such embodiments, the fastener may be inserted through a preformed hole in the steel and driven into the solid wood as described above to affix the members together. In such an embodiment, the preformed hole may be the same diameter as the outer diameter of threads 114 and unthreaded shank portion 116. Alternatively, the preformed hole in the steel plate may have a diameter larger than the diameter of the shank portion 116 in alternative embodiments.
Although the invention has been described in detail herein, it should be understood that the invention is not limited to the embodiments herein disclosed. Various changes, substitutions and modifications may be made thereto by those skilled in the art without departing from the spirit or scope of the invention as described and defined by the appended claims.
