Patent Application
20080000015
1. Technical Field
The present invention relates generally to safety fasteners for ballistic resistant helmets. More particularly, it encompasses fasteners designed to reduce the likelihood of secondary projectiles forming due to a nearby ballistic incursion.
2. Description of Prior Art
Ballistic helmets, primarily for military use, are capable of resisting ballistic threats through the design and manufacture of the helmet shell. One approach involves several layers of composite materials that are impregnated with resin and laminated together under pressure and heat. For example, layers of Kevlar® fabric may be impregnated with a thermoset resin that is molded to form the helmet shell. Steel helmet shells constitute another approach.
In order to adequately protect the wearer, helmets must remain strapped on through the use of various suspension, retention and chinstrap fitting equipment. Mounting of fitting equipment to the helmet shell requires bolting to bores formed in the helmet shell. The creation of these bores, and the external exposure of the bolt heads, compromises the ballistic safety of the helmet shell. In particular, instances of ballistic incursion in the vicinity of this mounting hardware presents an elevated threat, in the form of secondary projectiles. If the fastener fails, secondary projectiles may be created within the helmet shell or exterior to the helmet shell.
U.S. Pat. No. 6,854,921 discloses a ballistic resistant cap nut for use with a conventional screw bolt for securing the flight deck door frame on a commercial airliner. The cap nut achieves its ballistic resistance by employing titanium or stainless steel in an overdimensioned configuration to provide greater robustness. Such an approach would be unacceptable in a helmet environment due to the increased weight and excessive internal protrusion.
U.S. Pat. No. 5,600,084 discloses a conventional armor plate bolt fastener that captures a ballistic material web. The web is wrapped around the bolt head to secure a thin shield therein. The web and shield would be too heavy and provide too great an external protrusion for a helmet.
U.S. Patent Applications 2004/0058125 and 2005/0022658 utilize conventional screw and bolt fasteners to add layers of ballistic resistant panels to protect underlying structures. U.S. Patent Application 2003/0104738 discloses a multi-layered composite laminate, wherein various layers have different mechanical properties to improve shearout resistance along with better distribution of cutting and impact forces.
Accordingly, it would be desirable to provide a safety fastener that is lightweight, meets tighter standards for internal protrusion and minimizes the deleterious effects associated with forming mounting bores though the helmet shell.
It is an object of the present invention to provide a fastener which can safely mount fitting equipment to a protective helmet.
It is another object of the present invention to meet stricter guidelines for inside-the-helmet protrusion limits and corresponding weight limits.
It is a further object to provide a fastener which improves the likelihood that the helmet can absorb a ballistic threat without the formation of secondary projectiles.
These and other related objects are achieved according to the present invention by a safety connector for mounting fitting equipment to a protective helmet shell and resisting fracture during nearby ballistic incursion. The safety connector is a fastener having a low, interior impact profile which facilitates the absorption of ballistic threat energy by the helmet shell. The absorption of ballistic threat energy by the helmet shell prevents the fastener from forming a secondary projectile. The fastener is adapted to disengage from the fitting equipment without fracturing. The fastener disengagement prevents the fastener from forming a secondary projectile. The absorption of ballistic threat energy by the helmet shell works in combination with the fastener disengagement, depending on the nature, angle of incidence and momentum of the ballistic incursion, to prevent the fastener from forming a secondary projectile.
The fastener includes an engagement surface for contacting the fitting equipment on an inner side. The engagement surface is shaped and configured to retain the fitting equipment up to a preset pullout force, which is less than the fastener fracture force. The engagement surface is ramped to allow for progressive deformation and energy dissipation of the fitting equipment. The fitting equipment has a strap bearing a grommet, and wherein the smaller end of the ramped surface fits within the grommet. The ramped surface is conical. The fastener includes a threaded body along its central core which carries the exterior-facing engagement surface.
The fastener comprises a screw that extends through a bore formed through the helmet shell and includes mating threads to engage the threaded body without exceeding pre-set limits on in-the-helmet protrusion. The screw includes a head with an increased diameter that contacts the exterior of the helmet shell over an enlarged surface area. The screw includes a non-threaded shank portion adapted for placement within the helmet bore. The shank has a cross-sectional surface area “a” and the head has a cross-sectional area “A”>4a. The head contacts a greater surface of the shell thereby increasing the transfer of any ballistic energy to said head on to the shell. The screw includes a fillet portion at the juncture of said head and said shank that increases the screw's resistance to fracture. The shank has a diameter “d” and said fillet has a radius “f”, in which 0.1d<f<0.2d. The threaded body is dimensioned larger than the bore, so that following fitting equipment disengagement, the fastener is retained from exiting the shell. The threaded body and said mating threads are substantially disposed concentrically within a grommet of the fitting equipment.
In an alternate description, the invention relates to a ballistic resistant safety fastener for mounting a grommet to a protective helmet shell that utilizes helmet shell deformation along with a grommet pull-out force to avoid creation of secondary projectiles. The fastener includes a screw component and a nut component. The screw extends through the helmet and terminates near the interior face of the grommet. The nut includes a lip portion to retain the grommet up to the pull-out force which is less than the fastener fracture force. The nut includes a sleeve portion disposed concentrically between the screw and the grommet. The lip portion flares outwardly from the sleeve portion.
The screw includes a non-threaded shank that extends through the helmet. The screw includes machine threads at its terminal end, and wherein the screw has a zero in-the-helmet protrusion beyond the grommet. The shank has a greater diameter than the thread crest. The lip portion flare has a conic shape. At least part of the lip portion extends slightly past the terminal end of the screw. At least part of the lip portion extends slightly past the interior face of the grommet. The lip portion that extends beyond the screw is adapted to receive a tool. The flare allows for progressive deformation and energy dissipation as the grommet eye opens under influence of the pull-out force. The lip portion flare is larger than the screw-receiving bore formed within the helmet shell. Where following pull-out of the grommet, the size of the lip portion flare prevents the fastener from outwardly exiting the helmet.
The screw includes a head with an increased diameter that contacts the exterior of the helmet shell over an enlarged surface area. The screw includes a non-threaded shank portion adapted for placement within the helmet bore and wherein the shank has a cross-sectional surface area “a” and the head has a cross-sectional area “A”>4a. The head contacts a greater surface of the shell thereby increasing the transfer of any ballistic energy to the head on to the shell. The said screw includes a fillet portion at the juncture of the head and the shank that increases the screw's resistance to fracture. The shank has a diameter “d” and said fillet has a radius “f”, in which 0.1d<f<0.2d.
These and other aspects, features and advantages of the present invention will be described or become apparent from the following detailed description of the preferred embodiments, which is to be read in connection with the accompanying drawings.
The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings wherein like reference numerals denote similar components throughout the views:
It should be understood that the drawings are for purposes of illustrating the concepts of the invention and are not necessarily the only possible configuration for illustrating the invention.
In ballistic helmet design, one approach is to create the helmet shell out of layers of composite material, laminated together. Ideally, this structure would present the maximum protection if it were to remain complete and unpierced. But to make the helmet usable, hole must be made in the shell to attach suspension, retention and chinstrap components. Part of this mounting hardware will be on the outside of the helmet and subject to the threat projectiles. In general, reference will be made in this document to “fitting equipment” that includes the suspension, retention and chinstrap components. However, it is also intended that the phrase “fitting equipment” should encompass any part that may be mounted to a helmet.
A composite is a material having two or more distinct components. Typically ballistic composites include a reinforcing component like carbon, graphite, boron or aramid fibers, such as KEVLAR®. The reinforcing component is combined with a thermoplastic or thermoset resin to form a prepeg or lay-up layer. Multiple layers are joined together under pressure and heat to form a laminate. Other varieties of protective helmets are made from steel. The fastener according to the invention may be employed with any type of helmet that has fitting equipment mounted thereto.
There are two different types of ballistic trauma caused by a projectile when impacting this mounting hardware. References to “projectile” include bullets, shrapnel or other objects capable of contacting the helmet. One type is a direct hit to a screw head. The other type is a glancing hit to the screw head or near miss where the screw head is pushed laterally and the composite is deformed by the projectile. In this instance, the mounting hardware can be fractured and cause the formation of a secondary projectile. The secondary projectile, being inside the helmet shell and still possessing ballistic impact energy, may impact and cause damage to the user's head.
Referring now in detail to the drawings, and in particular
The prior art technology does not consistently stop a ballistic threat and results in secondary projectiles penetrating the witness plate when tested against 9 mm 124 grain FMJ threats. The prior art does not provide sufficient ballistic threat protection as secondary projectiles occasionally result from fastener failure during testing. Prior art technology depends on increasing the size (and thereby the strength) of the fastener. The larger fasteners may exceed the inside-the-shell protrusion requirements. In addition, larger nuts provide a greater surface area on which to receive impact energy, which increases strain and leverage applied to the screw. Prior art technology does not enlist the ballistic shell composite structure itself to absorb a portion of the impact energy, reducing the strength requirements (and thus) size of the fastener hardware yet consistently demonstrating no penetration of the witness plate during ballistic testing.
To defeat a glancing hit, as shown in
and can be applied to a wide variety of fastener configurations.
Several key features of the invention are as follows. Safety nut 60 does not possess a clamping surface that faces the inside of the helmet. Therefore, it is dimensioned and oriented to provide adequate space for the composite to deform or expand without interference, even at the very edge of the bore. Composite deformation or delamination may also cause displacement of the strap or grommet. The strap is free to move or bend the outer flange or flanges of the grommet. Energy acting upon the grommet will gradually force it up the conical surface. The grommet will progressively stretch or break as it rides up, causing a dissipation of energy in the process. Even if the grommet cracks, its separate piece(s) will remain clipped to the strap. Once released from the grommet, the safety nut will catch onto the edges of the delaminating composite. Accordingly, the formation of secondary projectiles will be avoided.
The safety fastener was tested against ballistic penetration using a 9 mm 124 grain FMJ bullet, tested in accordance with NIJ Level IIIA 9 mm threat. Surprisingly, the safety fastener employs smaller, lighter weight materials to achieve a higher ballistics rating than any known prior art fastener. The invention is well suited for ground troop ballistic helmets that face a ballistic threat and require the attachment of fitting equipment. However, the key concept developed herein is that of retaining fitting equipment rather than clamping. The retention is carefully engineered to allow fitting equipment pull-out well before fastener failure. This concept can be applied to a wide variety of connection elements other than grommets. For example, brackets or plates with conic shaped holes or other types of apertures, slots, etc. In general, we refer to the application of this concept as providing a fastener with a low, interior impact profile.
We define “low, interior impact profile” according to location, structure and performance as follows. Location wise, this means a portion of the fastener that resides interior of the helmet shell. Structurally, the interior portion of the fastener is shaped and configured to avoid interfering with the protective nature of the helmet shell, that is, to absorb ballistic threat energy in the vicinity of the fastener. By allowing the shell to absorb a greater proportion of ballistic energy, the fastener realizes improved performance in its function of safely securing the fitting equipment and reducing the risk of creating secondary projectiles.
Turning now to safety screw 62 of
For laminated helmets, there is provided a screw-bolt and nut fastener that safely dissipates energy directed at the screw-bolt when the fastener is used to mount grommetted fitting equipment to a protective helmet. The screw-bolt passes through a closely tailored bore formed through the layers of the protective helmet. A low intrusion nut having a Y-shaped cross sectional shape that fits substantially within the grommet is threaded onto the screw-bolt to retain the fitting equipment adjacent the helmet shell. Ballistic energy directed at the screw-bolt is primarily transferred via the screw head to said helmet causing said layers to absorptively delaminate. Further energy is dissipated as the delaminating layers force the fitting equipment away from the head causing the grommet to expand. Additional energy would cause the grommet to shear open and slip over the major diameter of the safety nut. The energy required to force the grommet off the safety nut is less than the fracture threshold of the fastener. As can be seen, the safety fastener represents an improvement over prior art approaches in that it has been reduced in weight and size. The head diameter, shank and fillet are proportionally larger and more robust. The low, interior impact profile, allows the fastener to interact more flexibly, thereby giving the shell greater latitude as it undergoes its transient deformation upon impact. The flexibility arises from the newly designed retaining lip, which overcomes the major problem of fastener failure resulting from the explosive release arising when equipment is clamped to the shell.
Although illustrative embodiments of the present invention have been described herein, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the present invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims. For example, it is expressly intended that alternated designs, dimensions and relationships between portions of the safety fastener which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or as a general matter of compatibility of application method. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
