Patent Application
20070243037
The present invention is generally related to blind fasteners, and more particularly to blind bolt fasteners having a body and a deformable sleeve.
Blind fasteners are used in a variety of applications to connect two or more workpieces together. In the construction of aerodynamic designs, such as control surfaces on aircraft and the like, a substantially flush surface is typically desired on the accessible side of the panels. Often, however, access to the blind side of the workpiece is not possible. Such one-sided access complicates the installation process. In these cases, the use of a blind fastener is appropriate and simplifies installation.
Typical blind fasteners comprise an internally threaded nut body and an externally threaded cylindrical core bolt in threaded engagement with the nut body. The inserted end of the core bolt has an enlarged core bolt head while the other end of the core bolt has a wrenchable portion. The fastener is inserted into aligned apertures of a pair of workpieces and the core bolt is rotated with respect to the nut body. The core bolt moves axially in an outward direction through the nut body. This axially outward movement typically causes a deformable sleeve around the core bolt and intermediate the nut body and core bolt head to deform about the nut body to provide a blind side head against the inner surface of the inner work piece. The core bolt further is provided with a localized weakened region or break groove adapted to sever the core bolt at a predetermined amount of torque and location.
It is advantageous that the break groove shears the core bolt in a substantially flush relation to the fastener body head after the fastener is fully set. Particularly, an accurate core bolt break is sought for fasteners having countersunk body heads to provide a flush relationship between the set fastener and the outer panel, thus providing a smooth aerodynamic surface after the fastener is set.
However, due to numerous factors including variations in combined panel thickness, sometimes the break groove on the core bolt extends beyond a flush position with the fastener body head. Therefore, when shear or breakage occurs at the break groove, a portion of the remaining core bolt may protrude beyond the fastener body head. Accordingly, it is often necessary to grind the protruding core bolt so that the core bolt is flush with the fastener body head. Prevention of such protrusion will provide a cost savings through the elimination of additional operations and manpower required in shaving, smoothing and trimming the protruding core bolt stem to provide a flush finish.
Conversely, positioning the break groove to break below the head surface can result in cavities that must be filled. Again, eliminating the need to fill such cavities will provide a cost savings through the elimination of additional operations and manpower required to provide a flush finish. In addition, low (below flush) breaks may result in some loss of strength in the fastener head.
Additionally, structural joints should have strengths at least equivalent to the panels in which they are installed. Otherwise, the fasteners will fail prior to panel failure in an overload situation. As most airframe joints are designed to carry shear loads, the joint shear strengths should be in line with the structure material bearing load strength. The shear load capability of a structural joint is usually measured using Metallic Materials Property Development and Standardization (FAA/DOD MMPDS) guidelines and testing in accordance with MIL-STD-1312 Test Method #4. A load versus elongation plot of a single fastener joint is shown in
Having a relatively large residual clamp load in the joint enhances structural strength. This allows fasteners to close gaps between panels and keep them tightly clamped together as desired. High residual clamp reduces microscopic movement between metal panels during flight operations, thereby minimizing the likelihood that fretting and fatigue cracks will develop.
Laminated carbon fiber composites are becoming increasingly prevalent in airframe structure because these composites provide lighter weight and accompanying fuel savings. Composites, however, cannot endure the high compressive stresses induced by the installation of conventional fasteners designed for metallic structure. It is, therefore, desired to spread the fastener clamp loads over a large region on the panels to minimize contact stresses while maintaining high clamp loads.
Additional information will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
The present invention relates to a blind bolt fastener having a core bolt in threaded engagement with a deformable sleeve. The fastener has a body with an enlarged head positioned between the deformable sleeve and wrenching flats of the core bolt. The bore of the deformable sleeve is tapered and has a leading edge that may abut the end of the body. An optional drive nut having protrusions may be provided for engagement with the head of the body. Bulbing of the sleeve causes the leading edge of the sleeve to move along the length of the body. The leading edge of the sleeve engages the blunt end of the body to flatten the deformable sleeve in a completely bulbed position. The core bolt has a break groove that fractures when installation of the fastener is complete.
Operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:
While the present invention is described with reference to the embodiments described herein, it should be clear that the present invention should not be limited to such embodiments. Therefore, the description of the embodiments herein is illustrative of the present invention and should not limit the scope of the invention as claimed.
Reference will now be made in detail to the embodiments of the invention as illustrated in the accompanying figures. Embodiments of a blind bolt fastener 10 are shown in
The core bolt 20 has a core bolt head 21 with a wrench engaging portion 22 on one end, as shown in
Further, the core bolt 20 has a core bolt break groove 26. The core bolt break groove 26 is a weakened region in the core bolt 20 that causes the core bolt 20 to fracture at a predetermined amount of torque or stress. For example, the break groove 26 may break or fracture the core bolt 20 when a preselected amount of torque is applied to wrenching flats 22 during installation of the blind bolt fastener 10. Such fracture is designed to occur upon completion of the installation of the blind bolt fastener 10, such as, when the torque required to deform the sleeve 40 exceeds the torsional strength of the break groove 26.
The core bolt 20 is insertable through the body 30. The body 30 may be sized to allow the core bolt 20 to rotate within the body 30. The body 30 has an enlarged head 32 (hereinafter “the body head 32”) at one end, such as, the end adjacent the wrenching flats 22 of the core bolt 20 when assembled. The body head 32 is capable of seating in a cavity in the access side of a pair of structural panels being fastened together. The body head 32 may be of a protruding type, setting on the surface of the access side panel.
The body 30 has a tapered nose 36. In a preferred embodiment, the tapered nose 36 is located at an opposing end of the body head 32. The outer diameter of the body 30 may taper or otherwise decrease toward the end of the body 30 adjacent the deformable sleeve 40. In an embodiment, the tapered nose 36 may abut the deformable sleeve 40.
The head 32 may have body-wrenching members 33, which are shown in
The deformable sleeve 40 may be positioned at the threaded portion 24 of the core bolt 20. The sleeve 40 may be made of a malleable material that has the ability to bulb or expand a predetermined amount without fracturing. For example, polished and annealed AISI 304 stainless steel is able to undergo a strain of approximately 100% without fracturing. Alternatively, Commercially-Pure Titanium, 300-Series Stainless steel, and A-286 Corrosion and Heat Resisting Steel can be used. The deformable sleeve 40 should not be deemed as limited to any specific material. One of ordinary skill in the art will appreciate the use of various materials for the deformable sleeve 40.
The deformable sleeve 40 has a tapered or stepped bore 42. The bore 42 may have threads 43 capable of threaded engagement with the core bolt 20. The threads 43 of the bore 42 may be buttress threads that matingly engage the threaded portion 24 of the core bolt 40. The buttress threads incorporate a steep pressure flank and a shallow non-pressure flank. For example, the pressure flank may be approximately between 75 and 90 degrees off the axis of the core bolt 20. In an embodiment, the non-pressure flank may be approximately 45 degrees. The pitch of the thread or threads per inch may be, for example, similar to that used for the 60 degree thread used on existing threaded blind bolts, such as, threads similar to MIL-S-8879 and MIL-S-7742. The buttress profile results in a lower radial component of force so that the female threads 43 of the sleeve 40 is not forced to expand radially as much as the sleeve 40 would be required to expand if a conventional thread form were used instead. A large radial component of force can cause the female threads 43 of the sleeve 40 to become disengaged with the male threads 24 of the core bolt 20 resulting in a weaker structural connection. Use of conventional threads would require more threads in engagement than the buttress threads to provide the same strain capability; however, additional threads requires additional length and, as a result, increased weight. The buttress threads minimize undesired radial expansion of the threaded portion 43 of the sleeve 40 with a minimal amount of thread engagement. The length of engagement is driven by the shear strength of the threads, rather than by concerns over radial expansion.
The sleeve 40 may have a counter bore or groove 44. The inner diameter of the groove 44 may be greater than the inner diameter of the threaded portion 43 of the sleeve 40. The sleeve 40 may be crimped at or around the groove 44 such that an edge 46 contacts or abuts the body 30. During installation, the edge 46 engages the outer surface of the tapered nose 36 of the body 30. To this end, the edge 46 and the groove 40 aid in allowing the sleeve 40 to slide or otherwise move on the body 30 toward the body head 32. As the sleeve 40 bulbs, the groove 40 is capable of causing the sleeve 40 to completely flatten against, for example, the blind side of a panel.
The drive nut 50 is positioned between the wrenching flats 22 of the core bolt 20 and the head 32 of the body 30. For example, the drive nut 50 is positioned at the break groove 26. The drive nut 50 is trapped axially between the body head 32 and the core bolt head 21 so that the drive nut 50 remains engaged with the body head 32 throughout the installation sequence.
The drive nut 50 may be provided with protrusions 52 that extend from an underside of the drive nut 50. The protrusions 52 extend toward the body 30. In an embodiment, the protrusions 52 correspond in size and shape to engage and fit into the wrenching members 33 of the body head 32. The drive nut 50 can be positioned such that the protrusions 52 engage the body head 32 upon assembly. Failure to preposition the protrusions into the recesses may prevent the core bolt head 21 from properly seating in the body head 32 throughout the installation process.
In an another embodiment, the protrusions 52 of the drive nut 50 may be a raised deformable portion adjacent the body head 32. The deformable portion may be capable of deforming into the body head 32 during assembly of the fastener components or during installation of the fastener 10.
The drive nut 50 can be used on other fasteners as will be appreciated by one of ordinary skill in the art, including, without limitation, other known fasteners. In an embodiment, a drive nut 150 is used with a blind bolt fastener 100 as shown in
The drive nut 150 may have features similar to the drive nut 50 as illustrated in
Turning to the fastener 10, an example of how to use the fastener 10 as illustrated in
The core bolt 20 rotates and is threaded into the deformable sleeve 40 causing the sleeve 40 to bulb. The tapered or stepped bore 42 controls the blind side formation of the sleeve 40. As the core bolt 20 is threaded into the sleeve 40, the sleeve 40 is driven against the tapered nose 36 of the body 30. In an embodiment, the sleeve 40 bulbs prior to moving up the body 30 toward the head 32. For example, the sleeve 40 buckles against the body 30, and then moves against the tapered nose 36 and along the body 30. In a preferred embodiment, the rotational friction force at the interface between the body 30 and sleeve 40 is greater than the rotational friction force between the threaded interface of the core bolt 20 and sleeve 40. Knurls on the body 30 may be used to enhance or increase frictional force.
The edge 46 of the sleeve 40 reaches the outer surface of the tail-side panel and as a result, the sleeve 40 flattens completely against the blind side surface and causes the strain on the core bolt 20 to increase such that the core bolt 20 fractures at the break groove 26. The residual clamp load is near maximum during formation of the blind side upset because there is no axial recoil upon torsional fracture at the break groove 26. The frangible portion of the core bolt 20 and the drive nut 50 are discarded upon completion of the installation.
The invention has been described above and, obviously, modifications and alternations will occur to others upon a reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.
This application claims priority from U.S. Provisional Patent Application No. 60/777,449, entitled “Blind Bolt Fastener” filed on Feb. 28, 2006, which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60777449 | Feb 2006 | US |
