Patent Description:
Fasteners are used in the aerospace industry to mechanically unite various structural components of an aircraft. For example, composite or metal panels that form a portion of a skin of an aircraft wing may be joined to one another via a fastener. In aircraft structures, it is often desirable to install fasteners in interference, meaning that the fastener diameter is larger than the diameter of the hole that receives it. Interference-fit installation of fasteners can facilitate aircraft assembly operations and improve joint performance. As fasteners are intended to enhance the structural strength of an aircraft, it remains desirable to ensure that the act of installing a fastener does not damage underlying structural components of the aircraft. In particular, if an interference-fit fastener is forced through a composite part with too much force, it may cause the composite part to delaminate or experience other issues. Thus, an interference fit fastener may utilize a lubricant that reduces the amount of force used to drive the fastener during installation. Excessive force may also result in damage to the fastener and, in joints comprised of metal and composite parts, it can additionally result in detrimental galling, scoring or excessive deformation of the metal parts. The magnitude of the fastener insertion force may be controlled by the application of lubricants to the fastener or to the hole, by limiting the amount of interference, or by using means other than driving or pulling the fastener into the hole to create the desired state of interference.

Although structural strength of a fastener is important to consider, it is also important for fasteners to adequately conduct and/or disperse electrical energy from the surrounding structure. Hence, it remains important for the fastener to efficiently disperse electrical energy to surrounding structural components in a manner that prevents energy from building at the fastener.

While both structural strength and electrical compatibility remain desirable, it is a complicated process to balance both of these requirements when designing a fastener. For example, a fastener may be coated in order to enhance lubricity and therefore reduce the amount of force used to install the fastener. However, coatings, and/or finishes may electrically insulate the fastener from its surroundings, thereby confounding the ability of the fastener to adequately dissipate electrical energy. As another example, using fasteners sheathed in a protective metal sleeve may provide an adequate level of energy dissipation and reduce the possibility of damaging composite parts during fastener installation. However, such fasteners can be generally costly.

For at least these reasons, designers continue to seek out fastener designs that strike a balance in fulfilling both structural and electrical design constraints.

The prior art includes <CIT>, which discloses a screw, whereby the underside of the screw head is provided with recesses- pockets- which are filled with lubricant. When the screw is tightened onto a joint, lubricant is released from the pockets in order to prevent wear and tear during multiple loosenings and tightenings. <CIT> is not an interference fit fastener.

<CIT> in accordance with its abstract states a metallic fastener for assembly by interference fit of at least two structural elements comprising a through drilling, the fastener comprising an enlarged head , a shaft having an external diameter before installation that is greater than an internal diameter of the bore, said shaft comprising a conductive surface. Before installation, at least one conductive surface is covered with a lubricating layer having sufficient adherence to prevent its abrasion by manual manipulation of the fastener and sufficiently weak to be at least partially torn from the conductive surface during mounting by interference fit of the fastener, a mixture of at least one polyolefin and one polytetrafluoroethylene for example.

<CIT> A1, in accordance with its abstract states a metal attachment for providing an electrically conductive surface through the entire thickness of structural elements to be assembled. The attachment thereby comprises a head and a smooth shank extending along an axis of revolution, characterized in that the shank comprising at least one conductive portion and one lubricating portion disposed along the axis of revolution of the attachment along at least one length of the shank.

The present disclosure concerns the application of irregular (e.g., speckled/spattered) coatings, for example metal pigmentation coatings onto fasteners in regions where the fasteners will be installed into a hole in a state of interference. For example, the fasteners may comprise pull-type lock bolts that include a shank dimensioned for an interference fit with structural components of an aircraft. The surface of the fastener may be bare, or may be coated partially or in its entirety with a finish (e.g., anodize, or a finish formed from bare metal via grit blasting etc.). Additionally, a portion of the fastener is coated in an irregular (i.e., unpatterned) high-lubricity coating, such as a metal pigmentation coating and/or a dry film lubricant coating, such that the coating is randomly interspersed with regions of bare (or finished) metal. In this manner, application of the coating enhances lubricity without substantially compromising electrical conductivity of the fastener. This is because remaining exposed regions not covered by the coating (e.g., regions of bare or anodized metal) provide desired levels of electrical energy dispersion at the fastener surface discontinuously along the entire circumference (i.e., all 360º) of the fastener.

In particular the present disclosure provides a method for providing a fastener for use in the aerospace or similar industry, the method comprising the steps of:.

The coating preferably comprises an Al or Cu metal pigment coating and/ or a molybdenum disulfide coating.

The surface of the shank may be anodized prior to spraying the coating over the shank.

A lubricant may be applied that covers the shank which lubricant is applied after the coating has been applied, which lubricant is preferably not fused, baked, or otherwise firmly adhered onto/or integral with the fastener and which lubricant can act as a sacrificial lubricant during installation of the fastener and which lubricant preferably comprises cetyl alcohol.

The step of coating spraying also preferably sprays coating onto a circumference of the fastener head.

Applying the coating may increase a width of the shank by less than <NUM> (one hundredth of an inch) and the radius of the coated shank is greater than the radius of the non-coated shank.

The coating sprayer is preferably calibrated to adjust the size of individual spots or speckles of coating sprayed onto the fastener wherein for example the sprayer is calibrated to spray spots of coating that are between approximately one tenth and one thousandth the circumference of the fastener.

The pressure, time of exposure and/or orifice size for the sprayer can be adjusted in order to adjust the percentage covering of surface area of the shank covered by coating as a matter of design choice to preferably between <NUM>% and <NUM>%.

The disclosure further specifically relates to a fastener, as preferably provided by this method, for use in the aerospace or similar industry, the fastener comprising:.

The surface of the shank preferably comprises a finish, for example an anodized surface.

The shank is preferably made of a material selected from the group consisting of titanium, ferrous alloy, and nickel alloy.

The coating may comprise an Al or Cu metal pigment coating and/or a molybdenum disulfide coating.

The fastener can further comprise a lubricant covering the shank and the coating, which lubricant is preferably not firmly adhered to the fastener so that the lubricant acts as a sacrificial lubricant during installation of the fastener and which lubricant preferably comprises cetyl alcohol.

The coating is also preferably in the non-uniform, discontinuous manner to the fastener head during spraying, to thereby leave the fastener head with irregular portions exposed and free of coating and also to thereby leave the fastener head with irregular portions covered in coating, preferably wherein areas of the fastener head that are not in contact with the hole after installation are not coated.

The coating can increase a width of the shank by less than <NUM> (one hundredth of an inch).

The radius of the coated shank can be greater than the radius of the non-coated shank.

The coating preferably covers between <NUM>% and <NUM>% of a surface area of the shank.

The fastener is also preferably unmasked.

The disclosure also comprises the following:
A fastener for use in the aerospace or similar industry, the fastener comprising:.

The fastener can further comprise a lead-in transition that extends from the shank, and
the lead-in transition can exhibit a gradually varying diameter that changes from being equal to a diameter of the shank to being less than the diameter of the shank.

The fastener may comprise
a pull-type lock bolt or a stump-type lock bolt.

The surface of the shank may comprise a finish, for example an anodized surface or bare metal.

The shank can be made of a material selected from the group consisting of titanium, ferrous alloy, and nickel alloy.

The fastener may further comprise a lubricant covering the shank and the coating, which lubricant preferably comprises cetyl alcohol, and which lubricant is not firmly adhered to the fastener so that the lubricant acts as a sacrificial lubricant during installation of the fastener.

The coating can be discontinuously speckled over an entire circumference of the shank.

The coating may cover between thirty and seventy percent of a surface area of the shank.

The coating can discontinuously cover a circumference of the head.

The head can comprise a hexagonal female slot.

Coverage of the surface by the coating can increase a width of the shank by less than <NUM> (one hundredth of an inch).

A percentage of surface area of the shank covered by the coating can remain constant as the circumference of the shank is traversed.

The coating can be a high-lubricity coating.

The coating may comprise a metal pigment coating and/or a dry film lubricant.

The fastener can further comprise a removable pintail, which pintail may be replaceable by a swagable collar, and/or where the fastener may be unmasked.

The disclosure also includes an aircraft comprising this fastener.

A method for providing a fastener speckled with a coating, the speckled fastener for use in the aerospace or similar industry, comprising:
providing a fastener comprising a head and a cylindrical shank that extends from the head and applying a speckled coating to the fastener in such a way as to leave irregular portions of a surface of the shank exposed and uncovered by the coating, which coating exhibits a higher lubricity than the surface of the shank and wherein the coating exhibits a higher dielectric withstand voltage than the surface of the shank.

Applying the coating can comprise forming an unpatterned mosaic.

Applying the coating can comprise spattering the coating over the surface.

The coating can be speckled discontinuously around an entire circumference of the shank.

The coating can comprise an Al or Cu metal pigment coating and/ or a molybdenum disulfide coating.

Applying the coating can comprise speckling the coating over a portion of the shank dimensioned for an interference fit with a hole.

The surface of the shank can be anodized prior to speckling the coating over the shank.

The coating can comprise discontinuously covering bare metal of the shank. heating the coating can be heated prior to application.

The method can further comprise applying a lubricant that covers the shank, which lubricant preferably comprises cetyl alcohol, and which lubricant is preferably applied after the coating has been applied, which lubricant is not fused, baked, or otherwise firmly adhered onto/integral with the fastener, and which acts as a sacrificial lubricant during installation of the fastener.

The irregular portions of the surface of the shank exposed and uncovered by the coating can be randomly shaped.

Applying the coating may comprise covering between thirty and seventy percent of the surface area of the shank.

The method can further comprise applying the coating over a circumference of the head.

Applying the coating can increase a width of the shank by less than <NUM> (one hundredth of an inch).

The coating can be firmly adhered to the shank for example by way of fusing or baking.

Applying the coating may comprise maintaining a constant percentage of surface area of the shank covered by the coating as the circumference of the shank is traversed.

Applying the coating may be carried out to fully cover an entire circumference of a lead-in transition of the fastener.

The disclosure also includes a method, preferably for fastening aircraft components together, comprising:.

The fit can be an interference fit formed by selecting the fastener as having a diameter larger than the hole and placing the fastener into the hole.

The pintail can be broken off of the fastener after forming the fit and providing a securement on the fastener, preferably by way of swaging a collar onto the fastener at a location where the pintail was broken off or by threading a nut onto threads of the fastener.

The hole can be a hole in a skin of an aircraft.

The hole can be at a fuel tank of an aircraft.

The shank can be held in the hole to prevent rotation of the shank during installation of a nut.

Driving the shank into the hole can comprise shearing the coating.

Dissipating the electrical energy between the hole and the fastener can comprise transferring current through the shank at locations on the shank that are anodized.

The disclosure also includes an aircraft fastener obtainable according to this method.

The disclosure also includes a method for providing a fastener for use in the aerospace or similar industry, the method comprising the steps of:
providing a fastener comprising:.

Other aspects (e.g., methods and computer-readable media relating to the disclosure) may be described below. The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings.

The present disclosure is now described, by way of example only, and with reference to the accompanying drawings.

The figures and the following description illustrate the present disclosure.

<FIG> illustrates the structure of an exemplary aircraft that may utilize fasteners according to the present disclosure. Specifically, <FIG> is a diagram of an aircraft <NUM>. Aircraft <NUM> includes nose <NUM>, wings <NUM>, fuselage <NUM>, and tail <NUM>.

<FIG> is a cut-through view of a section of wing skin <NUM> of aircraft <NUM> indicated by view arrows <NUM> in <FIG>. As shown in <FIG>, wing skin <NUM> comprises multiple composite or metal parts (<NUM>, <NUM>), which include holes <NUM>. Fasteners <NUM> are driven through holes <NUM> (e.g., to form an interference fit), and are secured with securements <NUM> (e.g., swaged-on collars or threaded nuts). Securements <NUM> may be swaged onto fasteners <NUM> at locations where pintails have been broken off of fasteners <NUM>. Fasteners <NUM> may comprise pull-type lock bolts, stump-type lock bolts, HI-LOK brand pins, or any other type of permanent pin-type fastener.

<FIG> is a zoomed-in view of a fastener <NUM> that attaches parts (<NUM>, <NUM>) forming a section of skin <NUM>. <FIG> corresponds with region <NUM> of <FIG>. <FIG> illustrates that each part may comprise a composite part including one or more layers/plies <NUM> (e.g., carbon fiber within a cured matrix of resin). In further examples, parts <NUM> and <NUM> are metal. <FIG> further illustrates that within region <NUM>, fastener <NUM> is placed into an interference fit with part <NUM>. Although an interference fit is achieved, small air gaps (not shown) may still remain at certain locations between fastener <NUM> and part <NUM>. These air gaps are caused by surface irregularities that may occur when drilling holes in composite materials. These small air gaps in turn insulate small portions of fastener <NUM> from composite part <NUM>. Fastener <NUM> may further include a coating that provides corrosion protection and enhanced lubricity. The coating may facilitate the installation of fastener <NUM>, but may also electrically insulate fastener <NUM> from composite parts <NUM> and <NUM>. Ideally, the level of insulation is low enough so as to ensure a low dielectric withstand voltage, thereby ensuring that no substantial electrical arcing occurs between wall <NUM> of fastener <NUM>, and wall <NUM> of composite part <NUM> in the event of an electrical discharge through region <NUM>. While <FIG> illustrates fastener <NUM> being used for a composite-composite joint, fastener <NUM> may also be used for a metal-composite joint, or any suitable location.

<FIG> is an additional view of fastener <NUM> and corresponds with region <NUM> of <FIG>. <FIG> illustrates a lock bolt configuration that is designed for installation with a swaged metallic collar (not shown). Fastener <NUM> further includes head <NUM>, shank <NUM>, lead-in transition <NUM>, break groove <NUM>, feature <NUM>, and pintail <NUM> (including feature <NUM>. Feature <NUM> and/or feature <NUM> can be annular shapes (e.g., rings), such that fastener <NUM> is forced into place by directly pushing fastener <NUM> into hole <NUM>. Features <NUM> and/or <NUM> can be threaded. <FIG> illustrates a fastener <NUM> implemented as a threaded fastener, including slots <NUM> and <NUM> (e.g., hexagonal female slots, prismatic "star drive"/TORX slots, or other suitable slots that facilitate installation of fastener <NUM> into hole <NUM>). Feature <NUM> can comprise threads designed to accommodate a threaded collar or nut (not shown). <FIG> further illustrate coating coverage area <NUM>, where a high-lubricity coating (e.g., a metal pigment coating or dry film lubricant) is applied in a non-uniform discontinuous manner to fastener <NUM>. Areas of fastener <NUM> other than the head <NUM> and shank <NUM> may be fully coated with high-lubricity coatings in order to facilitate fastener installation.

<FIG> is a close-up view of coating coverage area <NUM> of fastener <NUM> of <FIG> and <FIG>. Specifically, <FIG> illustrates that coating <NUM> (e.g., an Al or Cu metal pigment coating, a coating of dry-film lubricant such as molybdenum disulfide, etc.) may be discontinuously applied within area <NUM>. The coating coverage area <NUM> may encompass the bearing portion of the fastener, including fastener head 430and shank <NUM>, stopping at fastener lead-in transition <NUM>. The surface of coating coverage area <NUM> may comprise bare metal (e.g., titanium or a ferrous or nickel-base alloy having enhanced properties of electrical conduction) or a finish (e.g., anodize). <FIG> illustrates regions <NUM> of the coating coverage area <NUM> where some of the original surface/finish <NUM> of fastener <NUM> remains visible after application of the high-lubricity coating. Areas of fastener head <NUM> that are not in contact with hole <NUM> after installation need not receive coating <NUM>.

Coating <NUM> beneficially increases the lubricity (and/or corrosion resistance) of fastener <NUM> when compared with commonly used fastener finishes or bare metal. As coating <NUM> is applied on the surface <NUM> of shank <NUM>, a majority of sliding friction force endured by fastener <NUM> during installation is borne by coating <NUM>. Fortunately, the portions of shank <NUM> covered by coating <NUM> are among the regions of the fastener <NUM> needing the highest amount of lubricity during fastener insertion into hole <NUM> for an installed state of interference. Hence, shank <NUM> may slide into hole <NUM> along its most lubricious portions, reducing the amount of force involved in installing fastener <NUM>.

While coating <NUM> provides the above-noted benefits relating to fastener installation, coating <NUM> may not provide sufficient conductivity to ensure that fastener <NUM> properly dissipates electrical energy. For example, a coating <NUM> formed from metal pigment coating may have a dielectric withstand voltage in the range of zero to two thousand volts, as compared with anodized metal having a dielectric withstand voltage in the range of zero to sixty volts, or some other intrinsically low voltage. This means that a uniform metal pigment coating on shank <NUM> would cause shank <NUM> to experience high voltage discharges of electrical energy (e.g., discharges of about two thousand volts), which is undesirable. Hence, although coating <NUM> is desirable to ensure compatibility with structural design constraints, coating <NUM> exacerbates issues pertaining to electrical constraints because it exhibits less than a desired level of electrical energy conduction.

To address this issue, instead of uniformly coating the entirety of fastener <NUM> (or shank <NUM> and head <NUM>) with coating <NUM>, or applying solid regions (e.g., stripes) of coating <NUM> to fastener <NUM>, shank <NUM> and/or head <NUM> are discontinuously coated in an irregular, non-uniform and/or unpatterned mosaic <NUM> application of coating <NUM>. This technique is beneficial in that it eliminates any need to mask fastener <NUM> (e.g., in order to apply solid stripes or strips of coating <NUM>), which removes a step involved in the fabrication of fasteners <NUM> and therefore reduces cost. Furthermore, this technique balances concerns related to electrical and structural design constraints.

<CIT> discloses a metal screw for interference fits in composite materials for aircraft, which fastener is provided with regularly alternating, separate electrically conductive strips and lubricating strips.

<CIT> discloses a fixing member for securing aircraft elements together, which fixing member is provided with separate strips of electrically conductive layers and lubricating layers made of different materials.

An additional benefit exists in the use of a non-uniform coating. Specifically, coating <NUM> may be applied via spraying to form a speckled/spattered film on head <NUM> and/or shank <NUM>. As used herein, a speckled/spattered arrangement comprises a random distribution of coating across the entire perimeter of head <NUM> and/or shank <NUM>. By adjusting a pressure, time of exposure, and/or orifice size for the sprayer, a percentage of surface area in area <NUM> covered by coating <NUM> may be carefully adjusted. This enables the electrical and physical properties of fastener <NUM> to be adjusted as desired. For example, the percentage of surface area of shank <NUM> (and/or head <NUM>) covered by coating <NUM> may be varied as a matter of design choice to between thirty and seventy percent. The percentage may even vary along the axial length of fastener <NUM>. The percentage area of coating <NUM> may be increased for fasteners that would otherwise require high amounts of force to install (e.g., fasteners having long grip lengths/axial lengths for shanks <NUM>, or fasteners having shanks with larger diameters). In contrast, the percentage area of coating <NUM> may be decreased for fasteners that are exposed to higher electrical currents and where fastener <NUM> insertion force requirements may be less demanding.

The percentage area of coating <NUM> on shank <NUM> may also vary depending on whether or not surface <NUM> features a finish (e.g., anodize) or is bare metal. If surface <NUM> is bare metal, more coating <NUM> may be allowed while still conforming with electrical constraints, but more coating <NUM> may also be needed to ensure that the amount of force used to overcome friction with laminate plies does not exceed a predefined amount. As noted above, if fastener <NUM> is driven with too much force, it may damage underlying structural components (e.g., by delaminating those components in the region around hole <NUM> and causing ply separation). In a similar fashion, if surface <NUM> is anodized, less coating <NUM> may be allowed to conform with structural constraints, but less coating <NUM> may also be needed to ensure sufficient dispersion of electrical energy.

In addition to coating <NUM>, an optional lubricant <NUM> is shown. Lubricant <NUM> is applied to fastener <NUM> (e.g., via a dip or spray). Lubricant <NUM> is applied after coating <NUM> is speckled on. Furthermore, lubricant <NUM> may be applied to shank <NUM> or the entirety of fastener <NUM>. However, lubricant <NUM> differs substantially from coating <NUM> in that lubricant <NUM> is not fused, baked, or otherwise firmly adhered onto/integral with fastener <NUM>. Instead, lubricant <NUM> comprises a traditional lubricant (e.g., an oil, cetyl alcohol, wax, sealant etc.) which acts as a sacrificial lubricant during installation. That is, the majority of lubricant <NUM> is rubbed away/off or otherwise dissipated during installation, and hence the insulating properties of lubricant <NUM> do not substantially impact the electrical properties of fastener <NUM>. Since the installation of fastener <NUM> involves an interference fit, lubricant <NUM> alone (e.g., without coating <NUM>) is insufficient to ensure that the driving force applied to fastener <NUM> will be below a desired threshold level of friction. Specifically, the interference contact involved in an interference fit ensures that lubricant <NUM> will be scraped off during installation, meaning that coating <NUM> defines in large degree the amount of installation force used for fastener <NUM>. Hence, lubricant <NUM> may be considered an optional complement to coating <NUM>, but does not replace the role of coating <NUM>. <FIG> further illustrates an additional and/or alternative coating/finish that may be applied to lead-in portion/transition <NUM>. Coating <NUM> may further continue into region <NUM>, including lead-in transition <NUM>. A different high-lubricity coating may be utilized in region <NUM> than coating <NUM>. The coverage of region <NUM> by coating <NUM> may be complete (e.g., as shown) in order to fully cover the circumference of lead-in transition <NUM>. In another embodiment, the coverage may be speckled in a similar manner as described above for shank <NUM>.

<FIG> illustrates a further view of fastener <NUM> after fastener <NUM> has been installed at skin <NUM> of aircraft <NUM>. As shown in <FIG>, the force of installing fastener <NUM> (by forcing fastener <NUM> in direction D into a hole of a smaller diameter has caused portions of coating <NUM> to shear, resulting in coating <NUM> having a smeared appearance <NUM>. This alters the arrangement of coating <NUM> on fastener <NUM>, but does not substantially change the amount of surface area occupied by coating <NUM>. For the purposes of discussion, notes regarding a desired percentage of surface area occupied by coating <NUM> are directed towards the amount of surface area occupied by coating <NUM> prior to installation.

Illustrative details of the fabrication and operation of fastener <NUM> will be discussed with regard to <FIG>.

<FIG> is a flowchart illustrating a method <NUM> for fabricating a fastener <NUM> in an exemplary embodiment. The steps of method <NUM> are described with reference to fastener <NUM> of <FIG>, but those skilled in the art will appreciate that method <NUM> may be performed for other fasteners (e.g., bolts, screws, rivets, etc.) in order to achieve a desired result. The steps of the flowcharts described herein are not all inclusive and may include other steps not shown. The steps described herein may also be performed in an alternative order.

According to method <NUM>, fastener <NUM> is acquired, including head <NUM> and cylindrical shank <NUM> (step <NUM>). Shank <NUM> includes surface <NUM>, which may be bare metal or anodized metal. In one embodiment, fastener <NUM> may be forged from a titanium, ferrous alloy, or nickel alloy form, or may be machined from a precursor part. Alternatively, fastener <NUM> may be acquired from a supplier that manufactures fasteners in volume. The fastener <NUM> may furthermore comprise a finish over part of, or the entirety of its surface.

A high lubricity coating <NUM> is speckled over shank <NUM> (step <NUM>). Coating <NUM> may further be applied to head <NUM> in a similar manner. Coating <NUM> is discontinuously applied (e.g. speckled) over irregular (e.g., randomly shaped) portions of head <NUM> and/or shank <NUM>, and leaving irregular portions of shank <NUM> exposed. This forms an unpatterned mosaic of spattered/speckled coverage arrangement across the surface of shank <NUM> (e.g., across the entire circumference of shank <NUM> and/or the entire circumference of head <NUM>, and extending axially), ensuring that a desired percentage of the surface area of shank <NUM> is covered by coating <NUM>. As discussed above, coating <NUM> enhances the lubricity of shank <NUM>, reducing the amount of effort used in installing shank <NUM> in an interference fit into hole <NUM>. This in turn helps in overcoming interference fit friction. However, if coating <NUM> were applied across the entirety of shank <NUM> in a manner that completely covered the entire surface of shank <NUM>, coating <NUM> would be electrically insulated from parts <NUM> and <NUM>, resulting in diminished capability of fastener <NUM> to disperse electrical energy.

Applying coating <NUM> in a non-uniform, irregular manner (e.g., an unpatterned manner) provides a further and seemingly paradoxical benefit in that coating <NUM> remains, on average, evenly distributed along the entire circumference of shank <NUM>, even though the specific regions occupied by coating <NUM> are random. In this manner, no portion of the circumference of shank <NUM> (e.g., 30º, 60º, 90º, 120º, etc.) has a substantially different ratio of coating <NUM> to exposed surface <NUM> (e.g., the ratio of these components remains substantially the same/constant as the circumference is traversed, such as within ten percent of a desired value). Thus, unlike a "striped" pattern of application for coating <NUM> along the circumference of fastener <NUM>, which would necessarily cause uncoated regions to experience greater local amounts of friction (and therefore higher risk of damage) during insertion, fastener <NUM> includes a more uniform lubricity (on an area-by-area basis). Fastener <NUM> therefore achieves better lubricity than striped fasteners, and the cost of manufacture is less than that for striped fasteners, which require masking. Furthermore, unlike a striped system, electrical current does not have to flow past large solid coated areas/stripes at fastener <NUM> in order to reach a region where dissipation of electrical energy may occur. Hence, fastener <NUM> exhibits more uniform/better electrical energy dissipation capability (e.g., dielectric withstand voltages) along its circumference (on average) than striped fasteners. Coating <NUM> marginally increases the width/diameter of fastener <NUM>. Accordingly the radius of the coated shank is therefore also greater than the radius of the non-coated shank.

For example, coating <NUM> may increase the width of fastener <NUM> by several thousandths of an inch (i.e., less than one hundredth of an inch). This means that the diameter of the coated fastener is not consistent as the coating is not consistent. However, the added thickness has no substantial impact on installation. The coating <NUM> increases the diameter of the shank, so that the coating <NUM> extends beyond the surface of the shank at the exposed, irregular portions of the shank free of coating. As such, the coating <NUM> creates peaks which project from and extend away from the surface of the shank. The shank is not polished after application of the coating. Coating <NUM> may consist of a high-lubricity metal-pigmented organic material or a dry-film lubricant. Coating <NUM> may be thermally processed (e.g., cured) after application to ensure that coating <NUM> is heat-fused to shank <NUM> and is not rubbed off or otherwise perturbed (except for example when shank <NUM> is fit into hole <NUM>). In one embodiment, speckling comprises physically spattering coating <NUM> over shank <NUM>. This means that no masking is needed.

With coating <NUM> successfully applied, lubricant <NUM> (e.g., cetyl alcohol) is applied to fastener <NUM> (step <NUM>). For example, fastener <NUM> may be dipped and/or uniformly sprayed with lubricant <NUM> (i.e., saturated, resulting in no irregularities in coverage) in order to cover fastener <NUM>. In embodiments where lubricant <NUM> is cetyl alcohol, lubricant <NUM> may be sacrificial in nature, rubbing off during the installation of fastener <NUM>, and thereby ensuring minimal interference with (e.g., insulation of) the conductivity of shank <NUM>.

Method <NUM> may be performed via batch processes of forming, machining, spraying, and/or dipping in order to ensure sufficient throughput of fasteners <NUM> for aircraft manufacturing. Since no masking is required (e.g., since fastener <NUM> is not masked and then sprayed with solid stripes of continuous coating <NUM>), the amount of labor and time involved in the manufacture of fasteners <NUM> is reduced.

With fasteners <NUM> successfully fabricated, an aircraft manufacturer may desire to install fasteners <NUM> on an aircraft (e.g., aircraft <NUM>). To this end, <FIG> is a flowchart illustrating a method <NUM> for installing a fastener <NUM> using a semi-automated or manual assembly process in an exemplary embodiment. According to method <NUM>, a hole of a predetermined diameter is drilled into an aircraft (e.g., by an automated machine in accordance with a Numerical Control (NC) program, by a technician, etc.) (step <NUM>). A fastener <NUM> is acquired by the machine having an appropriate diameter and grip length (step <NUM>). The diameter may be chosen such that it is larger (e.g., by several thousandths of an inch) than the hole. This may be performed, for example, by loading a fastener (or batch of fasteners) into a fastener insertion machine (not shown). The fastener insertion machine may consist of a machine end effector that may operate according to a Numerical Control (NC) program implemented by a controller/processor in order to select a hole <NUM> into which to install fastener <NUM>. In one embodiment, hole <NUM> is located at a fuel tank of an aircraft. The diameter of the hole <NUM> is sized such that when fastener <NUM> is installed, a prescribed level of interference is attained within tolerance. With a hole <NUM> chosen, the fastener insertion machine forms an interference fit between shank <NUM> and hole <NUM>, by driving/inserting shank <NUM> into hole <NUM> (step <NUM>). This operation may be performed by torqueing shank <NUM> into hole <NUM> via feature <NUM> (e.g., threading), or by directly forcing shank <NUM> into place. Shank <NUM> may be held in hole <NUM> after insertion to prevent rotation. This may be performed for example during installation of a securement <NUM> (e.g., a nut) onto fastener <NUM>. After a number of fasteners <NUM> have been installed in this manner, an operator then follows by installing collars or nuts using hand-held or automated tools as desired. In all cases, since shank <NUM> is partially covered with coating <NUM>, the lubricity of shank <NUM> is enhanced, reducing the amount of installation force used. With shank <NUM> in place in an interference fit, fastener <NUM> performs its role of fastening desired structural components (e.g., composite or metal parts that each form a portion of skin <NUM>). Steps <NUM>-<NUM> may be repeated multiple times (e.g., hundreds or thousands of times) during the manufacturing of aircraft <NUM>, in order to fasten the various components of aircraft <NUM>. Upon aircraft <NUM> being fully manufactured, it may then be operated in flight.

Methods <NUM>-<NUM> utilize fasteners that exhibit an advantage over prior coated fastener systems in that fasteners <NUM> ensures that structural requirements of lubricity are met, while also ensuring that fasteners <NUM> safely dissipate electrical energy. Compared to fasteners sheathed in metallic sleeves, another system sometimes used in similar applications, methods <NUM>-<NUM> provide a one piece fastener at much lower cost.

<CIT> discloses a modified shank fastener, the object of which is to do away with the need for extra sleeves provided between the fastener and the hole in a composite joint, for dissipating electrical/lightning strike energy. The shank disclosed in <CIT> is provided with a roughened surface, formed by knurling/machining, which roughened surface is then provided with a dry lubricant coating. The knurled shank is then polished to remove coating from the knurl peaks, but to leave a regular coating in the knurl valleys. The coating and the knurl peaks are evened out to be level with each other on the polished surface of the shank fastener, whereby the peaks are removed.

The disclosure also relates to a method for fastening aircraft components together, comprising forming a fit between a fastener and a hole by driving a shank of the fastener into the hole, which fastener comprises a head, a cylindrical shank that extends from the head and is dimensioned to engage in a fit with a corresponding hole, which cylindrical shank has a surface, a diameter, a circumference, a coating wherein the coating is applied in a non-uniform, discontinuous manner to the fastener to thereby leave the surface of the shank with irregular portions exposed and free of coating, and also to thereby leave the surface of the shank with irregular portions covered in coating. The coating increases the diameter of the shank and forms peaks on the surface of the shank. The coating exhibits a higher lubricity than the surface of the shank so that the irregular, coating covered portions of the surface of the shank exhibit a higher lubricity than the irregular portions of the surface of the shank exposed and free of coating. The coating exhibits a higher dielectric withstand voltage than the surface of the shank so that charge dissipates via the irregular portions of the surface of the shank exposed and free of coating before the dielectric withstand voltage of the coating covering the irregular, coating covered portions of the surface of the shank is met.

The pressure, time of exposure and/or orifice size for the sprayer is adjusted in order to adjust the percentage covering of surface area of the shank covered by coating as a matter of design choice.

Electrical energy is built up between the shank and the hole.

The electrical energy is traversed around the coating and
the electrical energy is dissipated at the irregular portions of the shank that are not covered by the coating, along the entire circumference of the shank.

The fastener can further comprise a removable pintail which is preferably broken off the fastener after forming the fit to provide a securement on the fastener and preferably further comprises swaging a collar onto the fastener at a location where the pintail was broken off.

A nut can be thread onto threads of the fastener.

The hole can be a hole in a skin of an aircraft, for example the hole at a fuel tank of an aircraft.

The coating can be sheared on driving the shank into the hole.

The surface of the shank can be anodized prior to spraying the coating over the shank.

Dissipating the electrical energy between the hole and the fastener may comprise transferring current through the shank at the anodized locations on the shank.

The shank can be made of a material selected from the group consisting of Titanium, ferrous alloy, and nickel alloy.

The coating can comprise an Al or Cu metal pigment coating and/or a molybdenum disulfide coating.

The fastener can further comprise a lubricant covering the shank and the coating, which lubricant is not firmly adhered to the fastener so that the lubricant acts as a sacrificial lubricant during installation of the fastener, which lubricant preferably comprises cetyl alcohol.

The disclosure also relates to an aircraft comprising a fastener which fastener comprises a head and a cylindrical shank that extends from the head and is dimensioned to engage in a fit with a corresponding hole.

The cylindrical shank has a surface, a diameter, a circumference, and a coating applied in a non-uniform, discontinuous manner to the fastener to thereby leave the surface of the shank with irregular portions exposed and free of coating, and also to leave the surface of the shank with irregular portions covered in coating whereby the coating increases the diameter of the shank and forms peaks on the surface of the shank.

Thee coating exhibits a higher lubricity than the surface of the shank so that the irregular, coating covered portions of the surface of the shank exhibit a higher lubricity than the irregular portions of the surface of the shank exposed and free of coating.

The coating also exhibits a higher dielectric withstand voltage than the surface of the shank so that charge dissipates via the irregular portions of the surface of the shank exposed and free of coating before the dielectric withstand voltage of the coating covering the irregular, coating covered portions of the surface of the shank is met.

The disclosure also related to a first aircraft component fastened to a second aircraft component by a fastener.

The fastener comprises a head, a cylindrical shank that extends from the head and which is dimensioned to engage in a fit with a corresponding hole.

The cylindrical shank has a surface, a diameter, a circumference, and a coating.

The coating is applied in a non-uniform, discontinuous manner to the fastener to leave the surface of the shank with irregular portions exposed and free of coating, and also to leave the surface of the shank with irregular portions covered in coating.

The coating increases the diameter of the shank and forms peaks on the surface of the shank.

The coating exhibits a higher lubricity than the surface of the shank so that the irregular, coating covered portions of the surface of the shank exhibit a higher lubricity than the irregular portions of the surface of the shank exposed and free of coating.

The coating exhibits a higher dielectric withstand voltage than the surface of the shank so that charge dissipates via the irregular portions of the surface of the shank exposed and free of coating before the dielectric withstand voltage of the coating covering the irregular, coating covered portions of the surface of the shank (<NUM>) is met.

With fasteners <NUM> installed, operation of the aircraft <NUM> (e.g., during flight) may cause electrical energy to build at fastener <NUM>. This energy may be the result of a lightning strike. This built-up charge continues to accumulate until it overcomes the dielectric withstand voltage of surface <NUM>, at which point electrical energy between hole <NUM> and fastener <NUM> is dissipated at locations on surface <NUM> When energy is dissipated at fastener <NUM>, dissipation occurs between exposed portions of surface <NUM> and hole <NUM>, and not between coating <NUM> and hole <NUM>. Coating <NUM> may possess a higher dielectric withstand voltage than surface <NUM>. Charge may therefore dissipate via surface <NUM> before the dielectric withstand voltage of coating <NUM> is met. This operation is beneficial, as dissipating charge through the relatively large area of surface <NUM> prevents the generation electrical arcs, for example limiting dissipation voltages to between about zero and sixty volts (preferably between zero and twenty volts).

<FIG> is a flowchart illustrating a method <NUM> for dissipating electrical energy between a fastener <NUM> and a hole <NUM>. Electrical energy builds up between fastener <NUM> and hole <NUM> (step <NUM>). This may occur for example during flight. Next, the built-up electrical energy traverses around coating <NUM>, which covers surface <NUM> of fastener <NUM> (step <NUM>). Since coating <NUM> exhibits a higher dielectric withstand voltage than surface <NUM>, electrical energy passes around coating <NUM> instead of passing into fastener <NUM> via coating <NUM>. Next, electrical energy is dissipated between surface <NUM> of shank <NUM> and hole <NUM> (step <NUM>). This action is performed discontinuously along the entire circumference (i.e., the entire 360º) of shank <NUM> at irregular portions, any may occur at each location that is not coated by coating <NUM>.

In a further embodiment, a sprayer may be calibrated to adjust the size of individual spots/speckles applied to fastener <NUM>. For example, in order to ensure that randomly applied speckles do not substantially vary in terms of the percentage of surface area occupied in a given portion of fastener <NUM>, it may be desirable to select spot sizes that are between approximately one tenth and one thousandth the circumference of fastener <NUM>.

In the following examples, additional processes, systems, and methods are described in the context of a semi-automated titanium pull-type lock bolt fastener installation on the structure of an aircraft.

<FIG> is a block diagram of a fastener <NUM> and accompanying machinery in an exemplary embodiment. Specifically, <FIG> illustrates a titanium pull-type lock bolt fastener <NUM> which includes head <NUM>. Fastener <NUM> further includes shank <NUM>, which itself includes surface <NUM> (e.g., an anodized or bare metal surface) and high-lubricity coating <NUM>. Coating <NUM> is applied discontinuously, irregularly, and or in a non-uniform fashion (e.g., randomly) upon head <NUM> and shank <NUM>, leaving irregular portions of surface <NUM> exposed while other portions of surface <NUM> are covered. Coating <NUM> application is accomplished during fastener <NUM> fabrication using automated spray equipment. Fastener <NUM> further includes lead-in transition <NUM>, which gradually reduces in diameter as it extends from shank <NUM> into features <NUM> (e.g., threading). Break groove <NUM> is also illustrated. Feature <NUM> and pintail <NUM> are separated by neck/break groove <NUM>. Pintail <NUM> includes feature <NUM> (e.g., annular rings) that allow a swaging tool to engage the fastener <NUM> and keep the joint clamped during collar <NUM> installation and swaging. Pintail <NUM> will be broken off after installation of fastener <NUM>, and pintail <NUM> includes feature <NUM> as well as slot <NUM>. Fastener <NUM> is covered in lubricant <NUM>. During installation, collar <NUM> will be swaged onto feature <NUM>, locking fastener <NUM> in place.

Further components illustrated in <FIG> include parts <NUM> comprising laminates <NUM> which each comprise individual plies <NUM>. In some areas, one or more of the structural parts <NUM> being joined may be metallic. Assembly equipment may include numerically-controlled drilling and fastener insertion machines <NUM> and hand-held fastener insertion tools and swaging tools <NUM>. Die <NUM> (from which fastener <NUM> is formed), sprayer <NUM> (which applies coating <NUM> to shank <NUM> by spraying metal pigment coating (e.g., an aluminum-pigmented coating) from reservoir <NUM> via orifice <NUM>), and fastener insertion machine <NUM> (which installs fastener <NUM>) are also illustrated. The operations of fastener insertion machine <NUM> and/or sprayer <NUM> may be directed by controller <NUM> in accordance with a stored NC program <NUM>.

Referring more particularly to the drawings, the present disclosure may be described in the context of an aircraft manufacturing and service method <NUM> as shown in <FIG> and an aircraft <NUM> as shown in <FIG>. During pre-production, exemplary method <NUM> may include specification and design <NUM> of the aircraft <NUM> and material procurement <NUM>. During production, component and subassembly manufacturing <NUM> and system integration <NUM> of the aircraft <NUM> takes place. Thereafter, the aircraft <NUM> may go through certification and delivery <NUM> in order to be placed in service <NUM>. While in service by a customer, the aircraft <NUM> is scheduled for routine maintenance and service <NUM> (which may also include modification, reconfiguration, refurbishment, and so on). Apparatus and methods disclosed herein may be employed during any one or more suitable stages of the production and service method <NUM> (e.g., specification and design <NUM>, material procurement <NUM>, component and subassembly manufacturing <NUM>, system integration <NUM>, certification and delivery <NUM>, service <NUM>, maintenance and service <NUM>) and/or any suitable component of aircraft <NUM> (e.g., airframe <NUM>, systems <NUM>, interior <NUM>, propulsion <NUM>, electrical <NUM>, hydraulic <NUM>, environmental <NUM>).

As shown in <FIG>, the aircraft <NUM> produced by exemplary method <NUM> may include an airframe <NUM> with a plurality of systems <NUM> and an interior <NUM>. Examples of high-level systems <NUM> include one or more of a propulsion system <NUM>, an electrical system <NUM>, a hydraulic system <NUM>, and an environmental system <NUM>. Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the automotive industry.

As already mentioned above, the present disclosure may be employed during any one or more of the stages of the production and service method <NUM>. For example, components or subassemblies corresponding to production stage <NUM> may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft <NUM> is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages <NUM> and <NUM>, for example, by substantially expediting assembly of or reducing the cost of an aircraft <NUM>. Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft <NUM> is in service, for example and without limitation, to maintenance and service <NUM>. For example, the techniques and systems described herein may be used for steps <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>, and/or may be used for airframe <NUM> and/or interior <NUM>. These techniques and systems may even be utilized for systems <NUM>, including for example propulsion <NUM>, electrical <NUM>, hydraulic <NUM>, and/or environmental <NUM>.

Fastener <NUM> may secure portions of airframe <NUM>, and is manufactured during component and subassembly manufacturing <NUM>. Fastener <NUM> may then be secured in order to fasten components of airframe <NUM> during system integration <NUM>, and then be utilized in service <NUM> until wear renders fastener <NUM> unusable. Then, in maintenance and service <NUM>, fastener <NUM> may be discarded and replaced with a newly manufactured part.

Claim 1:
A method for fastening aircraft components together, comprising:
forming a fit between a fastener (<NUM>) and a hole, the fastener (<NUM>) comprising:
a head (<NUM>);
a cylindrical shank (<NUM>) that extends from the head (<NUM>) and is dimensioned to engage in a fit with a corresponding hole; and
a discontinuously speckled coating (<NUM>), wherein,
the coating (<NUM>) exhibits a higher lubricity than a surface of the shank (<NUM>), and wherein
the coating (<NUM>)exhibits a higher dielectric withstand voltage than the surface of the shank (<NUM>),
driving the shank (<NUM>) of the fastener (<NUM>) into the hole,
building up electrical energy between the shank (<NUM>) and the hole,
traversing the electrical energy around the coating (<NUM>); and
dissipating the electrical energy at irregular portions of the shank (<NUM>) that are not covered by the coating (<NUM>), along the entire circumference of the shank (<NUM>),
wherein the discontinuously speckled coating (<NUM>) is obtained by applying a speckled coating to the fastener (<NUM>) in such a way as to leave irregular portions of a surface of the shank (<NUM>) exposed and uncovered by the coating.