Metal attachment

0001The present invention relates to a metal fastener with partial lubricating coating. The technical field of the invention is generally that of fasteners. More particularly, the invention relates to screws, fasteners with crimping grooves and any other metal fastener for assembling structural elements, for example aircraft type apparatus previously bored. Such fasteners are generally made from a metallic material such as stainless steel or titanium alloy.

0002The new generation of aircraft uses a composite material structure that has the advantage of significantly lightening the mass of the aircraft and not being sensitive to the phenomenon of fatigue. However, unlike a metal structure, composite materials have the disadvantage of misbehaving electricity and pose problems of resistance to lightning impacts. In order to resist lightning, it is necessary to fill any gap that exists between the fixing device and the bore to prevent breakdown of the air.

0003In the state of the art, the teaching of the document is known <patcit id="pcit0001" dnum="WO2011050040A"><text>WO 2011/050040</text></patcit> which discloses jacketed fasteners installed in interference in composite materials. The interference is defined by the installation of a fastening device having an outer diameter greater than the bore diameter which will receive it, which causes an expansion of the bore during the installation of the device. The fastener includes a shaft comprising a lubricant coating on the threaded portion and a transition portion between the straight shaft and the threaded portion to facilitate insertion of the shaft into the shaft. The disadvantage of this attachment is to include very little lubricant coating on the barrel, which may require very significant effort of installation of the fastener.

0004In fact, the greater the thickness of the structures to be assembled, the greater the insertion forces of an interference-installed fastening device. They may exceed the maximum force of installation of the fastener beyond which the fastener breaks, either at the level of the threads, or at a traction groove, or at the level of the installation tool.

0005The teaching of the document is also known <patcit id="pcit0002" dnum="GB2212580A"><text>GB 2,212,580</text></patcit> which discloses a non-jacketed metal fastener installed in a composite material structure. This attachment includes on a portion of its drum a lubricant coating - TEFLON ™ type or MoS₂ - Dielectric allowing the insertion of the fixation in the bore. The portion of it was connected to the head, resting on an outer surface of the structure of the aircraft, which can be struck by lightning, is left bare to allow electric currents to pass from the head of the attachment into the structure of the aircraft via the bare metal barrel portion, and therefore conductive. The disadvantage of this fixation is to present a single conductive portion placed arbitrarily near the head, which requires that the adjacent structure is relatively conductive, or includes a conductive wire mesh. The document<patcit id="pcit0003" dnum="DE202006008314U"><text>DE 20 2006 008 314 U</text></patcit> discloses another attachment of the prior art.

0006An object of the present application is to solve the problems of the fasteners of the aforementioned prior art. An object of the invention is to obtain a fixture capable of conducting electricity in all the layers of a structure comprising at least one low-conductive layer when the element is struck by lightning, without requiring too great efforts of 'installation.

0007The invention therefore relates to a metal fastener comprising an enlarged head and a smooth drum extending along an axis of revolution, the barrel comprising at least one conductive portion and a lubricant portion disposed continuously along the axis of revolution of the fastener. .

0008The binding according to the invention makes it possible to ensure an electrical contact surface on all the structural elements to be assembled - whether they are conductive or little conductors - whatever the composition of the structure, depending on the dimensions of the conductive strips. The fastener can therefore be installed in any structure, without requiring a conductive layer at a specific location.

0009The binding according to the invention can be installed in structures made entirely of composite materials, or in mixed structures, ie using composite and metal materials of the aluminum alloy or titanium type, for example. In these two cases, the insertion of the fastener according to the invention can be carried out in interference in a metal jacket previously arranged in a bore of the structures, in order to avoid any risk of delamination of the composite during the installation of the fastener. , and fill any air pockets that may exist between the structure and the fastener.

0010Fixation of the invention can also be installed in only metal structures of the aluminum or titanium type, for example.

0011The binding according to the invention may comprise one or more of the following features:<ul><li>the conductive and lubricant portions are arranged parallel to the axis of revolution of the fastener,</li><li>at least two lubricating strips are arranged diametrically opposite to the axis of revolution of the fastener,</li><li>the conductive and lubricating portions are arranged helically around the axis of revolution of the fastener,</li><li>the conductive portion is a portion of bare metal, or a portion of bare metal covered with a conductive layer of electricity,</li><li>the lubricating portion is a solid lubricating film or an organic resin coating comprising aluminum pigments,</li><li>the barrel of the fixing is cylindrical or conical,</li><li>the fastening further comprises a metal liner whose inside diameter is smaller than an outside diameter of the barrel of the fastener,</li><li>the locking portion is a thread or a plurality of crimping grooves.</li></ul>

0012The invention and its various applications will be better understood by reading the following description and examining the figures that accompany it. These are presented only as an indication and in no way limit the invention. The figures show:<ul><li><figref idref="f0001">figure 1A</figref> : a schematic representation of an end view of a first example of attachment according to the invention;</li><li><figref idref="f0001">Figure 1B</figref> : a schematic representation of a side view of a first example of attachment according to the invention;</li><li><figref idref="f0001">Figure 2A</figref> : a schematic representation of an end view of a second fixing example according to the invention;</li><li><figref idref="f0001">Figure 2B</figref> : a schematic representation of a side view of a second example of attachment according to the invention;</li><li><figref idref="f0002">figure 3A</figref> : a schematic representation of an end view of a third embodiment of attachment according to the invention;</li><li><figref idref="f0002">figure 3B</figref> : a schematic representation of a side view of a third embodiment of attachment according to the invention;</li><li><figref idref="f0002">figure 4</figref> : a schematic representation of a side view of a fourth fixing example according to the invention.</li></ul>

0013The <figref idref="f0001">Figures 1A and 1B</figref> schematically represent a first embodiment of fastening according to the invention. The<figref idref="f0001">Figure 1B</figref> represents a metal fastener 10, for example titanium alloy TiA6V, comprising an enlarged protruding head 12, a cylindrical smooth barrel 14 and a locking portion 16 in the form of a thread on which a nut (not shown) can be screwed . The attachment further comprises a pulling tail 18 for pulling on the rod to insert it into a previously bored structure. This shank 18 may be an integral part of the rod 10, or removable by means of a threaded end inserted in a threaded end made inside the threaded portion 16. This pulling tail is optional, and may be omitted.

0014The smooth drum 14, intended to come into contact with a structure made of one or more materials, metal or composite materials, is bare. Preferably, it is oxidized by sulfuric anodizing to improve the resistance to galvanic corrosion. The threaded portion 16 is fully coated with a layer of lubricant 20, to prevent jamming of the nut during installation. The smooth drum also comprises two portions 22 of lubricant coating, arranged in the form of strips parallel to the axis of revolution A of the fastener. Each lubricating portion 22 extends on the smooth drum 14 and a portion under the head 12. The two lubricating strips 22 are arranged diametrically opposite to the axis of revolution A, with an angle of 90 ° between each band as represented in<figref idref="f0001">figure 1A</figref>. The barrel surfaces 14 left between the lubricant strips 22 are conductive, and extend over a length X of the barrel 14. In this example, the barrel 14 has 50% lubricated contact surface and 50% conductive surface. electricity. The conductive portions 24 being disposed parallel to the axis of revolution A, the conduction is ensured regardless of the composition of the structure in which the attachment is inserted. The lubricating portions 22 allow to ensure a minimum lubrication of the smooth drum 14 over its entire height so that the attachment can be installed without requiring too much effort and without risk of breaking the fastener.

0015The lubricant used to cover the thread 16 and the lubricating portions 22 may be a solid lubricant film, of the MoS type₂, or more generally in accordance with SAE AS5272. The lubricant may also be an organic aluminum pigmented resin, such as HI-KOTE ™ 1 or HI-KOTE ™ 1NC coating marketed by Hi-Shear Corp. in Torrance, California. This type of coating is for example described in patents<patcit id="pcit0004" dnum="US3983304A"><text>US 3,983,304</text></patcit> or <patcit id="pcit0005" dnum="EP2406336A"><text>EP2 406 336</text></patcit>. The lubricant may also be of the type described in NAS4006. The lubricant is deposited on a thickness of 5 to 13 microns, for example by spraying. The rod may be covered with a second layer of lubricant, for example, ethyl alcohol. This second layer facilitates the insertion of the fastener in the structure, and is removed as and when the installation because it adheres very little to the surface.

0016The <figref idref="f0001">Figures 2a and 2b</figref> represent the same attachment 10 as that described with reference to <figref idref="f0001">Figures 1A and 1B</figref>. The only difference lies in the number of lubricating portions 22 applied to the smooth drum 14. In these figures, the shaft 14 is covered with four lubricating portions 22, arranged diametrically opposite to the axis of revolution A, with an angle of 45 ° between each band as shown in FIG.<figref idref="f0001">Figure 2A</figref>. In this example, the total conducting surface of conductive strips 24 represents 50% of the contact area between the shaft of the fastener and the structure.

0017The <figref idref="f0002">Figures 3a and 3b</figref> represent the same attachment 10 as that described with reference to <figref idref="f0001">Figures 1A and 1B</figref>. The difference lies in the fact that the attachment is entirely covered with a first conductive layer 26, and comprises two lubricating portions 22 applied on the first metal layer 26. The conductive layer 26 improves the electrical conduction between the attachment 10 and the structure. It can be made in the form of an aluminum metal deposit, or a conductive organic coating.

0018The <figref idref="f0002">figure 4</figref> represents a fourth embodiment of the invention. Fixation is identical to that described with reference to<figref idref="f0001">Figures 1A and 1B</figref>. The difference lies in the fact that the lubricating portion 22 is in the form of a helix winding around the axis of revolution A of the fastener 10. The conductive portion 24 is also helical.

0019The invention is of course not limited to the examples given above and the attachment may vary in its forms and / or materials used. Thus, the number of lubricating portions 22 may vary, and the proportion of surface area covered may also vary depending on the expected performance of the attachment. A proportion of conductive surface of 50% of the contact surface between the barrel of the fastener and the structure is preferable, but this proportion can vary between 20% and 80% of the contact surface between the barrel of the fastener and the structure . The length X may vary, and cover only a portion of the smooth drum 14, the width of the conductive portions 24 and lubricant 22 to be adapted according to the desired proportion of total conductive surface.

0020Alternatively, the locking portion 16 may consist of crimping grooves on which a metal ring is crimped. In this case, the lubrication of this portion is not necessary.

0021The smooth drum 14 may be conical, and installed either in a conical bore, or in a liner whose inner surface is conical and the outer surface is cylindrical, so that the assembly is installed in a cylindrical bore, easier to perform than conical drilling. When the device of the invention is used with a jacket, the latter comprises an inside diameter smaller than an outside diameter of the barrel of the fastener so that the fastener, when it is inserted, radially expands the jacket against the walls of the pierced hole. in the structure. This variant is particularly suitable for structures comprising at least one layer of composite material.

0022The head 12 of the fastener may be countersunk and fit into a complementary countersink in the structure.