BACKGROUND

The present invention relates to a fastener intended to be interference-fitted into an aperture of an aircraft structure and to a method for fitting such a fastener in an assembly of structures.

Certain aircraft structures are highly stressed in shear loading, and experience highly variable loading cycles, these typically being the structures of the wings or the joints between wing structures and fuselage. In order to assemble these structures use is made of interference-fitted fasteners, which means to say fasteners fitted with a negative clearance between the shank of the fastener and the aperture in the structure intended to accept the fastener. In other words, the outside diameter of the fastener is greater than the diameter of the aperture before the fastener is fitted. This method of interference-fitting fasteners makes it possible to improve the strength of the structure, notably its ability to withstand cyclic loadings.

Interference-fitted fasteners currently in use are of the pull type or of the push type. A third type of fasteners referred to as sleeved or sleeve fasteners allows an interference to be created in the structure without having friction against the latter during the fitting phase.

Pull-type fasteners comprise an enlarged head intended to rest against one face of the structure, referred to as the “front” face, or in a countersink formed in this face, a shank, a locking portion in the form of a screw thread or of crimping grooves, and a pulling stem comprising pulling grooves. The fastener is dimensioned so that the pulling stem protrudes from the opposite face of the structure—referred to as the “rear” face. These grooves are gripped by a tool which pulls on the pulling stem and inserts the fastener until the head comes up against the front face of the structure. The stem is then broken or removed. These fasteners are typically the GP™, LGP™ or XPL™ lockbolts by HUCK Manufacturing, or the PULL-IN™ or PULL-STEM™ fasteners by LISI AEROSPACE. These fasteners are for example described in patents U.S. Pat. Nos. RE30445, 6,702,684, 5,098,238 or 6,665,922.

Push-fit fasteners are made up of the same elements as pull-fit fasteners except for the pulling stem. They are interference-fitted in the structure by pushing them in using a hammer, a hammer gun, or a ram if it is possible to position the structure in an assembly jig. These fasteners are typically HI-LITE™ fasteners described in patent U.S. Pat. No. 4,326,825.

Sleeved fasteners are provided which a sleeve which accepts the shank of the fastener, and can be installed by pushing, by pulling or even by screwing. In this case, the fastener is kept rotationally fixed and a nut is screwed on to the threaded end. The turning of the nut forces the body of the screw, which is slightly tapered, into the sleeve which is held axially on the structure by its head, so that the sleeve expands radially in the structure. Such a fastener is described for example in patent application WO 2010/142901.

In order to protect the contacting parts from galvanic corrosion and in order to avoid any seizure between the thread of the screw and the tapping of a nut or alternatively between the shank and the structure during interference fits, the fasteners may also be covered with an organic coating containing aluminum pigment, typically from five to thirteen microns thick, of the HI-KOTE™ type, which has been developed and marketed by the company HI-SHEAR Corporation, and which are described for example in patents U.S. Pat. No. 3,979,351 and EP 2 406 336 B1. These coatings have a certain lubricating ability, reducing the loads of installing the fastener during the fitting into the structure or into the sleeve in the case of a sleeved fastener.

Aeronautical fasteners are generally lubricated using cetyl alcohol applied to the entire exterior surface of the fastener. Cetyl alcohol is a fatty alcohol with the semi-developed formula CH3(CH2)15-OH. At ambient temperature, cetyl alcohol adopts the form of a waxy white solid or of flakes. In order to be able to be applied to the fastener, the flakes are diluted in a solvent and the fastener is dipped in the solution. Once the solvent has evaporated, the solid flakes of cetyl alcohol cover the fastener.

FIG. 1schematically depicts an insertable fastener of the prior art, during the course of being interference-fitted into a structure S featured in cross section, comprising a aperture8of diameter DS. The fastener1comprises a protruding head, a cylindrical shank2of outside diameter D1greater than DS, and a threaded end3the maximum diameter of which is smaller than the diameter D1of the cylindrical shank and smaller than the diameter of the aperture Ds. A transition zone4connects the shank2and the threaded end3. The transition zone may have various geometries, such as one or more radii, or even an angle. The fastener here comprises a pulling stem5equipped with pulling grooves. The fastener is entirely coated with an anticorrosion organic coating of the HI-KOTE™ 1 type.

InFIG. 2the fastener ofFIG. 1is partially engaged in the structure, creating interference.

As depicted inFIGS. 1 and 2, in which the relative differences in size have been exaggerated for the sake of greater clarity, a chamfer or a radius is generally made on the entry to the aperture of the first layer S1of the structure, so as to accommodate the radius under the head of the fastener. This chamfer does not exist on the other layers S2and S3in the middle of the assembly when the drilling is made through all the layers. During the transition from one layer of structure to the next, a geometric staircase effect arises because the aperture of the first layer is expanded radially by the fastener shank already inserted, whereas the diameter of the aperture of the next layer is smaller.

The applicant company has discovered that the staircase effect accentuates the fitting force when the fastener1enters a layer comprising a material that is more rigid than the material of the layer previously passed through, for example when the fastener passes from a layer of composite material to a layer made of metal, for example titanium, aluminum or steel, or when the fastener passes through a first layer made of aluminum followed by a layer made of steel.

The staircase effect is weakly accentuated when the fastener enters a layer comprising a material that is not as rigid as the material of the layer previously passed through, for example when the fastener passes from a layer made of metal into a layer made of composite.

The staircase effect is depicted inFIG. 3which shows a schematic example of the force required to insert a fastener1of the prior art interference-fitted into the structure S ofFIGS. 1 and 2, in the case of a total structure thickness of 60 mm, made up of three layers S1, S2, S3respectively made of aluminum, carbon fiber composite and titanium, each layer having a thickness of 20 mm. The fastener in this example is able to withstand a maximum insertion force equal to 56 kN, represented by the dotted horizontal line. What that means to say is that, if a force greater than 56 kN is imparted to the fastener, the pulling stem of the fastener will break. When the fastener1enters the first layer S1, the fitting force increases substantially linearly with a first gradient. On entering the second layer S2, the gradient of the fitting force decreases slightly because the composite material of the layer S2deforms more readily under the imparted force needed to interference fit the fastener into the aluminum.

As the fastener enters the third layer S3, the gradient increases significantly because the layer S3is made of titanium. The insertion force here reaches the maximum threshold before the shank2of the fastener is completely inserted into the entire structure S. The fastener is not fitted all the way in, which means to say that the head of the fastener is not in contact with the front face of the structure S and part of the shank2is outside the structure S. To fit the fastener all the way in a fitting force greater than the maximum strength of the fastener would have to be imposed on the fastener and that would cause the fastener to break inside the structure before it has been correctly installed.

The applicant company has observed that the insertion into the first two layers substantially tears away the very fine layer of lubricant and/or of anticorrosion coating on the transition zone4and on the shank2. In effect, the transition zone4is the first zone of the fastener to come into contact with the structure S. This zone is the most highly loaded in friction against the structure because during fitting it passes through the entire thickness of the structure. In addition, the transition zone4will experience the geometric staircase effects as it passes through the layers which tend to localize the contact forces and lead to rapid deterioration of any coatings there might be or even of its geometry.

When the fastener enters the third layer S3, the rigidity of the titanium, combined with the direct contact between the fastener and the structure lead to a sharp increase in the force needed to insert the fastener as an interference fit into the third layer.

The applicant company has also observed that the apertures made in different materials had different diameters, because of the different conditions in which the materials are cut during the drilling with the one same tool. The difference in diameter may therefore accentuate the geometric staircase effect phenomenon. The staircase effect may also be created when the layers are predrilled and then aligned, and the apertures of the various layers have an alignment defect.

Moreover, the more the thickness of the assembly increases, the more the area for friction between the shank of the fastener and the aperture increases. At high levels of interference, there is therefore increased probability that the fastener will not be inserted all the way in for thick assemblies because of the greater probability of the anti-corrosion and/or lubricating coating being torn off.

Document U.S. Pat. No. 3,779,127A describes a fastener intended to be interference-fitted into an aperture to assemble a structure. The fastener comprises two bulbs of diameters greater than the nominal diameter of the shank of the fastener and which are intended to expand the aperture to a diameter greater than the diameter of the shank. When the fastener previously covered with a lubricant is forcibly introduced into the aperture, the back surface48of a bulb28allows the aperture wall to smoothly elastically recover into a lubricant reservoir52, and a quantity of lubricant trapped in the hollow part between the two bulbs is picked up and carried along and eases the progress of the larger-diameter second bulb. With such a solution, the expansion of the aperture beyond the diameter of the shank of the fastener is a necessity that does not apply to all materials and in any event is not necessarily desired.

SUMMARY

It is an object of the invention to overcome these problems. Thus, one subject of the invention is a fastener for assembling at least two structural elements, the fastener comprising an enlarged head and a cylindrical or tapering smooth shank exhibiting, prior to fitting, an outside diameter greater than an inside diameter of a destination aperture, a locking portion comprising a screw thread or annular grooves and exhibiting an outside diameter always smaller than the inside diameter of the destination aperture, and a transition portion between a distal end, with respect to the enlarged head, of said smooth shank and a proximal end, with respect to the enlarged head, of said locking portion.

The fastener furthermore comprises at least one sacrificial ring containing a lubricant, arranged on an exterior surface of the fastener, positioned on an exterior surface of the smooth shank and or of the locking portion and or of the transition portion, said sacrificial ring exhibiting, prior to the fitting of the fastener into the structure, an outside diameter greater than a maximum outside diameter of the locking portion and greater than a smallest diameter of the aperture with which the smooth shank will interfere after the fastener has been fitted, said sacrificial ring exhibits, prior to the fitting of the fastener into the structure, a toroidal shape.

Thus, the sacrificial ring is first to come into contact with the wall of the aperture when the fastener is being interference-fitted into the structure—or into the sleeve if the fastener is interference-fitted into a sleeve—and with each of the layers of the structure which are formed by the structural elements that are to be assembled. The nature of the ring allows to store lubricant and to release the lubricant during insertion when the ring is compressed and worn between the wall of the aperture and the fastener shank surface.

The sacrificial ring corresponds to an additional deposit of material, separate from the structural part of the fastener formed by the head, the smooth shank, the locking portion and, where appropriate, a pulling stem, the ring having a toroidal shape. In the present description “toroidal shape” means a torus with a generatrix cross section that may be but is not necessarily circular in a generalized acceptance of the idea of a torus, the central hole of such torus being the structural part of the fastener passing through it. A reason the cross-section may not be circular is because the interior portion of the ring surface will generally follow the profile of the underlying surface of the fastener. Another reason the cross-section may not be circular is because an exterior portion of the ring surface may form a bead over 360° around the fastener, the bead having a larger cross section in the radial direction than the other part of the ring at the same angle. Another reason the cross-section may not be circular is because the sacrificial ring may include segments separated from an adjacent segment or segments around the fastener at least 50% or greater of the perimeter of the fastener, so that an exterior portion of the ring surface may form a wave over 360° around the fastener.FIGS. 4A to 4E,FIGS. 7 and 8show rings having a toroidal shape according to the meaning of the present description.

The ring has the ability to wear or erode easily when rubbed in contact with another material. The sacrificial ring guarantees the reliability of the insertion of the fastener during the force-fitting into the structure: the ring will be gouged or eroded or worn by the structure or by the sleeve, thereby, gradually as the fastener is inserted, leaving the lubricating material of the sacrificial ring at the interface between the smooth shank and the structure or the sleeve, and the fastener will experience practically no wear. The term “lubricant” is to be understood as meaning any material or substance which, interposed between two surfaces, reduces the friction or wear between these two surfaces in contact and in relative motion. Thus, the lubricating sacrificial ring wears away or becomes destroyed during insertion, and in so doing lubricates the interior surface of the aperture throughout the insertion.

The fastener according to the invention may also comprise at least one or more of the following features, considered in isolation or in combination where these combinations are technically feasible:the lubricant comprises a synthetic or mineral oil containing at least 80% in volume of oil,the lubricant comprises a mixture of synthetic or mineral oil and cetyl alcohol,the dispersant molecules surround the lubricant molecules,the polymer resin comprises acrylic polymer,the lubricant comprises a polymer resin forming a matrix encapsulating dispersant molecules and lubricant molecules,the sacrificial ring comprises polytetrafluoroethylene,the sacrificial ring further comprises graphite and or bronze,the sacrificial ring comprises a lubricating polymer,the sacrificial ring further comprises dry particles of graphite and or of cetyl alcohol,the sacrificial ring comprises a first ring comprising a pasty lubricant covered by a second ring forming a sacrificial solid casing,the pasty lubricant comprises cetyl alcohol and an oil,the pasty lubricant comprises a grease with an NLGI grade of between 00 and 3, preferably between 00 and 2,the grease comprises a mixture of wax and of oil, the wax representing between 20% and 50% by weight of the grease,the second ring comprises an acrylic polymer, a silicone or a polysulfide polymer,the second ring is a fluidtight film resulting from an application of a liquid sealant, hardened by evaporation in the air,the sacrificial ring is discontinuous and exhibits peripheral segments following on from one another to form a segmented ring.the sacrificial ring exhibits, prior to the fitting of the fastener into the structure, an outside diameter greater than a largest diameter of the aperture with which the smooth shank will interfere after the fastener has been fitted,the sacrificial ring exhibits, after the fastener has been fitted into the structure, an outside diameter smaller than the outside diameter it had prior to fitting,the sacrificial ring is sheared or cut into at least two portions after the fastener has been fitted into the structure;a sheared or cut portion of the sacrificial ring is positioned between an exterior surface of the structure and a surface of the head of the fastener after the fastener has been fitted into the structure,the sacrificial ring is worn during the fitting of the fastener into the structure;a circular groove is formed so that it is recessed with respect to the exterior surface of the smooth shank, in the smooth shank or in the region of the transition portion, in which circular groove the sacrificial ring is positioned, the sacrificial ring having a shape that complements the circular groove so as to ensure retention of the sacrificial ring in the circular groove;

Another subject of the invention is a method of assembly implementing a fastener of the invention.

For assembling at least two structural elements comprising a aperture exhibiting, before fitting, an inside diameter, the method comprises the steps ofsupplying a fastener comprising an enlarged head, a smooth shank exhibiting, prior to fitting, an outside diameter greater than the inside diameter of the aperture or than an inside diameter of a sleeve that is to be inserted into said aperture, a locking portion comprising a screw thread or annular grooves exhibiting an outside diameter always smaller than the inside diameter of the destination aperture, and a transition portion between a distal end, with respect to the enlarged head, of said smooth shank and a proximal end, with respect to the enlarged head, of said locking portion, and a lubricating sacrificial ring, arranged on an exterior surface of the fastener, positioned on an exterior surface of the smooth shank and or of the locking portion and or of the transition portion, said ring exhibiting, prior to the fitting of the fastener into the structure, a diameter greater than a maximum outside diameter of the locking portion and greater than a smallest diameter of the aperture or of the sleeve with which the smooth shank will interfere after the fastener has been fitted, said sacrificial ring exhibiting, prior to the fitting of the fastener into the structure, a toroidal shape,forcibly fitting the fastener into the aperture or into the sleeve placed in the aperture, during which step the lubricating sacrificial ring is worn through interference with an interior wall of the aperture or of the sleeve, so as to release a lubricating substance between a distal end of the smooth shank and a proximal end of the smooth shank.

Another subject of the invention is a water-based composition for forming a ring of a fastener according to the invention, comprising 20% to 60% by volume of water based resin, 35% to 75% by volume of water insoluble an oil-based lubricant, and 1% to 10% by volume of dispersant.

In other embodiments, the water-based composition may comprise:25% to 50% by volume of water based resin, 45% to 70% by volume of water insoluble an oil-based lubricant, and 4% to 6% by volume of dispersant.10% to 60% by volume of water based resin, 2.5% to 20% by volume of cetyl alcohol, 35% to 70% by volume of water insoluble an oil-based lubricant, and 1% to 10% by volume of dispersant.25% to 45% by volume of water based resin, 2.5% to 10% by volume of cetyl alcohol, 45% to 70% by volume of water insoluble an oil-based lubricant, and 5% to 10% by volume of dispersant,the water insoluble lubricant comprises at least 80% in weight of oil in the total composition of the water insoluble an oil-based lubricant,the water-based composition has a viscosity between 20 000 and 50 000 Cp, measured with a RVT Brookfield viscometer, with a Mobile 7 needle, at 20 rpm and 25° C.

Another subject of the invention is a method of forming a fastener according to the invention, for assembling at least two structural elements, the method comprising the steps of:provide a fastener comprising an enlarged head and a cylindrical or tapering smooth shank exhibiting, prior to fitting, an outside diameter greater than an inside diameter of a destination aperture, a locking portion comprising a screw thread or annular grooves exhibiting an outside diameter always smaller than the inside diameter of the destination aperture, and a transition portion between a distal end, with respect to the enlarged head, of said smooth shank and a proximal end, with respect to the enlarged head, of said locking portion,apply a water based composition according to any water-based composition of the invention on an exterior surface of the fastener, positioned on an exterior surface of the smooth shank and or of the locking portion and or of the transition portion,dry the water-based composition such as the water evaporates and forms a sacrificial ring exhibiting, prior to the fitting of the fastener into the structure, an outside diameter greater than a diameter of the aperture with which the smooth shank will interfere after the fastener has been fitted, said sacrificial ring comprising a polymer resin forming a matrix encapsulating dispersant molecules and lubricant molecules.

Oil-based lubricant substance, stored in the sacrificial ring prior to the fitting of the fastener, is thus distributed gradually as the fastener is inserted, on to the contacting walls of the fastener and of the structure or of the sleeve.

The insertion forces are thereby reduced thereby guaranteeing that the fastener can be inserted with acceptable force and preventing the fastener or structure from being damaged.

In one embodiment, the lubricating sacrificial ring is sheared or cut into at least two portions during the fitting step.

In one embodiment, the method comprises a later step of eliminating at least one sacrificial-ring portion.

