Patent Application
20170175794
The present invention relates to a fastener system and an assembly. The assembly can be used on an aircraft, the assembly comprising a first aircraft structure held to a second aircraft structure by the fastener system.
Conventional fasteners such as rivets require significant space to apply a tool for collar swaging and breaking the pin tail. The pin tail must be discarded. Complex drilling is often the only way to remove a rivet for repair, inspection or installation purposes. The rivet must then be replaced and cannot be re-used.
As an alternative to a rivet, U.S. 2014/0356094 A1 describes an assembly comprising first and second parts made of composite material held one against the other by at least one non-removable fastener system. The assembly described therein is designed to be used in an afterbody of an aeroengine, wherein an inner peg with a higher thermal expansion coefficient expands upon the high temperatures in the afterbody to secure the fastener system.
It is therefore an object to provide an improved fastener system which is secure and removable.
In accordance with a first aspect of the present invention, there is provided a fastener system for fastening a first structure to a second structure, said fastener system comprising
This fastener system requires less tooling space and it is removable and optionally re-usable. No complex drilling is needed to remove the fastener for repair, inspection or installation purposes. This fastener system is secure, because the at least one cavity is able to hold a locking material in place. It may be easier and therefore optional to manufacture a cavity in the threaded male portion of the bolt than in the female shank portion of the nut. Furthermore, applying or inserting of a locking material may be easier and therefore optional for a cavity in the threaded male portion of the bolt than the female shank portion of the nut.
Optionally, the at least one cavity has a distance to the countersink bolt head or the nut tip, wherein the distance is at least one thread pitch. In particular when using a viscous locking material, this distance may prevent locking material from being squeezed out at the nut tip. In addition or alternatively the at least one cavity may have a distance to the bolt tip or countersink nut head, wherein the distance is at least one thread pitch. This distance may prevent locking material from being squeezed at the bolt tip into a residual space in the female shank portion.
It is optional that the at least one cavity extends over 25% or more of the threads of the threaded male portion of the bolt and/or the inner thread of the female shank portion of the nut. This option may allow use of a wider variety of locking materials and shapes thereof.
It is also optional that at least one of the at least one cavity has a longitudinal shape extending in form of a helix section crossing the threads of the threaded male portion of the bolt and/or the inner thread of the female shank portion of the nut. The helix section may cross the threads at a crossing angle to maximise the contact surface between the locking material and the threads. This crossing angle may be between 10 and 80 degrees. The helix shaped cavity may be manufactured by a linear cut in the thread at a helix angle between 10 and 85 degrees with respect to the longitudinal axis of the threaded male portion.
In a first optional embodiment, the fastener system comprises at least two cavities having a longitudinal shape extending in form of separate and parallel helix sections crossing the threads of the threaded male portion of the bolt and/or the inner thread of the female shank portion of the nut. This option may allow a redundancy of locking material to increase the security the fastener system. Furthermore, the locking material can easily be dosed by the number of identical plastic inserts or tabs used as locking material. The at least two cavities may be arranged on opposite sides of the threaded male portion of the bolt and/or the inner thread of the female shank portion of the nut as sections of a double-helix. For N cavities, where N>2, the cavities may be arranged evenly distributed with an angular distance of 360°/N.
The local diameter of the threaded male portion of the bolt may be reduced by the at least one cavity by 10% or less, optionally 8%, with respect to the general diameter of the threaded male portion of the bolt. Thereby, the cavity is deep enough to secure a locking material without compromising the structural integrity of the bolt.
To facilitate the installation of the fastener system it may be optional that the locking material is already preinstalled and part of the fastener system. It may be partially located in at least one of the at least one cavity and may partially protrude out of said at least one of the at least one cavity such that the locking material increases the friction between the threaded male portion of the bolt and the threaded female shank portion of the nut when the threaded male portion of the bolt is fully screwed into the female shank portion of the nut. “Fully” shall mean herein with a sufficient amount of rotations to engage with at least 50% of the threads to establish a secure connection between the bolt and the nut.
With the preinstalled locking material as part of the fastener system, the locking material may increase the friction between the threaded male portion of the bolt and the threaded female shank portion of the nut when the threaded male portion of the bolt is fully screwed into the female shank portion of the nut with a locking torque below a maximum locking torque and a breakaway torque above a minimum breakaway torque depending on the diameter of the threaded male portion according to the following table:
including interpolations therefrom, optionally linear or other suitable interpolations. “Locking torque” herein refers to the torque required to screw the bolt into the nut and out of it. “Breakaway torque” herein refers to the torque required to start the unseated nut in the removal direction. The exact definitions and test conditions can be found in the Aerospace Industries Association of America's National Aerospace Standard Document No. NASM25027, first revision of 12 Dec. 2012.
The locking material may be a plastic insert or a thread-locking fluid like Loctite® or another suitable material for increasing the friction between the threads. A plastic insert may be optional for re-using the fastener system as the plastic insert can be easily renewed. A thread-locking fluid like Loctite® may provide more frictional contact surface with the threads and may need to be cleaned away before it is re-applied for re-using the fastener system.
The countersink bolt head and/or the countersink nut head may comprise engagement surfaces for rotating the bolt and/or the nut by a tool configured to engage with the engagement surfaces. Thereby, less tooling space is needed compared to a rivet with a breakable pin tail.
The countersink bolt head and/or the countersink nut head may comprise countersink surfaces for bearing against the countersink of the first structure and/or the countersink of the second structure, respectively. The countersink surfaces optionally have countersink angles of about 100 degrees or more, particularly of about 130 degrees. The countersink angles of the fastener system and the countersink angles of the countersinks of the first and second structure are optionally the same to distribute the induced stress on the structure on as much surface area as possible while benefitting from a centring effect of the countersinks.
In accordance with a second aspect of the present invention, there is provided an assembly comprising
This assembly requires less tooling space for installation and can easily be unassembled by removing the fastener system. No complex drilling is needed to remove the fastener for repair, inspection or installation purposes. This assembly is secure, because the fastener system is secured by the locking material fixed in the at least one cavity. It may be easier and therefore optional to manufacture a cavity in the threaded male portion of the bolt than the female shank portion of the nut. Furthermore, applying or inserting of a locking material may be easier and therefore optional for a cavity in the threaded male portion of the bolt than the female shank portion of the nut.
The first structure and/or the second structure may comprise composite material. The fastener system does not laterally expand during installation and is therefore particularly suited to fasten structures comprising composite material.
Embodiments will now be described by way of example only with reference to the following figures of which:
The cavities 33, 35 have a longitudinal shape extending in form of separate and parallel helix sections crossing more than 25% of the threads of the threaded male portion 19 of the bolt 11. Both cavities 33, 35 have a distance to the countersink bolt head 15 and to the bolt tip 17 of at least one thread pitch. The helix chirality of the cavities 33, 35 is different from the chirality of the threads.
The cavities 33, 35 are configured to fix a locking material partially located in them and partially protruding out of them such that the locking material increases the friction between the threaded male portion 19 of the bolt 11 and the threaded female shank portion 27 of the nut 13 when the threaded male portion 19 of the bolt 11 is fully screwed into the female shank portion 27 of the nut 13. The locking material may be a plastic insert or a thread-locking fluid like Loctite® optionally pre-installed as part of the fastener system 9. The locking material can increase the friction such that the fastener system has a locking torque below a maximum locking torque and a breakaway torque above a minimum breakaway torque depending on the diameter of the threaded male portion 19. For instance, with a general diameter of 4.83 mm of threaded male portion 19, the locking torque is 1.4 Nm or below and the breakaway torque is 0.2 Nm or above. This means that it requires a torque of 1.4 Nm or less to screw the bolt 11 into the nut 13 (and out of it) and 0.2 Nm or more to start the unseated nut 13 (or bolt 11) in the removal direction.
The countersink bolt head 15 and/or the countersink nut head 23 comprise engagement surfaces (not shown) for rotating the bolt 11 and/or the nut 13 by a tool (not shown) configured to engage with the engagement surfaces. These engagement surfaces can be part of a slot, a cross-recess, a square, a hex, a pentagon, torx drive or another suitable feature for receiving a mating tool like a screw driver, wrench or key to apply the necessary torque. The countersink bolt head 15 and the countersink nut head 23 both comprise countersink surfaces 37, 39 for bearing against countersinks of structures to be held together by the fastener system 9. In this example, the countersink bolt head 15 and the countersink nut head 23 both have a countersink angle β of about 130 degrees.
The threaded male portion 19 of the bolt 11 has a general diameter D and a reduced local diameter Dred where the cavity 33 is. To guarantee the structural integrity of the bolt 11, the local diameter Dred is only reduced by 10% or less, optionally about 8%, with respect to the general diameter of the threaded male portion of the bolt. This means (D−Dred)/D≦0.1, optionally (D−Dred)/D≈0.08.
Where, in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as optional, preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.
The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. While at least one exemplary embodiment has been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art and may be changed without departing from the scope of the subject matter described herein, and this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.
In addition, “comprising” does not exclude other elements or steps, and “a” or “one” does not exclude a plural number. Furthermore, characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. It should be understood that there should be embodied within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of the contribution to the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims and their legal equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1522497.5 | Dec 2015 | GB | national |
