In the aircraft construction and repair industry, fasteners are critical tools needed to assemble detailed parts and combine them with other parts to form assemblies. These assemblies are in turn combined to form installations which eventually make up a complete aircraft. Fasteners are found throughout all parts of an aircraft. For instance, there are approximately six million parts used to fabricate a Boeing 747, over half of which are fasteners. Fasteners are used to connect parts in primary and secondary structural areas, pressurized and unpressurized applications, as well as to transfer loads from one part to another in production and repair applications. This blog will provide a thorough look at aircraft fasteners and their uses, types, codes, designs, and more.
Fastener Uses and Types
There are two main factors to consider when choosing the correct fastener type. These are the type of joint a fastener will be exposed to, shear or tension, and what types of loads will be transferred through the joint. Aircraft loads can include those experienced during towing, normal flight operations, high winds, pressurization, engine-out operations, landing, and others. Both factors determine how thick or thin the structure must be, the material used in the structure, and the necessary fasteners. Fasteners must be able to transmit loads from one part to the next. For instance, the engine transfers a load to the pylon, which transfers a load to the wing, which transfers a load to the fuselage. Many calculations are done to determine which fasteners are capable of transferring these loads. Other criteria including weight, inspectability, tooling requirements, aerodynamic smoothness, access, corrosion protection, and cost are also considered.
Fasteners can be divided into many groupings and types such as those used for structural load-bearing applications as well as nonstructural fasteners that connect non load-bearing components. There are also fasteners for restricted-access applications known as blind fasteners. These include Huck Lock Bolts, Composi-Locks fasteners, and CherryMAX fasteners. For applications where both sides are accessible, standard rivets, structural bolts, and Hi-Lok fastening system fasteners are used. These fasteners are made from aluminum, steel, and titanium and are coated to prevent dissimilar metal corrosion. Styles of fastener heads differ based on the aerodynamic requirements of the aircraft. Other types of aerospace fasteners include Dzus Fastener, Zipper Fastener, Wing Nut Fastener, and more.
Fastener Codes and Orientation
A fastener code can be designated by the fastener manufacturer, airframe manufacturer, or industry standards. The coding description can be found in fastener booklets, on the repair drawing, or the production blueprint. These codes are often used to simplify repair and production drawings, where they are found in the upper-left part of the drawing quadrant. Standard installation for the head of a fastener is head-up, or head-forward. Despite this, the blueprints will provide specific orientation in the upper-right corner of the fastener quadrant, and will call-out the fastener head as near or far based on the drawing.
In the design of a fastener joint the fastener must remain intact and not fail when it is placed under a load. If the fastener fails, the joint comes apart. In a total joint failure, the joint will tear apart. In an improper joint, this can result in material shear-out failure, material tension failure, or fastener shear failure caused by the wrong diameter, wrong material, or insufficient fastener edge-margin. The edge-margin refers to the distance from the center of the fastener to the edge of the part. Requirements for the edge-margin as well as the distance from the center of one fastener to the next must be carefully observed to ensure a proper joint. Throughout aircraft fatigue testing it was determined that equally-spaced fastener joints will perform better than staggered fastener joint designs. Despite this a staggered joint design is ideal for fuel tank applications as they provide a larger faying surface sealing area.
Adhering to proper processes is crucial during the installation of aircraft fasteners. While inspections are helpful to find faulty parts, an inspection alone cannot fix the part. That is done during installation. For example, when a solid rivet is bucked, the hi-lok collar is tightened, or the blind-rivet is pulled, the quality of both the hole and countersink are invisible. When countersinking, it is ok for the fastener to be slightly above flush rather than below flush after installation. The sole exception would be an area where specific aerodynamic smoothness is required. Countersinking below flush can also cause problems between the layers of materials and lead to a crack or shearing of the fastener head. Proper fastener-hole size and finish must be maintained based on the fastener type and the material the fastener is installed in. Rivets are commonly installed in a clearance-fit where the hole is larger than the fastener.
Criteria for the inspection, removal, and substitution of aircraft fasteners must also be closely followed to ensure the load can be carried throughout the aircraft structure. If the removal of a fastener is required for additional access or because it was improperly installed, it is also critical to take care to prevent damage to the surrounding structure and fastener hold itself. Furthermore, certain fastener substitutions are allowed by the airframe manufacturer if the new fastener meets or exceeds the original strength, diameter, and corrosion protection characteristics. Proper assembly and installation of parts and components is obviously critical, but adherence to the maintenance and repairs process is every bit as important to ensure the fastener joint will properly load and unload over its entire service life.
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