The Types and Applications of Aircraft Bolts

Aircraft bolts are a type of fastener that is widely used across the assembly of an aircraft. Fasteners are a type of hardware that allow for two or more components to be secured together either non-permanently or permanently, and they are greatly utilized for aircraft construction. Aircraft fasteners such as the bolt are used in areas where there is a need for high strength that typical screws cannot fulfill. Depending on the application and need, different types of aircraft bolts with varying properties may be used. In this blog, we will discuss some of the main bolt types that are used for aircraft assembly.

The general purpose bolt is an aircraft hardware part that provides for tension and shear loads. A general bolt consists of a cylindrical shaft that is partially threaded, as well as a shaped head. To secure components together, the shaft is passed through a hole, and a nut is tightened to create compression forces between the nut and bolt head. Typically, the general purpose bolt is used in many areas and structures, though some smaller alloy steel bolts will not be used for primary structures. The material of a bolt is also important to consider, as aluminum alloy should not be used when the fasteners will often be removed for MRO, as well as should not be used in seaplanes due to possible galvanic corrosion.

Close tolerance bolts are aircraft fasteners that are engineered with much more precision as compared to the general purpose bolt. Close tolerance bolts are installed into an assembly with the use of a 12 to 14 ounce hammer and provide well for applications where a tight drive fit is desired. The head of a close tolerance bolt may either be hex headed or feature a 100 degree countersunk head. To identify the material that a close tolerance bolt is made from, one can refer to a triangle mark that is present on the bolt head.

Internal wrenching bolts are often present in areas of the aircraft where there are mainly tension loads. Such bolts are manufactured with high tensile strength steel and are considered to be close tolerance. For easier installation and removal of internal wrenching bolts, the head is recessed which allows for the insertion of a wrench. For optimal installation, high strength nuts are mated with the internal wrenching bolt.

Due to their many applications, bolts may come in a variety of shapes and designs. Because of this, it can sometimes be difficult to identify types, especially without a standard classification method. To discern between bolts, the head shape, securing method, manufacturing materials, and usage may all be considered. If the bolt is an AN type bolt, there are code markings that will be present on the bolt head that will provide information such as materials, manufacturer, and whether or not the bolt is special purpose. Close tolerance bolts may also be identified similarly, featuring the presence of a triangle on the bolt head as discussed beforehand.


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