Nuts intended for use in aviation are designed in a wide range of sizes and styles and are also made from a variety of materials such as hardened steel, corrosion resistant steel, stainless steel, tempered aluminum alloy, and more. Aviation nuts can generally be grouped into one of two categories: non-self-locking nuts, also called standard nuts, and self-locking nuts. In this blog, we will discuss these two categories and the types of nuts belonging to each.
The plain nut is a common nut designed to specific aviation requirements and with aviation grade materials. It looks like a nut you would find in everyday items but is built to aviation standards relating to hardness and thread/hole tolerances. They are also far more corrosion-resistant than everyday hardware nuts.
Castle Nuts and Castellated Shear Nuts
Castle nuts are used in conjunction with AN bolts, clevis bolts, eyebolts, and studs with holes drilled in the shanks. It looks similar to a plain nut but features a crown on top with slots resembling a medieval castle. Nuts of this type are designed to be rugged and handle large tension loads. The slots allow for the use of a cotter pin or safety wire, providing additional retention properties. Castellated shear nuts are a thinner version of the castle nut. The thin design makes them weaker than castle nuts and means they can only be used in applications where shear stress forces are encountered.
Plain Hex Nuts and Light Hex Nuts
The plain hex nut is a plain nut with coarse threads. They are extremely rugged and capable of use in large tension load areas. However, because it has no locking features, they are not widely used on aircraft. A common application of plain hex nuts is in magnetos where they are used with a plain and lock washer combination. The light hex nut is a thinner version of the plain hex nut which requires the use of an auxiliary safety device. Light hex nuts can be found used in various light tension applications throughout an aircraft.
Plain Check Nuts
Plain check nuts are used as locking nuts for plain nuts, threaded rods, set screws, and similar devices. These nuts are thinner than a standard plain nut but otherwise are similar in appearance.
The wing nut is a very distinctive nut that looks like a small cone with large wings set 180 degrees apart. There is commonly a hole in one of the wings for the use of a safety wire. Wing nuts are used in applications where an item may be frequently removed and extreme tightness is not necessary.
A self-locking nut is a nut that resists loosening under vibrations or torque. Some types of self-locking nuts deform elastically to provide a locking action. The most common types of self-locking nuts are boot nuts, stainless steel nuts, and elastic stop nuts.
Boot nuts are metallic, one piece designs used to hold components tight under severe vibrations. It is essentially two nuts in one unit - a load carrying portion and a lock nut portion. The two portions are joined by a spring which, when installed properly, applies a constant locking effect on the bolt. They come in varying styles and materials to fit many applications, though they are not commonly used on new aircraft.
Stainless Steel Self-Locking Nuts
Nuts of this type can be tightened and loosened by hand. The locking features of this nut are such that locking action can only occur when the nut is seated against a surface and tightened. Similar to a boot nut, these are two pieces. Because the nut and insert are not deformed and are of the same size as the bolt, they tighten and loosen easily.
Elastic stop nuts are similar to a plain nut but feature an additional head to allow room for a fiber to be installed and retained. These nuts must be extremely tight for their intended applications. If it can be moved by hand, it should be replaced.
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