A typical aircraft is composed of countless fasteners and hardware pieces, all of which work together to maintain a secure and operable assembly for the means of undertaking flight operations. Across the millions of fasteners that are situated across a fuselage, many come in the form of washers. While varying in type to accommodate different needs and requirements, a conventional washer typically consists of a thin plate with a hole in its center. Generally, such components are implemented alongside a threaded fastener for the means of distributing loads, though they may also act as a spacer, wear pad, spring, locking device, vibration reducer, or preload indicating device. Due to their common implementation in aircraft and the important roles that they serve, it can be useful to have a general understanding of what types are often found in aircraft assemblies.
The plain washer is the most basic type, and the ones used for aircraft construction will often be AN960 and AN970 variations. Generally, these washer components are placed under hex nuts, offering a smooth bearing surface. Plain washers also act as a shim, allowing for the optimal bolt and nut grip length to be achieved. Additionally, they can help adjust the positioning of castellated nuts so that they sit correctly with drilled cotter pin holes.
When plain washers are used under bolt heads or alongside nuts on an aluminum alloy or magnesium structure, aluminum and aluminum alloy materials are the most optimal washer materials. By pairing an aluminum or aluminum alloy washer with a nut placed on a dissimilar metal, corrosion can be avoided. For wooden structures, the AN970 steel flat washer is known for providing an increased bearing area over other steel flat washer parts.
A lock washer, or washer lock component, is a specialized washer that can prevent the loosening of a component or fastener through the use of its shape or features. For aircraft applications, the AN935 and AN936 variations are the most common, and they both may be used alongside screws or bolts when a self-locking or castellated nut is unable to be relied on. To prevent the loosening of fasteners, lock washers such as the AN935 variation take advantage of their spring action to increase friction. It is important that lock washers are always used in the correct instances. As such, they should never be paired with a screw that is regularly removed, placed where washers are exposed to airflow or corrosive materials, installed in an area failure can cause hazards, or implemented in a way that causes the washer to gouge surfaces.
Shakeproof Lock Washers
Shakeproof lock washers are another specialized washer type, coming in the form of a rounded component with tabs or lips that are bent upward. These tabs or lips reach across the sides of a hex nut or bolt fastener, ensuring that they are locked in place. To ensure that the shakeproof lock washer itself does not turn, an external tab is bent down at a 90 degree angle and into a hole situated on the face of the unit or an internal tab is placed within a keyed bolt. As compared to other various aircraft washer types, the shakeproof lock washer is capable of undertaking a high amount of heat and vibration with ease. Due to the use of tabs that are bent for securing the washer and other fasteners, shakeproof lock washers should be replaced after use to ensure that the tabs do not break during a second installation.
While the aforementioned fasteners are all common to aircraft, there are also various specialized washers that are typically used for aircraft construction as well. AC950 and AC955 washers are ball socket and seat washers respectively, and such components may be paired with a bolt that has been installed in a surface at an angle or where there is a need for perfect alignment. Generally, both washers are used in unison for such means.
NAS143 and MS20002 washers are also both specialized types, often finding use alongside internal wrenching bolts that are a part of the NAS144 through NAS158 series. Both of these washers may come in plain or countersunk shapes, the latter being used to seat a bolt head shank radius. Plain washers, meanwhile, are placed under the nut. Alongside such specialized washer types, there are numerous fasteners that benefit the overall structure and assembly of the aircraft.
With all washer types, it is important to conduct ample research before making a purchasing decision to ensure that all parts work well together and serve the roles they are assigned to optimally. As discussed before, it is crucial to correctly pair materials as dissimilar metals can often cause corrosion or other detrimental effects that can quickly cause structures and parts to wear down and fail. Corrosion should always be prevented or attended to as soon as possible as it can cause surrounding areas to degrade as well. Whenever a washer, fastener, or other aircraft component reaches the end of its service life, one should rapidly repair or replace them so that the aircraft can remain airworthy and safe to operate.
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