Selecting the Right Fasteners, Bearings, and Structural Hardware for High-Stress Aircraft Applications

The reliability and performance of countless aircraft systems often hinges on the use of fasteners, hardware, and bearings that are engineered to accommodate varying loads, vibration levels, temperatures, and environmental conditions common to flight. To meet these expectations, such items are often manufactured in accordance with NAS, MS, or AS standards, all of which strictly define allowable materials, dimensional tolerances, and performance characteristics. In more operationally-demanding areas like structural joints, landing gear systems, engines, flight control linkages, and beyond, the expected conditions and functional requirements are further amplified, making it necessary for professionals to seek the right options for any assembly. In this blog, we will offer some basic guidance for selecting fitting fasteners, bearings, and hardware for demanding aircraft applications, so read on if you are interested in learning more.

A Guide to Selecting Heavy-Duty Fasteners

Fasteners placed in any aircraft assembly are tasked with maintaining joint integrity while transferring complex shear and tensile loads. In the most high-stress areas of a typical aircraft, engineers will often turn to specialized subtypes like:

• Close-Tolerance Bolts: Close-tolerance bolts are manufactured with minimal clearance between the shank and installation hole for accurate load transfer and alignment.
• Hi-Lok and Hi-Lite Pins: Hi-Lok and Hi-Lite fasteners are two-piece pin-and-collar systems that provide consistent preload and fatigue resistance.
• Solid Structural Rivets: Solid rivets are permanent fasteners that present excellent shear strength and fatigue resistance. Usually, they are employed in layered assemblies where loads are distributed across multiple fasteners, rather than concentrated at a single point.

While there are many different considerations to make when evaluating different fastener types, the choice of material is always one of the first decisions to make for its major impact on performance and longevity. In the present, titanium and aluminum have largely become the standard for fasteners used in challenging aircraft areas, a result of their exceptional strength-to-weight ratios. However, in hot sections near engines or other assemblies, Superalloys like Inconel 718 are typically better suited to maintain tensile strength at temperatures exceeding 650°C.

Bearings for Load-Bearing and Motion-Critical Systems

Bearings in landing gear joints, flight control hinges, actuator interfaces, and other such areas are tasked with simultaneously supporting controlled motion and handling substantial loads. Aside from these basic functions, they may also serve to prevent excessive friction and stress transfer into surrounding elements. Different bearing types suit specific load and motion characteristics, with the most common choices for aviation including:

• Spherical Plain Bearings: Spherical plain bearings are used in locations where angular misalignment or structural flexure is expected, as they allow multi-directional rotation without binding.
• Needle Roller Bearings: Needle roller bearings are favored in sections where space is limited but radial load demands are high, such as in flap tracks and compact actuator housings, as their elongated rollers increase contact area and cause less concentrated contact stress.
• Ball Bearings: Ball bearing are efficient at handling radial and axial loads with low friction and great accuracy, provided that space and lubrication requirements can be met.

When it comes to materials, bearings placed in areas that face exposure to moisture, temperature extremes, hydraulic fluids, fuel, or de-icing chemicals are typically manufactured from hardened or vacuum-melted alloy steels. The vacuum melting process is important, as it reduces non-metallic inclusions that could otherwise initiate fatigue damage under repeated loading. Additionally, many aircraft designs might specify the use of self-lubricating bearings with PTFE-based liner materials, as they are chemically inert to common aviation fluids and remove the need for conventional greased bearings that may be impractical or prone to contamination.

Structural Hardware for Demanding Areas

While fasteners and bearings carry the majority of applied loads, other forms of structural hardware are necessary to add a layer of assurance that forces will be properly distributed and aligned to isolate wear from vital components. The categories of hardware that are routinely utilized for high-stress interfaces in aircraft assemblies include:

• Spacers and Sleeves: Spacers and sleeves achieve separation between structural members and control clamp-up during fastener installation, ensuring that stressors are transmitted through the intended load path, rather than inducing bending or localized crushing forces.
• Pins: Various forms of pins help to keep structures aligned while resisting the shear loads caused by vibration or other dynamic forces, helping to mitigate fretting, hole elongation, and other forms of joint degradation.
• Washers: Placed beneath bolt heads or nuts, washers prevent localized deformation under high clamp loads.
• Threaded Inserts: Threaded inserts reinforce tapped holes in softer materials, allowing fasteners to be installed, removed, and reinstalled without degrading the parent structure.

In applications where relative motion, vibration, or repeated loading is present, these components are often specified as sacrificial wear elements so any fretting or surface damage occurs to a lower-tier, replaceable item, instead of the primary structure or fasteners. Due to this express role, materials for certain pieces of hardware may be selected based on controlled hardness relationships.

For instance, sleeves and washers might be manufactured from alloys that are softer than the steel pins or bolts they interface with, yet still sufficiently durable to resist deformation and abrasion when installed in aluminum or composites. Furthermore, if any piece of hardware is likely to be exposed to high temperatures, it must exhibit a coefficient of thermal expansion that is compatible with all surrounding materials to avoid unintentionally increasing fastener loads and causing premature issues.

Find High-Caliber Aircraft Fasteners, Bearings, and Hardware on ASAP Aerospace

Although we only covered a few basic pointers for selecting fasteners, bearings, and structural hardware, the takeaway we wish to impart is to always source aerospace-grade options that are carefully matched to the distinct stressors they are expected to encounter. Luckily, much of the decision-making may be taken off your shoulders seeing as variants, dimensions, and other such details for parts are often noted in the Aircraft Maintenance Manual (AMM), especially for demanding areas. However, even the most carefully specified component can become a liability if it is sourced improperly.

To aid in your procurement endeavors, ASAP Aerospace offers leading procurement options on countless forms of fasteners, bearings, and hardware that come from trusted manufacturers and meet strict aviation standards. Aside from serving as a one-stop shop for all your needs, our competitive advantage stems from our dedication to quality, affordable pricing, and prompt order fulfillment. Bearing all of this in mind, be sure to peruse our selection and connect with our expert team at your convenience.