Threaded fasteners play an integral role in the operations of an aircraft. Though small in size, these fasteners are just as highly specified as any other aircraft component. There are various classifications that categorize types of fasteners and where they can be used on an aircraft.
Threaded aircraft bolts, screws, and nuts are classified by their thread series or threads per inch. The classifications include the American National Coarse (NC), American National Fine (NF), American Standard Unified Coarse (UNC), and American Standard Unified Fine (UNF).
The UNF thread series is commonly seen in the aerospace industry and designates a fastener that has better load carrying capacities and better torque locking than UNCs. They have a fit that is specific to tighter fastening and finer tension adjustments. Threaded fasteners are also categorized by “Class of Fit”— which designates the tolerance allowed during manufacturing of a threaded fastener. Aircraft bolts are usually a Class 3 (medium fit) and aircraft screws are usually a Class 2 (free fit).
The dual series— National and Standard Unified, differ in one important way when it comes to fine thread fasteners. NFs have a 1-inch diameter size, and thread specifications of 14 threads per inch. It’s labeling looks like this: (1-14 NF). UNFs specify 12 threads per inch, and its labeling is as follows: (1-12 UNF). Both are also categorized by the number of times the threads rotate around 1 inch of a given diameter; 4-48 thread means a ¼ inch diameter bolt with 48 threads in 1 inch of its length.
Overall, aircraft threaded fasteners must be able to hold aircraft parts under high loads without being riveted or welded. This requirement is due to the need for efficient airflow, and redundancy of aircraft parts. A fastener that has been too tightly secured to an airframe or is the incorrect fastener for the applied load and receiving hole, can cause severe breakdowns and weakening of materials over time. Selecting the appropriate threaded fasteners can streamline dismantling and replacement of aircraft parts that are frequently reassembled or replaced. As such, proper threaded fastener selection is a very important aspect of aircraft manufacturing and maintenance.
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