Structural screws are stronger aircraft screws that conform to military standards. They have higher tensile strength requirements than non-structural screws and are used when strong fastening is needed. Structural screws are identified by markings on the head and have an unthreaded shank to prevent threads from acting as a shear plane. They also have modified thread designs and tighter tolerances that increase fatigue strength over standard screws. Common structural screw part numbers are provided.

1. Aircraft Structural Screws What are they and when to use? Brought to you by Mechanic's Toolbox Software for the Mechanic www.mechanicsupport.com

2. What are Structural Screws? Structural Screws are those screws that conform to MIl-S-7839B or latest version. Structural Screws are stronger and have more rigid manufacturing and inspection requirements than non-structural screws. Structural Screws are used where a stronger screw is required or when it is necessary to keep threads out of the shear plane.

5. Better Fatigue Strength Aerospace-fasteners must have high fatigue strength. Most use a modified thread called a "J" thread. The UNJ thread has a rounded root radius that increases thread strength. Failures in threaded fasteners often start as a crack at the thread root where the stresses are the highest. Rounding the thread root greatly increases fatigue strength. Aerospace fasteners also use a tigher tolerance (class 3 instead of class 2) than commercial fasteners, resulting in greater thread engagement. UNF UNJF

6. Screws used as Shear Pins Hole edges should not press against threads. Screws used as pins (subject to shear stress) should not have threads in the shear plane In the picture above, a fully-threaded screw has been used on an aircraft spinner. Screw threads against a vibrating surface acts like a saw and cuts away the hole boundry. A structural screw with unthreaded shank would have protected the hole boundry from wear.

7. Common Structural Screw Part Numbers AN525 Washer Head Screw fine thread AN502 Fillister Head fine thread AN503 Fillister Head coarse thread MS27039 Pan Head fine thread NAS517 100 degree countersink fine thread MS24694 100 degree countersink fine thread

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