Fasteners_02_Design

Fastener Design Considerations
  • When fastener procurement is not a consideration, then fasteners should be designed and manufactured. The following are some design considerations for fasteners (specifically nuts, bolts, and rivets)
  • Design of nuts, bolts, and rivets may at first appear like a trivial task, but many considerations must be taken into account. These considerations range from, but are not limited to, shock, vibration, shear, temperature, corrosion, fatigue, stress load (axial, radial), etc.
  • Materials: Most bolts are made from carbon steel, alloy steel, or stainless steel
  • Coating: For most aerospace applications, cadmium coating is used for carbon and alloy steel. Coating is normally not required for stainless steel
  • Other coatings/platings used are zinc , phosphate, nickel, chromium.
    • They are many more available coatings/platings. Usage usually depends on environments and cost
  • Carbon and Alloy steel is generally not used since it becomes brittle at -65° F
  • Series 400 stainless steel contains a low amount of chromium which can lead to corrosion
  • Always remember that contact of dissimilar materials can cause galvanic corrosion (more information in the appendix)
  • Stress corrosion is also possible. This ocurrs when a tensile stressed part is placed in a corrosive environment
  • Lubricants: Used to coat threads. Normally are oil, grease, wax, graphite, or molybdenum disulfide
  • Embrittlement: When free hydrogen is present near metal, embrittlement of the metal may occur. Bake out per MIL-N-25027 will prevent this issue from occurring
    • Cadmium embrittlement is also a concern
  • Locknuts: Used to bind the nut thread to the bolt thread. In aerospace applications, the deformed thread locknut is common due to ease of manufacturing, reuse and use in a high range of temperatures
    • Many other locking nuts are available for use
  • Locking Adhesives, such as Loctite may be used.
    • Note: Adhesives have little resistance to temperatures even as low as 200°F
  • Washers: Generally used to ensure a uniform tension load on the bolt
  • Inserts: Inserts are threaded on the inside diameter and locked with threads on the outside diameter.
    • Generally used inside tapped holes in order to utilize small high strength fasteners to save weight (works like a wall anchor)
  • Threads: Thread information can be found in the supplemental information entitled “Thread and End Connection Identification Guide”
  • Rivets: Low-cost, permanently installed fasteners that are lighter and bolts
  • Lockbolts: High strength fastener that has either a swaged collar or a type of threaded collar to lock it in place
  • As always, be aware of counterfeit parts! This includes materials in addition to parts. Improperly manufactured parts and materials can cause failures. These failures may be mission critical.

Fastener Installation
  • Fastener Torque: determining the proper torque is the most difficult problem in fastener installation
    1. The coefficient of friction between mating threads
    2. The coefficient of friction between the bolthead (or nut) and its mating surface
    3. The effect of bolt coatings and lubricants on the friction coefficients
    4. The percentage of bolt tensile strength to be used for preload
    5. Once agreement is reached on item 4, how to accurately determine this value
    6. Relative spring rates of the structure and the bolts
    7. Interactions formulas to be used for combining simultaneous shear and tension loads on a bolt (should friction loads due to bolt clamping action be included in the interaction calculations?)
    8. Whether "running torque" for a locking device should be added to the normal torque
  • Torque tables such as listed in the appendix may be used. Alternatively, the following equation may be used for quick calculations:

\( K = \left( \frac{d_m}{2d} \right) \frac{\tan \psi + \mu \sec \alpha}{1- \mu \tan \psi \sec \alpha}+0.625 \mu_c \)

Where,

\( d_m = \) thread mean diameter
\( \psi = \) thread helix angle
\( \mu = \) friction coefficient between threads
\( \alpha = \) thread angle
\( \mu_c = \) friction coefficient between bolthead (or nut) and clamping surface