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Items that increase loosening resistance:

1. Tighter hole tolerances.
2. Increased bolt preload (tension); increasing bolt diameter, yield strength, preload.
3. Smaller thread helix angle. Fine thread bolts. Fine thread lock nut
4. Minimize joint relaxation
5. Maintain large friction forces - high preload, don't lubricate threads and mating surface.
6. Increased surface contact.
7. Increase thread contact area - Class 3 (aerospace fasteners) fit instead of Class 2 (commercial) fasteners.
8. Galling susceptible materials such as uncoated stainless steel.
9. Corrosion susceptible materials such as uncoated and unlubricated steel
10. Minimize transverse movement of faying surfaces - increase joint friction by not lubricating joint surfaces.
11. Increased contact between mating surfaces - smooth surfaces.
12. Nylon lock nut instead of all-metal lock nut to dampen resonant vibrations.
13. Aircraft lock nut instead of commercial grade lock nut.
How to prevent vibration induced loosening of bolts and nuts
MS21044 nylon lock nut
steel lock nut
Experimental Analysis of Thread Movement in Bolted Connections Due to Vibrations,
Auburn University, Prepared for George C. Marshall Space Flight Center Research Project NAS8-39131, March 1995
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