Preventing vibration induced bolt loosening

MS21045 Lock Nut comes in two styles -- spring beam (left) or elliptical beam (right)

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.


References:
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