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Reason for continuous thread failure
The water screw invented by Archimedes (287-212 BC) led to the development of the threaded fasteners commonly used for all joining technologies nowadays. The threaded assemblies today are the most important detachable parts used in mechanical components, installation and repair. Since people are very familiar with such applications, the function has never been analyzed in sufficient depth. The two major causes for the failure of a threaded assembly are: Relaxation and Self-loosening
 
Relaxation
A threaded assembly “relaxes” and the bolt tension is reduced when a permanent change in the axial length of bolt occurs or the attached object relaxes on a heated gasket surface, which also reduces the residual gripping force. The following factors generate permanent changes in length:
Settling: The rough surfaces of the contiguous parts (e.g. nut, washer) become smoother under the pressure of bolt tension.
Creeping: The surface pressure on the supporting surface of bolt or nut exceeds the compressive strength of the material.
Preventing relaxation
If the elasticity of the assembly can be increased so that the expected amount of settling and creeping can be compensated, then basically the reduction of pre-stress can be prevented and this can be achieved via the following method:Bolts with high i/d ratio (i = bolt length; d = bolt diameter);
Collar bolts, collar nuts, quenched and tempered washers can reduce the surface pressure and further reduce the settling of supporting surface;
Bolts and nuts carrying washers with pressed spring or concave supports;and
Washers with rigid conical spring or cup spring.
 
Self-loosening
When slippage occurs between the contact surfaces, self-loosening of the threaded assembly starts, overcoming the friction within the assembly; this further degrades the thread locking mechanism. Such rotation of ML moment that loosens the assembly can only be overcome with sufficient gripping force.
Where
ML = Moment of self-loosening torque ρ = Friction angle of the thread
FV = Effective pre-stress μA = Coefficient of friction for the supporting surface
d2 = Pitch diameter of the thread ra = Lever arm of friction for the supporting surface
ψ = Helix angle of the thread  
If the binding of the threaded assembly cannot stop the relative rotation between the exerted parts, the bolt rotates; the thread flanks slide past each other and the bolt is no longer frictional. The moment of loosening torque is then presented as:
The moment of loosening torque Mi is only determined by the pre-stress, pitch diameter of the thread and helix angle of the thread. The direction of action is opposite to the tightening direction and loosens the threaded assembly. The following factors may cause slippage between contacting surfaces:
Axial dynamic load: The axial overload of pulse causes the relative rotation of the flanks.
Dynamic load perpendicular to axial load: The difference in thermal expansion rate, ejection or vibration of bending material can overcome the friction between supporting parts.
 
Preventing self-loosening
The following measures can prevent free loosening of the bolt under adequate load:
Use highly tensile bolts since the greater pre-stress is sufficient to prevent relative rotation.
Increase the i/d ratio ((i = bolt length; d = bolt diameter) to enhance the elasticity of the assembly (preferably with i/d ratio not less than 6 according to experience).
The surface finish and structure of the supporting surfaces of the bolts and nuts can increase the friction.
Use adhesive to thoroughly fill the gap for eliminating access of horizontal rotation and enhancing the thread friction with interface connection after the adhesive is cured.
Form a reliable connection (e.g. adequate size of bolt and welding spot) to limit the thread slippage.

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