Why Do Bolts Become Loose

I. Main Causes of Bolt Loosening
Insufficient Tightening

Bolts with insufficient tightening or pseudo-tightening inherently lack adequate preload. If further loosening occurs, the joint will not possess sufficient clamping force to secure components together. 

This may trigger lateral sliding between mating parts, subjecting the bolt to undesired shear stress and potentially resulting in bolt fracture in the end.

Vibration

Tests on bolted joints under vibration indicate that numerous minor lateral movements cause relative displacement between the two connected components, 

as well as movement between bolt heads/nuts and clamped members.

Such repeated motion offsets friction between bolts and connected parts. Eventually, vibration leads to rotational loosening of bolt threads, and the joint loses clamping force.
Embedment
Engineers who design bolt tension allow a running-in period accompanied by certain preload loss, during which bolt tightness relaxes.

This relaxation arises from embedment at contact surfaces including bolt heads, nuts, threads and mating faces of connected components.

It occurs both on soft materials (such as composites) and hard polished metals.If the joint is improperly designed or the bolt fails to reach specified tension upon initial tightening, 

embedment may cause clamping force loss such that the required minimum clamping force cannot be maintained.Microscopic irregularities exist on mating surfaces. Under the action of bolt preload after tightening, 

surface asperities are crushed and permanent plastic deformation takes place. This reduces the bolt clamping length and ultimately leads to declining bolt preload.

Gasket Creep and Thermal Expansion

Many bolted joints are fitted with thin, soft gaskets between bolt heads and joint surfaces to seal connections and prevent gas or liquid leakage. 

The gasket itself acts like a spring and rebounds under compression from bolts and mating surfaces.

Over time, especially when exposed to high temperatures or corrosive chemicals, the gasket may creep, meaning it loses elasticity and causes clamping force attenuation.

If bolts and joint components are made of dissimilar materials, drastic temperature fluctuations caused by rapid environmental changes or cyclic industrial operation will lead to rapid thermal expansion or contraction of bolt material, 

which may loosen the bolts.

Impact
Impact refers to the condition where large impact loads exceed the friction generated by bolt preload and induce sliding.

Dynamic or alternating loads from machinery, generators, wind turbines and other equipment can create mechanical shocks. 

Impact forces applied to bolts or joints trigger relative sliding of bolts.Similar to vibration, such sliding will eventually loosen bolts. Frequently, 

such large loads are not taken into consideration during joint design.II. What is Preload?This term carries multiple definitions in engineering. 

One definition is the tension (load) generated in a fastener upon initial tightening. As the bolt elongates, components sandwiched between the bolt and nut are compressed, 

establishing the so-called clamping load until tightening is completed.III. Hazards Caused by Loose Bolts

Flange leakage
Separation and falling-off of wind turbine rotor from nacelle
Falling-off of vibration-induced connecting bolts on marine engines

Falling-off of connecting bolts on marine engines, aggravated by vessel rolling, will cause further damage to equipment.In bolted connections, 

tightening the nut stretches the bolt, similar to pulling a spring. This tensile force generates a counteracting clamping force to firmly fasten two parts of the connected assembly.

If the bolt loosens, the clamping force diminishes.Loose bolts are far more than a trivial nuisance. If the joint is not retightened promptly, 

liquid or gas leakage may occur, bolts may fracture, equipment may be damaged, or catastrophic accidents may happen.IV. Conclusion“The best anti-loosening measure is to ensure sufficient preload to avoid joint sliding, 

separation and other defects.”As demonstrated in the above analysis, three types of loosening mechanisms stem from insufficient or declining preload. Therefore, special control over bolt preload is required to mitigate loosening risks.

Provided the preload meets requirements and the clamping length is not excessively short (e.g., \(L_k≥3d\)), bolts generally will not self-loosen even under certain vibrational loads.

Well-designed bolted joints combined with proper establishment of clamping force and appropriate bolt anti-loosening devices can reliably secure bolted connections against 

many loosening challenges mentioned above.A well-designed bolted joint adopts appropriately sized and typed bolts and nuts, 

with specified optimal tension to deliver the clamping force necessary for maintaining joint integrity.For practical applications, 

adequate clamping force requires the tension (preload) inside each bolt to reach the correct level and sustain this level throughout its service life.Hence, 

maintaining appropriate tension in bolts is critical. Ultrasonic technology can be adopted during design to measure bolt axial force and ensure bolt preload complies with design specifications.

The Zimao ultrasonic bolt axial force measuring instrument is specially developed for measuring bolt axial force. It also supports QR code function to realize real-time traceability of axial force data for individual bolts.