Bolts are getting tighter and tighter? Revealing the reasons and the key role of thread anti-loosening

Bolts are getting tighter and tighter? Revealing the reasons and the key role of thread anti-loosening

In daily life, we often encounter situations where bolts are getting tighter and tighter, especially in some equipment that needs to be frequently disassembled or in special environments. This phenomenon not only brings great inconvenience to maintenance and disassembly, but may also cause equipment failure. So, why do bolts become tighter and tighter? At the same time, what important role does thread anti-loosening play? This article will explore from multiple aspects.

1. Reasons why bolts are getting tighter and tighter

(I) The role of friction

During the tightening process of bolts, large friction will be generated between the threads. When the bolts are tightened, the contact surface between the threads will be under pressure, and this pressure will cause slight deformation and wear on the thread surface. Over time, these slight deformations and wear will gradually accumulate, resulting in a tighter fit between the threads and further increase in friction. Therefore, when we try to tighten the bolts again, we need more force to overcome this friction, giving people a feeling that the bolts are getting tighter and tighter.

(II) Elastic deformation of materials

Bolts and nuts will undergo elastic deformation during the tightening process. When the tightening force reaches a certain level, the materials of the bolts and nuts will undergo a slight elastic deformation, which will reduce the gap between the threads. This elastic deformation is irreversible during the tightening process, so the bolts will feel tighter and tighter in subsequent use.

(III) Environmental factors

Humid environment: In a humid environment, a layer of water film is easily formed on the surface of the bolts and nuts. The moisture in the water film will react chemically with the metal on the surface of the thread to generate oxides or corrosion products. These corrosion products will fill in the thread gap, making the fit between the threads tighter, causing the bolts to be tightened more and more.

High temperature environment: High temperature will cause the materials of the bolts and nuts to expand thermally. When the temperature rises, the gap between the threads will become smaller and the friction will increase. In addition, high temperature will accelerate the oxidation process on the thread surface, forming a dense oxide film, which will further increase the bite force between the threads.

(IV) Improper assembly

Excessive preload: During the assembly process, if the preload of the bolt is too large, the stress between the threads will exceed the elastic limit of the material, causing the threads to undergo plastic deformation. This deformation will make the fit between the threads tighter and increase the friction, which will cause the bolts to tighten more and more.

Insufficient lubrication: If the bolts are not properly lubricated during assembly, the friction coefficient between the threads will increase significantly. During the tightening process, high friction will cause the surface temperature of the threads to rise, accelerate the wear and deformation of the thread surface, and eventually make the bolts tighten more and more.

2. The role of thread anti-loosening

(I) Prevent loosening

The main function of thread anti-loosening is to prevent the bolts from loosening due to factors such as vibration, impact or temperature changes during use. In many mechanical equipment and structures, the reliability of bolt connections is crucial. If the bolts are loose, it may cause unstable equipment operation and even cause safety accidents. By taking effective anti-loosening measures, it is possible to ensure that the bolts maintain a stable connection under various complex working conditions.

(II) Extend service life

Anti-loosening measures can effectively reduce the wear and deformation of the bolts during use. By reducing the friction and stress concentration between the threads, the service life of the bolts can be extended, the frequency of repairs and replacements can be reduced, and the maintenance cost of the equipment can be reduced.

(III) Improve safety

In some key parts, such as aerospace, automobile engines, bridges, etc., the loosening of bolts may lead to serious consequences. By adopting reliable anti-loosening measures, the safety of the equipment can be significantly improved and its reliability in long-term operation can be ensured.

(IV) Reduce maintenance workload

Anti-loosening measures can effectively reduce the loosening and damage of bolts during use, thereby reducing maintenance workload. In some parts that are difficult to disassemble or maintain, the use of anti-loosening measures can avoid frequent disassembly and inspection, and improve the operating efficiency of the equipment.

III. Common thread anti-loosening methods

(I) Mechanical anti-loosening

• Spring washer: Spring washer is a commonly used mechanical anti-loosening device. It uses the elastic deformation of the washer to generate a reverse force to offset the loosening tendency of the bolt during vibration.

  
  

• Double nut locking: Double nut locking is a simple and effective anti-loosening method. By tightening two nuts on the bolt, a mutual squeezing force is generated between the two nuts, thereby preventing the nut from loosening.

  
  

• Stop washer: The stop washer bites the nut and the connection surface through its serrated edge to prevent the nut from loosening during vibration.

(II) Chemical anti-loosening

• Thread locking glue: Thread locking glue is a chemical anti-loosening material. It prevents the nut from loosening by forming a sticky film on the surface of the thread to increase the friction between the threads.

  
  

• Anti-loosening coating: Anti-loosening coating can form a protective film on the surface of the thread to prevent oxidation and corrosion of the thread surface, while increasing the friction between the threads.

(III) Structural anti-loosening

• Self-locking nut: The self-locking nut forms a conical locking surface inside the nut through its special structural design. When the nut is tightened, the conical locking surface will tightly bite the thread of the bolt to prevent the nut from loosening.

  
  

• Nylon insert locking nut: The nylon insert locking nut has a piece of nylon material embedded inside the nut. When the nut is tightened, the nylon material will expand and tightly bite the thread of the bolt to prevent loosening.

IV. Summary

The phenomenon of the bolt being tightened more and more is mainly due to the combined effect of multiple factors such as friction, material deformation, environmental factors and improper assembly. The thread anti-loosening measures effectively prevent the bolt from loosening during use through various methods such as mechanical, chemical and structural methods, extend the service life of the bolt, and improve the safety and reliability of the equipment. In practical applications, the appropriate anti-loosening method should be selected according to the specific working conditions to ensure the stability and reliability of the bolted connection.