Basic knowledge of bolts

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Are bolt axial force and preload the same concept? 01. Bolt axial force and preload are not exactly the same concept, but they are related to each other to some extent. Bolt axial force refers to the tensile or compressive force generated in a bolt, which results from the action of torque and preload on the bolt. When the bolt is tightened, torque and preload act on the bolt, generating axial tensile or compressive forces, which is known as the bolt axial force. Pre-tensioning force is the initial tensile or compressive force applied before tightening the bolt. When the bolts are tightened, the preload generates axial tensile or compressive forces on the bolts, pressing the connected parts together. The magnitude of the preload is usually determined by torque or stretch amount. Therefore, the pre-tightening force is one of the reasons for the axial tension or compression force in bolts, and it is also one of the key factors in controlling such axial tension or compression force. The relationship between bolt preload and yield strength 02. There is a certain relationship between the preload of bolts and their yield strength. Pre-tension plays a very important role in the tightening and bonding of bolts; its magnitude should be sufficient to generate an axial tensile force in the bolts, thereby ensuring the tightness and safety of the connections. The yield strength of a bolt refers to the force at which it undergoes plastic deformation or failure when subjected to axial tensile stress. If the pre-tension force exceeds the yield strength of the bolt, the bolt may suffer permanent deformation or failure, resulting in the loosening or failure of the connection. WeChat official account “Mechanical Engineering Highlights” – the fuel station for engineers! Therefore, the preload of the bolts should be kept within an appropriate range – neither too high nor too low. It needs to be determined based on factors such as the yield strength of the bolts, material properties, the stress conditions of the connectors, and the operating environment. Generally, the bolt preload should be kept within the range of 70% to 80% of the bolt’s yield strength to ensure the safety and reliability of the connection. What is the yield strength of bolts? 03. The yield strength of bolts refers to the minimum force required for a bolt to undergo plastic deformation when subjected to axial tensile forces; it is usually expressed as a force per unit area (N/mm2 or MPa). When the tensile force acting on a bolt exceeds its yield strength, the bolt will suffer permanent deformation, meaning it cannot return to its original shape; meanwhile, the joint connected by the bolt may also become loose or fail. The yield strength of bolts is determined by factors such as the properties of the material and the manufacturing conditions. When designing and selecting bolts, it is necessary to choose bolts with sufficient yield strength based on factors such as the requirements of the connection and the working environment. At the same time, when tightening bolts, it is also necessary to determine the preload level based on the yield strength of the bolts, in order to ensure that they can withstand the working loads without experiencing excessive plastic deformation or failure. The effective period for applying molybdenum disulfide to the surface of bolts is 04 months. Applying molybdenum disulfide (MoS2) to the bolt surface can reduce the frictional resistance during tightening and loosening, thereby extending the bolt’s service life. However, the effective lifespan of a coating depends on many factors, such as the quality of the coating, the operating environment of the bolt, and the level of tightening force. Generally, the effective lifespan of a bolt’s surface coating is usually between 3 months and 1 year. If the bolts are exposed to humid or high-temperature environments for extended periods, or are frequently subjected to working loads such as vibration and impact, the effective lifespan of the coating may be reduced. Furthermore, if the coating is damaged or worn out during use, its effectiveness and lifespan will also be reduced. Therefore, when using coated bolts, it is necessary to regularly check the condition of the coating based on actual conditions, and re-coat or replace the bolts as needed. Furthermore, the type and quality of coating must also be selected based on factors such as the operating environment of the bolts and their functional requirements, to ensure that they achieve the desired performance and have a sufficient service life.