Causes and control measures of bolt preload attenuation
- What is the main cause of bolt preload attenuation?
The relaxation of the thread pair is the main cause. The threads of bolts and nuts will produce slight plastic deformation under load, leading to a gradual decrease in preload, especially in a vibrating environment. The creep phenomenon of bolt materials, the material deforms slowly after long - term stress, resulting in preload attenuation. High - temperature environments will accelerate this process, such as when the rail temperature rises in summer, the attenuation speed accelerates.
- Under different environmental conditions, how does the bolt preload attenuation speed differ?
In a humid environment, bolts are easy to rust, the thread friction increases, and the preload attenuation speed is 15% - 20% faster than in a dry environment. Rust may also cause bolts to seize and cannot be retightened. High - temperature environments (such as rail temperature exceeding 40℃ in summer) will soften the bolt material, and the preload attenuation rate is 10% - 15% higher than that at room temperature, while low - temperature environments have little impact on the attenuation speed, usually within 5%.
- How to detect the attenuation of bolt preload?
Use a torque wrench to regularly detect the actual torque of the bolt, calculate the current preload through the "torque - preload" conversion relationship (the torque coefficient needs to be known), and compare it with the initial value. For bolts in important parts, preload sensors can be installed to monitor and transmit data in real - time, and alarm in time when the attenuation exceeds the limit, which is more efficient and accurate.
- What is the relationship between the strength grade of bolts and preload attenuation?
High - strength bolts (such as grade 10.9) have better creep resistance than ordinary bolts (such as grade 8.8). Under the same conditions, the preload attenuation rate is 5% - 8% lower within one year. However, high - strength bolts have higher installation requirements. When the preload exceeds 75% of the yield strength, the attenuation speed will increase significantly, and there may even be a risk of fracture.
- What measures can be taken to effectively control bolt preload attenuation?
Use lock nuts or apply thread locking adhesive to increase the friction of the thread pair and slow down relaxation. Anti - loosening measures can reduce the attenuation rate by 20% - 30%. Regularly retighten the bolts, once every 3 months for ordinary railways, and once every 1 - 2 months for high - speed and heavy - haul railways. When retightening, use calibrated tools to ensure that the preload meets the standard.