Bolt Selection: Strength Grade and Tightening Standards
Why is the 10.9-grade bolt preferred for 27t axle load heavy-haul railways?
The 8.8-grade bolt has a tensile strength of 800MPa and is prone to deformation when subjected to a tensile force exceeding 60kN, failing to meet the heavy-haul requirements of 27t axle load. The 10.9-grade bolt has a tensile strength of 1000MPa and a yield strength of 900MPa, which can withstand greater impact loads and ensure connection stability. Bolts on heavy-haul lines need to withstand frequent vibration and impact; the 10.9-grade bolt has better hardness (32-39HRC), toughness, and fatigue resistance. The torque decay rate of this grade bolt is no more than 8% within one month, which is more stable than the 8.8-grade bolt, reducing maintenance frequency. The tightening torque of the 10.9-grade bolt needs to be controlled at 500-550N·m, 150-200N·m higher than that of the 8.8-grade bolt, providing more reliable clamping force.
What are the basis for determining the bolt tightening torque?
The bolt tightening torque must be determined according to the bolt strength grade; the higher the grade, the larger the torque value. For example, the torque of 8.8-grade bolts is generally 350-400N·m, and that of 10.9-grade bolts is 500-550N·m. It is necessary to consider the material and thickness of the connected parts; rigid components such as rails and fishplates require higher torque to ensure close fit. The line type is also an important basis: the bolt torque for conventional railways is 80-120N·m, for high-speed railways it needs to be increased to 100-150N·m, and for heavy-haul lines it is even higher. Environmental factors cannot be ignored; the bolt material becomes brittle in low-temperature environments, and the torque needs to be appropriately reduced by 10%, while maintaining the standard value in high-temperature environments. The torque must be accurately controlled with a torque wrench, applying force evenly in stages to avoid thread damage caused by one-time tightening.
What are the core requirements of the bolt tensile test?
The bolt tensile test shall be carried out in batches, with 3 samples taken from each batch, and the loading rate controlled at 5mm/min to ensure accurate test data. The breaking force of the 10.9-grade bolt must be not less than 120kN; if it fails to reach this value, the entire batch of products shall be scrapped. During the test, it is necessary to observe the deformation of the bolt; the deformation in the yield stage should be within the specified range without sudden brittle fracture. After the test, it is necessary to check the hardness change of the bolt to ensure that the hardness does not exceed the standard or decrease due to stretching. The tensile test shall be carried out in a standard environment (temperature 23℃±5℃, humidity 50%±10%) to avoid environmental factors affecting the results.
What are the common types and effects of bolt anti-loosening measures?
Anti-loosening washers are the most commonly used measure, generating continuous preload through the elastic deformation of the washer to prevent bolt loosening, suitable for conventional and heavy-haul lines. Thread locking adhesive technology applies thread locking adhesive to the thread surface, which fills the thread gap after curing, providing long-lasting anti-loosening effect, suitable for scenarios with strong vibration. Double nut anti-loosening locks each other through two nuts, using friction to prevent loosening, with high reliability but cumbersome installation and disassembly. Cotter pins cooperate with slotted nuts, mechanically locking the bolt by inserting the cotter pin into the nut slot and bolt hole, suitable for key parts. Different anti-loosening measures can be combined, such as anti-loosening washers + thread locking adhesive, which can further improve the anti-loosening effect and extend the maintenance cycle.
What are the common failure modes of bolts during service and preventive measures?
Common failure modes of bolts include thread wear, fracture, corrosion, and loosening, among which fatigue fracture accounts for the highest proportion, mostly caused by vibration and insufficient torque. Regularly inspect the bolt appearance; if the thread wear exceeds 30% or thread stripping occurs, replace it immediately. Treat the bolt surface with anti-rust coatings (such as sherardizing, dacromet); replace the bolt when the corrosion area exceeds 50%, and apply oil for protection after derusting for slight corrosion. Regularly review the tightening torque, conduct a comprehensive inspection every quarter, and re-tighten in a timely manner when the torque drops by more than 10%. Avoid overloading bolts, ensure the torque meets the standard during installation, do not arbitrarily increase the load, and prevent material fatigue.