Bolt Strength Grades and Anti-Loosening and Anti-Corrosion Design
- What are the differences in material and mechanical properties among 4.8-grade, 8.8-grade, and 10.9-grade bolts, and what are their application scenarios?
4.8-grade bolts are mostly made of Q235 steel, with a tensile strength ≥400MPa and a yield strength ≥320MPa. They have low strength and are suitable for sleeper bolts and pressing plate bolts in ordinary railways, bearing small loads with low cost; 8.8-grade bolts are made of 45# steel after quenching and tempering, with a tensile strength ≥800MPa and a yield strength ≥640MPa. They have good strength and toughness, suitable for elastic clip fastener bolts and fishplate bolts in high-speed railways, capable of withstanding high-frequency vibrations; 10.9-grade bolts are made of alloy steel (such as 35CrMoA), with a tensile strength ≥1000MPa and a yield strength ≥900MPa. They have high strength and excellent fatigue resistance, suitable for fishplate bolts and spike bolts in heavy-haul railways, resisting tension generated by large axle loads.
- What are the common anti-loosening methods for bolts, their advantages and disadvantages, and how to select a suitable anti-loosening method?
Anti-loosening nuts achieve anti-loosening through double-thread cooperation between nuts and bolts, with advantages of reusability and easy installation and disassembly, but the disadvantage is that the anti-loosening effect is greatly affected by the tightening torque, and they are easy to loosen under severe vibration, suitable for detachable connection parts in ordinary railways (such as pressing plate bolts). Thread glue (anaerobic glue) achieves anti-loosening through adhesive force formed by glue curing, with advantages of long-lasting anti-loosening effect and waterproof and anti-corrosion properties, but the disadvantage is non-reusability, requiring residual glue cleaning after disassembly, suitable for elastic clip fastener bolts in high-speed railways to cope with high-frequency vibrations. Elastic washers achieve anti-loosening through pre-tightening force generated by elastic deformation, with advantages of low cost and easy installation, but the disadvantage is weak anti-loosening effect, suitable for spike bolts in ordinary railways with small loads and vibrations. Selection should be based on line vibration intensity, maintenance frequency and cost: anti-loosening nuts or thread glue are preferred for high-frequency vibration lines, and elastic washers for simple connections.
- How to select anti-corrosion treatment methods for bolts in different environments, and what is the basis?
Hot-dip galvanizing (coating thickness 8-12μm) can be selected in dry inland areas, with low cost and a maintenance cycle of 1-2 years. The basis is that the corrosion in this environment is weak, and hot-dip galvanizing can meet basic anti-corrosion needs; Dacromet treatment (coating thickness 5-10μm) is required in coastal humid areas, with salt spray resistance of more than 500 hours. The basis is that seawater contains salt and has strong corrosion, and Dacromet can effectively resist salt spray corrosion; stainless steel bolts (304 material) are recommended in chemical-polluted areas, with acid and alkali corrosion resistance. The basis is that chemical environments contain acid and alkali media, ordinary anti-corrosion treatments are prone to failure, and stainless steel can withstand corrosion for a long time; bolts in alpine areas are selected with galvanizing + anti-rust grease. The basis is that grease can prevent moisture penetration at low temperatures, avoid bolt freeze-thaw rust, and improve low-temperature toughness.
- How to detect whether the tightening torque of bolts meets the requirements, and what problems will insufficient torque cause?
A torque wrench is used to sample and test the bolt torque regularly. The torque deviation of 4.8-grade bolts in ordinary railways is allowed to be ±15%, the torque deviation of 8.8-grade bolts in high-speed railways must be controlled within ±10%, and the torque deviation of 10.9-grade bolts in heavy-haul railways is ≤±8%. During detection, the torque wrench should be adjusted to the standard value, and torque should be applied slowly. When a "click" sound is heard, the torque is confirmed to be up to standard. Insufficient torque will lead to insufficient bolt pre-tightening force, unable to effectively fix components. During train operation, bolts are easy to loosen, which in turn causes rail displacement and gauge deviation. In severe cases, components may fall off, affecting driving safety.
- After bolts are rusted, how to judge whether they need to be replaced, and what are the rust treatment steps?
Rust grade 1 (slight surface rust, no rust pits): No replacement is needed, and rust is removed with sandpaper and then anti-rust paint is applied; grade 2 (rust pits on the surface, depth ≤0.1mm): For 4.8-grade ordinary bolts, they can be reused after rust removal; for high-strength bolts of grade 8.8 and above, strength testing is required, and they can be reused if the strength does not decrease; grade 3 (rust pit depth >0.1mm, decreased thread accuracy): Regardless of the bolt grade, replacement is required; grade 4 (risk of bolt rust penetration or fracture): Immediate replacement. Treatment steps: First, clean the debris on the bolt surface, then select grinding or replacement according to the rust grade, and finally perform anti-corrosion treatment (painting or galvanizing) on reusable bolts and re-tighten them according to the standard torque.