Bolt Anti-Loosening and Anti-Corrosion Design
- What are the strength differences and application scenarios of 4.8-grade, 8.8-grade, and 10.9-grade bolts?
4.8-grade bolts have a tensile strength ≥400MPa and a yield strength ≥320MPa, mostly made of Q235 steel, suitable for sleeper bolts and pressing plate bolts in ordinary railways, bearing small loads with low cost; 8.8-grade bolts undergo quenching and tempering, with a tensile strength ≥800MPa and a yield strength ≥640MPa, suitable for elastic clip fastener bolts and fishplate bolts in high-speed railways, capable of withstanding high-frequency vibrations; 10.9-grade bolts have a tensile strength ≥1000MPa and a yield strength ≥900MPa, made of alloy steel, suitable for fishplate bolts and spike bolts in heavy-haul railways, resisting tension generated by large axle loads and avoiding bolt breakage.
- What are the common anti-loosening methods for bolts, and what are their advantages and disadvantages?
Anti-loosening nuts achieve anti-loosening through double-thread cooperation between nuts and bolts, with the advantage of reusability and easy installation and disassembly, but the disadvantage is that the anti-loosening effect is greatly affected by the tightening torque, and may still loosen under severe vibration; thread glue (anaerobic glue) achieves anti-loosening through adhesive force formed by glue curing, with the advantage of long-lasting anti-loosening effect and waterproof and anti-corrosion properties, but the disadvantage is non-reusability, requiring residual glue cleaning and re-gluing after disassembly; elastic washers achieve anti-loosening through pre-tightening force generated by washer elastic deformation, with the advantage 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.
- How to select anti-corrosion treatment methods for bolts in different environments?
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, requiring only regular anti-rust paint touch-ups; Dacromet treatment (coating thickness 5-10μm) is required in coastal humid areas, with salt spray resistance of more than 500 hours, capable of resisting seawater corrosion, with a maintenance cycle of 3-5 years; stainless steel bolts (304 material) are recommended in chemical-polluted areas, with acid and alkali corrosion resistance and a service life of more than 10 years, but the disadvantage is high cost, only used in special corrosive environments; bolts in alpine areas need to balance anti-corrosion and low-temperature toughness, and a combination of galvanizing + anti-rust grease can be used to prevent low-temperature rust and brittle fracture.
- How to judge whether bolts have fatigue loosening, and what are the detection methods?
Regularly use a torque wrench to sample and test the bolt torque. If the torque decreases by more than 15% from the initial value, it indicates that the bolt may be fatigued and loose, requiring re-tightening and observation of repeated loosening; simulate train operation vibration through vibration testing (frequency 10-50Hz). If the bolt torque attenuation rate exceeds 20% after vibration, it is determined as fatigue loosening, and the bolt needs to be replaced; the gap between the bolt head and the connected part can also be checked. If there is an obvious gap (>0.2mm), it indicates that the bolt has undergone plastic deformation and cannot provide sufficient pre-tightening force, which is a case of fatigue loosening and needs to be replaced in a timely manner.
- To what extent does a bolt need to be replaced when rusted, and how to judge the rust grade?
Bolt rust grades can be divided into 4 levels: Grade 1 is slight surface rust with no obvious rust pits, which can be treated by rust removal + anti-rust paint without replacement; Grade 2 has obvious rust pits on the surface, but the rust depth ≤0.1mm and the thread is still intact, which can be treated by rust removal, replacement of anti-loosening nuts, and enhanced monitoring; Grade 3 has a rust depth >0.1mm, decreased thread accuracy, and a bolt diameter reduction >1%, requiring replacement; Grade 4 has obvious rust penetration or fracture risk of the bolt, which must be replaced immediately regardless of the rust depth. During judgment, a vernier caliper can be used to measure the change in bolt diameter, combined with the depth of surface rust pits, to determine the rust grade, avoiding safety accidents caused by reduced bolt strength due to rust.