Bolt Loosening Prevention Measures and Installation Specifications
- What are the common anti-loosening measures for bolts?
Common anti-loosening measures for bolts include: using lock nuts, which have a certain interference with the bolt threads to increase friction; adopting double nut locking, first tightening the main nut, then the secondary nut, which exerts a reverse force on the main nut; applying thread locking adhesive (such as Loctite 243), which forms a film after curing to prevent loosening; installing spring washers, which use the elastic deformation of the washer to generate continuous pressure to prevent bolt loosening; for important parts, using a combination of cotter pins and slotted nuts for mechanical locking to prevent loosening.
- What are the inspection contents before bolt installation?
Check the appearance of the bolt, which must be free of cracks, thread damage and rust, and the thread accuracy should meet the 6g level requirements in GB/T 197; measure the diameter and length of the bolt, with deviations within ±0.2mm to ensure matching with nuts and mounting holes; check the hardness of the bolt, 10.9-grade bolts should have a hardness of HRC32-39, and 8.8-grade bolts should have a hardness of HRC22-32. Bolts with unqualified hardness must not be used; clean the debris and oil in the threaded holes of bolts and nuts to ensure smooth installation.
- How to determine the preload of bolts?
The bolt preload is determined according to its strength grade and diameter. The preload of 8.8-grade M24 bolts is 200-250N·m, and that of 10.9-grade M24 bolts is 300-350N·m; for bolts bearing alternating loads (such as fishplate bolts), the preload should be 60%-70% of their yield strength to reduce the impact of load fluctuations on the bolts; in low-temperature environments (below -20℃), the preload needs to be increased by 10%-15% to compensate for the strength reduction caused by the low-temperature brittleness of the material; the preload must be precisely controlled with a torque wrench, and the deviation must not exceed ±5%.
- What are the requirements for the order of bolt installation?
When installing multiple bolts, the principle of symmetry and uniformity should be followed. For example, fishplate bolts should be tightened from the middle to both ends in sequence, with each tightening torque being 50% of the final torque, reaching the specified preload in 2-3 times; it is not allowed to tighten from one end to the other in sequence to avoid uneven stress and deformation of the component; when installing bolts for flange connections, tighten them diagonally in clockwise or counterclockwise order to ensure uniform stress on each bolt; after each tightening, check the torque of adjacent bolts to prevent preload changes due to mutual influence.
- What are the acceptance criteria after bolt installation?
After bolt installation, recheck the preload with a torque wrench, the qualification rate should be ≥95%, and the preload deviation of each bolt must not exceed ±10%; the bolt head should be close to the surface of the connected part, with a gap ≤0.3mm and no skew; the exposed length of the thread should be 2-3 pitches, and both too long and too short need to be adjusted; the anti-loosening measures are effective, such as lock nuts without loosening, cotter pins fully inserted into the grooves and opened, and spring washers flattened without breaking.