Bolt Thread Accuracy and Connection Reliability
- How are the thread accuracy grades of bolts divided? What are the characteristics of each grade?
Bolt thread accuracy is divided into grade 3, 4, and 5 according to GB/T197, with smaller numbers indicating higher accuracy. Grade 3 precision threads have a tooth profile error and pitch error of ≤0.02mm, surface roughness Ra≤1.6μm, small fitting clearance, suitable for key connections in high-speed and heavy-haul railways. Grade 4 precision threads have an error of ≤0.05mm, roughness Ra≤3.2μm, used for general force-bearing connections, such as fishplate connecting bolts. Grade 5 precision threads have larger errors, suitable for temporary connections with small stress, such as auxiliary bolts for sleeper fixing.
- What impact does thread accuracy have on the preload of bolt connections?
Grade 3 precision threads fit closely with nuts, with small preload loss. The preload loss rate is only 5%-8% in a vibrating environment for 1000 hours. Grade 4 precision threads have a preload loss rate of 10%-15%, requiring regular re-tightening. Grade 5 precision threads have large gaps, so preload loss is fast, with a loss rate of more than 20%, and are prone to loosening in long-term use. For bolts of the same specification, grade 3 precision can increase the initial preload by about 15% compared to grade 5 precision, and maintain more stability.
- How to test the thread accuracy of bolts?
Use thread gauges for testing. The go gauge can be smoothly screwed in without obvious looseness, and the no-go gauge screwed in no more than 2 threads is qualified. Use a thread micrometer to measure the thread pitch diameter. A deviation within ±0.03mm is grade 3 precision, and within ±0.05mm is grade 4 precision. For high-precision bolts, a coordinate measuring machine is needed to detect parameters such as pitch and thread angle to ensure that all errors meet the grade requirements. The testing frequency is 5% sampling of each batch of bolts, and no less than 10 pieces.
- What are the different requirements for bolt thread accuracy in different connection scenarios?
High-speed railway turnout connecting bolts need to use grade 3 precision threads to ensure stable connection under high-frequency vibration and avoid turnout displacement. Grade 4 precision threads are used for fishplate connections in ordinary railways, which can meet daily load requirements with relatively low cost. Grade 5 precision threads can be used for fixing bolts between sleepers and ballast, due to small stress and short maintenance cycle. For coastal railways with high anti-corrosion requirements, the bolt thread accuracy needs to be increased by one grade, such as grade 3 instead of grade 4, to reduce corrosive media entering the gap.
- What connection problems can insufficient thread accuracy cause?
Insufficient thread accuracy will cause excessive fitting clearance between bolts and nuts, resulting in relative sliding under vibration loads, leading to rapid attenuation of preload and loose connections. Uneven clearance will cause local stress concentration, making thread teeth wear or break prematurely, reducing the service life of bolts. In addition, threads with insufficient accuracy are prone to "seizing", making installation and disassembly difficult and increasing maintenance costs. In severe cases, it will lead to connection failure, such as loosening of fishplate bolts causing rail joint misalignment, endangering driving safety.