Bolt thread accuracy grade and application
- What are the common thread accuracy classes for railway bolts?
Common accuracy classes are 6g (external thread) and 6H (internal thread), which are medium accuracy and meet most railway connection needs. High-speed railway key parts (such as turnouts) use 5g/5H high-precision classes, with smaller thread tolerance zones and 30%-40% smaller fitting gaps than 6g/6H. 7g/7H classes can be used for secondary parts of ordinary railways, with lower cost but slightly poor interchangeability.
- How does thread accuracy class affect preload?
The preload fluctuation range of 6g/6H threads is ±8%, 5g/5H can be controlled within ±5%, and 7g/7H reaches ±12%. Low-precision threads (7g/7H) have large tooth profile errors, resulting in significant preload deviation under the same torque, easily causing some bolts to be overloaded and others to loosen. High-precision threads ensure uniform preload and reduce connection failure risks.
- What are the differences in thread processing technology for different accuracy classes?
5g/5H threads must be processed with precision thread rolling machines, with rolling wheel accuracy one level higher than 6g, processing speed reduced by 20% to ensure tooth profile error ≤0.02mm. 6g/6H threads can be processed with ordinary thread rolling machines, with tooth profile error controlled within 0.03mm. 7g/7H threads can use thread rolling processes, with high efficiency but low precision, allowing tooth profile error ≤0.05mm. After processing, thread gauges must be used for inspection; 5g/5H requires 100% go-no-go inspection.
- What is the relationship between thread accuracy and bolt corrosion resistance?
High-precision threads (5g/5H) have smoother surfaces, with roughness Ra≤1.6μm, which are less likely to accumulate moisture and corrosive media than 7g/7H (Ra≤3.2μm). The zinc coating adheres more uniformly, extending corrosion resistance life by 20%-30%. Low-precision threads have rough surfaces, leading to uneven zinc coating thickness and local corrosion, especially in coastal humid environments.
- How to select thread accuracy classes for bolts?
High-speed railways and heavy-haul railway key connections (such as turnouts) use 5g/5H classes to ensure stable preload and long-term reliability. 6g/6H classes are used for ordinary railway straight sections, balancing performance and cost. 7g/7H classes can be used for temporary repairs but must be replaced with 6g/6H within 3 months to avoid long-term loosening.