Thread matching accuracy of bolts and nuts
- What is the thread matching accuracy standard for bolts and nuts?
National standard 8.8 grade bolts and nuts use 6H/6g matching (internal thread accuracy 6H, external thread 6g), pitch deviation ±0.02mm, tooth angle deviation ≤0.5°. 10.9 grade high-strength bolts use 5H/5g matching, with stricter deviation control to ensure that the preload loss is ≤5%.
- What are the effects of insufficient thread matching accuracy?
A pitch deviation exceeding 0.03mm will cause poor thread engagement, accelerated preload attenuation, and a loss of 20% within 1 month; a tooth angle deviation exceeding 1° will cause local stress concentration and increase the risk of bolt breakage by 30%, especially in heavy-duty railways.
- How to detect thread matching accuracy?
Use a thread gauge to detect the tooth profile, a pitch gauge to measure the pitch, and a three-needle measurement method to measure the mean diameter deviation. Each pair of bolts and nuts needs to be tested for meshing, and the torque fluctuation during the tightening process is measured with a torque wrench. If the fluctuation exceeds 5%, it is judged as mismatched.
- What are the production measures to improve the thread matching accuracy?
Bolts use rolling thread technology, which is 2 levels higher than cutting threads; nuts are precisely processed by tapping machines to ensure the consistency of internal threads. During production, the same batch of bolts and nuts are processed in pairs to avoid the accumulation of deviations from different batches.
- Are there any differences in the matching accuracy requirements for bolts and nuts of different strength grades?
4.8-grade ordinary bolts can be matched with 6H/6g; 8.8-grade bolts need to be controlled at 6H/6g but the deviation range is reduced by 50%; 12.9-grade high-strength bolts must be matched with 5H/5g, and 100% meshing detection is required to ensure that each pair meets the standard.