Bolt Thread Accuracy and Connection Reliability
- What are the thread accuracy grades of bolts and how are they divided?
Bolt thread accuracy grades are mainly divided into precision grade (grade 3), medium grade (grade 4) and rough grade (grade 5), divided according to GB/T 197-2003. The accuracy grade is determined by the tolerance zone and the length of engagement. Grade 3 precision threads have small tolerance zones and high processing accuracy, suitable for occasions requiring tight fit and stable preload; grade 4 precision has medium tolerance and is the most widely used; grade 5 precision has large tolerance zones and low processing requirements, used in scenarios with low requirements for connection accuracy.
- What impact will insufficient thread accuracy have on connection reliability?
Insufficient thread accuracy will lead to excessive thread fit clearance, easy relative sliding between bolts and nuts, causing preload attenuation and connection loosening; uneven clearance will concentrate stress on local thread teeth, leading to overload fracture of individual thread teeth; low-precision thread surfaces are rough, easy to cause seizing during installation, increasing tightening resistance and making it difficult to control preload; in addition, it will affect the interchangeability of threads, bringing difficulties to installation and replacement.
- How to detect the accuracy of bolt threads?
Use thread gauges for detection. The go gauge can be smoothly screwed into the thread, and the no-go gauge can be screwed in no more than 2 threads, indicating that the thread size is within the allowable range; measure the pitch diameter of the thread with a thread micrometer, compare with the standard pitch diameter, and the deviation must be within the specified tolerance; measure parameters such as thread angle and pitch with a thread profilometer to check whether they meet the accuracy requirements; for important bolts, actual assembly tests can also be carried out to check the fit tightness with nuts and preload stability.
- What are the differences in requirements for bolt thread accuracy in different connection scenarios?
High-speed railway fastening connections have high requirements and need to use grade 3 precision threads to ensure stable preload and prevent loosening caused by train vibration; grade 4 precision threads can be used for general connections in ordinary railways to meet basic connection reliability; temporary fixing or connections with small stress, such as temporary fixing bolts for sleepers, can use grade 5 precision threads; in occasions with frequent vibration, such as turnout areas, thread accuracy needs to be improved to reduce the risk of loosening.
- What are the processing technologies to improve the accuracy of bolt threads?
Adopt precision rolling process to process threads. During rolling, the metal fibers are continuous, the thread surface roughness is low, the accuracy is high, and the strength can also be increased by 10%-15%; after thread processing, grinding is carried out to remove surface burrs and defects, reduce roughness and improve accuracy; strictly control the accuracy of processing equipment, calibrate the machine tool regularly to ensure accurate parameters such as pitch and thread angle; select high-quality cutting tools to reduce the impact of tool wear on thread accuracy and ensure stable processing quality.