Torque Coefficient, Preload Control Accuracy, and Compatibility Selection of Fastening Systems
Why will the preload deviation exceed 15% when the torque coefficient fluctuation of the fastening system exceeds 0.05?
The calculation formula of the torque coefficient K is K=P/(πdμ), where μ is the friction coefficient, and fluctuation will directly affect the K value. When the K fluctuation exceeds 0.05, the bolt preload will have a significant deviation under the same tightening torque. For example, if the torque coefficient fluctuates from 0.18 to 0.23, the preload deviation can reach 18%. Insufficient preload will make the elastic clips and bolts unable to effectively lock the rail, leading to loosening under train vibration; excessive preload will exceed the yield strength of the material, causing bolt fracture. Therefore, the torque coefficient must be controlled within ±0.02 to ensure the preload accuracy.
Why is the torque coefficient of hot-dip galvanized bolts 0.03-0.05 higher than that of blackened bolts?
The surface zinc layer thickness of hot-dip galvanized bolts reaches 60-80μm, the roughness of the thread contact surface is higher, and the friction coefficient μ is larger. According to the torque coefficient formula, an increase in μ leads to a higher K value, so the torque coefficient of hot-dip galvanized bolts is usually 0.22-0.25. The surface of blackened bolts is smooth, with low roughness and small friction coefficient, so the torque coefficient is only 0.18-0.20. The difference in torque coefficient between the two will cause the preload of hot-dip galvanized bolts to be 15%-20% lower under the same torque. When selecting models, the tightening torque parameters need to be adjusted according to the surface treatment method to avoid preload deviation.
Why do high-speed line fastening systems require a torque coefficient accuracy of ±0.01 instead of ±0.03 for ordinary lines?
The train speed on high-speed lines is fast, and the frequency of wheel-rail alternating loads is high, so the requirements for preload accuracy are extremely high. When the torque coefficient accuracy is ±0.01, the preload deviation can be controlled within 5%, which can effectively ensure the long-term stability of elastic clips and bolts. Ordinary lines have low load frequency, and the preload deviation within ±10% can meet the use requirements. If ordinary precision torque coefficient control is adopted for high-speed lines, the preload fluctuation will exceed 15%, which is easy to cause lateral displacement of the rail and affect driving safety. Therefore, high-speed lines need to choose fastening systems with high-precision torque coefficients.
Why is the impact of lubrication state on the torque coefficient of the fastening system more significant in heavy-haul lines?
The preload borne by bolts in heavy-haul lines is large, and the pressure on the thread contact surface is high. The viscosity and thickness of lubricating grease will directly affect the friction coefficient. When lubrication is insufficient, there is dry friction between threads, and the friction coefficient μ can reach 0.18-0.22, leading to a high torque coefficient K; when lubrication is excessive, grease overflows, causing the friction coefficient μ to drop to 0.12-0.15, and the K value is low. The torque fluctuation range of heavy-haul lines is large, and the change in lubrication state will cause the K value to fluctuate by more than 0.08, and the preload deviation can reach more than 25%. The preload of ordinary lines is small, so the lubrication state has little impact on the K value, and the fluctuation is usually no more than 0.03.
How to calibrate the torque coefficient of the fastening system on site to ensure the preload control accuracy?
On-site calibration requires the cooperation of a torque wrench and a tension meter. Select three gear positions: 10%, 50%, and 90% of the rated torque for testing. First, tighten the bolt to the target torque, then use a tension meter to measure the actual preload, and calculate the torque coefficient K=P/(πdμ). If the K value exceeds the design range by ±0.02, the tightening torque parameters need to be adjusted. At the same time, it is necessary to regularly detect the thread surface treatment state and lubrication condition, replace the failed lubricating grease, and clean the thread impurities. Through dynamic calibration and state maintenance, the torque coefficient accuracy can be controlled within ±0.01, meeting the needs of high-grade lines.