Bolt Strength Grade and Anti-Loosening Design
What are the core judgment indicators of bolt strength grades?
The judgment of bolt strength grades has clear core indicators, mainly including tensile strength, yield strength and hardness. The 6.8-grade bolt has a tensile strength ≥600MPa, yield strength ≥480MPa, and hardness HRC20-25; the 8.8-grade bolt has a tensile strength increased to 800MPa, yield strength ≥640MPa, and hardness HRC24-30; the 10.9-grade bolt has a tensile strength ≥1000MPa, yield strength ≥900MPa, and hardness HRC32-36. In the grade mark, the number before the decimal point is 1/100 of the tensile strength, and the number after the decimal point is the yield ratio (yield strength/tensile strength). For example, 8.8 grade means tensile strength 800MPa and yield ratio 0.8. In addition, the elongation and reduction of area of the bolt need to be tested; the elongation of the 8.8-grade bolt is ≥12% to ensure sufficient plastic deformation before fracture and avoid brittle fracture. These indicators must be verified by a third-party testing agency before they can be put into use.
What are the performance advantages of foreign standard 10.9-grade bolts over national standard 8.8-grade bolts?
Foreign standard 10.9-grade bolts are far superior to national standard 8.8-grade bolts in strength and durability, adapting to the needs of high-grade lines. Its tensile strength reaches 1000MPa and yield strength 900MPa, which are 25% and 40% higher than 8.8-grade bolts respectively, and can bear larger loads, adapting to heavy-haul and high-speed lines. The yield ratio of 10.9-grade bolts is 0.9, with higher material utilization rate and excellent fatigue resistance, which can pass 30 million fatigue tests, 1.5 times that of 8.8-grade bolts. The bolt is made of low-carbon alloy steel, and after carburizing and quenching treatment, the thread accuracy reaches 6g grade, and the installation preload is more stable with a deviation ≤±5%. Its corrosion resistance is stronger, and the Dacromet coating has a salt spray life ≥3000 hours, 50% higher than 8.8-grade galvanized bolts. The 10.9-grade bolt also has better anti-relaxation performance; the preload attenuation in high-temperature environments is ≤8% per year, while that of 8.8-grade bolts can reach 15%, with better long-term stability.
What is the working principle and application precautions of double nut anti-loosening?
Double nut anti-loosening is the mainstream anti-loosening scheme for national standard lines, realizing anti-loosening through mechanical locking, with clear application points. Its principle is that after tightening, axial pressure is generated between the nuts to form frictional locking of the thread, and at the same time, the threads of the upper and lower nuts move relative to each other to increase the resistance of the thread pair and prevent bolt loosening. During installation, the main nut must be tightened to the rated preload first, and then the secondary nut is tightened with a torque of 60-70% of the rated value to avoid bolt tensile deformation caused by excessive tightening. Double nut anti-loosening is suitable for conventional speed lines with vibration frequency ≤10Hz, and its effect is limited in high-speed rail lines with high-frequency vibration. It is necessary to regularly check the loosening of nuts; if there is a gap, it needs to be retightened, and the combined thickness of double nuts is large, so sufficient installation space must be ensured. This scheme is low-cost and easy to operate, but needs to be used with anti-loosening glue in extreme environments to improve anti-loosening reliability.
What are the anti-loosening advantages and applicable scenarios of foreign standard self-locking thread bolts?
Foreign standard self-locking thread bolts achieve anti-loosening through special thread design, with significant advantages and adapt to specific lines. Its thread profile is wedge-shaped; after tightening, the wedge surface generates radial pressure to make the thread pair fit closely to form self-locking, with no loosening risk under vibration, and the anti-loosening reliability is 3 times higher than that of double nuts. The bolt does not require additional anti-loosening components, has a compact structure and low installation height, adapting to the low headroom requirements of high-speed rail lines. The anti-loosening performance of self-locking threads is not affected by temperature, and is stably effective in the environment of -40℃ to 150℃, suitable for extreme climate areas. It is mainly suitable for high-speed rail lines with a speed ≥250km/h and urban rail lines with high-frequency vibration, which can maintain stable preload for a long time and reduce maintenance frequency. Compared with traditional anti-loosening schemes, the service life of self-locking thread bolts is doubled; although the procurement cost is 10-20% higher, the comprehensive maintenance cost is lower, making it the first choice for foreign standard high-end lines.
What is the integrated anti-corrosion and anti-loosening treatment process of bolts?
The integrated anti-corrosion and anti-loosening treatment of bolts is a new process that can achieve both anti-corrosion and anti-loosening, improving comprehensive performance. The process first applies Dacromet anti-corrosion coating treatment to the bolt, with a zinc-aluminum coating thickness of 60μm to ensure basic anti-corrosion performance. Then, high-temperature resistant anti-loosening glue is coated on the thread surface, with a glue layer thickness of 5-10μm, which forms an elastic locking layer after curing to achieve anti-loosening. During the treatment process, the temperature must be controlled; the curing temperature of the Dacromet coating is 300℃, and the curing temperature of the anti-loosening glue is 120℃ to avoid high temperature damaging the performance of the anti-loosening glue. The bolt after integrated treatment has a salt spray life ≥3000 hours, and the preload attenuation in vibration test is ≤5%, far exceeding the bolts with traditional separate treatment. This process is suitable for coastal high-speed rail and heavy-haul lines with high corrosion and high vibration, which can reduce later maintenance procedures, improve installation efficiency, and achieve the effect of "one-time treatment, double protection".