Rail Bolt Anti-Loosening Technology and Adaptation Solutions for Different Track Conditions
What are the core anti-loosening technologies for rail bolts in high-speed railway lines?
Rail bolts in high-speed railway lines are subject to high-frequency and low-amplitude vibration loads, and the core anti-loosening technology needs to be optimized simultaneously from both thread structure and anti-loosening accessories. First, fine-thread bolts are selected. Fine threads have a smaller pitch and a smaller thread lead angle, and their self-locking performance is more than 30% higher than that of coarse threads, which can effectively resist the loosening trend caused by vibration. Second, anti-loosening nuts are used with them. The inner part of the nut is equipped with a nylon locking ring. The inner diameter of the nylon ring is slightly smaller than the major diameter of the bolt thread. After tightening, the nylon ring will undergo elastic deformation and tightly wrap the thread, forming a continuous locking force to prevent the bolt from loosening due to vibration. At the same time, pre-applied adhesive is coated on the contact surface between the bolt and the nut. The pre-applied adhesive is an anaerobic adhesive, which cures in an oxygen-free environment after tightening, fills the thread gaps and forms a firm bonding force, further enhancing the anti-loosening effect. During installation, the tightening torque must be strictly controlled. The design torque of bolts in high-speed railway lines is usually 350-400N·m. Insufficient torque cannot provide enough preload, while excessive torque is likely to cause thread slipping. In addition, after the bolts are installed, anti-loosening marks must be made. Special paint is used to mark the relative position of the bolts and nuts, which facilitates quick judgment of whether loosening occurs during later inspections.
What is the anti-impact anti-loosening scheme for rail bolts in heavy-haul freight lines?
Rail bolts in heavy-haul freight lines bear large impact loads and high vibration amplitudes, and the anti-impact anti-loosening scheme must take into account both high strength and strong locking capacity. First, 10.9-grade high-strength bolts are selected. Compared with ordinary 8.8-grade bolts, their tensile strength is increased to more than 1000MPa, and the yield strength reaches 900MPa, which can withstand greater impact tension without plastic deformation. Second, a double-nut anti-loosening structure is adopted. After the main nut is tightened, the auxiliary nut is screwed on. When the auxiliary nut is tightened, it will generate a reverse preload on the main nut, so that a continuous friction force is generated on the thread contact surface between the two nuts, offsetting the loosening torque caused by the impact load. At the same time, a disc spring washer is installed between the bolt head and the fish plate. The disc spring has good elastic recovery ability. When the bolt undergoes slight deformation due to impact load, the disc spring can timely compensate the preload to avoid loosening caused by preload attenuation. During installation, the torque-angle method is used to control the tightening process. First, tighten to the basic torque of 200N·m, then rotate 60°-90° to ensure that the preload of the bolt is uniform and stable. In addition, the torque of the bolts is re-inspected regularly, once every 3 months, and the bolts with torque attenuation exceeding 10% are re-tightened to ensure the long-term stability of the anti-loosening effect.
What is the economical anti-loosening optimization technology for rail bolts in ordinary-speed mixed-traffic lines?
Ordinary-speed mixed-traffic lines have high requirements for cost control, and bolt anti-loosening optimization needs to reduce investment on the premise of ensuring performance, adopting an economical combination scheme of "mechanical anti-loosening + surface treatment". First, washers with anti-loosening teeth are selected. One side of the washer is provided with serrated protrusions. After tightening, the serrations will be embedded into the surface of the fish plate to form mechanical engagement, preventing the bolt from rotating due to vibration. The cost of this type of anti-loosening washer is only 1/3 of that of nylon lock nuts, with extremely high cost performance. Second, the bolts are subjected to hot-dip galvanizing and passivation treatment, with a zinc layer thickness ≥80μm. The passivation film can improve the corrosion resistance of the zinc layer, prevent thread seizure or preload attenuation caused by bolt corrosion. The cost of hot-dip galvanizing treatment is much lower than that of high-end surface treatment processes such as zinc infiltration. At the same time, the bolt installation process is optimized, and the "diagonal tightening method" is adopted, that is, the bolts at the joint are tightened sequentially according to the diagonal order, avoiding uneven stress caused by improper tightening sequence and reducing the probability of bolt loosening. In addition, standardized bolt specifications are selected, and M24×180mm bolt models are uniformly adopted to realize batch procurement and replacement, further reducing maintenance costs. The anti-loosening effect of this scheme can meet the operation requirements of ordinary-speed lines, and the overall cost is reduced by more than 40% compared with high-end anti-loosening schemes.
What are the detection methods and qualification standards for the anti-loosening performance of rail bolts?
The detection of the anti-loosening performance of rail bolts needs to simulate the vibration conditions of actual lines, and bench tests are carried out using a vibration testing machine. The detection methods mainly include vibration loosening test and preload retention test. The specific steps of the vibration loosening test are: fix the installed bolt-nut assembly on the vibration table, apply the same vibration frequency and amplitude as the target line, simulate a frequency of 50Hz and amplitude of 0.1mm for high-speed lines, and a frequency of 20Hz and amplitude of 0.5mm for heavy-haul lines, and measure the torque attenuation rate of the bolts after continuous vibration for 2 hours. The preload retention test is to place the tightened bolts in a constant temperature and humidity environment, measure the change of preload regularly, and monitor continuously for 30 days. The qualification standards are divided according to line types: the torque attenuation rate of bolts for high-speed lines should be ≤5%, and the preload retention rate ≥95%; the torque attenuation rate of bolts for heavy-haul lines should be ≤8%, and the preload retention rate ≥92%; the torque attenuation rate of bolts for ordinary-speed lines should be ≤10%, and the preload retention rate ≥90%. In addition, on-site sampling inspection is required. 5 groups of bolts are sampled per kilometer of line, and the actual torque is measured with a torque wrench. The sampling qualification rate must reach 100%. If unqualified items appear, the sampling is doubled to ensure that the overall anti-loosening performance of the line meets the standard.
What is the integrated anti-loosening and anti-freezing technology for rail bolts in alpine regions?
Rail bolts in alpine regions face the dual challenges of low-temperature frost heave and deicing agent corrosion. The integrated anti-loosening and anti-freezing technology needs to be upgraded simultaneously from three aspects: material, protection and structure. First, low-temperature resistant bolt material is selected, using 40CrNiMoA alloy steel. The impact energy of this material in a low-temperature environment of -40℃ is ≥34J, avoiding the risk of low-temperature brittle fracture. At the same time, its thread precision is higher, which can improve the adaptability of anti-loosening accessories. Second, the bolts are subjected to zinc infiltration treatment, with a zinc infiltration layer thickness ≥60μm. The corrosion resistance of the zinc infiltration layer is more than twice that of hot-dip galvanizing, which can effectively resist the corrosion of deicing agents. Moreover, the zinc infiltration layer has good low-temperature stability and will not fall off due to sudden temperature changes. At the same time, anti-freezing anti-loosening glue is used. The freezing point of the glue is -50℃, and it can still maintain good viscosity in low-temperature environments. After tightening, it fills the thread gaps, which not only enhances the anti-loosening effect, but also prevents ice and snow from penetrating into the thread gaps and causing frost heave. During installation, anti-freezing lubricant must be applied to the bolt holes. The lubricant can reduce the friction force when the bolts are tightened, ensure accurate preload, and prevent thread seizure at low temperatures. In addition, a comprehensive inspection of the bolts is carried out before winter, and loose or rusted bolts are replaced in a timely manner to ensure the winter operation safety of lines in alpine regions.