Bolt Strength Grades and Anti-Loosening Techniques
What are the differences in applicable scenarios of different strength grades of track bolts?
The strength grade of track bolts is determined by tensile strength and yield strength, and different grades adapt to different line loads and scenario requirements. Grade 8.8 bolts have a tensile strength of ≥800MPa and a yield strength of ≥640MPa, with moderate cost, suitable for fastening general parts of conventional speed railways and urban rail lines, such as pad fixation and pressing plate connection. Grade 10.9 bolts have a tensile strength of ≥1040MPa and a yield strength of ≥940MPa, with higher strength, suitable for key parts of heavy-haul lines and high-speed railways, such as fishplate connection and core nodes of fastening systems, and can withstand greater impact loads. Grade 12.9 bolts are high-strength products with a tensile strength of ≥1220MPa and a yield strength of ≥1100MPa, mainly used in extreme load scenarios such as heavy-haul special lines and long-span bridges to ensure long-term stable joint connection. Foreign standard ASTM A325 bolts (equivalent to grade 8.8) are suitable for foreign conventional speed lines, and ASTM A490 bolts (equivalent to grade 10.9) are used for foreign standard high-speed lines, with strength grades corresponding to domestic standards but more refined anti-loosening design. The selection of strength grade should avoid "overconfiguration waste" or "underconfiguration risk" and must be accurately matched according to the line design load.
What are the common anti-loosening technologies for track bolts? What are their characteristics?
Common anti-loosening technologies for track bolts include mechanical anti-loosening, friction anti-loosening and chemical anti-loosening, each with adaptive scenarios and performance characteristics. Mechanical anti-loosening limits bolt loosening through mechanical structures, commonly using cotter pins, lock washers, series wires and other methods, with simple structure and low cost, suitable for non-critical parts of conventional speed lines, but the anti-loosening effect is greatly affected by installation accuracy. Friction anti-loosening achieves anti-loosening by increasing the friction between bolts and nuts, such as spring washers, lock nuts, thread lockers, etc. Spring washers generate preload through elastic deformation, and lock nuts have built-in nylon rings or metal teeth, with more durable anti-loosening effect, suitable for high-speed and urban rail lines. Chemical anti-loosening uses anaerobic adhesive applied to the thread, which forms adhesion after curing to completely lock the bolt, with the best anti-loosening effect, but difficult to disassemble, suitable for key parts that are not inspected for a long time, such as bridge track foundation fastening. The service life of different anti-loosening technologies varies significantly: friction anti-loosening and chemical anti-loosening can last for 5-8 years, and mechanical anti-loosening for about 3-5 years, which should be selected according to the maintenance cycle.
What are the unique features of the anti-loosening design of foreign standard ASTM bolts?
The anti-loosening design of foreign standard ASTM bolts focuses on high-frequency vibration scenarios of high-speed lines, with refined and long-term characteristics. ASTM A490 high-strength bolts are often matched with nylon lock nuts. The nylon ring inside the nut fits closely with the bolt thread to generate continuous friction, which can effectively prevent loosening even under high-frequency vibration, and the anti-loosening effect is 30% better than that of ordinary lock nuts. Some models adopt fine thread design, with the thread profile angle optimized to 60°, increasing the thread contact area, improving vibration resistance and reducing thread wear. The bolt head adopts a twelve-point plum blossom design, which is convenient for tightening with a special wrench, ensuring uniform preload and avoiding anti-loosening failure caused by improper installation. Some ASTM bolts adopt Dacromet coating on the surface, which not only has excellent corrosion resistance but also can increase the friction coefficient between threads, indirectly improving the anti-loosening effect. In addition, the anti-loosening design of foreign standard bolts must pass strict vibration table tests (1000Hz high-frequency vibration for 24 consecutive hours) to ensure long-term reliability in high-speed train operation scenarios.
Why are bolts on heavy-haul lines prone to loosening? How to solve it targetedly?
The core reason why bolts on heavy-haul lines are prone to loosening is that they are subjected to long-term large loads, high-frequency impacts and severe vibrations, leading to attenuation of bolt preload and thread wear. The axle load of heavy-haul trains is ≥25t, and the impact load generated when passing is 2-3 times that of conventional speed lines, causing the bolts to be subjected to repeated tensile and compressive stresses, accelerating thread fatigue and loosening. The line vibration frequency is high (50-200Hz), exceeding the adaptive range of ordinary anti-loosening technologies, leading to the failure of mechanical anti-loosening structures. Targeted solutions include: selecting grade 12.9 high-strength bolts to improve the fatigue resistance of the bolts themselves and reduce preload attenuation; adopting a combined anti-loosening scheme of "lock nut + spring washer + thread locker" for multiple guarantees; optimizing the bolt installation process, using the torque-angle method for tightening to ensure the preload meets the design requirements (usually ≥1000N·m); conducting regular bolt torque detection, inspecting every 3 months, and retightening in a timely manner if loosening is found; performing surface treatment on bolts and nuts, adopting phosphating + oiling process to reduce thread wear and extend the anti-loosening validity period.
How does the anti-corrosion treatment of bolts affect the anti-loosening performance?
The anti-corrosion treatment of bolts not only affects the service life but also is directly related to the anti-loosening performance. Improper anti-corrosion treatment will increase the risk of bolt loosening. High-quality anti-corrosion treatment (such as hot-dip galvanizing, Dacromet coating) can form a dense protective film on the bolt surface, preventing thread jamming or wear caused by rust, ensuring stable bolt preload and indirectly improving the anti-loosening effect. If the anti-corrosion coating is too thick or uneven, the thread fit gap will increase, and the bolt is prone to micro-displacement after tightening, accelerating loosening; if the coating is too thin, it cannot effectively prevent corrosion, and the surface roughness of the thread increases after rusting, which may make the bolt impossible to disassemble, but will not directly affect the anti-loosening performance in the short term. Bolts in heavy-haul and coastal areas need to adopt composite anti-corrosion treatment (hot-dip galvanizing + passivation + oiling), which can pass more than 5000 hours of salt spray test, avoiding corrosion medium erosion of threads and ensuring the long-term effectiveness of the anti-loosening structure. In addition, the bolts after anti-corrosion treatment need to be subjected to torque testing to adjust the tightening torque parameters, ensuring that the preload is not affected by the coating thickness, and avoiding anti-loosening failure caused by insufficient torque.