High-Strength Thread Forming and Anti-Loosening/Anti-Slip Structure Design of Rail Bolts
What are the core advantages of forming threads of track bolts by cold heading and thread rolling process?
Compared with the traditional turning process, forming threads of track bolts by cold heading and thread rolling process has core advantages in improving thread mechanical properties, ensuring dimensional accuracy, increasing production efficiency and reducing material loss, which fully meets the requirements of high strength and high consistency of bolts in track engineering. The cold heading and thread rolling process is a non-cutting processing method. Threads are formed by extruding the bolt shank with molds. The metal fibers of the steel are not cut off during processing, but instead distributed continuously along the thread profile, making the tensile strength and yield strength of the thread part 20%-30% higher than that of turned threads, and the fatigue resistance more than 40% higher. It can bear the large pre-tightening force and repeated vibration load of track fastening, avoiding thread fracture. In terms of dimensional accuracy, the cold heading and thread rolling process is controlled by CNC molds, with a thread pitch deviation ≤±0.03mm and a tooth profile angle deviation ≤±0.5°, which is much higher than the accuracy of turned threads, ensuring precise meshing between bolts and nuts, avoiding stress concentration caused by meshing gaps, and improving fastening stability. In terms of production efficiency, the cold heading and thread rolling process realizes automatic continuous processing. A single equipment can process 800-1000 bolts per hour, which is 5-6 times that of the turning process, meeting the supply demand of large-scale railway construction. In terms of material loss, the turning process forms threads by cutting the shank with a material loss rate of 15%-20%, while the cold heading and thread rolling process is plastic forming with a material loss rate ≤3%, greatly reducing the cost of raw materials. In addition, the surface roughness of the thread after cold heading and thread rolling is Ra ≤1.6μm, and the surface is smooth, which can reduce the frictional resistance during thread meshing, facilitate the control of pre-tightening force during construction, and reduce the probability of thread corrosion.
What core criteria should be followed in the selection of high-strength base materials for track bolts?
The selection of high-strength base materials for track bolts follows four core criteria: mechanical performance adaptation, good processability, excellent corrosion resistance and controllable cost. It is necessary to accurately select materials according to line working conditions (heavy-haul/high-speed/ordinary-speed) and service environments (coastal/inland/alpine) to avoid over-selection or insufficient selection of materials. Mechanical performance adaptation is the primary criterion. Steel materials of corresponding strength grades should be selected according to the use position and pre-tightening force requirements of the bolts. High-strength alloy structural steels of grade 8.8 and 10.9 are required for core track fastening bolts (fish plate bolts, elastic strip bolts). Grade 10.9 bolts have a tensile strength ≥1000MPa and a yield strength ≥900MPa, adapting to the large pre-tightening force requirements of heavy-haul and high-speed lines; carbon structural steels of grade 4.8 and 6.8 can be used for auxiliary bolts of station and branch lines to reduce costs. Good processability is the basic criterion. The selected base material must have good cold heading, thread rolling and heat treatment process performance, not easy to crack during cold heading, full thread forming during thread rolling, and good hardenability during heat treatment, ensuring uniform overall performance of the bolts. For example, 42CrMo steel commonly used for grade 10.9 bolts has excellent hardenability, and the core and surface strength are consistent after heat treatment without performance deviation. Excellent corrosion resistance is the environmental adaptation criterion. Corrosion-resistant alloy structural steels should be selected for coastal high-salt spray environments, or high-end anti-corrosion treatment should be added on the basis of ordinary high-strength steels; steels with good low-temperature toughness such as 35CrMnSi steel should be selected for alpine areas to avoid brittle fracture of bolts at low temperatures; conventional high-strength steels can be selected for inland dry environments to simplify the anti-corrosion process. Controllable cost is the economic criterion. On the premise of meeting mechanical performance and environmental requirements, priority should be given to selecting base materials with high cost performance. For the same strength grade, priority should be given to selecting mature domestic steel materials to reduce import costs, and at the same time avoid selecting over-specification steel materials for high performance, resulting in cost waste.
What are the core methods of thread modification for anti-loosening and anti-slip of track bolts?
The core methods of thread modification for anti-loosening and anti-slip of track bolts include thread gluing, thread carburizing, thread knurling and nylon insert meshing. All of them increase the friction between threads by changing the surface characteristics or meshing structure of threads, offset the loosening torque caused by vibration, and adapt to the anti-loosening requirements of different lines. Thread gluing is the most widely used modification method. A special anaerobic anti-loosening adhesive is coated on the surface of the bolt thread with a coating thickness of 5-10μm. After tightening, the anti-loosening adhesive cures in the thread meshing gap to form a high-strength adhesive layer, transforming the mechanical meshing of threads into "mechanical meshing + gluing", and the friction force is increased several times. Even under strong vibration conditions, the bolt will not loosen, adapting to core lines such as high-speed and heavy-haul lines. The gluing process is simple and can be processed in batches. Thread carburizing is an enhanced modification method. Carbon atoms are infiltrated into the thread surface through high-temperature carburizing process to form a high-hardness carburized layer with a thickness of 0.1-0.2mm, and the surface hardness reaches more than HV800, which not only improves the wear resistance and fatigue resistance of the thread, but also increases the friction coefficient during thread meshing to achieve anti-loosening and anti-slip, adapting to urban rail lines with frequent vibration and easy thread wear. Thread knurling is a physical modification method. Tiny diamond patterns with a depth of 0.05-0.1mm are rolled on the tooth side surface of the bolt thread. After tightening, the patterns interlock with the tooth side of the nut thread to form mechanical occlusion, increasing frictional resistance. At the same time, the patterns can break the oil film between threads to avoid slipping caused by oil contamination, adapting to station and branch lines with low pre-tightening force. Nylon insert meshing is a structural modification method. A nylon ring is inlaid in the threaded hole of the nut, and the inner diameter of the nylon ring is slightly smaller than the major diameter of the bolt thread. When tightened, the bolt thread extrudes the nylon ring to form elastic deformation, and the nylon ring is closely attached to the thread to generate continuous elastic holding force to offset the loosening torque. In addition, the nylon ring has a damping effect, which can reduce the impact of vibration on the thread, adapting to high-speed lines with high requirements for anti-loosening accuracy.
What are the anti-loosening principle and adaptation requirements of anti-loosening washers in track bolt fastening?
The core anti-loosening principle of anti-loosening washers in track bolt fastening is elastic deformation compensation + reverse friction locking. Through the structural characteristics of the washers, when the bolt has a slight loosening trend due to vibration, the pre-tightening force is compensated in time and reverse friction is generated to prevent the bolt from continuing to loosen, which is an important auxiliary structure for track bolt anti-loosening. Most anti-loosening washers are disc-shaped or double-stack self-locking structures. The disc-shaped anti-loosening washer is compressed to produce elastic deformation when the bolt is tightened, storing elastic potential energy. When the bolt loosens due to vibration and the pre-tightening force decreases, the elastic potential energy of the washer is released to generate a reverse pressing force to compensate for the loss of pre-tightening force. At the same time, the friction surface between the washer and the bolt head, and the connected part surface will generate reverse friction to offset the loosening torque; the double-stack self-locking washer is composed of two washers with helical teeth. When tightened, the helical teeth of the two washers mesh with each other. When vibrating, the self-locking structure of the helical teeth will prevent the relative rotation of the washers, thereby locking the bolt, and the anti-loosening effect is better than that of the disc-shaped washer. The adaptation of anti-loosening washers must follow three requirements: matching with bolt strength, matching with working conditions and matching with installation surface. In terms of strength matching, high-strength anti-loosening washers of corresponding grades should be selected for grade 8.8 and 10.9 high-strength bolts. The tensile strength and elastic limit of the washers must match the bolts to avoid deformation and failure of the washers before the bolts; in terms of working condition matching, double-stack self-locking high-strength anti-loosening washers should be selected for heavy-haul and high-speed lines, disc-shaped damping anti-loosening washers can be selected for urban rail lines, and ordinary spring anti-loosening washers can be selected for ordinary-speed and branch lines; in terms of installation surface matching, flat anti-loosening washers should be selected when the surface of the connected part is a smooth plane, and anti-loosening washers with anti-slip teeth should be selected when the surface is a rough or curved surface to increase the friction with the installation surface and avoid slipping of the washers. In addition, the inner and outer diameters of the anti-loosening washer must be precisely matched with the bolt specifications, with a gap ≤±0.1mm, to ensure centering and positioning during installation and avoid anti-loosening failure caused by eccentric load.
How does the anti-slip tooth design of the track bolt head improve fastening stability?
The anti-slip tooth design of the track bolt head improves the overall fastening stability from the source of bolt fastening through three aspects: increasing the friction coefficient with the installation surface, preventing the relative rotation of the bolt head, and dispersing the pre-tightening force, effectively offsetting the loosening risk caused by train vibration, and is the basic anti-slip structure of track bolts. The anti-slip teeth are designed on the lower surface of the bolt head (the side in contact with the installation surface), mostly triangular or strip teeth distributed in a ring, with a tooth height of 0.2-0.5mm and a tooth pitch of 1-2mm. When the bolt is tightened, the anti-slip teeth will be embedded into the surface of the connected part (such as pressure plate, fish plate) to form mechanical occlusion, transforming the simple surface contact between the bolt head and the connected part into "surface contact + tooth occlusion", the friction coefficient is increased by 2-3 times, which greatly reduces the relative rotation of the bolt head caused by vibration and prevents bolt loosening from the source. At the same time, the anti-slip teeth are evenly distributed in a ring, which can make the pressure of the bolt head evenly transmitted to the surface of the connected part, disperse the pre-tightening force, avoid the deformation of the installation surface caused by local stress concentration, and ensure the stable transmission of the pre-tightening force. If there are no anti-slip teeth on the bolt head, the head is easy to slip with the installation surface during vibration, leading to a rapid attenuation of the pre-tightening force and failure of the fastening system. In addition, the size of the anti-slip teeth is accurately designed according to the bolt specifications and use positions. The anti-slip teeth of large-specification bolts (M24 and above) are higher and denser, adapting to the demand for large pre-tightening force; the anti-slip teeth of small-specification bolts (M16 and below) are shallower and sparser, avoiding excessive damage to the surface of the connected part due to too high teeth; the anti-slip teeth of fish plate bolts are strip teeth, adapting to the flat installation surface of the fish plate, and the anti-slip teeth of elastic strip bolts are triangular teeth, adapting to the curved installation surface of the elastic strip. The accurate size and form design maximize the effect of the anti-slip teeth. The anti-slip teeth cooperate with thread anti-loosening and anti-loosening washers to form a triple anti-loosening system of "head anti-slip + thread anti-loosening + washer compensation", so that the track bolts always remain fastened under long-term vibration conditions.