High-strength Anti-loosening Design of Rail Bolts and Adaptation Technology for Different Fastening Scenarios
What are the core points of high-strength anti-loosening design for track bolts in turnout areas?
The core of high-strength anti-loosening design for track bolts in turnout areas is to cope with the high-frequency vibration and impact loads of turnout switching. First, 42CrMo high-strength alloy steel is selected as the material, which has a tensile strength ≥1080MPa, yield strength ≥930MPa and hardness reaching HRC32-36 after quenching and tempering, meeting the heavy-haul stress requirements of turnout areas. The thread of the bolt adopts a fine thread design, with the pitch reduced from 3mm to 2mm. The fine thread has stronger self-locking performance, which can effectively resist loosening caused by vibration, and at the same time increase the contact area of the thread to reduce stress concentration at the thread part. The anti-loosening structure adopts a combined scheme of double nut anti-loosening + lock washer. After the main nut is tightened, a lock nut is added, and the preload of the lock nut is controlled at 80% of that of the main nut. The lock washer adopts a disc spring washer, which uses the elastic deformation of the spring to compensate for the preload attenuation of the bolt. The surface of the bolt is nitrided, with a nitrided layer thickness ≥0.15mm, and the surface hardness is increased to above HV900, enhancing wear resistance and corrosion resistance to adapt to the complex working conditions of turnout areas. In addition, the head of the bolt adopts a hexagonal flange design to increase the contact area with the fishplate, prevent the bolt head from sinking into the plate, and ensure fastening stability.
What are the installation adaptation and adjustment measures for pre-embedded bolts in ballastless tracks?
The installation adaptation of pre-embedded bolts in ballastless tracks needs to solve the problems of pre-embedding accuracy and long-term anchoring stability. First, the pre-embedding depth of the bolt is controlled at 200-220mm. During pre-embedding, a positioning mold is used to fix the bolt position to ensure that the verticality deviation of the bolt is ≤0.5°, avoiding uneven fastening stress caused by bolt inclination. Three annular ribs are arranged on the rod of the bolt, with a rib height of 5mm and a width of 8mm, which can enhance the grip between the bolt and the concrete base and prevent the bolt from pulling out under long-term loads. Before pre-embedding, the bolt surface must be derusted to a derusting grade of Sa2.5, and then epoxy steel bar anchoring adhesive is applied with a glue layer thickness ≥2mm to further improve the anchoring strength of the bolt, with an anchoring force ≥120kN. During installation, the preload of the bolt is applied in three times: 50% of the design preload is applied for the first time, increased to 80% after 24 hours, and to 100% after 72 hours, avoiding cracking of the concrete base caused by one-time application of preload. In addition, the exposed part of the bolt is subjected to hot-dip galvanizing + sealant anti-corrosion treatment, with a zinc layer thickness ≥120μm. The sealant fills the pores of the zinc layer to prevent water vapor from invading, adapting to the open-air environment of ballastless tracks.
What is the integrated anti-corrosion and anti-loosening scheme for high-strength bolts in ballasted tracks?
The integrated anti-corrosion and anti-loosening design of high-strength bolts in ballasted tracks needs to balance the anti-corrosion requirements of the humid track bed environment and the anti-loosening requirements of vibration conditions. First, weather-resistant steel bolts with the grade Q450NQR1 are selected. This material contains corrosion-resistant elements such as copper and chromium, with a corrosion rate ≤0.03mm/year in humid environments, 70% lower than that of ordinary carbon steel bolts. The anti-loosening structure adopts a combined scheme of chemical anti-loosening adhesive + self-locking thread. Anaerobic anti-loosening adhesive is applied to the thread part, which cures in an oxygen-free environment, fills the thread gap and forms a rigid connection. The thread angle of the self-locking thread is adjusted to 30°, and its self-locking performance is 50% higher than that of ordinary 60° threads. The anti-corrosion treatment adopts hot-dip galvanizing + dacromet coating double protection, with a hot-dip galvanizing layer thickness ≥100μm and a dacromet coating thickness ≥8μm. The dacromet coating has excellent corrosion resistance and heat resistance, with a salt spray test corrosion resistance time ≥2000 hours. The installation torque of the bolt is strictly controlled according to the design requirements. The torque of the bolts matched with 60kg/m rails is controlled at 400-450N·m to ensure the stability of the bolt preload. At the same time, a torque wrench is used for regular re-inspection every 3 months to supplement the preload in time. In addition, a waterproof cap is installed on the bolt head, which is made of polyethylene to prevent rainwater and ballast debris from entering the thread part.
What is the dynamic testing method for the anti-loosening performance of track bolts?
The dynamic testing of the anti-loosening performance of track bolts needs to simulate the vibration conditions of train operation. First, build a vibration test bench, install the bolts on the test fixture simulating rails and fishplates, and apply the same preload as that of the actual line. The vibration parameters of the test bench are set as follows: vibration frequency 10-50Hz, amplitude 0.5-1mm, vibration time 100 hours, simulating the impact of high-frequency train vibration. During the test, the preload attenuation of the bolt is monitored in real time. A torque sensor is used to measure the bolt torque every 10 hours, and the torque attenuation rate is recorded. The torque attenuation rate after 100 hours of vibration is required to be ≤5%. At the same time, impact load test is carried out, applying an impact load of 30t axle load for 1×10⁵ times. After the test, check the thread damage and anti-loosening structure failure of the bolt. The thread shall not have slipping or deformation, and the anti-loosening structure shall not have loosening. In addition, on-site dynamic testing is required. Install torque monitors in the turnout area and curve section of the operating line, continuously monitor for 3 months, record the changes of bolt torque when trains pass, and the torque fluctuation range shall be controlled within ±10%.
What are the selection guidelines and quality acceptance standards for bolts in different track scenarios?
The selection of bolts in different track scenarios should follow the principle of "working condition adaptation and strength matching". For turnout areas, select 42CrMo high-strength bolts with double nut anti-loosening, with a tensile strength ≥1080MPa; for ballastless tracks, select pre-embedded rib bolts with an anchoring force ≥120kN; for ballasted tracks, select Q450NQR1 weather-resistant steel bolts with an anti-corrosion grade ≥2000 hours of salt spray test; for urban rail transit, select stainless steel bolts (grade 2Cr13) to meet insulation and anti-corrosion requirements. The quality acceptance standards are divided into three levels: first, material acceptance, the chemical composition and mechanical properties of the bolts must comply with GB/T 3077-2015 standard, and the deviation of tensile strength and yield strength is ≤±5%; second, anti-loosening performance acceptance, the torque attenuation rate after vibration test is ≤5%, and no loosening after impact test; third, anti-corrosion performance acceptance, the surface rust area after salt spray test is ≤5%, and the coating adhesion is ≥5MPa. During acceptance, 20 bolts are sampled per batch, 5 for material testing, 5 for anti-loosening testing and 5 for anti-corrosion testing. If any index is unqualified, double sampling shall be carried out; if the double sampling is still unqualified, the batch shall be judged as scrapped. After passing the acceptance, the material grade and strength grade shall be marked on the bolt head to facilitate on-site identification and selection.