Fishplate and Bolt Connection Matching Requirements
Why should the material of fishplates match the rails?
Fishplates need to transmit tension, pressure and shear force at rail joints. Only when the material strength matches the rails can damage caused by stress concentration be avoided. If the material strength of the fishplate is lower than that of the rail, it will first deform or fracture under the action of train load, causing loosening of the rail joint. Matching materials can ensure uniform stress at the joint and reduce joint diseases caused by wheel-rail impact. At the same time, the thermal expansion coefficients of the same or similar materials are consistent, which can avoid additional stress during temperature changes. In addition, matching materials are convenient for welding and maintenance, extend the service life of joints, and ensure the overall stability of the line.
What are the special requirements for fishplate bolts used in heavy-haul lines?
Fishplate bolts for heavy-haul lines need to have higher tensile strength and yield strength, with tensile strength not less than 800MPa to bear huge train loads. The fatigue performance requirements of bolts are more stringent, and professional fatigue tests are required to ensure no failure under long-term high-frequency vibration. The thread accuracy is higher, adopting coarse thread or extra fine thread design to ensure tight meshing with nuts and prevent loosening. The surface treatment adopts anti-corrosion processes such as thickened hot-dip galvanizing to resist corrosion from rainwater, dust, etc., and adapt to harsh operating environments. At the same time, the bolts need to have good toughness to avoid brittle fracture under heavy load impact and ensure reliable joint connection.
Why can't the bolt torque value be adjusted arbitrarily?
The bolt torque value is the key to ensuring the clamping force of the fishplate. Excessive torque will cause damage and fracture of the bolt thread, or deformation of the fishplate, affecting the connection effect. Too small torque cannot provide sufficient clamping force, and train vibration will cause the bolts to loosen, leading to displacement of rail joints, which seriously threatens driving safety. Different line grades and bolt specifications have clear torque standards, such as 800N·m±10% for high-speed railways, which must strictly follow the design requirements. Uneven torque will lead to unbalanced stress at the joint, local stress concentration, and accelerate component wear. In addition, arbitrary adjustment of torque will damage the overall stability of the joint and increase maintenance costs and safety risks.
What are the common standards for foreign standard fishplate bolts?
Foreign standard fishplate bolts mainly follow the American AREMA standard and ASTM standard, with threads adopting unified threads (UN series), such as coarse thread (UNC) or extra fine thread (8UN). These standards clearly specify the dimensional parameters, material properties and surface treatment requirements of bolts to ensure product compatibility and reliability. Common materials include medium carbon steel or alloy steel such as ASTM A307 Grade B and A193 Grade B7, which meet the performance requirements after quenching and tempering heat treatment. The standard also has strict specifications for the size of the bolt head shape, thickness, width across flats, etc., to adapt to special installation tools. In addition, foreign standards will specify material and process requirements under special working conditions according to different operating environments.
How to judge whether the connection between the fishplate and the bolt is reliable?
First, check whether the bolt torque meets the design standard, use a torque wrench to test one by one, and ensure the value is within the allowable deviation range. Observe the fitting condition between the fishplate and the rail, ensuring no gaps, loosening or deformation, and the rail at the joint is smooth without height misalignment. Check the state of bolts and nuts, ensuring no thread damage, slipping, fracture and other defects, and the lock washer is intact. Analyze the vibration and displacement of the joint through track dynamic detection data to judge the connection stability. Conduct regular flaw detection to check for internal cracks in bolts and fishplates, and handle abnormalities in a timely manner to ensure reliable connection.