Causes of Misalignment in Fishplate Joints and Its Impact on Bolt Stress
Q1: What are the main causes of rail joint step?
A1: Uneven rail head wear at rail ends and inconsistent settlement of under-rail pads cause height difference between adjacent rails, forming vertical step. Lateral rail displacement caused by lateral force in curves, or loose bolts and worn fishplates, result in horizontal step. Uneven temperature force and uneven subgrade settlement also push rails out of position, forming step. Once step appears, it continuously expands under train impact, forming a vicious circle.
Q2: How does the force on fishplates change under step condition?
A2: Under normal condition, fishplates mainly bear tension and shear force. After step occurs, they additionally bear large bending moment. The bending moment causes tension in the middle and compression at the edges of fishplates, resulting in highly uneven stress distribution and easy crack initiation in the middle. Larger step leads to greater bending moment, faster fatigue damage and even bending deformation or fracture of fishplates, losing joint connection function.
Q3: How does joint step aggravate bolt fatigue fracture?
A3: Step changes bolt loading from simple tension to combined tension-bending, greatly increasing stress amplitude. Strong impact during train passage imposes repeated dynamic loads on bolts, significantly shortening fatigue life. Step also causes uneven bolt force, overloading some bolts and loosening others. Overloaded bolts are prone to necking and cracks, eventually leading to sudden fracture and endangering operation safety.
Q4: Why are joints on sharp curves more prone to step?
A4: Sharp curves bear huge wheel-rail lateral forces. Rails are continuously pushed outward, making joints highly susceptible to horizontal dislocation. Frequent gauge variation and complex fastener force in curves make bolts and fishplates easier to loosen. Meanwhile, speed variation and greater impact during train passage accelerate joint defect development. Therefore, sharp curves are high-incidence areas for joint step and require key monitoring.
Q5: How to rectify and prevent joint step?
A5: Promptly retighten joint bolts to ensure close fitting between fishplates and rails. Adjust under-rail pad thickness to eliminate vertical step. Grind or replace severely worn rails to restore joint smoothness. Strengthen subgrade maintenance to prevent uneven settlement. Add reinforcement measures at curved joints to improve overall joint stiffness. Regularly inspect joint status and handle slight step immediately to avoid defect expansion.