Date: 2025/7/21
Category: Metallurgical encyclopedia terms
Views: 56
(1) High-speed rail straightness requirements
High-speed rail straightness requirements
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Notes:
End Upward Curvature: The upward curvature at the rail end.
End Downward Curvature: The downward curvature at the rail end.
End Horizontal Curvature: The horizontal curvature at the rail end.
Body Vertical Curvature: The vertical curvature along the rail body.
Body Horizontal Curvature: The horizontal curvature along the rail body.
Overall Vertical Curvature: The vertical curvature across the entire rail length.
Overall Horizontal Curvature: The horizontal curvature across the entire rail length.
(2)Steel Rail Straightening Principle
Steel rails must undergo straightening of both the rail body and rail ends to meet straightness requirements. Generally, the rail body is straightened using a rolling straightener, while the rail ends are straightened using a four-sided straightener. The rail body undergoes multiple repeated elastoplastic bending deformations in the rolling straightener to complete one straightening process.
Modern heavy rail rolling straighteners are typically composite rolling straighteners, consisting of a flat straightener and a vertical straightener. The flat straightener uses horizontal rolls, with the straightening force acting on the rail head and rail base to correct vertical bending (up-down bending) of the heavy rail. The vertical straightener uses vertical rolls, with the straightening force acting on the rail waist to correct horizontal bending (side bending) of the heavy rail. The differences between flat straightening and vertical straightening are shown in Figures 1 and 2.
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(3) Straightening Force Analysis
During the straightening process, a 3-point bending force model is applied to analyze the forces. Between every three adjacent rolls, a 3-point bending plastic deformation zone is formed. The horizontal straightener undergoes 7 reverse bending plastic deformations (7 plastic deformation zones) to achieve straightness for up-down bending, while the vertical straightener undergoes 5 reverse bending plastic deformations (5 plastic deformation zones) to achieve straightness for side bending. The rail is subjected to pressure by the alternating arrangement of straightening rolls in the rolling straightener. Plastic deformation occurs when the actual stress on the rail exceeds its yield strength.
In the initial stages of straightening, such as the first and second bends, a larger reduction force is applied to create a significant reverse bending rate. This quickly reduces the unevenness of the original curvature, transforming the original bending with varying sizes and directions into residual curvature with consistent direction and approximately similar magnitude, making the rail curvature more uniform. In subsequent bending stages, the reduction force is gradually reduced, primarily to minimize the residual curvature that has become uniform, ultimately achieving straightness.
The straightening process of steel rails is a complex elastoplastic deformation process. This process can be divided into two stages: the reverse bending stage and the elastic recovery stage. During the reverse bending stage, the rail undergoes elastoplastic deformation due to external forces and moments. In the elastic recovery stage, the rail, under its own elastic deformation energy, attempts to return to its original equilibrium state. Straightening involves repeatedly undergoing reverse bending and elastic recovery to overcome internal elastic counter-moments, ultimately achieving straightness through yielding.
During the straightening process, different sections of the rail cross-section experience varying degrees of force and deformation. Near the neutral axis, elastic deformation predominates, while plastic deformation occurs farther from the neutral axis. The extent of plastic deformation depends on the straightening pressure. The deformation conditions for straightening are as follows: the straightening stress must be at least equal to the yield strength of the rail to ensure plastic deformation; however, the straightening stress must not exceed the rail's tensile strength, as this could result in fracture.
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(4)Residual Stress After Composite Straightening
Residual stress reduces the service performance of steel rails, particularly their fatigue strength. The magnitude of residual stress primarily depends on the straightening process and the initial state of the rail before straightening (especially its pre-straightening curvature). If the initial state of the rail is similar, the straightening process significantly influences residual stress. Flat straightening increases residual stress in the rail, while composite straightening can improve residual stress in the rail after straightening.
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