Rail Joint Gap Design and Temperature Stress Relief
What is the design basis for the rail joint gap?
The core design basis for the rail joint gap is the extreme temperature difference in the area where the line is located and the linear expansion coefficient of the rail, and these two parameters directly determine the expansion and contraction of the rail. The linear expansion coefficient of the rail is about 0.0118mm/(m·℃), which means that for every 1℃ change in temperature, each meter of rail will expand or contract by 0.0118mm. During the design, it is necessary to first determine the difference between the local maximum rail temperature and minimum rail temperature, then calculate the maximum expansion and contraction combined with the rail length. The joint gap must be slightly larger than this maximum expansion and contraction to fully release the temperature stress. In addition, the track type of the line needs to be considered: ballasted tracks have more space for rail expansion and contraction, so the gap can be appropriately reduced; ballastless tracks have strong constraints, so the gap needs to be appropriately increased. Only by comprehensively considering these factors can a joint gap suitable for a specific line be designed.
What are the differences in rail joint gaps in different climatic regions?
The extreme temperature difference in high-temperature and high-humidity areas is relatively small, and the expansion and contraction of the rail is limited, so the design value of the joint gap is small, generally controlled at 4-6mm. In cold regions, the temperature difference between winter and summer can reach more than 60℃, and the thermal expansion and contraction range of the rail is large. The joint gap needs to be increased to 8-10mm to meet the stress release demand. In plateau areas with large temperature differences between day and night, the temperature of the rail changes frequently. The gap design must take into account both the daily expansion and contraction and seasonal expansion and contraction, usually ranging from 6-8mm. Coastal areas have high humidity, and rail joints are prone to rust. The gap design needs to reserve a rust margin, which is 1-2mm larger than that in inland areas with the same temperature difference. The different temperature differences and environmental characteristics of different climatic regions directly lead to the differentiated design of joint gaps.
What hazards will be caused by an excessively small rail joint gap?
If the rail joint gap is too small, it cannot fully release the expansion stress of the rail caused by temperature rise, and the rail will experience longitudinal extrusion deformation, which will in turn lead to increased side wear of the rail head. Long-term extrusion stress will be transmitted to the fasteners and sleepers, causing diseases such as loose fasteners and cracked sleepers, increasing line maintenance costs. In the high-temperature period of summer, an excessively small gap may also cause the rail "buckling" accident, that is, the rail undergoes lateral bending deformation, which seriously threatens the safety of train operation. In addition, a too-small gap will increase the rigidity at the rail joint, and the vibration and impact when the train passes will increase, which not only reduces the riding comfort, but also shortens the service life of the joint components. These hazards will directly affect the stability and safety of the track system.
What is the difference in joint gap design between seamless tracks and ordinary tracks?
Ordinary tracks adopt a short rail splicing mode. Each rail has a limited length, and a joint gap must be set to release temperature stress. The gap size must be determined according to the temperature difference calculation. Seamless tracks, on the other hand, weld short rails into long rail strips and resist temperature stress through the constraints of fasteners and ballast beds, so there is no need to set joint gaps. The joint gap of ordinary tracks is a passive way to release stress, while seamless tracks are an active way to constrain stress, and the stress control logic of the two is completely different. In the expansion zone and buffer zone of seamless tracks, a small number of joints will be set, and their gap design needs to combine the expansion characteristics of long rail strips, and the value is smaller than that of ordinary tracks. This design of seamless tracks greatly reduces the number of joints and improves track smoothness, while the joint gap design of ordinary tracks focuses more on the complete release of stress.
How to maintain the effectiveness of the rail joint gap?
To maintain the effectiveness of the rail joint gap, first of all, it is necessary to regularly detect the gap size using special measuring tools. If the gap becomes smaller due to rust or deformation, it is necessary to carry out slot expansion in a timely manner. Secondly, keep the joint area clean, remove sediment, rust and other debris in the gap to avoid debris blocking and preventing the rail from expanding and contracting freely. It is also necessary to regularly check the status of joint components. Wear or looseness of fish plates, bolts and other components will affect the stability of the gap and need to be replaced or tightened in a timely manner. Before the arrival of extreme weather, such as summer high temperature and winter severe cold, it is necessary to check the joint gap in advance to ensure that it meets the temperature stress release demand. In addition, anti-corrosion lubricant can be applied to the joints to reduce component rust and extend the effective service life of the gap. Only by doing a good job in these maintenance tasks can the joint gap always play its due role.