Before the invention of continuous welded rail (CWR), traditional railway tracks are lengths of rails joined by rail joints (joint bars, fishplate). If you look closely at the rails, you will find that every certain distance, that is more than 12 or 24 meters, there will be a small gap between the two rails.
It turned out that this was done to solve the problem of thermal expansion and contraction of railroad rails. If there is no such gap, the steel rails will squeeze, twist, and buckle when they are extremely heated, and deform the entire railway. When the weather is hot in summer, the length of the rails increases and the rails without reserved gaps can only bulge upwards, which is not good for driving safety. To avoid this phenomenon, a gap must be reserved between the rails.
With the increase of transportation tasks of a railway line, the impact of the wheels on the rails will be greater when the train is running. To ensure driving safety, the gaps on the line should be set evenly. If the rail gap is too large, the train will produce low joints on the line. Under the effect of the impact changes of the wheels, the wear of the joints in the rail will be accelerated, which can seriously cause the crack of railhead or derail of the train. However, if the rail gap is too small, under the action of temperature changes, internal stress is generated inside the rail. When this stress exceeds a certain level, the rail will move laterally and destroy the normal state of the line. The size of the rail gap is very important to the safety of the train. Therefore, when the rail gap changes beyond the normal range, the rail gap must be adjusted in time.
How long should the gap between the rails be?
To drive safely, rail gaps generally cannot exceed 18mm. It is determined by experiments: every 1 ℃ change in the temperature of the rail, the rail will expand and contract by 0.0118 mm per meter. In China, the temperature difference between winter and summer on railway lines in the south and north is usually about 80℃. According to the calculation of the solid linear expansion relationship, the length of each section of rail is preferably 12.5m.
In daily inspections, sometimes it is found that there is no gap between the two rails. To eliminate such potential safety risks, the rail gap adjusters will be used for processing. Under normal circumstances, there is a certain amount of clearance between the front and rear rail joints. It is necessary to move two rails to adjust to the proper rail gap. Before the operation, measure the rail temperature, then loosen the rail clips and remove the gauge block to reduce the resistance of the rail to move. Next, place the rail gap adjuster at the joint to toggle the clamp to the fix the rail, and finally shake the joystick and adjust the rail gap to the standard range of 6-18mm.
Special attention should be paid that the gaps are between millimeters. When shaking the operating lever, you must always pay attention to the distance between the front and rear joint gaps. Do not excessively shake the control lever to cause the front and rear joint gaps to be too large or too small. Tighten all the bolts after completion. In this step, you must carefully check each bolt to prevent loosening and creeping of the rail. The precise control of rail gaps in millimeters is related to the service life of the rail and the safety of train operation.
Why do high-speed rails have no gaps
The reason why we choose to use no gap rails is because of the requirements of high-speed railways. As the wheels impact the rails, when the train runs too fast, the gaps between the rails may cause the train to derail. When the train speed exceeds 140 kilometers per hour, it is necessary to use continuous welded rails.
The continuous welded rail controls the deformation of the rail due to thermal expansion and contraction between the two sleepers. The specific method is to firmly press the rails on the sleepers with tie plates and bolts. Because the fixing device is elastic (up and down direction), when the lifting force generated from the upward of the steel rail, the fixing device transfers and absorbs the force to itself, and finally transmits the force to the sleeper through the screw. The expansion coefficient of the rubber pad under the rail is larger than that of the rail, so the bottom of the rail can always be kept in a non-empty state.