rail tracks warp

Why Rail Tracks Warp in Extreme Heat

When the rail is placed freely, it will expand and contract freely when the rail temperature changes. It will be stretched when it is heated in summer, and it will be shortened when it is cooled in winter, that is, “heat expands and contracts with cold”. To connect multiple steel rails into a track, obviously there will be a joint every 12.5m or 25m. there should be a rail gap between the joints, about 8mm. The purpose of keeping the rail gap is to prevent the temperature force generated when the rail expands with heat and contracts with cold to damage the rail. Generally speaking, for every 33.8℉ change in rail temperature, each rail will bear 1.645tons of pressure or tension. When the rail temperature varies by 122℉, a steel rail with bear up to 82.25tons of pressure or tension. Such a huge temperature force is enough to destroy the normal state of the railway track switch. Therefore, such a large amount of expansion and contraction on the seamless railway is absolutely not allowed. It is necessary to use anti-climbing equipment to lock both ends, or set a strong line resistance on the line track to lock the track to limit the free expansion and contraction of the rail.

If both ends of the rail are fixed within a certain length range to limit the free expansion of the rail, when the rail temperature changes, the line rail will appear internal stress, this force is caused by the rail temperature change, so it is called temperature force. Specifically, after the seamless line is locked, when the temperature rises in summer, the rail is heated and expanded, but the constraint cannot be extended, and the internal compressive stress is generated; when the temperature is reduced in winter, the rail is shortened by cold, but is can not be shortened by constraint, and the internal tension is generated. Because the rail is so firmly locked on the sleeper, the rail can receive such a large temperature force without deformation, which is the basic principle of seamless lines.

When the rail temperature is higher than the lock rail temperature, the rail section of the seamless line should bear the temperature pressure. The temperature pressure is proportional to the number of degrees of positive change of the rail temperature. When the rail temperature rises to the maximum maxt, the temperature pressure reaches the maximum maxPt.

On the other hand, because of the joint resistance and the longitudinal resistance of the ballast, most of the temperature and pressure are limited to the rail section, and only a small part is released in the expansion zone. The temperature and pressure limited to the rail section must always follow the laws of nature and seek to be released in order to completely balance. When it reaches a certain value and still can’t find a way out in the longitudinal direction, it will go to the horizontal to seek a way out, and the curve of the seamless line just provides it with this opportunity, that is, the radial component of the longitudinal temperature and pressure synthesis Pr just points to the direction of the outside of the curve, so that the curve is bent upward. And the straight line can not be absolutely straight, once a certain bend, longitudinal temperature and pressure will also be along the direction of the bending of the radial component Pr, resulting in the direction of the bending of the straight track deformation.

In this way, as long as the temperature and pressure a certain value, the transverse deformation of the seamless line track is inevitable.

A large number of tests show that there are certain rules occurrence and development of this deformation, which can be basically divided into three stage: holding stage, expanding stage and runway stage.

1. Hold steady phase

The holding stage is the initial stage of the seamless line to withstand temperature pressure, in this stage, although the temperature pressure increases due to the rise of the rail temperature, the track does not deform and still maintains the initial state, and the temperature force is completely “stored” in the rail section in an elastic state. The smaller the initial bend of the rail, the higher the temperature and pressure value corresponding to this state. If the rail is an ideal geometric straight line, this state may continue until the temperature and pressure reach a considerable value, and then suddenly bulging will occur under the interference of external forces. However, due to various reasons, the rail can not be an ideal geometric straight line, there will always be some degree of bending; Therefore, the temperature and pressure of the rail in the stable stage cannot reach the aforementioned “considerable value”, on the contrary, the smaller the line resistance, the worse the track geometry, especially the direction, the lower the temperature and pressure resulting in the bulging deformation of the track.

Whether the track of the seamless line is “stable” depends on whether the temperature pressure has reached a critical value, that is, whether the rail temperature has reached a critical rail temperature. The critical temperature pressure or the critical rail temperature is high or low depending on the state of the line. For the same wireless line, as long as the temperature exceeds expansion stare from the stable state.

The temperature pressure that makes the seamless line enter the expanded rail from the holding state is called the first critical temperature pressure. In the holding stage, the seamless line is relatively safe.

2. Expansion stage

When the temperature of the rail continues to rise and the temperature pressure exceeds the first critical value, the expansion stage begins. At this stage, the increasing temperature and pressure make the track produce transverse deformation from small to large, from less to more, and sometimes can be clearly perceived by the naked eye – the curved line is becoming more and more obvious, the variant vector degree is becoming larger and larger, and the track direction is significantly bad.

But the temperature of the track cannot rise indefinitely. When it rises to a certain extent (as long as it is within the bearing range of the track) and begins to fall, as the temperature and pressure are gradually lifted, it may be seen that the deformation and bending of the track are also reduced until it returns to the initial state. In other words, in the expansion stage, the deformation of the track is elastic deformation.

The frontal elastic deformation of seamless track under the action of temperature and pressure is called expansion rail.

In the expansion stage, after the temperature and pressure are relieved, the elastic deformation of the track that can be restored to the initial state is only 2mm. In theory, the orbital elastic deformation of more than 2mm cannot be fully recovered after the temperature and pressure are lifted, and some residual deformation will always be left. The rail temperature changes repeatedly, and this residual deformation will accumulate and cause serious bad direction. Therefore, it is necessary to limit the expansion of rail in time.

3. Runway stage

In the expansion stage, the temperature and pressure did not exceed the bearing capacity of the seamless line, but it was possible to reach the limit of capacity. At this time, the relative stability of the seamless line has been barely maintained and security is at stake.

When the rail temperature increases slightly, the temperature and pressure continue to increase. If the track is slightly disturbed by external forces (such as train braking, construction influence, hammer rail, etc.), the excessive temperature and pressure accumulated on the rail section will suddenly cause a malignant change in the track geometry – the deformation degree in the expansion stage suddenly increases significantly, sometimes up to hundreds of millimeters, and the track makes a huge sound in an instant, seriously bulging, and the track is separated from the track and pulled down the track bed. Or the rail is separated from the sleeper, resulting in a complete loss of driving conditions. It can be seen from the severely distorted rail that its deformation has exceeded its elastic limit and become plastic deformation. The temperature force on the rail section has been released. The rail is in a “zero stress” state in its natural state, and the temperature pressure and line resistance are eliminated at the same time, and the line has been seriously damaged.

The destructive deformation of seamless track under the action of temperature and pressure is called runway.