Strength and stability of seamless rails
Expansion zone, fixed zone and buffer zone
The joint resistance at both ends of the welded long rail is a concentrated force, while the resistance of the ballast along the line is a distributed force. If the longitudinal resistance of the ballast is expressed as the resistance per meter of rail (kN/m), then at the end of the rail, the sum of the two is +. This value increases with the increase of. If the length of the welded long rail is, its maximum value can reach +/2. However, the local maximum rail temperature is always limited, so the maximum rail temperature change range [max1] will also be limited.
Therefore, to balance the maximum temperature force [max1]=[max1] generated thereby, it is not necessary to use all the ballast resistance on the welded long rail, but only a part of the length close to the rail end. At this time, the balance formula of temperature force and resistance is [max1]=+, and the value obtained from this.
In the range, a part of the free expansion and contraction cannot be achieved due to the limitation of joint resistance and ballast resistance, resulting in different degrees of restricted expansion and contraction. This area is called the expansion zone. The middle part outside the expansion zone at both ends is completely restricted in free expansion and contraction, and is therefore in a completely fixed state. This area is called the fixed zone. It can be seen that the maximum temperature force on the seamless line occurs in the fixed zone, which is only related to the maximum rail temperature change amplitude of the rail.
Theoretically, the length of the welded long rail can be free of any restrictions, but other factors should be considered during actual laying. For example, at the boundary between two automatic block areas, the long rails should be disconnected to install insulating joints; and when approaching the turnout groups at both ends of the station, the long rails should also be interrupted to facilitate the maintenance and repair of the turnouts. A buffer zone should be set at the interruption of the welded long rails. The buffer zone consists of 2 to 4 or more standard rails. The purpose is to facilitate the adjustment of the rail joint, release stress, and repair and replace insulating joints and turnouts.
To prevent rail breakage, the seamless line should have sufficient strength. The requirements for the strength calculation of the seamless line are: under the action of the train power, the sum of the bending stress, temperature stress and braking force of the welded long rails shall not exceed the allowable stress of the rail steel material.
In addition to meeting strength requirements, seamless tracks must also meet stability requirements. Practice and theory show that the possibility of seamless tracks buckling on a vertical plane is very small, and track expansion always occurs on a horizontal plane, starting at the original bend of the track.
When the track temperature is not high and the temperature pressure is not large, the track buckling deformation is minimal; as the track temperature and temperature pressure continue to increase, the track deformation will gradually increase, but will not cause sudden damage; once the temperature pressure rises to a certain critical value, if the pressure increases slightly or is disturbed by external forces, the track deformation will suddenly increase sharply, and finally lead to a complete loss of stability. The gradual stage of track buckling is called track expansion. The sudden change stage is called track.
