Continuously welded track
Producing closure welds below neutral temperature
Continuous welded track is certainly one of the greatest achievements of railway engineers in the last century. Besides the many advantages, such as a higher travelling comfort and reduction of noise generated, the introduction of the continuous welded track is linked with great economical benefits. The DB AG, for example, estimates that the maintenance costs have been reduced by 30% compared to jointed track and those for the vehicles by 5%. Furthermore, the continuously welded track is an essential requirement for high-speed lines.
Significance of the neutral temperature
On continuous welded track forces occur in the rails as the temperature changes: tensile stresses occur at low temperatures, compressive stresses occur at high temperatures as the expansion is prevented. Therefore a neutral temperature is determined for the rail at which the rail is tension-free. To perform closure welds at temperatures differing from this, an interrelation of forces has to be produced in the rail which corresponds to the present temperature conditions. This ensures that the continuous rail is not subjected to any undue tensions when the temperature fluctuates.
Hydraulic rail tensioning device from Plasser & Theurer
Several years ago,
The technique: the hydraulic rail tensioning unit is lowered onto the rails being welded. The rail ends are held in four clamping jaws of the tensioning device. Four pulling cylinders draw the rail ends to the required spacing and produce the necessary tension in the rail. The tensile force is spread over four pulling cylinders enabling the pulling forces to be transmitted to the axis through the centre of gravity of the rails. So there is no momentum in rail and tensioning device. To achieve exact alignment of the rail ends, four lifting and four lining cylinders are applied. Now the welding head can be lowered onto the rail gap. The welding process is carried out in the usual way, the pulling movements of the rail tensioning device are synchronised automatically with the movements of the tensioning cylinders of the welding head. A microprocessor controls the entire welding procedure. After completion of the weld and removal of the welding seam, the rail tensioning device remains in position until the weld has cooled down sufficiently.
APT 600 - the new welding machine
The latest flash-butt welding machine APT 600 made by