In rail engineering, the joint whereby the ends of two rails are connected to each other, by means of fish plates, causes dynamic impact on loads resulting in, among other things, rail failure, geometric unevenness, crushing of the ballast at the joint and a great deal of maintenance effort and cost, Plasser South Africa’s Linda Ndlovu tells Engineering News.
Further, the publication of the EN 14587-2 standard ‘Railway Applications – Track – Flash Butt Welding of Rails’ in 2009 had new requirements for flash butt welding, which available welding heads globally do not meet.
Rail track maintenance and track laying machines manufacturer Plasser & Theurer therefore developed a welding robot that would fully comply with this standard. The APT 1500 R welding robot has all the features which have become the norm for mobile flash butt welding machines, together with a number of unique additional features.
“Arguably the most important new and unique feature of the APT 1500 R is its ability to execute the welding of any length of long welded rails (LWR) on site and to perform the closure weld to connect two sections of continuous welded rail (CWR) without an additional external pulling device, thereby eliminating the closure Thermit weld. All welds can now be carried out at lower temperature ranges, allowing more efficient track occupations and a drastic increase in production,” Ndlovu notes.
This is made possible by, among other things, the rail clamps of the APT 1500 R that grip the rail at a force of 2.5 times that of current welding heads and the pulling cylinders that exert a traction force of 1500 N at a stroke distance of up to 200 mm, which allows welds to be made at rail temperatures up to 20 ºC below neutral rail temperature. After closure welding at below neutral temperatures, the rails are held by the rail clamping jaws until the weld can withstand the pulling forces in the rail.
Neutral Rail Temperature
To avoid rail stress exceeding the limits for breaking or buckling with changing tempera-tures, the last weld to join two sections of CWR are therefore carried out during the ‘neutral rail temperature’, which is midway between the rail temperature extremes experienced at that location, Ndlovu explains.
The duration of a ‘neutral rail temperature range’ is short and on many cold days, it may not be reached at all. This complicates work planning, which leads to lower production. Two sections of CWR can be welded at temperatures below the neutral rail temperature by stretching the rails with hydraulic pullers, which will artificially stress the rail to the required tensile stress for that temperature. However, this does not seem to be the practice, since the pulling equipment is cumbersome to transport and to handle.
“Another problem that arises, is despite great strides in Thermit welding material and practice, a Thermit weld will still have up to a third of the fatigue life of a flash butt weld; therefore, increasing the track life-cycle cost with every Thermit weld installed. Thermit welds are also time consuming and take 45 minutes to 60 minutes on each weld, compared with eight to ten flash butt welds an hour,” Ndlovu says.
History of Welded Rails
The first iron rails were cast in the 1770s but could only be made in short lengths, which required many rail joints. The first steel rails were made in 1857 which were possible to roll in longer lengths, thereby reducing the number of joints in the rails.
In an attempt to reduce the number of rail joints even further, rails were welded for the first time in 1899 using the Thermit welding process developed by rail networks developer Goldschmidt Thermit Group.
Flash butt welding was developed in 1930 and is based on the fusion principle, which does not require any additional welding material as the rail itself is used as a welding compound. This process, with its limited change to the metallurgical character of the rail material, results in a near flawless weld.
Flash butt welds were initially used - and still are today to a large extent - to weld the standard rolled lengths of up to 60 m into LWR exceeding 200 m in a stationary depot. The long welded rails are then transported to the installation site to be Thermit welded into CWR.
Plasser & Theurer developed the first mobile flash butt welding machine in 1973 capable of welding the standard rolled rails into LWR on site. It is used as an alternative to stationary welding depots. The last weld of two sections of CWR, referred to as the closure weld, is however, still Thermit welded as current flash butt welding heads do not have the necessary gripping or pulling force to pull and stretch two CWR sections.
Though CWR reduced maintenance costs associated with rail joints, the thermal expansion gaps provided with rail joints are no longer there. At high temperatures, CWR will experience compressive forces, which, if it exceeds the track’s resistance to lateral displacement, will cause the track to buckle, also referred to as a temperature kick-out.
At low temperatures, the CWR will cause tensile stresses. If there are any internal rail fissures, the rail could break in cold weather.
“The stresses associated with CWR must therefore be carefully managed,” Ndlovu concludes.