It’s an odd fact that some of the best inventions are based on the simplest of ideas. We might look at some brilliant new solution and wonder why no one thought of it before. But then, perhaps they did?
Sometimes the real trick in problem solving is in bringing together old ideas. A valuable new addition to Network Rail’s arsenal of plant machinery makes use of a very old technique… with some added oomph!
Forge welding has been used to join metals for millennia. Blacksmiths since ancient times have been familiar with the technique of heating metal parts to a high temperature and then hammering them together. In today’s techno- speak we might term this a solid-state diffusion process.
Importantly, the result is a welded joint that comprises only the original metals, with no fillers or bridging materials. Since the industrial revolution, this method has been superseded for convenience by gas and electrical welding – processes that add material. This added material may have different physical and chemical properties to the metals it joins.
The welding of railway lines might seem to be a modern idea, but continuous welded rail (CWR) has been used in the USA since the late 1890s. Here in the UK, though, it didn’t find favour until the 1960s. Welding techniques and the rails themselves have improved steadily over the past five decades, but alumino- thermic welding remains the most popular method. Molten iron produced by an exothermic chemical reaction is cast into a ceramic mould that surrounds the rail ends. In other words, metal is added to fill a gap. An advantage is that no complex or heavy equipment is required, but great care needs to be taken to eliminate voids and slag inclusions. Even a perfect alumino-thermic weld has properties that differ from those of the rails themselves.
The time available for track maintenance and renewals is ever-more- constrained by the drive to increase capacity on the rail network. Accordingly, in Control Period 4, Network Rail made a commitment to order ten Mobile Flash Butt Welders (MFBW). Finance came from the £220 million seven-day-railway fund, established to support schemes offering substantial improvements in network availability.
The MFBW equipment uses an electric current to heat up the rail ends which are then hydraulically pressed, or forged, together. Sean Heslop is Network Rail’s programme manager for rail services (Network Operations Delivery Services). As he puts it: “It used to take up to four shifts to fix a defective rail because of the time it took to weld and stress the new sections of rail in the short midweek possessions that were available.”
Rails are normally delivered to site in 216 metre lengths where they have to be stressed – stretched to the length they would be at 27°C – and welded together to form CWR. “We knew there was a piece of equipment out there that could deliver this work in a fraction of the time,” said Sean. “But there were a lot of problems with older types of MFBW. They couldn’t do the stressing job and there were also issues with them interfering with the signalling and telecommunications systems.” The equipment was also large and difficult to transport. With Network Rail undertaking approximately 60,000 welds per year (2012 figure) there is clearly a need for MFBW equipment that is self- contained, fast to operate and easy to move from site to site.
The MFBW solution now being adopted by Network Rail marries the latest K945 flash butt welding head developed by Holland Company of Illinois, USA, with a modified Doosan DX170W wheeled excavator. An on-board Marathon Electric Magnaplus three-phase generator, powered by a Deutz V6 diesel engine, provides the electricity for the boom-mounted welding head.
The vehicle’s rail controls are managed through the GOS ‘Rail Safe’ Canbus system which controls the rail lighting (four white and four red LED sets) complete with auto directional switching, auto horn sounding when the machine starts to move, speedometer, extra boom services and extra working lights. GOS Engineering, based in Blaenavon, is also responsible for fitting the Holland welding equipment and its computerised control system.
The first machine was delivered in 2012 and four are now in service. Another six machines are undergoing approvals.
Flash butt welders have been used on the UK rail network for about 15 years, but the key advancement now is in the ability of the new equipment to stress the rails as they are being welded. Using the new machines, up to 600-metres of track can be re-railed, stressed and welded in a single eight-hour possession. Previously the whole operation could take up to four shifts.
Stressing and welding can be accomplished in-track, from the lineside, or from adjacent lines. Safety check-valves fitted to the boom cylinders even allow the MFBW to be operated under live overhead lines. The three-piece offsettable knuckle boom also allows rail welding to be undertaken with the adjacent lines open to traffic.
When stressing is involved, there is a waiting time of just eight minutes from completion of the stress weld – primarily the time taken for the rail weld to cool to below 400°C. With the alumino-thermic welding process, this waiting time is 30 minutes.
Rail Engineer was recently invited by Network Rail to view an MFBW machine in action on the High Marnham test track. The time-saving benefits were obvious, but Bob Hervey, Network Rail’s project manager for the MFBW programme, was keen to highlight the other important benefit.
“Flash butt welding offers better performance and fatigue strength than alumino-thermic welding,” he commented. “Because no material is added, the rails and the weld are homologous. With no resultant hardness differences and the virtual elimination of inclusions and flaws, these welded joints can be bent and flexed in the same manner as the original rail.”
Using the MFBW, the stressing and welding process itself takes just two minutes, meaning that a defective rail can be changed in less than one hour. Once the rails have been aligned and clamped in the welding head, a pulsed AC current of typically 600-700 Amps (800 Amps peak) is passed through the rail ends to heat them. When the rail end temperature has risen to 800- 900°C, hydraulic rams bring the rails together with a typical force of about 40-50 tonnes (100 tonnes max). It is this movement that can be used to simultaneously provide the rail stressing.
During the forging (or ‘upset’) process, a further 27mm of rail length is lost. The excess metal is squeezed outwards and this flashing is trimmed off when still soft by a shear die that closely corresponds to the rail profile. Once cooled, the railhead can then be ground to a perfect finish.
The maximum rail pull for stressing is 900-metres of unclipped rail. Working alongside the MFBW on the High Marnham test track was a Rosenqvist CD501 high-output clipper. This self-propelled machine, produced in Sweden, is designed to work with Pandrol Fastclips and SHC clips. It will unclip or re-clip 900-metres of rail in 25 minutes. By hand this would take an eight-man team about two hours to complete.
Working together, the MFBW and clipper machines make an impressive team, typically giving a completion time from burn-in to fully stressed and clipped up of less than two hours.
The Holland computerised weld management system calculates the required starting rail gap and tonnage required to achieve the correct degree of stressing for a given rail temperature and rail profile. Sensors within the weld head measure distance, current, voltage, temperature and pressure in order to control the welding process. There is continuous monitoring and recording as the weld progresses, plus an analysis of the completed weld. The results are stored in a database and historical weld data can be viewed either as a full report or a one-line summary.
There is an alternative method for stressing the rail, which Bob Hervey was keen to show us during the High Marnham demonstration. It offers a further saving of time when replacing short lengths of rail, as only about 90-metres of rail needs to be unclipped. The freed rail is then barred into the four foot to form a loop. Cut lengths of scaffold pole act as runners to reduce the manual effort involved. The loop creates the rail end gap needed for the stressing and welding process. Again, the on-board computer system calculates the exact gap required. As the weld is made the unseated rail is pulled back into position with the correct tension.
A key feature of the butt welding process is the need for one rail to move. The technique cannot therefore be used for welding within switches and crossings, but it is suitable for all other rail welds.
Network Rail has negotiated terms and conditions with the trade unions so that its own staff can run and operate the MFBW machines rather than making use of contractors. Two teams of five men are assigned to each machine, geared to delivering seven shifts per week from a base of 250 shifts per year.
Simple is best
The MFBW initiative makes use of a simple idea, albeit with a high degree of precision and control. As Bob Hervey put it: “The craftsmen who fashioned medieval Samurai sword blades would recognise the principle of what we’re doing.” Simple in essence perhaps, but Bob cannot hide his enthusiasm for the benefits of this system, not only because of the time and track access savings it provides, but also because of the increased performance of the welds themselves.
“Even the worst flash butt weld is superior to the best alumino-thermic weld, which we’ve been relying on for the past 50 years,” he said.
In view of the 60,000 alumino-thermic welds undertaken each year on the network, the deployment of these new MFBW machines seems set to revolutionise rail welding within the UK.