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the rail engineer • October 2014
Innovative solutions from Sheffield
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tretcher bars are important components in the design of any set of points. They are the bars that lock the blades into place relative to each other. The whole blade assembly then moves as one unit.
The stretcher bars have to permit adjustment, so that the points can be set up properly, but then be locked to prevent movement. If the assembly goes wrong, as has happened on a couple of occasions, the results can be disastrous. Two major accidents over the last twelve years were put down to failure of this assembly, although in one case the direct cause was actually retaining bolts rather than the stretcher bars themselves. Be that as it may, the whole assembly has been under Network Rail scrutiny for some time as ways were sought to improve both the design and the inherent reliability of the system. Now, Network Rail has begun implementing a new design of high-integrity stretcher bars. Design and development of the new bars, which for the first time use tubular components, was undertaken in collaboration with Balfour Beatty Rail.
Rigorous testing
A new stretcher bar in situ.
Once design was complete, Network Rail went out to tender and appointed just three manufacturers for the new product. Being safety critical components, they had to go through several years of testing and refinement prior to entering manufacturing stage. The new tubular stretcher bars, which consist of a tube with articulated motion units, will replace both ‘fixed’ and ‘adjustable’ stretcher bars in new and existing installations. The tube has been designed to allow the overall length to be adjusted - one 360º rotation of the bar changes the length by 4mm. The motion units are articulated to enable the stretcher bar to cope with relative slip and twisting when the switches are thrown, while the bars furthest from the pivot also feature kicking straps which limit the amounts the switch rail can rise.
Tinsley Bridge Rail is one of just three suppliers to have been selected by Network Rail to manufacture the new design of tubular stretcher bars. It has been developed to give greater safety, improved performance and a reduced whole-life cost. In addition, it is simpler to install, requires less maintenance and is designed to withstand the rigours of the twenty-first century rail network. Requiring high levels of manufacture and quality assurance, this project followed a rigorous process to ensure that the completely-redesigned safety-critical equipment could be successfully introduced onto the passenger-carrying railway. It was developed and tested under the most challenging conditions, safely supporting 91 million tonnes of traffic in a testing environment before being introduced onto the UK rail infrastructure. “The group has a solid track record of working with world-class companies, but to be one of only three manufacturers in the UK to be chosen by Network Rail is a real achievement,” said Tinsley Bridge’s managing director Mark Webber.
Automotive and military experience The engineering group, which is over 150 years old, originally manufactured vehicle suspension springs and now manufactures anti-roll bars for truck producers such as Volvo and Renault, rolling stock parts for railways, and suspension components for military vehicles. It has a turnover of £16 million and employs 180 people. “Our automotive and military experience in the highrisk, zero-tolerance environment of truck suspensions gives us the right skillset to transfer to the safety critical rail project which demands the highest levels of quality and precision,” Mark Webber explained.