APPLICATION TECHNOLOGY
Arc suppression applications In addition to the standard switches and crossing, lockbolting has been used in some unique applications to overcome a specific engineering challenge. One such recent application was the design of arc suppression rails on the London Underground. On the London Underground, track fires had been a problem
for a number of years in some cases causing line closures and delays for commuters. The fires were being caused where there were connections and disconnections of the trains to the live power rail, leading to energy discharges that were arcing over and igniting litter and debris in the tube tunnels. In an effort to stop this, an ingenious solution was developed by the engineers at Metronet. A special arc suppression rail was created to capture
and dissipate this energy discharge in a controlled and safe manner. The theory behind the rail is to channel the energy and subsequent fireball from the contact between the live rail and contact shoe, down a sloped piece of rail and into a chamber that is enclosed with insulating material.
correctly torqued to ensure joint integrity. The solution to this challenge was solved by using Huck C50L fasteners. As in the standard rail switches and crossings, their vibration resistance made them the ideal solution to the problem. Since switching to these fasteners there have been no further issues with joint integrity, meaning only a visual inspection is required giving significant time savings.
Cold hole expansion A problem that has come to light in the rail industry is with
star cracking of drilled holes in rails, which is predominantly a problem where fishplates are used. The problem is with micro defects in the material created during the drilling process that act as stress concentration sites that can lead to eventual fatigue failures where the mechanical fasteners are used. To eliminate this problem, fish plate holes must now be cold hole expanded. During this process, an oversized tapered mandrel is drawn through the drilled hole that essentially irons out any surface defects leaving a smooth surface that is free from stress concentration sites. There is specialist equipment available to complete this task,
however Huck distributor Permalok has developed a special mandrel that can be used with existing Huck track bolting installation tooling to perform the task of cold hole expansion. Using the existing installation equipment offers significant logistical and cost benefits.
This cold hole expansions kit consists of the tapered mandrel
and ironing sleeve as pictured above. The sleeve is placed inside the drilled hole and the mandrel
After the contact shoe loses connection with the live rail after
passing over the insulated chamber, any bounce will be levelled out by the sloped lead-on rail before reconnecting with the live supply rail. The greatest challenge faced by the engineers in designing a functioning arc suppression rail was in finding the correct angle so that the arc could be reliably channelled into the discharge chamber. A number of test revisions were made before the final design was realised. This was not the only challenge faced however. It was found
that over time the bolts in the rail were working loose due to vibration. This meant extra maintenance to keep the bolts
124 Fastener + Fixing Magazine • Issue 72 November 2011
slid through. The operation is then to simply draw the mandrel through the hole using a standard Huck track bolting tool.
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