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Reliability through redundancy

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Tradition can be a great thing. Most of us enjoy our traditional Christmas lunch and are amused by such quirky British traditions as Morris dancing, Gloucestershire cheese rolling and piping in the haggis. The changing of the guard at Buckingham Palace and the Tower of London’s Beefeaters certainly boost the British tourist industry. However, some traditions are not so great – sitting in a traffic jam on a bank holiday Monday, for example.

In railway engineering, many things are produced in a traditional way for very good reasons. They incorporate years of hard-won experience and there has to be a very good case to change the proven design of a safety-critical component. Railway switches and crossings are a good example. In the very early years, trains were generally switched from one track to another by stub points, in which a pair of rails was slid to align with the selected diverging rails. In addition to the impact load at the resultant gap, these points suffered from thermal expansion that led to a wide gap in winter and tight switches in summer.

One of the first recorded uses of switch rails was on the Elgin Railway in West Fife in 1821. As the early railways were built, various types of points were subject to patents, including one of 1838 by Charles Fox for a design with a single switch rail. Today’s basic switch first appeared in patent filed in 1843 by Charles Wild. Since then, such switches have been developed into a robust design used worldwide, although they remain a significant cause of operational failures.

Now for something completely different

Trains running on plain line at two to three minute intervals can carry high volumes of traffic. However, somewhere along the line are nodes such as junctions and stations that significantly reduce capacity. In 2010, this issue was the subject of a call for proposals for research funding to overcome constraints caused by such nodes, issued by a joint initiative of the Engineering and Physical Sciences Research Council (EPSRC) Strategic Partnership, Department for Transport (DfT) and RSSB (Rail Safety and Standards Board).

This resulted in the award of five grants to investigate innovative radical approaches to reduce the impact of nodes (issue 101, March 2013). Most of these concerned the manner in which movement authorities through junctions were issued. Loughborough University’s proposal was the only one that that concerned the more effective design and operation of points. Its remit was to undertake a fundamental re-think of railway track switching.

Loughborough’s approach was to ask interested parties what they want from a set of points. The answer was instantaneous switching, no maintenance, no failures, no space requirement, zero energy usage, no speed restriction and zero cost. Achieving all these wishes is, of course, impossible – but well worth aiming for.

After considering around 400 separate ideas, it became apparent that it might be possible to break with tradition by developing a radical new switch design that goes a long way to meeting these requirements. This included the reduction of switch movement time to under a second from around four seconds for a conventional switch, so offering a capacity benefit.

Inspired by redundancy

Professor Roger Dixon is acting Dean of the Loughborough University’s School of Electronic, Electrical and Systems Engineering. He is also head of its Control Systems Research Group, which Roger describes as an interface between academia and industry as the focus is on applied research. This group is undertaking research projects for the rail, aerospace and energy industries.

One example of its rail research is the development of a low adhesion detection system which monitors and analyses vehicle running dynamics to detect a low wheel / rail coefficient of friction before brakes are applied. A simulation using Vampire software has shown the viability of this concept. The researchers have a vision that, in the future, control engineering should be built into rail vehicle design using actively guided wheelsets that would provide extra space by eliminating bogies and give better ride performance.

On the permanent way, the group’s revolutionary points mechanism, REPOINT, may well start to replace the traditional switch design in the near future. REPOINT (Redundantly Engineered POINTs) is the result of Loughborough University’s research into improved switches to address the junction node problem. Roger advised that this work is inspired by his group’s research into redundancy in aerospace systems.

REPOINT’s development has been in two stages. From May 2011 to May 2013, the concept was developed using the grant from EPSRC, DfT and RSSB. In 2013, Future Railway gave the initiative an award from its ‘always open’ Rail Innovation Support Engine scheme. From June 2013 this award, combined with funding from HEFCE (The Higher Education Funding Council for England’s Higher Education Innovation Fund) has been used for laboratory work, the management of intellectual property issues and exploring how best to implement the concept.

21st century stub switch

Commercial aircraft typically have triplex or quadruplex redundancy built into flight control systems. However, unlike flight control surfaces, a railway switch has to be locked in position for passing traffic. Hence, even if there were multiple actuators, these would need to act through a common locking mechanism that is a single point of failure. To avoid this problem, the Loughborough group has devised, and patented, a switch mechanism that is inherently failsafe.

This uses a stub switch in which the full- section switch rails mounted on locking blocks are lifted by cams and moved laterally from one locked position to another. A motor-driven actuator rack drives these cams. When lifted out of these blocks, the force required to back-drive the motor is less than that required to bend the rails during the switch movement. Hence, a power failure will result in the switch falling back to a safe locked state. The use of a stub switch with full section rails also eliminates the failure mode of a blockage between switch and stock rails.

The cams and locking blocks are contained within an actuator bearer, the size of a sleeper, at the end of which is the drive motor. It is envisaged that there would typically be three actuator bearers at the end of the switch. Between these actuators and fixed sleepers there would be passive locking bearers and passive non-locking bearers (i.e. sliding bearers). The locking blocks would be positioned to allow the rail to bend into a transition curve and also provide turnouts with different cants if required. Another feature of this design is that the switch can have multiple turnouts.

As a roller bearing forms an integral part of the cam, there are virtually no friction losses. Unlike a conventional switch, there are no sliding surfaces. Hence, energy use is low. After the switch is lifted to its high point, the raised switch rails drive it down to its new position. As they do so, the actuator motor acts as a brake to reduce impact on the locking blocks. In this locked position, the cams are a few millimetres below the locking block and so take no impact load from passing trains. This arrangement offers maintenance advantages as bearings have a long life and their wear is predictable.

In addition to other advantages listed above, this mechanism can move the switch in a fraction of a second. Yet despite all these advantages, it seems that Loughborough has just re-invented the stub switch which was not suitable for the traffic of the 19th century, let alone that of the 21st century.

Interlocking rail ends

Research and development engineer, Sam Bemment, explained that this is not a problem for REPOINT as the group has designed and patented an arrangement of interlocking rail ends. These incorporate a sliding arrangement similar to a breather switch. The fixed rail end incorporates a raised V-section onto which REPOINT’s lift and drop mechanism places a recessed V-section in the switch rail end. This arrangement both allows for thermal expansion, avoids impact load and provides an additional locking mechanism.

Maintenance and installation of REPOINT has been carefully considered. The actuator bearer is a sealed unit containing the actuation rack, cams and locking blocks. The motor and rack drive is at the end of the bearer and so could be in the cess.

The motor can be replaced whilst the switch is in service, as this would not affect operation of the other actuators. Replacement of the actuator bearer would be a similar operation to replacing a sleeper although the switch would first have to be raised off its locking blocks to the mid-point position. For this, the control system would incorporate a maintenance position.

During tamping operations, a locking pin would be inserted so that the actuator bearer would lift with the rail. These locking pins would also be used during installation to enable the switch unit to be handled as a track panel.

Getting to TRL9

Loughborough University’s Control Systems Research Group has developed the REPOINT concept to the stage that the concept has been validated through simulations and laboratory demonstration of an actuator bearer and its control system. It is thus at technology readiness level (TRL) 4 – “Technology component validation in lab”. Much needs to be done before it reaches TRL9 – “Actual technology system qualified through successful operation in railway environment”.

The layout of the laboratory demonstrator is 384 mm gauge, which is that of the Romney, Hythe and Dymchurch Railway (RHDR) that had been identified as a possible technology demonstrator site, although the actuation has been designed for a standard gauge railway.

Manufacturers have also shown an interest in producing a standard gauge REPOINT prototype and both Network Rail and Transport for London have expressed significant interest in the concept. As a result, the RHDR demonstrator step may not be required.

The production of a prototype switch is likely to cost a large six-figure sum and further work to get REPOINT to TRL9 will cost much more and take years. Roger has no doubt that the benefits are massive but obtaining the required investment to realise these benefits is a challenge.

For this reason, the University has commissioned Interfleet Technology to produce a business case for REPOINT.

Roger confirmed that the rail industry has expressed significant interest in the REPOINT concept. This has resulted in a suggestion that this radical concept would be best trialled in stages. For this reason, the University is developing “REPOINT Light” in which actuator bearer actuator units will move switch blades in a convention point design. This will lose some of the stub switch benefits, but allows demonstration of the new bearer concept as the first step with the possibility of developing the full REPOINT vision later.

Ticking the boxes

REPOINT ticks all the boxes from the points wishlist derived at the early stage of the project and so has the potential to deliver huge cost savings with a significant increase in reliability and safety. Yet it has a long way to go to prove that it offers a robust alternative to conventional switches. The investment to do this through trials, testing and certification must be justified.

When certification is obtained, it is highly likely that, after 150 years, it will be time to break with tradition to change the design of railway switches. Who knows what other traditional designs might benefit from the application of systems engineering practice used in other industries? No doubt, the answer lies with Universities and other research institutions which are challenging tradition as have Roger Dixon, Sam Bemment and the team at Loughborough’s Control Systems Research Group.

David Shirres BSc CEng MIMechE DEM
David Shirres BSc CEng MIMechE DEMhttp://therailengineer.com

Rolling stock, depots, Scottish and Russian railways

David Shirres joined British Rail in 1968 as a scholarship student and graduated in Mechanical Engineering from Sussex University. He has also been awarded a Diploma in Engineering Management by the Institution of Mechanical Engineers.

His roles in British Rail included Maintenance Assistant at Slade Green, Depot Engineer at Haymarket, Scottish DM&EE Training Engineer and ScotRail Safety Systems Manager.

In 1975, he took a three-year break as a volunteer to manage an irrigation project in Bangladesh.

He retired from Network Rail in 2009 after a 37-year railway career. At that time, he was working on the Airdrie to Bathgate project in a role that included the management of utilities and consents. Prior to that, his roles in the privatised railway included various quality, safety and environmental management posts.

David was appointed Editor of Rail Engineer in January 2017 and, since 2010, has written many articles for the magazine on a wide variety of topics including events in Scotland, rail innovation and Russian Railways. In 2013, the latter gave him an award for being its international journalist of the year.

He is also an active member of the IMechE’s Railway Division, having been Chair and Secretary of its Scottish Centre.


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