February 04, 2013

Is customised maintenance the answer to track stress?

Written by  Dr David Rhodes
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With heavy-haul operations placing significantly more stress on track structures and components, railways should consider customised maintenance schedules and practices, as Dr David Rhodes, Pandrol's technical director, explains.

ON the face of it, rail fastenings are not required to do anything more in heavy-haul applications than they are in any other railway. Heavier rail sections and closer sleeper spacing can compensate for higher axleloads and the loading on the fastenings does not need to be any higher than usual. However, in practice the demands of the specialist heavy-haul operators are such that quite different approaches to track component design are usually necessary.

The detailed design of rail fastening systems affects parameters such as track gauge, track stiffness and rail inclination, all of which affect, in turn, wheel-rail interface mechanics. Of course this makes fastening specifications a critical factor in keeping railways running smoothly. However, in the case of heavy-haul railways three additional factors are highlighted which combine to make these requirements even more challenging.

First, compared with conventional railways, the annual passing tonnage on heavy-haul lines can be very high. In conventional applications, 20 million tonnes per annum constitutes a busy line, but in heavy-haul operations 200 million tonnes per annum is not unusual.

This means that if components still require maintenance or replacement after the same accumulated passing tonnage, maintenance intervals are naturally much shorter. What is less obvious is the effect on life-­­­cycle cost analysis. Maintenance activities which would be discounted to a negligible net present value on a conventional railway because they occur so far in the future are much more significant on a heavy-haul railway.

SAM 2886At the detailed level of the rail fastenings the cost of maintenance and replacement does not lie in the cost of the components themselves but in the cost of labour and track closures. There are considerable economic benefits in replacing small track components only when other maintenance work, such as re-railing, is being carried out.

This suggests that considerable savings can be made by ensuring the life of fastening components exceed the life of the rail. Inevitably this becomes more of a challenge as rail life is extended, with the major US heavy-haul railways now expecting rail to last for 3 billion tonnes of traffic in tangent track. In the case of a heavy-haul line with high annual passing tonnage the net present value of such savings is high enough to justify investment in more durable components.

Secondly, the effects of traction forces on longitudinal track stresses can be very significant. For most railways, when track is designed to cope with forces related to thermal expansion and contraction, and the maximum expected braking forces, then it will by default be strong enough to withstand the applied traction forces.

High traction forces

However, in heavy-haul applications this is not the case. On uphill grades the sustained application of high traction forces from train after train can induce track failure modes not experienced elsewhere. Usually the first sign of failure is uneven movement of sleepers which become skewed and displaced relative to the rails and the ballast.

The solution to the problem lies in the design, construction and maintenance of a high quality track bed and selecting rail fastenings with appropriate longitudinal shear elasticity. Tests carried out on several railways over a number of years have shown that different types of rail pad which have similar performance in conventional type approval tests do, nevertheless, give quite different performance in terms of mitigating the effects of high traction forces.

Finally heavy-haul railways are often unusual in demanding all of these things in some of the most hostile environments our planet has to offer.

As deposits of minerals - especially iron-ore - are exploited in ever more inaccessible places it becomes necessary to construct railways which can be built, operated, and maintained in extreme climatic conditions. For components made from steel and concrete this is not too difficult, but there are two things in particular which function quite differently at extremes of temperature and humidity.

The biggest technical problems are those associated with plastics. Within the rail fastenings system plastics are used to provide electrical insulation, resilience and sometimes sacrificial wear elements. Materials such as nylon function well in most climates but are softened in hot, wet conditions and become brittle in dry conditions. A significant amount of work is being undertaken to find additives which can mitigate these effects or to evaluate the use of completely different engineering polymers which may not give the best performance in average conditions but work acceptably well in a range of extreme environments.

Another key factor that is constrained by extreme climates is the human body itself. This may not sound like a technical issue, but a great deal of manual labour is critical for track construction and maintenance. When heavy-haul lines are built in inaccessible and inhospitable places the pressure to introduce more automation and to extend maintenance intervals is increased because of the additional human factors.

Most of today's heavy-haul railways are proposed not by established railways but by mining companies. As the most economical and reliable way to move bulk commodities from source to consumer, railways have become an essential part of many of the world's most important mining projects.

As a result, in most cases, the whole job of designing, building and even operating the railway is put out to competitive tender in the same way as any other capital investment. This process requires specifications of technical performance to be written into a commercial contract. However, no established technical standards take into account the kind of factors discussed above.

To make it even more difficult, technical standards which exist within individual railway networks are closely interdependent. For example, the calculation of the loads which are applied to test a sleeper or rail fastening system is based on assumptions about the stiffness and consistency of the track bed. This in turn is based on another assumption; that track maintenance limits specified elsewhere in the system will be observed, and that these limits are based on empirical assessment of particular track and traffic conditions.

Simply adopting technical specifications from one railway and applying them to another in a different part of the world is rarely sufficient. The heavy-haul railway industry has an enviable record of sharing technical knowledge through organisations such as the International Heavy Haul Association. It is through this process that best practice can be exported to new and more challenging projects.

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