Автор: Dr. David Rhodes, Former Technical Director at Pandrol

Rail fastenings for heavy haul applications

Rail fastenings expert Dr David Rhodes looks at how the special requirements of heavy haul railways can be met with the fastenings specification

 

«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 axle loads 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 detail 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 that all makes the fastening specification a critical factor in keeping railways running smoothly, but in the case of the Heavy Haul railways three additional factors are highlighted which combine to make these requirements even more challenging.

High Annual Passing Tonnage

Firstly, compared with conventional railways, the annual passing tonnage of Heavy Haul railways can be very high. In conventional applications 20 million tonnes per annum constitutes quite a busy line. In Heavy Haul operations 200 million tonnes per annum is not unusual. The obvious effect of that is that if components still require maintenance or replacement after the same accumulated passing tonnage, that maintenance requirement is reached in a much shorter elapsed time. What is less obvious is the effect which that has on life cycle cost analysis. Maintenance activities which would be discounted to negligible net present value on a conventional railway, because they would occur so far in the future, are much more significant on the Heavy Haul railway.

At 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. That suggests that considerable savings can be made by ensuring that the life of fastening components exceeds the life of the rail – and that becomes more of a challenge as rail lives are 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.

High Traction Forces

Secondly, the effects of traction forces on longitudinal track stresses can be very significant. For most railways, once track has been designed to cope with forces due to thermal expansion and contraction of the track and the maximum expected braking forces then it will by default be strong enough to withstand the applied traction forces. In Heavy Haul applications that is not the case. On uphill grades the sustained application of high traction forces, train after train, can induce track failure modes not seen 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 in attention to the selection of 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.

Extreme Climates

Finally the Heavy Haul railways are 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 that 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 which work acceptably well across a wide range of extreme environments.

The other thing which does not function so well in extreme climates is the human body! This may not sound like a technical issue but the fact is that we still expect to use a great deal of manual labour for track construction and maintenance. When Heavy Haul lines are built in inaccessible and inhospitable places the pressures to introduce more automation and to extend maintenance intervals are increased because of the additional human factors which must be taken into account.

Specifications and Standards

Most Heavy Haul railways which are being planned or built today are proposed not by established railways but by mining companies. The railway becomes a part of the mining project because it is the most economical and reliable way to move bulk commodities from source to consumer and so 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. That process requires a specification of technical performance which can be written into a commercial contract. The problem is that no established technical standards take into account the kind of factors which have been 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. That in turn is based on another assumption, that track maintenance limits specified elsewhere in the system will be observed – and those 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 IHHA. It is through that process that best practice can be ‘exported’ to new and more challenging projects.