Track
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.
We take care of life’s essentials. Virtually every product ingredient is transported in our rail freight wagons and tank containers.
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. IRJ
This article summarises work presented at the International Heavy Haul Association (IHHA) meetings in Calgary in 2011 and New Delhi in 2013, and at the 3rd Railway Gathering organised by Brazil’s National Association for Transport Operators (ANTF) at Juiz de Fora, Brazil, in October 2011.
We lease rail freight wagons, provide rail forwarding services throughout Europe and tank container transports worldwide. Safe and reliable.
42
info@vtg.com •
www.vtg.com IRJ February 2013
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52