04 TRACK SYSTEMSSUPPLEMENT


components to exploit new designs. Production is now taking place of a hydro-dynamic suspension bush which is able to combine the best of both worlds: a soft suspension characteristic when negotiating curves and a much stiffer response to prevent wheelset instability when running at higher speeds on straight track. Computer simulation demonstrated that such a bush could reduce RCF damage by a factor of at least 10 on many curves. This has resulted in trials of suspension modifications on a number of vehicles, and another fleet of vehicles is in the process of undergoing modifications to fit the new bush design to all its bogies. This also brings the benefit of a significant reduction in track access charges for the train operator: a saving of many millions of pounds to the train operator and a reduction in the amount of rail maintenance and replacement on the route expected over the life of the vehicles. Although we don’t often have the oppor -


tunity to remove curves from the railway we can influence the basic geometric design of curves where the track is canted to reduce the lateral forces experienced by the passenger. The design of such curves is often a compromise since not all trains will negotiate a curve at the same speed: freight is often limited to run slower than passenger trains, whilst suburban passenger trains accelerating away from a station stop will be running slower than inter- city traffic over the same curve. Vehicle dynamics simulations show that running at slower speeds on canted track results in much higher forces being generated in the contact patches of the leading wheelset of each bogie than in other wheelsets. Increasing cant deficiency (either by increasing vehicle speed or reducing the installed cant on a curve) better distributes the forces between the two wheelsets, actually reducing the forces on the leading wheelset of a bogie, which, in turn, reduces the amount of fatigue damage to the wheel and rail. Although it is rare to be able to increase vehicle speed on a curve to increase the amount of cant deficiency, it is often possible to reduce the amount of cant installed on the curve, increasing the amount of cant deficiency for all traffic. Again, a systems approach is required to understand the implications of such changes and whether they are possible: the change in track position could affect


European Railway Review Volume 18, Issue 2, 2012


structural gauge clearances, and the maximum cant deficiency must be controlled to ensure that passenger ride comfort is maintained. Tools such as Track-Ex allow track engineers to quickly assess whether changing cant deficiency will make a difference to the amount of damage on a curve and whether changes to the curve design will contravene any limits on cant deficiency or rate of change of cant, which can affect passenger comfort. This can then be used to help build a business case for making changes to the track design for the long-term good of the railway. Track geometry alignment is also very


important. Small variations in the lateral position of the track can trigger dynamic forces in the behaviour of the vehicle that are sufficient to cause damage to the rail surface. Although track maintenance standards specify maximum values for the allowable


Over the last 10 years Network Rail has led the world in research into the causes of wheel and rail damage, particularly rolling contact fatigue (RCF)


variation in track alignment, our research has found that the wavelength of the alignment variation is an important factor in controlling dynamic behaviour: vehicles can negotiate quite large lateral changes in track position without generating large wheel/rail forces or passenger discomfort if they occur over a relatively long wavelength. But our current standards do not account for the wavelength of the alignment fault, so we may be maintain- ing alignment tolerances for some defects which are not causing problems for the vehicle, track or passenger. Alternatively, some small alignment variations at quite short wave- lengths may fall within the track maintenance limits but cause wheel/rail damage and ride discomfort. Using Track-Ex and detailed vehicle dynamics modelling, we are in the process of reviewing our track standards to better account for these factors to ensure that track maintenance is focused on those locations where the combination of track alignment variation, amplitude and wavelength are causing problems. Maintaining wheel and rail profiles is also important; the shape of the wheel and rail


running surfaces provide a significant contribution to the dynamic behaviour of a vehicle and the forces it generates on a given piece of track. One example of this is the use of the relatively new P12 wheel profile as an alternative to the traditional P8 wheel profile. The P12 profile was developed to have a shape which helped reduce the forces causing rail damage. The characteristics of this profile have shown that for some train operators it can have significant benefits for train maintenance as well as reducing track damage: it not only reduces damage (RCF) to wheels in the same way as it benefits rails, but its shape means that it can run for higher mileages before it reaches limits where it requires reprofiling. For some operators it has also improved vehicle dynamic stability and reduced the forces generated whilst running through switches and crossings. Once again the benefits can be seen by those responsible for both sides of the wheel/rail interface. In conclusion, the wheel/rail interface is now


much better understood and with the use of detailed computer simulation, we have been able to increase our understand- ing and identify the factors that can influence the forces generated. Reducing the forces benefits everyone – the infrastructure main - tainer, the train owner and maintainer, and the passenger. We now have tools which track engineers can use to optimise the wheel/rail interface for their specific track and traffic conditions. Overall, we cannot ignore the wheel/rail interface and we cannot hope to manage it efficiently by looking at the characteristics of one side of the interface alone. We need to consider the behaviour of the vehicles and the track as a system.


BIOGRAPHY


Steve Yianni graduated with an Engineering Degree from Cambridge University in 1983. He is a chartered engineer and a fellow of the Institution of Mechanical Engineers. In 1990, he attained an MBA from London Business School. He has gained experience in a


variety of roles in engineering, business management and leadership at the Ford Motor Company (from 1983-1991), JCB (from 1991-2007) and Network Rail (from 2008). He is the Director of Engineering at Network Rail.


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