Martin Hiensch, Specialist Mechanics – Dekra Rail
Improving the structural performance of the switches
Turnouts are known operational bottlenecks in the railway network. The nature of the wheel/rail forces, acting as a result of changing contact conditions and route of travel, contribute strongly to this critical behaviour.
The resulting contact forces and slip levels at the wheel/rail interface cause the switch panel rails to suffer from wear, plastic deformation and rolling contact fatigue damage. Consequently, the importance of switch inspection and maintenance activities is high, whereas the available maintenance windows are limited. Since switches are key assets at the junction of individual routes, their nonavailability often leads to a major disruption of the train service.
The forces and resulting stresses acting at the switch panel running band are determined by the characteristics of the vehicle, the track, the interface and operational parameters. The research presented here focuses on improvement of the switch panel performance by optimisation of the steering behaviour of the railway vehicle and related wheel/rail contact stresses. More specific on the modification of vehicle characteristics and particularly on the possible contribution of frequency selective stiffness at the wheelset primary suspension.
Vehicle stability on straight track and vehicle behaviour in switches and curves impose a conflict upon the vehicle designer. To guarantee stability on straight track at high speed, a high rotational stiffness of the wheelset within the bogie frame is necessary, resulting in high values of primary yaw suspension stiffness design. However, to limit wear and rolling contact fatigue (RCF) damage at switches and curves, a low rotational stiffness is required, since this facilitates steering of the wheelset, reducing the lateral forces.
New elastic components
Recent developments within bogie design are aiming, among others, at the application of new elastic components with a characteristic that is dependent on the frequency of loading. These so-called frequency selective stiffness (FSS) elements can possibly provide the required high stiffness at high frequencies to ensure stable running together with the low stiffness at low frequencies resulting in moderate loading when negotiating a curve or switch. These elements can be retrofitted within the existing train concept by simply replacing the conventional bushes or dampers.
The assessment of this possible design modification will be carried out by means of track/train simulation and field measurement. Using the VAMPIRE multi body simulation software, the impact of modification in vehicle characteristic will be quantified for a number of wheel/rail interface aspects: wear index (represented by Tγ), RCF damage coefficient and lateral forces. The track model consist of the most common switch type applied in Dutch track (1:9). The vehicle model used is the Dutch double deck passenger train VIRM.
The research will show the potential benefits of the application of frequency selective stiffness in relation to switch panel damage and associated performance. It will further provide insight in the possible contribution that track friendliness of rolling stock can have to reach a more sustainable rail transport system and weather it can result in a win-win situation for the whole system.