TRANSIT systems usually operate in dense urban areas, with tight curvature that can increase the potential for flange climb derailments and increase wheel and rail wear. In general, these systems are locally operated and often have wheel/rail standards that are specific to their system.
While cylindrical wheels with poor steering characteristics are still used on some networks, the wheel flange angles for North American transit systems range from 60o to 75o. Independently rotating wheels (IRW), which do not steer, are often used in low-floor vehicles. Older systems may have wheel profile standards and designs that were established many years ago, which may not work well with new LRVs equipped with modern suspension systems.
Due to the wide range of vehicle types and suspension designs available, together with different wheel and rail profiles used, and varying track conditions, a systemic level analysis is required for solving issues related to wheel/rail interaction. Transportation Technology Center Inc (TTCI) is conducting research for a number of transit systems, both in the United States and around the world, to improve operational safety both through optimisation of wheel and rail profiles and improvement of wheel and rail maintenance.
Figure 1 shows the forces acting on a wheel at flange climb and the maximum wheel flange angle. The figure also shows the equation for Nadal's limit for the lateral wheel force to vertical wheel load ratio (L/V ratio). If the L/V exceeds Nadal's limit, there is risk of flange climb. Figure 2 shows that for a friction coefficient of 0.5 and a flange angle of 75o, the Nadal limit is 1.13.
For 63o, the limit is 0.73. Wheels with low flange angles have a higher risk of flange climb. Figure 2 also shows that higher friction increases risk of flange climb. Research conducted by TTCI for the Transit Cooperative Research Programme of the mechanics of three transit vehicle designs has shown several other factors that affect flange climb including wheel profile shape, flange height, IRWs and suspension design.
Increasing wheel flange angle is common practice for reducing the risk of flange climb derailment. A wheel profile with a higher flange angle can reduce the risk and can offer better compatibility with any new designs of vehicle or bogie that may be introduced in the future compared with wheels with lower flange angles. Also, with a higher L/V ratio limit, high flange angles will tolerate greater levels of unexpected track irregularity and high friction conditions. The American Public Transit Association (Apta) recommends a minimum flange angle of 72o.
Traditional railway wheelsets, with two tapered wheels mounted on a common axle, develop longitudinal steering forces at the wheel/rail interface to help steer the bogie around curves and to develop a self-centring motion on tangent track. Flange climb studies indicate that as the longitudinal steering force increases, the wheel L/V ratio required to develop flange climb also increases above the Nadal limit.
IRWs are not forced to rotate at a common speed, so no longitudinal forces can be developed to assist steering. This means wheel flange climb is likely to occur when the L/V ratio is just above the Nadal limit. Hence, for the same friction levels, IRWs have a greater risk of climb than standard wheels.
The recent introduction of a low-floor LRV with IRWs into a mature system where some parts are more than 100 years old, and wheel profiles with 63o flange angle, resulted in multiple derailments. TTCI recommended a 75o flange angle wheel profile and a programme of rail grinding to reshape the gauge corner to assist in maintaining the angle as the wheels and rails wear. Combined with other improvements in track maintenance, the derailments caused by the low flange angle were eliminated. Systems that have introduced low-floor LRVs with IRWs but with a higher 75o flange angle have not experienced derailments.
Due to their tendency to derail, vehicles with IRWs need to be carefully designed to control flange climb and wheel wear. Additional control mechanisms, such as linkages or active control systems, can be used to steer wheelsets on curves and to react to track perturbations. Without such control mechanisms, the wheel/rail profiles and vehicle/track maintenance will need to be much more strictly controlled and monitored to prevent wheel flange climb.
While most transit systems use tapered wheels on traditional wheelsets, several networks use wheels with cylindrical profiles. Standard wheelsets generate longitudinal steering forces to assist in steering because of the radius differential that develops on the wheel treads as the wheelset moves laterally.
The lack of taper and consequent lack of steering also prevents the cylindrical wheelset from tracking well in straight track. Figure 3 shows that lateral movement occurs with a long and irregular wave, initiated by track irregularities, instead of the normal cyclic action of tapered wheelsets.
The flanges of cylindrical wheels provide the main mode of steering and guidance on curves. Wheel flange contact is similar for both tapered and cylindrical wheels. However, the cylindrical wheels tend to produce strong two-point contact resulting in higher lateral forces. Slightly widening the track gauge, which is a practice commonly applied in operation, can increase the rolling radius difference for tapered wheels on curves to assist wheelset steering, but has no effect on cylindrical tread wheels.
Field observations show that cylindrical wheels tend to experience more flange wear. Newly machined cylindrical wheels tend to wander and contact tangent rail in the gauge face and also wear quickly to a slightly hollow (worn) tread. They then stabilise in this shape for a reasonably long period. Replacing cylindrical wheel profiles with tapered wheels can offer improved steering and reduced wheel and rail wear.
A new wheel or rail profile introduces new wheel and rail wear patterns. Consequently, the system requires a transition period to bring the wheel/rail system to a new level of equilibrium. Wheels and rails generally wear into conformal (matching) profile shapes.
If a new wheel or rail profile is introduced and the new profile has a significantly different shape compared with the existing one, the existing conformity will be lost.
For example, if a new wheel profile is introduced with a higher flange angle, the contact with the existing worn rail with a lower gauge face angle might be like that shown in Figure 4, resulting in strong two-point contact, high contact stress, and high wear rates. The contact near the flange tip can also increase the risk of flange climb. Therefore, to reach a new equilibrium of wheel/rail contact, and reduce flange climb risk, a programme that includes both wheel truing and rail grinding should be implemented for a smooth transition. If necessary, an interim wheel profile may be introduced to smooth the process of transition.
Implementing new profiles can incur high initial costs for the purchase of new wheel profile cutters, performing rail grinding, and monitoring system performance. Therefore cost-benefit analysis of the proposed recommendations may be appropriate as part of any recommendations for new profiles.