INTRO: Two methods of achieving economic benefits in the management of rolling contact fatigue were examined in studies carried out by TTCI for the UK railway industry during 2005. Both the use of premium rail steels and changes to vehicle suspensions have the potential to deliver cost savings
BYLINE: Steve Clark, Curt Urban, and Firdausi Irani*
BYLINE: * Steve Clark and Curt Urban are Principal Engineers at TTCI. Firdausi Irani is Managing Director of TTCI (UK)Ltd
AMONG Network Rail’s objectives are commitments to deliver cost-effective services to the passenger and freight operating companies who are its customers. Since its inception in October 2002, Network Rail has consistently sought to drive down costs on its 16116 route-km network, and further economic and financial benefits are in prospect.
One area where this may be possible is in the management of rolling contact fatigue. Among potential improvements are the use of premium rail steels that resist RCF crack initiation and the modification of rolling stock suspensions to reduce primary yaw stiffness. Studies into both areas were conducted during 2005 for the Vehicle/Track System Interface Committee (V/T SIC), producing interesting results. The studies were funded and managed by Network Rail and the Rail Safety & Standards Board (RSSB).
Premium rail steels
It was assumed that premium rail steels - defined for this study as head-hardened and mill heat-treated steels with hardness values generally around 350 Brinell or higher - would have the greatest potential value on the busiest, fastest, and more maintenance-intensive lines. Sections of track were therefore studied on the East Coast Main Line, the West Coast Main Line, and the main line from London to Brighton. If premium rail steels could not demonstrate value on these routes, then they would be unlikely to show benefits on less busy lines requiring lower levels of maintenance.
Based on the data collected and given the assumptions used, the study concluded that use of premium rail steels has potentially significant long-term economic benefits. It was assumed that benefits would be generated by premium rail steels offering longer service life thanks to lower wear, reduced rail grinding requirements and improved surface fatigue durability.
Fig 1 presents the potential steady-state, per-km benefits estimated from using premium rail steels on curves of 1800m radius and less, and indicates the estimated benefit of premium rail steels with lives of 1·5, 2·0 and 2·5 times standard rail life. The values presented are uniform annual benefits over a five-year period and pertain to replacing only the rail affected by RCF - this is usually the high rail.
There is some incremental benefit from reducing the grinding frequency, but the main economic benefit from using premium rail steels is in deferring rail replacement. However, the benefit does not appear until the rail requires replacement.
Table I presents the estimated average wear and RCF life for rails on the East Coast Main Line, based on current average rail life in Equivalent Million Gross Tonnes. This shows that it would be 16 years or more before the rail replacement benefits were received.
Therefore, if an infrastructure owner is substituting higher cost premium rails for standard rails that already have long lives, the replacement benefits from the premium rail will not be realised until far into the future. For an infrastructure owner required to make economic decisions on five to 10-year time horizons, the increased cost of premium rail steels would not be recouped that quickly.
The same analysis was carried out on two curves on the West Coast Main Line near Weedon and Wolverton. The rail on these curves has suffered from chronic RCF and has recently required replacement after only two to three years in service. In this case, the benefits from premium rail steels would be realised quickly.
Fig 2 presents the estimated benefits from the Weedon curve, where approximately 65% of the curve suffered RCF. Fig 3 presents the estimated disbenefits from Wolverton, where only about 35% of the curve was affected by RCF.
The results for these curves are very different because when rail is replaced, it is not good practice to mix premium rail steels with standard rail steels. Consequently, when premium rail is substituted for standard rail, the entire curve must be replaced. When replacing standard rails, only the affected parts of the curve need replacing, so reducing the total amount of replacement rail.
Fig 4 shows a five-year break-even sensitivity for these two curves based on premium rail steel having twice the life of standard rail steel. On both, the break-even point is reached when the proportion of RCF-affected rail is around 60%. In other words, unless at least 60% of the curve suffers from RCF, then no economic benefit accrues from using premium rail steels.
Reducing yaw stiffness
Work was also conducted to develop a methodology and perform a preliminary cost-benefit analysis of the effects of changing primary yaw stiffness on vehicles operating on Network Rail infrastructure.
Vehicle curving theory suggests that primary yaw stiffness has a direct relationship with the incidence and severity of rolling contact fatigue and wear on wheels and rails. In this analysis, changes in this damage mechanism were estimated based on wear index values calculated for different primary yaw stiffness values.
The wear index (also known as T-gamma) is a measure of the energy being expended at the wheel-rail contact patch. This energy is converted to any one of, or a combination of, heat, noise, surface fatigue, and wear, in both the wheels and the rails.
The objectives were to determine if savings could be achieved in the cost of vehicle and track by reducing the primary yaw stiffness of suspension systems and to quantify the estimated benefits from a stakeholder perspective, and to determine a break-even cost for modifying vehicle suspensions.
The study indicated that increasing primary yaw stiffness can increase stakeholder costs, while reducing primary yaw stiffness can generate stakeholder benefits. Furthermore, the analysis suggested that the allocation of benefits and costs falls differently across the vehicle-track system interface, and the allocation of costs and benefits will be specific to route and fleet.
To illustrate the methodology and apply it to a preliminary analysis, a generic vehicle model was used to produce wear index values for vehicle primary yaw stiffness values ranging from 8 to 128MN-m/rad, with 20MN-m/rad set as the base case. Three Southern Region routes were selected for this analysis.
To estimate a per-vehicle break-even cost, a notional fleet size was generated based on the route Equivalent Million Gross Tonnes per Annum. This notional fleet represented the number of vehicles required to operate the routes at their current traffic densities.
Benefit and cost estimates were based on steady-state rail replacements and grinding costs for track and on steady-state wheel reprofiling and wheel replacement costs.
Table II presents the five-year present value of savings (or cost) to the industry stakeholder groups (track and vehicle), based on the wear index analysis compared to the base case.
Fig 6 presents the break-even cost of modifying each vehicle in the notional fleet to achieve the respective primary yaw stiffness value. These values represent a cost ceiling for the cost of modifying each vehicle in the notional fleet to achieve a respective primary yaw stiffness value. For example, given a five-year time horizon, and changing from a primary yaw stiffness value of 20MN-m/rad to 16MN-m/rad, the break-even cost is approximately £18000 per vehicle.
The potential savings are time-sensitive, with the longer periods generating stronger savings. Fig 5 presents the combined industry stakeholder savings or costs for five-year, 10-year and 15-year time horizons.
Harnessing the benefits
The conclusions drawn from this preliminary study depend on a number of general assumptions. But a process or tool has been established for the industry to use that provides a starting point from which more specific, well-defined analyses could be performed.
There is every indication that premium rail steels and changes to vehicle primary yaw stiffness could generate positive economic benefits for the industry. However, it is not clear that the incentives to initiate the necessary changes are in place or that they are even available.
In the case of vehicle suspension modifications, the majority of the benefits fall to the infrastructure owner, yet the costs fall to the rolling stock stakeholders. It is possible that the vehicle modification cost could exceed the benefits available to the vehicle owner or maintainer. In addition, there are other drivers that may provide equal or larger benefits such as changes to wheel profiles, cant deficiency, and the use of lubrication.
Likewise, in the case for premium rail steels, if benefits are only to be received in the very long term, then decision-making processes required by the infrastructure owner and driven by short-term metrics may conflict with the overall cost reductions possible for the industry.
The authors wish to stress that these are only two possible means of reducing RCF, and it is possible that other changes could produce greater results. In addition, these findings should only be considered as a starting point for further analysis and discussion. To establish more robust results, a more rigorous study using specific routes and vehicle input parameters should be carried out.
TABLE: Table I. Estimated average rail life for wear and RCF on a test section of NR’s East Coast Main Line
Curvature m Wear RCF
r > 2500 >50 years 44 years