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23 About x% wheels are replaced 0.6 or re- re-profiled annually 0.5 (New wheel profile) About s % wheels are 0.4 Gap (mm) worn into critical 0.3 conditions Worn Wheels 0.2 About y% rails are 0.1 Worn into critical 0 Worn Rails conditions 1 21 41 61 81 101 Number of Wheels About z% rails are About n% rails are ground annually newly laid annually (Ground rail profile) (New rail profile) Figure 3.20. Example of distribution of contact Figure 3.21. Illustration of distribution of wheel and rail conformality. conditions in a system. 3.4 UNDERSTANDING IMPORTANT STAGES OF WHEEL/RAIL CONTACT IN A SYSTEM which will lead to the condition of a new wheel contacting a As listed in Table 3.3, the wheel/rail contact situations in new rail. Of course, it is also the contact condition of a newly a system can generally be categorized into several important opened line. stages. Those stages usually exist in parallel in a system due The initial condition determines the likely wear patterns of to different life and wear levels of wheels and rails, different wheels and rails, the wear-in period, and the effects of loads or capacities between lines, and different maintenance wheel/rail profiles on vehicle performance. The initial con- processes. dition should be carefully considered and analyzed; espe- Appreciating the conditions of these important stages of cially for a new rail system. All contact parameters discussed wheel/rail contact in a system can provide insight into the in Section 3.3 should be assessed and documented. Simula- improvement of wheel/rail interaction and can assist in the tion and track test should also be performed to ensure that the management of wheel/rail maintenance. Figure 3.21 illus- new wheels and rails provide desirable dynamic performance trates distribution of wheels and rails assumed in a system. under specified vehicles and tracks. Desirably, the dominant contact condition in a system Some transit systems have wheel/rail profile standards that should be stable, worn wheels contacting stable, worn rails. were established many years ago. Awareness of the initial Starting with compatible new wheel and rail profiles, contact contact conditions of those profiles would contribute to an of stable, worn wheels and rails should produce desirable understanding of what can be expected in wheel/rail interac- contact features and should last a relatively long period with- tion and wear and what improvements in profiles can out other disturbances. enhance wheel/rail interaction. The contact conditions, listed in Table 3.3, are further dis- Any new wheel and rail profile combinations starting with cussed in the following sections to emphasize their distin- severe two-point contact will produce higher wear rate, guishing features and the attention that may be required. longer wear-in period, and poorer curving in the initial stage. Possibly the new combinations provide better lateral stability. Section 4.5.1 of Appendix A gives examples of 3.4.1 Initial Contact Conditions--New Wheel three types of initial contact conditions in surveyed transit Contacting New Rail systems. Every year in a system, new wheels will replace con- demned wheels, and some sections of rail may be re-laid, 3.4.2 Stable Contact Conditions--Stable Worn Shapes of Wheel and Rail TABLE 3.3 Important stages of wheel/rail contact Important Stages Related Wheeland Rail Profiles Stable contact is considered to be the desirable equilib- rium condition. When this stage is reached after the wear- Initial contact conditions New wheels contact new rails in period, wear rate and contact stress should be relatively Stable contact conditions Stable worn shapes of wheel and rail low due to a conformal contact situation at both wheel Contact conditions of new New wheels contact rails from or newly trued wheels new to worn tread/rail crown and wheel flange throat/rail gage areas. Contact conditions after Wheels from new to worn contact Without disturbances from sudden changes on vehicles and rail grinding ground rails tracks (such as changes in vehicle yaw stiffness due to dam- Wheel/rail shapes indicate risk of aged dampers or rail cant changing due to tie plate cutting), Critical contact conditions derailment and cause significant the stable condition should continue for a reasonably long damage to the system period.