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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.
