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24 Note that different initial contact conditions may lead to Low rail gage angles different equilibrium situations. These conditions likely Low rail with field side contact inherit problems from the initial contact, such as low flange Significant loss of rail cross section angle. When wheels and rails wear into the critical shape, they should be either re-profiled or replaced. 3.4.3 Contact Conditions of New or Newly Trued Wheels in Worn Rails 3.5 WHEEL RE-PROFILING Transit operations have to true wheels (return them to the shape of a new wheel) somewhat more often than freight ser- Wheel truing is a process for re-profiling the wheel shape vice due to wheel flats. Wheel flats can be caused by frequent and removing surface defects like flats, spalls, and shelling. braking and acceleration or by wheel sliding due to contam- Two types of wheel truing machines are commonly used. inated track (see Appendix A, Section 4.7). The milling type has a cutting head with many small cutters. The equilibrium of stable wheel/rail contact is lost once The arrangement of the cutters forms the wheel profile. The new wheel profiles are introduced. New wheels need a wear- lathe type has a wheel profile template. The single cutter cuts in period to reach the equilibrium state with existing rail pro- the wheel by following the shape of a template. files. During this period, the vehicle curving performance is Three major aspects require special attention in wheel tru- likely to be poorer than during the stable stage because of the ing: tolerances, surface finishing roughness, and lubrication likelihood of two-point contact conditions. The vehicle lat- after truing. Here, it is assumed that the profile accuracy of eral stability is likely to be better than at the stable stage due the cutting tools or template has been reached since they are to lower effective conicity. usually professionally preset. 3.4.4 Contact Conditions after Rail Grinding 3.5.1 Tolerance between Wheels, Axles, and Trucks Rail grinding is often conducted in transit operations to remove rail corrugations and surface defects and sometimes In the transit systems involved in this survey, the wheel to improve wheel/rail contact. Like wheel truing, rail grind- diameter truing tolerances ranged from 1/16 to 1/8 in. for ing may also change the equilibrium contact conditions. wheels on an axle, 1/4 to 1 in. for axles within a truck, and Rails are usually not ground back to the new rail shape. 1/4 to 1 in. for trucks within a car. Therefore, the contact condition after rail grinding is influ- In general, the manufacturer's specification on wheel diam- enced by the designed ground rail shapes and the accuracy of eter differences for axles within a truck and for trucks within a rail grinding. After rail grinding, the wheel/rail contact could car should be followed for both powered and unpowered axles. be completely different from the previous three conditions. The difference in diameter for wheels on a (coupled) axle Sometimes, the contact condition could be even worse than could either lead to the truck running off-center if two axles that before grinding due to improperly ground rail tem- within a truck have similar patterns of diameter difference or plate(s) or poor grinding accuracy. cause the truck to rotate or yaw if the two axles within a truck Assessment of the contact conditions of grinding tem- have different patterns of diameter difference. plates (designed ground rail profiles) and rail profiles after The wheel diameter difference from truck to truck within grinding should be done using the representative wheels that a car may affect the load sharing patterns at the truck center run past the grinding sections. This will ensure that the grind- pivot and produce different wheel-wear patterns, but only if ing templates are adequate for the grinding sections and the the diameter difference is significant. Smaller tolerances shapes of templates have been closely reproduced. would provide a better defined vehicle running behavior. The diameter difference for wheels in a coupled axle is most crit- 3.4.5 Critical Contact Conditions and ical for truck performance. Some European railway systems Associated Wheel/Rail Profiles only allow a 0.02 in. (0.5 mm) difference in diameter for wheels within a coupled axle. Considering the capacity of Critical contact conditions are defined as wheel/rail pro- wheel truing machines currently available in some transit files that may cause significant damage of wheels and rails or systems, this tolerance should not exceed 1/16 in. (1.5875 considerably increase the risk of derailment. The associated mm) difference in wheel diameter. The diameter difference wheel and rail profiles may include these conditions: for axles within a truck is critical for the powered axles. Under the same axle rotating speed, both axles may slide due Thin flange to the wheel diameter differences. This is especially true for Low wheel flange angles mono-motor trucks, because mechanical coupling between Hollow wheels axles force the axles to rotate at the same speed. Therefore,