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25 the truck manufacturer's specified tolerances on the wheel 3.5.3 Lubrication after Profiling diameter for powered cars should be strictly followed. The profiling tolerances should not be difficult to achieve if the Lubrication after truing can also be an effective way to pre- truing machines are properly maintained. vent flange climb derailment with newly trued wheels. Reduc- ing the friction coefficient at the wheel/rail interface can increase the L/V limit for flange climb. The sharp asperities 3.5.2 Surface Finish Requirements on the wheel surface after truing may quickly deform or wear off in operation due to very high locally concentrated contact Several systems have reported flange climb derailments stresses. After operating for some time, the wheel surface occurring at curves or switches in yards just after the wheels should be in a smoother condition. Light lubrication can help had been trued. This type of derailment may have been a wheels safely pass through this rough to smooth transition. result of the required maximum flange angle not being Lubrication can be performed as one of the procedures of obtained, but was more likely caused by excessive wheel sur- wheel truing or applied using wayside lubricators installed face roughness after wheel truing. Figure 3.22 shows exam- on the curve in yards. Other techniques, such as onboard ples of wheel surfaces just after truing and after many miles lubrication systems, can also be employed. of running. Generally, the coefficient of friction for dry and smooth steel-to-steel contact is about 0.5. The effective coefficient 3.6 WHEEL PROFILE DESIGN for a rough surface could be much higher. For example, if the coefficient reaches 1.0, the L/V limit (Nadal criterion) would Given the effect of wheel profiles on vehicle performance be 0.5 for a 75-degree flange angle and 0.3 for a 63-degree and wear discussed above, the requirements for wheel profile flange angle. Therefore, the rough surface produced by wheel design are clear and can be summarized as follows: truing could significantly reduce the L/V limit for flange climb. The low flange angle further increases the derailment · The design must meet the dimension requirements for a risk. specific system. Several remedies may improve the surface condition: · Higher flange angle is necessary to reduce the risk of flange climb derailment on curves. · Frequently inspect the cutting tools, especially for the · Effective conicity must be selected to give the optimum milling type machine. Dulled tools can produce a very compromise between curving and stability requirements rough surface. Sometimes, the grooves are obvious. for the particular vehicle design and transit system. · Address the final surface turning. In this step, there is no · Severe two-point contact with rail to improve curving significant material removal but a light cut is used for and reduce wear should be avoided. smoothing the surface. · High contact stress should be avoided. a. b. c. Figure 3.22. Comparison of wheel surface roughness. ([a] surface after wheel re-profiling from milling type machine, [b] surface after wheel re-profiling from lathe type machine, and [c] surface of wheel back from operation with a flat spot.)