Click for next page ( 27

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 26
26 Note that the designed wheel flange angle currently used relocating the contact band. Under these conditions, the in North American transit operation ranges from 60 to 75 ground rail design must take the wheel shapes in the system degrees. TCRP Report 57: Track Design Handbook for Light into consideration to ensure that the objectives will be Rail Transit (11) proposed a wheel flange of 70 degrees achieved. based on Heumann's design. The APTA Passenger Rail Ground rail profiles can be designed differently for Safety Standard Task Force Technical Bulletin (12) provided straight and curved tracks. This is likely to be the case for guidance on reducing the probability of wheel-climb derail- high speed operations because of a strong emphasis on lat- ment, suggesting a minimum wheel flange angle of 72 eral stability on straight track. On straight track, the goal is degrees (suggested tolerances are +3.0 degrees and -2.0 mainly to achieve low effective conicity, thereby raising degrees). the speed at which vehicle hunting could start. In curved A new wheel profile may be requested for a completely track, the main emphasis may be placed on improving new rail system starting with new wheels and new rails. The vehicle steering and reducing lateral forces and rolling design emphasis under this condition is to establish desirable resistance. starting wheel/rail contact features to help new vehicles meet their performance requirements. Simulations and trial tests should be conducted on vehicles with the new wheel profiles 3.7.1 Ground Rail Profile for Straight Track under the specified operating conditions to ensure that the specified requirements have been meet. The likely wear In straight track, high values of effective conicity can lead patterns may be predicted to further determine the vehicle to hunting at lower speeds for lightweight vehicles. Hence, performance under worn wheel/rail shapes. one goal of profile design should be that a general low effec- A new wheel profile may also be requested for an existing tive conicity trend is maintained for a large population of rail system with worn wheels and rails. Under this condition, passing wheels that may have varying tread slopes due to dif- the existing worn wheel/rail shapes should be taken into con- ferent levels of wear. sideration when designing the new wheel profile, for exam- One way to lower conicity (actually, to lower the RRD of ple when adopting a new wheel profile with a higher flange two wheels) from the ground rail is to reduce the contact at angle to replace the existing low-flange-angle wheel. If the the wheel flange throat by producing a strong, two-point con- profile change is significant compared to the existing design, dition when the wheel flanges, as illustrated in Figure 3.23. it is likely that an interim profile (more than one, when nec- Then, the rail has no chance to contact the flange throat, thus essary) will be needed to gradually approach the desired pro- reducing the variation of rolling radius. file. Further, a transition program should be carefully Care needs to be taken when grinding the rails in straight planned by considering the capacity of both wheel truing and track to avoid concentrating contact in just one portion of the rail grinding on the system. An example is discussed in Sec- wheel tread. Concentrated contact can lead to excessive tion 4.2.3 of Appendix A. wheel hollowing. 3.7 GROUND RAIL PROFILE 3.7.2 Ground High Rail Profile In general, North American transit operations use Ameri- The ground high rail shapes generally should be close can Railway Engineering and Maintenance of Way Associa- to the stable, worn high rail shapes at the rail gage shoul- tion (AREMA) standard rails (13). The AREMA 115-pound der and corner. Severe two-point contact should be (115RE) and the 132-pound rail (132RE) are two types of rail avoided because it produces poor steering. that are commonly used on transit lines, especially for newer systems. Some old systems have lighter weight rails (90- to 110-pound rail) still in use on their lines. Rail profiles are generally not ground back to the new rail shapes. The ground rail shapes determine the contact condi- tion after rail grinding or determine the variations of contact condition compared to the situation prior to grinding. There- fore, properly designed ground rail profiles are critical for producing minimum disturbances to a profile-compatible system. This would result in a short wear-in period to reach the equilibrium stage. Sometimes, rail grinding is conducted for a major cor- Figure 3.23. Example of reducing effective conicity by rection of rail shapes, such as increasing contact angle or controlling contact position.

OCR for page 26
27 If grinding were only conducted on the stable, worn rail shapes to remove corrugations or surface defects, maintain- ing these stable shapes would lead to a minimum disturbance to the existing conformal contact. Such grinding requires the removal of only a thin layer of metal--not more than the depth of corrugation and surface defects. 3.7.3 Ground Low Rail Profile Figure 3.24. Example of grinding tolerance for a high The ground low rail profiles should be designed to avoid rail. contact positions that significantly face toward the field side. This is especially important for the low rails in curves, where hollow-worn wheels can give very high contact stresses on to a point with an angle of 10 degrees on the field side and to the field side. Fieldside contact can also increase the risk of a point with an angle of 40 to 60 degrees on the gage side. rail rollover or gage spreading. The angle on the gage side is based on the capacity of the Varying low rail shape by grinding can alter the RRD by grinders (the maximum angle that can be reached by the intentionally moving the contact positions on wheels to a grinder). desirable area, such as from the gage shoulder to the crown Much work has been done in recent years to set tolerances area by lowering the gage shoulder. Because the contact on profiles given by rail grinding. Based on a survey of these radius in the rail crown area is larger than on the rail shoul- references (14, 15, 16), the recommended tolerance for the der, this adjustment reduces contact stress. ground rail transverse profile should be from -0.4 mm to In designing ground rail profiles for curves, the contact +0.3 mm. Negative tolerances mean that the ground rail conditions of both leading and trailing wheels should be con- sidered. The leading wheels on the high rail are generally shape is below the design rail shape. Positive tolerances flanging for curves above 3 degrees. Vehicles that are mean that the ground rail shape is above the design rail designed with soft yaw suspension that allow the axles to shape. The example in Figure 3.24 shows negative toler- steer may flange at higher degree curves. Therefore, it is rec- ances, that is, the measured rail profile is inside the template. ommended that the assessment of vehicle curving perfor- Some grinders can reach even better accuracy with careful mance be conducted for the ground rail design. The trailing control of the grinding stone patterns. wheels are generally not flanging. They usually have a small Positive tolerances in the gage corner can be much more lateral shift relative to the track center, depending on vehicle detrimental than negative tolerances. A large positive toler- and track conditions. ance in the gage corner can lead to high contact stress and consequent high wheel- and rail-wear rates and the potential for crack formation. With good grinding accuracy, the 3.7.4 Grinding Tolerance ground rail shape will quickly wear to a profile that is con- formal with the wheels passing over it. During rail grinding, the transverse rail profile is produced Rail template gages are also commonly used to inspect the by a series of straight facets from the individual grinding rail shapes during routine checks or after rail grinding. Expe- units. Thus, the grinding process unavoidably produces a rienced inspectors can estimate differences by looking at the polygon-curve approximation to the desired profile, and this gaps between the template and the actual rail shape (Figure causes variance between the actual ground rail profile and the 3.25). Note that in order to allow the gage to slide over the target design rail profile. The stone pattern selections and set- head of the two rails (even under the wide gage condition), tings can also cause deviations of ground rail shape from the the template gages usually do not have the whole shape of the target shape. rails. Thus, a grinding tolerance needs to be specified to limit the variation from the target shape. The tolerance is defined as the radial distance between the measurements of the ground 3.7.5 Rail Lubrication after Grinding rail profile and the target design profile. To check whether grinding has produced the design ground rail profile within Slight lubrication immediately after rail grinding can the specified tolerance, the ground rail profile should be over- reduce the wheel flange climbing potential, just as lubri- laid on the target rail profile. cation after wheel truing can. The rough rail surface after Tolerances should be assessed as shown in Figure 3.24. grinding can reduce the limiting L/V ratio for flange Tolerance is evaluated from the highest point on the rail top climb.