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28 - FIELD GAGE + 2 0 2 4 6 8 15 25 35 45 60 Figure 3.25. Example of ground rail template, "bar gage" and measurement. 3.8 EFFECT OF GAGE AND FLANGE f = the variation of wheel flange thickness from CLEARANCE ON WHEEL/RAIL CONTACT the value for the new wheel, which is generally negative due to wear, and 3.8.1 Effect of Rail Gage and Wheel Flange Ga and fa = the actual gage and flange thicknesses, Clearance respectively. Wheel flange clearance is the wheel lateral shift limit rela- tive to the rail prior to wheel climb (Figure 3.26). It is directly In general, the wheel back-to-back spacing is constant. It related to rail gage, flange thickness, and wheel back-to-back is obvious that the clearance increases with wide gage and a spacing. Equation 3.3 computes the flange clearance (C0) thin flange. under the condition of designed gage and new wheel. As discussed in Section 126.96.36.199, the flange clearance has an influence on the RRD in curves. Too narrow a gage can Gs - Bs - 2 f s limit the RRD (and therefore the yaw displacement of the C0 = (3.3) wheelset in curves), inducing wear, especially to high rails in 2 curves. However, too wide a gage can increase the risk of where Gs and Bs are standard gage and wheel back-to-back gage widening derailment. spacing, respectively, and fs is new wheel flange thickness. Figure 3.27 illustrates that the gage widening criterion is Equation 3.4 computes the actual flange clearance (C) related to the wheel, rail geometries, and their relative positions. under the worn wheel/rail shapes and varied gage conditions. When a wheel drops between the rails, as in Figure 3.28 the geometry of wheel and rail must meet the following G G - Bs - 2 f a expression: C = C0 + + f = a (3.4) 2 2 Ga > B + W + f a (3.5) where G = the variation of rail gage from the standard value and where W is wheel width and B is the wheel back-to-back can be both positive and negative based on the vari- spacing. ation direction, Flange Clearance Figure 3.27. Wheel and rail geometry related to gage Figure 3.26. Illustration of flange clearance. widening derailment.
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29 Figure 3.28. Derailment due to gage widening. Figure 3.30. Restraining rail. Therefore, a safety margin (S), expressed in Equation 3.6, 3.8.2 Effect of Clearance between Low Rail represents the minimum overlap of wheel and rail required and Restraining Rail on the nonflanging wheel when the flanging wheel contacts the gage face of the rail. In this circumstance, the instanta- Restraining rails and guard rails have been frequently neous flangeway clearance on the flanging wheel is zero. applied on sharp curves in transit operations to prevent flange climb derailment and to reduce high rail gage face wear. As illustrated in Figure 3.30, the restraining/guard rails are gen- ( B + W + fa ) - G > S (3.6) erally installed inside the low rail. In extremely sharp curves, restraining rails are sometimes installed on both inside and where G is the gage spacing. outside rails in order to also reduce low rail flange contact In general, the wheel back-to-back spacing (B) is a constant with the trailing wheelset. for a solid axle wheelset, and so is the wheel width (W). The current practices of restraining rail installation vary, as However, different designs of wheel have different values for shown in Table 3.4. The extension length of restraining rails at these two parameters. The flange thickness (fa) is gradually the ends of curves and the clearance to be set for the restrain- reduced as the wheel wears. The track gage variations are ing rails may also vary within different transit systems. influenced by multiple factors. Rail roll and lateral movement The clearance between the low rail and the restraining rail due to wheel/rail forces and weakened fasteners can widen the is critical for the effectiveness of restraining rails. A clear- gage. Rail gage wear can also contribute to gage widening ance that is too tight reduces wheelset RRD required for (Figure 3.29), which results in a gage wear of about 6 mm. truck curving while limiting flange contact on the high rail. Widening the gage on sharp curves is a practice that has Clearance that is too wide will cause a complete loss in the been adopted in some transit operations for improving vehi- function of the restraining rail. cle curving. The limit to which the gage can be widened Wear at the wheel flange back and the contact face of the should be assessed carefully by considering the worst possi- restraining rail can change the clearance between the low ble condition on that section of track, including both wheel rail and the restraining rail. The wheel flange and high rail and rail wear and wheel/rail lateral force. For example, a widened gage combined with hollow-worn wheels could TABLE 3.4 Examples of restraining rail installation cause rail roll or high contact stress. Transit System Practice Massachusetts Bay Restraining rail is installed on curves Transportation Authority with a radius less than 1,000 ft. ( MBTA) Newark City Subway Restraining rail is installed on curves (Light Rail Line) with a radius less than 600 ft. Southeastern Pennsylvania Restraining rail is installed on curves Gage Widening Due to Wear Transportation Authority with a radius less than 750 ft. (SEPTA) (Rapid Transit Line) WMATA Restraining rail is installed on switches (Washington Metropolitan corresponding to less than 500-ft radius Area Transit Authority) and curves with a radius less than 800 ft. CTA Restraining rail is installed on curves (Chicago Transit with a radius less than 500 ft. Figure 3.29. Gage widening due to wear. Authority)