Interface - The Journal of Wheel/Rail Interaction

WAYSIDE LOAD DETECTION

Using Wayside Load Detectors for Preventive Vehicle Maintenance - Part 1 of 2 (continued)

— Wheel and Rail Profiles. Wheel and rail profiles, the resulting contact geometry and changes due to wear were determined to be the second most critical influential factor. The simulation assumed that wheel wear tends to increase flange angle and reduce tread taper. A greater flange angle is accompanied by some increase in flange clearance. Wear tends to transfer the wheel flange contour to the high rail (consistent flange contact) and wheel tread contour to the low rail (consistent tread contact). High rail wear tends to reduce the rail gauge-face angle, while low rail wear tends to flatten the rail crown. Both effects — increased flange angle and smaller tread conicity (due to reduced tread taper / flange side tapeline wear) — tend to reduce wheelset steering. WMATA’s nominal track gauge, which is 1/4-inch tighter than the North American standard, results in normal tread contact shift toward the flange side of tapeline. The rail is AREMA 115 pound at 1:40 cant; the wheel is a British worn profile (63-degree nominal flange angle) at 1:20 taper. The nominal pairing was a worn wheel with 69-degree flange angle on curve-worn rails at standard gauge. Simulation indicates that both angle of attack and single-wheel L/V ratio tend to increase with flange angle (wheel wear). However, it should be noted that the Nadal limit value for single-wheel L/V ratio increases significantly as flange angle increases, as does the safety margin against wheel climb derailment.

— Air Spring Imbalance. The effect of air spring imbalance was analyzed as the third most critical influential factor. The modeled car was equipped with a four-point leveling system. The disadvantage of this arrangement is the potential for a diagonal air spring load imbalance. In this case, air spring loads and pressures are increased at two diagonal corners and reduced at the opposing corners. The diagonal load imbalance does not create a car pitch or roll imbalance. Instead, the forces are reacted by carbody twist about the length axis. The effect of the diagonal air spring load imbalance is that static vertical loads per truck side are no longer uniform. Instead, truck side vertical loads under static conditions are roughly proportional to the diagonal variation in nominal air spring pressures. A further effect is that air spring stiffness is also roughly proportional to air spring pressure. Thus, the car is supported on a pair of “stiff” springs at two diagonal corners, and a pair of “soft” springs at the opposing corners. Under dynamic conditions, this leads to further reduction in vertical load at the lightly loaded corners. The total air spring imbalance represents a combination of the car's weight distribution, the static imbalance occurring on a level truck (due to improper adjustment of the car leveling system) and the effect of track twist (i.e., the spiral). In addition to the diagonal imbalance, improper car leveling may result in further static imbalance (car roll) due to uneven air spring height. This illustrates that the air spring condition has significant influence on vehicle curving performance.

— Side Bearing Friction. The effect of side bearing friction coefficient on the curving performance was investigated as a minor causal factor. Investigation indicates that an increase in side bearing friction will proportionally increase truck-turning resistance.

Perhaps unexpectedly, maximum L/V ratios are predicted for the lowest friction value for the trailing truck. An increase in side bearing friction reduces the predicted L/V ratio. The results for the leading truck are the opposite. For a vehicle’s leading truck, the direction of truck rotation and the resulting turning moment increase the lateral force at the leading-axle, high-rail wheel. For a trailing truck, the direction of truck rotation and resulting turning moment are opposite to the leading truck; the turning moment tends to reduce the lateral force at the leading-axle, high-rail wheel. The lower the turning moment, the lower this beneficial effect and the greater the potential for derailment of the leading axle. The influence on single-wheel L/V ratios are not significant. Side bearing friction appears to have limited influence.

— Truck Radial Misalignment and Wheel Mismatch. The impact of truck radial misalignment and wheel mismatch was also investigated. Truck misalignment refers to defects when wheelsets are not parallel. Misalignment values of 1 to 2 milliradians were considered. Wheel diameter mismatch is a defect when the left and right wheels have diameter mismatch. Diameter mismatch of ± 2 mm was considered.

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