Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

Effects of Secondary Suspension Imbalance on Wheel-Climb Derailment (continued)

The total air spring imbalance is a function of the car weight distribution, the static imbalance occurring on level track (due to improper adjustment of the car leveling system) and the imbalance due to track twist (i.e., the spiral). Further static imbalance, such as car roll, may result from uneven air spring height.

Four levels of air spring imbalance, with 0-, 4-, 8- and 12-kip loads applied, were evaluated to determine their effect on dynamic performance. The diagonal air spring imbalance represents the sum of the increase and decrease in air spring loads at a car end. For example, a 4-kip imbalance represents a 2,000-pound increase in load on one spring and a 2,000-pound decrease in load on the opposite spring, or 4,000-pound total difference. Calculated air spring loads and pressures are given in Table 1 for the four cases. The nominal case includes the effect of a car weight distribution imbalance. (Air spring loads are not uniform even under ideal conditions.)

Tests indicate that predicted L/V ratios consistently increase as the air spring load imbalance increases. Figure 1 shows predicted single wheel L/V ratios at the derailing wheel for the four cases. (The distance axis is relative to the track geometry reference point.) The worst-case for the 12-kip imbalance produces an L/V ratio of 0.75 or greater.

Since the critical single wheel L/V ratio for a wheel with a 70-degree flange angle and wheel/rail coefficient of friction (COF) of 0.4 is 1.12, the peak value in this case does not indicate a significant derailment risk. The maximum predicted value occurs about 50 feet beyond the observed point of derailment (POD). One explanation for this is that the track geometry data available was limited to gauge and crosslevel measurements at 15.5-foot intervals. Alignment data were available in the form of stringline measurements only.

Effect of Varying Truck Turning Resistance
Investigators also looked into the role of side bearing conditions and truck turning resistance in the actual derailment. Figure 2 shows predicted single wheel L/V ratios at the critical trailing truck wheel as a function of the side bearing COF. The results are shown for the nominal case (no air spring load imbalance beyond that due to car weight distribution).

The maximum L/V ratios are predicted for the lowest friction value. That is, an increase in side bearing COF lowers the predicted L/V ratio. This is easily explained. At the leading truck of a vehicle, the direction of truck rotation and the resulting turning moment increase the lateral force at the truck leading axle high rail wheel. At the trailing truck, the direction of truck rotation and resulting turning moment are opposite to the leading truck. As a result, the turning moment tends to reduce the lateral force at the leading axle, high rail wheel. The lower the turning moment, the lower the beneficial effect, and the greater the potential for derailment of the leading axle.

For vehicles where a portion of the car weight is carried by the side bearings, the effect of the side bearing COF is important. For a trailing truck, an increase in side bearing COF reduces the predicted L/V ratio, while for a leading truck, the effect is the opposite. Under normal operating condition, the L/V ratios obtained are not significant. A seized side bearing, however, may create a significant increase in truck turning moment.

Effects of Wheel/Rail Friction

Yet another influence in the actual derailment was the potential effect of wheel/rail friction. Field observations indicated dry wheel/rail conditions. A high COF increases the lateral wheel forces while reducing the critical L/V ratio. As indicated earlier, a wheel with a 70-degree flange angle and a COF of 0.4 generates an L/V ratio of 1.12. The same wheel with a COF of 0.6 reduces the critical L/V ratio to 0.81.

Figure 3 shows predicted single wheel L/V ratios for the derailing wheel. These are again results for the nominal case (no air spring load imbalance beyond that due to car weight distribution). As expected, values consistently increase as the COF increases. The worst case produces an L/V ration of less than 0.7 with a COF of 0.6.

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