Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

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

Results with Combined Effects

Since the actual derailment was caused by multiple factors, the final simulation examined a combination of effects, including an air spring load imbalance of 8 kips, a side bearing COF of 0.1, and a wheel/rail COF of 0.6.

Figure 4 compares predicted single wheel L/V ratios at the derailing wheel for the nominal and combined case. The predicted L/V ratio for combination of factors is approximately 0.8 to 0.9. Since 0.81 represents the critical single wheel L/V ratio (with a COF of 0.6), the predicted values imply a substantial risk of derailment. And since the L/V ratios for the derailing wheel are consistently high, no specific track location could be expected to initiate the actual wheel climb.

Rather, simulations indicate that the primary factor leading to a wheel-climb derailment was an air spring load imbalance, particularly for high values of imbalance, i.e., those with pressure differences of 25 to 30 psi and higher. Table 2 compares worst-case L/V ratios at each wheel of the vehicle for a range of factors. The shaded cells show the worst-case L/V ratio for each case. In most instances, the critical location is the high rail wheel of the leading truck, leading axle. Air spring load imbalance is the sole input making the high rail wheel of the trailing truck, leading axle the critical location for derailment.

The actual air spring imbalance is caused by a combination of: the static imbalance occurring on level tangent truck (due to the car weight distribution), improper adjustment of the car leveling system, and track twist or superelevation. If the car comes to rest in a spiral, the car leveling system will attempt to compensate for the superelevation ramp. This will increase air spring pressure at the leading truck (high rail side) and trailing truck (low rail side), and reduce pressure at the leading truck (low rail side) and trailing truck (high rail side). Derailment then occurs in the body of the following curve. The worst case occurs when the imbalance created by improper adjustment of the leveling system and the effect created by the entry spiral are additive.

The principal effect of diagonal air spring load imbalance is that static vertical loads per truck side are no longer uniform. In that air spring stiffness is roughly proportional to air spring pressure, the car is supported on “stiff” springs at two diagonal corners, and “soft” springs at the opposing corners. Under dynamic conditions, this can lead to further reduction in vertical load at the lightly loaded corners and a potential increase in single wheel L/V ratios. Based on the simulation results, the effect of air spring imbalance begins to become important at approximately 25 to 30 psi differential pressure. Air spring load imbalance alone is seldom enough to lead to a wheel climb derailment. However, when combined with other factors, such as wheel/rail friction coefficient and flange geometry, and operational issues (such as stopping in a curve entry spiral and then proceeding at slow speed), air spring load imbalance will significantly increase the potential for wheel climb.

Radovan Sarunac is Lead Mechanical Engineer in the Washington, D.C., office of Booz Allen Hamilton; Peter Klauser is a Vehicle Dynamics - Engineering Consultant.

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