Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

Effects of Secondary Suspension Imbalance on Wheel-Climb Derailment
(Part 2 of 2)
By Radovan Sarunac and Peter Klauser • October 2007

Part 1 of this article examined the effects of track geometry, wheel/rail profiles, friction, and wheel flange angle on wheel-climb derailment potential. Part 2 examines the effects of wheel unloading due to air spring imbalance on derailment potential.

The secondary suspension of a modern rail car, which incorporates two air springs per truck and a four-point leveling system, has the potential for a diagonal air spring load imbalance. When this happens, air spring loads and pressures are increased at two diagonal corners, and reduced at the opposing corners, resulting in reduced vertical wheel loads.

The actual air spring load imbalance is a combination of static imbalance due to the car weight distribution, leveling system misadjustment, and the effect of track twist on superelevated curves. The principal effect of the imbalance is that static vertical loads per truck side are no longer uniform. Instead, truck side vertical loads are increased or reduced in rough proportion to the diagonal variation in air spring pressures. A further effect is that air spring stiffness is also roughly proportional to air spring pressure. Consequently, 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 may further result in reduced vertical force and increased wheel unloading. Wheel unloading, combined with high lateral forces due to curving and high rail/wheel friction levels, leads to increased wheel climb derailment potential.

In this study, vehicle dynamics and an actual derailment were modeled using a VAMPIRE® vehicle dynamics simulation program. The modeled vehicle was based on a vehicle equipped with powered two-axle, articulated frame trucks. Typical worn wheel and rail profiles were used.

A nominal case was established to evaluate the response of the vehicle without air spring diagonal load imbalance, but with a realistic amount of carbody weight imbalance. The individual and combined effects of air spring load imbalance, truck rotational resistance, and wheel-to-rail friction on single wheel L/V ratios were modeled. Diagrams were developed to compare predicted single wheel derailment potential at the derailing wheel for the nominal and combined cases.

The results show that a combination of factors result in a significant increase in predicted single wheel L/V ratios. The predicted values imply an increased risk of derailment, and identify air spring imbalance as the predominant risk factor; in particular when high rail/wheel friction conditions are experienced. The importance of proper adjustment of the car leveling system is better quantified and understood as a result of this analysis.

As part of this study, engineers investigated a representative derailment that occurred on a transit system to determine the influence of air spring load imbalance. The derailment occurred at the trailing truck of the vehicle in the body of a curve. The derailing wheel was the outside (high rail) wheel on the leading axle of the truck. The vehicle had previously stopped in the entry spiral to the curve.

In addition to investigating the derailment, engineers used a VAMPIRE® program to model the dynamic response of a representative vehicle — a 75-foot vehicle with 50-foot truck centers and an empty weight of approximately 80,000 pounds. The trucks were a two-axle, two-part frame design with cast side frames and bolster, rubber primary suspension, and air spring secondary suspension. Both axles were powered; wheel flange angle was 70 degrees.

Effect of Varying Air Spring Load Imbalance
The modeled car was equipped with a four-point leveling system — an arrangement with the potential for a diagonal air spring load imbalance. In this design, air spring loads and pressures are increased at two diagonal corners and reduced at the opposing corners. This diagonal load imbalance does not create a car pitch or roll imbalance. Instead, the forces are counteracted 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 configuration leads to a further reduction in vertical load at the lightly loaded corners.

If the car comes to a stop in a curve entry 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). It will also reduce pressure at the leading truck (low rail side) and trailing truck (high rail side). The opposite effect will occur in a curve exit spiral. Under dynamic conditions, this effect is less likely due to a delayed response (by design) of the leveling valve.

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