Effect of Secondary Suspension Air Spring Imbalance on Wheel Climb Derailment Potential

This paper describes how the secondary suspension of a modern rail car, with two air springs per truck and a four-point leveling system (two control valves per truck), has 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, 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. This paper discusses results of a study into the effects of wheel unloading due to air spring imbalance on derailment potential. Vehicle dynamic response and an actual derailment were modeled using a vehicle dynamics simulation (VAMPIRE®). The vehicle model was based on a detailed model of a car equipped with powered twoaxle 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.


  • English

Media Info

  • Media Type: Print
  • Features: Figures; Tables;
  • Pagination: 8p
  • Monograph Title: Rail Transit Conference Proceedings, 2005

Subject/Index Terms

Filing Info

  • Accession Number: 01002186
  • Record Type: Publication
  • ISBN: 1931594155
  • Files: TRIS
  • Created Date: Jul 14 2005 4:25PM