Interface - The Journal of Wheel/Rail Interaction

SUSPENSION IMBALANCE

Effects of Secondary Suspension Imbalance on Wheel-Climb Potential
(Part 1 of 2)
By Radovan Sarunac and Peter Klauser • July, 2007

Low-speed wheel-climb derailments are certainly not a new phenomenon. Investigations into the likely causes have long since identified the primary factors. Sometimes, however, old lessons need to be relearned. While all aspects of wheel/rail interaction, which includes wheel/rail profiles, friction, track geometry, and vehicle suspension systems, must be considered (APTA RP-M-010-98 Recommended Practice for Derailment Investigation Reports provides a good guide), this two-part article examines an issue specific to modern rail transit cars. That is, the effect of wheel unloading due to air spring imbalance.

In this study, 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, two-axle articulated frame trucks. Typical worn wheel and rail profiles were used.

Studies have shown that the forward rolling motion of the wheelset, combined with a high lateral to vertical (L/V) wheel load ratio, can induce this type of derailment. Two-point contact between the wheel flange and rail gauge face may create an additional moment that can lift the flanging wheel.

For each low-speed derailment, the following information is typically gathered and analyzed:
— Train Data: train speed, leading or trailing, manual or ATC mode of operation, unusual stops, and suspension behavior.
— Vehicle Data: condition of primary and secondary suspension (inflated/deflated and each air spring pressure), wheel diameter, wheel out-of-roundness, wheel profile measurements (MiniProf®), flange condition and wheel hardness, wheel back-to-back spacing, truck tram, production tolerances of the carbody bolster and center pin, force required to rotate truck, truck equalization, coupler condition, and brake condition.
— Track Data: curve, spiral, tangent, dry or lubricated, high or low rail, track gauge, gauge variations, gauge-face wear, crosslevel, rail profile measurements (MiniProf®), type of ties (wood, concrete), type of fasteners.

Low-speed wheel-climb derailments are not usually the result of a single deviation or cause; they typically are caused by multiple factors. The most common include: dry wheel/rail conditions (high coefficient of friction (COF) between wheel and rail, high lateral forces), newly trued wheels (high COF between the wheel flange and rail gauge face), shallow wheel flange angle (low Nadal criterion), track perturbations, excessive gauge-face wear (conditions for two-point contact), absence of restraining rail or guard rail, and low vehicle speed or sudden change in vehicle speed. Air spring imbalance recently has been added to the list.

The effect of flange angle and coefficient of friction can be assessed by analyzing the wheel climb derailment safety limit. Nadal’s criterion (expressed as L/V = [tan (δ) – μ] / [1+μ tan (δ)]) was used in this study. (The impact of angle of attack was not considered). Nadal’s criterion is based on the ratio of lateral (L) and vertical (V) forces acting on the wheel and rail just prior to derailment.

Nadal’s criterion, which is generally conservative, is dependent only on the wheel flange’s maximum angle of contact with the rail (δ, relative to horizontal) and the amount of friction (μ) between them. The minimum single wheel L/V ratio that can sustain a wheel climb is the derailment limit criterion used to indicate the risk of single wheel-climb derailment. In other words, when the L/V ratio is greater than the right side of the above equation, wheel climb may occur. The forward rolling motion of the wheelset, combined with a high value of lateral to vertical wheel load (L/V ratio), will induce this type of derailment. These conditions typically last for a minimum of approximately 5 feet before a wheel-climb derailment will occur.

PAGE 1 OF 3 | NEXT PAGE >