Interface - The Journal of Wheel/Rail Interaction

W/R INTERACTION ON TRANSIT SYSTEMS

Managing Wheel/Rail Interaction on Rail Transit Systems (continued)

Vehicle Parameters

Vehicle design and maintenance practices also impact wheel/rail interaction and derailment potential, as Peter Klauser, a vehicle dynamics engineering consultant, pointed out in a presentation on “the Effects of Secondary Suspension Air Spring Imbalance on Wheel Climb Derailment Potential.” Air spring imbalance typically results from a combination of static imbalance due to car weight distribution, leveling system defects, and the effect of track twist. The result, Klauser said, is that truck side vertical loads are increased or reduced in rough proportion to the diagonal variation in air spring pressures. And since air spring stiffness is roughly proportional to air spring pressure, the car can be supported on “stiff” springs at two corners and on “soft” springs at the opposing corners. Under dynamic conditions, this may result in further unloading. Wheel unloading, combined with high lateral forces due to curving, leads to increased wheel climb derailment potential.

Klauser discussed results of a study of wheel unloading due to air spring load imbalance and its effect 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 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 load imbalance, but with a realistic amount of carbody weight imbalance. Consulting engineers modeled the individual and combined effects of air spring load imbalance, truck rotational resistance and wheel/rail friction on a single wheel. The results showed that a combination of factors can significantly increase single wheel L/V ratios and the risk of derailment.

Air spring load imbalance alone is seldom enough to lead to a wheel climb derailment, Klauser said. But when combined with other causal factors, such as the coefficient of wheel/rail friction, flange geometry and operational factors (such as the effects of stopping in a spiral before proceeding at low speed through the body of a curve), air spring load imbalance will significantly increase the potential for wheel climb.

Measurement Requirements
Measurement is an essential aspect of optimizing wheel/rail interaction on Wiener Linien, the Vienna (Austria) urban transit system. Markus Ossberger, assistant program manager of Wiener Linien's Department of Construction and Subway Planning, described how Wiener Linien closely monitored wheel/rail contact as part of an integrated rail grinding strategy. The strategy focused on measurement data to monitor wear and the grinding process, the capabilities of the grinding equipment and the experience of the staff that controlled the grinding program.

Even before the program began, however, Wiener Linien examined its track and vehicle designs and vehicle steering capabilities. The railway also determined the limits of accuracy regarding wheel and rail profile measurement, and overall optimization.

“Precise measurements and comprehensive databases provide tools that allow engineers to manage wheel and rail assets — particularly in systems with grooved rails,” Paul Mittermayr, Managing Director, Mittermayr Scientific Consulting GmbH, Bureau of Applied Mechanics and Mathematics, said regarding methods that can be used to optimize rail (and wheel) profiles in the urban rail context.

Wiener Linien varied rail profiles to improve contact geometry. By distributing the contact patch locations, the railway was able to keep the equivalent conicity within a favorable range, which improved rolling radius difference and vehicle curving capabilities. As a result of regular measurement and rail-profile analysis, Wiener Linien is able to better evaluate rail conditions and plan grinding programs. It is also better able to estimate the remaining rail life on segments of the system.

Overall, Ossberger said, "preventive grinding with precisely defined quality standards may reduce the grinding budget by 10 to 20 percent.” Preventive grinding has also reduced the number of rail fractures from more than 30 to six per year.

While all agree that optimization is a good thing, wheel/rail interaction is hard to see. To do so, Gordon Bachinsky, president of Advanced Rail Management, presented an animated three-dimensional picture of what wheel/rail interaction looks like during various operating conditions. During wheel/rail contact, rails undergo successive rolling, lateral, and vertical motions, while the wheelsets undergo successive lateral and yawing motions, Bachinsky said. “You can’t really see or address lateral or vertical motion, or flange contact’s effect on wheel/rail contact at the tread surface.” An animated three-dimensional look makes it easier to understand the importance of having a properly matched rolling radius differential in order to promote healthy steering in curves.

Measurement is also required to ensure that good contact geometry is maintained between third rail power systems and vehicle-mounted collection systems. Ta-Lun Yang, vice president of Ensco Inc., discussed a third-rail measurement system that has been deployed by the Beijing Metro, China. In order to ensure smooth contact of the vehicle-mounted contact shoes and the third-rail power source, geometry is referenced to the position of the running rails.

Beijing Metro recently deployed a modern track inspection vehicle, installed on a custom-built metro coach to perform automated track inspection. The inspection system, which includes full track geometry and third-rail geometry, uses image-based sensors for rail positioning and inertial-based reference for track geometry. The inspection car is used on existing lines as well as on a new line equipped with linear induction motor technology.

< PREVIOUS PAGE | PAGE 2 OF 3 | NEXT PAGE >