Interface - The Journal of Wheel/Rail Interaction

RAIL TRANSIT

Examining wheel/rail interaction on rail transit systems
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Wheel and rail wear rates are affected by rolling contact fatigue and, on a practical basis, by the ability to produce and maintain the desired profiles. "In order to design a matching wheel/rail profile pair it is necessary to consider a number of factors relating to its anticipated performance," Roy Smith pointed out in his introduction to the "Principles of Wheel/Rail Profile Design and Maintenance."

Each vehicle design has dynamic performance characteristics, which relate primarily to the effective conicity that will be created, and curving performance characteristics, which affect the lateral forces and L/V ratios that are generated. "In order to optimize these factors the designer must have a complete knowledge of the vehicle design parameters, the track characteristics and the performance expectations for the two working together," Smith said.

Leading wheelsets tend to generate angles of attack toward the high rail, while trailing wheelsets generate angles of attack toward the low rail. These opposing lateral forces produce a net turning moment in the direction of contacting the high rail flange. "If the wheel/rail profile will not allow sufficient rolling radius difference to be produced before the flange is reached, the moment on the leading axle becomes negative and combines with that of the trailing axle to create a total moment that increases the flange force," Smith said. Effective rolling radius difference on a given wheelset will minimize, or eliminate, wheel-flange/gauge-corner contact in all but the sharpest of curves. Restraining rails are typically used to prevent contact when sufficient rolling radius difference cannot be obtained to effectively steer through the curve.

North American transit systems have a variety of design flange angles, ranging from 60 degrees to 75 degrees. While increasing the maximum contact angle decreases the potential for derailment, the contact angle is function of both wheel and rail, said John Elkins, President, RVD Consulting, Inc. Consequently, transit systems must have consistent wheel and rail maintenance policies for maximum effectiveness.

Recent research by Transportation Technology Center, Inc., conducted on behalf of the Association of American Railroads, Federal Railroad Administration and the TCRP to develop flange climb distance and L/V criteria indicates that the lower the wheelset angle of attack, the higher L/V ratio required to derail, Elkins said. "The greater the wheel/rail flange contact angle, which is preferably greater than 70 degrees, the lower the derailment potential," he said.

In order to improve overall wheel/rail interaction, rail transit systems should design vehicles with good steering and a soft primary longitudinal suspension with low sidebearing friction. They should also design high-conicity wheel/rail profiles and avoid large angles of attack, Elkins said.

These comments represent a glimpse of the insight into many of the issues that were covered at this groundbreaking Rail Transit Wheel/Rail Interaction Seminar. Interface will provide more details from the seminar in these pages over the next several months.

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