Interface - The Journal of Wheel/Rail Interaction

FLANGE CLIMB

Flange Climb and Independently Rotating Wheels
December 1, 2004

Investigations have shown that wheel-flange/gauge-face angle and the coefficient of friction (COF) play significant roles in contributing to or preventing flange-climb derailments. Investigations have also shown that Light Rail Vehicles with Independently Rotating Wheels (IRWs) have a greater propensity for flange-climb derailment than vehicles with conventional wheelsets and rigid axles. While not an industry-wide issue, this affects a number of transit systems using various types of articulated two-car and articulated three-car low-floor vehicles, which incorporate trucks with IRWs in the center low-floor section of the vehicle.

What are the differences between conventional, rigid axles and IRWs, and why are IRWs more prone to flange-climb derailment?

"Independently rotating wheelsets tend to climb the rail more easily than conventional solid wheelsets due to a lack of self-steering capability," John Elkins, president of RVD Consulting, Inc., told delegates at Interface Journal and Advanced Rail Management's Rail Transit '04 Wheel/Rail Interaction Seminar. (See "Examining wheel/rail interaction on rail transit systems.") Flange climb occurs at lower L/V ratios with IRWs than with conventional wheelsets, and the flange climb distance is shorter for IRWs than for conventional wheelsets, so the L/V must be sustained for a shorter length of time to cause a derailment.

When a conventional wheelset takes an angle of attack, it creates lateral creepage, which generates a lateral force. As the wheelset moves laterally, rolling radius difference generates longitudinal forces, or steering moments, on the wheelset. "The wheel that is larger in radius tries to pull the wheelset forward while the wheel that is smaller in radius tries to pull it back. That creates a turning moment," Elkins said.

A conventional, rigid-axle wheelset that is in flange contact with an angle of attack generates lateral and longitudinal forces. The longitudinal force will reduce the possible magnitude of the lateral force, he said. "The larger the longitudinal force, the smaller the lateral force can be."

IRWs, on the other hand, generate no steering moment—even when shifted laterally. While the two IRWs have different rolling radii, they are able to rotate at different rotation speeds without taking an angle of attack or generating a steering force. This feature plays a significant role in IRWs propensity for derailment.

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