Interface - The Journal of Wheel/Rail Interaction

WHEEL RAIL INTERACTION

Tuning in to the Systems Approach (continued)

Out of Round Wheels
Out-of-round railway wheels represent another source of problems in railway operations. Paul Mittermayr, Managing Director of Mittermayr Scientific Consulting GmbH, pointed out that out-of-round wheels increase wear, cause damage to the track structure, compromise comfort on passenger lines, generate noise and increase costs. As a result, there is a need to be able to reliably detect cars that do not meet the operational specifications. “With the liberalization of railway traffic in Europe, this will be important for the calculation of usage fees in the near future,” Mittermayr said.

Identifying out-of-round wheels is one thing, understanding the causes and their impact on wheel/rail interaction is another, though essential aspect of identifying and controlling maintenance and life-cycle costs. New simulation tools such as simOOR (Simulation of Out-of-Round Railway Wheels) model rolling motion under realistic operating conditions, using representative out-of-round wheels, rather than new wheel conditions, as is done on some existing models, to simulate vehicle/track interaction, Mittermayr said. The actual wheel/rail contact is characterized as non-elliptical contact, which provides deeper insight into the causes of out-of-roundness and the mechanisms associated with it.

Friction Management
Whenever railroaders talk about wheel/rail interaction, friction management is part of the discussion. Over the past several years, interest has grown in the use of top-of-rail friction modifier technology as a means to control friction at the wheel tread / railhead interface. Benefits associated with controlling friction at this interface include reductions in lateral curving forces, gauge widening, fastener fatigue, rail wear and rail rollover potential.

In tests on a full scale rail/wheel test rig, Kelsan Technologies and Voestalpine Schienen GmbH examined the relationships between rail strength, contact geometry and friction conditions to determine wear rates and the initiation of rolling contact fatigue (RCF). They found that in addition to reducing lateral loads in curves, the application of a TOR friction modifier every 50, 250 or 500 cycles per 100,000 cycles simultaneously reduced rail wear, surface and sub-surface plastic flow, and RCF crack generation, said Richard Stock, Voestalpine Schienen’s Manager of Research & Development. While cracks and some wear developed on “dry” rail (friction modifier was applied every 500 cycles), no cracks developed after 100,000 cycles with friction modifier applied every 50 or 250 cycles. And since the friction modifier is a dry film, though applied in a water-based solution, the risk of hydro-pressurization of cracks is eliminated.

Still, applying TOR friction modifier technology to heavy axle load traffic in heavy-grade, high-curvature areas presents challenges. NS installed 24 Portec wayside systems to apply Kelsan Technologies’ Keltrak®, TOR friction modifier at 14 locations on its Virginia Division Bluefield-Narrows coal route, on which trains apply air brakes through the application zone, much of which has sustained, greater than 1% descending grades. Curves range from 3 to 12 degrees, with very short spirals and very little tangent between the curves. NS makes extensive use of dynamic and air brakes and sanding to hold the speed on the descents on this route.

It’s a high stress environment with plenty of evidence of high lateral loadings, including wide gauge, negative cant, gauge corner cutting and low rail wear in curves, said NS Research Engineer Kevin Conn. “The lateral force not only transmits itself into the tie plate, but into the fasteners, resulting in a large number of broken spikes and broken screw spikes.”

NS wanted to determine whether TOR material would withstand the high heat generated by nearly continuous braking on descending grades, and under heavy sanding on ascending grades. NS also looked at the effect of superelevation on train handling.

Data from the NS track geometry car showed a reduction in loaded gauge in the test area on descending grades. Additional measurements conducted on back-to-back 6.3- and 6.8-degree reverse curves (in conjunction with the TTCI’s Eastern Heavy Axle Load “Megasite” monitoring program) initially indicated that reductions in lateral loads were not in the anticipated 30% range, however. After experimenting with the application rates, NS, along with Kelsan and Portec, found that an application rate of 140% of the normal rate enabled NS to achieve the desired 30% reduction in lateral loads.

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