Stress reduction, railroad style: the contact area where wheel meets rail is about the size of a dime. Making it as stress-free as possible can save big dollars

Railway Age, July, 2002 by Eric Magel, Joe Kalousek, Mike Roney

Since the first days of railroading in the 1800s, optimizing the steel wheel/steel rail contact area has been problematic. On the plus side, steel-on-steel contact accounts for a 10% to 17% reduction in rolling resistance compared with rubber tires on asphalt. But under increasing demands to improve railway performance, axle loads and speeds have steadily increased while available time for maintenance has decreased.

However, rather than simply making things "stronger" in response, John Samuels, chairman of the Association of American Railroads Railway Technology Working Committee, suggests that the industry needs to focus on reducing the "stress state" of the railroads. Science and technology are playing a strong supporting role in making that possible.

Wheel/rail contact

The vertical load of a typical train is carried over several small contact patches, each about 8-12 mm in diameter--roughly the size of a dime. At each contact patch, slip occurs between the wheel and rail surface while at the same time a large normal stress arises from bearing the heavy load of the car over a small contact region. These two elements--slip and stress--are essential (the train won't move without them) and unavoidable in the wheel/rail system. The modern railway environment of increasing axle loads, faster and longer trains, higher-adhesion locomotives, and greater cant deficiencies are all increasing the demands on the wheel/rail interface. At the wheel/rail contact, reducing the stress state means developing or refining maintenance practices that address the elements of contact stress and slip.

Contact stress is the inevitable result of pressing the wheel and rail surfaces together. The normal load, tangential (sliding) load, and profiles of the wheel and rail govern the value of stress, with shape having a much more dramatic impact than load. For this reason, wheel and rail profiles are a key method to control contact stress.

The ongoing effort in North America to establish wheel hollowing limits is an important step in the right direction. On the track side of the interface, rail grinding continues to have as a key function the application and maintenance of proper rail shapes. It is crucial that the wheel and rail profiles be properly mated.

Reducing dynamic loads (for example, from shelled wheels or track irregularities) is also an effective way to minimize contact stress.

Slip (sometimes called creep) refers to the small relative displacement between the wheel and rail surfaces in the contact zone. A locked wheelset clearly slides over a rail, but under acceleration, braking, or curving (in fact, even under "pure rolling") there is also some slip between wheel and rail.

Slip has a dramatic influence on wear and contact fatigue. Even in cases of relatively low stress, slip can contribute to wear and corrugation. But in combination with high stress, the state of wear can change from relatively mild wear to a more severe state, as shown in the illustration on p. 29.

At the wheel flange/gauge-face contact, the levels of slip are very high, and the frictional energy dissipated there is tremendous. Lubrication is crucial in such cases to provide a protective layer between the two metal surfaces. Proper profile shapes, in combination with steerable trucks, are effective in preventing gauge face contact in the first place.

As an extreme example, the Vancouver Skytrain, with its body-steered trucks, can curve through 16-degree curves without flanging. In heavy-haul systems, Canadian Pacific Rail credits frame-braced trucks with a 36% improvement in wheel life.

Improving technology

Fortunately, stress and slip are increasingly manageable. Our understanding of the wheel/rail interface steadily progresses year by year, thanks in part to models and experiments in contact mechanics, material response, and vehicle dynamics. At the same time, advances in technology further our capacity to measure, analyze, and rectify inefficiencies in wheel/rail interaction.

Of particular note are instruments that measure rail profiles, lateral track strength, and friction conditions with good accuracy' and at high speed.

On the vehicle side, wayside instruments that measure wheel profiles, angles of attack, and lateral forces from each passing wheelset are extremely useful in evaluating the performance of the vehicle/track combinations in curves. Profiles, rail grinding, lubrication, metallurgy, and non-destructive inspection systems are continuously improving to meet the challenge of reducing the stress state.

As one of many examples of how technology has benefited the railways, consider the tools now available to accurately monitor fuel consumption.

A senior railway manager once noted that if we believed all the predictions about fuel savings, we'd have to "build locomotives with expandable fuel tanks" because they'd be gaining fuel with every mile traveled. The many optimistic predictions of the past may have instilled some skepticism in the operating department, a barrier that is being overcome through the application of advanced instrumentation.