Tuesday, May 31, 2011

Stress state balancing act

Written by  Mischa Wanek-Libman, Engineering Editor
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NS discovers that managing the wheel/rail dynamic in a heavy haul environment comes down to finding equilibrium among several factors.

What goes on in the tiny contact patch between the wheel and rail has a big effect on a railroad’s fuel economy, wheel life and track performance. To optimize this dynamic, Norfolk Southern focuses on a particular goal that it attributes to Dr. John Samuels: Reducing the stress state of the railroad.

Samuels, then a vice president at NS, now retired, drew several bell curves. The horizontal axis of the graph reflected stress and strength, while the vertical axis showed frequency of occurrence. One series of curves represented the stress caused by equipment of different weights, and another curve represented the strength of the track. Ideally, the curve for track strength should be further out on the X-axis than the curve for equipment-induced stress. Track degradation and risk of derailment increase when the stress curve overlaps the strength curve (in other words, when the track is stressed beyond its strength). This can happen when wheels or trucks are worn, tie condition degrades, rail profiles wear to less than optimum contact, or lubrication and top-of-rail friction control (TOR) are out of adjustment.

“We are on a mission to reduce the wheel/rail forces, particularly in curving, that contribute to poor fuel economy, worn wheels and, most importantly to engineering folks, degraded track,” says Brad Kerchof, director research and tests at Norfolk Southern. “We are employing a number of strategies to accomplish this goal.”

“When applying these strategies, we have found that our approach needs to be balanced, for, like many things with the railroad, changing one factor can affect other things,” says Kevin Conn, research engineer at NS.

One of those factors is friction management. NS wants to control the coefficient of friction at the wheel/rail interface to be between 0.30 and 0.35.

“We want the coefficient of friction low enough to reduce the lateral curving forces,” says Conn, “but high enough to maintain locomotive adhesion and preserve the steering moment generated between the wheel and rail.”

Conn notes this can be done with two different types of friction modifier: One achieves the desired friction by controlling the amount of friction modifier that is applied. The other achieves its effectiveness due to its material properties (and the amount applied is less important).

Two other big factors are the wheel and rail profiles.

“Rail profile is important, but so is the wheel profile,” says Walter Rosenberger, research engineer at NS. “I think everybody in the industry knows that you have to manage carefully the shape of the rail profile for metallurgical damage. But it is also important to maintain a profile to govern the wheel’s interaction with the rail. Curving forces are a function of where the contact patch is on the rail, which in turn is a function of the rail profile and the wheel profile.”

Rosenberger also points out that wheel profiles vary. Three profiles that are frequently observed in the field are new wheel, stable worn, and hollow worn, and each performs differently. Each pair of wheel profiles generates a different rolling radius differential as it contacts a given pair of rail profiles. Rolling radius differential is critically important, because that is what steers the truck through a curve.

Relying on good rail profile

While wheel profiles are difficult to manage (though NS does has a wheel profile detector initiative that is intended to address this challenge), rail profiles can be managed more easily, through rail grinding.

“Our grinding objective on tangent track is a profile that carries the wheels in the center third of the head. In curves, we’re looking for the high side rail to carry the wheels on the gauge third of the head, which matches up nicely with the part of the wheel that has the larger rolling radius. On the low side, we want contact in the middle third, similar to tangent track. If you manage your rail profiles on a curve properly, you allow wheels to steer around that curve, taking advantage of the conical shape of the wheel and the differential rolling radius. The high wheel, rolling on the fatter part, travels further than the low wheel, which if rolling on the middle part of the tread has a smaller diameter,” says Kerchof.

Adds Rosenberger, “We have found that very small changes in contact patch location can have a dramatic impact on rolling radius differential and thus on curving performance.”

NS performs regular inspections to help monitor some of these factors. NS’s geometry cars use a laser-optic system that saves a digital profile of both rails every 10 feet. The system reports the amount of wear on the rail, the orientation (cant) of the rail, and its shape. The digital profiles can be used in vehicle dynamics software to simulate curving performance, including lateral forces and wear rates.

The railroad’s grinding contractor uses a similar system to determine what combination of grinding passes and patterns are needed in order to achieve the desired rail profile. A pre-grind inspection defines the existing rail profiles using a high-rail truck equipped with the same laser-optic system used on the geometry cars. A software program then determines which grinding patterns and how many grinding passes are needed to reshape the rail head to the desired profile. Another laser-optic system on the grinder serves to confirm that the desired profile was obtained.

Concerning grinding away rolling contact fatigue (RCF), the industry’s detection is only skin deep. “We know quantitatively what the profile of our rail looks like, but in terms of the amount of surface damage, our assessment is only qualitative. We can see the cracks on the rail head, but we really don’t know how deep they are. As a result, we are looking at technology that can tell us how deep the cracks are so that we can grind them out while balancing this with the desired rail profile,” says Conn. “We talk about the magnitude of forces that go into the rail and wheel, but we do not necessarily talk about where on the rail or the wheel these forces are occurring. We’re learning that in order to understand what is happening at the rail/wheel interface, we need to talk about both.”

In addition to identifying the extent of RCF and the point of contact force, where, how, and under what conditions rail cant is measured is another issue the industry will need to look into, Rosenberger says. “Everybody is measuring cant, but I think we as an industry are trying to figure out what cant value is meaningful under various load conditions and how cant relates to the risk of rail roll-over. We’re into the classic physics lab case where you’re trying to measure something and the act of measuring it has an effect on it. How do you determine what the legitimate, reliable and sensible way to measure rail cant is? And, having measured it, what is an appropriate alarm criteria on the geometry car? And we need to agree on an industry standard for the reference plane that defines zero cant: either the rail tilting 1:40 inward or vertical. Railroads currently use both.”

Dynamic test site

The rail industry has studied the larger wheel/rail dynamic category for a number of years. Across North America, there are dozens of wayside detectors measuring the force that equipment puts into the track. Each railroad also has the means to measure the strength and condition of its track. But to focus on the interaction between the two, NS established a test site at Wills, W.Va., in April, specifically to study the relationships between wheel/rail forces, rail profiles, rail deflections (lateral and rotational) and friction control.

“The purpose of our test site is to measure force levels and rail displacement under a number of maintenance conditions. The conditions include worn rail profiles, improved rail profiles after grinding, with and without TOR friction control, in a canted orientation and finally, in an upright orientation behind a rail gang,” says Kerchof.

“We’re satisfied, based on our field observations, that rails do cant out dynamically under trains. At our test site, we have already measured rail force and displacement in both the pre- and post-grind conditions and with and without TOR. Our next step is to have a rail gang address the open gauge and high-side rail cant. We’re going to remove the existing rail, adze the ties to eliminate any tie-plate cutting and set the rail back down in its original 1:40 inward cant. The combination of rail profile and rail orientation determines the wheel/rail contact point. Setting the rail upright, and holding it there with liquid spike-hole filler and new spikes, can move the wheel/rail contact point from the gauge corner toward the field side. Our mission is to answer this question: Does any of this work adversely affect axle steering such that it increases wheel/rail lateral forces? If so, is the magnitude of the increase enough to cause a loss of rail stability? In other words, does any of this work increase the likelihood that the rail will roll over?” says Kerchof.

As far as a timeline for the research, Kerchof sees it lasting through mid-June. Whatever the results, Kerchof says the information learned will not be proprietary. “This is important, safety-related research. We intend to share our findings with the rest of the industry.”

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