Interface - The Journal of Wheel/Rail Interaction

REDUCING DERAILMENTS

Reducing Broken Rail Derailments in Dark Territory (continued)

Another issue is the effect of residual stresses on TD growth rates. In simple terms, residual stresses occur at a right angle to the plane of a developing TD. A rail with high residual stresses will have a higher TD growth rate. BC Rail worked with one manufacturer that produced rail that remained neutral when saw cut, and with another manufacturer that produced rail that closed when saw cut, exhibiting desirable compressive stresses in the head. Of course, wheel and rail profiles, rail grinding and lubrication practices can also impact the formation of TDs. All other factors being equal, however, BC Rail found that the use of rail with lower residual stresses generally resulted in fewer TDs.

Large Railways continue to push for the development of harder, higher-strength steels to reduce rail wear costs. There is a trade-off, however, between hardness/high-strength and toughness — the ability to absorb energy without brittle fracture. Railways operating in dark territory must weigh the additional cost of “special specifications” relating to residual stress, against improved performance that is measured by wear and crack-growth rates. (1) Railways should also examine their bonding practices at crossings and other troublesome locations. Railways that do any cad-welding to the middle of the rail web should use rail without high residual stress.

The Effect of Curvature

Research by David Jeong and the Volpe Center on analytical modeling indicates that TD growth rate increased with curvature (2). In looking at TDs and curvature, BC Rail found some curious data on TD formation. The data in Figure 4 shows the number of TD defects and service failures between 1997 and 2001 on a subdivision with 58% curvature. The number of defects per mile of curve formed a bell curve around 7 degrees. Analysis indicates that this was caused by sub-optimum lubrication and grinding practices (over-lubricating and under-grinding). In the case of over-lubrication, as the curves got sharper, the lubrication was less effective and the “magic wear rate,” as characterized by Dr. Joe Kalousek, re-established itself and reduced the number of defects per mile. BC Rail’s experience indicates that lubrication is required to reduce wear; grinding is required to reduce the number of defects. Other railways may obtain different results, but regardless of the conditions, the point is: identifying anomalies and trends in the data helps to improve maintenance practices, extend rail life and reduce risk.

Rail Section
Heavier axle loads have led to accelerated fatigue growth in 100-pound and smaller rail sections, which were produced when steel making practices were not as good as they are today. The 100-pound ARA rail section, which has a tighter radius under the head, for example, is prone to developing head-web separations at 200 MGT and about 10 mm of vertical wear. The 85-pound rail sections are subject to bolt-hole cracks. Often, the relay rail available to short line (dark-territory) railways may have been changed out because of fatigue problems. It should be monitored for “early” failures, particularly if the pre-existing worn profiles have not been reground to the current standard.

Optical rail measurement systems can accurately determine existing (and the desired) profiles. Data from these systems (see Figure 5), which utilize software that can forecast rail wear, can be integrated into a railway’s existing database.

Vehicle and Wheel Issues
Optimized wheel/rail interaction can generate benefits and costs across department lines. Poor rail conditions, such as corrugation, can impact Mechanical Departments’ maintenance costs. Impacts caused by poor rail conditions can lead to cracked housings, plasma arc and bearing problems on traction motors, particularly under increasing axle loads. The life of roller bearings on vehicles can be reduced by poor rail/track conditions, as well.

Cold weather is hard on equipment in general. But winter operation is especially hard on wheels. BC Rail, for example, spent $4 million - $4.5 million per year on wheels, against a $13-million budget to maintain 10,000 cars. Most of these wheels were prematurely taken out of service because of shelling, out-of-round or other high-impact conditions that hammered the rail during the coldest (highest tensile stress) time of the year. Overall, only about 14% of the wheels were removed for high flange, thin flange or thin rim wear conditions. About 86% of the wheels were removed prematurely for high-impact, shelled tread or slid-flat defects. Imagine if only 14% of the rail was wearing to full life while 85% was being removed prematurely because of rail defects, railways would do something about it. Yet, these types and number of wheel defects are tolerated even though the causes are generally understood.

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