Railway Track & Structures - July 2008 - (Page 35)

The plastic ties continue to withstand HAL traffic at FAST and in revenue service. They are installed out of face and intermixed with wood ties, fastened with both cut spikes and screw spikes. So far, there has been more tie plate breakage than on wood ties. The lower bending strength of plastic ties appears to allow more plate bending. In 2008, the tests will include Vossloh 18-inch plates with drive spikes and screw spikes; MSFR 16-inch plates with screw spikes; NorFast 18inch plates with both drive spikes and screw spikes, leading edge plates with screw spikes, conventional 128-inch tie plates with cut spikes and Pandrol rolled plates with screw spikes (replicate from previous test) and drive spikes. The researchers will be looking at component evaluation, system performance, loads and load paths. According to a recent Technology Digest, premium test rails at both mega sites, one on Union Pacific and one on Norfolk Southern, continue to show excellent wear resistance, losing less than 2.1 percent of the total head area after 114 mgt in the 10-degree test curves at the eastern site and less than 2.4 percent after 494 mgt in the twodegree test curves at the western site. Without preventive grinding in the test curves, rolling contact fatigue developed after 300-350 mgt on the low rails in the two-degree test curves. Corrective grinding was required at 375 mgt to remove RCF. At the eastern site, wide-gap welds continue to show good performance with no surface or internal defect identified after 123 mgt of traffic. Track fasteners Tim Austin of Lewis Bolt & Nut Co. described improvements in track fasteners for heavy-haul areas in a paper presented at AREMA in September 2007. “The increasing stresses of heavy axle load traffic have placed greater demands on the fasteners used in track compo- nents,” Austin said. “Just as all other components have needed to adjust to the higher stress environment, there has been a movement to upgrade standards for spikes, bolts and screws that were in place for many years to more robust designs. In addition to the proliferation of 286,000-pound cars and the potential for 315,000-pound, higher speeds, more traffic and the use of continuously welded rail have also put increased demands on the fastening systems. The increased lateral loads from higher speeds and modern equipment place demands on special trackwork and curves, leading to new systems to resist the loads and fight rail rollover. The widespread use of continuously welded rail has placed thermal loads on components in some locations. “The need to resist greater loads has led to a general trend to make the fasteners larger and make them out of higher-strength and better-controlled materials,” he noted. “Many fasteners traditionally used in limited applications have found more widespread use as more places began to see the vulnerabilities of the legacy fastening systems. “Maintenance-of-way practices have changed to accommodate and make better use of the new fasteners and applications,” he pointed out. “In some cases, maintenance practices have needed to change significantly to properly install and take advantage of higher strength fasteners. In other cases, wider usage of parts has forced a look at installation methods to keep work crews productive.” Austin continued: “The track spike has seen modest changes. It has been replaced by screw spikes in some special trackwork and curves on some roads, but the track spike is still the norm in the vast majority of track. In 2005, AREMA Committee 5 updated the track spike specification, which had been unchanged since 1968.” The 2005 specification applied upper limits on the carbon and more control over other elements in the steel. Limits were applied to manganese, silicon phosphorus and sulfur, while vanadium was added to the chemistry. The optional minimum copper content for corrosion resistance was made mandatory. The minimum yield strength was effectively increased and an on-request impact property test was added. The dimensions remained unchanged. Overall, the 2005 version of the standards eliminated a lot of potential variability that was allowed under the 1968 version and resulted in more consistent performance. “Probably no fastener has seen more changes in usage and design than the screw spike (a.k.a. lag screw or coach screw),” Austin said. “The higher lateral loads of special trackwork and curves frequently exceed the ability of a track spike to maintain gauge and hold the rail in place. Screw spikes are typically used to affix plates when elastic fasteners are used. Under static conditions a track spike can be pulled out with about 7,000 pounds. A 15/16-inch screw spike typically will take 15,000 pounds or more. Moving the spikes farther from the rail base, as on a typical elastic fastener plate, also gives them more leverage to prevent rail rollover. How well the spikes stay in place under dynamic loads is probably more important. However, very little work has been done to quantify such conditions.” The only standard currently addressing screw spikes is ASTM A66. It specifies minimum 60,000 psi tensile and gives some guidance on testing and tolerances. It does not give any specifics on configuration, dimensions, or chemistry. Screw spikes are typically available in many head and thread configurations and diameters from 3/4-inch to one inch. Three common combinations account for most of the market. There are three typical ranges in which material properties are generally specified, ranging from 60,000 to www.rtands.com Railway Track & Structures July 2008 35 http://www.rtands.com

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Railway Track & Structures - July 2008

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