Nylon Tiedowns

 Nylon Tiedowns for Rail Movement - Why not in CONUS?

A description and comparison of the rail transport shock environment in US (North America), Europe, and Korea.

29 September, 1998
  • Purpose: To explain why the shock level in CONUS precludes the use of nylon straps as tiedowns for rail movement (and a cursory look at other factors).

  • Facts: The shock environment is more severe in North America than in Europe because of several factors:

    • Railcars in North America use automatic couplers while in Europe railcars are linked together manually. Automatic couplers tend to encourage impacts to ensure engagement of the coupler mechanism. The railroad companies admonish their employees not to impact cars faster than 4 mph, but higher impacts sometimes occur. (The Association of American Railroads (AAR) impact test, used to verify the safe loading procedures for most commodities, calls for impacts of 4, 6, and 8 mph and then 8 mph in the opposite direction. Military vehicles must pass the MIL-STD-810 version of the AAR test to receive transportability approval.)

      The automatic couplers are attached to the car frame through a draft gear that may be standard or cushioned. While the division between standard and cushioned is travel of less than 5 inches for standard and greater than 5 inches for cushioned, standard draft gear travel is usually about 2 to 3.5 inches and cushioned is 9 to 15 inches. The draft gear provides shock mitigation in both buff (compression) and draft (tension) but also adds slack to a train. Slack is the difference in length of a train between being bunched together and being stretched out.

      The European cars do not have automatic couplers. Instead, the European cars are coupled by hand using a turnbuckle-like device that draws the cars together. European cars have buffers, one at each corner on the ends, which are pulled together and compressed by the coupling device. This arrangement limits the slack in European trains. The Korean railroad uses automatic couplers similar to those in North America, but the flatcars are typically loaded and unloaded as a unit train, so the cars are not as likely to be uncoupled and coupled as in North America.

    • Railroads in North America operate significantly longer trains than in Europe. The longer trains combined with the major difference in coupling systems lead to higher shocks in moving trains. As a train goes up and down hills, accelerates, and applies the brakes, the slack runs in and out propagating shocks as the cars rapidly change velocity. In some areas of North America, trains are limited to 60 cushioned cars because of the severity of the shocks produced by the terrain and the slack. Those cushioned cars are typically used for military moves. For other operational reasons the railroads would run trains of over 100 cars. We believe European trains usually consist of 20 to 40 cars. Korean trains are typically limited to 22 cars due to the length of passing sidings, so in-train shocks due to slack would be significantly less than in North America.

    • Heavier loads are transported on trains in North America. Allowable axle loads in North America are roughly twice as high as those in Europe and Korea. That means that the mass (weight) of each car in North America may be significantly higher than in Europe or Korea, therefore, contributing to higher shock levels. This is true of military trains, since a North American 89-foot car will hold more vehicles than a typically used European 61-foot (18 500-mm) flatcar with the same number of axles. Korean cars are shorter than North American cars, also having the effect of lowering the total mass of a train.

    • Hump operations can cause severe shocks. A hump is a hill down which cars are allowed to roll into one of multiple tracks and is used to sort cars based on destination. Several humps do exist in Europe. Perhaps, the biggest difference is that in North America the cars are expected to couple and in Europe excess impact could cause the cars to bounce apart.

    • The distances traveled by rail in North America are vastly greater than those in Europe and Korea, therefore, the cumulative effect of shocks is greater.

  • Most of the flatcars that the military uses in North America are equipped with integral tiedown chains (and cushioned draft gear). Because the chains are available and compare favorably with nylon straps for ease of application, nylon straps would not be economically justified in North America. Wire rope tiedown for vehicles in North America has virtually disappeared, leaving only the threat of use in the case of a general deployment, which may bring plain, non-chain-equipped flatcars back into military use. In Europe and Korea the shipper provides tiedown materials.

  • 4. Nylon straps have been used (though not officially recognized nor approved by the AAR) as secondary cargo tiedown in the beds of trucks transported by rail in North America. The rail officials in North America (AAR) are worried about stretching, cutting, ultraviolet degradation, and theft while still in use, leaving cargo unsecured. Therefore, they are not inclined to approve the use of nylon straps.

  • 5. Since no AAR rules exist governing the use of nylon straps, the widespread use of nylon straps for rail tiedown would require changes to the AAR rules. The rules for loading flatcars are written and maintained by the Open Top Loading Rules Committee of the AAR, and all North American railroads accept these rules for unrestricted interchange. The military is bound to follow these rules in order to procure railroad transportation. When the military needs a rule change or a new rule, the request should be sent to the DOD-AAR Representative for coordination with the AAR. We would expect rail impact testing and extensive over-the-road testing would be required to support a change in the AAR rules.

  • For information, contact the DOD-AAR Representative at usarmy.scott.sddc.mbx.tea-dpe@mail.mil or give us a call at (618) 220-5263 or DSN 770-5263.

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