Interface - The Journal of Wheel/Rail Interaction

TRACK BUCKLE

Preventing Track Buckles
By Gary Wolf • March 10, 2005

With the approach of spring, a trackman's thoughts turn to the potential for sun kinks. Some call them thermal misalignments, some call them track buckles, some call them sun kinks. But no matter what you call them, their effects can be characterized in a word: Disastrous. There are, however, telltale signs that a misalignment is about to occur, and things that can be done to prevent track buckles from occurring.

Any piece of steel when heated tends to expand. For a mile length of rail steel, that expansion can be substantial. The amount of expansion depends on the rise in temperature, and the coefficient of thermal expansion. For most rail steels, the coefficient of expansion is roughly .0000065 of its length per degree (F) of temperature rise. That may not seem like much, until you figure that there are 63,360 inches in a mile of rail steel. Thus, a rise in temperature from 40 degrees F to 140 degrees F can cause a one- mile strand of rail to grow by 41 inches! If left unchecked, that expansion would cause the track to buckle. On tangent track, the rail typically buckles in a "S" pattern, while in a curve it typically buckles in a "C" pattern (see Figure 1). In order to control that expansion, rail anchors are used to firmly anchor the rail to the crossties, and prevent any longitudinal movement.

Rail Neutral Temperature

Continuous welded rail, or CWR, is laid in a stress-free state at a given temperature. This becomes the Rail Neutral Temperature, or RNT. In most of the Midwest and upper plains of the U.S., rail is laid at a RNT of anywhere from 95 to 100 degrees. That means simply that the rail is stress free when the rail temperature is around 100 degrees. If it gets colder, the rail goes into tension and shrinks, causing potential pull aparts. If it gets hotter than 100 degrees, the rail goes into compression and expansion, causing a potential buckle or kink. Research has shown that a rise in rail temperature of anywhere from 40 to 50 degrees above the RNT can cause a potential buckle. You might say that the rail never gets as hot as 150 degrees, therefore, track buckle should not be a problem. But there are other, related factors to consider. RNT changes constantly for a given section of track. Over winter cycles, curves can pull in, thereby effectively lowering the RNT. At Rail Sciences, we have been actively using Vortok's VERSE equipment to measure RNT (see Figure 2). The VERSE equipment measures the amount of force required to lift a section of rail after spikes or clips have been removed. The lower the RNT, the more the rail is in tension, and the more force required to lift the rail a given displacement. Thus it is possible to correlate the required lifting force to the RNT.

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