Interface - The Journal of Wheel/Rail Interaction

TRUCK WARP

Truck Warp: The Causes and Cures – Part 1 of 2 (continued)

Remember Newton’s third law of motion? "For every action or force, there is an equal and opposite reaction or force." In addition to the flanging forces on the high rail, a warped truck also generates a lateral creeping or sliding force across the head of the low, or inner, rail of a curve as the inner wheel attempts to keep up with the speed of the outer wheel. The magnitude of this creep force, which acts toward the field side, is largely dependent upon the coefficient of friction between the wheel and the rail. The net effect is that a warped truck produces gauge-spreading forces against both rails. While many track maintenance supervisors think that it is primarily the outer rail, or high side, of a curve that receives gauge-spreading forces, the inner, or low, side of a curve also tends to roll outward under truck warp forces. Figure 4 shows the forces developed against the head of the low rail at all four wheels of a vehicle with warped trucks.

A common characteristic of the warped truck is that the trailing wheel of the truck produces equal, or in many cases higher, lateral forces than the leading wheelset. This rarely occurs in a normally behaving truck. Also, the wheels on the trailing truck of the car produce excessively high forces on both axles. On a normally tracking car, the trailing truck generally produces much less lateral force than the lead truck. But when the trucks warp, both the leading and trailing trucks produce excessive lateral forces. This means that the track structure must be able to withstand the attack of four wheels on a vehicle, instead of just the lead axle of the lead truck on a normally steering car. This creates an opportunity for gauge widening as all four wheels are pushing on the rail simultaneously, trying to roll it outward. The track structure must be solidly spiked, or anchored to prevent a rail rollover derailment.

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