Interface - The Journal of Wheel/Rail Interaction

TRUCK WARP

Truck Warp: Causes and Cures (Part 2 of 2)
By Gary P. Wolf • November 1, 2004

Part 1 of Truck Warp: Causes and Cures covered the three primary causes of truck warp:
• An insufficient longitudinal steering moment is developed at the wheelset due to poor profile characteristics, typically, hollow-worn wheels.
• An insufficient warp stiffness or moment is present at the bolster-to-sideframe interface, otherwise known as the friction castings, or friction wedges. Or, an insufficient stiffness is present at the wheelset bearing adapter-to-sideframe interface.
• A significant resistance to bolster rotation, or turning moment, is developed due to friction in the centerbowl, or rotational resistance due to tight side bearings.
Part 2 of this article explores these causes in more detail and, more importantly, what can be done to prevent trucks from warping.

Lack of sufficient steering moment

Hollow-worn wheels are the primary reason that wheelsets fail to develop sufficient steering force to steer the bolster and truck through curves. For many years, the industry lacked a hollow wear standard in the AAR Field Manual. (Figure 1 shows an extremely hollow-worn wheel that was found running in the North American fleet.) Typically, hollow-worn wheels are changed out due to a high flange, a vertical flange, or excessive tread wear. In mid 2004, however, a new hollow wear standard was inserted into AAR Rule 41, and a new gauge was designed to check for hollow wear. The current hollow wear standard allows up to 4 mm of tread hollow before the wheel is condemnable.

In addition to poor steering, hollow wheels also create excessive contact stresses on the rail that can lead to head checking or shelling of the high rail and surface spalling of the low rail in curves. Hollow-worn wheels also cause trucks to hunt at less than normal critical speeds. Any way you slice it, excessively hollow-worn wheels increase the stresses and forces that are transferred to the track structure. Some railroads – especially captive heavy-haul systems – have recognized the poor characteristics of hollow wheels and change or re-profile wheels at approximately 2 mm of tread hollow. They have found that this practice actually lengthens the life of the wheelset and the rail.

Poor rail maintenance practices can also cause poor steering conditions. Railroads that excessively grind the gauge corner of the high rail of curves, for example, can create two-point contact, which inhibits wheelset steering. Similarly, failure to relieve the profile build-up on the field side of the low rail can result in poor wheelset steering. Finally, excessive negative rail cant (see the "Effects of Rail Cant on Wheel/Rail Forces and Derailment Potential") can reduce the effective steering of the wheelset.

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