Interface - The Journal of Wheel/Rail Interaction

TRUCK WARP

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

What causes trucks to warp?
There are several causes of truck warp. First, we need to understand how a truck steers through a curve. In a normal situation, as the leading wheels of a wheelset enter a curve, the taper on the wheelset causes the outer wheel to develop a larger radius than the inner wheel. That is, the outer wheel moves toward the throat of the flange, where the radius is greater, and the inner wheel moves toward the field side of the tread, where the radius is smaller. Thus, the wheel becomes conical in shape, with the outer side rolling on a larger radius than the inner side. Visualize the typical foam coffee cup. The top is larger in diameter than the bottom. If placed on its side and rolled, it will tend to roll in a circle. A wheelset does the same thing. The larger diameter of the outer wheel creates a steering moment that pulls the wheelset in a curved path. This steering moment that is generated by the diameter (radius) differences in the outer and inner wheel then creates a turning moment that tends to pull, or steer, the truck bolster through the curve. The forces from the wheel are transferred through the bearing adapters, into the side frame, through the friction damping elements (friction castings or wedges), and eventually into the bolster. The bolster must then rotate on the truck center bearing (centerbowl) to complete the sequence. If the bolster rotates freely, the truck steers properly, and curving forces are minimal. If the bolster does not rotate freely, the truck attempts to "warp" at the bolster to sideframe interface to accommodate the curve of the track.

Among the potential causes of truck warp are:

• An insufficient longitudinal steering moment is developed at the wheelset due to poor wheel 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.
Figure 5 shows how the steering moment of the wheelset, the warp moment of the bolster to sideframe interface, and the turning moment of the truck bolster interact to form a force balance.

Gary P. Wolf is President of Rail Sciences Inc.
Part 2 of this two-part sequence will drill down into the potential causes of truck warp, and examine proactive measures to prevent it.

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