Interface - The Journal of Wheel/Rail Interaction

TRUCK WARP

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

Insufficient Warp Stiffness
The friction casting, or friction shoe, or friction wedge provides vertical damping as well as warp stiffness. The friction wedge rides in the bolster pocket with its back surface rubbing against the roof of the bolster pocket and its front surface rubbing against the column wear plate. A large control spring under the friction wedge pushes the wedge up into the pocket, with a resulting friction force developed against the column wear plate. It is this friction force that dampens out road shocks and resists the tendency of the bolster to warp, or lose its 90-degree alignment with the side frame. There are two primary types of trucks used in North America: the constant-damped truck, typically called the Ride Control Design; and the variable-damped truck, typically called the Barber Design.

After many miles of operation, the constant frictional rubbing of the friction wedge against the pocket and column wear plate creates wear on the steel surfaces. The result of this wear is that the bolster pocket gets larger, the column wear plate wears deeper into the side frame and the friction wedge itself gets smaller in dimension. The net result of wear on these three surfaces is that the friction wedge rises higher in the pocket, pushed upward continually by the control spring force. As the control spring expands to push the wedge higher into the pocket, the force on the spring diminishes, resulting in less friction, and inevitably, less damping and warp stiffness. This rise of the friction casting above the top of the bolster is termed "wedge rise." (Figure 2 shows an example of a ride control truck with extreme wedge rise of nearly 3 inches.)

Each truck manufacturer specifies the maximum amount of wedge rise permissible. Generally, most Barber designs can tolerate an approximately 3/4 inch wedge rise before the truck loses its damping and warp stiffness. By contrast, most Ride Control designs can tolerate approximately 1-3/4 inches of wedge rise. (Other truck designs such as National and Buckeye have other criteria for evaluating the loss of truck damping.) While wedge rise condemning limits apply when a car is being re-built to AAR Rule 88 standards, there currently is no AAR interchange standard for condemning a truck for wedge rise when it is on the rip track for maintenance. Some railroads recognize the importance of wedge rise and maintain their own fleets to the manufacturer’s standards. (Figure 3 shows the use of the "moustache" gauge to check for excessive wedge rise.)

< PREVIOUS PAGE | PAGE 2 OF 3 | NEXT PAGE >