Interface - The Journal of Wheel/Rail Interaction

TRUCK WARP

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

Excessive Truck Turning Moment
or Bolster Rotational Resistance
A truck or bogie should be free to turn or steer on the car body center casting as it negotiates curvature. Conditions between the car body centerplate and the bolster bowl can inhibit free steering of the bolster. First, we need to recognize that the centerplate is actually a center "bearing." With any bearing, lubrication is essential. AAR Rule 47 specifies that the center casting should be lubricated or an approved centerbowl liner should be used whenever the car body is raised for maintenance. Tests have shown that many of the plastic style liners can maintain their lubrication effectiveness longer than conventional greases.

Another common cause of excessive friction at the centerplate is contact, or fouling, between the centerbowl rim and body centerplate. AAR Rule 47 specifies that a minimum clearance of 1/16-inch should be maintained between the centerbowl rim and body center casting. Excessive friction or rubbing on the bowl rim will cause resistance to turning, or worst case, can tend to lock up the bolster from turning. (Figure 4 shows evidence of a center casting rubbing on the bowl rim.) Center casting fasteners such as huck bolts that rub on the side of the bowl rim also can inhibit bolster steering.

Finally, tight side bearings can cause resistance to bolster rotation. AAR Rule 60 specifies that a clearance of 3/16 to 5/16 inches should be maintained at each conventional side bearing location. The clearance should be checked for proper dimension on both conventional roller or steel block side bearings, and the set-up height on constant contact designs. (Figure 5 shows an example of a conventional roller side bearing with insufficient clearance.)

AAR Rule 60 also specifies that a minimum set-up height of 4-7/8 inches should be maintained for constant contact designs. Manufacturers have traditionally recommended a minimum set-up height of 5 inches. Go/no-go gauges, or calipers, can be used to check set-up heights. Failure to maintain proper dimensions will result in added resistance to turning and the potential to cause truck warp. (Figure 6 shows an example of a constant contact side bearing measured with calipers.)

Truck warp has been shown to cause excessive gauge-widening forces, which increase wheel, rail and fastener wear, and tie degradation. At worst, truck warp can result in rail rollover or gauge spreading. The root cause of most truck warp lies in excessive wear of truck components, or failure to maintain proper clearances as the truck wears. Knowledge of the causes of truck warp and attention to inspection and maintenance procedures can help reduce its negative effects on vehicle and track components and operating conditions.

Gary P. Wolf is President of Rail Sciences Inc.

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