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The truck bolster bowl: Is it a bowl or a bearing? (continued)
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Evidence of rubbing or fouling between the bowl rim and the body centerplate indicates a violation of this AAR rule, and can lead to poor steering, truck warp and excessive gauge spreading forces at the wheelsets. Figure 2 shows evidence of fouling on the centerbowl rim. Figure 3 shows an extreme example where fasteners used to secure the centerplate to the body bolster have dug into the rim of the bowl preventing proper truck rotation. Another condition that can increase turning moment is where fasteners used to connect the body centerplate to the body bolster foul, or rub against, the outer rim of the centerbowl. Figure 4 shows an example of Huck bolts rubbing a shiny mark into the edge of the centerbowl rim, greatly increasing the truck turning moment.
During curving on superelevated track, a roll moment of the body centerplate can cause an unequal pressure distribution on the surface of the centerbowl. Figure 5 illustrates this condition. When this occurs, the turning moment is substantially increased as the effective turning radius is moved toward the outer edge of the centerbowl. Tipping of the car body can also move the centerplate into edge contact with the vertical surface of the centerbowl, further increasing turning moment. Thus, operation of high center of gravity cars on over-elevated curves or below balance speed can affect the ability of the cars to effectively steer through the curve. Also, operation through spirals with significant track twist can produce a similar effect. Testing by Rail Sciences has shown that the effect of simulated track twist can increase turning moment by 100% to 200%, depending on the severity of the twist.
In order to reduce the chance of increased turning moment and rotational resistance, AAR rule 47 lists several dimensional tolerances and maintenance requirements for the centerbowl and centerplate. In summary, these are:
• 1/16-inch clearance must be maintained between the centerbowl rim and the base of the body centerplate.
• The maximum amount of wear in the depth of the centerbowl is 5/16 inches.
• The maximum amount of allowable diameter increase in the centerbowl is 7/8 inches beyond the nominal diameter (e.g., 16-7/8 inches for a nominal 16-inch centerbowl).
• The difference in diameters of the body centerplate and the bolster bowl must not exceed 1-3/8 inches (check both fore and aft, and side to side).
• Body centerplate diameter must not be reduced more than 1 inch (Rule 60)
• Top surface of the bolster bowl rim must not be in contact with the centerplate horizontal surface.
• Lubrication or a lubricating liner is required any time a car is on a repair track and the car body is raised off the trucks.
Those responsible for maintenance should become familiar with these requirements for the centerbowl and centerplate. Excessive truck turning moment can be the root cause of excessive wheel flange wear, and even gauge-spreading or rail rollover derailments. In order to improve overall performance, maintenance managers must start thinking of the centerbowl as a bearing, and ensure that it is properly lubricated during any maintenance activity.
Gary P. Wolf is President of Rail Sciences Inc.
