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The truck bolster bowl:
Is it a bowl or a bearing?
by Gary Wolf • October 2005
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Like many aspects of the three-piece truck, performance issues surrounding the truck bolster bowl and body centerplate are often misunderstood. One of the biggest misunderstandings is that the bolster bowl is actually a bearing. Years ago at a meeting where truck performances issues were being debated, an especially astute engineer from one of the car builders made the simple statement: “We keep calling it a bowl, when in reality it is a bearing. And what do you do with a bearing? You lubricate it!” That simple, yet profound, statement contained much wisdom.
Car designs reaching back to the 1800s all had some semblance of a bolster pivot, or center bearing. Initially quite small in diameter, as the size and weight of freight cars increased, the size of the bearing surface gradually increased. Today, most centerbowls are either 14 inches or 16 inches in diameter. The function of the centerbowl is twofold: 1) To distribute the weight of the car body into the truck bolster. 2) To provide a central pivot point for the truck bolster to turn on the car body.
When a car is loaded to a gross rail load of 286,000 pounds, there is approximately 246,000 pounds of weight that bears onto the two truck bolsters. Since there a two bolsters, simple math tells us that each bolster must support roughly 123,000 pounds. If the body centerplate is nominally 16 inches in diameter, the stress per unit area is approximately 600 psi. Further, if the body centerplate must rotate within the bowl, the turning moment of two surfaces in contact is defined mathematically as:
Centerbowl Turning Moment = R (2/3) µ (N)
Where:
R = radius of the centerbowl
µ = coefficient of friction between the centerplate and centerbowl
N = Normal force on the centerbowl
If we assume that µ is 0.40 for a dry bowl condition, and the normal force in the bowl is roughly 123,000 pounds, then for an 8-inch radius centerbowl, the turning moment would be approximately 260,000 in.-lbs. The steering moment generated by the wheelset radius differential must overcome this substantial turning moment in order to turn the bolster on the body centerplate, and steer the truck through the curve.
If we consider the condition of a lubricated centerbowl, with a µ of 0.10, then the turning moment drops to roughly 65,000 in.-lbs., and the required steering moment drops accordingly. AAR interchange rule 47 requires that any car on a repair track that is raised off the trucks must have the centerbowl lubricated with either an AAR-approved lubricant or an approved centerbowl liner. It is easy to see why centerbowl lubrication greatly improves the steering ability of the truck by a reduction in rotational resistance. Rail Sciences' field testing has shown that a dry centerbowl can increase turning moment by approximately 50% above a nominally lubricated centerbowl.
Effects of Wear
This analysis assumes that there is an even distribution of vertical load across the surface of the centerbowl. This may not always be the case. If there is substantial wear on the surface of the centerbowl, or corresponding wear on the surface of the centerplate, then the centerplate will naturally sit deeper in the centerbowl. In cases of extreme wear, this will cause contact to occur around the rim of the centerbowl and against the base of the body centerplate. This condition is illustrated in Figure 1. When this occurs, the turning moment radius is moved to the extreme radius of the centerbowl rim, and the equation for total turning moment becomes: Centerbowl Turning Moment = R µ (N)
Thus, assuming a µ of 0.40, and a normal force of 123,000 pounds, the total turning moment becomes approximately 390,000 in.-lbs. This is a substantial increase in turning moment of 50%, which can cause the wheelset steering moment to be insufficient to steer the truck bolster through the curve. AAR interchange rule 47 requires that a minimum clearance of 1/16 inches must be maintained between the centerbowl rim and the body centerplate base at all times.
