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Switch Point Derailments: Is it the point or the wheel? (continued)
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A common cause of switch point gapping is pumping in the heel block area. If the heel block is moving vertically due to loose bolts, poor tie condition, or mud in the ballast, it can dynamically allow the point to rise up in the air in a seesaw fashion. As the wheels of a preceding car negotiate over the heel block, the wheels of a trailing car may be approaching the point. Depending on the length of the couplers, the car lengths, and the coupler overhang, the timing may be such that the trailing wheels hit the gapped point. (See figure 5. for an illustration of how this action might occur.)
Figure 6 shows a switch point gapped up in the air due to heel block pumping. The only way to spot this condition is to watch the point dynamically as cars pass over it, or carefully inspect for weak conditions in the heel block area. The root cause may not be the point, but rather a condition 16 feet, 6 inches from the point at the heel block.
Thin flange wheels are a common cause of picked switch points. A condemnable thin flange wheel may also exhibit heavy metal flow on the tip of the flange that presents a sharp knife edge to a slightly gapped switch point. Figure 7 shows a condemnable thin flange wheel as verified with an AAR wheel gauge. Wheels that are worn to thin flange also are likely tread hollow, which means they may not be steering correctly, and may be crowding the gauge corner of the stock rail upon approach to the point area. A common situation is that a severely worn wheel is gauged, but in the words of the mechanical department “it won’t take the gauge.” Thus, the wheel is AAR compliant, but still shows signs of heavy flange wear and picked the point.
Alternately, you might notice that the opposite wheel on the same axle does not appear worn. This condition is called asymmetrical or uneven wheel wear. A good investigator will look beyond this condition for truck performance issues that might be causing the asymmetrical wear. Upon a detailed inspection, he might find tight side bearings, fouling between the centerbowl rim and the body centerplate, mismatched side frames, fastener contact around the bowl rim, worn or uneven centerplate conditions, or other reasons causing a stiff truck in either a right or left hand curve. The bottom line is that the root cause of the non-steering truck is not wheel flange wear, but rather an AAR condemnable condition elsewhere on the truck. That's why it is important to inspect the entire car as a system, not just focus on wheel wear.
Another common cause of gapped switches is heavy metal flow on the lip of the stock rail gauge corner. This flow can either gap the point, or hold it off the gauge side of the stock rail, or alternately, the flow will cause the point to become chipped and broken. Grinding of switch stock rails is as important as grinding the mainline. Metal flow must be periodically removed in order to maximize the life of the point and prevent gapped points. Figure 8 illustrates how metal flow interferes with proper fitting of the points.
In Part 2 of this article, we'll examine wheel climb derailments at switch points, and look at frog conditions that can lead to derailments.
Gary P. Wolf is President of Rail
Sciences Inc.
