Interface - The Journal of Wheel/Rail Interaction

SIDE BEARINGS

Vehicle Side Bearings: Function, Performance and Maintenance (Part 1 of 2)
(continued)

Side Bearing Clearance
Normally there is a gap, or clearance of approximately 1/4 inch between the top of the side bearing roller and the car body wear plate. In fact, AAR interchange rules specify that the clearance gap should be no wider than 5/16 inches and no smaller than 3/16 inches. When traveling on level, tangent track there is no useful role for the side bearing. If the car encounters rough track and the car begins rolling side-to-side, contact will occur. Or if the car enters a curve and leans over, side bearing contact will occur. If the side bearing gap, or side bearing clearance, is too wide (>5/16 inches), it will allow excessive rolling or leaning of the car body. If the gap is closed up tighter than 3/16 inches, premature contact will occur during curving. Neither of these conditions is desirable. If side bearing clearance is too tight, two conditions can develop:
• The car will exhibit excessive resistance to curving.
• The car will not be able to accommodate normal track twist conditions without exhibiting an excessive amount of wheel unloading.

Both of these conditions can directly lead to a derailment. Any resistance to curving will cause higher than normal lateral force, with an attendant increase in L/V ratio (assuming the vertical force remains constant). Thus, cars with tight side bearings often cause wheel-climb or rail-rollover derailments. Heavy commodity cars with tight side bearings can generate large lateral forces due to truck warp. This in turn leads to high truckside L/V ratios and a tendency to roll rail or spread gauge. In addition, cars with tight side bearings will cause excessive wheel unloading, especially when negotiating curve spirals or low joints in the track. This can cause cars with long truck centers, or torsionally stiff cars such as empty tank cars, to climb the high rail of curves during curve entry or exit.

As the car encounters the entrance spiral, the bolster will start to rotate against the car body. If adequate clearance is present at the side bearing, there is no resistance to turning due to side bearing friction or slip-stick. As the car continues to enter the body of the curve, contact might occur at some point on the inside or outside side bearing. By this time, the bolster has likely rotated to its full position, and the effect of side bearing friction is minimal.

If side bearing clearance is tight as the car enters the spiral, even a small amount of leaning can take up the slight clearance, leading to friction on the top of the roller and the potential for slip-stick to occur. As the car moves deeper into the spiral, crosslevel differences increase the twist on the car and the resultant normal force on the side bearing. At this point, the potential for excessive turning resistance is increased to the point of causing a stiff truck condition. Thus, tight side bearing clearance is a prime cause of high lateral wheel forces due to truck turning resistance. Rail Sciences' tests and inspections of freight cars developing high lateral forces, or cars with warped truck condition, have shown that tight side bearings were present in roughly 50% of the cars exhibiting curving problems. Figure 3 shows an example of a tight side bearing condition.

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