Interface - The Journal of Wheel/Rail Interaction

SIDE BEARINGS

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

Designers went back to the drawing board and developed long-travel, metal-capped, and roller-assist CCSB designs. These new concepts largely mitigated the problem and are now commonplace on Doublestack and intermodal equipment (see Figure 3).

How CCSBs Work
The theory behind the CCSB is fairly simple. To prevent a truck/bogie bolster from hunting, a small amount of restraint is necessary between the bolster and car body. Placing a compressed “spring” (either metal or elastomeric) between the bolster and the car body creates such a restraining force. By limiting the upward force against the wear plate to 3,000 - 4,000 pounds, the necessary restraint against hunting is developed. However, when the bogie bolster needs to rotate through a large angle while curving, the large steering forces of the wheelset will “break” the frictional force at the top of the side bearing, and allow it to freely rotate. To establish the correct amount of frictional force, the CCSB element is designed around the clearance envelope of a conventional roller side bearing design. With a conventional roller side bearing, the nominal clearance from the top surface of the bolster to the side bearing wear plate is 5-1/16 inches in order to provide for 1/4-inch clearance above the top of the roller. The CCSB manufacturers decided to work with this dimension and design the metal spring or elastomeric spring around it. Thus, when the setup height between the top surface of the truck bolster and the wear plate is 5-1/16 inches, the correct amount of pre-load on the wear plate will be developed when the elastomeric block is placed in the side bearing cage.

When Wear Takes Place
As you might expect, wear takes place over time and closes up the setup height from 5-1/16 inches to less than 5 inches. The primary source of wear is the combined wear on the surface of the body centerplate and the wear in the centerbowl. The AAR allows a nominal 5/16 inches vertical wear in the centerbowl (Rule 47). Also, a minimum of 1/16-inch clearance must be maintained between the bowl rim and the centerplate base. Over time, the body centerplate gradually sits deeper and deeper into the bowl, effectively closing up the clearance between the bolster top surface and the side bearing wear plate. The effect of this wear is to cause the elastomeric element, or spring, to shorten, or become more compressed. The more compression on a spring, the more force is developed within the spring. As the CCSB setup height is reduced below 5 inches, more and more frictional force is developed at the top of the elastomeric block and the wear plate. This frictional force resists turning of the truck bolster and in severe conditions can lead to a stiff truck condition. A stiff truck can lead to gauge-spreading forces under loaded cars, and wheel climb on empty cars. Maintenance of the correct setup height is important, or derailment can occur. Figure 4. shows the setup height being measured with a set of calipers. Alternately, go/no-go gages are available for a quick check on condemning limits.

In the early days, and before the critical importance of setup height was understood, many derailments occurred on long intermodal flats and autorack cars due to tight CCSBs. The long (66-foot) truck center length and stiff turning resistance was a two-fold problem. When encountering track twist, the vertical wheel load would be reduced, while at the same time, the lateral wheel force would be high due to turning resistance. The vertical force would go down while the lateral force went up. This combination would cause a high L/V ratio.

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