Interface - The Journal of Wheel/Rail Interaction

CONSTANT CONTACT SIDE BEARINGS

Making the Case for Long Travel Constant Contact Side Bearings
by Bill O'Donnell • July 4, 2005

As part of its efforts to reduce the stress state on North American railroads, the Association of American Railroads (AAR) mandated that all new cars ordered after January 1, 2002, must be equipped with constant contact side bearings. Constant contact side bearings (CCSBs) raise the threshold speed at which lateral instability, or hunting, takes place. As seen in Figure 1, CCSBs can raise this threshold from 40 mph, on cars equipped with conventional gap style side bearings, to more than 70 mph. Lateral acceleration, which is a measure of hunting severity, increases rapidly once hunting starts and speed increases.

In any mechanical system there typically are trade offs in different areas of performance. Three-piece trucks are no different in that an improvement in hunting usually comes at the expense of curving performance. The dramatic improvement in hunting control also has the negative effect of increasing turning resistance in curves. As seen in Figure 2, the CCSB pushes against the underside of the car body with a given force. If the truck begins to rotate, the frictional force at the CCSB/car body wear plate interface will begin to resist this movement. If truck rotation continues, the CCSB will continue to resist rotation and dissipate energy in the form of heat generated by the friction. While this is beneficial from a hunting standpoint, truck turning resistance has a negative effect on negotiating curves.

In an effort to understand the effect of the CCSB mandate and the effect of side bearing design on railcar performance, the AAR conducted a series of tests at Transportation Technology Center, Inc., (TTCI) involving different car types and side bearing designs. Side bearing manufacturers were asked for side bearing design recommendations for different car types. The cars chosen were intended to represent the fleet from the standpoint of truck centers, stiffness, center of gravity and propensity to hunt. The performance of metal and non-metal capped, long and standard travel CCSBs was compared to the performance of gap style roller side bearings (see Figure 3).

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