Figure 1: Journal Bearing System

These adapter roof geometry features provide for the best possible internal roller load distribution with such a small adapter. The rolling contact fatigue service life of the bearing is exponentially dependent on maximum roller loads (see Technical Forum 90-3). We therefore want to share the total load among as many rollers as possible, thus reducing the peak roller load.

STRUCTURAL LOADING

To understand the relationship between adapter crown wear and bearing fatigue life, one must first look at how crown wear affects the distribution of the applied load across the bearing and the resulting peak roller loads. Figure 4 shows several adapter configurations and their resulting peak roller loads. Case A represents the ideal configuration of a rigid "wrap-around" adapter. This tight fitting "full bore housing" surrounds the entire bearing thus making the adapter/cup combination very rigid. This provides the best load distribution possible.

Figure 3: Longitudinal Section showing Tapered Roller Bearing Components and Adapter Crown Radius.

Figure 4

The remaining configurations show the narrow adapter. Dimensions and tolerances allow the adapter bore diameter to be slightly larger than the cup outside diameter. The adapter must therefore flex slightly to prevent point loading the cup. Case B represents a new narrow pedestal adapter. Note how the load is distributed uniformly on either side of the relief groove. This helps to spread out the load over most of the upper rollers.

Figure 5

Case C and Case D represent two examples of an adapter which has worn beyond the depth of the relief groove. Case C represents the assumption that the load is distributed only in the releif groove area. Case D makes a conservative assumption that the load is distributed uniformly across the entire adapter crown. The actual worn situations found in service fall somewhere between these two extremes. Note how in both of these cases, more of the load is located directly over the top center roller. The result is less adapter flex, less load distribution, and higher loading of the top, center roller.

RESULTING ROLLER LOAD DISTRIBUTIONS

Computer modeling was used to simulate the interaction of the adapter with the cup and the individual rollers. Load cases similar to those shown in Figure 4, were then studied. The model was able to estimate the distribution of the applied load and predict the bearing internal load on each roller. The model's accuracy was confirmed with an actual test of a benchmark case using specially instrumented bearings. The results of the simulations are shown in Figure 5 (click to see). Note that the distance of the symbol from the cup's outside diameter represents each roller's relative load.

The ideal adapter, Case A, shows the best distribution where the load is shared by the maximum number of rollers and the peak roller load is a minimum. The load with the new adapter, Case B, is not distributed as well and thus has higher loads on the top center roller. This is however, a good balance between design efficiencies and the economics of the small adapter size as described earlier. The load distribution degrades significantly with worn adapters as shown in both Case C and Case D. Notice that the peak roller loads are much higher.

Figure 6	Figure 6

Figure 6 shows a direct graphical comparison of the peak roller loads for the four adapter configurations relative to the new adapter. Here we see that the peak roller load for the ideal adapter, Case A, is about 19% lower than for the "new" adapter, Case B. The worn adapters, Case C and Case D, have 21% and 46% higher loads (respectively) than the new adapter case. Once again, the actual worn conditions found in service fall somewhere between the two extremes. The relative increases in peak roller loads of the worn adapter cases over the new adapter case can be used to predict an expected service life of 53% or less based on the conventional load/life relationship discussed in Technical Forum 90-1 and illustrated in Figure 7.

OTHER CROWN WEAR CONSEQUENCES

Wear also affects the division of load between the bearing races. As suggested in the longitudinal section shown in Figure 3, the presence of the adapter crown radius helps to keep the load centered on the adapter so that half of the load is supported by each race or cone. If the crown is worn down and the side frame tilts to either side of the vertical centerline, the cones will be subject to high overloads because of a greater side frame load offset. See Figure 8 (click to see). These cyclic overloads contribute to reduced bearing fatigue life.

Figure 8

OTHER CONSIDERATIONS

From the preceding discussion, it can now be understood that even minor adapter design changes could adversely affect roller and race load distributions. For instance, the introduction of an elastomeric pad centered on the top of a flat (not crowned) adapter can potentially degrade the roller load distribution. There may be significant benefits in such a design, but the full consequences of any proposed change to the adapter/bearing system must be assessed and the impact on bearing performance included in any analysis.

In addition, wear limits are specified in the Field Manual of the A.A.R. Interchange Rules for other areas of the adapter (i.e., thrust lugs and bore) that can also affect bearing performance. Inspection of these areas can be accomplished when the adapter is removed from the side frame. The condition (wear) of the matching side frame pedestal roof opening is also a factor which can influence load distribution.

SUMMARY

Wear of the adapter crown can adversely affect bearing performance. With a new adapter, the load is distributed equally on either side of the axle centerline on raised pads. This produces the best possible distribution of load among the rollers that this somewhat "truncated" adapter design is capable of delivering. If these raised pads are worn down to the bottom of the center relief groove, thus allowing side frame contact, then load distribution will suffer, peak roller loads will increase, and service life will be reduced. In addition, such extreme wear will completely remove or flatten the longitudinal adapter crown radius allowing side frame rocking to cause higher cyclic overloads of the bearing races.

This Technical Forum has emphasized that the adapter is an important, integral, although frequently overlooked, part of the bearing system, and must be carefully inspected for signs of excessive wear. Replacement of worn adapters is essential if optimum bearing service life is to be realized. Evidence of excessive crown wear may be gathered from field inspections as described in previous Technical Forum 89-2. When evidence of such excessive wear appears, arrangements should be made to remove, inspect and replace the adapters according to the AAR Field Manual Rule 37 at the earliest practical time.

The Technical Forum is an information resource for the rail industry and is provided as a courtesy of Amsted Rail Group. Suggestions, inquiries or comments are welcomed and should be directed to:

Editor, Technical Forum
BRENCO, Incorporated
P.O. Box 389
Petersburg, Virginia 23804
804-863-1713

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