Figure 1	Figure 2

Benefits of Proper Bearing Lateral

Improved rolling contact fatigue life is one of the benefits of proper mounted lateral.

Figure 1 and Figure 2 show radial roller load distributions for two mounted lateral; zero (free turning) and 0.030" lateral. The theoretical contact fatigue life of the bearing can be determined from such roller load distributions.

As explained in BRENCO Technical Forum 90-1, "Bearing Life", fatigue life is highly dependent on load. Excessive mounted lateral can cause high peak roller loads, as a result of fewer rollers sharing the load, which can significantly reduce expected fatigue life. This is illustrated graphically in Figure 3 wherein the bearing fatigue life for a range of mounted laterals is expressed as a ratio of the rated L10 life at zero mounted lateral. Note that the theoretical life for an in service mounted lateral of 0.020" is approximately 43% of the life predicted for zero lateral.

In fact, it may be seen that some small amount of preload gives maximum fatigue life. However, because of the rapid drop in predicted life as preload increases, spacers are selected during bearing assembly to give a mounted lateral within the allowable range of zero (free turning) to 0.020".

Figure 3

Fundamentals

Several factors, some within the control of the shop assembling the bearing and some within the control of the shop installing the bearing, determine mounted lateral. These factors include:

* spacer size selection * journal diameter
* cone bore diameter * mounting practice

When new or reconditioned railroad bearings are assembled, the cone spacer width is selected to produce a predetermined amount of clearance (C1) between the roller sets and the cup rolltracks (Figure 4). This clearance permits a small lateral movement (LB) of the cup over the roller sets. This movement is called bench end play or bench lateral.

When the bearing is pressed on the axle journal producing an interference fit, there is an increase in the cone race diameter. For the typical railroad bearing cone sizes, approximately 80% of the fit is transferred into cone diameter expansion. The increase in cone race diameter reduces the clearance (C2) between the roller sets and the cup rolltracks, in effect reducing the mounted lateral movement (LM) of the cup over the roller sets (Figure 5).

Taking into consideration the nominal 20° included angle of the railroad bearing cup rolltracks, the 80% press fit to cone diameter change, and two cone assemblies per bearing, each 0.001" of interference fit will reduce bench lateral by approximately 0.0045".

Mounted Lateral Limits vs. Bench Lateral Limits

The manufacturers of freight car roller bearings have established the acceptable mounted lateral range as zero (free turning) thru 0.020". This lateral range and the acceptable cone bore to axle journal interference fit range (0.0045" maximum with new cones and 0.0015" minimum with reconditioned cones) dictates the bench lateral range.

The 0.0045" interference fit will reduce bench lateral by 0.020"; therefore, minimum bench lateral was established at 0.020" to avoid a harmful preload on the bearing. The 0.0015" interference fit will reduce bench lateral by 0.0067"; therefore, maximum bench lateral was established as 0.026". The relationship of bench lateral and interference fit to mounted lateral is shown graphically in Figure 6. Upper limits on bench lateral are established with a view towards minimizing the loss of life potential reflected in Figure 3, but with practical regard given to the fact that reconditioned cone bores and used axle journal diameters produce a trend toward minimum interference fit.

It should be noted at this point that lateral ranges for journal roller bearings are given for both hand operated lateral devices (0.020" - 0.026") and power driven lateral devices (0.023" - 0.029"). The higher range for power lateral devices recognizes the ability of these devices to thoroughly seat the roller of the cone assembly against the cone thrust rib and measure the full movement of the cup with respect to the rollers. The bench lateral range used in this Forum refers to hand lateral.

Figure 6

How to Achieve Proper Lateral

With this understanding of the benefits of proper mounted lateral and the theoretical bench lateral limits, how can these limits be achieved in practice?

Both hand operated and power driven lateral equipment require lightly lubricated bearing components and a moderate thrust load to "seat" all of the rollers in the roller set against the cone thrust rib. Fully seating the rollers permits approaching the true geometrical (or minimum) outside diameter of the cone assembly. It is more difficult to achieve the fully seated roller condition with the hand lateral than with the power lateral. In addition, the application of a generally higher thrust load with power lateral will usually cause some small elastic enlargement of the bearing cup. In recognition of these differences, the limits for the power driven equipment are set 0.003" higher than those for hand operated devices.

As a practical guide to achieving consistent and accurate bench lateral measurements, care should be taken to insure that cup and cone assemblies are clean and lightly oiled rather than dry when measurements are made. Furthermore, regular quality control checks should be made to verify that lateral equipment and dial gages are in proper working order and calibrated. It is also good practice to maintain a "master" bearing of known lateral to serve as a reference check for both the operation and the equipment.

Cone Bore Fit

It should be clear from the preceding discussion that mounted lateral is directly related to cone bore fit, which is determined by both the cone bore and the journal diameter. These dimensions must be within the specified tolerance ranges if proper mounted lateral is to be achieved. As previously pointed out in Technical Forum 88-3, "Proper Mounting of Bearing Enhances Retention", undersize journals have been identified as a substantial problem area in service. Likewise, oversize journals have the potential for causing an undesirable preload in some bearings. Therefore, careful checking of journal size using a snap gage is a prerequisite to achieving both proper mounted lateral as well as good bearing retention.

Bearing Installation Practice

In order to achieve full bearing performance and service life, checking the bearing's mounted lateral upon installation is an absolute must. The correct procedure for checking mounted lateral is shown in BRENCO's video tape titled "Proper Bearing Mounting Procedures" which is available upon request.

Maintaining Proper Lateral in Service

Any evidence of loss of bearing retention in service, such as loose backing rings or loose or missing cap screws, is an indication that excessive lateral has probably developed as well. Furthermore, as required by the AAR Field Manual, whenever wheelsets are removed from cars, the bearing laterals must be checked and bearings removed from service if the laterals are greater than 1/16" (0.061").

In summary, correct lateral is essential to bearing fatigue life in service. In order to maximize this fatigue life, the mounted lateral should be as close as possible to zero without any preload. Such mounted laterals are dependent upon close control of bench lateral as well as careful attention to correct size cone bores, axle journals, and correct bearing installation practice.

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

BACK TO TECH SHEETS