At the November 6 meeting of the AAR Mechanical Division General Committee in Pueblo, Colorado, the Roller Bearing Manufacturers' Engineers' Committee (RBMEC) presented a briefing on several recommendations it has made to the AAR to improve bearing retention. Bearing retention is the ability of the bearing components to remain properly clamped together with a tight fit on the axle journal. BRENCO's joint inspection statistics indicate that loose components account for more than a third of all set outs and up to one half of the bearing failures in service.
As part of the RBMEC's presentation, BRENCO showed a video on research it had conducted into the interrelationship between axial and radial clamp in the bearing. Specifically, BRENCO engineers investigated three aspects of bearing retention:
- The effect of the cap screw seal rings on clamp
- The effect of lubricating the cap screw on cap screw tension, and
- The effect of various clamping forces on the ability of cones to resist rotation on the axle journal.
CAP SCREW SEAL RING
The cap screw seal ring (o-ring), which has been a part of the bearing assembly since the inception of the NFL bearing in 1977, was investigated for its effect on the clamping of the bearing by the end cap. Although the cap screw seal rings were originally incorporated in the bearing design to improve the bearing's resistance to water intrusion, the benefit of the seal rings in this respect is not considered to be significant. The effect on achieving proper clamp in the bearing however, can be dramatic. As the cap screws are tightened during bearing installation the end cap deflects and acts as a lock washer. BRENCO's tests revealed that initial torque to the cap screws is consumed in compressing the cap screw seal rings into the space between the inside of the end cap and the end of the axle. The gap between the end of the axle and end cap varies according to axle tolerances and bearing tolerances. As the gap becomes smaller, more compression of the cap screw seal ring must occur. Increasing compression of the cap screw seal ring consumes more of the torque applied to the cap screw reducing the amount of torque available to deflect the bearing end cap and produce a secure clamp of the bearing on the axle. BRENCO's tests indicated that installations requiring heavy cap screw seal ring compression exhibit as much as 50% reduction in end cap deflection when compared to the same amount of applied torque when cap screw seal rings are not used. This loss of clamp force on the bearing can lead to loose components and overheated bearings in service.
Based on these findings, the RBMEC has recommended to the AAR that cap screw seal rings be eliminated from the bearing assembly.
LUBRICATING THE CAP SCREWS
The second area investigated was the effect that lubricating the cap screw threads has on the amount of tension developed in the cap screw for a given amount of applied torque. Compared to dry threads on cap screws, lubricating the threads produces a substantial increase in the amount of tension developed in the cap screw. This has been known for sometime, but the difference has never been measured. BRENCO measured the difference by developing a series of cap screw tension values for a given amount of torque, first with non-lubricated cap screws and then with lubricated cap screws. From these experiments, conducted both under laboratory conditions and in actual railroad shop applications, BRENCO found that lubricating the cap screws achieved a more uniform distribution of cap screw tension and also increased cap screw tensions by as much as 2 to 1, thus improving the clamp of the end cap on the bearing assembly.
These findings prompted to RBMEC to recommend to the AAR that cap screws be lubricated at the time of bearing installation.
CLAMPING FORCES
The final area investigated as part of this study was the relationship between clamping forces on the bearing and resistances to cone rotation on axle journal. A simple cone rotation test was conducted using three different clamp conditions:
- Un-clamped
- Clamped with cap screw seal rings in place
- Clamped without the use of cap screw seal rings
BRENCO engineers demonstrated that the un-clamped bearing cone required the least amount of force to cause cone rotation. Increasing amounts of force were required to rotate the cone as the cap screws were properly torqued, first with the cap screw seal rings in place, and finally without the use of the cap screw seal rings. This test further supported the recommendation made to AAR to eliminate the cap screw seal rings from the bearing assembly.
Full details of these test procedures and the results obtained from these tests are available on a 15 minute video tape which can be obtained upon request by writing BRENCO, Bearing Retention Tape, P.O. Box 389, Petersburg, Virginia 23804 or calling 804/863-1713.
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
Additional copies provided upon request.
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