Wrong bearing direction causes failure

Problem: NTN was approached to examine a failed 7016 angular contact ball bearing from a pump assembly. Rather than achieving the 60,000-hour design life, the bearing was removed from service after only 6,500 hours. The catastrophically failed bearing displayed a circumferentially fractured inner and outer ring. Spalling was also evident adjacent to the location of each fracture site.

Solution: Angular contact ball bearings are manufactured to accept a combination of radial load and axial load. However, a single bearing is designed to accept axial load, or thrust, from one direction only. Thrust from the wrong direction can result in disassembly and catastrophic failure. Consequently, these bearings are commonly utilized in a multiple arrangement that will accept axial loads in either direction (i.e., back to back, face to face).

See Fig. 1: Left: Acceptable direction of thrust. Right: Incorrect direction of thrust.

In this particular example, the location of the spalling and wear indicated the rolling elements were running completely outside of the intended raceways.

The bearing was installed contrary to the direction of thrust and lasted an amazing nine months in a partially disassembled state.

The heavily preloaded rolling element, trapped between the inner and outer ring lands, led to wear, spalling and subsequent inner ring fracture.

Mr. O’s thanks go to NTN Bearing Corp. of Canada for this tip.

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