Reliability Engineering Snapshot TM

The 10.5" ID double row spherical roller bearing shown to the left, came out of a failed trunnion assembly on a rotary kiln. It was one of two bearings that straddle either side of a 30" diameter roller that the 8 ft. diameter kiln rotates upon. There are four of these trunnions. Rotary kilns need to be periodically aligned for thrust control reasons. In doing so, the trunnion assemblies are purposely skewed in order to impart a thrust force on the kiln. Thrust is necessary in order to keep the kiln from literally moving "downhill" and falling off the trunnions.

In this particular failure, the trunnion had been skewed in one direction far too much over a given period of years. This was due to normal wear on the roller assembly. Nobody kept track of the amount of skew, and eventually the misalignment limit was reached on this bearing and it failed.

In the picture to the upper left, the ballpaths on the outer race can be seen for both sets of spherical rollers. The left set of rollers ran off the edge of the outer race and created tremendous edge loads. These loads were great enough to cause yielding of the material, that eventually spalled off, as can be seen in the picture to the upper right (yellow circled region, arrow denotes crack where the edge will finally spall off). Over time, huge chunks broke away. The reason the left raceway is spalled and the right raceway is not, is because the normal thrust loaded the left set. These raceways should be evenly worn and are obviously not. Unfortunately, this trunnion was used to carry an abnormally high amount of thrust, and is a good example for promoting periodic alignment checks by qualified kiln servicemen.

The change in the alignment can be seen in the upper left picture. The yellow bars approximate the distance between the edge of the raceway and the edge of the roller at the initial installation of the trunnion assembly. There is a very slight angularity set on it due to setting a slight thrust on the trunnion assembly itself. As the roller wore down due to natural causes, the maintenance crews adjusted the roller assembly accordingly by skewing the roller. This action skewed the bearings; the blue bars denote the final running position of the rollers' edge in the outer race prior to failure.

The sister bearing that did not fail is shown below. The edges of the roller paths with respect to the edge of the outer race, are highlighted in yellow. The edges of the roller paths are denoted by the arrows. The reason this bearing did not fail is because it did not carry a thrust load, the other bearing carried the thrust load.

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