Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

Using a solid graphite block for lubricating kiln trunnion rollers is no big deal. However, attention to how the block is placed means the difference between long "tire" riding ring life and premature spalling of the tire. This is a good case where sweating the details was overlooked.

The blocks were too short. The lubrication mechanic couldn't get the correct size because there weren't any in the warehouse. The ones in the warehouse were either too big or too small, so this block was the "closest" thing that came to fitting correctly. Was it close enough? Not a chance!

This little oversight by the lubrication mechanic allowed the trunnion roller and the tire to contact each other in this region with absolutely no lubrication. It doesn't sound like a big deal. If you treat solid lubrication as though it were a liquid and assumed that the graphite would spread out over the entire roller area you might be correct. However in this case graphite doesn't flow well. In this application the asperities of the trunnion roller grind away at the graphite block and the graphite fills the voids between the asperities. When I saw this block placement from a distance I suspected something. Of course while the unit was running, as depicted in the first two pictures above, it was hard to see any damage. When the unit was shut down for routine inspection an additional inspection was made on the tire surface. The picture to the lower left shows most of the cross section of the tire. Notice in the circled region how there is a fish scale appearance. This is not an optical illusion because there are actually thousands of tiny cracks joining together. The picture to the lower right is a closer view. The fish scale is unmistakable.

An even closer view in the picture below put a sickening feeling in the pit of my stomach. This tire had been conditioned about three years ago. It could have easily gone a minimum of another seven to ten years before its surface was ground again. Instead it is easy to see how the thousands of tiny cracks joined together to allow pieces of the tire surface to fall away. The cracks are shallow because the stresses are very high at the surface. The pits that are created when the cracks join together and a piece falls out make it difficult to provide adequate lubrication and proper distribution of the load bearing forces. Wherever there is a pit the area around that pit is stressed that much more because now it must carry the additional load where the pit is located. Therefore that overloaded region will fatigue even sooner than the first pit. Eventually the pitting accelerates as the neighboring deteriorating regions are overstressed at greater levels. Notice how there is fish scaling next to the pits in the picture below. These cracks will join together quicker as the pits begin to grow and the metal pops out.

For the people that can't believe their eyes the final test was to use liquid dye penetrant to verify that the fish scale appearance was in fact thousands of tiny cracks. The liquid penetrant was sprayed onto the surface of the tire in the picture to the lower left. It was allowed to penetrate and then was wiped off. The white developer was then sprayed on and immediately the dye in the cracks bled out into the white developer. The results can be seen in the picture to the lower right.

When it comes to using solid graphite lubrication blocks size does matter. Don't be cheap and get the "one size fits all" block. Make sure that the block covers every inch of the trunnion roller and make sure your lubrication mechanic is aware of this little detail that must not be overlooked. Otherwise tire grinding will come quicker than you'll want.

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