What’s Up Doug? Problem with fan’s oscillating axial load
I was asked to come on site to a petrochemical facility to investigate problems with a fan. It was a pretty standard design fan. They had a recent rash of failures and the problem was brought to the attention of the global maintenance group.
The last failure had been only months ago and the problem was diagnosed as improper installation. I had worked with the maintenance group to ensure that the installation instructions were accurate and easily useable by the millwrights.
Inner Ring Fracturing
Working with the metallurgist on site, we determined that the last failure was a result of the inner ring fracturing and that fracture was the result of the bearing cage “bashing” into the inner ring of the bearing.
As it turned out, the past failures had caused the rotating assembly to become deformed and, instead of the impeller being a flat “plate,” it had become twisted with 1/4 inch of “wobble” and when operating at 1,760 rpm there was an axial shaking that occurred.
Since spherical roller bearings are not axially stiff, this axial wobble allowed the shaft assembly to shake back and forth. In doing so it caused the cage to contact the inner ring and over time (1,760 times a minute) wore a groove that weakened the ring.
Although there was a lot of discussion about “can we have a different cage design?” the real problem is the deformation of the rotating assembly causing a “wobble.”
Oscillating Axial Load
At any time that a mechanical system has an oscillating load you are setting up the system for a fatigue failure. So in this case, if the user attempts to increase mean time between failure by different bearing cage design, they then do not fix the real problem, the oscillating axial load. If they do not fix the oscillating axial load, but perhaps find a more durable bearing cage design, they pass the problem on to the next mechanical component. What will be the next component to fail from cyclic (Hertzian) failure? The bearing housing? The fixing bolts? Will damage be caused to the coupling? The drive motor?
Solve the Problem
The better solution would be to eliminate the oscillating axial load and then increase the life of all the associated components.
As a footnote, the user reduced the speed of the fan from within 8 per cent of the critical speed to lesser/greater than 20 per cent of the critical speed. By running farther from the rotating assembly’s critical speed the effects of imbalance or “wobble” were reduced.
Douglas Martin is a heavy industry engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at firstname.lastname@example.org.
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