Reliability Engineering Snapshot TM

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



Corrosion - Case Study No. 18: Steam From Defective Shaft Packing Causes Bearing Failure


Packing can be the most neglected, discounted, underrated,mechanical component.Yet it can do the most damage.

Corroded Ball Bearing

The ball bearing shown to the left came out of the bottom of an agitator gearbox. This bearing was sealed. Between it and the top of the tank, was a packing box. The service of the agitator gearbox generated a lot of steam which had two ways of egress. One was through the vent, the other was through the packing box. While watching this unit in operation, it was always engulfed by a cloud of steam. This steam continuously impinged upon the shield of the lower bearing.

When the agitator gearbox finally failed and was removed for repair, it was noticed that the lower bearing on the low speed shaft had failed. This is the shaft that is directly connected to the agitator shaft itself. When viewing the inner raceway of this bearing and noticing the buildup of crud, it's hard to believe that any ball bearing traveled over that crud. They didn't. The ball cage which holds and separates the balls failed long ago. This allowed the balls to just pretty much sit there and do nothing.

We got our "money's worth" out of this gearbox, it did last for many years. However, instead of just replacing the lower bearing shown here, the low speed gear and the intermediate gear were replaced at a premium price; because, since the bearing could no longer do its job of stabilizing the agitator shaft, the low speed gear did the job instead. Gear mesh was no longer the operative word. It was more like "gear crunch."

This inner raceway hasn't seen a ball run on it for a very long time. Corroded Inner Raceway of Ball Bearing

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