SUBJECT: Shaft fretting 10-3
The next time you remove a grease or lip seal (the rubber seal located next to the bearing) you'll note that the shaft is grooved and damaged under the rubber lip. You'll see this same damage in a few other locations also:
This shaft or sleeve damage is called fretting and it will cause you several problems:
What causes this fretting problem? How can a soft piece of rubber or a slick wedge of Teflon cut a hard shaft? It doesn't seem to make any sense.
Surprisingly, it has nothing to do with dirt in the air or abrasives in the fluid. The problem will occur even if you are pumping a filtered, clean lubricant in a sterile atmosphere.
To understand fretting you must first understand the term "corrosion resistant". Some materials resistant corrosion others do not. What's the difference? We say that iron rusts, but aluminum oxidizes. A look at any dictionary will verify that these terms mean the same thing. So why do we use different terms to describe the same problem?
The answer lies in the way a metal rusts or oxidizes. If the oxide layer is protective we say that the material is corrosion resistant. Take aluminum as an example:
After this dense, ceramic layer is formed on the surface of the aluminum, the oxidation or rusting rate is slowed down to less than 0.002 inches (0,05 mm) per year, and this is the definition of corrosion resistant.
If this protective oxide layer is rubbed or polished off by the packing, lip seal or Teflon wedge the oxide will immediately reform to protect the base material. It is this constant oxide removal and reforming that causing the shaft grooving that is so visible. We get the same reaction when we polish silver. The "tarnish" replaces its self to protect the silver.
Shaft vibration and end play causes a constant axial movement of the shaft through the mechanical seal dynamic rubber or Teflon® part. Bearing grease seals and stuffing box packing are stationary, so the rotating shaft is constantly being polished by these materials when the pump is running
There is a second problem associated with fretting. The ceramic oxide that is removed imbeds its self into the rubber part causing a wear or grinding action on the base metal.
Stainless steel protects its self by forming a protective oxide called chrome oxide, one of the hardest ceramics. When this oxide forms we say that the active stainless steel is now "passivated". It is this chrome oxide imbedded into the packing, Teflon®, or rubber lip that does so much damage to the shaft sleeve.
So now we have two causes of fretting:
Now that we know the causes of fretting, how do we prevent it?
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