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:
- On the sleeve, under the stuffing box packing, if you’re still using packing in your pumps.
- On the sleeve under the Teflon® wedge, “U’ cup, or “V” rings if you are using original equipment type mechanical seals.
- Underneath the spring loaded o-ring found on many popular single and double mechanical seals.
- Underneath the rubber bellows of the type #1 seal, if the rubber bellows did not vulcanize to the shaft.
This shaft or sleeve damage is called fretting and it will cause you several problems:
- Sleeve replacement is costly.
- The pump bearings are often destroyed in the process of removing the damaged shaft sleeve.
- The shaft diameter was reduced to accommodate the wear sleeve. This reduction weakened the shaft, raised the L3/D4 number, and increased shaft deflection problems.
- The seal can “hang up” in the fretted groove, opening the lapped seal faces.
- The fretted grove becomes an additional leak path for the fluid. This is a major cause of seal failure
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:
- Aluminum metal protects its self by forming a layer of aluminum oxide on the surface when it is exposed to oxygen. It’s very visible and looks almost white in color.
- A more common name for aluminum oxide is ceramic; a dense, hard, corrosion resistant material.
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:
- The removal of the passivated layer by the rubbing action of the rubber or Teflon®.
- The hard ceramic that we removed sticking into the rubber or Teflon causing a grinding action.
Now that we know the causes of fretting, how do we prevent it?
- Replace bearing, lip or grease seals with labyrinth seals, or the newer positive face seals.
- Stop putting cut packing into pumps. You don’t need that kind of leakage any more.
- Do not use mechanical seals that are designed with a dynamic elastomer positioned on the pump shaft or sleeve. Most original equipment seals are designed this way. Stationary cartridge seals, most balanced o-ring seals and all properly installed bellows seals eliminate the shaft dynamic elastomer and the fretting associated with it.
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