Buna-N Swelling in Oil 1

[rconnor]

We've got issues with our oil baffles in a bearing for a hydroelectric units shaft. The baffle is made of Buna-N and after about a year the baffle swells, becomes wavy and, in some cases, shears off at its mounting location. Also, the rubber that has been in-service can be torn by hand. The image attached shows a baffle that was removed from service after a 8000 hr (~1 year) inspection.

The baffle is attached at one end (you can see the drill holes in the image) and protrudes into the bearing to deflect the oil back into the oil pot. The oil is Teresso 32 turbine oil and the temperature would be around 50-75[C]. The forces on the baffle shouldn't be of any great impact.

Before I saw the baffle, I thought it might be ozone cracking because it's buna-n and is located near the generator. However, it doesn't appear to be the case after seeing it. They also thought that they just over-tightened the clamp but on the next unit they loosened it and the same thing happened.

I have very limited experience with rubbers and I'm not sure what is happening and why. According to some research I've done, Buna-N should be resistant to oil swelling but it is definitely obsorbing oil (the thickness increased significantly).

Any thoughts on what might be causing this and would a material change (to viton or something else) be needed?

Thanks in advance for any advice!

 

[DRWeig]

Can you tell us exactly what type of oil it's in contact with?

Good on ya,

Goober Dave

 

[rconnor]

The oil is Teresso 32 turbine oil. As for the actual chemical composition I'm not quite sure. A search indicates that turbine oils contain "diester, polyolester, and polyalphaolefin".

I have also attached the product data sheet, although it is more or less a sales brochure.

If you need any further clarification, let me know and I'll look into finding you the answer.

 

[DRWeig]

Thanks --

Unfortunately, I can't find anything that can be definitely attributed to the oil... Perhaps someone else here will have an idea, or you might consider re-posting in the mechanical engineering or chemical engineering forums. There are some really good materials-compatibility people in there.

Alternative: Contact Esso. They should be able to tell you compatible materials.

Sorry I wasn't more help!

Good on ya,

Goober Dave

 

[rconnor]

Thanks for the effort DRWeig,

I'm still confused as to why it's swelling so much. According to everything I've read, Buna-N is resistant to swelling in this type of oil.

Would other factors, such as stress or ozone, cause the rubber to be more susceptable to swelling? Or does the swelling simply mean the wrong type of rubber is being used for this type of oil?

 

[GrumpyPlasticBastard]

There is either a contamination in the oil causing the attack... an undocumented additive... or substandard rubber.


Send the oil to a lab check for contamination, and have another lab verify that you are getting the right kind of seals made of the right kind of rubber.

----
BFL Dreamworks
"If we don't have an answer we'll get one"

 

[GrahamBennett]

RConnor - can you tell us which grade of NBR (Buna-N) is used in the seals? The reason I ask is the oil resistance increases with increasing acrylonitrile content (so does the price/unit unfortunately). Usually you will find a 34% ACN grade has been used but for cost purposes an 18% ACN grade could have been used (less oil resistant). 48% ACN has the best oil resistance of the available NBR grades.

You mention trying a viton-based seal. This polymer should give excellent service in your application but, obviously, you will never know until you try it. You can expect to pay far more for viton-based seals.

 

[tom1953]

Graham, good point re: the ACN level of the NBR.

Also, some "commercial grade" (read: inexpensive) NBR compounds may even have some other, non-oil resistant, rubber blended in with the NBR. Your supplier have have a higher quality, more oil-resistant, NBR available.

Another polymer that has good oil resistance, similar or slightly better than a medium-ACN Nitrile, is urethane (polyester types).

 

[rconnor]

Thanks for all the advice,

To answer Graham's question, this was one of those things that originaly was a small field fix and didn't get documented properly. Now that it's a problem, I can't find any info on what exact material was used.

This leads me to think you guys are right on it being a lower grade compound, as it was probably thought the cheaper the better.

And Tom, thanks for the suggestion, I'll do some research on that material as well.

Any other factors that I should look into besides the rubbers resistance to the oil? Does the wavy shape or swelling of the rubber indicate anything else (see attachment in original post for an image of the rubber)?

Thanks again; I'm always impressed by the members of eng-tips!

 

[Compositepro]

After a year in any liquid, including water, I would expect changes in most any rubber. It's a question of how much. And yes there are vast differences in rubber quality and formulation.

 

[btrueblood]

Star for Graham for nailing the problem.

 

[GrahamBennett]

Thanks for the star btrueblood, it's much appreciated.