Nitrile Seal Problem

[BrainFailure]

Howdy:

My company builds hydraulic dampers for prosthetic knees. I am having a problem with nitrile seals (60A durometer) that are shrinking about 3% over a period of about 2-4 months. When we build the dampers we 100% inspect them. We are having units that pass when they are built, but after they sit in the warehouse for a couple months they will fail. When I take apart the unit I find that the main piston seal is shrunk about 3% and the seal feels harder than a fresh one. The hydraulic oil we are using is Mobile DTE11.

I am currently sending some seals back to the supplier for testing. I would also like to find an easy way to test to see if my hydraulic oil is bad.

Has anyone heard of a problem like this?

 

[hydtools]

I have two questions.
Have you changed o-ring suppliers?
Have you changed oil suppliers?

I have not experienced nitrile shrinking or hardening in hydraulic oil in that short of a time. I have experienced purchasing buying 'substitutes that are the sames as' causing problems.

Ted

 

[BrainFailure]

Ted:

No and No. Also note that it is not an oring, its a custom double cup seal (cross section looks like an "N").

I did some research and I found that in 2006 we switched from Viton to Nitrile to save money. I wouldn't think that this would cause a problem because the other seals have always been Nitrile.

-Daniel

 

[israelkk]

Look for the "compression set" data as functio of time for the rubber compound in your environment, temperature, squeeze, etc of the seal.

 

[BrainFailure]

Isrealkk:

Thats a good thought, but in this case my squeeze is like 3%. Plus remember its a cup seal so the material doesn't actually squeeze, it flexes.

 

[budt]

How long were the seals setting on the shelf before using them?

I have heard shelf life is important on Nitirle seal material.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[hydtools]

BrainFailure:
That is a good idea to have the seal manufacturer verify that their seal has been compounded and cured correctly.

It would be a good idea to have the oil sampled to verify qualities and additives.

Have the seals been exposed to ozone or ultra violet during storage before being used? Both at your facility and at the supplier's facility. Does the seal go through a sterilization process that has changed?

Ted

 

[BrainFailure]

BTW thanks everyone for your responses.

Bud:
Cure date on the batch I currently have failures from is 4th quarter 2008.

Ted:
The seals are shipped in clear bags but as soon as we get them we store them in dark bags. Another thing to consider is that the seals are nice and stretchy before they are installed. After they sit in oil for a while they get a little less stretchy. These are not sterilized. Not sure about ozone at all.

Do you have any tips on how to sample my oil? Is it cheaper to send the oil somewhere to get tested or is it cheaper to buy a test kit?

 

[hydtools]

I'm not aware of easy testing you can do to determine the correctness of the oil chemistry.
Seals normally swell if they do not tolerate the oil, not shrink.
It sounds like perhaps the plasticizer is leaving the seal. Could happen if the manufacturer did not cure the seal properly.
If you happen to have a old seal from a batch known to not shrink and harden, but both it and a new seal in a cup of the current oil and see how they both react. If both shrink and harden, the problem is the oil. If only the new seal shrinks and hardens, then it is the new seal manufacture. A similar test would be done with a sample of old oil.

Ted

 

[BrainFailure]

Ted:

I had the same thought. Unfortunatly the assembly manager already threw out the oil that was in the failed units. I am currently soaking seals in new oil and it appears to have no effect. Unfortunately I am not sure if it really is having no effect or if it just takes 3 months for the effect to occur.

Thanks for the tips.

-Daniel

 

[BrainFailure]

Here is a follow up:

I talked to a few seal experts and they are saying that non-synthetic (patroleum-based) hydraulic oil can contain small amounts of plasticizer which can cause nitrile to shrink a small amount. For most applications 3% shrink is not a problem but for my application it is. Ironically he said that high quality nitriles are more suceptible to this than cheap nitriles. So my options right now are to switch to Viton or switch the oil to synthetic.

Does anyone have a recommendation where I can get a small sample of synthetic hydraulic oil that has a viscosity of 15 cst in a very short amount of time (tommorrow)?

 

[flexibox]

a good Synthetic oil is Royal Purple! After reading all above I would stick with Viton because the cost of the oil may be too high

 

[matkenmd]

if you have 3% squeeze and then 3% shrink of course you'll have trouble. How about bumping up the squeeze to 10%, then when you loose 3% for whatever reason, you still have a seal?

 

[BrainFailure]

MatkenMD:

At first we wanted to avoid making a new seal mold, but we ended up doing exactly what you suggested. The seal manufacturer is remaking the mold to give us the 13% squeeze that we originally asked for.


Here is another update:

After trying synthetic oil, we actually still had a Nitrile seal shrink. After some experimentation I found that not only can patroleum based hydraulic oil cause a small amount of shrinkage, but heptane the heptane we use for cleaning can also cause shrinkage. So right now I beleive that the heptane is also a factor. The only remedy that I have high confidence in right now is switching to Viton.

 

[hydtools]

Viton will serve you well.

Ted