Smallest O Ring Cross Section for Face Seals - Parker O Ring Handbook 1

[innominata]

On page 98 of the Parker Handbook, section 4-18, there are listed cross sections for o-rings and groove dimensions.

The smallest cross section listed is 1.78mm, the handbook doesn't list any smaller.

I tried 1.78mm and made and tested a working seal. I wanted to decrease the wall thickness of my part so I used the apple rubber calculator to get the groove dimensions for a 1mm cross section o-ring but was not able to make it seal.

The apple rubber calculator gives me the same values for the 1.78mm cross section so I don't think it's faulty. The reason the 1mm o-ring didn't seal is I was not able to fit the o-ring, there just wasn't enough retention as the o-ring would fall out of the groove while I was seating it. The corner radius was 5-6x the cross section of the groove too. I attempted to use silicone grease on a nitrile o-ring and it seemed to help but ultimately was unsuccessful.

For those who know, did Parker simply exclude any o-ring cross sections smaller than 1.78mm because they would be a hassle to assemble by hand?

If not does anyone have any experience fixing retention issues?

 

[hydtools]

How did you create the gland? Machining method? Does the surface finish have features that run across the seal?
I have used grease to retain the seal during assembly.

Ted

 

[3DDave]

O-rings are great in o-grooves. Their resilience makes it difficult to put into not-o-grooves. I swore never to try that again and to use deeper edge-on rectangular gaskets for non-o-groove sealing. Rectangular edge-on gaskets allows for far more compliance than o-rings can have as well as being self-retaining.

Alternatively one can create molds for round-section non-o-groove use, but that's more expensive though also common.

 

[innominata]

I noticed what the issue must be. The two o-rings actually had a different material. I compared a handful of o-rings I had and the FKM material is somewhat set in place, that is the shape tends to have no memory while the nitrile o-rings love to return to their O shape.

edit:
Actually the material wasn't relevant, I wouldn't want anyone getting the wrong idea. I made the mistake of only testing with one design and changing the material. After testing with many different designs it became apparent that the existence of a lip on the part surrounding the o-ring almost entirely determines how easy it is to assemble first and foremost. After this other factors have some level of importance but seem trivial, e.g. without a lip adding grease isn't very helpful.

 

[desertfox]

Hi innominata

Just an observation but if the o’ring section is only 1mm then shouldn’t the depth of the o’ring groove be shallower than the groove for a 1.78mm o’ring section? In your post you’re saying both groove dimensions are the same.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

He was using the Apple Rubber calculation at 1.78 to confirm that Parker's recommendation at 1.78 mm was the same; Parker didn't go below 1.78mm and Apple did.

It was to confirm that the Apple Rubber calculator was reliable to make the calculation and then to use it for the 1mm ring.

 

[desertfox]

Have a look at this link it shows groove dimensions for a 1mm and 1.78mm section of o’ring and compare it with the Apple calculator.
Thanks 3DDave I misread the post.

https://www.allorings.com/o-ring-groove-design-metric

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Prometheus21]

innominata:

This answer might be late, but Trelleborgs O-ring catalogue (and online calculator) is a valuable resource when it comes to smaller O-ring diameters. I believe the smallest cross section diameter they describe is 0.5mm, for both static and dynamic uses (radial and axial installation).

Speaking from experience, a half-dovetail groove is a great way to ensure O-ring retention, while allowing for a slightly smaller groove. Although for cross sections smaller than 1.78 mm you will need a custom made cutter in solid carbide, and a good machinist. Sharp edges alone can easily cut into O-rings with ShA of <90 regardless of the material, creating leakage and/or contamination. Zurcon Z22/24 series type materials are a great way to alleviate this problem, due to the materials abrasion resistance.

A good rule of thumb (at least in the industrial gas industry) is that a well-designed O-ring face seal should be easily installed and create a perfect seal without the use of any lubrication. I cant tell you how many times I've seen a product relying on an abnormal amount of lubrication to perform the sealing action properly, instead of the actual O-ring. That can be very dangerous, especially when dealing with high-pressure oxygen systems.