O-ring tolerance

With O rings I think the standard bible for applications is the Parker O ring design guide which is easy to find online.

Chapter 9 has the sizes and tolerances

I'm making a part that has an O-ring in a seat. I.e., not in a groove. The part will be "loose" most of the time & I want the O-ring to remain in its seat by having the seat diameter smaller than the O-ring's nominal diameter. I.e., it'll be squeezed into the seat. How much smaller can the seat be? The O-ring's nominal diameter is 0.7 & its thickness is 0.1

Thanks, Bob ==========================================================

Orings are not compressible so the cross-sectional area of the groove while sealed must be at least as large as the cross-sectional area of the oring. Usually the groove depth is made 75-85% of the oring diameter to give 15-25% crush for sealing. For really demanding designs Parker has tables of different groove shapes for different oring materials used at different pressures, but for normal not-really-critical stuff 80% works. Of course, that wide groove lets the oring fall out when not sealed :-). The best answer for that is a dovetail shaped groove with the top width a little less than the oring diameter. The narrow top keeps the oring in place on disassembly and the extra width at the bottom of the groove keeps the oring from being extruded or cut.

For really demanding applications anti extrusion rings are available to support the O ring and prevent it being nipped, I think the details are in the Parker manual, certainly a mate that worked in truck air brake systems was very familiar with the details of their use.

Thanks for the replies.

I got the Parker book and couldn't find the "style" that I want to use: an O-ring in the bottom of a well, with a cylinder pressing on it. As:

||_______|| |O_______O|

I thought that I had seen such a use, but maybe not.

Not that it matters, cause I don't really have any flexibility as far as the sizes go. So, I'm going to make it and see what happens. I'll let you know.

Can you machine a groove just over half the thickness (deep) of the o-ring in the bottom of the well? Make it tight to retain the o-ring and leave enough protruding to seal .

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Snag

I probably could have made a groove, but I didn't need to. The diameter of the well had to be the same as the fitted cylinder (0.71) and the closest O-ring was 0.74, or so. That .030 squeeze holds it nicely.

It also seals nicely, holding 900 psi of CO2.