O-Ring groove question Caution! Metalworking content! ;-)

The machinist called me over and asked me to help him design an o-ring groove. We looked up a chart at

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and on page 89 of the above PDF, there's a chart: Design guide 4-2.

It's clear that for nominal 1/4" (actual width .275 ± .006) the gland depth (dim. L) is .226-.229.

But on the chart, it seems to indicate that the groove width, G, for a nominal 1/4 width, is .375-.380, which he thinks (and I tend to agree, but I'm not the machinist - I just draw the pretty pictures) is too wide and the o-ring would flop around or something. He thinks the groove width should come from the line above, and be .281-.286.

Anyone got the definitive answer for that?

Thanks 10E9 in advance! Rich

Reply to
Rich Grise
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Well, this is the cheat sheet I've been using for decades for inch-sized o-rings on vacuum systems. Since it clearly doesn't match any of what you posted, perhaps our inch-sized o-rings were really the size they claimed to be...

Anyway, you don't need (or want) to confine the ring 4 ways. You need the ring to squish and seal (in the "depth" dimension), and you need to give it room to squish (in the "width" dimension.)

All measurements are in inches. Measurements in brackets [] are -0.000, my preferred end of the scale on these.

1/16" width .085 + or - .003 [.082 +.006] depth .052 + or - .002 [.050 +.004] 3/32" width .120 + or - .003 [.117 +.006] depth .077 + or - .003 [.074 +.006] 1/8" width .160 + or - .003 [.157 +.006] depth .104 + or - .003 [.101 +.006] 1/4" width .325 + or - .003 [.322 +.006] depth .206 + or - .005 [.201 +.010]
Reply to
Ecnerwal

Roughly speaking, the o-ring will increase in width by the inverse of the fraction it is compressed. In other words if it is compressed by a factor of 0.8, the width will expand by a factor of 1.25 .

In your case the maximum diameter is .281 and the minimum compressed height is .226, So the maximum width is .281(.281/.226) = .349 so a groove of .375 makes sense.

Reply to
anorton

You're not working at TCFNA Morton-Thiokol are you?

I think the number is about right. The O-ring squishes out sideways, and you have to consider the tolerances of all dimensions and the O-ring itself. A maximum size O-ring has to fit a minimum size groove and gap.

Not much bad will happen if the groove is a bit wider than required for the particular O-ring, but bad things will happen if the depth is out of tolerance or the groove is too narrow.

Here's another source with exactly the same numbers for radial squeeze applications (see page 9). Smaller numbers for axial and smaller again for vacuum axial.

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Reply to
Spehro Pefhany

You wouldn't happen to know anything about a frozen o-ring back in the early

80's , would you ? I'm pretty sure I was involved with the standardization mixes for the propellant on that one ... And I know for sure I was on the crew that poured STS1A and 1B .
Reply to
Snag

Thanks! Rich

Reply to
Rich Grise

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Thanks! Rich

Reply to
Rich Grise

Thanks! Rich

Reply to
Rich Grise

Got my answer, so I guess we can retire the thread.

Thanks again to all! Rich

Reply to
Rich Grise

PURE GOLD! Thanks, I wish I had this years ago!

Reply to
Tom Gardner

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