Turning an O-ring groove in piston

sp>proscribed bottom. What's the best way. Make a rough semi V lathe bit, and then

LOL.

Silicone o-ring for modest heat. Upto about 325 degrees.

Most O-rings I look at are seated in square grooves. I believe you need the extra space afforded by the square groove to allow the O-ring to deform and seal properly. For a piston though, you should probably be looking at cup seals and the like on McMaster.com.

Most o-ring grooves are square - there's little or no need to match the profile of the ring, just get the size right so the ring compresses adequately to work...

If you want a round bottom, and you are HSMing rather than production, just use a round-nosed tool and take it easy, unless you've gone and decided to work in stainless where you can't take an easy cut, in which case you'll need a machine that will stand a real cut. Roughing out with a V does not reduce the power needed to make the final cut at full radius, and making the cut in any normal material is not likely to dull the tool much.

The Parker O-Ring Handbook (9 Mb) discusses some of the design criteria for o-ring grooves. See Section 4:

Not an engine piston. A modest temp single stroke hand injection pump piston. After I posted I was thinking a square groove might work better. Thanks. I have not grooved the piston itself yet.

Check this out:-

Pretty much all you need to know about static and dynamic O-ring seals.

"The shape of the seal groove is unimportant as long as it results in proper compression of the seal between the bottom of the groove and the cylinder wall, and provides room for the compressed material to flow so that the seal is not solidly confined between metal surfaces"

Not engine piston, they use very different seals, think hydraulic piston as most of those use some variety of cup seal.

The other big name in o-rings is Apple. I am sure they also have information on the web about grooves for O-rings.

=20 Dan

Correct. Most O-rings are intended to be seated in nearly square (rectangular) grooves. Rubber is essentially like a fluid ... it is nearly incompressable. When pressure is applied it DEFORMS, but does not compress significantly. That's why it's used for some kinds of one-sided-mold stamping operations. It applies a uniform pressure just like a hydraulic fluid.

Assuming a square groove with a round O-ring, the groove should be about the same width as the O-ring's diameter, and bit less in depth than the ring's diameter. Then, when the cover-plate (or whatever) moves to put pressure on the O-ring, it deforms, swelling sidewise to press against the sides of the groove as well as the bottom. It effectively makes a three-point seal in the groove, with one seal against the closing element.

A round-bottom groove doesn't allow the ring to deform properly, and it will "mushroom" out of the groove, deforming oddly, since there's no other place for it to deform into.

Now, it's true that a round bottom groove CAN work, for low-pressure or unusal applications, where the O-ring is not overly deformed, but it's not what's usually recommended. Check most any o-ring manufacturer for specs.

Dan Mitchell ============

That paragraph pretty much sums it up and describes one of my problems on my first tests.

I guess I need to go make some square lathe tips in the right sizes. I was going for an exact fit round groove.

Ok... if I ever go to high production (if I do remind me I'm a hack) I'll look at that. Most of the "commercial" models of this are using an o-ring.

Use a rectangular groove.

Lots of information (11 meg download) in "The Sealman's O-Ring Design Handbook"

Thanks, I found them and bookmarked them for future reference (though you have to register to read their technical library).

What everybody said. I've had good luck with a cutoff tool ground to the proper width and the corners lightly stoned.

If you want the seal to hold a high vacuum, the bottom of the groove will probably have to be polished.

In general I prefer to design with metric O-rings as there's a much wider range of sizes.

No..use the round one to start and finish with.

If you have a tool and cutter grinder and can make some fixtures..you can turn out a decent half round tool

On the other hand...unless you are handling really high pressures...you CAN often times get away with a simple V groove.

Gunner

It has been a few years, but like others have said, just go with a square groove. Seems to me we would make the groove same width as the o-ring, and the depth 90% of the thickness, .250 thick ring, cut the groove .225 deep. Now that depends on your clearance from the piston and cylinder wall too, so I suppose you should go with the cylinder diameter, minus 2x of 90% of the o-ring thickness. so .250x90%x2 subtracted from the cylinder diameter.

like this site. Click on "O-Ring Gland Design" Randy

What's that Lassie? You say that Greg O fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Fri, 7 May 2010 18:03:41 -0500:

No. The width of the groove should be equal or greater than the width of the O-ring.