Tank o-ring "grooves"

[ClayJar]

HMMMMMMMMMMMM.......my 40 cft luxfer has the cone shape........2002 manf date

Okay, call it *almost* an annular disc but actually the space between a narrow-angle truncated cone and a cylinder (i.e. the valve). The cross section is trapezoidal, but it much more resembles a rectangle than a triangle.

(Hopefully that's properly descriptive. :biggrin

 

[pescador775]

Square, trapezoid, cone, cylinder--it is all irrelevant. An O ring is cast or extruded from an elastomer whose properties allow it to assume any regular shape into which it is forced. Modulus causes the O ring to be "springy" and exert tension in several directions when compressed. After a year or more the new shape will become permanent. The doughnut has assumed the shape of the tank neck groove after the cross linked polymers rearrange themselves and memory is lost causing a reduction of elasticity. At that point, the O ring should not be reused but replaced if disturbed. Otherwise, it is subject to the problem of any packing which is no longer under tension, leakage.

 

[DepthCharge]

Okay, call it *almost* an annular disc but actually the space between a narrow-angle truncated cone and a cylinder (i.e. the valve). The cross section is trapezoidal, but it much more resembles a rectangle than a triangle.

(Hopefully that's properly descriptive. :biggrin

OK, but that gives only 3 sealing surfaces. Square cross section groove gives 4 sealing surfaces, so should be a better seal. Also square cross section would possibly give a more accurate, or consistent % compression range over the entire ring

 

[fppf]

The only true sealing surface is the gap between the parts.

 

[ClayJar]

OK, but that gives only 3 sealing surfaces.

Obviously, you're not understanding the geometry I described in straightforward geometric terms, since there's no way to get the number three out of it. As I'm not inclined to CAD out a model for you at this hour, I'm going to recuse myself and get some sleep.

 

[WD8CDH]

OK, but that gives only 3 sealing surfaces. Square cross section groove gives 4 sealing surfaces, so should be a better seal. Also square cross section would possibly give a more accurate, or consistent % compression range over the entire ring

But only 2 of the surfaces matter, the flat bottom of the valve seat and the outer part of the tank groove. Tank pressure holds the o-ring against those two surfaces by forcing the o-ring toward their junction.

 

[sphillips3]

In a static (no moving parts) application the 0-ring seals against two surfaces of the space - which is properly called a "gland". In the example of a scuba cylinder neck the tank pressure forces the o-ring "up" against the bottom of the valve assembly and "out" against the machined surface in the tank neck internal diameter. Obviously the pressure tries to extrude the o-ring through the gap between the valve and the tank, but in a properly designed sealing system no extrusion will occur. Since any extrusion damages the o-ring and potentially compromises the seal it is to be avoided by properly managing the size of the gap, the diameter of the gland space and the o-ring and the hardness of the o-ring. If any extrusion occurs in a scuba tank application either you did not tighten the valve down enough (allowing an excessive gap to exist) or your o-ring material is too soft (wrong durometer) or something is damaged.

Static O-Ring Gland Design - Static Radial Applications

In a dynamic application (piston or shaft) the o-ring moves back and forth in the gland, alternating sealing surfaces - this is the type of appliction where you will usually find a "square" o-ring gland. The principle is the same, but you need a different size gland to allow the o-ring to work properly.

 

[DepthCharge]

But only 2 of the surfaces matter, the flat bottom of the valve seat and the outer part of the tank groove. Tank pressure holds the o-ring against those two surfaces by forcing the o-ring toward their junction.

OK, I'll buy that. This was a kind of off-shoot of the original question. Seems as though either method will work, but WHY is is the groove an "annular disk" (or what ever)? Is it just easier to manufacture that way, or is there another reason?

 

[Twistedarts]

You could allways just use a little permatex......

 

[DepthCharge]

Obviously, you're not understanding the geometry I described in straightforward geometric terms, since there's no way to get the number three out of it. As I'm not inclined to CAD out a model for you at this hour, I'm going to recuse myself and get some sleep.

Actually no I don't understand. Here I've attached a graphic representation as to what I'm describing. Unfortunately I'm doing this from work so no AutoCad.............but I think you'll get the idea.