Basic question (I think) why an o-ring?

[gcbryan]

I have a reason for asking them but it sounds so basic and obvious yet it's not quite obvious to me or I wouldn't ask it.

In a front lens/head design where there is a ledge behind the front lens why is an o-ring even needed?

A general non-diving example would be the Surefire 6P, the "ugly light" would be another example in a different way.

The 6P has a recessed retaining ring at the front of the head that you take off and then you drop in the lens from the front. Just behind the lens is a ledge so the lens can drop no further into the head that this. In my 6P there is no o-ring here but then again it's not a dive light.

In the ugly light the ledge is a little further down but the concept is the same but there is an o-ring. The initial flooding of this light was due to the lens not being able to get to this ledge because of a spring that wouldn't allow it.

Once a lens does get to such a ledge it can't go any further so how could it leak? Would the o-ring be mainly a cushion to soften the impact of hard glass coming into contact with hard aluminum?

Am I missing something?

 

[jspeybro]

is there also an O-ring around the lens, or only behind the lens?

anyway, if there is no o-ring, both the lens and the ledge would need to be perfectly flat to make the 2 surfaces touch on the entire circle. This is hardly ever the case.
If there is an O-ring behind the lens (on the ledge), you can get around this 'not-perfectly-flat' issue. Once the lens is pressed against the O-ring it will be more or less sealed since it will fill the air spaces at the non-flat parts. In case of a dive light, this is an interesting concept, since the lens will press harder on the O-ring when going deeper. This should make it seal better as you go deeper.

there is some theory about how much compression of the o-ring is needed to get it really seal, but I don't know the details about that.

If you look at a watch, there is only 1 o-ring, but it is actually around the lens, not below the lens. This seems to work to, at leat to some extend, so I guess there is always more than 1 solution to a problem.

johan

 

[Packhorse]

As J said.

Some lights may use multiple O rings. These can be for redundancy or for cushioning glass on metal contact. But there will always be one that does the sealing.

 

[gcbryan]

So I guess the specific answer to my question is without any o-ring but with a ledge it would leak eventually after a certain pressure because in reality the two seemingly flat surfaces are never going to really be flat enough.

Thanks. This isn't a dive light and I'm looking at how to make it one. I like the design because it does have a ledge and didn't come with an o-ring but does have a retaining ring.

I have put an o-ring on the ledge but the glass lens is much thinner than I would like. With the o-ring that I've added the retaining ring doesn't screw down as far as it originally did. That's still workable but there is no extra room.

I was thinking about the option of removing the o-ring for more space to be able to use a thicker glass lens but I knew that probably wouldn't work but I wasn't positive as far as the physics and thought I'd get some feedback here just to help clarify it in my mind a bit more.

Thanks.

 

[hanachan]

strictly speaking, the function of O ring is somewhat different from
what Johan says.

Water pressure directly compresses the o ring and the deformed o ring by
water pressure seals the gap between the lens and body.

An o ring under lens can be replaced with a gasket (flat rubber ring).
The design that the o ring around the lens is correct.

 

[Packhorse]

strictly speaking, the function of O ring is somewhat different from
what Johan says.

Water pressure directly compresses the o ring and the deformed o ring by
water pressure seals the gap between the lens and body.

An o ring under lens can be replaced with a gasket (flat rubber ring).
The design that the o ring around the lens is correct.

Well the pressure caused by a retaining ring being threaded down on the lens will also cause pressure on the O ring and make it seal too.

Im no O ring expert but both axial and radial O rings have their advantages. I would probably pic a axial (face ) Oring over a radial for sealing a dive light lens. The face of the lens will probably be smoother than the edge.

Here is some more info on O rings. Pity its in imperial. Anyone got a metric version?
http://www.rlhudson.com/O-Ring Book/designing-tables.htm

 

[JaguarDave-in-Oz]

An o ring under lens can be replaced with a gasket (flat rubber ring)..

yes, if the surfaces are flat then that's a great idea. They work well on water resistant watches and if you find a watch makers supply house that will deal with you (they are notoriously tetchy with non-trade buyers) you can get kits of flat o rings in myriad diameters and thicknesses. That could save you a heap of space.

 

[wquiles]

You seed "something", o-ring, gasket, etc., to make sure the two surfaces are in fact creating complete contact/seal to prevent the water from coming inside. In the most common setups the material can be on the sides of the lens or beneath the lens, or a combination. This older thread has some good discussion and a couple of diagrams that might be useful in this discussion:
link ...

And for those wanting to learn more about o-rings, most of my searches on the subject consider the Parker O-Ring Handbook to be "the" reference on the subject, explaining "exactly" what o-rings do, how they operate (and fail), different materials, etc.:
http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf

Will

 

[Klem]

I remove all O rings that are not meant for regular access, replacing them with glues.

From strongest through to weakest;
a. 2-part Epoxy putty or glues (Selleys Aqua-Knead-it, Devcon, JB Weld), for permanent seals.
b. Polyeurathane (Sikaflex Marine), for rare need to re-open.
c. Silicone sealant (Silastic), for occassional need to open and service.
d. O-ring, for regular need to re-open.

A screw thread enables you to use a weaker seal as the thread is the bind.

I try to design torches with only one O ring.

Your seal between the aluminium bezel ring and glass is an example of what I would permanently epoxy putty together. Using putty you can squeeze it together, and this takes care of any irregularity between two seemingly flat surfaces.

 

[OS74]

So I guess the specific answer to my question is without any o-ring but with a ledge it would leak eventually after a certain pressure because in reality the two seemingly flat surfaces are never going to really be flat enough.
Thanks.

Slightly OT, but illustrative.

For the first bathysphere (http://en.wikipedia.org/wiki/Bathysphere_(vessel)) The entrance hatch was oring-sealed for the first hundred meters or so, but the next eight hundred by metal against metal. O-ring free seals worked, but apparently needed quite a few tons of pressure to create the seal.

 

[gcbryan]

Slightly OT, but illustrative.

For the first bathysphere (http://en.wikipedia.org/wiki/Bathysphere_(vessel)) The entrance hatch was oring-sealed for the first hundred meters or so, but the next eight hundred by metal against metal. O-ring free seals worked, but apparently needed quite a few tons of pressure to create the seal.

I like that. Thanks!