Sealing screw holes with o-ring and washer

[nopaddle]

I frequently have low pressure sealing problems where the screws joining the parts are the main trouble. Dowty seals are the obvious solution but at small sizes (M3, for example), I can only find the non-self-centring type. The trapezoidal rubber section does not seem to offer enough rubber deformation for all but the most precise holes.

Why is there so little rubber on a typical bonded seal when compared to a similar o-ring joint? I understand the advantages with a precision machined part where a decent counterbore can centre the seal, or on larger parts where a self-centring version is offered. But what about low-pressure (IPX5 to IPX7) type applications where adaptability and simplicity are principle considerations?

Standard o-rings fitted inside a stainless steel washers to limit the compression seems like a passable solution. However, I have a some questions:
[ol 1]
[li]Why is this solution not more commonly used?[/li]
[li]Why are sealing washers with more rubber so hard to find? (I know there are some solutions, but I can find very little suitable for small diameters.)[/li]
[li]Does anyone know of a supplier offering M3 self-centering sealing washers?[/li]
[li]Is this o-ring and washer combination a bad idea for some reason that I have not thought of?[/li]
[/ol]

 

[hydtools]

Create a counterbore for an M3 oring and use a plain flat washer to complete the oring gland.

Or use something like these:

http://www.longlok.com/site/pdf/selfseal.pdf

Ted

 

[dgallup]

Sealing at small sizes always gets more problematic. For one thing, the tolerances on rubber parts does not scale. As the dimensions get smaller the rubber tolerances become a bigger percentage on the squeeze calculations until there is no tolerance left for the mating parts. We once had a customer who had a standard requiring all o-rings to be at least 2.5 mm cross section. The more I work with small o-rings the more I appreciate the wisdom of that rule.

Those self sealing fasteners linked above look good but I do see potential pitfalls. Installing the o-rings over the threads will require some sort of sleeve over the threads to prevent cutting the o-ring. Probably not a problem in production but could be a problem in service. Also the hole size and concentricity of the piece under the bolt head will have a big affect on the sealing ability. The o-ring extruding into the gap could certainly compromise the sealing on a small thread size.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Tmoose]

If the leak is thru the threads on the tapped part any old Loctite will probably do a great job of sealing if carefully applied.
Wicking ( 290 ?) might even do a great job after the fact.

As always, this old man needs pictures and drawings of your arrangement with arrows and explanations to help understand what it is you are REALLY asking.

 

[hydtools]

Also see Dowty rubber washers with a metal insert.
As noted, Loctite 242 would work to seal the threads.

Ted

 

[chez311]

Standard o-rings fitted inside a stainless steel washers to limit the compression seems like a passable solution.

What exactly are you envisioning here? An extra thick washer with o-ring grooves and o-rings on either side? Seems like it would be more difficult to handle for assembly/service, more cumbersome, and more expensive. A sealing washer with a single integral molded sealing element like the ones you described likely would beat it in all those categories.

Why are sealing washers with more rubber so hard to find? (I know there are some solutions, but I can find very little suitable for small diameters.)

I was actually just looking at these the other day, something I noted was that Inch sized sealing washers seemed to have larger molded rubber o-ring elements than the nearest size Metric ones (ie: 1/4" vs M6). Maybe something to look at.

 

[IRstuff]

I think we just used RTV.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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