Interferance Fit

[nornrich]

All,

Is a steel to steel Interferance Fit going to be water proof? I am not talking about any pressure, submerged in about 6" of water.

Regards,

Rich.....

Richard Nornhold, PE

 

[CoryPad]

Are you asking because of corrosion resistance of the fit area? Possible water transport from the fit area to a hollow? Something else?

In general, the fit is not permanently (i.e. long time duration) water proof unless it is sealed.

Regards,

Cory

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[JakeAdkins]

How water proof are you talking? you want to leave it a the bottom of a 6 foot pool and never have it leak any?

If it is a moving part you could do a mechanical seal. If not I would think about a weld bead or an O-ring.

 

[JakeAdkins]

The o ring would work for a moving part too.

 

[GregLocock]

No, in fact even your O ring is a bit dodgy because six inches of head isn't enough pressure to ram it into place. Can you use grease, rust, or RTV?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[arunmrao]

To begin with, why use steel,you could think of rubber or plastic based material. This could eliminate many variables too.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[hydtools]

GregLocock, you know better. O-rings don't have to be 'rammed' into place. Installed in a standard squeezed condition the o-ring will result in an effective seal.

Pressfit the parts then add Loctite 409. It will wick into the small crevices and seal the joint.

Ted

 

[hydtools]

Sorry. 409 should be 290.

Ted

 

[GregLocock]

No, an O ring will not seal properly in waves, or near the surface of open water. It will actually pump water across the seal line.

If you pressurise the O ring, typically by using several atmospheres of nitrogen behind it, then it will remain watertight so long as it remains pressurised.






Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Timelord]

Sorry Greg, Hydtools is right.
Your leaking o-ring must have been a very special case with motion (this case is a press fit). I design vacuum equipment and I've never "rammed" an o-ring. O-rings will seal down to their permeability in a properly designed groove with less than 14 psi across them.

Timelord

 

[KENAT]

I once spent a bunch of time designing a rig to do what Greg is talking about.

Fond as the MOD is of wasting money, I don't think this was one of the cases.

KENAT,

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[GregLocock]

I'm talking about submersing very expensive electronics in seawater for weeks at a time. O rings alone were /not/ good enough. Plugs and sockets were O ringed and greased, and then their backplanes were encapsulated, and the the electronics chamber behind them was pressurised. I strongly suspect the pressure was monitored (not my system )

I was told that splashing and wave action were the hardest situation to seal, as the pressure varies locally on the seal the O ring rolls backwards and forwards imperceptibly, squeegeing water molecules through the seal. BTW this was for high end commercial sonar gear, not military.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MotorVib]

Just as a side note welding an interferance fit is not a good idea. You are going to lose some if not all of your friction fit because of the heat affected zone around the weld.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[chicopee]

MotorVib, is that all three laws of thermo.

 

[MotorVib]

yes it is

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[electricpete]

I have no practical experience on that question. I would imagine the answer would depends on how smooth the surfaces are (smoother surfaces seal better) and how long a distance it is from one side of the fit to the other (longer seals better) and possibly the presence of vibration and thermal cycles.

If you want to improve the sealing performance, why not put in a non-hardening self-forming gasket compound like Permatex 2.




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[electricpete]

Sorry - I got pulled away by my wife before I got to finish my thought.

The finish of my thought was: for a smooth cylindrical fit we can of course calculate the contact pressure. Wouldn't the joint behave similar to what you get if you cut the seam of the cylinder of contact and unreolled it into a flat bolted flange joint (with similar sealant) under the same contact pressure?

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[nornrich]

All,

Thanks for all your input.....

Regards,

Rich....

Richard Nornhold, PE

http://www.ovenind.com