Please advice on this o-ring seal design

[RayJohnson2]

Hi all,

I am considering the following design to seal a chamber (holding compressed air of 70 bar (~1000 psi)) using an o-ring.
I would like your advice on this.
See the attached sketch.
I have only sketched half the construction. It is pretty much symmetrical around the vertical axis, so imagine the same construction on the right side (not shown).

The chamber (inside diameter 9 mm) has a threaded rod inserted. Rod diameter 8 mm and screwthread M8 at both ends.
So the compressed air is in this circular 'gap' of 0.5 mm wide.
The chamber is sealed off at both ends with a nut and o-ring combination. The o-ring sits compressed in the groove in the nut.
Nothing spectacular there.
My question is about what could happen during assembly.
The nut is screwed on the threaded rod. At a certain point, the o-ring will start being compressed while the nut is still being rotated.
At that (short) time, the o-ring will get compressed while the nut is rotating, relative to the chamber.
I was wondering.. could the rotation and compressing at the same time cause a bad seal? I am not sure what the o-ring will do, due to the friction of the rotation of the nut.
Perhaps just being compressed (as I hope it will).
Maybe putting some grease in the groove might help to reduce friction on the o-ring to prevent it from being damaged by the rotation of the nut...

What do you all think? Am I worried for nothing..?

PS. Once assembled, this is a completely static construction. No movement at all.

Kind regards
Ray

 

[jmec87]

It's probably fine. Lubricating the o-ring is a good idea. I recommend the Parker O-ring Handbook for guidance on the dimensions and lubricant to use. It's available for free from Parker at the link below (or easily found with a quick internet search).

https://www.parker.com/Literature/O-Ring Division Literature/ORD 5700.pdf

 

[506818]

You'll be fine. Ensure you lubricate correctly. Silicone grease will be fine with compressed air unless you're concerned about contamination. If that's a coarse M8, it's probably about one full turn of the nut so little friction once lubricated. The mating face should be machined to a decent finish depending on what standard you use. It's all very common what you're doing - think anything with a hydraulic/pneumatic port connection.

Regarding groove design - can't go wrong with Parker. Also ISO 3601 if you're UK/EU-based (it's very similar to Parker anyway).

 

[RayJohnson2]

Thanks a lot, both, for your input.
I will use this design then.

Thanks also for the link to the Parker O-ring handbook.

Most of my o-ring seals are for static applications, so I usually take a fairly high compression ratio. Usually from 15 to 25% (o-ring thickness vs groove depth). Air Pressure is usually in the range from vacuum to around 70 bar, from the inside of the o-ring. That's why I let the o-ring touch the groove outer diameter. The groove inner diameter, I take small enough to give the o-ring plenty of room to deform (as compensation for the compression).

 

[506818]

If it's a standard o-ring size, just follow the Parker or other standard groove designs. You'll typically make the groove OD the same as the o-ring's OD anyway. Then the groove width is standard based on o-ring section.

 

[hydtools]

Much like ORFS, oring face seals.

Ted