O'Rings 1

[gatz]

For some time, I've wondered how O'Rings actually work.
It seems almost magical that an O'Ring can be compressed a small amount (given ring vs groove dim's) and they seal up to 3,000 PSI and even beyond.
What is actually taking place?
Why doesn't the fluid just raise the point of contact and merely go by ?
Is the fluid pressure deforming the O'Ring by pushing on the "face" even more and thus it seals even better? What takes place when an O'ring fails ?

I can understand that fluid pushing on a shaft seal face forces the lip of the seal down onto the shaft.
Do O'rings behave the same way?
What about dynamic compared to static?

Thanks in advance
Gatz

 

[byrdj]

o-rings are a very interesting area that is taken for granted when they work, which is most of the time. The answer to your question could be a semester course.

I had a referance book from Parker Hannifin that was good. I thought a google search would easliy bring up some good basic info, but that was not case. However I did find this site that does OK in the basic, it should then help with the information found on other sites

(

http://www.americanseal.com/technical/oring_function_1.html)

Good luck,

 

[monkeydog]

Byrdj is correct on the semester course on o-rings.
I found the Parker website, but my slow modem at home does not like downloading documents. Check out this web site, I think the O-Ring Handbook is what you are looking for.

http://www.parker.com/Catalogs/

Monkeydog

 

[hydromech]

Hey,

In basic terms...O rings are made from varying types of rubber, normally nitrile of varying hardness and some times viton.

In any application, the pressure that hits the O ring is applied at right angles. The pressure is applied evenly to the surfaces that are exposed (to the pressure).

The O ring will be deformed by the pressure and in the correct housing this deformation is controlled, forcing the O ring against the sealing face.

The sealing force is proportionally equal the pressure applied to the O ring.

The sealing face needs to be flat and smooth so as not to cut the O ring.

The housing needs to correct too. At high pressure, it is possible to extrude a 0.25" section O ring through a 0.025" gap.

In static applications the load is always applied from the same direction and the forces applied to the O ring are hydraulic. In dynamic applications the forces applied to the O ring vary constanly and are also mechanical. This puts a lot of additional strain on the O ring and therefore requires more attention to the design of the housing.

Regards

 

[danthemeterman]

2 cents worth.
Any machinist will tell you that in a perfect environment, there is no need for a seal, be it mechanical, flat gasket, oring or onion seal. In other words if you could machine 2 or more perfect opposing faces there is no need for an intrusive seal. This doesn't answer your question but may open further thought to the function of the oring.

 

[dtberry]

Very good explanation from hydromech. An even better type of seal can be used in replacement of an o-ring. They are called R-rings or square seals. You will get a better seal using r-rings because they give you an almost perfect fit in the area to be sealed. They aren't influenced nearly as much as the traditional o-ring when subjeted to higher pressures.

 

[danthemeterman]

dtberry. I whole heartedly. We started using "r" rings approx. 15 yrs. ago as a replacement for OEM orings. They are much easier to insatll and a greater sealing surface. Now my question for you. I never heard them called "R" rings. What does the R stand for. Radial? Just a dumb guess.

 

[gatz]

Thnx for all the replies. Think I'll check into that Parker site.
danthemeterman.....
I recall (as best as memory will allow) that on the CH47 Chinook Helicopter engines, there were "no seals" on the internal main shaft.
Lycoming had developed what was called air seals on the T55L7C Turbine engine.
Nothing more than a balanced pressure load between areas in the engine that would allow only a small clearance for oil to provide lubrication.
To consider an engine to be using too much oil when this equalibrium became unbalanced, there was a maximum amount of oil usage before it could be considered "Red X" (not flyable)

I think the amount was something like 7 qts per hour.
Our crew had one that was using 5+ qts per hour and we had a hard time getting the TI to Red X it. Made for some interesting stops.