O-ring contact stress v's fluid pressure

[EngineerPhil]

Can anyone help?

Is there a rule of thumb that roughly describes the relationship (for a seal to perform) between the contact sealing force/stress on the o-ring to the applied fluid pressure?

I have a seal that doesn't always work and FEA suggests it does. If the applied fluid pressure is too great then it could blow past the contact sealing pressure, however, there must be some sort oflimit to this?

Just a bit stuck.!

 

[Tmoose]

If the design is under 1500 psi and uses name brand commerical o-rings in grooves designed AND MANUFACTURED with specs from the catalog I'd expect them to work just fine. I have battled a few High, and especially low pressure leaks and the problems so far have been groove finish, or some feature ( hole, shoulder, groove) that hurts the O-ring during assembly.


O-rings are self pressure balancing and self sealing, in that the higher the pressure the tighter they grip.

The the upper pressure limit the no-longer-shaped-like-an-O-ring is being stuffed hard into the far corners of the gland. Smaller escape clearances and even plastic backer rings keep the O-ring from oozing into trouble.

 

[vanstoja]

Hertz,D.L.,1979, "O-Rings for Low Pressure Service", Machine Design, 4/12/79, pp.94-98 gives an equation for peak contact stress(psi) that must be greater than system pressure for sealing a joint:
f'=4F/pi^2*b*Dm where b=seal contact area=2.4x
Dm=Seal mean diameter(in)
x=Seal deflection(in)
F=pi*d*Dm*E[1.25*(x/d)^1.5+50*(x/d)^6]
d=Seal thickness(in)
E=Young's Modulus(psi)
The opening paragraph says "O-rings normally operate with about 15% squeeze to ensure a tight seal. But at system pressures below 400 psi, this amount of squeeze can cause high friction and excessively high actuating forces."
Feeley,J.E.,1981, "Economical Approach to O-Ring Seal Design",Machine Design, 12/10/81,pp.169-172 mentions a minimum squeeze of 12% and says "Maximum squeeze is not as critical as minimum squeeze, but may lead to difficult assembly if it exceeds the following values:
Size(in) Max Squeeze
0.103 25%
0.139 24%
0.210 22%
0.275 20%