Sealing

Considering the scarce information you got I wanted to explain in detail and I made a few computations to give you values.

First of all you have to take into account that when an O-ring is its grove it is compressed and thus a force between the O-ring and the wall appears. In th Parker catalogue you can find specific value in N/cm as function of compression degree and compound Shore hardness.

A second point to consider is how the ring deforms under pressure. Following picture shows an O-ring in its grove under pressure (in your drawing you figured an O-ring in its grove BUT in absence of pressure).

When under pressure the O-ring TRANSMITS the pressure to its contact surfaces. In your assembly it will apply a force to the hole wall which is proportional to the applied pressure "p" and the area of this surface A= π*d4*b. The force you measured is the sum of the pressure force Fp= π/4*d4^2*p and the friction force which is Ffr= µ*(A*p+F1*π*d4) where F1 is the specific compression force I mentioned above.

You used -I assume- the O-ring 3.68x1.78 so that I made the computations accordingly. Since I did not have an information about the Sh values I presumed further that you used either HSh 80 or HSh 90 but most probably due to the pressure HSh90 to reduce the extrusion risk.

The results are in following pictures :

The values for the friction coefficients are from same document and are 0.3 for a waiting time of 1 minute and 0.4 for 1 hour.

As you see the 590 N are in the middle of the field for HSh 90 and µ=0.4. The horizontal scale is the compression as (d-h)/d. If the d value was almost at highest value it could have been possible that the O-ring had 80 Shore. In fact all hardness values are average since from the process deviations as high as 5 to 10 Shore can occur.

I hope this explanation is what you expected.

From my own experience I had when I designed a pressure sensitive by-pass to feel how important is this computation and how sensitive the small diameters are to the different "hidden" force and associated friction.

If you need something more please ask.

With respect to the other case I have to know how you made the measurement and how high was the sampling rate for the acquisition since you had to record a very short in time force peak and if the sampling rate was too small you measured AFTER it opened and since the pressure can fall very quick the result was lower than expected.