O-Ring Sizing

[joe123456]

Could anyone tell me how to select the proper O-Ring size necessary to hold back a specfic pressure? There are formulae everywhere telling the dimensions of the gland, the dimensions of the clearance, the material to be used etc. but all the calculations I have found assume that you have already selected the proper O-Ring size.

I have an axial, externally sealing O-Ring application, I know the pressure which I want to seal, but I don't know what O-Ring diameter (thickness) I need to do the job. I know that this will depend upon durometer and clearance as well, but I can't find a formula anywhere to calculate this with.

 

[unclesyd]

There is lot involved it determining the maximum allowable pressure that a particular O-ring will provide sealing capability. Provided you have the temperature, fluid compatibility, and operation covered here is a chart of max pressure based on calculated clearances of the O-ring gland. If there is any doubt contact the technical dept of the O-ring manufacturer.

http://www.marcorubber.com/clearance_gap.htm

I like to remind everyone that an O-ring is compound and as such that all are not created equal in terms of fluid compatibility. If your find a specific O-ring that is compatible with your process fluid don’t change to another compound of the same family without checking compatibility.

 

[joe123456]

I have seen this chart, but this only relates clearance gap to pressure rating. Can I then assume that any O-Ring diameter (thickness) will work as long as I have the proper clearance?

I was thinking that the diameter of the O-Ring material would also be important seeing as larger diameter O-Rings would have a longer contact surface than smaller diameters.

If I have a 1/8" diameter O-Ring in a current application could I replace it with a 1/16" diameter ring as long as I modify the gland so as to have the same clearance?

 

[unclesyd]

If you modify your gland to the recommended clearances for a specific cross section O-ring using the manufacturer's specification then the chart comes into play in respect to the hardness of the O-ring vs your clearance. In general you want to stay with the lowest hardness O-ring that meets your design criteria.

Remember this is just a guide and if there is any doubt contact the manufacturer.

 

[joe123456]

I see what you're saying, you can get the clearance from the chart, and then find a specific O-Ring with that recommended diametrical clearance. My problem is, the recommended clearance for different sizes is a fairly large range.

If, for example, you look at the chart on this page

http://www.marcorubber.com/glands_static_radiall.htm

O-Rings with a diameter of 0.07" have a recommended diametrical clearance of 0.002" - 0.005". From the same chart, O-Rings with a diameter of 0.275" have a recommended clearance of 0.004" - 0.007". What I'm wondering is if I determine (from the graph you posted) that I need a clearance of 0.005" can I use either a 0.07" or a 0.275" diameter O-Ring (of the same material) and achieve the same allowable pressure?

 

[israelkk]

Try Parker Hannifin Inphorm 2 program it is free.

http://www.parker.com/ead/cm2.asp?cmid=2973

they have an Americam and a Europien versions

 

[bigTomHanks]

Use the inphorm program first as israelkk suggested. I have used it a few times already for the exact same situation that you have. It only takes a few minutes to use and it gives you everything that you need to know.

 

[Cockroach]

As suggested by Isrealkk, Parker-Hannifin is perhaps the best in the business. I totally recommend downloading the Inphorm program from their website. There are several, polypak for example, in addition to elastomers (o-rings).

I would typically give you the equations that govern stretch and compression, but the Parker reference pretty much covers this and much, much more.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[rorschach]

You can improve your sealing capability by a huge margin by using back up rings (we used 30% glass filled PEEK, far superior to the 90 durometer rubber ones that Parker sells) to prevent the o-ring from extruding into the clearance gap. You should always strive to keep that gap just as small as practical and to keep the corner radius/chamfer as small as possible. I've designed o-ring seals that function far beyond what is generally considered possible by following these rules of thumb (20KSI nitrogen/water). you generally want about a 20-30% crush on the o-ring. Large cross section o-rings make that much easier to accomplish with conventional manufacturing tolerances, small cross section o-rings should be used only when larger cross section o-rings won't work due to geometry limitations.