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• # O ring calculations.

Discussion in 'Calculations' started by Bert, Apr 6, 2013.

1. ### proinwvMember

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I remember in the distant past of an article explaining that in gas service, extreme pressures could develop between the two o-rings. I don't recall where that was written. You might want to confirm or deny that information.

www.ostand.com

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4. ### proinwvMember

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I suggest that you call Parker. They would have the answer and my experience with them is positive.

5. ### proinwvMember

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Oh, if you get an answer post it for all to see. Even if I am wrong

6. ### Paul.RMember

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I've done a lot of design with o-rings (and other kinds of seals) for pressures as high as 30ksi. A few things to consider in any sealing application (besides cost) would be:

1. What is the sealing pressure?
2. Will there be any relative motion between the seal and any sealing components, and if so, is the motion translational or rotational?
3. Operating temperature
4. Chemical compatibility

Regardless of your answers to the three questions above, you will need to cut a groove in either the bore or the shaft - this is called the gland (it's usually much easier to cut a seal gland into a shaft). You will also need to specify chamfers on both the shaft and the bore, and the transition of the chamfers to the bore and shaft surfaces should be rounded as well - this is necessary to prevent the o-ring from being cut as it is assembled. The surface finish of the shaft up to the seal gland should be 32 rms or less, and the surface finish of the bore should also be 32 rms or less as well - any rougher and you'll risk damaging the seal on installation.

The extrusion gap (this is one-half the bore diameter minus one-half the shaft diameter) depends on the pressure you need to seal against - and whether or not there is any relative motion between the sealing parts under pressure. If there is relative motion (which I don't think there is in your application) under pressure - sealing gets a little more complicated...

7. ### proinwvMember

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And after all your research and design, TEST.

8. ### craigwatkinsonNew Member

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Having worked as a seal design engineer for over 17 years, I would advise against a double o-ring in this type of application, as stated previously even in liquid service a pressure trap can occur. Not only does this increase the removal force but once the trapped fluid is released it can be very dangerous with the risk of injury. Depending upon the fluid I would opt for a PTFE finger spring type seal as the primary and elastomeric o-ring or T-ring as secondary.

if it's a gas media then check compatibility and permeation but a urethane lip seal could replace the PTFE lip seal as a primary with the same elastomer contact secondary.

One last general tip, use the largest cross section of seal you can, unless it's a gas media (large cross sections will increase permeation) if you can use a compatible grease between the seals to help with a gas barrier.

You can download the Parker o-ring handbook, or Apple rubbers handbook or alternately contact a reputable sealing company for more advice. I work for Greene, Tweed and with offices all round the world you can get the advice and seals you need, gtweed.com is their homepage.