O - Rings, bit of help needed

So, my "Get-out-of-Jail-Free" card has expired. ie. I can't find the chap I normally call for advice with this is MIA and I suspect he left his present job and has gone elsewhere. No surprise really as he was very good. But that now leaves me with the problem that I have pretended would never arise: Which 'O' ring to use for a specific job. I throw myself at the mercy of the forum!

The problem: I have a piston (in brass for the sake of pedantry) which measures out at 29.5mm, the Bore is 30.5mm (Chrome). Which size 'O' ring do I use to create a seal? The piston is compressing air to approx 750psi and I would normally on something this diameter use two 'O' rings one right behind the other and a nylon skirt ring for smoothness.

I prefer to use Viton rings but now, as my contact has left, I have no reference. If someone could point me in the right direction for some specs. I would be grateful. Depth and width of groove are also quite critical.

Thank you,
graham.

James Walker web site offers a free 'O Ring Guide' covering materials, housings etc. Other manufacturers may also offer similar technical material.

The ERIKS "O" ring calculator is very good, just select material, size etc and it does the rest, an online search for Eriks O rings, should bring it up

Took some finding but HERE is the calculator for anyone looking in future.

graham.

Has anyone actually used the Eriks online calculator successfully I wonder?

I need to design several O ring sealed joints and have just spent the last hour struggling to make sense of how it works. I am baffled as to how, once having saved a design (after registering and logging in etc) that design is reloaded to edit or print out.

I can find no instructions in how to use the calculator but I think there may be a clue to its failings in that the features of this free design calculator include the ability to SAVE designs. However nowhere do that say it can load designs!

I think I might be able to get the details and dimensions I want by writing down the various intermediate results and then entering them again on the keyboard when needed for the next step.

No wonder it took you a while to find it Graham, maybe they hide the results as well.

Ian P

Just tried it, seems to work.

What you will find is if buy three o rings from three sources to a BS standard most likely they will actually measure different.
So what you have inputted will change when o rings come.

It would be quite usable if the link provided off the shelf o ring sizes rather than input size of parts only to find nothings remotely available. For this to work would have to input actual o ring size.

Very rarely I draw things out and tend to either compress the o/d or expand the bore or even both. Some try to tell you to create clearance on a piston bore so floats with minimal contact on I/d of bore, the pressure creates o ring to contract and expand to create the seal.