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O Ring Groves

Grove size calculations

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Geoffery Mordan20/09/2009 12:39:12
3 forum posts

I am a beginner about to use "O"rings on pistons for the first time.

Piston diameter is 1.23"
and the" O" Ring outside diameter is 1.25".>>
 

Inside diameter of the ring is 1" and the ring cross section diameter is 0.125">>

 >>

Is it correct that the diameter of the grove in the piston should be the 1" I/D of the ring + 2% (to provide stretch)  i.e. 1.02"  and should the grove width be 1.5 X the ring cross section i.e. 0.125" X 1.5 = 0.1875"

> >

Confirmation or otherwise of suitable dimensions would be much appreciated>>

mgj20/09/2009 13:41:50
1017 forum posts
14 photos
The figure for groove width that you are using is the SAE figure.
 
For model engineering (low pressure) applications, a groove width for a 1/8 cord ring is .160"(Model engineers handbook).
 
Depth should be .132. (MEH).
 
The SAE figures which you quote,  should seal to 1500 psi, but at the cost of increased pinch and hence friction.
 
So says Tubal Cain. And he'm pretty well always right!
 

JasonB20/09/2009 14:08:49
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25215 forum posts
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Are you sure of those O ring sizes a BS214 1" ID ring would have a cross section of 0.139" not 0.125
 
Jason
Geoffery Mordan20/09/2009 14:20:15
3 forum posts
Thank you Meyrick.
Very useful information.
I obviously need to purchase the ME Hand Book.
 
Jason
With regards to the actual "O" rings -  I have purchased SR 214 1" ID 1.25OD from Chronos would have the 0.139 cross section?
 
Thanks for the advice.
JasonB20/09/2009 17:35:53
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25215 forum posts
3105 photos
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I would have thought so, the SR part of the code is likely to be Chronos own for "silicon rings" and the number is the B.S. of which the size ref of which the 2 represents the cross section, in this case 0.139" and the 14 is the number for a 1" ID ring of that section.
 
Gently put a mic or calliper on it and you should get a good idea of what the section is
 
Jason
mgj20/09/2009 19:46:00
1017 forum posts
14 photos
Are we not in danger of getting in a tiswas over actual and nominal.
 
Jason is quite right- actual should be.139 +/- whatever the tolerance is.. However the groove size quoted is for 1/8 nominal. (.139 actual)
 
Thus the .132 depth is giving the pinch/stretch and the .160 the ability to roll slightly?
 
Geoffrey - you want that MEH - its just so useful.
 
I'm doing piston rings - same chapter. So it has all that sort of useful guff in it.(I'm delighted to say that, having solved that horrendous equation in there, my answer came out to within .003 of the graphical solution plotted in ME a little while ago.  Thankyou Casio.

Edited By meyrick griffith-jones on 20/09/2009 19:46:30

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