# O-ring compression?



## milkyspit (Mar 19, 2010)

Please excuse what may possibly be a silly question, but when I'm making provision for the thickness of an o-ring in compression, is there a rule of thumb I should use, like 50% the uncompressed thickness? Does o-ring material impact the formula?

Thanks guys. Have wondered about this for a long while. :bow:


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## savumaki (Mar 19, 2010)

This page has a lot of great info on o'rings, type and material.

http://www.rlhudson.com/O-Ring Book/designing-dimensions4.html

Hope it helps. It's been many years since I was involved in the industry.

Karl


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## unterhausen (Mar 19, 2010)

rubber is incompressible, so if you want the o-ring to squish, there has to be a place for the displaced material to go.


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## PhotonFanatic (Mar 19, 2010)

Using Buna-N with a 70 Shore hardness A value, I usually plan on 20% of the CS width to be reduced. If you were making a dive light, then even more, say, 30-40%, might be warranted.

And, unterhausen makes a good point--leave room for the CS to expand. Nothing worse than cutting the groove equal to the width of the O-ring, only to find that the O-ring will be too tight. :devil:


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## savumaki (Mar 19, 2010)

PhotonFanatic said:


> Using Buna-N with a 70 Shore hardness A value, I usually plan on 20% of the CS width to be reduced. If you were making a dive light, then even more, say, 30-40%, might be warranted.
> 
> And, unterhausen makes a good point--leave room for the CS to expand. Nothing worse than cutting the groove equal to the width of the O-ring, only to find that the O-ring will be too tight. :devil:



Oh you'll find out- all that rubber has to go somewhere and it finds its way between the surfaces you want to seal :green:


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## precisionworks (Mar 19, 2010)

The Parker O-Ring Handbook is *the *bible for this topic ... here's the table of contents:

Introduction – Section I
Basic O-Ring Elastomers – Section II 
O-Ring Applications – Section III 
Static O-Ring Sealing – Section IV 
Dynamic O-Ring Sealing – Section V
Back-up Rings – Section VI
Compatibility Tables for Gases, Fluids, Solids – Section VII
Specifications – Section VIII
Sizes – Section IX
Appendix – Section X
Index – Section XI 

292 pages but it loads fast. An awesome reference 

http://www.parker.com/literature/ORD 5700 Parker_O-Ring_Handbook.pdf


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## PEU (Mar 21, 2010)

Barry beat me to post the same, I have the spanish version and was lazy to find the link for the english one. 
I recall without reading it again, for moving parts compression should be around 10~20% for fixed sealing it can go to 30%.


Pablo


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