Insane Hydraulics

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If You Use O-rings Every Day...

Everybody likes o-rings. What? You don't? Well, you may not like o-rings, but if you work in this business, I bet you use them all the time. I have found over the years that all techs follow more or less the same "evolutional pattern" in regards to the ubiquitous o-ring seal. I want to share my observations (note that these observations are only valid for "classic" hydraulics - i.e. mineral oil and industrial temperature and pressure ranges):

The starting point for all hydraulic techs:

O-rings are doughnuts molded from rubber that seal the gaps between surfaces so that our beloved hydraulic oil stays confined.

Evolution in regards to

O-ring Material:

1) O-ring material? Is this even a thing? All o-rings are made from rubber, aren't they?

2) There are "normal" o-rings and "high temperature" o-rings. Also - very important - green o-rings should be replaced with green o-rings and black o-rings should be replaced with black ones.

3) Viton, FPM, and FKM are the same thing!

4) Just read bout the synthetic rubbers used for o-rings. Damn! ACM, CSM, EPDM, FPM, FEPM, CO, ECO, BR, NBR, SBR, XNBR, FMQ... I'll stop now.

5) You know what? I think I'll go back to categorizing my o-rings as normal (NBR) and high-temperature" (Viton) o-rings.

O-ring Hardness:

1) O-ring hardness? Is this even a thing? Aren't all o-rings the same?

2) There are "hard" o-rings and "soft" o-rings.

3) There's actually this thing called shore-A hardness scale, and the o-rings that I used to call "hard" are 90-shore, and "soft" are 70-shore. Who could have thought?!

4) Saw my first durometer today. Such an interesting contraption!

5) Just read about harness of things. Damn! Aside from Shore-A, there are also B, C, CF, D, E, DO, M, OO, OOO, OOO-S, and RR. I'll stop now.

6) You know what? I think I'll go back to categorizing my o-rings as "soft (shore-70)", "hard(shore-90)", and "in-between" (shore-80-ish)!

O-ring Sizing:

1) Sizing? Easy! Just put one o-ring next to another and keep comparing them till you find one that kind of looks the same...

2) I learned to use a caliper, and I know that o-rings are sized by their cross-section and internal diameter.

3) It is much easier to measure grooves than o-rings (that is - when you have access to the respective grooves). And then choose o-rings that are a "tiny bit " fatter than the groove width.

4) Just read an o-ring groove sizing manual. Damn! Static sealing, dynamic sealing, recommended o-ring squeeze, recommended gland fill, concentric and diametric clearances, hardness vs pressure, and clearance gap... I'll stop now.

5) Just learned about o-ring size standards. Apparently, manufacturers and various international organizations, in their attempt to standardize o-ring sizes, managed to create a "standard mess" that can only be compared to (and, possibly, surpassed by) the standards for fastener threads.

6) You know what? I'll do my best to stick to standard cross sections our supplier stocks and uses the most (possibly the AS568) for my designs and choose standard sizes for my orders. Also, 15% squeeze and 75% fill is a good enough ballpark for most of my applications.

Now let me explain my point - if you use o-rings all the time, and by "use" I mean - not taking o-rings from a seal kit, but finding yourself in a situation where you need to determine the correct size for an o-ring (or an o-ring+ back-up seal combination) so that you could order a correct replacement, you can't solely rely on a "gut feeling" that tells you to use a "tiny-bit fatter than the groove o-ring".

Educate yourself about o-ring/groove size recommendations and standard sizes - and you'll see that choosing and ordering o-rings becomes a lot easier, and you'll also learn that a lot of o-rings that you require all the time (or that come in OEM seals kits) are, actually, standard. For example, the NG06 (CETOP 3) interface o-ring is the AS568 number 12, or those o-rings for the ORFS fittings - AS568 numbers 11, 12, 14, 16, 18...

This is why I uploaded the standard o-ring sizes table last week - to be able to check if an o-ring I am looking for belongs to a certain standard. Why? Because standard o-rings are easier and cheaper to source. Also - asking suppliers for standard sizes usually results in faster quotes.

Now, with the fast-changing internet, I don't want to place links to resources that may disappear tomorrow. But what you can do (if you haven't done this already) is Google for things like "o-ring handbook", "o-ring groove design guide" and, maybe "standard o-ring size chart" - and you will find tons of good usable info provided by seal-selling companies and OEMs. Tons!

So, I guess this post should be considered as something along the lines of "I dare you to learn something new about this because it's important and good for you".

What? You know everything about o-rings? Can you tell me what the Joule Effect is? No? It's time to go and search for that handbook then...