Insane Hydraulics

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About the O-ring Patent and More...

I like making interactive pages (like the gear pump displacement calculator, the two-spring torque limiter simulator, the interactive pilot-to-open check valve pressure chart, the hose sizing nomogram, the 7-cavity orbital motor animation, and so on...), and so I decided to make one that would help me with sizing o-rings for given grooves - something that I do quite often, usually employing my notoriously inaccurate ball-parking skills.

Measuring hard grooves is much easier than measuring flimsy deformed o-rings, but then choosing the right o-ring size by the gut feeling alone often feels inappropriate, at least to my "scientific self", and my initial idea was to code a page that would make a dimensionally accurate drawing of a gland from a user's input, and then place an o-ring of a chosen size in it accounting for any stretching or compression, and also provide the numbers that will allow the user to decide if a given o-ring is the right choice for the given cavity.

I am assuming that the o-ring volume stays the same when it is stretched or compressed over or into a gland - and this is a simplification, of course, but I think it's still a better way to calculate the squeeze, stretch and the gland fill percentage than the traditional eyeballing method - and it also should be a great visual aid for teaching folks about them o-ring seals.

I first thought this would be a matter of quickly hacking a few lines of code together - but it turned out to be more challenging than I anticipated (because there are actually quite a lot of things one needs to account for - male gland vs female gland vs face seal, dynamic vs static application, presence of 0/1/2 back-up rings, the recommended (by the ISO 3601 standard) o-ring deformation to determine how good (or bad) a fit an o-ring is (plus more stuff I can't think of right now). In other words - this will definitely take me more than a weekend (especially if I want to plug in at least one standard o-ring table) so I won't be presenting a finished app today. I did, however, upload a temporary un-styled prototype that calculates only a face seal - and you can try it out here, the full version (whenever it's ready) will, of course, be linked in the Library.

But there's something else I wanted to talk about today - and it's the first (American) patent for the o-ring, which I came across when I was researching on o-ring seals for the said app. It is extremely cool! It is so interesting to see how people used to look at things at the dawn of industrial hydraulics and pneumatics. And the language is beautiful, too. (I secretly wish people talked like this today).

So, the big name for us to remember is Niels Anton Christensen - the Danish-American engineer who invented and patented the o-ring seal. You can check out the full patent text here, but I still want to comment on some abstracts that I found particularly interesting:

"...An important object of the invention is to provide a resilient packing ring associated with the packed members in a manner that it will be continually kneaded or worked to enhance its life..."

Can you imagine your life without an o-ring seal? I certainly can't! But that was the reality back then - and stuff like leather or rope was commonly used for the so-called "stuffing box service" in hydraulic and pneumatic cylinders.

"...My improved packing rings to seat in such grooves... ...are made of solid rubber or rubber composition very dense and yet possessive of great liveliness and compressibility..."

This charming style is something that I love about old patents!

"...Each packing ring is perfectly circular in cross section prior to its assembly and insertion in a cylinder, and possesses a normal circular cross section..."

Guys! He's explaining what an o-ring looks like to people who have never seen or heard of an o-ring in their life!

"...it is advantageous that the packing ring be not compressed solid within the groove but that the dimensions of the latter be such that when the ring is assembled with the piston, and cylinder, it has slight freedom of movement to be worked or kneaded during the piston reciprocation in order to maintain the rubber in a live state and to prolong its serviceable life..."

Mr Christensen realized that an o-ring gland should never be filled at 100%. This is common knowledge today, but back then it was a genius foresight, in my opinion. He also, quite correctly, imagined that an o-ring would be shifting in its cavity under pressure and that if the cavity is sized correctly - this shifting would prolong the seal's life (in comparison to static rubber seals).

"...I. have found in one embodiment of the invention, that a circular section ring having a section diameter of three-sixteenths of an inch serves admirably for the purposes of a packing of small hydraulic cylinders, when the ring is compressed about one sixteenth of an inch when installed thereby causing the normal circular section of the ring to flatten out to the extent of about three-thirty seconds of an inch on its opposite sides..."

I suppose they used to work with a pretty soft rubber back then - because he's talking about a 30% squeeze here!

"...In cases where the packing ring almost fills the groove as shown in Fig. 7, and when the piston is moved in the direction of the arrow H, adhesive tendencies of the ring to the cylinder wall tends to twist the ring counter-clockwise and create an annular fin or projection 27 on the trailing side of the ring between the piston and cylinder wall..."

This is the first description of o-ring extrusion in the world! I am sure that Mr. Christensen would love modern plastic backup rings!

Anyhow - I thought this would be something cool to share and I hope that at least for some of you this was new. And yes, do give the o-ring sizing prototype app a try and let me know what you think.