O-Ring Squeezes- Static or Dynamic? Does it really manner?
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    Default O-Ring Squeezes- Static or Dynamic? Does it really manner?

    I copied this from another thread to here so the other thread does not get cluttered up from this being off topic. There has been several already mentioned comments made on the other thread. Maybe one of the moderators can move the comments over to this thread. Ken

    Does it really matter if it is a static seal? Or a dynamic seal? It really doesn't really matter...


    I've been doing engineering for nearly 40 years in the profession I work in. Only had one or two fellow engineers I had to sit down and explain why you can't do it that way. And after they fix their mistake, turned right around and did the same mistake again! Eventually layoffs got them. I thrive on the shop giving me feedback on just about anything that is questionable as to changing a feature to make it easier to machine to why is it designed this way? Why can't it be done this way? My door is always open for the shop guys to come in and visit! And yes, I have machine shop at home where I do a variety of things, too. Ken

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    I've been using a O-Ring squeeze chart that originated from BakerHughes way back in the 1960's. It is based on the dynamic seal dimensions from the Parker book. The main differences is, it has much more tolerance allowed for machining to and also old school 63 finish in the bottom of the groove or sealing surfaces and 125 finish on the sides of the grooves. Still it has a 10% minimum squeeze designed into it and additional squeeze for adjacent seal surface misalignment. It is the standard we use in the work we do and is shared with over two dozen oilfield equipment manufactures world wide. Normal design parameters are 10,000 psi, and temperatures up to 275 F in Nitrile, 350 F in Aflas and certain Viton compounds. Of course, temperatures over 225 F we use anything from glass filled Teflon to PEEK for backup rings. I can provide the charts if anyone is interested. It won't happen today. Ken

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    Yes please - can you scan and post the info?
    Thanks

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    Download Parker o - ring handbook. Pdf

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    I guess you need to read and understand that other thread in order to understand this one. Since the contents of that thread are not here, could you at least post a LINK to it?

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    I think the most important factor of a dynamic O-ring is having a groove designed with enough length so that the ring can roll slightly for each direction reversal. This is intended to bring a lubricated surface into the wipe zone. If the groove is too narrow, or stuffed with backup rings that prevent this, then the O-ring will simply wear out in short order.

    Naturally, the clearance between piston and wall will affect the minimum effective compression of the ring should the piston be forced off center. So sufficient compression needs to be spec'd into the groove depth to ensure that some compression remains in that side loading situation.

    Static rings are pretty simple in comparison, since the assembled clearance never changes, and lubrication is unnecessary.

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    I've been using the Trelleborg o-ring calculator for years. It's great. Google it and download it. Just a useful exe file to have on your desktop.

    Note that if you are used to NBR O-rings then the you are in for a surprise when installing higher grade materials such as Viton o rings. To say they are harder to stretch and compress would be an understatement 😀

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    Quote Originally Posted by deltap View Post
    Download Parker o - ring handbook. Pdf
    Request a hardcopy, they sent me 2 for free

    Sent via CNC 88HS

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    Quote Originally Posted by HuFlungDung View Post
    I think the most important factor of a dynamic O-ring is having a groove designed with enough length so that the ring can roll slightly for each direction reversal. This is intended to bring a lubricated surface into the wipe zone. If the groove is too narrow, or stuffed with backup rings that prevent this, then the O-ring will simply wear out in short order.

    Naturally, the clearance between piston and wall will affect the minimum effective compression of the ring should the piston be forced off center. So sufficient compression needs to be spec'd into the groove depth to ensure that some compression remains in that side loading situation.

    Static rings are pretty simple in comparison, since the assembled clearance never changes, and lubrication is unnecessary.
    Hadn't heard about rolling the o-ring within the groove. Lubrication is also dependent on surface finish. The idea is that a finish that is too smooth will be wiped entirely clean, preventing lube from getting into the seal interface zone at all.

    And as to gap and misalignment, that's why I thought .063" depth was so scary in a dynamic application. You should never go less than .007" squeeze on an o-ring of any size, or 5% (whichever is greater). .063" depth on your groove for an .070" o-ring is already the bare minimum, if you know you'll never get a ring in the lower half of the tolerance band, and your piston is RIGIDLY held on the center of an infinitely stiff bore. Ouch. The other thing to note about piston-bore clearance is that it affects maximum pressure. More gap = less max pressure.

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    Here's the link to the other thread where this thread was started from.

    Convincing the new engineer his idea is a waste of time

    I'm still working on putting the document into a file PM will allow to be posted. Wish they would open up the maximum file size for a pdf file to something like 500K mybts.

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    Do you have a link to request a hard copy, all I could find was a PDF. Thanks
    Quote Originally Posted by Cole2534 View Post
    Request a hardcopy, they sent me 2 for free

    Sent via CNC 88HS

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    Quote Originally Posted by true temper View Post
    Do you have a link to request a hard copy, all I could find was a PDF. Thanks
    Gotta call or email.



    Sent via CNC 88HS

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    Oh, great. It would be that one.

    OK, I asked for it so I will read it.

    Thanks!



    Quote Originally Posted by 4GSR View Post
    Here's the link to the other thread where this thread was started from.

    Convincing the new engineer his idea is a waste of time

    I'm still working on putting the document into a file PM will allow to be posted. Wish they would open up the maximum file size for a pdf file to something like 500K mybts.

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    After reading both this thread and the other, somewhat long one, I still have a question.

    What is the exact meaning of the terms "static" and "dynamic" when referring to o-rings? The simple answer seems to be that dynamic would refer to a situation where one part would be moving across the o-ring which would be in use as a seal against some internal or external pressure. And static would refer to their being used as a seal between two parts that do not move in relation to each other. But is that correct? And do these terms only apply when actually creating a seal is necessary? What about OTHER uses for o-rings?

    I can think of at least one additional use for o-rings that no one even considered in that other thread. An o-ring can be included in an assembly to PREVENT movement of one of the parts relative to the other one. The other thread was talking about a piezo switch, which was probably just a push button style switch. Having worked in electronics, I have seen many places where switches have become loose and would rotate in their mounting holes: this applies to push button switches as well as rotary switches. Many switches have a mechanical feature in that mounting hole like a key or a D shaped hole to prevent this rotation. But many do not and they depend only on friction of the tightened nut. I can see a possible, legitimate need for using an o-ring to prevent this rotation. AND, that o-ring would need to be deformed a lot more than an analysis of it as a seal would indicate. I have seen things like coaxial connectors that had optional o-rings included with them and I almost always used the o-rings, as much to prevent rotation as for any sealing that may or may not have been needed.

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    In the O-Ring seals I use and the rest of the down hole oil tool industry uses, there is no "static" or "dynamic" O-Ring squeezes. Yes, we use both but the dimensions are the same, or the same squeeze i used for both. Yeah I know, that is not so with Parker or Parco and others out there. I do know this, you cannot use a static sealing O-ring squeeze in a dynamic application. The O=Ring will roll on you. I've seen the aftermath of this. I've seen almost all of the O-ring failures shown in the Parker handbook at one time or another in tests I've run that had O-Rings installed. In fact, one test I did had 12 of the failure modes shown in the Parker handbook. Of course the Parker handbook I had back then would be 30 years old today.

    Anyways, here are the pictures of the design guide I use today for O=Ring groove design. Have fun guys!!!!
    Attached Thumbnails Attached Thumbnails baker-o-ring-design-4.jpg   baker-o-ring-design-3.jpg   baker-o-ring-design-2.jpg   baker-o-ring-design-1.jpg  

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    Here's the rest of them
    Attached Thumbnails Attached Thumbnails baker-o-ring-design-8.jpg   baker-o-ring-design-7.jpg   baker-o-ring-design-6.jpg   baker-o-ring-design-5.jpg  

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