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Thread: Threading and Oring gland help

  1. #1

    Threading and Oring gland help

    Orings
    For standard -0xx orings in dynamic applications what is considered a generally acceptable gland spec? Yes, I know they vary by circumstance, but what is considered about average?

    Stretch % -
    Squeeze % -
    Extrusion Gap -
    Break edge radius (top) -
    Bottom edge radius -
    Gland fill % or gland width (I built a calculator that derives gland width from the desire fill %) -

    Threading
    I don't know why I am struggling with this so much... When I used to do machine design we always bought these components and since we had an in-house machine shop, we just specified the thread (they determined class, etc). I need a super shallow pitch... something similar to the DM bolt can or the MacDev valve can. I have limited depth so I was thinking a custom ACME thread would work, but I don't know if most machine shops could make it?

  2. #2
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    I usually default to a 28 or 32 TPI thread size for an internal part like your assembly, which tends to be okay as long as you can get 0.150-0.200 length of threaded section. For stability it's good the have more threads available, but you loose actual threaded length when you add in any relief grooves at the start or end of the threaded area. But having a full 0.150 section threaded with a 32-TPI thread will give you a good 5 rotations which might be enough. Good place to start anyway.

    You can see if something like that will fit by taking your male threaded component and sizing its diameter to the nominal major diameter (1/2" dia, 7/16" dia, etc) and assume the threads would be [1/TPI] deep on the radius. In reality the threads won't be that deep so it leaves some wiggle room.
    The male side is easy because the diameter is nominal, minus a small allowance if you really care. But the female size you'd have to look up in machinist handbook (or calculate if you're into that sort of thing). I prefer to look it up in a spreadsheet that has the allowance fits. Lemme see if I can attach it to this post

    Acme can be tricky because there's supposed to be a proportional link between the thread depth and thread pitch, specifically pitch is supposed to be 2x the depth (before all the allowances and other details). It'd definitely give you a lot of strength but it might not be necessary. Some shops hate dealing with parts liek that since you have to get creative to measure the thread, if it's out of their comfort zone...
    Attached Files Attached Files

  3. #3
    I am actually less concerned with the can... I have reworked my design almost entirely to make it easier to machine. You guys have been fantastic in that regard. I am actually looking to make the bolt itself 2-pieces so the pocket can be made MUCH easier. The other option I'm considering there is a heat releasable chemical bond. I would add a couple tiny grooves to let the adhesive bond things better or I have even been contemplating an interference fit there too. The seam will be under the spring and no seal would touch the joint on the ID.

    If you look at a Pilot G-2 pen then you can see some ULTRA shallow threads that are actually quite strong and that's in plastic... They appear to be true square threads which I've read are a beast to make. I would supplement the threading with an epoxy anyway, but it just means they won't pull apart. It also makes the post-anodizing assembly process easier.

    The latest rev of the can actually did away with the threading altogether until I realized then people can remove the assembly while it's aired up. Doh! Needless to say it has to be threaded.



    Any guidance on the oring glands?

    - - - Updated - - -

    I am actually less concerned with the can... I have reworked my design almost entirely to make it easier to machine. You guys have been fantastic in that regard. I am actually looking to make the bolt itself 2-pieces so the pocket can be made MUCH easier. The other option I'm considering there is a heat releasable chemical bond. I would add a couple tiny grooves to let the adhesive bond things better or I have even been contemplating an interference fit there too. The seam will be under the spring and no seal would touch the joint on the ID.

    If you look at a Pilot G-2 pen then you can see some ULTRA shallow threads that are actually quite strong and that's in plastic... They appear to be true square threads which I've read are a beast to make. I would supplement the threading with an epoxy anyway, but it just means they won't pull apart. It also makes the post-anodizing assembly process easier.

    The latest rev of the can actually did away with the threading altogether until I realized then people can remove the assembly while it's aired up. Doh! Needless to say it has to be threaded.



    Any guidance on the oring glands?

  4. #4
    Yeah, I'm not a huge fan of ACME threads either, and I tend to run with 28 or 32 tpi too. I have often adjusted diameters to fit what ever I need and not worried about it. You don't need to fit a certain size if you are making both parts.

    O-rings are a tricky one and I have adjusted them depending on pressures, size, material, use etc. I tend to err on a slightly wider groove than most and fit to the moving surfaces with more back clearance (again depending on circumstances) I find they seal more reliably on the breaking edge of the way we use o-rings.

    What sizes and pressures are you looking at? Might be able to give you more specific help then.

  5. #5
    o-ring fits are so hard to get right on the first try.

    that being said, i don't design oring fits too often. what the "book" says is almost always wrong though haha. in paintball, i'd assume much much bigger, with less stretch.

    - - - Updated - - -

    o-ring fits are so hard to get right on the first try.

    that being said, i don't design oring fits too often. what the "book" says is almost always wrong though haha. in paintball, i'd assume much much bigger, with less stretch.
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  6. #6
    Quote Originally Posted by Simon View Post
    Yeah, I'm not a huge fan of ACME threads either, and I tend to run with 28 or 32 tpi too. I have often adjusted diameters to fit what ever I need and not worried about it. You don't need to fit a certain size if you are making both parts.

    O-rings are a tricky one and I have adjusted them depending on pressures, size, material, use etc. I tend to err on a slightly wider groove than most and fit to the moving surfaces with more back clearance (again depending on circumstances) I find they seal more reliably on the breaking edge of the way we use o-rings.

    What sizes and pressures are you looking at? Might be able to give you more specific help then.
    It's been a long time since I spec'd a thread... Even at my old company I didn't have to say thread class or clearances. The machine shop knew what we needed, but when outsourcing I need that information included. What I'm looking at is if you look at my axe design, I need a shallow thread for the can so I can keep it thin.

    As for the orings, I have 2 that are identical to the stock axe bolt application, but I'm not overly concerned with the seal fidelity in the rear location. I have another one that seems to be an ideal candidate for a floating seal. I don't need great seal fidelity there either, but less friction is important. Then I have the powertube orings that'll be ~-011. Those are dynamic and seal critical. All will be ~130-150psi.


    Quote Originally Posted by cockerpunk View Post
    o-ring fits are so hard to get right on the first try.

    that being said, i don't design oring fits too often. what the "book" says is almost always wrong though haha. in paintball, i'd assume much much bigger, with less stretch.
    That's exactly my problem. I came from industrial automation design and either the oring vendor designed the gland or we followed Parker's. In the case of paintball, it's low pressure and friction sensitive. In a general sense that means that I want as little of the cross section to rub the part, but I don't know if I do that with high stretch, high gland fill and low squeeze or what. It's always the little things.

    - - - Updated - - -

    Quote Originally Posted by Simon View Post
    Yeah, I'm not a huge fan of ACME threads either, and I tend to run with 28 or 32 tpi too. I have often adjusted diameters to fit what ever I need and not worried about it. You don't need to fit a certain size if you are making both parts.

    O-rings are a tricky one and I have adjusted them depending on pressures, size, material, use etc. I tend to err on a slightly wider groove than most and fit to the moving surfaces with more back clearance (again depending on circumstances) I find they seal more reliably on the breaking edge of the way we use o-rings.

    What sizes and pressures are you looking at? Might be able to give you more specific help then.
    It's been a long time since I spec'd a thread... Even at my old company I didn't have to say thread class or clearances. The machine shop knew what we needed, but when outsourcing I need that information included. What I'm looking at is if you look at my axe design, I need a shallow thread for the can so I can keep it thin.

    As for the orings, I have 2 that are identical to the stock axe bolt application, but I'm not overly concerned with the seal fidelity in the rear location. I have another one that seems to be an ideal candidate for a floating seal. I don't need great seal fidelity there either, but less friction is important. Then I have the powertube orings that'll be ~-011. Those are dynamic and seal critical. All will be ~130-150psi.


    Quote Originally Posted by cockerpunk View Post
    o-ring fits are so hard to get right on the first try.

    that being said, i don't design oring fits too often. what the "book" says is almost always wrong though haha. in paintball, i'd assume much much bigger, with less stretch.
    That's exactly my problem. I came from industrial automation design and either the oring vendor designed the gland or we followed Parker's. In the case of paintball, it's low pressure and friction sensitive. In a general sense that means that I want as little of the cross section to rub the part, but I don't know if I do that with high stretch, high gland fill and low squeeze or what. It's always the little things.

  7. #7
    I tend to rarely if ever stretch an o-ring, unless I absolutely HAVE to fit a weird size. With the low friction we use in paintball that tends to leaky o-rings from my experience.

    For an external o-ring riding in a bore. I tend to size the bore to the stated OD of the o-ring. So for say an 015 that would be a bore of 0.6875", I would make the groove 0.075" wide to start with and open up if needed to maybe 0.085" and the internal diameter of the groove 0.075" from the bore the o-ring is sealing against so 0.5375". Pick your clearances between parts and see what happens.

    No idea if that conforms to the book but it means I have had great luck with getting o-rings that seal and work first time in paintball guns. I will adjust from there if I have issues.

    I reverse the process for internal o-rings.

    A 1** series I make the groove 0.11 and use 0.11 as the distance from the riding bore to the groove bottom surface.

    Different o-ring materials and manufacturers will cause inconsistencies.

  8. #8
    I just got the measurements from a MacDev Clone GT powertube and they use a -011 and the bore hole through the middle of the tube is .315." The ID of a -011 is .301". That means they're running an insane amount of stretch.

    The method you described is almost per the book for a floating seal. I have no idea how that even works when the CS of a -0xx oring is only .070". You have no squeeze to even hold the seal so you're basically relying on extrusion to do the job. Even still "the book" says that you can't use that type on internal (rod) seals. That just agrees with cockerpunk that the book is almost always wrong.

    I read Ydna's guide, but it gets to a point where he says try and see. Lol, NOT a cheap method.

  9. #9
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    LOL yeah the "try and see" is only applicable for situations where you can make some test parts on a lathe and see how things go. Nothing functional of course but still it requires some machine time. I wrote that page of the site just because I received the question a lot and never really had an answer. I originally just yanked dimensions from other parts in use, piggybacking on their R&D, but that only gets you so far. Especially in the case of the Clone powertube like you mentioned...they have to stretch the o-ring in that spot because if it were a looser fit then the o-ring gets physically blown out of its groove by the air trapped under/behind it. Porting would trap the seal and keep them from getting blown out, but it's not possible to port a bolt like that internally. Well, not cost effectively anyway

    Anyway, as general rule I'd say when in doubt make the o-ring fit tighter, rather than looser. If it's too loose the components might leak while moving...a problem that extremely hard to pinpoint! If the fit is too tight, it generally works okay but requires more lubrication. Of course there's exceptions but it gets back to the "try and see" methods..

    Myself I don't follow the precribed rules for o-ring stretch and compression either. I find that some designers do, some don't, some like Dye like going to next size up, etc. I don't like stretching them unless required. Like you mentioned, the industrial side of things work VERY differently.
    Besides for performance, I find the more difficult it is to remove the seal, the more prone to "operator induced" damage there will be. You have to take out a dental pick or small allen key and dig at the o-ring to remove it, stuff just gets scratched easier.

  10. #10
    Insider PBSteve's Avatar
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    Actually I'm gonna hijack this thread a little, I've got a question on threads as well.

    Could anyone give me guidance on how to indicate to a shop where to "start" a thread so two parts align in a somewhat specific way? I know this can't be used to locate parts to significant accuracy, I've just got a part here where I'd like the part to align to any point in an approximately 90* range.

    Any help on what I'd put on the callout would be greatly appreciated.
    Last edited by PBSteve; 12-11-2013 at 10:54 AM.
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