Page 2 of 4 FirstFirst 1234 LastLast
Results 11 to 20 of 36

Thread: Threading and Oring gland help

  1. #11
    Quote Originally Posted by blueshifty View Post
    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.
    As Andy points out there are times when you do things differently for specific reasons and that looks to be the case with the Macdev situation (it also was when I was doing a version of the Freestyle valve system with the tip o-ring on the center shaft that always wanted to come off). Sometimes you are constrained in the space/location and it causes different decisions. I haven't found a perfect science to it, but have found my method is usually a good starting point for most of the things I design.

    I pretty much always rely on extrusion to seal the o-rings with the lower durometer o-rings, and it seems to work well for the low forces and low frictions we find in many paintball situations.

    Can't say I have looked at the book for a very long time. Much of the standard industrial methods they specify just simply doesn't work for paintball. Much of what I do comes from years of doing it and some amazing conversations with someone I consider to be a pneumatics Guru, Mike Woods (of Nova, SuperNova, Assault etc. fame)

    I've had many o-rings that ended up with a little too large a groove diameter and too thin a groove cross sections such that they leaked, and simply putting the parts back on the lathe and opening them up got everything to seal up perfectly.

    Quote Originally Posted by PBSteve View Post
    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.
    That's called indexing. You need to specify the parts that need to align and give an angular tolerance. Some machine shops will hate you for it, others won't care. It can be a HUGE PITA to get right after you take into account tolerances and finishes. Try not to make it overly critical if you can.

  2. #12
    Insider PBSteve's Avatar
    Join Date
    May 2012
    Location
    Los Angeles
    Posts
    2,757
    Quote Originally Posted by Simon View Post
    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.
    I may or may not be telling you something you already know, but Figure 3-3 in the bible is actually pretty good for making estimates on loss of diameter due to stretch. I follow the "observed" curve.
    A Radiant Purple Sky Ribbon That Defies Explanation
    I work for the company building the Paragon...once we figure out a name

  3. #13
    Insider
    Join Date
    Feb 2014
    Posts
    1,578
    So, here I am again trying to learn from those of you with much more experience.

    If you had a dimension on a dynamic seal that fell right in between 2 oring sizes how would you handle it? Stretch and the smaller size or just make the gland deeper and go up a size?

  4. #14
    I usually look for a metric size in the middle, increase the bore if possible to properly fit the larger size, or stretch the smaller size ( in that order of preference). Unless im trying to prevent blowout like mentioned in the MacDev or freestyle variant mentioned where stretching gives an advantage other than fit.

  5. #15
    Insider
    Join Date
    Feb 2014
    Posts
    1,578
    In the event of needing to go with the stretched option, do you reduce the gland depth? Running a .075" depth worked fantastic when I could control the bore and not stretch the oring, but since the bore is dictated in my case I need to stretch to get the right fitment. I am not sure a whether a floating type gland is still appropriate or if I'd go with a traditional bore squeeze gland spec.

  6. #16
    Depends on the amount of stretch and the elasticity remaining after stretching. The smaller the percentage of stretch the more elasticity remains available for a semi-floating seal to work. If you stretch the equivalent of several sizes like the clone the more you want crush.

    One of the more useful things found in the parker guide, and of the few that pertains directly to us, is a chasrt that compares percentage stretch to percent loss of cross section. You can work that backwards to determine gland diameter based on you chosen o-ring, bore diameter, and desired squeeze.

  7. #17
    Insider
    Join Date
    Feb 2014
    Posts
    1,578
    I actually designed that into a spreadsheet, then I started this thread. Now I have the "Simonized" oring gland spec table.

    I suppose I could run a metric, but I just hate them unless absolutely needed. It limits the material selection.

  8. #18
    Insider
    Join Date
    Apr 2012
    Location
    Manchester, England
    Posts
    699
    Supports Inception Designs
    You have to registar, but I've used this in the past: http://www.tss.trelleborg.com/global...alculator.html

  9. #19
    Insider
    Join Date
    Feb 2014
    Posts
    1,578
    In the instance where I need to rely on crush, what % do you guys like for low friction and an adequate seal?

  10. #20
    Insider
    Join Date
    Apr 2012
    Location
    Manchester, England
    Posts
    699
    Supports Inception Designs
    Are you talking about having an internal groove, with an o-ring sat in it, and a shaft running on the ID of the seal? But where the shaft is a smaller diameter than the ID of the seal, so you reduce the ID of the internal groove to place crush on the o-ring to bring it down to the shaft size you are running through it? Is that the scenario you are asking about?

    Jack

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •