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  • Locking a male and female thread...?

    Discussion in 'Machining' started by john12, Dec 5, 2018.

    1. john12

      john12 Well-Known Member

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      I'm working on something that two circular parts - a front and back.

      The front body just has a female thread and the back body has a male thread. So they screw together into a fixed assembly.
      The rear part also has another male thread at the other end that attaches to another connector which is frequently removed and reattached.

      The problem I'm finding is that over time the rear thread gets quite sticky (it's used in a dirty environment) and so when people try to screw off the rear connector they're accidentally unscrewing the front and back parts from each other and damaging the internal parts.

      I'm really pushed for space so can't make anything too complex... but I was thinking of just cutting a screw thread, down through the mated male and female parts, so each has a threaded hole, perpendicular to the main threads. A small grub screw would be screwed in here that essentially locks the parts together.

      The problem is - the two holes then have to line up exactly, so I guess the main threads always have to be cut in the same place. Am I setting myself up to fail here? Has anyone got any better ideas?

      Would it possibly to machine everything else, then screw the two halves together, then drill and tap the locking thread, so the holes always line up?

      Sorry for the vagueness - I can't disclose too many details!
       
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    3. GoodCat

      GoodCat Active Member

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      «The problem I'm finding is that over time the rear thread gets quite sticky (it's used in a dirty environment) and so when people try to screw off the rear connector they're accidentally unscrewing the front and back parts from each other and damaging the internal parts.»

      If this is a small connector that is often connected and disconnected, then it would be reasonable to use a non-threaded connection here. I think a bayonet connection would be the best solution. Does this solve your problem?

      Can you use an extra nut to lock?

      If the connections you want to strengthen are not disassembled, then it is probably easier to use a nylon or fluoroplastic ring to strengthen the connection.
       
    4. s.weinberg

      s.weinberg Well-Known Member

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      You don't need a hole through both parts. Just put the hole through your female portion, and put in your set screw to lock with friction. If you can afford a flat in a section of your male thread, all the better.
      I assume you are using some kind of locking something on the screw connection that you don't want to disassemble? Loctite, or a nylon patch or something?

      I actually have something of the opposite reaction as GoodCat. It's not your front thread I wonder about. If you don't want to be disassembling your two halves, why are they threaded together? Use a joining method that isn't meant for constant disassembly.
       
    5. john12

      john12 Well-Known Member

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      Thanks for the answers guys. The front body is occasionally disassembled (for the electronics to be serviced) so a permanent locking solution/thread lock can't really be used. The back thread (connector) is basically used every time the device is used.

      We have previously looked at a bayonet connector for the back part but couldn't get a decent and secure connection in the small space available (and also it means changing the entire existing ecosystem of connectors).

      I'd actually started a full redesign to remove the main threads from the front two halves but we just don't have the space (very thin walls) to replace the thread with parallel bolts.
      Now I'm thinking about just having the two halves push together (sort of like a Kinder Egg plastic shell) and then have a few small grub screws that go down through both the male and female overlapping wall section.
       
    6. s.weinberg

      s.weinberg Well-Known Member

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      If it's only serviced occasionally, something like a shaft with a retaining ring would be a very common and compact solution that should hold much better than a thread.

      EDIT: And thread locker (liquid or patch) doesn't prevent occasional service. It may need to be reapplied after a few uses is all.

      EDIT2: And if you don't want to change too much, another simple fix is to use a left-hand thread on the more permanent portion. That would make it tighten, rather than loosen, when you remove the part that you want.
       
      Last edited: Dec 6, 2018 at 8:03 AM
    7. john12

      john12 Well-Known Member

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      Left-hand thread is a great idea! I'll run that by the user group but I think that might be winner.

      For the retaining ring - would that be something like a circlip you mean?
       
    8. s.weinberg

      s.weinberg Well-Known Member

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      Yeah, I mean something like a circlip.

      And with the left-hand thread, you just have to worry about the opposite problem - loosening the left-hand thread when you tighten the right-hand one.
       
    9. john12

      john12 Well-Known Member

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      Yeah, this is the problem... maybe I'm just making it so problem will occur when adding the connector rather than removing it.

      I think I may have to try just having a push-fit with three small grub screws.
       
    10. s.weinberg

      s.weinberg Well-Known Member

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      If you want to over-complicate things, you can put a left-handed external thread on your central piece, and hollow out the center, putting an internal right-handed thread inside the externally threaded rod.
      Then (left handed) thread your front piece on, and finish with a (right-hand) thread fastener through the front piece into the inner thread on the the central piece.

      That way, you'll be tightening one or the other, whichever way you turn the back piece, but you can remove by removing the fastener first, then the front piece.
       
    11. john12

      john12 Well-Known Member

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      Hmmm, yeah, possibly. The centre is already quite packed with electronics and so on but I could make this work somehow. Thanks for the inspiration anyway!
       

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