I can't figure out how to keep this screw from loosening!

  1. I'm making a simple "thing" to test out a simple part of something much bigger I'm designing, but am not experienced enough to figure out what I can do to keep the main screw from loosening. Is it even possible to keep the screw from loosening under vibration? If so, what can I add (and where) to keep this screw in place under moderate vibration?

    It's basically a screw going through a threaded piece of metal. The metal strip is attached at it's center to a bow-like apparatus (two strips of thin metal attached to each other that will bend). The threaded metal piece is attached to the inner strip and the screw, coming through a hole in the inner strip, is attached to the outer strip. When the screw is tightened, it will expand the bow. The formatting wouldn't work, so imagine the s's are empty space.
    sssssssssssss| ___|
    sssssss __---sss|sss--- __
    __ ---sssssssss|sssssss--- __
    sss --- __ssssss|ssssssss---
    sssssssssss--- _|_---
    There's probably a very simple solution to this, but I have no experience with any of this stuff. I'm just a math guy LOL.
    Last edited: Mar 8, 2010
  2. jcsd
  3. Borek

    Staff: Mentor

  4. I feel stupid saying this, but I have no idea what that is. I can only think of a pin as in the sewing device.

    Maybe I should take some engineering classes when I enter college next year. :rofl:
  5. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Is the screw into a tapped hole in the other part, or is it a nut and bolt?
    Easy option is screwlock, like superglue that you put in the hole first. There are different types, some can be easily removed by a bit more force, others are almost impossible - check which type first!

    For a nut and bolt there are more options.

    Lock nuts - if you put two nuts and tighten them against each other (not necessary to tighten them to the part) they are much less likely to undo

    Vibration nuts have a rubber ring inside the thread that stops them unwinding under reasonable vibration - look for shake-free or anti vibration.

    Lock washer - a serated washer that bites into the nut and the part to stop it turning, means the nut must be tight against the part and it will damage the surface.

    If the nut is big enough you could have a pin through it as Borek said.
    edit - you drill sideways through the bolt and the material and put a pin through so the bolt can't rotate
  6. I'm not worried about the nut loosening, it's the screw that's in the threaded metal piece I'm worried about. I'm pretty sure that vibration pushing on the nut will cause the screw to loosen, which will cause the bow to retract. The bow needs to stay in place for long periods of time, but also be adjustable.
  7. stewartcs

    stewartcs 2,244
    Science Advisor

    Use Loctite on the threads as mgb suggested.

  8. Wouldn't Loctite make tightening and loosening the screw difficult? I always thought Loctite was a more permanent thing. This screw will be adjusted all the time, so it can't be that difficult to use.
    Last edited: Mar 8, 2010
  9. turbo

    turbo 7,063
    Gold Member

    There are nuts with nylon inserts that are VERY resistant to loosening. Can you put the bolt through the bow in such a way that such a nut would prevent loosening? If you could draw a picture, scan it, and post it, it would be a lot easier to figure out what you're trying to accomplish.
  10. stewartcs

    stewartcs 2,244
    Science Advisor

    If it requires constant tightening and loosening then Loctite is a bad choice.

  11. stewartcs

    stewartcs 2,244
    Science Advisor

    What type of head is on the screw? You might be able to make a latching device that locks the screw in place and is easy to remove when no in use.

  12. Q_Goest

    Q_Goest 2,989
    Science Advisor
    Homework Helper
    Gold Member

  13. [​IMG]

    The top thin metal strip has a hole for the screw and is attached to the threaded piece of metal and the bottom strip is attached to the nut. The two thin strips are also attached to eachother at their ends. When the screw is tightened, it will push the bottom strip farther down. Since the goal is to put adjustable pressure against something (which will vibrate), a quick release, which pushes down farther when engaged, will be a pain to use. Is there some type of quick release that will not push down farther, keeping the lower strip in the same place?
  14. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Classic locknut case

    Put another nut between the head end of the srcew and the threaded metal.
    You tighten the new nut against the threaded metal surface to lock the bolt.
  15. That will work, but for the final product I'm looking for something with a knob or lever that you can do with one hand. It needs to take a few seconds, just tighten or loosen the knob or lever and lock it somehow.
  16. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper


    On a small precision screw you would put a thin metal plate spring (or a spring washer) between the threaded metal and the head.
    This also makes sure that the same face of the thread is in contact with the metal and so avoids 'backlash error'
    Last edited: Mar 8, 2010
  17. With just a wing nut it would become loose and with a wingnut and a nut it would take too much time to do everything. I'm looking for a two-step method, just tighten/loosen and lock. Is there anything in existence that can do that?
  18. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    I meant the wing not as a lock nut on the top, it then just takes a flick to unlock the wing nut and a flick to retighten it.

    Other option depends on scale,
    For a little gadget you would put something very like the thin metal strips on the top of the threaded plate - that will effectively pull the bolt tight (upward) against the metal - but that only works for fine low force applications.

    For bigger loads you could have a bent piece of metal bolted/welded to the top plate and coming up at an angle - with a slightly oversized hole drilled for the bolt.
    You would push the bar down flat so that the bolt was free to rotate - then when you let go it springs up and the hole is now at a slight angle to the bolt and so grips it.
    It will damage a precision thread but it's fine for something chunkier.
  19. Your screw threads are lightly loaded against the threads of the plate hence there's little stiction and it's easy for the threads to turn in respect to the plate.

    One really secure solution would be to drill out the threaded plate and press in something like a CN, PL, or PLC series PEM fastener. These have packing material that resists vibration.
  20. What about a cam lever on top?
  21. Maybe its just you who has the screw loose :P

    - Welltec
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