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Threaded stop without locking?

  1. May 20, 2016 #1
    Is it possible to use a thread to stop the rotation of a nut without locking the nut? For instance, I want a nut to rotate 90 degrees only. That's the easy part. But I want to be able to reverse the rotation without having to apply force to break the nut.

    In other words, I want to nut to rotation CW or CCW 90 degrees with locking.

    I'm unaware of a threaded solution and that makes sense as locking a nut is generally desirable and the purpose of a threaded nut. But perhaps there's a special thread of which I'm unaware. I achieve the action now with a pin a slot.
     
  2. jcsd
  3. May 20, 2016 #2

    berkeman

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    Welcome to the PF.

    Maybe use a wingnut with stops?

    http://ecx.images-amazon.com/images/I/41OKwQ0IBSL._SX300_.jpg
    41OKwQ0IBSL._SX300_.jpg
     
  4. May 20, 2016 #3
    I'm confused. What's unique about the threads of a wingnut? Are you suggesting I use the "wings" against a stop?

    I'm using a pin on the axle and a slot in the nut now.
     
  5. May 20, 2016 #4

    Bandit127

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    Bayonet fittings are better designed to do what you require.
     
  6. May 21, 2016 #5
    Indeed, what I described is basically a bayonet fitting. It just seems to me a threaded union without locking would be better. Again, I have no idea how to accomplish that. I was hoping there may be some obscure thread type unknown to me.
     
  7. May 21, 2016 #6

    Baluncore

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    The OP describes the movement of a nut. It does not say if the nut has any resistance at any point. We need more information about the application and environment before we can give sensible solutions.

    There is a whole field of unusual special purpose threads. For example ...
    https://en.wikipedia.org/wiki/Interrupted_screw
    https://en.wikipedia.org/wiki/Welin_breech_block

    There is also the 'quick release fastener' that has two short spiral grooves that engage with bayonet pins.
     
  8. May 21, 2016 #7

    CWatters

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    Perhaps use a pin that is smaller in diameter than the notch in the nut.

    Diagram to follow.
     
  9. May 21, 2016 #8
    Now we're talking. An interrupted screw is worth looking into.

    To answer the question, no, the nut has no resistance while rotating. I use the term nut for illustration. The actual application is a tooth which rotates 90 degrees on and off. At 0 degrees rotation (off) it's not engaged while at 90 degrees (on) it's engaged and there's a constant rotational force applied. If I use a standard thread that force will lock the tooth, requiring a force to break the lock. I want to avoid any force to reverse the rotation.

    Currently, the tooth has a smooth bore rotating about a smooth shaft. I stop the rotation at 90 degrees with a pin on the shaft moving in a slot in the tooth.
     
  10. May 21, 2016 #9
    I just saw this response. I'll look forward to the diagram.
     
  11. May 21, 2016 #10

    CWatters

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    Don't get too excited. Not sure it's accurate enough for what you want. I was just thinking of enlarging the notches in a castellated nut to allow it to rotate 90 degrees...

    Nut.jpg
     
  12. May 21, 2016 #11

    Baluncore

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    You have not identified the application or the scale of the device.
    What does the "tooth" do ?
    How big is it ?
     
  13. May 21, 2016 #12
    Small. It engages a bicycle chain.
     
  14. May 22, 2016 #13

    Baluncore

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  15. May 22, 2016 #14
    No, it move 90 degrees CW and then 90 degrees back CCW.
     
  16. May 22, 2016 #15

    Baluncore

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    I suspect you are a paranoid inventor who wants us to do your inventive work for you, or maybe you have a technical communication problem? Either way, you are being so secretive about the application that I really cannot help you.
     
  17. May 22, 2016 #16
    How in the world am I being secretive? I'm simply wondering if there's a better way than a pin and slot.
     
  18. May 22, 2016 #17

    Baluncore

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    To do what ?
    How many axes, how many degrees of freedom ?
    Under what loads in either state ?
     
    Last edited: May 22, 2016
  19. May 23, 2016 #18
    I do not mean to be rude and I'm sincerely not trying to be snarky. But I just don't know how I can be more clear. You're making this too complicated.

    I initually used the nut example as I thought it clear. I still do. I then explained it's a tooth that engages a bicycle chain as I feared I was being taken too literally. How that's being secretive totally escapes me.

    Q: To do what?
    A: To engage and move a bicycle chain. It might be used as an expanding chain wheel. However, to refer to me as an inventor is a stretch at best. I simply have an honest question which should be proper on a form designed for questions. I consider your # 15 post in rather bad form.

    Q: How many axes, how many degrees of freedom?
    A: Again, it rotates about a shaft just as a nut. It moves 90 degrees either CW or CCW.

    Q: Under what loads in either state?
    A: Again, It's at load in at only one extreme of rotation when it engages the chain. As to how great a load. I don't know.

    My guess is this thread has run it's course, and that's fine. I do appreciate the interrupted thread response and consider it a very good reply. It does seem an interrupted thread still requires a force to break, but it was a good idea.
     
  20. May 23, 2016 #19

    Nidum

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    There may be solutions to this design problem using :

    A controlled application of friction. Possibly based on the Nyloc nut principle .

    A two action motion . Something like a lever which is pressed down to release before turning .
     
  21. May 23, 2016 #20

    Baluncore

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    If the thread pitch is fine and the angle is very small, the nut will lock when tightened. If the pitch is coarse and the angle too steep, the nut will never lock when tightened. All simple fastener threads are fine enough to lock. If you know the pitch of a thread and you measure the diameter of a threaded bolt at the half depth points you can calculate the angle of the thread, α, as if it was a wedge wrapped around the bolt. Tan( α ) = pitch / ( π * diameter).
    The critical difference between a locking thread and a freely unlocking thread is determined by the coefficient of static friction between the nut and screw thread. The coefficient of static friction is then Tan( α ). For steel on steel lubricated by oil you have a coefficient of about 0.16 The critical angle is then Atan( 0.16 ) = 9.1° It needs a steeper angle than α = 9.1° to prevent locking.

    A stepped interrupted thread can be made to seat against the step. It does not have to lock.
     
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