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Need guidance in thread design

  1. Jun 7, 2012 #1
    Hi all,
    I would like to know how to design a thread/screw to induce a specific stress on an object. What I have in numerical data is the stress itself that is required.
    The application is very similar to that of a threaded car jack. By turning the thread, it will lift up the load that was placed on the jack. In my case, what I have is a cylindrical structure with 4 threads (which I need to design) placed parallel and symmetrically around the cylinder. These threads will be strapped onto the cylinder so that by turning a nut on each of the threads, it will put the cylinder into tension and eventually stretch it apart.
    I would like to know if anyone here is familiar with such thread design to get me started on this project.
    I apologize for the poor explanation as it is quite difficult without a visual diagram.

    Thank you!
     
  2. jcsd
  3. Jun 7, 2012 #2

    Danger

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    Gold Member

  4. Jun 7, 2012 #3
    Thank you for the welcome,
    I have actually browsed through the link you provided and it is still not clear to me as all I see are formulas.
    To those familiar, correct me if I'm wrong but as I see from a machine design text book, it seems to me I must use the equation stress = force / tensile stress area of the threaded rod. This is where I am a bit thrown off, so if I have the stress already and I want to find the tensile stress area in order to find the diameters of thread that I want, do I need to randomly select a value of force and keep iterating until I come to a certain thread diameter suitable for my application?
     
  5. Jun 7, 2012 #4

    Danger

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    Gold Member

    I see what you mean now. I'm afraid that I know nothing about the subject. The best that I can think of is that you just treat the thread as an inclined plane and use whatever calculations are appropriate to that.
    Your mention of having the thread "strapped on" to the cylinder really puzzles me.
    Anyhow, I've about outlived my usefulness. I hope that someone else can be of more help. Meanwhile, perhaps the following can be of assistance in modeling your device:
    http://www.thingiverse.com/thing:8793
     
  6. Jun 8, 2012 #5
    Thanks for the reply and model Danger.
    Anyone else know how to design threads???
     
  7. Jun 8, 2012 #6
    What you are looking to design is basically what Instron (and similar companies) have designed. Stress and Strain go pretty well hand in hand, and what you are going to be doing is imparting a stress on the cylinder by way of strain. Really, all you need for this is the pitch of the thread so that you can determine how much rise you get per turn. if you get 2 mm per turn, you can determine how much strain you've imparted on the object and from that calculate the stress (more like estimate closely, since you'll need the modulus of elasticity).

    Pick your thread diameter for well above the maximum stress the thing is expected to experience. You'll want to balance the size of the rod with the thread pitch angle. You want a nice thick peice of metal so it doesn't snap (and also because you don't want your stretching device to stretch itself! That would throw off everything) but you also want a shallow pitch so that you can control the accuracy of the strain being applied.

    Edit:
    These may help:
    http://staff.fit.ac.cy/eng.la/AMEM316/MDESIGN%20Power%20Screw.pdf [Broken]
    Link 2
     
    Last edited by a moderator: May 6, 2017
  8. Jun 12, 2012 #7
    That was of great help, thank you Travis King and Danger !
     
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