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Need help manufacturing something with threads. THREAD FAILURE problem.

  1. Aug 26, 2009 #1
    hello everyone, this is my first post.

    Right now I am trying to figure out which equations to use to solve the following.

    I will have a block that will a threaded rod through it. This block will be next to another larger block. In between these blocks are going to be metal samples.

    Now here is the cool part, when I turn the thread via gears and such, the block that is engaged with the threaded rod will move and help deform the separate metal samples till failure occurs.

    I need to know how much engagement (Le) the block has to have with the threaded rod so failure will not occur.

    Some helpful givens:
    -I am using steel
    -The amount of force needed to break these samples is approximately 300lbs, that is 150 in the opposite directions of the sample.

    assem2.jpg
     
  2. jcsd
  3. Aug 26, 2009 #2

    FredGarvin

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    Length of engagement is based on material (obviously) as well as shear are of the thread form. Most tables list shear stripping area per 1 diameter of length. You can use the 1D stripping area as a starting point to calculate the stripping load over the stripping area and compare to the material allowables for the stresses.

    The tough part is really properly identifying ALL of the forces on your thread, i.e. not just the forces due to the load (preload, etc...).
     
  4. Aug 26, 2009 #3
    Yeah, this is why I decided to use steel.

    It is going to be 4o to 80 threads per an inch. The guy asking me to produce this wants it to be very very very precise...

    Well the equation I have found from McGraw-Hill's series of ME is

    Tao(Shear stress) = [2 * F(force)] / [pi * d(diameter) * Le]

    I'm a little rusty at finding the shear stress of steel, I have all I need using poisson's ratio except the shear strain.

    Does anyone know the average shear strain of steel?

    I can already predict a factor of safety will probably be lower than 2 sadly. But the guy wants it, and it does something pretty simple.

    I can even change the diameter if I want to to decrease Le because I am basically controlling every variable via manufacturing.
     
    Last edited: Aug 26, 2009
  5. Aug 27, 2009 #4
    Can anyone help please? I'm stuck with this problem for my research.
     
  6. Aug 27, 2009 #5

    FredGarvin

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    A rule of thumb for most ductile metals is that ultimate shear stress allowable is 50% of ultimate tensile stress (think of a Mohr's Circle).
     
  7. Aug 27, 2009 #6
    Thanks Fred. Right when you said Mohr's circle, I remembered right away.

    Seeing that I can't send you a PM Fred, I just wanted to ask you on what you think of going straight to work and then possibly getting a MS, or just going straight for the MS?
     
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