Shear failure in threaded fasteners

  1. Hi
    How can i derive an equation to prove that a bolt wont fail due to shear stress if the length of thread engagement of the bolt with nut is equal to the diameter of the bolt.

    I got a formula, but i dont know whether it is useful in this case.

    A_s,e = pi*n*L_e*K_nmax*[1/2n + 1/sqrt3*(E_s,min-K_nmax)]
    where
    n=threads/inch
    L_e=Length of engagement
    K_nmax=maximum inner diameter of nut
    E_s,min=minimum pitch diameter of bolt

    thanks
     
  2. jcsd
  3. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Are you talking shear stresses in the threads or in the bolt shank? You are listing the shear area for failure of the threads.
     
  4. The shear stress in thread.

    Thanks for your response :)
     
  5. FredGarvin

    FredGarvin 5,087
    Science Advisor

    The equation you state is the calculation of the shear area of an external threaded fastener. It is in a slightly different form than what I am used to, but it is fine the way you have it. The easy thing about your question is that you are putting the constraint that the engagement is equal to 1D. That is the way most shear area tables for fasteners is made. The tables list shear area for threads when Le = 1D. Tables like this are available in texts like Bickford and others on fastener strength.
     
  6. Thanks for your reply. I think the formula what is stated (FOR Shear area) is of no use. What i'm looking for is a derivation or a formula which can say that the bolt wont fail due to shear stress, if Length of engagement between = the dia of the bolt. But unfortunately i couldn't find it anywhere :(.

    Thanks for your support
     
  7. Gokul43201

    Gokul43201 11,141
    Staff Emeritus
    Science Advisor
    Gold Member

    Think about it first.

    What does it mean if the bolt does not fail by shear in the threads?
     
  8. Need to take into account the material fastner is made out of, nylon, mild steel, galvanised, zinc coated, high tensile, stainless steel.
    Being able to shear will depend on how maluble the material is
     
  9. To make things simple.
    the length of engagement of bolt = diameter of the bolt.

    But i dont know how to prove that it wont fail. I couldn't find a concrete material for this.
    I am a kid in this field. So i need your advice.
     
  10. The minimum recommended length of engagement is 0.75 to 1 times the bolt major diameter for threading into steel and other materials of comparable hardness; and 1 to 1.5 times the bolt major diameter for threading into brass, cast iron, and aluminum. These lengths of engagement specified are for full threads engaged, not overall bolt length.
     
  11. could someone please help me in this regard.
     
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