How much weight can a bolt take before the threads pull out?

  1. Hi All,
    I was browsing the internet and came across this forum, I was thinking to myself one day and decided to do a google search.
    My question is basically, how do I determine how much weight a bolt can take before it pulls out of the thread.

    For example, I want to attach Part A to Part B with bolts and I am trying to determine the proper size. I am sure there is a formula that will tell me how much weight a bolt can take based on what material it is threaded in to, but I cant seem to find it.
    I have my machinists handbook out but am having no luck, a point in the right direction would be much appreciated. I also have an old physics book and even if you could point me to the section of the book that deals with this concept, that would be appreciated also.

    Thanks
    Rich
     
  2. jcsd
  3. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    Generally to determine pull out strength of a thread you will basically calculate the shear area (pi * shear diameter * length of engagement), and multiply by the shear strength of the softer component in the bolted joint.

    You must decide which is the stronger material (or if they're both about the same)- the internal thread or external thread. If the internal thread (threaded hole) is stonger, your shear diameter should be about the minor diameter of the thread. If the external thread (bolt) is stronger, the shear diameter will be the major diameter of the thread. If both materials are about the same in terms of strength, use the pitch diameter of the thread.

    This is a quick estimate, so make sure and build some safety factor in there as well. Generally as long as your length of engagement is >= 1.5x the nominal diameter of the thread, the fastener will snap before the threads pull out. However this is dependent on the materials being used.
     
  4. Q_Goest

    Q_Goest 2,966
    Science Advisor
    Homework Helper
    Gold Member

  5. thanks a bunch guys
     
  6. MIL-HDBK-60 is good place to start.
     
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