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Impact Load Due for Dropped Mass onto Bolt Threads

  1. Mar 18, 2016 #1

    I have an installation tool (which basically consists of a standard long bolt and nut) and a 100 lbs mass object would drop onto it (as if it were a hook) from about 1 - 2 inches. There are many forces to consider and I am looking into that. Since this installation tool will be reusable, I prefer not to have the threads deform too much over time. So I would like to know if there is a way to quantify the deformation on the threads as I drop the object onto it. It seems over my head. Thanks in advance for the help. See the attached image to get a better visual.

    Attached Files:

  2. jcsd
  3. Mar 18, 2016 #2


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    Staff: Mentor

    Welcome back to the PF. :smile:

    Seems like a bad setup. Is there no other way you can do things? Dropping a hard weight onto bare threads, with the resulting thread deformation and bending moment on the bolt, would seem to be a bad design (IMO). Why does it have to be that way?
  4. Mar 19, 2016 #3


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    Science Advisor

    The profile of the thread will be important. You show trapezoidal thread, a bit like acme, on the bolt with a flat tip profile, while real threads that are easier to cut, will appear sharper with less area and so be damaged more easily.

    You show the installation nut as a long nut, a bit like a coupler nut.
    The quick solution to the problem is to make a mounting stud that has:
    1. Only a short thread at one end to fully engage with the body.
    2. An exposed section of bolt that is not threaded, and;
    3. A threaded section for the coupler nut.
    The coupler nut is then partly bored out so it is always runs on and is supported on the section of the stud without thread. Sufficient thread is kept in the coupler nut to carry the clamping load.

    The load then drops onto the un-threaded shank of the mounting stud. Two short nuts locked together on far end of the stud aid insertion and extraction of the threaded tool into the body. Those nuts also prevent exposure of the thread if the coupler nut was withdrawn beyond the un-threaded portion of the shank.
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