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Calculating maximum load on a thread

  1. May 7, 2012 #1
    Can any one offer a method or location of a method for determining the permissible axial load that can be applied to an internal thread?
    i.e. Take a nut and screw into it two bolts, one from either end so they meet in the middle of the nut, then apply a tension force across the two bolts. How much tension can be applied before the thread brakes?
    The nut in this case is not stamped with its rating or grade etc. Its of a known material with a known tensile and proof strength.
    I'm not interested in pre-tension caused by tightening, as it is hand tightened, really just like to know how much load can be applied in direct tension.

    Any offers?
  2. jcsd
  3. May 7, 2012 #2
    The answer is totally variable, as bolts aren't designed to be half screwed into a nut, you'll get massive variation between similar bolts due to tolerances.

    When a bolts thread is fully engaged, the threads can deform to allow load sharing. If you only engage a couple of threads, they will just strip. So the answer will be 'not a lot' compared to bolt used properly.

    The only way to find out is to test it. Why on earth are you screwing two bolts into one nut anyway?
  4. May 7, 2012 #3
    Sorry for the confusion. The bolt example is only given as an analogy.
    What I have is in fact a components that goes on an underground drill rig.

    The analogy is correct in that what I have is an internally threaded component. There are two internal threads, each of a different diameter, each at a different end of the device.
    What I'm looking for is a recommendation for a method, or source of method to work out how much load can be applied to this device in axial tension before the thread is stripped. i.e. component critical failure.

    For structural or civil engineers it's similar to a tie between two threaded tie rods.

    Round two?
  5. May 8, 2012 #4


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    If the internal thread is deep enough to develop the full strength of the threaded rod screwed into it, then the axial load is limited by the strength of the smaller diameter rod, or the strength (wall thickness) of the threaded device (sleeve). Typically, a 5/8" diameter bolt needs to have a minimum of 5/8" internal thread in the nut (or sleeve), a 2 inch diameter bolt needs 2 inches of thread, etc.
  6. May 9, 2012 #5
    If you are designing this there may or may not be a standard that applies to your industry.
    I know the type of component though as I used to design something similar for penetrators, a female/female adapter (and to make things fun usually a metric to imperial thread).

    As you are not talking about screwing in bolts the bolt tension formula is irrelevent, your axial load for breakage will depend on what's being screwed in, so your own your own with figuring out if that will break.
    However for thread shear, in lieu of having a proper standard, I use the following.


    I don't think this applies to ACME or similar thread forms.
    Last edited: May 9, 2012
  7. May 9, 2012 #6
    In every job I have ever had it was a requirement that the male member had to fail before the female. But if the designer had other intentions, that may not be so. Not enough information here to know for sure one way or the other.
  8. May 9, 2012 #7
    Sorry that I do not answer your qwestion - because I do not have an answer, but I have thought about this also. I presume, at first, the area of the contacting surfaces (nut thread/bolt thread) need to be calculated.
    From the force applied, friction loss is to be subtracted. Then you look up at the material (metal) strength.
    Last edited: May 9, 2012
  9. May 9, 2012 #8
    I also do not have the exact answer, but just suggesting one thing that consider the fact that initial threads engaged are the ones which bear maximum load. So you may want to focus on first few engaging threads instead of whole length of threads
  10. May 9, 2012 #9


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    This is true that the first few threads see the greater part of the load, but standard (Code) thread depths and thread spacing account for this. A 1 inch diameter high strength bolt requires about a 1 inch (heavy hex nut ) depth of thread. Properly threaded, the bolt will probably break before the threads strip.
  11. May 9, 2012 #10
    If you want to get serious about doing this analysis, all of the following are excellent sources, but not the only ones: Shigley, Machinery's Handbook, Bickford. (Actually, the first two site the work of the third.) Every engineering office I've worked in for the last 35 years has had multiple copies of all these available because they are frequently refered to.
  12. May 10, 2012 #11
    Just the thought comes to my mind. I do not know what the type of the thread you work with, we here are basically metric and BSP (G). I would make an experiment with the metric, for instance. Would take M4, M5, M6, M8, M10, measured areas, applying the tensionmeter (forgot the proper name) make the break, would put the results in the form of a diagram. Extending the curve I would have made an estimation what the maximal tension the other (greater sizes) could bear.
  13. May 10, 2012 #12
    Yuri: We had one of those in my last job. We simply called it "The Tensile Tester."

    You don't actually have to do all that testing because all that data is published. But we did it anyway on critical applications because large uncertainties in the input data can cause a large variation from published data.

    But the one thing you don't have to measure is the stress area of the bolt thread. That is good data published in many different reference books.
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