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Contact pressure - screw pretension

  1. Jun 26, 2016 #1
    Hello,
    I'd like to calculate the max contact pressure in the below assembly, just before contact loss between the beam and the wall. The beam is clamped to the wall with a pretensioned screw, and sustaining a force at his gravity center.

    I'm assuming all bodies dimensionnaly stable (no distorsion).

    I wonder about few questions... how to calculate the maximum force at center of gravity before contact loss ? how to work out the composed pressure profile...

    Here are some sketches that may help understanding the problem :

    5K3J9.jpg

    VZEoh.jpg

    Thanks a lot for any kind of help (calculation steps...) !
     
  2. jcsd
  3. Jun 26, 2016 #2

    JBA

    User Avatar

    The beam will not lose contact with the end plate until the screw exceeds its screw thread tensile failure stress (assuming the screw nut is properly sized so the thread shear failure load in the nut is greater than the tensile failure load of the screw).
    As long as the initial friction force due to the pretension on the retaining screw is greater than the lateral beam load, the friction load on the end plate will remain constant but the contact pressure area on the wall will progressively move toward the bottom edge of the beam as the pretensioned screw is stretched.

    Edit: In my above initial post I misspoke when I stated: (assuming the screw nut is properly sized so the thread shear failure load in the nut is greater than the tensile failure load of the screw).
    That should have read: (assuming the bolt's engaged thread depth is correctly designed so the thread shear failure load of the engaged threads is greater than the tensile failure load of the screw).

    An added note: The above statement about the initial preload vs friction force is correct, there are two stages of beam end to mounting plate contact loading as the bolt is stretched. Within the elastic range of the bolt stretch the beam end to mounting plate contact force; and, therefore the friction force will increase until the bolt's elastic limit is reached; which at that point, that level of increased amount of contact load will persist until the point of tensile failure of the bolt.
     
    Last edited: Jun 26, 2016
  4. Jun 28, 2016 #3
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