# Contact pressure - screw pretension

## Main Question or Discussion Point

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 :

Thanks a lot for any kind of help (calculation steps...) !

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JBA
Gold Member
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).