I have a required clamping force for fastening two pieces of metal together, and am trying to work out if an m4 bolt will be sufficient. The pieces will be fastened without a nut, just a hole through one piece and a threaded hole in the aluminium block it needs clamping to.
The clamping force that the fastener must be able to withstand is 3400N, so I have found that an m4 bolt should be suitable at grade 8.8 or higher (with an allowance for 75% of proof strength) https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html.
However, how can I verify that the thread in the aluminium plate will be able to withstand this force, or does the size of the hole not effect this (eg. if it can withstand it for an m6 bolt it will be fine with an m4)?
Diagram to show hole placement and depth in aluminium.enter image description here

  • $\begingroup$ You interchange the words block and plate where you say the thread will be, so state the thickness of this part as the number of threads will be crucial to the strength needed. $\endgroup$
    – Solar Mike
    Aug 2 '19 at 8:54
  • $\begingroup$ The threaded hole in the aluminium can be up to 10mm deep, the aluminium itself is 140mm deep. The width of the plate where the hole will be needs to be as narrow as possible, which is why I would like to use a smaller size bolt. I was thinking of 8mm wide plate if an m4 bolt can be used. $\endgroup$
    – Sam B
    Aug 2 '19 at 9:44
  • $\begingroup$ Add a diagram with the detail you have just explained to your original question. $\endgroup$
    – Solar Mike
    Aug 2 '19 at 9:48
  • $\begingroup$ Be sure to lubricate the threads as aluminum is prone to galling. $\endgroup$ Sep 1 '19 at 14:38
  • $\begingroup$ There are two failure mechanisms on the aluminum plate - thread and plate. For checking the threads of the aluminum you need to compare the tensile strength of the bolt and the plate. Yes, the hole size matters. It will affect the tensile capacity of the aluminum plate as the available thickness to resist the load is reduced for larger holes. $\endgroup$
    – r13
    Aug 21 at 16:07

You did not give any information regarding the choosing strategy of the bolt. Thus, we will have to assume you chose it correctly.

What you are specifically asking for is the "pull-out strength" of the thread. In case your bolt is quite strong (let's say, made of 180 ksi alloy steel) relatively to the plate material, it is indeed important to verify that the thread would not shear, or pulled out. This pull-out force calculated as follows: the shear strength of the material X pi X thread diameter x thread depth x thread efficiency.

It is a simple one: pi * d is the perimeter of the thread, multiply it be thread depth will give you the total shear area. This area times the shear strength of the material (aluminum in your case) gives you the total shear force needed to pull this thread up. Of course, all of this area works together against the bolt only theoretically. Practically, only a part of it actually works. This is why a thread efficient is added to the formula. Try 1/3 if you want to be on the very safe side. However, you may take a glance over this article - by which a factor of 2/3 was found to be quite realistic.

As long as the clamping force is lower than the pull out force, your aluminum thread will survive.

  • $\begingroup$ I chose the bolt based off of the required clamping force and the proof strength of the bolt, and consideration towards the width of the aluminium it is going into is only 8mm. Is there anything else I need to consider? $\endgroup$
    – Sam B
    Aug 2 '19 at 12:34
  • $\begingroup$ So this computation? $\endgroup$
    – joojaa
    Sep 1 '19 at 13:25

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