Torquing Stainless screws into Aluminum

[KeninCA]

I'm torquing a stainless steel 4-40 socket head machine screw into a 6061-T6 aluminum threaded hole.

I've located some torque tables that give differing values for different screw materials, but I'm wondering how this applies since I'm going into a threaded hole of a softer material.

We've chosen to use torque values as a manufacturing standard, and I don't want to over-torque what the aluminum threads can handle.

We are using a small amount of blue thread-locker, which may act as a lubricant during torquing.

Does anyone out there have experience in this application?

 

[Mech_Engineer]

You need to calcuate the pull-out strength of the aluminum threads. Depending on your thread engagement, it could be the bolt will fail first, or it could be the threads will pull out.

At my work our standard is preload to 60% of whichever is lower. Here's an older thread on thread pullout strengths: https://www.physicsforums.com/showthread.php?t=389146

Keep in mind that the shear area should be considered (0.5 * pi * shear diameter * engagement). I missed that factor of two, note my signature