What is the problem under the current loading of the screw?
What is your thread engagement length?
What temper is the 6061?
Normally a screw torque is given in in/lbs.
The site below says it is 19. There is also a formula in Machinery Handbook for length of engagement. Al has a problem of threads pulling out if thickness is too thin, especially if screw is to be removed after. If material is thin it is better to use a nut plate or rivnut.
You said your units were ft.lb. I am pretty certain you meant in.lb.
Recently, I did a screw torque chart mostly for stainless steel screws. I did not worry about the material of the tapped holes. I made a whole bunch of assumptions and I got 17 lb.in for 8-32UNC.
These assumptions are as follows...
- The holes are deep enough that the screw will fail in tension rather than shear.
- The threads will be lubricated by something, probably Loctite 242.
- The yield stress for 18-8 stainless steel is 55ksi.
That last assumption is complicated. All sorts of stress values are quoted for this material. I used 55ksi because a local vendor quoted it for his stainless steel socket head cap screws, and this was the lowest value I found. Metric cap screws in stainless are usually supposed to be A2 stainless with a tensile strength (ultimate?) of 800N/mm^2 (~115ksi). When I torque down metric screws as per my torque charts, they feel much stronger than 55ksi.
The effect of using Aluminium 6061-T6 for your tapped hole that you need to exceed some minimum thread engagement length to ensure a tensile failure of the screw. As noted above, this is worked out in the Machinery's Handbook.
