Shear strength, yield strength

I can not sem to find an answer to my question in the 26th addition of the machinery handbook.

We have an aluminum nut (threaded onto a steel shaft) that is 16mm with a .1 thread pitch and has 7 threads of engagement. This nut holds it all together . it is a shock absorber of sorts. The spring pulls the shock to full extension and a top out plate that the nut holds on hits the seal head and it is at full travel and the only pressure on the nut -- besides the 25 ft lbs it is torqued to is the stored energy of the spring.

So my question is how much spring force will the aluminum (7075) nut take before the threads shear off ???

Thanks for your help!

Simplify and use equivalents.

I would take the thickness of a single thread along the pitch line (from the handbook) - add up 7 of them and that would be equal to one ring of aluminum that size placed in a ring groove on a steel shaft. Then apply the shear strength of 7075 to the cross sectional area of the ring groove (this is the same as the measured distance along the line where it would tear if it did fail).

Alternatively - Can you put it in a hydraulic press and rig it up to press to failure while reading the pressure gauge? It is exciting - wear safety glasses!