Skill and experience are the foundation of most good luck.

Your measurement ratio is not figured correctly. First, when considering the 'meat' of the screw, you need to consider the root diameter. For a #6 screw that is 0.1065. Next, when comparing area you don't do it by adding diameters, you need to add up area. The area of metal of a #6 screw is 0.1065sq x 3.14/4 = 0.0089sq in, or 0.0267 for three of them. The area of your main shaft (5/16 x 24), at the root, is 0.26" dia, about 0.053 sq in, or about twice the area of the screws used. Don't forget the pulse loading and severe vibration going on there either. Fatigue failure will be the problem and that is very hard to predict, particularly with those stress risers in the v-groove of the #6 threads. I think most manufactures use at least a high strength #10 screw and that is a good guide.

Keep at it!

Bedford