grandnat_6: I have almost always used Ssu = shear ultimate strength = 0.60*Stu, and Ssy = shear yield strength = 0.577*Sty, where Stu = tensile ultimate strength, and Sty = tensile yield strength. Most textbooks claim the above. I currently do not know why Machinery's Handbook instead says Stu = 0.75*Stu.
The above values are shear strength, and tensile strength, the point where the material shears or ruptures. These material strength values do not include a safety factor. They are material strength values, not allowable stress.
I am still currently leaning toward a yield factor of safety of FSy = 3 for your arm beams, and perhaps FSy = 5 for the pins. The factor of safety is the same in tension and compression. The allowable tensile (or compressive) stress is Sta = Sty/FSy. The allowable shear stress is Ssa = Ssu/FSu = 0.60*Stu/FSu, where FSu = ultimate factor of safety. Because your current FSy values are so high, you can just use FSu = FSy, for now.
We do not yet know the tensile yield strength (Sty) of your A513 steel tubes, because you did not state an SAE steel grade designation yet. A513 covers a lot of SAE steel grade designations (SAE 1008, 1010, 1020, 4130, 4140, just to name a few). You (your supplier) must state the SAE steel grade designation, before we can look up the strength of your A513. If no SAE grade designation for A513 tubes is stated, then we would be forced to assume SAE 1008.
And, your supplier must state whether the A513 steel tube thermal condition is as-welded (not annealed), normalized, DOM, or DOM stress-relieved, before we can look up the strength. If no thermal condition for A513 tubes is stated, then we would be required to assume normalized, or perhaps as-welded, depending on the SAE grade designation. However, A500, grade B, on the other hand, specifically defines a strength.
