$2.-$The stepped shaft has a fillet $r=0\mathrm{.}045in$ and is fixed at $A.$ A torque of $155$ lbfxft is applied at C $.$ The maximum shear stress is $7\mathrm{.}9$ ksi $.$ Determine de diameter $d_1$ for a material which an Brinell hardness test is $210.$ Consider $d_2=2\mathrm{.}5in,L_1=9$ in and $L_2=4$ in and the geometric stress concentration factor $($shear stress$)$ is $k_{ts}=1\mathrm{.}85.$