Q$7.$ The two $3-$ft$-$long shafts are made of $2014-$T$6$ aluminum. Each has a diameter of $1\mathrm{.}5$ in $.$ and they are connected using the gears fixed to their ends. Their other ends are attached to fixed supports at A and $B.$ They are also supported by bearings at $C$ and $D,$ which allow free rotation of the shafts along their axes. If a torque of $600lb*ft$ is applied to the top gear as shown, determine the maximum shear stress in each shaft.